Binaural response patterns in subdivisions of the medial geniculate body

Auditory evoked potentials (AEPs) to binaural click stimulation were examined in the ventral (MGv) and caudomedial (MGcm) subdivisions of the medial geniculate body (MG) in guinea pigs. Binaural stimulation caused a decrease in amplitude for the response component recorded from the MGv, but an increase in amplitude for the AEP component recorded from the MGcm. Findings suggest that the evoked responses recorded from MGv and MGcm are functionally distinct. The inhibitory binaural response (BR) pattern seen in MGv was similar to that of the middle latency response (MLR) component recorded over the temporal cortex, while the additive BR pattern typical of the MGcm was similar to that of the surface midline MLR component. Furthermore, these data imply that the binaural response patterns seen in the primary and non-primary auditory cortex may be processed and encoded at the thalamic level. It is concluded that the distinct BR patterns noted for the two MG subdivisions reflect the predominant type of binaurally responsive neurons within the respective pathways.     

Spinal strychnine alters response properties of nociceptive-specific neurons in rat medial thalamus

Experiments in both conscious and anesthetized animals indicate that intrathecal (i.t.) strychnine (STR; glycine receptor antagonist) produces acute, reversible allodynia, as evidenced by inappropriate behavioral and autonomic responses to cutaneous tactile stimuli. Although STR is known to produce disinhibition of afferent input to the spinal cord, changes in spinal reflexes cannot fully explain the complex behaviors observed following i.t. STR. Which supraspinal sites are involved in STR-dependent allodynia and how this abnormal somatosensory message is relayed to these sites remain to be determined. The medial thalamus contains many nociceptive-specific (NS) neurons and is believed to be involved in mediating the affective-motivational aspects of pain. It is thus important to determine whether spinally administered STR elicits changes in the responses of medial thalamic NS neurons. Extracellular single-unit recordings were conducted in urethan-anesthetized rats (290-490 g). A detailed characterization of 20 thalamic NS units (1 per rat; 2 in 1 case) was conducted before and immediately after i.t. STR (40 microg). Initially, all of the units in this study were classified as NS, because they were excited by noxious pinch but not by innocuous tactile stimuli. After i.t. STR, all (formerly NS) units exhibited significant responses to innocuous tactile stimuli (brush and/or air jet) applied to lumbar or sacral dermatomes. This effect of STR on thalamic NS neurons was acute and reversible. The majority of units (11 of 20) also exhibited an increase in spontaneous firing rate. Although the complete pinch receptive field (RF) could not be determined for all units, the available data indicate that the RFs for brush stimulation after i.t. STR were substantially different from the pre-STR pinch RFs for all but three units. The same i.t. STR injection that caused the observed changes in medial thalamus also produced allodynia, in the form of brush-evoked cardiovascular or motor responses, in 18 of the 19 rats. The ability of NS cells in medial thalamus to respond to tactile input after i.t. STR suggests that the STR lowers the threshold of nociceptive neurons that project directly and/or indirectly to medial thalamus. These observations suggest that ascending nociceptive pathways and medial thalamic structures contribute to the expression of STR-dependent allodynia.     

Auditory evoked potentials in a patient with a unilateral lesion of the inferior colliculus and medial geniculate body

To reveal any association between the histological type and grade of intraductal breast neoplasms and the manner of accumulation of gene alterations, eight types of gene alterations, i.e., loss of heterozygosity (LOH) on chromosomal arms 16p, 16q, 17p, 17q, and 18q, amplification of the c-erbB-2 and hst-1/int-2 genes, and mutation of the p53 gene, were examined by Southern blot analysis or single-strand conformation polymorphism analysis in a total of 60 cases of intraductal breast cancer and 18 nonmalignant proliferative lesions. Among the histological types and three histological grade groups of intraductal carcinomas, the gene alterations which occurred most frequently were LOH on 16q alone in non-comedo type and Grade 1, alterations of c-erbB-2, 17p, and 16q in comedo type and Grade 2, and alterations of 17q and p53 as well as those of 16q, 17p, and c-erbB-2 in Grade 3. LOH on 16q and 18q was frequent in intraductal carcinoma of the intracystic papillary type, whereas LOH on 18q alone was detected in 27% of papillomas. Among intraductal carcinomas, the mean number of gene alterations was largest in comedo type and Grade 3, whereas it was smallest in non-comedo type and Grade 1. It was possible that LOH on 18q and 16q was involved frequently in papillary tumori-genesis and acquisition of malignant phenotype, respectively, whereas most of the other gene alterations were involved in acquisition of aggressive biological properties by intraductal carcinoma cells. It was also possible that the phenotype of breast neoplasms was determined by the combination of gene alterations at a relatively early developmental stage.     

