A cytoarchitectonic atlas of the medial geniculate body of the opossum, Didelphys virginiana, with a comment on the posterior intralaminar nuclei of the thalamus

The lateral thalamus: a bridge between multisensory processing and naturalistic behaviors

The lateral thalamus (LT) receives input from primary sensory nuclei and responds to multimodal stimuli. The LT is also involved in regulating innate and social behaviors through its projections to cortical and limbic networks. However, the importance of multisensory processing within the LT in modulating behavioral output has not been explicitly addressed. Here, we discuss recent findings primarily from rodent studies that extend the classical view of the LT as a passive relay, by underscoring its involvement in associating multimodal features and encoding the salience, valence, and social relevance of sensory signals. We propose that the primary function of the LT is to integrate sensory and non-sensory aspects of multisensory input to gate naturalistic behaviors.

Evaluation of medial division of the medial geniculate (MGM) and posterior intralaminar nucleus (PIN) inputs to the rat auditory cortex, amygdala, and striatum

The medial division of the medial geniculate (MGM) and the posterior intralaminar nucleus (PIN) are association nuclei of the auditory thalamus. We made tracer injections in these nuclei to evaluate/compare their presynaptic terminal and postsynaptic target features in auditory cortex, amygdala and striatum, at the light and electron microscopic levels. Cortical labeling was concentrated in Layer 1 but in other layers distribution was location-dependent. In cortical areas designated dorsal, primary and ventral (AuD, Au1, AuV) terminals deep to Layer 1 were concentrated in infragranular layers and sparser in the supragranular and middle layers. In ectorhinal cortex (Ect), distributions below Layer 1 changed with concentrations in supragranular and middle layers. In temporal association cortex (TeA) terminal distributions below Layer 1 was intermediate between AuV/1/D and Ect. In amygdala and striatum, terminal concentrations were higher in striatum but not as dense as in cortical Layer 1. Ultrastructurally, presynaptic terminal size was similar in amygdala, striatum or cortex and in all cortical layers. Postsynaptically MGM/PIN terminals everywhere synapsed on spines or small distal dendrites but as a population the postsynaptic structures in cortex were larger than those in the striatum. In addition, primary cortical targets of terminals were larger in primary cortex than in area Ect. Thus, although postsynaptic size may play some role in changes in synaptic influence between areas it appears that terminal size is not a variable used for that purpose. In auditory cortex, cortical subdivision-dependent changes in the terminal distribution between cortical layers may also play a role.

Listening to another sense: somatosensory integration in the auditory system

Conventionally, sensory systems are viewed as separate entities, each with its own physiological process serving a different purpose. However, many functions require integrative inputs from multiple sensory systems and sensory intersection and convergence occur throughout the central nervous system. The neural processes for hearing perception undergo significant modulation by the two other major sensory systems, vision and somatosensation. This synthesis occurs at every level of the ascending auditory pathway: the cochlear nucleus, inferior colliculus, medial geniculate body and the auditory cortex. In this review, we explore the process of multisensory integration from (1) anatomical (inputs and connections), (2) physiological (cellular responses), (3) functional and (4) pathological aspects. We focus on the convergence between auditory and somatosensory inputs in each ascending auditory station. This review highlights the intricacy of sensory processing and offers a multisensory perspective regarding the understanding of sensory disorders.

The cocaine- and amphetamine-regulated transcript, calbindin, calretinin and parvalbumin immunoreactivity in the medial geniculate body of the guinea pig

The purpose of this study was to describe the distribution and colocalization of cocaine- and amphetamine-regulated transcript (CART) and three calcium-binding proteins (calbindin, calretinin and parvalbumin) in each main division of the medial geniculate body (MGB) in the guinea pig. From low to moderate CART immunoreactivity was observed in all divisions of the MGB, although in most of its length only fibers and neuropil were labeled. A small number of CART immunoreactive somata were observed in the caudal segment of the MGB. The central parts of all divisions contained a distinctly smaller number of CART immunoreactive fibers relative to their outer borders, where CART fibers formed patchy clusters. As a whole, the intense CART immunoreactive borders formed a shell around the weakly CART labeled core. Double-labeling immunofluorescence showed that CART did not colocalize with either calbindin, calretinin or parvalbumin, whose immunoreactivity was predominantly restricted to perikarya. The distribution pattern of calretinin was more similar to that of calbindin than to that of parvalbumin. Calretinin and calbindin exhibited higher immunoreactivity in the medial and dorsal divisions of the MGB, where parvalbumin staining was low. In general, although parvalbumin exhibited the weakest immunoreactivity of all studied Ca(2+) binding proteins, it was most highly expressed in the ventral division of the MGB. Our results indicate that CART could be involved in hearing, although its immunoreactivity in the medial geniculate complex was not as intense as in other sensory brain regions. In the guinea pig the heterogeneous and complementary pattern of calbindin, calretinin and parvalbumin is evident, however, the overlap in staining appears to be more extensive than that seen in other rodents.

Subcortical auditory structures in the Mongolian gerbil: I. Golgi architecture

By means of the Golgi-Cox and Nissl methods we investigated the cyto- and fiberarchitecture as well as the morphology of neurons in the subcortical auditory structures of the Mongolian gerbil (Meriones unguiculatus), a frequently used animal model in auditory neuroscience. We describe the divisions and subdivisions of the auditory thalamus including the medial geniculate body, suprageniculate nucleus, and reticular thalamic nucleus, as well as of the inferior colliculi, nuclei of the lateral lemniscus, superior olivary complex, and cochlear nuclear complex. In this study, we 1) confirm previous results about the organization of the gerbil's subcortical auditory pathway using other anatomical staining methods (e.g., Budinger et al. [2000] Eur J Neurosci 12:2452-2474); 2) add substantially to the knowledge about the laminar and cellular organization of the gerbil's subcortical auditory structures, in particular about the orientation of their fibrodendritic laminae and about the morphology of their most distinctive neuron types; and 3) demonstrate that the cellular organization of these structures, as seen by the Golgi technique, corresponds generally to that of other mammalian species, in particular to that of rodents.

A morphometric comparative study of the medial geniculate body of the rabbit and the fox

SUMMARY

Unbiased stereological methods were used to morphometrically examine and compare the medial geniculate body (MGB) of two species from different mammalian orders. The MGB had a similar nuclear pattern, and it was parcelled into three major cytoarchitectural areas: the dorsal nucleus (MGd), the ventral nucleus (MGv) and the medial nucleus (MGm). The MGd was predominant in the fox, where it contributed nearly 50% to the total MGB volume, while in the rabbit, the MGv was insignificantly larger than the MGd. In both species, the percentage contribution of the MGm was the lowest. The MGd in the fox was also characterized by twice as many neurons per mm(3) as in the rabbit, whereas a reverse proportion was observed in the MGm, although the numerical density in the MGv was very similar in both species. The total number of MGB neurons in the fox was over twice higher than that in the rabbit. The variability in the percentage contribution of the MGd, MGv and MGm cells to the total neuronal population of the MGB was different in both mammals. In the rabbit, there was a larger contribution from the MGv and MGm, while in the fox, the MGd was predominant. These data demonstrate that the main areas of the MGB complex differ in terms of the morphometric characteristics in both species. Our results also show that the negative correlation between the volume and numerical density in the sensory centres of the brain might not be as distinct as in the non-sensory brain structures.

