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

Medial Auditory Thalamus Is Necessary for Expression of Auditory Trace Eyelid Conditioning

Transforming a brief sensory event into a persistent neural response represents a mechanism for linking temporally disparate stimuli together to support learning. The cerebellum requires this type of persistent input during trace conditioning to engage associative plasticity and acquire adaptively timed conditioned responses (CRs). An initial step toward identifying the sites and mechanisms generating and transmitting persistent signals to the cerebellum is to identify the input pathway. The medial auditory thalamic nuclei (MATN) are the necessary and sufficient source of auditory input to the cerebellum for delay conditioning in rodents and a possible input to forebrain sites generating persistent signals. Using pharmacological and computational approaches, we test (1) whether the necessity of MATN during auditory eyelid conditioning is conserved across species, (2) whether the MATN are necessary for the expression of trace eyelid CRs, and if so, (3)whether this relates to the generation of persistent signals. We find that contralateral inactivation of MATN with muscimol largely abolished trace and delay CRs in male rabbits. Residual CRs were decreased in amplitude, but CR timing was unaffected. Results from large-scale cerebellar simulations are consistent with previous experimental demonstrations that silencing only CS-duration inputs does not abolish trace CRs, and instead affects their timing. Together, these results suggest that the MATN are a necessary component of both the direct auditory stimulus pathway to the cerebellum and the pathway generating task-essential persistent signals.SIGNIFICANCE STATEMENT Persistent activity is required for working memory-dependent tasks, such as trace conditioning, and represents a mechanism by which sensory information can be used over time for learning and cognition. This neuronal response entails the transformation of a discrete sensory-evoked response into a signal that extends beyond the stimulus event. Understanding the generation and transmission of this stimulus transformation requires identifying the input sources necessary for task-essential persistent signals. We report that the medial auditory thalamic nuclei are required for the expression of auditory trace conditioning and suggest that these nuclei are a component of the pathway-generating persistent signals. Our study provides a foundation for testing circuit-level mechanisms underlying persistent activity in a cerebellar learning model with identified inputs and well defined behavioral outputs.

Organization of the Zone of Transition between the Pretectum and the Thalamus, with Emphasis on the Pretectothalamic Lamina

The zone of transition between the pretectum, derived from prosomere 1, and the thalamus, derived from prosomere 2, is structurally complex and its understanding has been hampered by cytoarchitectural and terminological confusion. Herein, using a battery of complementary morphological approaches, including cytoarchitecture, myeloarchitecture and the expression of molecular markers, we pinpoint the features or combination of features that best characterize each nucleus of the pretectothalamic transitional zone of the rat. Our results reveal useful morphological criteria to identify and delineate, with unprecedented precision, several [mostly auditory] nuclei of the posterior group of the thalamus, namely the pretectothalamic lamina (PTL; formerly known as the posterior limitans nucleus), the medial division of the medial geniculate body (MGBm), the suprageniculate nucleus (SG), and the ethmoid, posterior triangular and posterior nuclei of the thalamus. The PTL is a sparsely-celled and fiber rich flattened nucleus apposed to the lateral surface of the anterior pretectal nucleus (APT) that marks the border between the pretectum and the thalamus; this structure stains selectively with the Wisteria floribunda agglutinin (WFA), and is essentially immunonegative for the calcium binding protein parvalbumin (PV). The MGBm, located medial to the ventral division of the MGB (MGBv), can be unequivocally identified by the large size of many of its neurons, its dark immunostaining for PV, and its rather selective staining for WFA. The SG, which extends for a considerable caudorostral distance and deviates progressively from the MGB, is characterized by its peculiar cytoarchitecture, the paucity of myelinated fibers, and the conspicuous absence of staining for calretinin (CR); indeed, in many CR-stained sections, the SG stands out as a blank spot. Because most of these nuclei are small and show unique anatomical relationships, the information provided in this article will facilitate the interpretation of the results of experimental manipulations aimed at the auditory thalamus and improve the design of future investigations. Moreover, the previously neglected proximity between the MGBm and the caudal region of the scarcely known PTL raises the possibility that certain features or roles traditionally attributed to the MGBm may actually belong to the PTL.