Morphological characteristics of layer II projection neurons in the rat medial entorhinal cortex

Idiopathic left ventricular tachycardia (ILVT) differs from idiopathic right ventricular outflow tract (RVOT) tachycardia with respect to mechanism and pharmacologic sensitivity. ILVT can be categorized into three subgroups. The most prevalent form, verapamil-sensitive intrafascicular tachycardia, originates in the region of left posterior fascicle of the left bundle. This tachycardia is adenosine insensitive, demonstrates entrainment, and is thought to be due to reentry. The tachycardia is most often ablated in the region of the posteroinferior interventricular septum. A second type of ILVT is a form analogous to adenosine-sensitive RVOT tachycardia. This tachycardia appears to originate from deep within the interventricular septum and exits from the left side of the septum. This form of VT also responds to verapamil and is thought to be due to cAMP-mediated triggered activity. A third form of ILVT is propranolol sensitive. It is neither or initiated or terminated by programmed stimulation, does not terminate with verapamil, and is transiently suppressed by adenosine, responses consistent with an automatic mechanism. Recognition of the heterogeneity of ILVT and its unique characteristics should facilitate appropriate diagnosis and therapy in this group of patients.     

Comparison of projection neurons in the pontine nuclei and the nucleus reticularis tegmenti pontis of the rat

Dendritic features of identified projection neurons in two precerebellar nuclei, the pontine nuclei (PN) and the nucleus reticularis tegmenti pontis (NRTP) were established by using a combination of retrograde tracing (injection of fluorogold or rhodamine labelled latex micro-spheres into the cerebellum) with subsequent intracellular filling (lucifer yellow) in fixed slices of pontine brainstem. A multivariate analysis revealed that parameters selected to characterize the dendritic tree such as size of dendritic field, number of branching points, and length of terminal dendrites did not deviate significantly between different regions of the PN and the NRTP. On the other hand, projection neurons in ventral regions of the PN were characterized by an irregular coverage of their distal dendrites by appendages while those in the dorsal PN and the NRTP were virtually devoid of them. The NRTP, dorsal, and medial PN tended to display larger somata and more primary dendrites than ventral regions of the PN. These differences, however, do not allow the differentiation of projection neurons within the PN from those in the NRTP. They rather reflect a dorso-ventral gradient ignoring the border between the nuclei. Accordingly, a cluster analysis did not differentiate distinct types of projection neurons within the total sample. In both nuclei, multiple linear regression analysis revealed that the size of dendritic fields was strongly correlated with the length of terminal dendrites while it did not depend on other parameters of the dendritic field. Thus, larger dendritic fields seem not to be accompanied by a higher complexity but rather may be used to extend the reach of a projection neuron within the arrangement of afferent terminals. We suggest that these similarities within dendritic properties in PN and NRTP projection neurons reflect similar processing of afferent information in both precerebellar nuclei.     

Involvement of the medial geniculate body in prepulse inhibition of acoustic startle

Prepulse inhibition of acoustic startle is the normal reduction in startle response to an intense auditory stimulus when this stimulus is immediately preceded by a weaker prestimulus. Previous studies have shown that several neuroanatomical structures and pathways in the brain are involved in the modulation of prepulse inhibition. In the present study, the functional importance of the medial geniculate body (MG) in the modulation of prepulse inhibition was investigated. To this end, in vivo brain microdialysis probes were used to infuse drugs locally into the MG of awake, freely moving rats simultaneously with startle response and prepulse inhibition measurements in the same animals. Intrageniculate infusion of the sodium channel blocker, tetrodotoxin, significantly reduced prepulse inhibition without affecting baseline startle amplitude. A similar effect was obtained after intrageniculate infusion of the GABA(B) receptor agonist, baclofen. In addition, intrageniculate infusion of muscimol, an agonist at the GABA(A) receptor complex, reduced prepulse inhibition, although this effect was obtained at a higher concentration of the drug compared to that of baclofen. These studies suggest that the MG is involved in the modulation of prepulse inhibition and that auditory signals relayed via the MG may be subjected to inhibitory control at this level, involving GABA neurotransmission.     

