Ultrastructural observations of synaptic connections of vibrissa afferent terminals in cat principal sensory nucleus and morphometry of related synaptic elements

Previous work suggests that slowly adapting (SA) periodontal afferents have different synaptic arrangements in the principal (Vp) and oral trigeminal nuclei and that the synaptic structure associated with transmitter release may be related directly to bouton size. The present study examined the ultrastructures of SA and fast adapting (FA) vibrissa afferents and their associated unlabeled axonal endings in the cat Vp by using intra-axonal labeling with horseradish peroxidase and a morphometric analysis. All SA and FA afferent boutons contained clear, round, synaptic vesicles. All the FA and most SA boutons were presynaptic to dendrites, but a few SA boutons were axosomatic. Both types of bouton were frequently postsynaptic to unlabeled axonal ending(s) containing pleomorphic, synaptic vesicles (P-ending). The size of labeled boutons was larger in FA than SA afferents, but the size of dendrites postsynaptic to labeled boutons was larger for SA than FA afferents. Large-sized FA and SA boutons made synaptic contacts with small-diameter dendrites. The size of FA and SA boutons was larger than that of their associated P-endings. A morphometric analysis made on the pooled data of SA and FA boutons indicated that apposed surface area, active zone number, total active zone area, vesicle number, and mitochondrial volume were highly correlated in a positive linear manner with labeled bouton volume. These relationships were also applicable to unlabeled P-endings, but the range of each parameter was smaller than that of the labeled boutons. These observations provide evidence that the two functionally distinct types of vibrissa afferent manifest unique differences but share certain structural features in the synaptic organization and that the ultrastructural "size principle" proposed by Pierce and Mendell ([1993] J. Neurosci. 13:4748-4763) for Ia-motoneuron synapses is applicable to the somatosensory system.     

Nucleus preeminentialis of mormyrid fish, a center for recurrent electrosensory feedback. I. Electrosensory and corollary discharge responses

1. The nucleus preeminentialis (PE) is a large central structure that projects both directly and indirectly to the electrosensory lobe (ELL) where the primary afferents from electroreceptors terminate. PE receives electrosensory input directly from ELL and also from higher stages of the electrosensory pathway. PE is thus an important part of a central feedback loop that returns electrosensory information from higher stages of the system to the initial stage in ELL. 2. This study describes the field potentials and single-unit activity that are evoked in PE by electrosensory stimuli and by corollary discharge signals associated with the motor command that drives the electric organ to discharge. All recordings were extracellular in this study. 3. Two types of negative-going corollary discharge-evoked field potentials were found in PE: 1) a shallow, long-lasting negative wave with a latency at the peak of approximately 11 ms, and 2) a more sharply falling and larger negative wave with a shorter latency at the peak of approximately 9 ms. The long-latency wave was predominant in the dorsolateral and posterior parts of PE, whereas the short-latency wave was predominant in the medial and rostral regions. Both waves were only found in PE and thus can serve for its identification. 4. Electrosensory stimuli given either locally to a restricted skin region or symmetrically to the entire body evoked characteristic field potentials in both regions of PE. The mean latency between the stimulus and the peak of the response was 6.9 ms in the early negativity region and 12.2 ms in the late negative region. The responses to such stimuli were strongly facilitated by the electric organ corollary discharge. 5. Field potential responses to the electric organ corollary discharge were markedly plastic. Responses to the corollary discharge plus a paired electrosensory stimulus decreased over time and the response to the corollary discharge alone was markedly enhanced after a period of such pairing. 6. Local electrosensory stimulation of the skin showed that the caudal-rostral body axis is mapped from dorsal-medial to ventral-lateral in PE. The same somatotopy was found in the regions of the early and late negatives. The ventral and dorsal body appeared not to be separately mapped in PE. The areas representing the head and chin appendage ("Schnauzenorgan") are especially large in PE, due presumably to the high density of electroreceptors in these areas. 7. Two main types of units were recorded in PE: 1) inhibitory (I) cells with a corollary discharge response that was inhibited by an electrosensory stimulus to the center of their receptive fields; and 2) excitatory (E) cells with an excitatory response to electrosensory stimuli that was facilitated by the corollary discharge. Some of the E cells responded to the corollary discharge alone and some did not. Most cells appeared to be responding to input from mormyromast electroreceptors, but a few cells were driven by ampullary electroreceptors and a few by Knollenorgan electroreceptors. 8. The corollary discharge effects on I cells and E cells were plastic and depended on previous pairing with a sensory stimulus. The corollary discharge facilitation of E cells and inhibition of I cells decreased during pairing with a sensory stimulus, and the corollary discharge-driven excitation of I cells was much larger after pairing than before. 9. The results provide an initial overview of a major component in the control of electrosensory information processing by recurrent feedback from higher stages of the system.     