Transformation of spatial sensitivity along the ascending auditory pathway

Locations of sounds are computed in the central auditory pathway based primarily on differences in sound level and timing at the two ears. In rats, the results of that computation appear in the primary auditory cortex (A1) as exclusively contralateral hemifield spatial sensitivity, with strong responses to sounds contralateral to the recording site, sharp cutoffs across the midline, and weak, sound-level-tolerant responses to ipsilateral sounds. We surveyed the auditory pathway in anesthetized rats to identify the brain level(s) at which level-tolerant spatial sensitivity arises. Noise-burst stimuli were varied in horizontal sound location and in sound level. Neurons in the central nucleus of the inferior colliculus (ICc) displayed contralateral tuning at low sound levels, but tuning was degraded at successively higher sound levels. In contrast, neurons in the nucleus of the brachium of the inferior colliculus (BIN) showed sharp, level-tolerant spatial sensitivity. The ventral division of the medial geniculate body (MGBv) contained two discrete neural populations, one showing broad sensitivity like the ICc and one showing sharp sensitivity like A1. Dorsal, medial, and shell regions of the MGB showed fairly sharp spatial sensitivity, likely reflecting inputs from A1 and/or the BIN. The results demonstrate two parallel brainstem pathways for spatial hearing. The tectal pathway, in which sharp, level-tolerant spatial sensitivity arises between ICc and BIN, projects to the superior colliculus and could support reflexive orientation to sounds. The lemniscal pathway, in which such sensitivity arises between ICc and the MGBv, projects to the forebrain to support perception of sound location.

Parvalbumin and calbindin expression in parallel thalamocortical pathways in a gleaning bat, Antrozous pallidus

The pallid bat (Antrozous pallidus) listens to prey-generated noise to localize and hunt terrestrial prey while reserving echolocation to avoid obstacles. The thalamocortical connections in the pallid bat are organized as parallel pathways that may serve echolocation and prey localization behaviors. Thalamic inputs to the cortical echolocation call- and noise-selective regions originate primarily in the suprageniculate nucleus (SG) and ventral division of medial geniculate body (MGBv), respectively. Here we examined the distribution of parvalbumin (PV) and calbindin (CB) expression in cortical regions and thalamic nuclei of these pathways. Electrophysiology was used to identify cortical regions selective for echolocation calls and noise. Immunohistochemistry was used to stain for PV and CB in the auditory cortex and MGB. A higher percentage (relative to Nissl-stained cells) of PV(+) cells compared with CB(+) cells was found in both echolocation call- and noise-selective regions. This was due to differences in cortical layers V-VI, but not layers I-IV. In the MGB, CB(+) cells were present across all divisions of the MGB, with a higher percentage in the MGBv than the SG. Perhaps the most surprising result was the virtual absence of PV staining in the MGBv. PV staining was present only in the SG. Even in the SG, the staining was mostly diffuse in the neuropil. These data support the notion that calcium binding proteins are differentially distributed in different processing streams. Our comparative data, however, do not support a general mammalian pattern of PV/CB staining that distinguishes lemniscal and nonlemniscal pathways.

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.

Thalamocortical mechanisms for integrating musical tone and rhythm

Studies over several decades have identified many of the neuronal substrates of music perception by pursuing pitch and rhythm perception separately. Here, we address the question of how these mechanisms interact, starting with the observation that the peripheral pathways of the so-called "Core" and "Matrix" thalamocortical system provide the anatomical bases for tone and rhythm channels. We then examine the hypothesis that these specialized inputs integrate acoustic content within rhythm context in auditory cortex using classical types of "driving" and "modulatory" mechanisms. This hypothesis provides a framework for deriving testable predictions about the early stages of music processing. Furthermore, because thalamocortical circuits are shared by speech and music processing, such a model provides concrete implications for how music experience contributes to the development of robust speech encoding mechanisms.

Correlation of neural response properties with auditory thalamus subdivisions in the awake marmoset

As the information bottleneck of nearly all auditory input that reaches the cortex, the auditory thalamus serves as the basis for establishing auditory cortical processing streams. The functional organization of the primary and nonprimary subdivisions of the auditory thalamus is not well characterized, particularly in awake primates. We have recorded from neurons in the auditory thalamus of awake marmoset monkeys and tested their responses to tones, band-pass noise, and temporally modulated stimuli. We analyzed the spectral and temporal response properties of recorded neurons and correlated those properties with their locations in the auditory thalamus, thereby forming the basis for parallel output channels. Three medial geniculate body (MGB) subdivisions were identified and studied physiologically and anatomically, although other medial subdivisions were also identified anatomically. Neurons in the ventral subdivision (MGV) were sharply tuned for frequency, preferred narrowband stimuli, and were able to synchronize to rapid temporal modulations. Anterodorsal subdivision (MGAD) neurons appeared well suited for temporal processing, responding similarly to tone or noise stimuli but able to synchronize to the highest modulation frequencies and with the highest temporal precision among MGB subdivisions. Posterodorsal subdivision (MGPD) neurons differed substantially from the other two subdivisions, with many neurons preferring broadband stimuli and signaling changes in modulation frequency with nonsynchronized changes in firing rate. Most neurons in all subdivisions responded to increases in tone sound level with nonmonotonic changes in firing rate. MGV and MGAD neurons exhibited responses consistent with provision of thalamocortical input to core regions, whereas MGPD neurons were consistent with provision of input to belt regions.

