Canal-specific excitation and inhibition of frog second-order vestibular neurons

Second-order vestibular neurons (secondary VNs) were identified in the in vitro frog brain by their monosynaptic excitation following electrical stimulation of the ipsilateral VIIIth nerve. Ipsilateral disynaptic inhibitory postsynaptic potentials were revealed by bath application of the glycine antagonist strychnine or of the gamma-aminobutyric acid-A (GABA(A)) antagonist bicuculline. Ipsilateral disynaptic excitatory postsynaptic potentials (EPSPs) were analyzed as well. The functional organization of convergent monosynaptic and disynaptic excitatory and inhibitory inputs onto secondary VNs was studied by separate electrical stimulation of individual semicircular canal nerves on the ipsilateral side. Most secondary VNs (88%) received a monosynaptic EPSP exclusively from one of the three semicircular canal nerves; fewer secondary VNs (10%) were monosynaptically excited from two semicircular canal nerves; and even fewer secondary VNs (2%) were monosynaptically excited from each of the three semicircular canal nerves. Disynaptic EPSPs were present in the majority of secondary VNs (68%) and originated from the same (homonymous) semicircular canal nerve that activated a monosynaptic EPSP in a given neuron (22%), from one or both of the other two (heteronymous) canal nerves (18%), or from all three canal nerves (28%). Homonymous activation of disynaptic EPSPs prevailed (74%) among those secondary VNs that exhibited disynaptic EPSPs. Disynaptic inhibitory postsynaptic potentials (IPSPs) were mediated in 90% of the tested secondary VNs by glycine, in 76% by GABA, and in 62% by GABA as well as by glycine. These IPSPs were activated almost exclusively from the same semicircular canal nerve that evoked the monosynaptic EPSP in a given secondary VN. Our results demonstrate a canal-specific, modular organization of vestibular nerve afferent fiber inputs onto secondary VNs that consists of a monosynaptic excitation from one semicircular canal nerve followed by disynaptic excitatory and inhibitory inputs originating from the homonymous canal nerve. Excitatory and inhibitory second-order (secondary) vestibular interneurons are envisaged to form side loops that mediate spatially similar but dynamically different signals to secondary vestibular projection neurons. These feedforward side loops are suited to adjust the dynamic response properties of secondary vestibular projection neurons by facilitating or disfacilitating phasic and tonic input components.     

Second-order vestibular neuron morphology of the extra-MLF anterior canal pathway in the cat

Second-order vestibular neurons form the central links of the vestibulo-oculomotor three-neuron arcs that mediate compensatory eye movements. Most of the axons that provide for vertical vestibulo-ocular reflexes ascend in the medial longitudinal fasciculus (MLF) toward target neurons in the oculomotor and trochlear nuclei. We have now determined the morphology of individual excitatory second-order neurons of the anterior semicircular canal system that course outside the MLF to the oculomotor nucleus. The data were obtained by the intracellular horseradish peroxidase method. Cell somata of the extra-MLF anterior canal neurons were located in the superior vestibular nucleus. The main axon ascended through the deep reticular formation beneath the brachium conjunctivum to the rostral extent of the nucleus reticularis tegmenti pontis, where it crossed the midline. The main axon continued its trajectory to the caudal edge of the red nucleus from where it coursed back toward the oculomotor nucleus. Within the oculomotor nucleus, collaterals reached superior rectus and inferior oblique motoneurons. Some axon branches recrossed the midline within the oculomotor nucleus and reached the superior rectus motoneuron subdivision on that side. Since these neurons did not give off a collateral toward the spinal cord, they were classified as being of the vestibulo-oculomotor type and are thought to be involved exclusively in eye movement control. The signal content and spatial tuning characteristics of this anterior canal vestibulo-oculomotor neuron class remain to be determined.     