Parallel projection of amplitude and phase information from the hindbrain to the midbrain of the African electric fish Gymnarchus niloticus

Two distinct sensory cues in electrosensory signals, amplitude modulation and differential phase modulation, are essential for an African wave-type electric fish, Gymnarchus, to perform the jamming avoidance responses. Individual neurons in the first brain station for central processing, the electrosensory lateral line lobe (ELL), were investigated by the in vivo whole-cell recording and labeling technique for their physiological responses, location, morphology, and projection areas. Neurons in the dorsal zone of the ELL responded selectively to amplitude modulation. Neurons in the outer cell layer of the medial zone were categorized physiologically into two groups: amplitude-sensitive and differential phase-sensitive. All but one neuron in the inner cell layer of the medial zone responded exclusively to differential phase modulation. All neurons recorded and labeled in the ELL had pyramidal morphology with large and extensive apical dendrites and less extensive basal dendrites. They were found to project to two midbrain nuclei: the nucleus praeeminentialis and the torus semicircularis. Amplitude-sensitive neurons in the dorsal zone projected exclusively to the lateral posterior subdivision, the torus semicircularis. Neurons in the medial zone projected to the medial dorsal and lateral anterior subdivisions of the torus semicircularis. Although some neurons in the ELL responded to both amplitude and differential phase modulation, they did not differentiate between temporal patterns of the two cues that encode necessary information for the jamming avoidance response. Overlapping projection of amplitude and differential phase-sensitive neurons to the torus semicircularis suggests integration of the two sensory cues in this nucleus.     

Inhibition evoked from primary afferents in the electrosensory lateral line lobe of the weakly electric fish (Apteronotus leptorhynchus)

Distal versus proximal inhibitory shaping of feedback excitation in the electrosensory lateral line lobe: implications for sensory filtering. J. Neurophysiol. 80: 3214-3232, 1998. The inhibition controlling the indirect descending feedback (parallel fibers originating from cerebellar granule cells in the eminentia posterior pars granularis) to electrosensory lateral line lobe (ELL) pyramidal cells was studied using intracellular recording techniques in vitro. Parallel fibers (PF) contact stellate cells and dendrites of ventral molecular layer (VML) GABAergic interneurons. Stellate cells provide local input to pyramidal cell distal dendrites, whereas VML cells contact their somata and proximal dendrites. Single-pulse stimulation of PF evoked graded excitatory postsynaptic potentials (EPSPs) that were blocked by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl--aspartate (NMDA) antagonists. The EPSPs peaked at 6.4 +/- 1.8 ms (mean +/- SE; n = 11) but took >50 ms to decay completely. Tetanic stimulation (100 ms, 100 Hz) produced a depolarizing wave with individual EPSPs superimposed. The absolute amplitude of the individual EPSPs decreased during the train. Spike rates, established by injected current, mostly were increased, but in some cells were decreased, by tetanic stimulation. Global application of a gamma-aminobutyric acid-A (GABAA) antagonist to the recorded cell's soma and apical dendritic region increased the EPSP peak and decay phase amplitudes. Tetanic stimulation always increased current-evoked spike rates after GABAA blockade during, and for several hundred milliseconds after, the stimulus. Application of a GABAB antagonist did not have any significant effects on the PF-evoked response. This, and the lack of any long hyperpolarizing inhibitory postsynaptic potentials, suggests that VML and stellate cell inhibition does not involve GABAB receptors. Focal GABAA antagonist applications to the dorsal molecular layer (DML) and pyramidal cell layer (PCL) had contrasting effects on PF-evoked EPSPs. DML GABAA blockade significantly increased the EPSP peak amplitude but not the decay phase of the EPSP, whereas PCL GABAA-blockade significantly increased the decay phase, but not the EPSP peak, amplitude. The order of antagonist application did not affect the outcome. On the basis of the known circuitry of the ELL, we conclude that the distal inhibition originated from GABAergic molecular layer stellate cells and the proximal inhibition originated from GABAergic cells of the ventral molecular layer (VML cells). Computer modeling of distal and proximal inhibition suggests that intrinsic differences in IPSP dynamics between the distal and proximal sites may be amplified by voltage-dependent NMDA receptor and persistent sodium currents. We propose that the different time courses of stellate cell and VML cell inhibition allows them to act as low- and high-pass filters respectively on indirect descending feedback to ELL pyramidal cells.     