Time course of cortically induced fos expression in auditory thalamus and midbrain after bilateral cochlear ablation

Expression of c-fos in the medial geniculate body (MGB) and the inferior colliculus (IC) in response to bicuculline-induced corticofugal activation was examined in rats at different time points after bilateral cochlear ablation (4 h-30 days). Corticofugal activation was crucial in eliciting Fos expression in the MGB after cochlear ablation. The pars ovoidea (OV) of the medial geniculate body ventral division (MGv) showed dense Fos expression 4 h after cochlear ablation; the expression declined to very low levels at 24 h and thereafter. In turn, strong Fos expression was found in the pars lateralis (LV) of the MGv 24 h after cochlear ablation and dropped dramatically at 14 days. The dorsal division of the MGB (MGd) showed high Fos expression 7 days after cochlear ablation, which persisted for a period of time. Using multi-electrode recordings, neuronal activity of different MGB subnuclei was found to correlate well with Fos expressions. The temporal changes in cortically activated Fos expression in different MGB subnuclei after bilateral cochlear ablation indicate differential denervation hypersensitivities of these MGB neurons and likely point to differential dependence of these nuclei on both auditory ascending and corticofugal descending inputs. After bilateral cochlear ablation, significant increases in Fos-positive neurons were detected unilaterally in all IC subnuclei, ipsilateral to the bicuculline injection.

Thalamic nuclei in the opossum Monodelphis domestica

We investigated nuclear divisions of the thalamus in the gray short-tailed opossum (Monodelphis domestica) to gain detailed information for further developmental and comparative studies. Nissl and myelin staining, histochemistry for acetylcholinesterase and immunohistochemistry for calretinin and parvalbumin were performed on parallel series of sections. Many features of the Monodelphis opossum thalamus resemble those in Didelphis and small eutherians showing no particular sensory specializations, particularly in small murid rodents. However, several features of thalamic organization in Monodelphis were distinct from those in rodents. In the opossum the anterior and midline nuclear groups are more clearly separated from adjacent structures than in eutherians. The dorsal lateral geniculate nucleus (LGNd) starts more rostrally and occupies a large part of the lateral wall of the thalamus. As in other marsupials, two cytoarchitectonically different parts, alpha and beta are discernible in the LGNd of the opossum. Each of them may be subdivided into two additional bands in acetylcholinesterase staining, while in murid rodents the LGNd consists of a homogeneous mass of cells. Therefore, differentiation of the LGNd of the Monodelphis opossum is more advanced than in murid rodents. The medial geniculate body consists of three nuclei (medial, dorsal and ventral) that are cytoarchitectonically distinct and stain differentially for parvalbumin. The relatively large size of the MG and LGNd points to specialization of the visual and auditory systems in the Monodelphis opossum. In contrast to rodents, the lateral dorsal and lateral posterior nuclei in the opossum are poorly differentiated cytoarchitectonically.

Multiple topographically organized projections connect the central nucleus of the inferior colliculus to the ventral division of the medial geniculate nucleus in the gerbil, Meriones unguiculatus

The ventral division of the medial geniculate nucleus (MGv) receives almost all of its ascending input from the ipsilateral central nucleus of the inferior colliculus (CNIC). In a previous study (Cant and Benson [2006] J. Comp. Neurol. 495:511-528), we made injections of biotinylated dextran amine into the CNIC of the gerbil and demonstrated that it can be divided into two parts. One part (zone 1) receives almost all of its ascending input from the cochlear nuclei, the nuclei of the lateral lemniscus, and the main nuclei of the superior olivary complex; the other part (zone 2) receives inputs from the cochlear nuclei and nuclei of the lateral lemniscus but few or no inputs from the main olivary nuclei. Here we show that these two parts of the CNIC project differentially to the MGv. Axons labeled anterogradely by injections in zone 1 project throughout the rostral two-thirds of the MGv, whereas axons from zone 2 project to the caudal third of the MGv. Throughout much of their extent, the terminal fields do not appear to overlap, although both parts of the CNIC project to medial and dorsal parts of the MGv, and there may be overlap in the most ventral part as well. The results indicate that two parallel pathways arising in the CNIC remain largely separate in the medial geniculate nucleus of the gerbil. It seems most likely that the neurons in the two terminal zones in the MGv perform different functions in audition.

Identification of subdivisions in the medial geniculate body of the guinea pig

The accurate and reliable identification of subdivisions within the auditory thalamus is important for future studies of this nucleus. However, in the guinea pig, there has been no agreement on the number or nomenclature of subdivisions within the main nucleus of the auditory thalamus, the medial geniculate body (MGB). Thus, we assessed three staining methods in the guinea pig MGB and concluded that cytochrome oxidase (CYO) histochemistry provides a clear and reliable method for defining MGB subdivisions. By combining CYO with acetylcholinesterase staining and extensive physiological mapping we defined five separate divisions, all of which respond to auditory stimuli. Coronal sections stained for CYO revealed a moderate to darkly-stained oval core. This area (the ventral MGB) contained a high proportion (61%) of V-shaped tuning curves and a tonotopic organisation of characteristic frequencies. It was surrounded by four smaller areas that contained darkly stained somata but had a paler neuropil. These areas, the dorsolateral and suprageniculate (which together form the dorsal MGB), the medial MGB and the shell MGB, did not have any discernable tonotopic frequency gradient and contained a smaller proportion of V-shaped tuning curves. This suggests that CYO permits the identification of core and belt areas within the guinea pig MGB.

Posttraining lesions of the auditory thalamus, but not cortex, disrupt the inhibition of fear conditioned to an auditory stimulus

The purpose of this study was to examine the effects of lesions within the auditory system in an effort to disrupt the processing of the noise stimulus conditioned to inhibit fear. To accomplish this, three experiments were conducted in which rats were first given feature-negative discrimination training in which a noise was conditioned to inhibit fear to a light that signals danger. Following training, rats were given lesions of the medial geniculate body (MGB), auditory thalamus (ADT), or auditory cortex (CTX). Next, rats were tested for the ability to inhibit fear in the presence of the noise safety signal. The results of these experiments indicated that bilateral lesions of ADT disrupted the ability of the noise inhibitor to inhibit fear. In contrast, lesions largely restricted to the MGB or CTX did not disrupt the inhibition of fear. Along with past studies, these results suggest that an auditory pathway(s), which includes projections from the tectum to the ADT, is used to detect the safety properties previously conditioned to an auditory stimulus.

Chemical divisions in the medial geniculate body and surrounding paralaminar nuclei of the rat: quantitative comparison of cell density, NADPH diaphorase, acetyl cholin esterase and basal expression of c-fos

Quantitative methods of cell density, the intensities of both acetyl cholinesterase (AChE) and NADPH diaphorase (NADPHd), as well as the basal expression of c-fos, have been carried out in order to study the anatomical divisions of the medial geniculate body (MGB) and the group of nuclei located ventromedially to the MGB called the paralaminar complex (PL). The MGB was composed of the dorsal (MGd), and the ventral (MGv) divisions. We included the medial, or the magnocellular division (MGm), in the PL complex. MGd was composed of a dorsolateral (DL) core and a belt. The belt was composed of the suprageniculate (SG), the deep dorsal (DD), the caudo-medial (CM) and the caudo-dorsal (CD) nuclei. In the MGv, the basal expression of c-fos was the only way to trace a clear boundary between the ovoid (Ov) and the ventrolateral (VL) divisions. However, the marginal zone (MZ) was clearly and contrastingly different. The PL was considered to be composed of: the MGm, the posterior intralaminar nucleus (PIN), the peripeduncular nucleus (PP) and the nucleus subparafascicularis lateralis (SPFL). The MGm and the PIN share most of the chemical features, meanwhile both SPFL and PP displayed different patterns of NADPHd reactivity. The study of cell density on Giemsa stained sections confirmed main divisions of the area. AChE and NADPHd methods allowed the main MGB divisions to be discriminated. The differences between subdivisions were emphasized when cell density and c-fos activity were quantified in each nucleus. Each MGB division displayed a different pattern of c-fos activity under basal conditions. Thus, c-fos basal expression was a particular feature in each MGB or PL nucleus.