Polyamine changes in the vestibular nuclei of guinea pigs following labyrinthectomy

Vestibular compensation is a process of behavioral recovery from ocular, motor and postural disorders following unilateral damage to the vestibular end-organ. Although restoration of the normal resting discharge rate in the ipsilateral vestibular nuclei is important in compensation, the biochemical and molecular mechanisms mediating recovery are largely unknown. The ornithine decarboxylase polyamine pathway is activated in the nervous system following axotomy or denervation. The authors postulate that changes in polyamines mediate vestibular compensation. Within 150-micron brain stem coronal section micropunches analyzed by high performance liquid chromatography techniques, the polyamine spermidine was significantly increased in the ipsilateral lateral vestibular nucleus 8 hours following labyrinthectomy in the guinea pig model. Because naturally occurring polyamines modulate excitatory amino acid receptors (N-methyl-D-aspartate [NMDA]) which in turn mediate neurotransmission between primary afferents and second order vestibular neurons, stimulation of polyamine pathways following neural injury may play a critical role in compensation.     

Response of the hypothalamic neurons to stimulation of the vestibular nerve and lateral vestibular nucleus in rabbits

Effects of single, double, and rhythmic stimulation upon hypothalamic neurons responding to the 1st excitatory phase of lateral vestibular nucleus stimulation, were studied. The data obtained show that activation of some hypothalamic neurons following stimulation of the lateral vestibular nucleus has a monosynaptic character. The findings suggest that ascending afferents from the lateral vestibular nucleus to the hypothalamus pass via oligo- as well as polysynaptic pathways.     

Uncrossed disynaptic inhibition of second-order vestibular neurons and its interaction with monosynaptic excitation from vestibular nerve afferent fibers in the frog

1. Eighth nerve evoked responses in central vestibular neurons (n = 146) were studied in the isolated brain stem of frogs. Ninety percent of these neurons responded with a monosynaptic excitatory postsynaptic potential (EPSP) after electrical stimulation of the ipsilateral VIIIth nerve. In 5% of these neurons, the EPSP was truncated by a disynaptic inhibitory postsynaptic potential (IPSP), and in 5% of these neurons a pure disynaptic IPSP was evoked. 2. Disynaptic IPSPs superimposed upon apparently pure EPSPs were revealed by bath application of the glycine receptor antagonist strychnine (0.5-5 microM) or of the gamma-aminobutyric acid-A (GABAA) receptor antagonist bicuculline (0.5-2 microM). The evoked EPSP increased in most central vestibular neurons (strychnine: 15 out of 16 neurons; bicuculline 26 out of 29 neurons). At higher stimulus intensities, the evoked spike discharge increased from 2 to 3 spikes before up to 8-10 spikes per electrical pulse during the application of blocking agents. The unmasked disynaptic inhibitory component increased with stimulus intensity to a different extent in different neurons. 3. Lesion studies demonstrated that these inhibitory components were generated ipsilaterally with respect to the recording side. The disynaptic strychnine-sensitive inhibition was mediated by neurons located either in the ventral vestibular nuclear complex (VNC) or in the adjacent reticular formation. The spatial distribution of the disynaptic inhibition was investigated by simultaneous recordings of VIIIth nerve-evoked field potentials at different rostrocaudal locations of the VNC. A significant strychnine-sensitive component was detected in the middle and caudal parts but not in the rostral part of the VNC. A bicuculline-sensitive component was detected in the rostral and in the caudal parts but not in the middle part of the VNC. In view of a similar rostrocaudal distribution of glycineor GABA-immunoreactive neurons in the VNC of frogs, our results suggest that part of the disynaptic inhibition is mediated by local interneurons with a spatially restricted projection area. 4. The monosynaptic EPSP of second-order vestibular neurons was mediated in part by N-methyl-D-aspartate (NMDA) and in part by non-NMDA receptors. The relative contribution of the NMDA receptor-mediated component of the EPSP decreased with stronger stimuli. This negative correlation could have resulted from a preferential activation of NMDA receptors via thick vestibular nerve afferent fibers. Alternatively, the activation of NMDA receptors became disfacilitated at higher stimulus intensities due to the recruitment of disynaptic inhibitory inputs. Comparison of data obtained in the presence and in the absence of these glycine and GABAA receptor blockers indicates a preferential activation of NMDA receptors via larger-diameter vestibular nerve afferent fibers. 5. The kinetics of NMDA receptors (delay, rise time) activated by afferent nerve inputs were relatively fast. These fast kinetics were independent of superimposed IPSPs. The association of these receptors with large-diameter vestibular nerve afferent fibers suggests that fast NMDA receptor kinetics might be matched to the more phasic response dynamics of the large diameter vestibular afferent neurons to natural head accelerations.     