Detection of auditory signals by frog inferior collicular neurons in the presence of spatially separated noise

Detection of auditory signals by frog inferior collicular neurons in the presence of spatially separated noise. J. Neurophysiol. 80: 2848-2859, 1998. Psychophysical studies have shown that the ability to detect auditory signals embedded in noise improves when signal and noise sources are widely separated in space; this allows humans to analyze complex auditory scenes, as in the cocktail-part effect. Although these studies established that improvements in detection threshold (DT) are due to binaural hearing, few physiological studies were undertaken, and very little is known about the response of single neurons to spatially separated signal and noise sources. To address this issue we examined the responses of neurons in the frog inferior colliculus (IC) to a probe stimulus embedded in a spatially separated masker. Frogs perform auditory scene analysis because females select mates in dense choruses by means of auditory cues. Results of the extracellular single-unit recordings demonstrate that 22% of neurons (A-type) exhibited improvements in signal DTs when probe and masker sources were progressively separated in azimuth. In contrast, 24% of neurons (V-type) showed the opposite pattern, namely, signal DTs were lowest when probe and masker were colocalized (in many instances lower than the DT to probe alone) and increased when the two sound sources were separated. The remaining neurons demonstrated a mix of these two types of patterns. An intriguing finding was the strong correlation between A-type masking release patterns and phasic neurons and a weaker correlation between V-type patterns and tonic neurons. Although not decisive, these results suggest that phasic units may play a role in release from masking observed psychophysically. Analysis of the data also revealed a strong and nonlinear interaction among probe, masker, and masker azimuth and that signal DTs were influenced by two factors: 1) the unit's sensitivity to probe in the presence of masker and 2) the criterion level for estimating DT. For some units, it was possible to examine the interaction between these two factors and gain insights into the variation of DTs with masker azimuth. The implications of these findings are discussed in relation to signal detection in the auditory system.     

Response characteristics of neurons in the medial component of the medial geniculate nucleus during Pavlovian differential fear conditioning in rabbits.

The amygdaloid central nucleus (ACE) may contribute significantly to Pavlovian fear-conditioned bradycardic responses during the presentation of conditioned emotional stimuli. Because the medial component of the medial geniculate nucleus (MGm) is a major source of input to the region of the ACE, the extracellular single-unit responses of MGm neurons were examined during Pavlovian differentially conditioned bradycardic responding in rabbits. Conditioning involved pairing one tone (CS+) with paraorbital shock and presenting another tone (CS–) in the absence of shock. Two general classes of MGm neurons were identified based on their conditioned-response characteristics. One group responded with greater increases in activity and at a shorter latency to the CS+ compared with the CS–, whereas the other group responded with greater increases in activity and at a shorter latency to the CS– compared with the CS+. Recordings from MGm neurons in naive rabbits prior to conditioning provided evidence that the acoustic stimuli used subsequently as the CS+ and CS– did not evoke differential responses. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Cholinergic responsiveness of neurons in the ventroposterior thalamus of the anesthetized rat