The ventral lateral geniculate nucleus, pars lateralis of the rat. Synaptic organization and conditions for axonal sprouting

The synaptic organization of the pars lateralis portion of the ventral lateral geniculate nucleus is similar to that of other thalamic nuclei. There are four types of synaptic knobs (RL, RS, F1, F2). RL knobs are large and irregularly shaped, contain round synaptic vesicles and make multiple asymmetrical junctions. They are found primarily in "synaptic islands" making contact with gemmules, spines, small dendrites, and other synaptic profiles containing pleiomorphic synaptic vesicles (F2). Smaller RS knobs contain round vesicles and make asymmetrical junctions with the same type of elements as RL knobs, with the exception of the F2 profiles, but are seldom found in synaptic islands. F1 knobs contain flattened synaptic vesicles and form symmetrical junctions with F2 knobs, gemmules, spines, and small-medium dendrites in synaptic islands, throughout the neuropil, and on the proximal dendrites and soma of the largest type of neuron. F2 knobs are irregularly shaped, contain pleiomorphic synaptic vesicles and make symmetrical junctions primarily with gemmules and spines in synaptic islands. They are postsynaptic to RL and F1 knobs. Occipital decortication indicates that cortical terminals are of the RS type. Bilateral enucleation indicates that retinal terminals are of both the FL and RS type. The large amount of geographic overlap of retinal and cortical terminals on gemmules, spines, and small dendrites found in the neuropil outside of synaptic islands logically would maximize axonal sprouting between these two sources.     

Mormyrid electrosensory lobe in vitro: morphology of cells and circuits

The electrosensory lobe (ELL) of mormyrid electric fish is a cerebellum-like brainstem structure that receives the primary afferent fibers from electroreceptors in the skin. The ELL and similar sensory structures in other fish receive extensive input from other central sources in addition to the peripheral input. The responses to some of these central inputs are adaptive and serve to minimize the effects of predictable sensory inputs. Understanding the interaction between peripheral and central inputs to the mormyrid ELL requires knowledge of its functional circuitry, and this paper examines this circuitry in the in vitro slice preparation and describes the axonal and dendritic morphology of major ELL cell types based on intracellular labeling with biocytin. The cells described include medium ganglion cells, large ganglion cells, large fusiform cells, thick-smooth dendrite cells, small fusiform cells, granule cells, and primary afferent fibers. The medium ganglion cells are Purkinje-like interneurons that terminate on the two types of efferent cells, i.e., large ganglion and large fusiform cells, as well as on each other. These medium ganglion cells fall into two morphologically distinct types based on the distributions of basal dendrites and axons. These distributions suggest hypotheses about the basic circuit of the ELL that have important functional consequences, such as enhancement of contrast between "on" elements that are excited by increased afferent activity and "off" elements that are inhibited.     