Modulation of calcium by inhibitory systems in the developing auditory midbrain

Inhibitory synaptic transmission is of fundamental importance during the maturation of central auditory circuits, and their subsequent ability to process acoustic information. The present study investigated the manner in which inhibitory transmission regulates intracellular free calcium levels in the gerbil inferior colliculus using a brain slice preparation. Inhibitory and excitatory postsynaptic potentials were evoked by electrical stimulation of the ascending afferents at the level of the dorsal nucleus of the lateral lemniscus. Pharmacologically isolated inhibitory synaptic potentials were able to attenuate a calcium rise in collicular neurons that was generated by depolarizing current injection. In addition, GABA(A) and glycine receptor antagonists typically led to an increase of calcium in collicular neurons during electrical stimulation of the ascending afferent pathway at the level of the dorsal nucleus of the lateral lemniscus. Bath application of GABA or muscimol, a GABA(A) receptor agonist, evoked a brief hyperpolarization followed by a long-lasting depolarization in inferior colliculus neurons. This treatment also induced a transient calcium increase that correlated with the membrane depolarization phase. Baclofen, a GABA(B) receptor agonist, had no effect on either membrane potential or calcium levels. Ratiometric measures indicated that the muscimol-evoked rise in calcium was approximately 150 nM above basal levels. The muscimol-evoked responses were completely antagonized by bicuculline and attenuated by picrotoxin. Together, these results suggest that inhibitory synaptic transmission participates in the regulation of postsynaptic calcium during the developmental period. Inhibitory transmission may attenuate a calcium influx that is evoked by excitatory synapses, but it can also produce a modest influx of calcium when activated alone. These mechanisms may help to explain the influence of inhibitory transmission on the development of postsynaptic properties.     

Contributions of regularly and irregularly discharging vestibular-nerve inputs to the discharge of central vestibular neurons in the alert squirrel monkey

The discharge of neurons in the vestibular nuclei was recorded in alert squirrel monkeys while they were being sinusoidally rotated at 2 Hz. Type I position-vestibular-pause (PVP I) and vestibular-only (V I) neurons, as well as a smaller number of other type I and type II eye-plus-vestibular neurons were studied. Many of the neurons were monosynaptically related to the ipsilateral vestibular nerve. Eye-position and vestibular components of the rotation response were separated by multiple regression. Anodal currents, simultaneously delivered to both ears, were used to eliminate the head-rotation signals of irregularly discharging (I) vestibular-nerve afferents, presumably without affecting the corresponding signals of regularly discharging (R) afferents. R and I inputs to individual central neurons were determined by comparing rotation responses with and without the anodal currents. The bilateral currents, while reducing the background discharge of all types of neurons, did not affect the mean vestibular gain or phase calculated from a population of PVP I neurons or from a mixed population consisting of all type I units. From this result, it is concluded that I inputs are canceled at the level of secondary neurons. The cancellation may explain why the ablating currents do not affect the gain and phase of the vestibulo-ocular reflex. While cancellation was nearly perfect on a population basis, it was less so in individual neurons. For some neurons, the ablating currents decreased vestibular gain, while for other neurons the vestibular gain was increased. The former neurons are interpreted as receiving a net excitatory (I-EXC) I input, the latter neurons, a net inhibitory (I-INH) input. When compared with the corresponding R inputs, the I inputs were usually small and phase advanced. Phase advances were larger for I-EXC than for I-INH inputs. The sign and magnitude of the I inputs were unrelated to other discharge properties of individual neurons, including discharge regularity and the phase of vestibular responses measured in the absence of the ablating currents. Unilateral currents were used to assess the efficacy of ipsilateral and contralateral pathways. Ipsilateral pathways were responsible for almost all of the effects seen with bilateral currents. The results suggest that the vestibular signals carried by central neurons, even by those neurons receiving a monosynaptic vestibular-nerve input, are modified by polysynaptic pathways.     