Acetylcholine has been implicated as an important neurotransmitter in the mechanisms of thalamic activation. Cholinergic mechanisms are thought to directly underlie the high level of excitability observed in thalamic relay neurons during waking and rapid eye movement sleep. We sought to determine if the cholinergic responsiveness of neurons in the ventroposterior nuclei of the thalamus in rat is consistent with this view. Neurons in the chloral hydrate-anesthetized rat were studied with extracellular recording and microiontophoretic application of cholinergic agents. In most cases (63% of 63 cells), the ejection of the agonist, carbachol, had no observable effect on spontaneous activity. Facilitation (25%), inhibition (8%) and inhibition followed by facilitation (3%) were also observed. Carbachol ejections that by themselves were ineffective in altering spontaneous activity proved capable, in 93% of 28 cells, of antagonizing the uniformly facilitatory responses produced by glutamate ejection. The putative M1-selective, cholinergic agonist, McN-A-343, was also ineffective alone in altering spontaneous activity in the majority of cases (74% of 27 cells) and produced only inhibitory responses in the remaining seven neurons studied. Interacting applications of McN-A-343 and glutamate resulted, in all cases, in antagonism of glutamate facilitation (N = 12). The various responses to applied cholinergic agonists were all capable of being antagonized by muscarinic receptor-blocking agents. Both the high proportion of inhibitory responses and the antagonism of glutamate facilitatory responses suggest that ventroposterior neurons in the rat differ from other thalamocortical relay neurons in the rat and cat with regard to cholinergic responsiveness. Additionally, the lack of predominantly facilitatory responding renders it unlikely that cholinergic mechanisms directly underlie increases in excitability of ventroposterior neurons observed during waking and rapid eye movement sleep.     

Electrophysiological study of the connection between medial thalamus and anterior cingulate cortex in the rat

We characterized the neuronal properties of the anterior cingulate cortex (ACC) evoked by electrical stimulation of the medial thalamus (MT). MT stimulation sites were found by their neuronal responses to noxious stimuli. Of 487 units identified histologically in the rat ACC, 94% were activated trans-synaptically at different areas of the ACC. Six percent of MT-evoked ACC units were activated antidromically and all of these units projected to a specific nucleus of MT. We suggest that MT nuclei mediate different aspects of nociceptive information to specific ACC areas, and that nociceptive information in the MT is modulated reciprocally by activities from the ACC.     

Excitatory interactions in neuronal networks which include cells of the auditory cortex and the medial geniculate body

The use of the method of cross-correlation analysis to elucidate the interactions between simultaneously recorded neurons from various loci of the auditory cortex (AC) and the medial geniculate body (MGB) has made it possible to identify the following characteristics of the functional organization of the excitatory interactions in the thalamocortical neuronal networks: the interdependant impulse action of neurons located at various loci of the AC and MGB was determined by reciprocal excitatory connections; the efficiency of the connections between neurons of the AC, 400-500 microns apart, and between tonotopically associated neurons of the AC and MGB was approximately identical (associations were identified in 12% of the cases); the "divergent" properties of the MGB (AC) neurons were manifested in the fact that one and same neuron could simultaneously excite both neighboring cells and neurons from one or several loci of the AC (MGB); the "convergent" properties of the AC and MGB neurons were manifested in the fact that cells located at various loci of the AC and MGB simultaneously excited one neuron. The results make it possible to explain the deviations observed in the investigation of RF of neurons of the AC and MGB from the principle of tonotopical organization. It is hypothesized that the character of the organization of the excitatory connections in the thalamocortical networks may promote the creation of the necessary conditions for the modification of the efficiency of synapses between all of the elements of the network during the stimulation of individual elements.     

Organization and morphologies of acetylcholinesterase-containing neurons in the thalamus and hypothalamus of the rat

The pineal complexes of two deep-sea fishes, Bathylagus wesethi (family Bathylagidae) and Nezumia liolepis (family Macrouridae), were studied with both light and electron microscopy. Receptor and supportive cells were identified in the pineals of both species. The presence of receptor cells suggests that the pineals function in photoreception. Ganglion cells could be identified only in B. wesethi. A dorsal sac and a paraphysis were found in B. wesethi; both structures are absent in N. liolepis. Several trends were found when the results of this study were compared with those of a study on the pineal complex of another deep-sea fish, the myctophid Triphoturus mexicanus (McNulty and Nafpaktitis, 1976). Two of these trends, which are correlated with the vertical distributions of the species studied, suggest an increase in the photosensitivity of the pineals. These are: 1) an increase in the average number of outer segment lamellar membranes per receptor cell, and 2) an increase in the ratio of receptor cells to nerve fibers in the pineal stalks. A functional relationship between the dorsal sac, paraphysis, and pineal central lumen was suggested. The relationship may involve secretory activities.     