Auditory thalamic organization: Cellular slabs, dendritic arbors and tectothalamic axons underlying the frequency map

A model of auditory thalamic organization is presented incorporating cellular laminae, oriented dendritic arbors and tectothalamic axons as a basis for the tonotopic map at this level of the central auditory system. The heart of this model is the laminar organization of neuronal somata in the ventral division of the medial geniculate body (MGV) of the rabbit, visible in routine Nissl stains. Microelectrode studies have demonstrated a step-wise ascending progression of best frequencies perpendicular to the cell layers. The dendritic arbors of MGV neurons are aligned parallel to the cellular laminae and dendritic tree width along the frequency axis corresponds closely with the frequency steps seen in microelectrode studies. In the laminated subdivision, tectothalamic axons terminate in the form of bands closely aligned with the laminae and dendritic arbors of thalamic relay neurons. The bands of tectothalamic axons extend in the anterior-posterior (A-P) plane forming a dorsal-ventral series of stacked frequency slabs. In the pars ovoidea region, the homologous spiraling of somata, dendritic fields and tectothalamic axons appear to represent a low-frequency area in this species. At least two types of tectothalamic terminals were found within the bands: large boutons frequently arranged in a glomerular pattern and smaller boutons arising from fine caliber axons. We propose that the rabbit is an ideal model to investigate the structural-functional basis of functional maps in the mammalian auditory forebrain.

Cytoarchitecture of the medial geniculate body and thalamic projections to the auditory cortex in the rufous horseshoe bat (Rhinolophus rouxi)

The auditory cortex in echolocating bats is one of the best studied in mammals, yet the projections of the thalamus to the different auditory cortical fields have not been systematically analyzed in any bat species. The data of the present study were collected as part of a combined investigation of physiological properties, neuroarchitecture, and chemoarchitecture as well as connectivity of cortical fields in Rhinolophus in order to establish a neuroanatomically and functionally coherent view of the auditory cortex in the horseshoe bat. This paper first describes the neuroanatomic parcellation of the medial geniculate body and then concentrates on the afferent thalamic connections with auditory cortical fields of the temporal region. Deposits of horseradish peroxidase and wheatgerm-agglutinated horseradish peroxidase were made into neurophysiologically characterized locations of temporal auditory cortical fields; i.e., the tonotopically organized primary auditory cortex, a ventral field, and a temporal subdivision of a posterior dorsal field. A clear topographic relationship between thalamic subdivisions and specific cortical areas is demonstrated. The primary auditory cortex receives topographically organized input from the central ventral medial geniculate body. The projection patterns to the temporal subdivision of the posterior dorsal field suggest that it is a "core" field, similar to the posterior fields in the cat. Projections to the ventral field arise primarily from border regions of the ventral medial geniculate body. On the whole, the organization of the medial geniculate body projections to the temporal auditory cortex is quite similar to that described in other mammals, including cat and monkey.

Thalamic projections to the auditory cortex in the rufous horseshoe bat (Rhinolophus rouxi)

In this study, we analyzed the thalamic connections to the parietal or dorsal auditory cortical fields of the horseshoe bat, Rhinolophus rouxi. The data of the present study were collected as part of a combined investigation of physiologic properties, neuroarchitecture, and chemoarchitecture as well as connectivity of cortical fields in Rhinolophus, in order to establish a neuroanatomically and functionally coherent view of the auditory cortex. Horseradish peroxidase or wheat-germ-agglutinated horseradish peroxidase deposits were made into cortical fields after mapping response properties. The dorsal fields of the auditory cortex span nearly the entire parietal region and comprise more than half of the non-primary auditory cortex. In contrast to the temporal fields of the auditory cortex, which receive input mainly from the ventral medial geniculate body (or "main sensory nucleus"), the dorsal fields of the auditory cortex receive strong input from the "associated nuclei" of the medial geniculate body, especially from the anterior dorsal nucleus of the medial geniculate body. The anterior dorsal nucleus is as significant for the dorsal fields of the auditory cortex as the ventral nucleus of the medial geniculate body is for the temporal fields of the auditory cortex. Additionally, the multisensory nuclei of the medial geniculate body provide a large share of the total input to the nonprimary fields of the auditory cortex. Comparing the organization of thalamic auditory cortical afferents in Rhinolophus with other species demonstrates the strong organizational similarity of this bat's auditory cortex with that of other mammals, including primates, and provides further evidence that the bat is a relevant and valuable model for studying mammalian auditory function.

Connections of the posterior thalamic region with the auditory ectosylvian cortex in the dog

The purpose of the present study was to define auditory cortical areas in the dog on the basis of thalamocortical connectivity patterns. Connections between the posterior thalamic region and auditory ectosylvian cortex were studied using axonally transported tracers: fluorochromes and biotinylated dextran amine. Cyto- and chemoarchitecture provided grounds for the division of the posterior thalamic region into three complexes, medial geniculate body (MGB), posterior nuclei (Po), and lateromedial and suprageniculate nuclei (LM-Sg). Distinctive cytoarchitectonic features and the distribution of dominant thalamocortical connections (determined quantitatively) allowed us to define four ectosylvian areas: middle (EM), anterior (EA), posterior (EP), and composite (CE). We found that each area was a place of convergence for projections from five to eleven nuclei of the three thalamic complexes, with dominant projections derived from one or two nuclei. Dominant topographical projections from the ventral nucleus to area EM confirmed physiological reports that it may be considered a primary auditory area (AI). We found the anterior part of the EM to be distinct in having unique strong connections with the deep dorsal MGB nucleus. Area EA, which receives dominant projections from the lateral Po (Pol) and medial MGB nuclei, as well as area EP, which receives dominant connections from the dorsal caudal MGB nucleus, compose two parasensory areas. Area CE receives dominant projections from the extrageniculate nuclei, anterior region of the LM-Sg, and Pol, supplemented with an input from the somatosensory VP complex, and may be considered a polymodal association area.

Immunolocalization of PICK1 in the ascending auditory pathways of the adult rat

Protein that interacts with C-kinase alpha (PICK1) is a PDZ domain protein that interacts with many binding partners in the central nervous system (CNS), including activated protein kinase Calpha and subunits of the AMPA subtype of glutamate receptor. Almost nothing is known about the anatomic distribution of PICK1 in the intact adult CNS. By using PICK1 antisera and peroxidase immunocytochemistry, we report on the distribution of PICK1 in the ascending pathways of the central auditory system of the adult rat. PICK1-immunoreactivity (ir) was observed in many component nuclei of the central auditory system, including the dorsal cochlear nucleus, anteroventral cochlear nucleus, posteroventral cochlear nucleus, some divisions of the superior olivary complex, inferior colliculus, medial geniculate body, and primary auditory cortex. The general staining pattern for PICK1-immunoreactivity was somatodendritic with scattered puncta in neuropil and somatodendritic regions. The distribution of PICK1 partially overlaps with PKCalpha and glutamate receptor subunits such as GluR2. These data suggest that PICK1 may function in the regulation of PKCalpha and GluR2 localization in components of the rat auditory system, which may be a fundamental mechanism of synaptic transmission and/or plasticity. J. Comp. Neurol.

Descending projections to the inferior colliculus from the posterior thalamus and the auditory cortex in rat, cat, and monkey

Projections from the posterior thalamus and medial geniculate body were labeled retrogradely with wheat germ agglutinin conjugated to horseradish peroxidase injected into the rat, cat, and squirrel monkey inferior colliculus. Neurons were found ipsilaterally in the (1) medial division of the medial geniculate body, (2) central gray, (3) posterior limitans nucleus, and the (4) reticular part of the substantia nigra. Bilateral projections involved the (5) peripeduncular/suprapeduncular nucleus, (6) subparafascicular and posterior intralaminar nuclei, (7) nucleus of the brachium of the inferior colliculus, (8) lateral tegmental/lateral mesencephalic areas, and (9) deep layers of the superior colliculus. The medial geniculate projection was concentrated in the caudal one-third of the thalamus; in contrast, the labeling in the subparafascicular nucleus, substantia nigra, and central gray continued much further rostrally. Robust anterograde labeling corresponded to known patterns of tectothalamic projection. Biotinylated dextran amine deposits in the rat inferior colliculus revealed that (1) many thalamotectal cells were elongated multipolar neurons with long, sparsely branched dendrites, resembling neurons in the posterior intralaminar system, and that other labeled cells were more typical of thalamic relay neurons; (2) some cells have reciprocal projections. Similar results were seen in the cat and squirrel monkey. The widespread origins of descending thalamic influences on the inferior colliculus may represent a phylogenetically ancient feedback system onto the acoustic tectum, one that predates the corticocollicular system and modulates nonauditory centers and brainstem autonomic nuclei. Besides their role in normal hearing such pathways may influence behaviors ranging from the startle reflex to the genesis of sound-induced seizures.