Expression and epitopic conservation of calponin in different smooth muscles and during development

The synaptic organization of the saccade-related neuronal circuit between the superior colliculus (SC) and the brainstem saccade generator was examined in an awake monkey using a saccadic, midflight electrical-stimulation method. When microstimulation (50-100 microA, single pulse) was applied to the SC during a saccade, a small, conjugate contraversive eye movement was evoked with latencies much shorter than those obtained by conventional stimulation. Our results may be explained by the tonic inhibition of premotor burst neurons (BNs) by omnipause neurons that ceases during saccades to allow BNs to burst. Thus, during saccades, signals originating from the SC can be transmitted to motoneurons and seen in the saccade trajectory. Based on this hypothesis, we estimated the number of synapses intervening between the SC and motoneurons by applying midflight stimulation to the SC, the BN area, and the abducens nucleus. Eye position signals were electronically differentiated to produce eye velocity to aid in detecting small changes. The mean latencies of the stimulus-evoked eye movements were: 7.9 +/- 1.0 ms (SD; ipsilateral eye) and 7.8 +/- 0.9 ms (SD; contralateral eye) for SC stimulation; 4.8 +/- 0.5 ms (SD; ipsilateral eye) and 5.1 +/- 0.7 ms (SD; contralateral eye) for BN stimulation; and 3.6 +/- 0.4 ms (SD; ipsilateral eye) and 5.2 +/- 0.8 ms (SD; contralateral eye) for abducens nucleus stimulation. The time difference between SC- and BN-evoked eye movements (about 3 ms) was consistent with a disynaptic connection from the SC to the premotor BNs.     

Capsaicin in the 4th ventricle abolishes retching and transmission of emetic vagal afferents to solitary nucleus neurons

Systemic tachykinin NK1 receptor antagonists and resiniferatoxin are known to abolish vomiting mediated by vagal afferents. Emetic vagal afferents have been shown to make synaptic contact with neurons in the medial solitary nucleus. These results suggest that substance P participates in the synapse as a mediator. To examine this possibility, the effects of 4th-ventricular application of capsaicin (0.033-33 mM, 20-30 microl) and resiniferatoxin (1.6-160 microM, 20-30 microl) on the activity of neurons in the medial solitary nucleus and fictive retching induced by vagal stimulation were observed in paralyzed decerebrate dogs. Capsaicin (33 mM) and resiniferatoxin (160 microM) initially increased the neuronal firing and occasionally produced retching, then abolished both neuronal and retching responses. However, stimulation of the medial solitary nucleus continued to provoke retching. Field potential changes in the medial solitary nucleus evoked by pulse-train vagal stimulation decreased in amplitude, but did not disappear. Latencies of neuronal firing and evoked potentials were about 300 ms. These results suggest that emetic vagal afferents are capsaicin-sensitive C fibers which may have substance P as an excitatory transmitter or modulator.     

Effects of botulinum neurotoxin type A on abducens motoneurons in the cat: ultrastructural and synaptic alterations