Bilateral simultaneous cerebellar infarction in the medial branches of the posterior inferior cerebellar artery territories

The contents of nitric oxide (NO) in the blood were measured by electron spin resonance (ESR) method in Wistar rats with 35% TBSA III degrees burn. NO is endothelium derived relaxing factor (E-DRF) released by vascular endothelial cells. The results showed that: 1. Blood NO contents were not found to be significantly increased (72 hours postburn); 2. Escherichia coli lipopolysaccharide (LPS, endotoxin) could induce excessive NO formation in early burns.     

Effect of a conditioned emotional state on the discharging behavior of single neurons of the rat dorsal lateral geniculate body

1. The spike activity of 155 cells of the dorsal geniculate body of rats was recorded under the influence of flash and the combination of flash with a tone. For test animals the tone has an emotional relevance (ERT), because they had been trained to learn a conditioned emotional reaction with the tone as conditioned stimulus. Controls had been sham trained, i.e. the tone was never reinforced and therefore without emotional relevance (EIT). 2. In the controls there was no difference in the quantity of facilitatory and inhibitory responses to the tone, whereas under ERT there were more often facilitations than inhibitions. 3. The tone changed the reaction to flash. This variation depends on the emotional relevance of the acoustic stimulus. The ERT evoked a stronger increase in the number of excitations than the EIT. The number of inhibitions decreased equally under both conditions. Also, the ERT changed the strength of the flash response more often than the EIT did. An increasing trend of excitation was found especially in primary positive cells, an increasing trend of inhibition especially in primary negative ones. The time course of responses shows these differences to occur in the primary positive cells essentially later than 200 mse cafter the stimulus, in the primary negative cells, however, within this interval.     

Contribution of NMDA and non-NMDA receptors to synaptic transmission from the brachium of the inferior colliculus to the medial subdivision of the medial geniculate nucleus in the rabbit

Previous work from this laboratory has demonstrated that monosynaptic inputs from the brachium of the inferior colliculus (BIC) to the medial subdivision of the medial geniculate nucleus (mMG) strengthen as a result of associative conditioning with an acoustic conditioned stimulus (i.e., fear conditioning). One model that has been proposed to underlie certain types of neuronal plasticity involves the recruitment of N-methyl-D-aspartic acid (NMDA)-type glutamate receptors. The purpose of the present study was to examine the relative contributions of glutamatergic NMDA and non-NMDA receptors to synaptic transmission within this pathway. Individual contributions of the specific receptor types were assessed through the use of 2-amino-5-phosphonovaleric acid (AP5), a selective NMDA receptor antagonist, and 6-cyano-5-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA receptor antagonist. Bipolar stimulating electrodes were stereotaxically implanted in BIC and recording electrodes (attached to dual 32-gauge cannulae for delivery of drug) were positioned in mMG of New Zealand albino rabbits. Single pulses (150 micros, 100-350 microA) delivered to BIC resulted in short-latency (<4 ms) responses in mMG. BIC-evoked single-unit activity was recorded from mMG before, during, and at several intervals after injection of AP5, CNQX, and/or artificial cerebrospinal fluid (ACSF). Injection of either AP5 or CNQX, but not ACSF, significantly attenuated the short-latency BIC-evoked responses in the vast majority of cells tested. These findings suggest that the monosynaptic pathway from BIC to mMG is glutamatergic and that this pathway frequently employs NMDA-type receptors during electrically stimulated synaptic transmission. Due to the NMDA receptors' proposed role in plasticity (e.g., long-term potentiation), these results may have implications for understanding the mechanisms of synaptic plasticity observed at this synapse during associative learning.     

Axonal projection patterns of neurons in the secondary gustatory nucleus of channel catfish