A regional ultrastructural analysis of the cellular and synaptic architecture in the chinchilla cristae ampullares

The chinchilla crista ampullaris was studied in 10 samples, each containing 32 consecutive ultrathin sections of the entire neuroepithelium. Dissector methods were used to estimate the incidence of various synaptic features, and results were confirmed in completely reconstructed hair cells. There are large regional variations in cellular and synaptic architecture. Type I and type II hair cells are shorter, broader, and less densely packed in the central zone than in the intermediate and peripheral zones. Complex calyx endings are most common centrally. On average, there are 15-20 ribbon synapses and 25-30 calyceal invaginations in each type I hair cell. Synapses and invaginations are most numerous centrally. Central type II hair cells receive considerably fewer afferent boutons than do peripheral type II hair cells, but have similar numbers of ribbon synapses. The numbers are similar because central type II hair cells make more synapses with the outer faces of calyx endings and with individual afferent boutons. Most afferent boutons get one ribbon synapse. Boutons without ribbon synapses were only found peripherally, and boutons getting multiple synapses were most frequent centrally. Throughout the neuroepithelium, there is an average of three to four efferent boutons on each type II hair cell and calyx ending. Reciprocal synapses are rare. Most synaptic ribbons in type I hair cells are spherules; those in type II hair cells can be spherical or elongated and are particularly heterogeneous centrally. Consistent with the proposal that the crista is concentrically organized, the intermediate and peripheral zones are each similar in their cellular and synaptic architecture near the base and near the planum. An especially differentiated subzone may exist in the middle of the central zone.     

Synaptic relationships between the chorda tympani and tyrosine hydroxylase-immunoreactive dendritic processes in the gustatory zone of the nucleus of the solitary tract in the hamster

The toxic lectin ricin was applied to the hamster chorda tympani (CT), producing anterograde degeneration of its terminal boutons within the gustatory zone of the nucleus of the solitary tract (NST). Immunocytochemistry was subsequently performed with antiserum against tyrosine hydroxylase (TH), and the synaptic relationships between degenerating CT terminal boutons and either TH-immunoreactive or unlabeled dendritic processes were examined at the electron microscopic level. Degenerating CT terminal boutons formed asymmetric axodendritic synapses and contained small, clear, spherical synaptic vesicles that were densely packed and evenly distributed throughout the ending, with no accumulation at the active synaptic. The degenerating CT terminated on the dendrites of TH-immunoreactive neurons in 36% (35/97) of the cases. The most frequent termination pattern involved the CT and two or three other inputs in synaptic contact with a single immunoreactive dendrite, resulting in a glomerular-like structure that was enclosed by glial processes. In 64% (62/97) of the cases, the degenerating CT was in synaptic contact with unlabeled dendrites, often forming a calyx-like synaptic profile that surrounded much of the perimeter of a single unlabeled dendrite. These results indicate that the TH-immunoreactive neurons of the gustatory NST receive direct input from the CT and taste receptors of the anterior tongue and that the termination patterns of the CT vary with its target neuron in the gustatory NST. The glomerular-like structure that characterizes many of the terminations of the CT provides an opportunity for the convergence of several functionally distinct inputs (both gustatory and somatosensory) onto putative dopaminergic neurons that may shape their responsiveness to the stimulation of the oral cavity.     

The efferents interconnecting auditory inner hair cells

The work describes the system of efferent terminals that interconnect inner hair cells through a chain of direct somatic synapses organized in repetitive patterns. The efferent boutons were discovered in the apical turns of 12-day-old (hearing) mice. Clusters or short rows of vesiculated boutons are located between adjoining hair cells at the lower half of the receptors, close to their modiolar side. The individual endings, about 1.2 microns in diameter, adjoin inner hair cells and form one synapse per hair cell. On the hair cell side, the synaptic contact is apposed by a classical postsynaptic cisterna. Within a cluster of endings, some synapse simultaneously with either or both neighbouring inner hair cells. The efferent boutons also connect synaptically with each other and with other--different in type--vesiculated and nonvesiculated endings. These endings seem to derive from the climbing collaterals of the inner spiral bundle, and we believe them to be GABAergic.     