The synaptic alterations induced in abducens motoneurons by the injection of 3 ng/kg of botulinum neurotoxin type A into the lateral rectus muscle were studied using ultrastructural and electrophysiological techniques. Motoneurons identified by the retrograde transport of horseradish peroxidase showed a progressive synaptic stripping already noticeable by four days post-injection which increased over the study period. By 35 days post-injection, the normal coverage of motoneurons by synaptic boutons (66.4 +/- 4.0%) significantly decreased to 27.2 +/- 4.0%. Synaptic boutons detached by a widening of the subsynaptic space but remained apposed by synaptic contacts and desmosomes to the motoneuron. Detachment did not affect equally flat and round vesicle-containing boutons. The control motoneuron had almost equal numbers of both types of boutons, but after 35 days post-injection the ratio of round to flat vesicle-containing boutons was 1.20 +/- 0.01. Synaptic boutons impinging on motoneurons showed signs of alterations in membrane turnover, as indicated by an increase in the number of synaptic vesicles and a decrease in the number of coated vesicles and synaptic vesicles near the active zone. Abducens motoneurons had a transient increase in soma size by 15 days that returned to normal at 35 days, but no signs of chromatolysis or organelle degeneration were seen. Accompanying the swelling of motoneurons, a 15-fold increase in the number of spines, very infrequent in controls, was observed. Spines located in the soma and proximal dendritic trunk received synaptic contacts from both flat and round vesicle-containing boutons that could be either partly detached or completely attached to the motoneuron. An increased turnover of the plasmatic membrane of the motoneuron was observed, as indicated by a four-fold increase in the number of somatic coated vesicles. Animals were implanted with bipolar electrodes in the ampulla of both horizontal semicircular canals for evoking contralateral excitatory and ipsilateral inhibitory postsynaptic potentials. Motoneurons were antidromically identified from the lateral rectus muscle. Synaptic potentials of vestibular origin were recorded in abducens motoneurons. In the period between two and six days post-injection, a complete abolition of inhibitory synaptic potentials was observed. By contrast, excitatory synaptic potentials remained, but were reduced by 82%. The imbalance between excitatory and inhibitory inputs to motoneurons induced a progressive increase of firing frequency within a few stimuli applied to the contralateral canal. Between 7 and 15 days post-injection, both excitatory and inhibitory postsynaptic potentials were virtually abolished and remained so up to the longest time checked (105 days). Some motoneurons recorded beyond 60 days post-injection showed signs of recovery of excitatory postsynaptic potentials. During the whole time-span studied, presynaptic wavelets were present, indicating no affecting of the conduction of afferent volleys to the abducens nucleus. Taken together, these data indicate that botulinum neurotoxin at high doses causes profound synaptic alterations in motoneurons responsible for the effects seen in the behavior of motoneurons recorded in alert animals.     

Inputs from three brainstem sources to identified neurons of the mouse inferior colliculus slice

A total of 40 neurons from of the central nucleus of the mouse inferior colliculus (IC) were recorded intracellularly from brain slices to determine input properties by electrical stimulation of the ipsilateral lateral lemniscus (LL), commissure of Probst (CP), and commissure of the IC (CoIC) together with cellular morphology (in 25 neurons) by biocytin injection and staining. Nine neurons had oriented (bipolar), 16 neurons non-oriented (multipolar) dendritic trees of various sizes. Axon collaterals of a given neuron often ran in several directions to provide multiple input to adjacent isofrequency laminae, the lateral nucleus of the IC, the brachium of the IC, the LL, the CP, and the IC commissure. Neurons were classified by spike response patterns to depolarizing current injection into onset- and sustained-spiking cells. The former had significantly shorter membrane-time constants, significantly less frequently and smaller hyperpolarizations after spike occurrence, and more Ca2+-humps. These properties and their preferred position in the dorsolateral ICC suggest a participation in binaural temporal processing. Almost all oriented cells showed only excitatory post-synaptic potentials (EPSPs) after LL stimulation, while in non-oriented cells inhibitory post-synaptic potentials (IPSPs) after the EPSPs were significantly more frequent. Neurons with largest dendritic trees and many dorsalward projecting axon collaterals were found in the ventral IC. There, neurons had average 4 ms (two synapses) shorter response latencies to LL stimulation than dorsally located neurons. Thus, neurons in the central and dorsal IC may receive mono- and disynaptic input from ventrally located neurons.     

Relative contributions of thalamic reticular nucleus neurons and intrinsic interneurons to inhibition of thalamic neurons projecting to the motor cortex