The second gustatory nucleus of teleost fishes receives ascending fibers from the primary gustatory center in the medulla and sends efferent fibers to several nuclei in the inferior lobe of the diencephalon. Similar to the corresponding parabrachial nucleus in birds and mammals, the secondary gustatory nucleus of catfish consists of several cytoarchitectonically distinct subnuclei which receive input from different portions of the primary gustatory nuclei. However, it is unclear how the subnuclear organization relates to the processing of gustatory information in the hindbrain and the subsequent transmission of that information to the forebrain. To determine whether cells within different subnuclei of the secondary gustatory nucleus of channel catfish project to different diencephalic targets, single cells were intracellularly labeled with biocytin. Three subnuclei have been identified in the secondary gustatory nucleus: a medial subnucleus spanning most of the rostrocaudal extent of the nucleus, a central subnucleus and a dorsal subnucleus, the latter two located in the rostrolateral portion of the complex. Cells throughout the secondary gustatory nucleus typically possessed similar collateral projections to several nuclei in the inferior lobe, although four of the six cells filled in the medial subnucleus projected only to nucleus centralis. The only apparent subnucleus-specific projection pattern involved cells at the rostral edge of the secondary gustatory nucleus and in the secondary visceral nucleus. Axons of these cells terminated only in restricted portions of nucleus lobobulbaris. These results suggest that efferents from different subnuclei of the secondary gustatory nucleus of catfish, like those of the parabrachial nucleus of birds and mammals, do not possess simple, topographical projections to target nuclei in the diencephalon.     

Role of platelet-activating factor in long-term potentiation of the rat medial vestibular nuclei

In rat brain stem slices, we investigated the role of platelet activating factor (PAF) in long-term potentiation (LTP) induced in the ventral part of the medial vestibular nuclei (MVN) by high-frequency stimulation (HFS) of the primary vestibular afferent. The synaptosomal PAF receptor antagonist, BN-52021 was administered before and after HFS. BN-52021 did not modify the vestibular potentials under basal conditions, but it reduced the magnitude of potentiation induced by HFS, which completely developed after the drug wash-out. The same effect was obtained by using CV-62091, a more potent PAF antagonist at microsomal binding sites, but with concentrations higher than those of BN-52021. By contrast both BN-52021 and CV-6209 had no effect on the potentiation once induced. This demonstrates that PAF is involved in the induction but not in the maintenance of vestibular long-term effect through activation of synaptosomal PAF receptors. In addition, we analyzed the effect of the PAF analogue, 1-O-hexadecyl-2-O- (methylcarbamyl)-sn-glycero-3-phosphocoline (MC-PAF) and the inactive PAF metabolite, 1-O-hexadecyl-sn-glycero-3-phosphocoline (Lyso-PAF) on vestibular responses. Our results show that MC-PAF, but not Lyso-PAF induced potentiation. This potentiation was prevented by D,L-2-amino 5-phosphonopentanoic acid, suggesting an involvement of N-methyl-D-aspartate receptors. Furthermore, under BN-52021 and CV-6209, the MC-PAF potentiation was reduced or abolished. The dose-effect curve of MC-PAF showed a shift to the right greater under BN-52021 than under CV-6209, confirming the main dependence of MC-PAF potentiation on the activation of synaptosomal PAF receptors. Our results suggest that PAF can be released in the MVN after the activation of postsynaptic mechanisms triggering LTP, and it may act as a retrograde messenger which activates the presynaptic mechanisms facilitating synaptic plasticity.     

Monaural spectral contrast mechanism for neural sensitivity to sound direction in the medial geniculate body of the cat