Contralateral cortical projection to the mediodorsal thalamic nucleus: origin and synaptic organization in the rat

The origin of the corticothalamic projections to the contralateral mediodorsal nucleus, the collateralization of cortical fibers and their synaptic organization in the ipsi- and contralateral mediodorsal nuclei were investigated in adult rats with double retrograde fluorescent and anterograde tracing. After tracer injections in the mediodorsal nuclei on either side, neurons were retrogradely labeled in all the areas of the contralateral prefrontal cortex in which ipsilateral labeling was also observed. Contralateral corticothalamic cells accounted for 15% of the labeled neurons in the orbital and agranular insular areas, while their proportion was lower (3%) in the anterior cingulate cortex. Up to 70% of the contralateral cortical neurons were double labeled by bilateral injections in the mediodorsal nuclei. At the electron microscopic level, unilateral injections of biotinylated dextran-amine in the orbitofrontal cortex resulted in anterograde labeling of small terminals and a few large boutons in the ipsilateral mediodorsal nucleus, while only small boutons were identified contralaterally. The diameter of postsynaptic dendritic profiles contacted by labeled small cortical endings was significantly larger in the ipsilateral mediodorsal nucleus than contralaterally. These findings demonstrate that dense contralateral cortical projections to the mediodorsal nucleus derive from the orbital and agranular insular areas, and that crossed corticothalamic afferents are mostly formed by collaterals of the ipsilateral connections. Our observations also point out the heterogeneity of corticothalamic boutons in the rat mediodorsal nucleus and morphological differences in the synaptic organization of prefrontal fibers innervating the two sides, indicating that ipsilateral cortical afferents may be more proximally distributed than crossed cortical fibers on dendrites of mediodorsal neurons.     

Simultaneous determination of thiabendazole and its major metabolite, 5-hydroxythiabendazole, in bovine tissues using gradient liquid chromatography with thermospray and atmospheric pressure chemical ionisation mass spectrometry

Modification of an existing neural structure to support a second function will produce a trade-off between the two functions if they are in some way incompatible. The trade-off between two such sensory functions is modeled here in pyramidal neurons of the gymnotiform electric fish's medullar electrosensory lateral line lobe (ELL). These neurons detect two electric stimulus features produced when a nearby object interferes with the fish's autogenous electric field: (1) amplitude modulation across a cell's entire receptive field and (2) amplitude variation within a cell's receptive field produced by an object's edge. A model of sensory integration shows that detection of amplitude modulation and enhancement of spatial contrast involve an inherent mechanistic trade-off and that the severity of the trade-off depends on the particular algorithm of sensory integration. Electrophysiology data indicate that of the two algorithms for sensory integration modeled here for the gymnotiform fish Brachyhypopomus pinnicaudatus, the algorithm with the better trade-off function is used. Further, the intrinsic trade-off within single cells has been surmounted by the replication of ELL into multiple electrosensory map segments, each specialized to emphasize different sensory features.     

Use of embryonal stem cells in studies of molecular haemopoiesis

We have examined the morphology of afferent endings that originate in three distinct cell groups and terminate in the lateral posterior nucleus of the thalamus (LP). Retino-LP projections were sparse, occurred throughout the nucleus, and could be classified into 1) simple en passant varicosities and terminal swellings found on poorly branched fibers in all LP subdivisions, 2) string-like configurations of varicosities detected largely in the medial subdivision of the LP, and 3) terminals resembling retinogeniculate endings occurring mainly in the rostral part of the superficial subdivision of the LP adjacent to the dorsal nucleus of the lateral geniculate body. Cortico-LP terminals fell into three classes: 1) single varicosities decorating the tips of short appendages on fine preterminal and terminal axons; 2) tiny, round varicosities studding the axon shaft; and 3) boutons of variable shape visible on medium-caliber corticothalamic fibers. Tecto-LP terminals exhibited a large variation in morphology and density. Those found most commonly could be classified into two groups: 1) individual swellings and 2) terminal clusters arranged in a tubular configuration that enclosed a central channel, most likely occupied by the dendrite of a postsynaptic neuron. An unusual tecto-LP terminal consisted of an ovoid swelling (up to 20 microns in the long axis) from which emerged several long, thin extensions and was seen at the tips of large-diameter axons. These results show that, despite having overlapping projection zones, each set of afferents that projects to the LP elaborates terminal specializations that are structurally distinct from others projecting to the same target area.     