1. Intracellular responses to stimulation of the cerebral cortex (Cx) and cerebellum were analyzed in thalamocortical neurons (TCNs) in the ventroanterior-ventrolateral (VA-VL) complex of the thalamus and neurons in the thalamic reticular nuclei (RNs) of anesthetized cats, and the contribution of reticular nucleus neurons (RNNs) and thalamic interneurons (TINs) to cerebral and cerebellar inhibition of TCNs was determined. 2. Single TCNs projecting to area 4 or 6 received convergent monosynaptic excitatory and disynaptic inhibitory inputs from both the dentate nucleus (DN) and the interpositus nucleus (IN). These TCNs also received monosynaptic excitatory postsynaptic potentials (EPSPs) and disynaptic inhibitory postsynaptic potentials (IPSPs) from the pericruciate cortex (areas 4 and 6). Each TCN received the strongest excitatory and inhibitory inputs from the cortical area to which that TCN projected, and weaker inhibitory inputs from adjacent cortical areas. 3. RNNs were identified morphologically by intracellular injection of horseradish peroxidase (HRP). Stimulation of the brachium conjunctivum (BC) evoked disynaptic EPSPs with a long decay phase in RNNs in the anterior ventrolateral part of the RN. Single RNNs received convergent disynaptic excitatory inputs from both the DNA and the IN. Stimulation of the Cx produced monosynaptic long-lasting EPSPs with two different latencies in these RNNs: early EPSPs with latencies of 0.9-2.1 ms and late EPSPs with latencies of 1.8-3.5 ms. Collision experiments with BC- and Cx-evoked EPSPs in RNNs indicated that BC-evoked disynaptic EPSPs and Cx-evoked early EPSPs were produced by axon collaterals of TCNs to RNNs. The latencies of the Cx-evoked late EPSPs in RNNs were almost identical to those of Cx-evoked monosynaptic EPSPs in TCNs, indicating that corticothalamic neurons (CTNs) exert monosynaptic excitatory effects on RNNs and TCNs. 4. Stimulation of the Cx produced IPSPs in TCNs with short latencies of 1.8-2.7 ms and longer latencies of > or = 2.8 ms. The Cx-evoked early IPSPs with latencies of 1.8-2.7 ms were mediated by RNNs. The origin of Cx-evoked late IPSPs with latencies of > or = 2.8 ms in TCNs was twofold, Cx-induced early IPSPs in TCNs were facilitated by conditioning cortical stimulation that induced late IPSPs in the TCNs. The same conditioning cortical stimulation also facilitated BC-evoked disynaptic IPSPs. The time course of this facilitatation indicated that CTNs produce long-lasting excitation in TINs. These results indicated that Cx-evoked IPSPs with latencies of > 2.7 ms were mediated at least in part by RNNs and inhibitory TINs in the VA-VL complex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Trigeminal disynaptic circuit mediating corneal afferent input to M. depressor palpebrae inferioris motoneurons in the pigeon (Columba livia)

Corneal afferent projections to the trigeminal brainstem nuclear complex (TBNC) and associated structures, as determined by transganglionic transport of various tracers, were found to be predominantly concentrated in two distinct patches in the dorsolateral medulla at periobex levels. One was in the external cuneate nucleus, and the other was in the ventralmost part of the ophthalmic division of the TBNC. The projections of putative second-order neurons in these regions, as determined by injections of wheat germ agglutinin conjugated to horseradish peroxidase into the dorsolateral medulla, were found to include the dorsal trigeminal motor nucleus (Vd), which innervates the M. depressor palpebrae inferioris. Electrical stimulation of Vd, which elicited lower eyelid movements, was then used to guide injections of tracer into Vd, which retrogradely labeled clusters of neurons in the corneal afferent recipient regions of the dorsolateral medulla. The lower eyelid of pigeons, unlike the nictitating membrane and upper lid, does not appear to be appreciably involved in either reflex blinking in response to relatively mild stimulation of the cornea (e.g., air puff), or in eye closure during the saccade-like head movements associated with walking, or in eye closure during pecking; but in response to a stimulus that makes corneal contact, an upward movement of the lower lid follows descent of the nictitating membrane and upper lid as part of a defensive eye-closing mechanism. The anatomical results thus appear to define a dedicated disynaptic trigeminal sensorimotor circuit for the control of lower eyelid motility in response to mechanical or noxious stimuli of the cornea. Injections of tracers into the lower and upper eyelids labeled palpebral sensory afferents that terminated predominantly in maxillary and ophthalmic portions, respectively, of the dorsal horn of upper cervical spinal segments. These terminal fields were therefore largely separate from those of corneal afferents. There were no specific corneal afferent projections upon accessory abducens motoneurons that innervate the two muscles controlling the nictitating membrane.     