Cell types and response properties of neurons in the ventral division of the medial geniculate body of the rabbit

Although there is evidence for multiple classes of thalamic relay neurons in the auditory thalamus, correlative anatomical and physiological studies are lacking. We have used the juxtacellular labeling technique, in conjunction with Nissl, Golgi, and immunocytochemical methods, to study the morphology and response properties of cells in the ventral division of the medial geniculate body of the rabbit. Single units in the ventral division of the medial geniculate body (MGV) were characterized extracellularly with monaural and binaural tone and noise bursts (100- to 250-msec duration). Characterized units were filled with biocytin and visualized with an antibody enhanced diaminobenzidine reaction. A total of 31 neurons were physiologically characterized and labeled with the juxtacellular technique. Labeled neurons were fully reconstructed from serial sections by using a computer microscope system. Three subregions of the rabbit MGV were identified, each characterized by differences in Nissl architecture, calcium-binding protein expression, and by the dendritic orientation of tufted relay neurons. In general, the dendritic fields of relay neurons were closely aligned with the cellular laminae. Qualitative and quantitative analyses revealed two types of presumptive relay neurons within the MGV. Type I cells had thick dendrites with a greater total volume and morphologically diverse appendages compared with the Type II cells whose dendrites were thin with a moderate number of small spines. Both classes were acoustically responsive and exhibited a variety of response patterns, including onset, offset, and sustained responses. In terms of binaural characteristics, most (ca. 53%) labeled neurons were of the EE type, with the remaining cells classified as EO (27%) or EI (20%) response types. Two types of presumptive interneurons were also seen: bipolar neurons with large dendritic fields and a small neurogliaform variety. Cell types and dendritic orientation within the MGV are discussed in terms of the physiological organization of the rabbit auditory thalamus.

Frequency organization and cellular lamination in the medial geniculate body of the rabbit

Cellular laminae within the tonotopically organized ventral division of the medial geniculate body (MGV) of the cat have been proposed as the anatomical substrate for physiologically defined isofrequency contours. In most species, the laminae are not visible with routine Nissl stains, but are defined by the dendritic fields of principal cells and the terminal arbors of afferents arising from the inferior colliculus. In the present study, we have used the rabbit to directly examine the relationship between the laminar and tonotopic organization of the MGV. Best frequency maps of the MGV in anesthetized adult New Zealand white rabbits were generated from cluster responses recorded at 30-100 microm intervals to randomly presented tone bursts. Parallel vertical penetrations, roughly perpendicular to the laminae, revealed a low-to-high frequency gradient within the MGV. Non-laminated regions of the ventral division, generally found at the rostral or caudal poles, did not demonstrate a systematic frequency gradient. In contrast to a predicted smooth gradient, best frequencies shifted in discrete steps across the axis of the laminae. A similar step-wise frequency gradient has been shown in the central nucleus of the inferior colliculus of the cat. It is proposed that the central laminated core of the MGV represents an efficient architecture for creating narrow frequency filters involved in fine spectral analysis.

Neural architecture of the rat medial geniculate body

The rat medial geniculate body was subdivided using Nissl preparations to establish nuclear boundaries, with Golgi-Cox impregnations to identify projection and local circuit neurons, and in fiber stained material to delineate the fiber tracts and their distribution. Three divisions were recognized (ventral, dorsal and medial): the first two had subdivisions. The ventral division had lateral and medial parts. The main cell type had bushy tufted dendrites which, with the afferent axons, formed fibrodendritic laminae oriented from dorso-lateral to ventro-medial; such laminae were not as regular medially, in the ovoid nucleus. The dorsal division contained several nuclei (dorsal superficial, dorsal, deep dorsal, suprageniculate, and ventrolateral) and neurons with radiating or bushy dendrites; the nuclear subdivisions differed in the concentration of one cell type or another, and in packing density. A laminar organization was present only in the dorsal superficial nucleus. Medial division neurons were heterogeneous in size and shape, ranging from tiny cells to magnocellular neurons; the various cell types intermingled. so that no further subdivision could be made. This parcellation scheme was consistent with, and supported by, the findings from plastic embedded or fiber stained material. There were very few small neurons with locally ramifying axons and which could perform an intrinsic role like that of Golgi type II cells. Their rarity was consistent with the small number of such profiles in plastic embedded or Nissl material and the few GABAergic medial geniculate body neurons seen in prior immunocytochemical work. While similar neuronal types and nuclear subdivisions are recognized in the rat and cat, there may be major interspecific differences with regard to interneuronal organization in the auditory thalamus whose functional correlates are unknown.

Pain pathways involved in fear conditioning measured with fear-potentiated startle: lesion studies

It is well established that the basolateral amygdala is critically involved in the association between an unconditioned stimulus (US), such as a foot shock, and a conditioned stimulus (CS), such as a light, during classic fear conditioning. However, little is known about how the US (pain) inputs are relayed to the basolateral amygdala. The present studies were designed to define potential US pathways to the amygdala using lesion methods. Electrolytic lesions before or after training were placed in caudal granular/dysgranular insular cortex (IC) alone or in conjunction with the posterior intralaminar nuclei of the thalamus (PoT/PIL), and the effects on fear conditioning were examined. Pretraining lesions of both IC and PoT/PIL, but not lesions of IC alone, blocked the acquisition of fear-potentiated startle. However, post-training combined lesions of IC and PoT/PIL did not prevent expression of conditioned fear. Given that previous studies have shown that lesions of PoT/PIL alone had no effect on acquisition of conditioned fear, these results suggest that two parallel cortical (insula-amygdala) and subcortical (PoT/PIL-amygdala) pathways are involved in relaying shock information to the basolateral amygdala during fear conditioning.

Differential projection patterns of superior and inferior collicular neurons onto posterior paralaminar nuclei of the thalamus surrounding the medial geniculate body in the rat

The thalamic nuclei at the medial border of the medial geniculate body (i.e. the suprageniculate nucleus, the medial division of the medial geniculate nucleus, the posterior intralaminar nucleus and the peripeduncular nucleus) which relay sensory information to the amygdala are thought to receive convergent input from multiple sites. In order to delineate the organization of these multimodal thalamic nuclei, the locations of superior and inferior collicular neurons projecting to these nuclei were studied by means of retrograde transport methods. Small injections of the tracer Miniruby were made into single paralaminar thalamic nuclei. Injections of Miniruby into the suprageniculate nucleus labelled predominantly neurons in the stratum opticum of the superior colliculus, whereas injections into the medial division of the medial geniculate body, the posterior intralaminar nucleus and the peripeduncular nucleus labelled predominantly neurons in the deep layers of the superior colliculus. These injections also labelled neurons in the inferior colliculus. The majority of retrogradely labelled neurons were found in the external nucleus of the inferior colliculus and here predominantly in layer 2. Injections focused onto the medial division of the medial geniculate body additionally labelled magnocellular neurons in layer 3 of the external nucleus and a few neurons in the central nucleus. More ventrally located injections, focused onto the posterior intralaminar and peripeduncular nucleus, almost exclusively labelled neurons in layer 1 of the external nucleus and the dorsal part of the dorsal nucleus. After injections into the suprageniculate nucleus, only neurons in layer 2 were found. Neurons in the central nucleus of the inferior colliculus were only found after injections that involved the medial division of the medial geniculate body. The present results suggest that, despite a considerable degree of convergence in this thalamic region, each of these thalamic nuclei receives a unique pattern of projections from the superior and inferior colliculi. It appears that the thalamic nuclei may be concerned mainly, but not exclusively, with a single sensory modality, and give rise to parallel multimodal and unimodal pathways to the amygdala.