Differential roles of Ca2+/calmodulin-dependent kinases in posttetanic potentiation at input selective glutamatergic pathways

The electrosensory lateral line lobe (ELL) of the electric fish Apteronotus leptorhynchus is a layered medullary region receiving electroreceptor input that terminates on basal dendrites of interneurons and projection (pyramidal) cells. The molecular layer of the ELL contains two distinct glutamatergic feedback pathways that terminate on the proximal (ventral molecular layer, VML) and distal (dorsal molecular layer) apical dendrites of pyramidal cells. Western blot analysis with an antibody directed against mammalian Ca2+/calmodulin-dependent kinase 2, alpha subunit (CaMK2alpha) recognized a protein of identical size in the brain of A. leptorhynchus. Immunohistochemistry demonstrated that CaMK2 alpha expression in the ELL was restricted to fibers and terminals in the VML. Posttetanic potentiation (PTP) could be readily elicited in pyramidal cells by stimulation of either VML or DML in brain slices of the ELL. PTP in the VML was blocked by extracellular application of a CaMK2 antagonist (KN62) while intracellular application of KN62 or a CaMK2 inhibitory peptide had no effect, consistent with the presynaptic localization of CaMK2 alpha in VML. PTP in the dorsal molecular layer was not affected by extracellular application of KN62. Anti-Hebbian plasticity has also been demonstrated in the VML, but was not affected by KN62. These results demonstrate that, while PTP can occur independent of CaMK2, it is, in some synapses, dependent on this kinase.     

Influence of the tectal zone on the distribution of synaptic boutons in the brainstem of goldfish

This study investigated whether the topographic differences in the functional properties of the tectal motor map of goldfish are related to particular patterns of connections with downstream structures. With this aim, the distribution of synaptic boutons in the mesencephalic and rhombencephalic structures was studied after discrete injections of the tracer biotinylated dextran amine were placed at separate sites along the tectal anteroposterior axis. Irrespective of the location of the injection site, the boutons were more abundant in the mesencephalon than in the rhombencephalon, and they were located chiefly ipsilaterally all throughout the brainstem. In the mesencephalon, the boutons were found in its ventrolateral reticular formation and, to a lesser extent, in the nucleus of the medial longitudinal fasciculus, the oculomotor and isthmi nuclei, and the torus semicircularis. In the mesencephalic reticular formation, the bouton location was distributed topographically with respect to the injection site. Terminals were also observed in the nucleus of the medial longitudinal fasciculus after injections into anteromedial or middle tectal zones. In the oculomotor nucleus, boutons were present exclusively in the case of the anteromedial injection. In the rhombencephalon, most boutons were found in the superior reticular formation, and their number decreased in the medial and inferior reticular formations. A topographic distribution could be observed within the superior reticular formation, although its density was attenuated compared with that observed in the mesencephalic reticular formation. The domains of synaptic endings on the ipsilateral side were different from those on the contralateral side: The ipsilateral synaptic endings were located more medially. Finally, a few boutons were also found in the vestibulocerebellar area on either the ipsilateral or the contralateral side, depending on the injection site. From these data, the authors conclude that, in goldfish, irrespective of the tectal injection site, the endings are in similar nuclei in the brainstem; however, the distribution of synaptic boutons within such nuclei can be related to the functional properties of each tectal zone.     

Sensitivity and response dynamics of elasmobranch electrosensory primary afferent neurons to near threshold fields

Elasmobranch fishes localize weak electric sources at field intensities of < 5 eta V cm-1, but the response dynamics of electrosensory primary afferent neurons to near threshold stimuli in situ are not well characterized. Electrosensory primary afferents in the round stingray, Urolophus halleri, have a relatively high discharge rate, a regular discharge pattern and entrain to 1-Hz sinusoidal peak electric field gradients of < or = 20 eta V cm-1. Peak neural discharge for units increases as a non-linear function of stimulus intensity, and unit sensitivity (gain) decreases as stimulus intensity increases. Average peak rate-intensity encoding is commonly lost when peak spike rate approximately doubles that of resting, and for many units occurs at intensities < 1 microV cm-1. Best neural sensitivity for nearly all units is at 1-2 Hz with a low-frequency slope of 8 dB/decade and a high-frequency slope of -23 dB/decade. The response characteristics of stingray electrosensory primary afferents indicate sensory adaptations for detection of extremely weak phasic fields near 1-2 Hz. We argue that these properties reflect evolutionary adaptations in elasmobranch fishes to enhance detection of prey, communication and social interactions, and possibly electric-mediated geomagnetic orientation.     