Structural characterization of three genetic variants of human serum albumin modified in subdomains IIB and IIIA

Sympathetic outflow is regulated by a direct pathway of the rostral ventrolateral reticular formation (rvlm) to the thoracic spinal cord. For the first time, a dual retrograde/anterograde transport technique was used to demonstrate by light microscopy, potential disynaptic pathways from the rvlm to the thoracic spinal cord in the rat. An anterograde tracer, biotinylated dextran amine (BDA) was injected into the rvlm and a retrograde tracer, FluoroGold (FG) deposited into the upper thoracic spinal cord in the same animal. Rostral ventrolateral medullary efferents labeled with BDA were apposed to thoracic reticulospinal neurons labeled with FG in the ventrolateral tegmentum, ipsilateral and contralateral to the injection site in the rvlm. Suggestive evidence was obtained of synaptic interactions with neuronal somata and proximal dendrites. The results support the idea that the rvlm projects to the thoracic cord via disynaptic, intrareticular pathways paralleling the well established monosynaptic projection.     

The differential expression of low-threshold sustained potassium current contributes to the distinct firing patterns in embryonic central vestibular neurons

The principal cells of the chick tangential nucleus are second-order sensory neurons that participate in the three-neuron vestibulo-ocular and vestibulocollic reflexes. In postnatal animals, second-order vestibular neurons fire repetitively on depolarization. Previous studies have shown that, although this is an important feature for normal reflex function, it is only acquired gradually during embryonic development. Whereas at 13 embryonic days (E13) the principal cells accommodate after firing a single spike, at E16 a few principal cells repetitively can fire multiple action potentials on depolarization. Finally, in the hatchling, the vast majority of principal cells is capable of nonaccommodating firing on depolarization. As a first step in understanding the mechanisms underlying developmental change in excitability of these second-order vestibular neurons, we analyzed the outward potassium currents and their role in accommodation, using brainstem slices at E16. The principal cells exhibited transient and sustained potassium currents, with both of these containing calcium-dependent components. Further, both high- and low-threshold sustained potassium currents have been distinguished. The low-threshold dendrotoxin-sensitive sustained potassium current (IDS) is associated with principal cells that accommodate and is not expressed in those that fire repetitively. Finally, blocking of IDS transforms accommodating cells into neurons capable of firing trains of action potentials on depolarization. These findings indicate that suppression of IDS during development is sufficient to transform accommodating principal cells into nonaccommodating firing neurons and suggests that developmental regulation of this current is necessary for the establishment of normal vestibular function.     

Secondary vestibulo-oculomotor projections in larval sea lamprey: anterior octavomotor nucleus

The ventral octavolateral area of lampreys contains three nuclei: the anterior, intermediate and posterior octavomotor nuclei, formed of large neurons that are contacted by thick primary vestibular fibres. We used horseradish peroxidase (HRP) or fluorescein-dextran-amine (FDA) labelling to study the projections of the anterior octavomotor nucleus (AON) in the larval sea lamprey, Petromyzon marinus. The tracers were injected either in the AON, the oculomotor nucleus or the rostralmost spinal cord. HRP injection in the AON labelled thick axons that coursed to the basal mesencephalic tegmentum, where most decussate and project to the oculomotor nucleus and the third Müller cell. Electron microscopy confirmed that AON axons contact with the contralateral third Müller cell and with oculomotor neurons. Some AON axons run in the mesencephalic tegmentum and the ventral diencephalon. An AON axon was observed to run close to the axon of the contralateral third Müller cell, establishing what appeared to be en passant contacts. HRP injection in the AON also revealed commissural fibres projecting to the contralateral octavolateral area. HRP or FDA injections in the oculomotor nucleus labelled both large and small neurons of the AON, mostly contralateral to the injection site, as well as of cells in the intermediate octavomotor nucleus, mainly ipsilateral. HRP injection in the AON or in the rostral spinal cord did not reveal any projections from the AON to the spinal cord. Our results indicate that the pattern of octavo-oculomotor connections in the lamprey is different from that observed in other vertebrates.     

Neural control of ventilation in the lamprey

Tidal breathing of adult lampreys is produced by a single phase of excitation that compresses the branchial basket for exhalation. Inhalation is due to passive recoil. Motoneurons are located primarily in the ipsilateral motor nucleus of the vagus and exhibit periodic bursts of excitatory synaptic potentials from unidentified generator cells in the medulla. Respiratory discharges continue in the isolated brain and can be recorded from the vagal roots. The frequency of bursts increases with temperature in a manner roughly similar to that of intact animals. However, the discharges of the isolated brain are not very sensitive to altered pH or oxygen content in the bathing fluid. Mechanoreceptors in the gill can initiate "cough" responses and may modulate normal breathing.     