Organization of the medial pulvinar nucleus in the macaque

BACKGROUND

The medial pulvinar appears to subserve the integration of associative cortical information and projects to visuomotor-related cortex. In contrast to the other pulvinar subdivisions, the medial pulvinar is a polymodal structure. Therefore, we studied the structural organization of the medial pulvinar to determine how it differs from the surrounding unimodal nuclei.

METHODS

Nissl-stained sections were examined to determine the boundaries of, and the distribution of neuronal sizes within, the medial pulvinar. In addition, Golgi-impregnated neurons were examined and drawn for analysis. Only rhesus monkey specimens were used, and the material had been prepared previously for other studies.

RESULTS

Projection neurons have round to oval somata and moderate numbers of primary dendrites that extend for short distances before branching into many secondary branches. Two variations of projection neurons (P1 and P2) were distinguished on the basis of the diameters of their dendritic tree. Both varieties have short dendrites that radiate in all directions. They differ in that P2 cells have longer second tier dendrites than P1 cells. Three types of local circuit neurons, tufted, radiating and varicose, were distinguished on the basis of their dendritic morphology. Four types of afferent fibers were identified. Type 1 afferents form cone-shape terminal arbors. Type 2 afferents are similar to those reported for retinal or cortical terminals. Type 3 afferents are of medium thickness and of an unknown origin. Type 4 afferents are thin and have small varicosities consistent with previously described cortical afferents. Afferent fibers are predominantly oriented along the mediolateral axis of the nucleus. We observed putative contacts between some afferents and local circuit neurons and between local circuit neurons and projection neurons.

CONCLUSIONS

Medial pulvinar neurons are generally smaller and rounder than those found in the adjacent pulvinar nuclei. These results provide additional evidence for structural distinctions between thalamic nuclei having different functions. However, the observed differences are subtle. In addition, the data in this report provide morphological evidence that cortical signals are likely to be integrated by means of the circuitry located within the nucleus.

Subcortical modulation of high-frequency (gamma band) oscillating potentials in auditory cortex

The purpose of this study was to use depth electrical stimulation and retrograde horseradish peroxidase (HRP) labeling to determine what role certain subcortical nuclei play in the neurogenesis of high-frequency gamma (approximately 40 Hz) oscillations in rat auditory cortex. Evoked and spontaneous electrocortical oscillations were recorded with the use of a high-spatial-resolution multichannel epipial electrode array while electrical stimulation was delivered to the posterior intralaminar (PIL) region of the ventral acoustic thalamus and to the centrolateral nucleus (CL) and the nucleus basalis (NB), which have been previously implicated in the production of cortical gamma oscillations. PIL stimulation consistently evoked gamma oscillations confined to a location between primary and secondary auditory cortex, corresponding to the region where spontaneous gamma oscillations were also recorded. Stimulation of the CL and NB did not evoke gamma oscillations in auditory cortex. HRP placed in the cortical focus of evoked gamma oscillations labeled cell bodies in the PIL, and in more lateral regions of the ventral acoustic thalamus, which on subsequent stimulation also evoked gamma oscillations in auditory cortex. No cells were labeled in either the CL or NB. These results indicate that the PIL and the lateral regions of ventral acoustic thalamus provide anatomically distinct input to auditory cortex and may play an exclusive and modality-specific role in modulating gamma oscillations in the auditory system.

Neuropeptide Y and enkephalin immunoreactivity in retinorecipient nuclei of the hamster pretectum and thalamus

This investigation was stimulated by the historical confusion concerning the identity of certain pretectal nuclei and by large differences reported between species with respect to which nuclei receive retinal innervation. Subcortical visual nuclei were studied using immunohistochemistry to identify retinal projections labeled following intraocular injection of cholera toxin, b fragment. In addition, neuropeptide Y (NPY) or enkephalin (ENK) immunoreactive cells and fibers were also evaluated in the retinorecipient pretectal and thalamic areas. The results confirm the established view that the retina directly innervates the nucleus of the optic tract (NOT), posterior (PPT), and olivary pretectal (OPT) nuclei. However, the retina also innervates the hamster medial (MPT) and anterior (APT; dorsal division) pretectal nuclei, results not previously reported in rodents. A commissural pretectal area (CPT) sparsely innervated by retina is also described. The data show for the first time that the posterior limitans nucleus (PLi) receives a moderately dense, direct retinal input. The PLi does not project to the cortex and appears to be a pretectal, rather than thalamic, nucleus. All retinal projections are bilateral, although predominantly contralateral. The PLi contains a moderately dense plexus of NPY- and ENK-IR fibers and terminals. However, peptidergic fibers also traverse the ATP and connect with the dorsomedial pretectium. The OPT contains ENK- and NPY-IR neurons and fibers, but is specifically identifiable by a moderately dense plexus of ENK-IR terminals. Numerous ENK-IR neurons are found in the NOT and PPT. The latter also has moderate numbers of ENK-IR fibers and terminals, but few NPY-IR neurons or fibers. The MPT contains modest numbers of ENK-IR fibers. The APT has no NPY-IR neurons or terminals, but an occasional ENK-IR neuron is seen and there is sparse ENK-IR innervation. Peptidergic innervation of the visual nuclei does not appear to be derived from the retina. The results show a set of retinally innervated, contiguous nuclei extending from the thalamic ventrolateral geniculate nucleus dorsomedially to the midbrain CPT. These nuclei plus the superior colliculus comprise a dorsal "visual shell" embracing a central core of caudal thalamus and rostral midbrain.

A single-cell study of the axonal projections arising from the posterior intralaminar thalamic nuclei in the rat

Thalamostriatal projections arising from the posterior intralaminar nuclei (P1; the parafascicular nucleus and the adjacent caudalmost part of the posterior thalamic group) were studied in rats by tracing the axons of small pools of neurons labelled anterogradely with biocytin. Thirteen P1 cells were also stained by juxta cellular application of the tracer. Relay cells of P1 nuclei have a morphology that differs radically from the classical descriptions of the bushy cells which represent the main neuronal type of the sensory thalamic relay nuclei. P1 cells have ovoid or polygonal somata of approximately 20-25 microm, from which emerge four or five thick, long and poorly branched dendrites bearing spines and filamentous appendages; their dendritic domains extend for up to 1.5 mm. Before leaving the nucleus 20% of axons give off collaterals that ramify locally. All axons course through the thalamic reticular nucleus, where they also distribute collaterals, and arborize massively in the striatum and sparsely in the cerebral cortex. At the striatal level four or five collaterals leave the main axon and terminate in patches scattered dorsoventrally within a rostrocaudally oriented slab. As revealed by calbindin D-28k immunohistochemistry, only the matrix compartment receives terminations from P1 axons. The cortical branch form small terminal puffs centred upon layer VI of the motor cortex. Before entering the striatum some axons of the parafascicular nucleus give rise to descending collaterals that arborize in the entopeduncular nucleus, in the subthalamic nucleus and in the vicinity of the red nucleus. Other axons arising from the caudal part of the posterior group send descending branches only to the entopeduncular nucleus. These findings show that P1 cells belong to a distinct category of thalamic relay neurons which, beside their massive projection to the striatum, also distribute collaterals to other components of the basal ganglia. Moreover, these results provide the first direct evidence that virtually all P1 cells project to both striatum and cerebral cortex. Finally, it is proposed on the basis of morphological, histochemical and hodological criteria that the caudal part of the posterior thalamic group in the rat is homologous to the suprageniculate-limitans nuclei of cats and primates.