Monaural spectral contrast mechanism for neural sensitivity to sound direction in the medial geniculate body of the cat. J. Neurophysiol. 78: 2754-2771, 1997. Central auditory neurons vary in sound direction sensitivity. Insensitive cells discharge well to all sound source directions, whereas sensitive cells discharge well to certain directions and poorly to others. High-frequency neurons in the latter group are differentially sensitive to binaural and monaural directional cues present in broadband noise (BBN). Binaural directional (BD) cells require binaural stimulation for directional sensitivity; monaural directional (MD) cells are sensitive to the direction of monaural stimuli. A model of MD sensitivity was tested using single-unit responses. The model assumes that MD cells derive directional sensitivity from pinna-derived spectral cues (head related transfer function, HRTF). This assumption was supported by the similarity of effects that pinna orientation produces on locations of HRTF patterns and on locations of MD cell azimuth function peaks and nulls. According to the model, MD neurons derive directional sensitivity by use of excitatory/inhibitory antagonism to compare sound pressure in excitatory and inhibitory frequency domains, and a variety of observations are consistent with this idea. 1) Frequency response areas of MD cells consist of excitatory and inhibitory domains. MD cells exhibited a higher proportion of multiple excitatory domains and narrower excitatory frequency domains than BD cells, features that may reflect specialization for spectral-dependent directional sensitivity. 2) MD sensitivity requires sound pressure in excitatory and inhibitory frequency domains. Directional sensitivity was evaluated using stimuli with frequency components confined exclusively to excitatory domains (E-only stimuli) or distributed in both excitatory and inhibitory domains (E/I stimuli). Each of 13 MD cells that were tested exhibited higher directional sensitivity to E/I than to E-only stimuli; most MD cells exhibited relatively low directional sensitivity when frequency components were confined exclusively to excitatory domains. 3) MD sensitivity derives from excitatory/inhibitory antagonism (spectral inhibition). Comparison of responses to best frequency and E/I stimuli provided strong support for spectral inhibition. Although spectral facilitation conceivably could contribute to directional sensitivity with direction-dependent increases in response, the results did not show this to be a significant factor. 4) Direction-dependent decreases in responsiveness to BBN reflect increased sound pressure in inhibitory relative to excitatory frequency domains. This idea was tested using the strength of two-tone inhibition, which is a function of stimulus levels in inhibitory relative to excitatory frequency domains. The finding that two-tone inhibition was stronger at directions where BBN responses were minimal than at directions where they were maximal supports the model.     

Modulation of bursts and high-threshold calcium spikes in neurons of rat auditory thalamus

Neurons in the ventral partition of the medial geniculate body are able to fire high-threshold Ca2+-spikes. The neurons normally discharge such spikes on low-threshold Ca2+-spikes after the action potentials of a burst. We studied membrane mechanisms that regulate the discharge of high-threshold Ca2+-spikes, using whole-cell recording techniques in a slice preparation of rat thalamus. A subthreshold (persistent) Na+-conductance amplified depolarizing inputs, enhancing membrane excitability in the tonic firing mode and amplifying the low-threshold Ca2+-spike in the burst firing mode. Application of tetrodotoxin blocked the amplification and high-threshold Ca2+-spike firing. A slowly inactivating K+ conductance, sensitive to blockade with 4-aminopyridine (50-100 microM), but not tetraethylammonium (2-10 mM), appeared to suppress excitability and high-threshold Ca2+-spike firing. Application of 4-aminopyridine increased the low-threshold Ca2+-spike and the number of action potentials in the burst, and led to a conversion of the superimposed high-threshold Ca2+-spike into a plateau potential. Application of the Ca2+-channel blocker Cd2+ (50 microM), reduced or eliminated this plateau potential. The tetrodotoxin sensitive, persistent Na+-conductance also sustained plateau potentials, triggered after 4-aminopyridine application on depolarization by current pulses. Our results suggest that high-threshold Ca2+-spike firing, and a short-term influx of Ca2+, are regulated by a balance of voltage-dependent conductances. Normally, a slowly inactivating A-type K+-conductance may reduce high-threshold Ca2+-spike firing and shorten high-threshold Ca2+-spike duration. A persistent Na+-conductance promotes coupling of the low-threshold Ca2+-spike to a high-threshold Ca2+-spike. Thus, the activation of both voltage-dependent conductances would affect Ca2+ influx into ventral medial geniculate neurons. This would alter the quality of the different signals transmitted in the thalamocortical system during wakefulness, sleep and pathological states.     

Associative retuning in the thalamic source of input to the amygdala and auditory cortex: Receptive field plasticity in the medial division of the medial geniculate body.

The medial division of the medial geniculate body (MGm) projects to the lateral amygdala and the upper layer of the auditory cortex and develops physiological plasticity rapidly during classical conditioning. The effects of learning on frequency receptive fields (RFs) in the MGm of the guinea pig have been determined. Classical conditioning (tone–footshock), as indexed by rapid development of conditioned bradycardia, produced conditioned stimulus (CS)–frequency specific RF plasticity: increased response at the CS frequency with decreased responses at other frequencies, both immediately and after a 1-hr retention period. Sensitization training produced only general changes in RFs. These findings are considered with reference to both the elicitation of amygdala-mediated, fear-conditioned responses and the mechanism of retrieval of information stored in the auditory cortex during acquisition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)