Regenerated dorsal root fibers form functional synapses in embryonic spinal cord transplants

1. The aim of the present study was to determine whether synapses formed by dorsal root afferents that regenerate into intraspinal transplants of fetal spinal cord are functional. Severed L4 or L5 dorsal root stumps were placed at the bottom of dorsal quadrant cavities made in the lumbar spinal cords of adult rats and juxtaposed to embryonic day 14 spinal cord transplants. 2. In animals examined 5-10 weeks later, we recorded extracellularly in transplants from 43 units that fired in response to electrical stimulation of the implanted dorsal root. Latency fluctuations of extracellular firing that increase with stimulus and failure to follow high-frequency and posttetanic potentiation of extracellular firing stimulation suggest that synapses with conventional properties are formed between regenerating afferents and transplant neurons. Limited intracellular recordings confirmed the existence of excitatory postsynaptic potentials in transplant neurons after dorsal root stimulation. 3. In 16 units, extracellular firing occurred in response to single shock stimulation. The remainder of the units required two or more dorsal root shocks to evoke firing; some of these connections also may be monosynaptic. 4. Under the assumption that single shock firing was most likely the result of monosynaptic connections between transplant neurons and regenerated dorsal root fibers, we estimated the conduction velocities of regenerated fibers. These estimates suggest that fibers with conduction velocities in the C, A delta, and A alpha/beta ranges regenerate into transplants of embryonic spinal cord. 5. The results demonstrate that regenerated dorsal root axons establish functional synaptic connections with transplant neurons. The implications for using fetal transplants to help rebuild spinal reflex circuits after spinal cord injury are considered.     

Convergence properties of solitary tract neurons responsive to cardiac receptor stimulation in the anesthetized cat

The convergence pattern of cardiac receptors, pulmonary C-fibers, carotid chemoreceptor, and baroreceptor afferents onto neurons within the nucleus of the solitary tract (NTS) was studied in the anesthetized (pentobarbitone sodium, 40 mg/kg,) paralyzed and artificially ventilated cat. Extra- and intracellular recordings were made from NTS neurons while stimulating both cardiac receptors by aortic root injections of veratridine (1-3 micrograms/kg) and pulmonary C-fibers by a right atrial injection of phenylbiguanide (10-20 micrograms/kg). The ipsilateral carotid body was stimulated by using arterial injection of CO2-saturated bicarbonate solution, whereas inflation of the ipsilateral carotid sinus was used to activate baroreceptors. The ipsilateral cardiac vagal branch, cervical vagus, and carotid sinus nerves were stimulated electrically (1 Hz, 0.2-1 ms, 1-35 V). In 78 NTS neurons recorded either extracellularly (n = 47) or intracellularly (n = 31), electrical stimulation of the cardiac branch of the vagus nerve evoked synaptic potentials (spikes and/or excitatory postsynaptic potentials) with an onset latency between 4 and 220 ms. Some neurons displayed both short and long latency inputs(15.5 +/- 1.8 and 160.0 +/- 8.5 ms; n = 14). Of these 78 neurons, 24 responded to veratridine stimulation of cardiac receptors (i.e., cardioreceptive neurons) by exhibiting an augmenting-decrementing discharge of 37 +/- 4 s in duration with a peak frequency of 30 +/- 5 Hz. Convergence from other cardiorespiratory receptors was noted involving either carotid chemoreceptors (n = 7) or pulmonary C-fibers (n = 4) or from both carotid chemoreceptors and pulmonary C-fibers (n = 6). In contrast, only one cardioreceptive NTS neuron was activated by distension of the carotid sinus. Recording sites recovered were confined to the medial NTS at the level of the area postrema and extended caudally into the commissural subnucleus. Our results indicate a convergence of carotid chemoreceptor and pulmonary C-fiber afferent inputs to cardioreceptive NTS neurons. With the paucity of baroreceptor inputs to these neurons it is suggested that sensory integration within the NTS may reflect regulatory versus defensive or protective reflex control.