Auditory thalamocortical pathways defined in monkeys by calcium-binding protein immunoreactivity

This study investigated differentiation of Macaca fuscata auditory thalamus into chemically defined nuclei forming relays to auditory cortical areas. The thalamus was stained immunocytochemically for parvalbumin and 28 kDa calbindin in normals and in brains in which retrogradely transported tracers were injected into middle layers of auditory cortical areas or applied to the cortical surface. Parvalbumin- and calbindin-immunoreactive cells show a complementary distribution in ventral, anterodorsal, posterodorsal, and magnocellular medial geniculate nuclei. The ventral nucleus has a high density of parvalbumin cells and few calbindin cells, and the anterodorsal nucleus has a high density of parvalbumin cells and moderate numbers of calbindin cells. Both nuclei have a dense parvalbumin-immunoreactive neuropil formed by terminations of fibers ascending in the brachium of the inferior colliculus. The posterodorsal nucleus has approximately equal proportions of parvalbumin and calbindin cells; neuropil staining is weak but contains terminations of calbindin-immunoreactive fibers ascending in the midbrain tegmentum. The magnocellular nucleus contains domains of parvalbumin and calbindin cells. Parvalbumin cells in the ventral nucleus project to a central core of auditory cortex with densest parvalbumin immunoreactivity. Those in anterodorsal and posterodorsal nuclei project to surrounding auditory fields with less dense parvalbumin immunoreactivity; those in the magnocellular nucleus project widely to auditory and other fields. Injections of middle cortical layers label a large majority of parvalbumin cells in the ventral, anterodorsal, or posterodorsal nuclei and in the magnocellular nucleus. Superficial deposits label calbindin cells only, usually in more than one nucleus, implying a widespread projection system.

Complementary expression of parvalbumin and calbindin D-28k delineates subdivisions of the rabbit medial geniculate body

The complementary pattern of immunohistochemical staining for the calcium-binding proteins parvalbumin (PV) and calbindin D-28k (CB) was used to delineate four major subdivisions of the rabbit medial geniculate body (MGB). PV immunoreactivity predominates in the ventral and medial divisions, whereas CB-immunoreactive cells characterize the dorsal and internal divisions. The ventral nucleus is strongly PV+ due to dense neuropil labeling and moderately labeled somata. The medial nucleus contains both medium-sized and large PV+ somata, as well as thick PV+ axons and terminals. The wedge-shaped internal nucleus composed of densely labeled CB+ cells, separates the dorsal and ventral nuclei rostrally, and expands caudally to encapsulate the posterior MGV. Large multipolar CB+ neurons with radiate dendrites characterize the dorsal nucleus. The differential expression of PV and CB also distinguishes the deep dorsal and superficial dorsal subnuclei in the dorsal division and a ventrolateral component in the ventral division. A comparison with studies of MGB connectivity in a variety of species suggests that PV immunoreactivity is highest in subdivisions that receive a substantial input from the central nucleus of the inferior colliculus and that project to primary auditory cortex. In contrast, CB immunoreactivity characterizes nuclei that receive input primarily from other sources, such as the paracentral nuclei of the inferior colliculus, the lateral tegmentum, and the spinal cord, and that project to secondary auditory areas. The ability of calcium-binding protein immunohistochemistry to delineate neuronal compartments across indistinct cytoarchitectonic borders makes it a powerful tool for guiding future connectional and physiological studies of the MGB.

Simultaneous single unit recording in the medial nucleus of the medial geniculate nucleus and amygdaloid central nucleus throughout habituation, acquisition, and extinction of the rabbit's classically conditioned heart rate

The present study examined single neuron activity in the medial nucleus of the medial geniculate (mMG) and amygdaloid central nucleus (ACe) simultaneously across several phases of differential heart rate conditioning (habituation, acquisition, and extinction). Within the same animals, the magnitude of mMG and ACe unit responses to two tone conditioned stimuli (CS) exhibited habituation, differential acquisition, and extinction. Neurons in each area developed a differential response latency to the CSs during acquisition, suggesting that mMG and ACe may be involved in changes of synaptic efficacy. Units in both areas rapidly developed a differential response magnitude to the CSs (< 6 acquisition trials), however, mMG units responded to the CSs with a shorter latency than ACe units across all phases of training. This suggests that unlearned and learned CS information may access mMG before ACe. These results are consistent with the notion that conditioning-induced plasticity which occurs in mMG may influence the conditioning-induced plasticity that occurs further downstream in the amygdala.

Retrograde axonal transport of different fluorescent tracers from the neocortex to the suprageniculate nucleus in the rat

The retrograde fluorescent tracers in a combination of two different dyes (Fluoro gold/Nuclear yellow or Fast blue/Fluoro gold) were used for the study of the projections from the medial geniculate body to the frontal and temporal cortices in rats. There were only single-labeled cells, no double-labeled ones in the medial geniculate body (MGB). The suprageniculate nucleus (SG) was considered to be the origin of the rat direct pathway to the frontal cortex. The present results suggest that projections from the suprageniculate nucleus to the frontal cortex and the temporal cortex consist of separate neuronal groups in the rat MGB and SG. The inputs to the SG from the auditory and oculomotor system may be processed in different ways.

Organization of the hamster intergeniculate leaflet: NPY and ENK projections to the suprachiasmatic nucleus, intergeniculate leaflet and posterior limitans nucleus

The intergeniculate leaflet (IGL) is an integral part of the circadian visual system. It receives direct retinal input and relays photic information to the circadian clock in the suprachiasmatic nucleus (SCN) through a geniculohypothalamic tract (GHT). In both rat and hamster, neuropeptide Y immunoreactive (NPY-IR) IGL cells project through the GHT to the SCN. However, the hamster GHT also contains enkephalin-IR (ENK-IR) fibers, presumably of IGL origin. In the present investigations, the IGL was examined for NPY-, ENK-, or dual-IR cells. Their projections to the SCN, contralateral IGL and pretectum were also studied. The results show that the hamster IGL contains both NPY- and ENK-IR neurons and that about 50% of these are immunoreactive to both peptides. Double-label retrograde analysis indicates that cells of each peptide class project to the SCN. Similarly, IGL neurons, many of which are NPY- and ENK-IR, project to the pretectum, particularly the posterior limitans nucleus. While numerous IGL neurons project contralaterally, very few are NPY- or ENK-IR. The distribution of SCN- and pretectum-projecting cells, in conjunction with the distribution of peptide-IR neurons, allows expansion of the IGL definition to include the region medial to the ventral lateral geniculate nucleus (VLG). The VLG is ventrolateral to the IGL and does not contain either neurons projecting to the SCN nor NPY- or ENK-IR cells, but does have numerous neurons projecting to the pretectum. The results substantiate and expand the previous definition of the hamster IGL, elaborate the species difference in IGL organization, and demonstrate the increased breadth of the circadian visual system.