Structural plasticity of optic synapses in the rat suprachiasmatic nucleus: adaptation to long-term influence of light and darkness

Synapses of optic afferents (optic synapses) in the suprachiasmatic nucleus of hooded rats were morphometrically evaluated after exposing the animals to 12 h, 14 days, 2 months, and 8 months of constant light (light rats) and darkness (dark rats). Compared with dark rats, optic synapses from light rats have larger boutons with larger mitochondria, more clear vesicles, fewer dense-core vesicles and front-line vesicles, smaller presynaptic dense projections, a smaller amount of postsynaptic density material, a smaller relative number of Gray-type I (asymmetric) junctions, a greater relative number of Gray-type II (symmetric) junctions, as well as more and larger mitochondria in the postsynaptic dendrites. Junctions of optic synapses are mostly straight, but the small number of positively curved contacts are more flattened in light rats than in dark rats. An age-related increase in the size of presynaptic dense projections was also observed. There are no changes in the sizes of clear and dense-core vesicles, in the size of synaptic junctions and their numerical density in area, and in the unspecific contact area between pre- and postsynaptic elements. The changes in optic boutons are characteristic for activated and relatively disused synapses with a slow, tonic firing rate. It appears that (1) the amount of postsynaptic density material is proportional to the strength of Gray-type I synapses, and that (2) some excitatory optic synapses become inhibitory after long-term activity, whereas some inhibitory synapses turn into excitatory contacts after long-term disuse.     

Synaptic frequency alteration on rat ventral horn neurons in the first segment proximal to spinal cord hemisection: an ultrastructural statistical study of regenerative capacity

To study the regenerative capacity of the spinal cord in adult rat, presynaptic boutons were classified as S (spherical vesicles), F (flattened vesicles) and C complexes, and analysed statistically on alpha-motoneuron somata and lamina VII interneurons on the operated side in the first segment rostral to a spinal cord hemisection. Following chloral hydrate anesthesia left spinal cord hemisections were made on twenty adult rats (225 gms) at vertebral level T-2. Animals were prepared for electron microscopy at 7, 14, 30, 45, 60 and 90 DPO and compared with normals. All counts were made on coded material and subjected to statistical analysis. The normal frequency of presynaptic bouton types on alpha-motoneuron somata at 30 DPO. At 45 DPO, massive degeneration with concomitant synaptic remodeling resulted in a return to near normal frequencies of S and F presynaptic boutons. At 60 and 90 DPO a gain in S presynaptic boutons and a concomitant loss in F presynaptic boutons resulted in frequencies different from normal and decreased absolute numbers of presynaptic boutons. The interneuron somata also exhibited alterations over the postoperative period. There was a reversal of frequency of presynaptic boutons at 45 DPO. However unlike on alpha-motoneuron somata the frequency of S and F presynaptic boutons returned to normal at 60 and 90 DPO. The alpha-motoneuron somata appeared to be cyclically and nonselectively reinnervated by ventral horn interneurons over 90 DPO.     

An autoradiographic study of the development of the entorhinal and commissural afferents to the dentate gyrus of the rat