The neurons of the medial geniculate body in the mustached bat (Pteronotus parnellii)

The neurons in the medial geniculate body were studied in Golgi preparations from adult mustached bats (Pteronotus parnellii). Their somatic and dendritic configurations were compared with those of cells in other, nonecholocating mammals. A second goal was to use the thalamic nuclear subdivisions derived from Golgi material to integrate the findings in parallel studies of cytoarchitecture, immunocytochemistry, and tectothalamic connections. Three primary divisions are defined. The ventral division is large and has a stereotyped neuronal organization. Medium-sized perikarya (about 10 microns in diameter) represent tufted neurons; the fibrodendritic plexus forms laminae in the lateral part along which midbrain axons terminate. A smaller, possibly intrinsic, neuron with thin, sparse dendrites is rarely impregnated. Neurons in the larger, medial part, which represents frequencies of 60 kHz and higher, have more spherical dendritic fields; their branching pattern remains tufted, and the laminar organization was less evident. The dorsal division is about equal in size, and it has many nuclei and a corresponding neuronal diversity. These neurons are medium-sized except in the suprageniculate nucleus, where many cells are larger. Four dorsal division nuclei are recognized. Each has neurons with radiate or weakly tufted dendritic arbors. Superficial dorsal nucleus neurons are oriented from medial to lateral, imparting a slightly laminated appearance to the neuropil. A few smaller, stellate neurons with modest dendritic domains are present. Suprageniculate nucleus neurons have radiating dendritic fields that project spherically; they have fewer branches than dorsal nucleus neurons. The posterior limitans nucleus is dorsomedial to the suprageniculate nucleus; it has small neurons with long, sparsely branched dendrites. The rostral pole nucleus, included in the dorsal division on cytoarchitectonic grounds, had too few neurons impregnated to reveal its neuronal architecture. The medial division, the smallest of the main parts, is one nucleus with at least six types of cells, including the magnocellular, bushy tufted, disc-shaped, medium-sized multipolar, elongated, and small stellate neurons. There is no laminar arrangement. Many of the neurons resemble those in rodent, marsupial, carnivore, and primate auditory thalamic nuclei. Despite such morphological correspondences, functional differences, such as the evolution of combination sensitivity, suggest that structurally comparable auditory thalamic neurons may subserve diverse physiological representations.

Cytoarchitecture of the medial geniculate body in the mustached bat (Pteronotus parnellii)

The cytoarchitectonic organization of the medial geniculate body and adjoining thalamic nuclei was analyzed in the mustached bat (Pteronotus parnellii). These subdivisions provide a reference for structural, physiological, connectional, and neurochemical work. Most nuclei recognized in other mammals exist in the mustached bat, although the relative volume of the three divisions was species specific. The ventral division contains medium-sized neurons and a few smaller cells and is well developed. Neurons in the lateral part lie in regularly aligned rows corresponding to the laminae in Golgi material; in the medial part, these laminae are obscured by fibers. The dorsal division has at least four nuclei, each with a unique cytoarchitecture and myeloarchitectonic organization. The suprageniculate nucleus is prominent and has many large radiate neurons. Cells in the superficial dorsal nucleus have weekly laminated dendrites, while dorsal nucleus neurons have spherical dendritic fields. There is a wide range of neuropil patterns within the dorsal division. The suprageniculate nucleus has thick myelinated axons, while the fibers in the superficial and dorsal nuclei are much thinner. The rostral pole nucleus becomes prominent in the anterior one-half of the auditory thalamus; its architectonic affiliation is equivocal, and connectional and immunocytochemical studies suggest that it may belong to the dorsal division. The medial division is one nucleus with many types of neurons, and it has coarse axons without laminar orientation. It is the smallest of the divisions and is present throughout the medial geniculate complex, except at the caudal tip and at the rostral pole. Many features of medial geniculate body organization evident in other mammals are recognized in the mustached bat. These include a prominent ventral division, some of whose neurons have a laminar organization, and a comparatively small medial division that is devoid of fibrodendritic laminae. Other features, such as the presence of a large rostral pole nucleus, whose homologue in other species is uncertain, or the sparse number or small cells that may participate in local circuits, set it apart from carnivores and primates and suggest that there are species specific patterns of medial geniculate body organization.

Response properties of single units in areas of rat auditory thalamus that project to the amygdala. I. Acoustic discharge patterns and frequency receptive fields

Projections from the auditory thalamus to the amygdala have been implicated in the processing of the emotional significance of auditory stimuli. In order to further our understanding of the contribution of thalamoamygdala projections to auditory emotional processing, acoustic response properties of single neurons were examined in the auditory thalamus of chloral hydrate-anesthetized rats. The emphasis was on the medial division of the medial geniculate body (MGm), the suprageniculate nucleus (SG), and the posterior intralaminar nucleus (PIN), thalamic areas that receive inputs from the inferior colliculus and project to the lateral nucleus of the amygdala (AL). For comparison, recordings were also made from the specific thalamocortical relay nucleus, the ventral division of the medial geniculate body (MGv). Responses latencies were not statistically different in MGv, MGm, PIN, and SG, but were longer in the posterior thalamic region (PO). Overall, frequency tuning functions were narrower in MGv than in the other areas but many cells in MGm were as narrowly tuned as cells in MGv. There was some organization of MGv, with low frequencies represented dorsally and high frequencies ventrally. A similar but considerably weaker organization was observed in MGm. While the full range of frequencies tested (1-30 kHz) was represented in MGv, cells in MGm, PIN, and SG tended to respond best to higher frequencies (16-30 kHz). Thresholds were higher in PIN than in MGv (other areas did not differ from MGv). Nevertheless, across the various areas, the breadth of tuning was inversely related to threshold, such that more narrowly tuned cells tended to have lower thresholds. Many of the response properties observed in MGm, PIN, and SG correspond with properties found in AL neurons and thus add support to the notion that auditory responses in AL reflect thalamoamygdala transmission.

Response properties of single units in areas of rat auditory thalamus that project to the amygdala. II. Cells receiving convergent auditory and somatosensory inputs and cells antidromically activated by amygdala stimulation

The purpose of this study was to further our understanding of the contribution of auditory thalamoamygdala projections to conditioned emotional memories formed when auditory and noxious somatosensory stimuli are associated. Single unit activity was recorded in the acoustic thalamus of chloral hydrate-anesthetized rats in response to auditory (white noise, clicks, tones) and somatosensory (foot-shock) stimulation. The thalamic areas focused on were the medial division of the medial geniculate body (MGm), the suprageniculate nucleus (SG), and the posterior intralaminar nucleus (PIN), thalamic areas that receive inputs from both the inferior colliculus and the spinal cord and that project to the lateral nucleus of the amygdala (AL). For comparison, recordings were also made from the specific thalamocortical relay nucleus, the ventral division of the medial geniculate body (MGv), which receives projections from the inferior colliculus but not from the spinal cord. Auditory but not somatosensory responses were recorded from MGv, while both auditory and somatosensory responses were frequently found in MGm, PIN, and SG. In these areas, convergent auditory and somatosensory responses were more frequently found rostrally than caudally. Within a thalamic subregion, the acoustic response properties of the convergence cells were not different from the response properties of unimodal auditory cells. Some cells that responded to somatosensory but not auditory stimuli showed a potentiated response when tested with simultaneous presentation of auditory and somatosensory stimuli. In some studies, thalamic cells that project to the amygdala were antidromically activated by stimulation of the AL. Consistent with anatomical tracing results, antidromically activated cells were found in MGm, PIN, and SG, but not in MGv. Antidromically activated cells were more likely to respond to auditory stimuli than to somatosensory stimuli, but unimodal somatosensory and convergence cells were also found. These findings, which provide the first characterization of acoustic response properties of multimodal cells in the auditory thalamus and of cells in the auditory thalamus that project to amygdala, suggest insights into the emotional functions of the thalamoamygdala pathway.