The development of the entorhinal, ipsilateral associational, and commissural afferents to the dentate gyrus have been studied autoradiographically, following the injection of small amounts of tritiated proline into the medial and lateral parts of the entorhinal cortex, and into fields CA3c and CA4 of the hippocampus, in a series of rats, on the third, sixth, and twelfth postnatal days. Clear labeling of the entorhinal afferents were found at the third postnatal day, and from the earliest stage studied the afferents from the two parts of the entorhinal cortex appear to be spatially segregated within the stratum moleculare of the dentate gyrus: the fibers from the lateral entorhinal area occupying the outermost one-third, or so, of this stratum, while those from the medial entorhinal cortex occupy its middle zone. The ipsilateral hippocampo-dentate associational pathway is present at the third postnatal day, but the commissural projection (which shares with it the inner part of the stratum moleculare) could not be labeled until the sixth postnatal day. By the twelfth day the characteristic adult pattern of distribution of the terminals of the two hippocampo-dentate pathways is established. Although this pattern is best accounted for on the basis of a temporal competition for the available synaptic sites on the proximal parts of the dendrites of the granule cells, the spatial segregation of these two fiber systems from those arising in the entorhinal cortex, is probably due to the selective fasciculation of fibers in each group of afferents and to their early cytochemical specificity.     

The synaptic organization of the cerebello-olivary circuit

Section of the superior cerebellar peduncle just rostral to the deep cerebellar nuclei results in degenerating axon terminals within the contralateral inferior olive. The nuclear origin of this fiber system and its distribution within the subdivisions of the inferior olive were described in a companion study (Martin et al., 1976). Precise localization of these degenerating terminals within the nucleus was accomplished by the examination of 1 mu plastic sections cut from each tissue block prior to thin sectioning. Degenerating axon terminals are present in all the nuclear subdivisions and when seen with the electron microscope they frequently are localized in the previously described synaptic clusters (King, 1976). These terminals demonstrate an electron dense reaction at survival times of 2 and 3 days. By day 4, they are shrunken and irregular in shape, and typically are surrounded by astrocyte processes. Cerebello-olivary axon terminals measure 1-3 mu, contain spherical, clear synaptic vesicles and typically contact spiny appendages within the synaptic clusters (glomeruli). Thus, we have demonstrated that one of the primary axon systems which terminates within the synaptic clusters is from the cerebellar nuclei. We have yet to determine the origins of the remaining terminals within the synaptic clusters which include endings with either smaller spherical, pleomorphic or numerous dense core vesicles.     

Innervation of serotonergic medullary raphe neurons from cells of the rostral ventrolateral medulla in rats

The rostral ventral medulla has been shown to consist of three distinct subregions: the midline or raphé region, the lateral paragigantocellular-gigantocellular region and the rostro-ventrolateral reticular nucleus. All three regions have been shown to contribute to central vaso-regulation and to project towards sympathetic preganglionic neurons of the thoracic spinal cord. Therefore it is of particular interest to describe the interconnections between the three regions and to see if local afferents reach cells which have been implicated in the regulation of descending inputs. Following injections of the anterograde tract tracer Phaseolus vulgaris leucoagglutinin into the lateral paragigantocellular nucleus or the rostroventrolateral reticular nucleus, labelled axons were traced into the medullary raphé nuclei and the contralateral rostral ventrolateral medulla. Efferents originating from both regions innervated the raphé pallidus, raphé obscurus and raphé magnus. However the distribution of terminals originating from the two regions was different in the contralateral ventrolateral medulla oblongata. The data indicate that the connection between the ipsi- and contralateral equivalents of both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus are stronger than the cross-connection between the ipsi- and contralateral parts of the two different regions. In the second part of the study, the existence of direct projections from the rostroventrolateral reticular nucleus and the lateral paragigantocellular-gigantocellular region onto serotonin-immunogold-labelled cells of the ventromedial medulla were investigated. The correlated light and electron microscopic analysis revealed direct synaptic contacts between axons originating from both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus, and serotonin-immunoreactive cells of the raphé obscurus and raphé pallidus. The results of the present light microscopic tract-tracing study revealed a different pattern of the intramedullary projection of the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus. These data are in support of the proposed parcellation of the two cytoarchitectonically different areas of the rostral ventrolateral medulla into two functionally distinct subdivisions. Furthermore, the direct anatomical connection revealed in the present study between cells of the rostral ventrolateral and ventromedial medulla oblongata indicates the possibility that vasoregulatory effects of some cells of the rostral ventrolateral medulla oblongata might be executed via direct projections onto serotonin-immunoreactive cells of the medullary raphé nuclei.