Somatovisceral projections from spinal cord and dorsal column nuclei to the thalamus in hedgehog tenrecs

In order first to overcome the difficulties in understanding the increasing amount of information available regarding the mammalian somatosensory thalamus, and then to correlate the findings among different species and integrate them into a general concept of thalamic organization, the present study investigated the spinothalamic and medial lemniscal projections in Madagascan hedgehog tenrecs (Echinops telfairi and Setifer setosus). Tracer substances were injected into the dorsal column nuclei and into spinal segments at various levels; additional injections were made into the inferior colliculus. The ascending somesthetic projections were to predominantly contralateral posterolateral target areas, and were almost mirror-like on both sides to intralaminar and medial thalamic nuclei. The densest and most extensive projections, originating mainly from the high cervical spinal cord and the dorsal column nuclei, reached the posterolateral thalamus caudal to the lateral geniculate nucleus. This region was difficult to subdivide cytoarchitecturally; nevertheless, on the basis of its labeling pattern, several subdivisions could be described and preliminary named. Some of them compared tentatively with the internal portion of the medial geniculate nucleus (GM) and the ventral posterior nuclear complex (VPC) in more differentiated mammals. The most prominent subdivision, however, located subjacent to the lateral surface of the brainstem, was shown to receive additional fibers from the inferior colliculus. This region might be considered a further subdivision of GM, VPC, a perigeniculate area, and/or a region of its own not comparable at present, with thalamic regions in other mammals. On the other hand, it may also be a remnant of the hypothetical, diffuse multimodal region from which GM and VPC have possibly evolved.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of somatic DNA recombination in the transgenic mouse brain

A DNA construct containing the bacterial beta-galactosidase gene (lacZ) was used to study somatic DNA recombination in the transgenic mouse brain. Recombination-positive areas of the adult brain were stained blue with X-gal, a substrate of beta-galactosidase. Blue-colored cells appeared soon after birth, and continued to emerge in postnatal tissue. Staining was prominent in sensory as opposed to motor regions of the brain, and was present in more than 70 discrete areas of the nervous system. The possibility of DNA rearrangement is discussed with respect to the development of the central nervous system.

The Organization of Projections from Subdivisions of the Auditory Cortex and Thalamus to the Auditory Sector of the Thalamic Reticular Nucleus in Galago

Anterograde and retrograde transport techniques were used to study the connexions between different subdivisions of the auditory cortex and thalamus with the thalamic reticular nucleus in the prosimian, Galago. In particular, the goal was to determine whether the primary auditory nucleus, GMv, and its cortical target, area I of the auditory cortex (A I), project to a different region of the auditory sector of the reticular nucleus from the secondary auditory nuclei, GMmc and Po and their cortical targets outside A I. The results show that the projections to and from the auditory sector are indeed segregated: injections of wheatgerm agglutinin-conjugated horseradish peroxidase into either GMmc or Po labelled cells and terminals along the medial, lateral and ventral borders of the auditory sector, forming a U-shaped pattern. Projections from area II of the auditory cortex produced almost an identical pattern of the terminal labelling in the auditory sector. In contrast, injections into GMv-labelled cells and terminals in the centre region of the auditory sector, in the 'interior' of the U-shaped region. Projections from A I were distributed to both the U-shaped border region and the central core of the auditory sector probably because A I received projections from GMmc, Po and GMv. The significance of these results depends on a comparison between the auditory and visual sectors of the reticular nucleus. Both sectors are divided into tiers or subsectors-one related to the primary relay nucleus, i.e. GLd or GMv, and the other related to the secondary relay nuclei, i.e. pulvinar nucleus, GMmc, Po, etc.

Acetylcholinesterase and NADH-diaphorase chemoarchitectonic subdivisions in the rabbit medial geniculate body

The distribution of acetylcholinesterase and NADH-diaphorase activities was studied histochemically in the rabbit medial geniculate body, yielding new data useful for the definition of the common structural pattern of this thalamic complex in mammals. Four chemoarchitectonic subdivisions could be detected in transversal, horizontal and sagittal sections that corresponded to the previously described ventral, dorsal and internal nuclei, and to a fourth subdivision, defined as the mediorostral nucleus of the medial geniculate complex in the rabbit. The topography and cellular typology of the mediorostral nucleus suggest its homology with the so-called magnocellular nucleus of other mammals, an identity that was previously assigned to the internal nucleus. The relative position of the rabbit internal and dorsal nuclei and comparative connectional data are combined to suggest their correspondence with the anterodorsal and posterodorsal subnuclei, respectively, of the cat and the monkey. Global functional interpretations of these nuclei as sites of visuoacoustic and somatoacoustic polymodal integration support the notion of a shell region of the medial geniculate, surrounding the principal cochleotopic ventral nucleus and interconnected to the cortical acoustic belt around the primary auditory area. Acetylcholinesterase and NADH-diaphorase chemoarchitectony may be useful for the detection of similar partitions in species where cytoarchitectonic differentiation of the medial geniculate is less clear.

Calcitonin gene-related peptide (CGRP) immunoreactive projections from the thalamus to the striatum and amygdala in the rat

The organization of calcitonin gene-related peptide-like immunoreactive (CGRPir) innervation of the amygdala and caudate-putamen in the rat was examined by using immunohistochemistry for CGRP combined with retrograde transport of the fluorescent dye fluoro-gold, as well as anterograde transport of Phaseoleus vulgaris leucoagglutinin (PHA-L). The lateral part of the central nucleus of the amygdala and the amygdalostriatal transition zone was densely innervated by CGRPir terminals at all anterior-posterior levels. More caudally, the lateral part of the caudate-putamen also had large numbers of CGRPir terminals. Injections of fluoro-gold into the amygdala and amygdalostriatal transition area followed by immunohistochemistry for CGRP revealed double-labeled neurons in the subparafascicular, lateral subparafascicular, and posterior intralaminar nuclei of the thalamus and peripeduncular nucleus. Injections into the caudate-putamen demonstrated double-labeled neurons in the more lateral parts of this same nuclear complex. PHA-L injections into the posterior thalamic nuclei from which the CGRPir projections arise confirmed the medial-to-lateral organization of the projections to the amygdala and striatum. The subparafascicular nucleus and the rostral portion of the lateral subparafascicular nucleus primarily projected to the medial amygdala and the amygdalostriatal transition area, while the more lateral cell groups, including the caudal part of the lateral parafascicular, posterior intralaminar, and peripeduncular nuclei projected to the lateral amygdala and the caudate-putamen. These CGRPir projections may be involved in mediating conditioned autonomic and behavioral responses to acoustic stimuli or somatosensory stimuli.

Calcitonin gene-related peptide immunoreactivity in the visceral sensory cortex, thalamus, and related pathways in the rat

It has been proposed that calcitonin gene-related peptide (CGRP) may serve as a major neuromodulator in visceral sensory pathways, but its exact role in the visceral sensory thalamus and cortex has not been determined. We therefore examined the distribution of CGRP-like immunoreactive (CGRPir) innervation of the insular cortex and the parvicellular division of the ventroposterior nucleus of the thalamus (VPpc) in the rat by using immunohistochemistry for CGRP combined with retrograde transport of the fluorescent dye fluoro-gold. Modest numbers of CGRPir fibers were distributed in the dysgranular and agranular insular cortex, but few were observed in the granular insular cortex. The density of CGRPir innervation increased caudally along the rhinal fissue and was considerably greater in the perirhinal cortex. When fluoro-gold was injected into the insular cortex numerous retrogradely labeled neurons were seen in the VPpc, but few of these were CGRPir. Retrogradely labeled CGRPir neurons were, however, seen in the ventral lateral and medial parabrachial (PB) subnuclei. Injection of fluoro-gold into the perirhinal cortex (which is just caudal to the insular cortex along the rhinal fissure) resulted in many retrogradely labeled CGRPir neurons in the posterior thalamic region, including the subparafascicular, the lateral subparafascicular, and the posterior intralaminar nuclei. The VPpc was heavily innervated by CGRPir fibers but contained few CGRPir cell bodies. Injection of fluoro-gold into the VPpc resulted in many retrogradely labeled CGRPir neurons in the external medial PB subnucleus bilaterally, but with a contralateral predominance. Smaller numbers of retrogradely labeled CGRPir neurons were also observed in the ventrolateral PB subnucleus, bilaterally with an ipsilateral predominance. These results suggest that CGRP may be a neuromodulator in the ascending visceral sensory pathways from the PB to the VPpc and the insular cortex, but not between the latter two structures.