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.

Core-and-belt organisation of the mesencephalic and forebrain auditory centres in turtles: expression of calcium-binding proteins and metabolic activity

The distribution of immunoreactivity to the calcium-binding proteins parvalbumin, calbindin and calretinin and of cytochrome oxidase activity was studied in the mesencephalic (torus semicircularis), thalamic (nucleus reuniens) and telencephalic (ventromedial part of the anterior dorsal ventricular ridge) auditory centres of two chelonian species Emys orbicularis and Testudo horsfieldi. In the torus semicircularis, the central nucleus (core) showed intense parvalbumin immunoreactivity and high cytochrome oxidase activity, whereas the laminar nucleus (belt) showed low cytochrome oxidase activity and dense calbindin/calretinin immunoreactivity. Within the central nucleus, the central and peripheral areas could be distinguished by a higher density of parvalbumin immunoreactivity and cytochrome oxidase activity in the core than in the peripheral area. In the nucleus reuniens, the dorsal and ventromedial (core) regions showed high cytochrome oxidase activity and immunoreactivity to all three calcium-binding proteins, while its ventrolateral part (belt) was weakly immunoreactive and showed lower cytochrome oxidase activity. In the telencephalic auditory centre, on the other hand, no particular region differed in either immunoreactivity or cytochrome oxidase activity. Our findings provide additional arguments in favour of the hypothesis of a core-and-belt organisation of the auditory sensory centres in non-mammalian amniotes though this organisation is less evident in higher order centres. The data are discussed in terms of the evolution of the auditory system in amniotes.

Evolutionary significance of delayed neurogenesis in the core versus shell auditory areas of Mus musculus

Early comparative embryogenesis can reflect the organization and evolutionary origins of brain areas. Neurogenesis in the auditory areas of sauropsids displays a clear core-to-shell distinction, but it remains unclear in mammals. To address this issue, [3H]-thymidine was injected into pregnant mice on consecutive embryonic (E) days (E10-E19) to date neuronal birthdays. Immunohistochemistry for substance P, calbindin, and parvalbumin was conducted to distinguish the core and shell auditory regions. The results showed that: 1) cell generation began at E13 in the external or dorsal nucleus of the inferior colliculus (IC), but it did not start in the caudomedial portion of the central nucleus of IC, and significantly fewer cells were produced in the medial and rostromedial portions of the central nucleus of IC; 2) cells were generated at E11 in the dorsal and medial divisions of the medial geniculate complex (MGd and MGm, respectively), whereas cell generation was absent in the medial and rostromedial portions of the ventral medial geniculate complex (MGv), and fewer cells were produced in the caudomedial portion of MGv; 3) in the telencephalic auditory cortex, cells were produced at E11 or E12 in layer I and the subplate, which receive projections from the MGd and MGm. However, cell generation occurred at E13-E18 in layers II-VI, including the area receiving projections from the MGv. The core-to-shell distinction of neurogenesis is thus present in the mesencephalic to telencephalic auditory areas in the mouse. This distinction of neurogenesis is discussed from an evolutionary perspective.

Different distributions of calbindin and calretinin immunostaining across the medial and dorsal divisions of the mouse medial geniculate body

We studied the distributions of calretinin and calbindin immunoreactivity in subdivisions of the mouse medial geniculate body and the adjacent paralaminar nuclei. We found that the vast majority of labeled cells in the dorsal division of the medial geniculate body were immunoreactive for calbindin-only, whereas most of the remaining labeled cells were double-labeled. Very few calretinin+ only cells were observed. By contrast, we observed significant proportions of calbindin+ only, calretinin+ only and double-labeled cells in the medial division of the medial geniculate body. Further, the distributions of calbindin-only, calretinin-only and double-labeled cells did not differ between the medial division of the medial geniculate body, the suprageniculate nucleus, the peripeduncular nucleus and the posterior intralaminar nucleus. We found essentially no somatic staining for either calbindin or calretinin in the ventral division of the medial geniculate body. These data suggest that there are distinct neurochemical differences between the two non-lemniscal auditory thalamic nuclei. In addition, these data extend previous observations that the medial division of the medial geniculate body shares many properties with the paralaminar group of nuclei.

An architectonic study of the neocortex of the short-tailed opossum (Monodelphis domestica)

Short-tailed opossums (Monodelphis domestica) belong to the branch of marsupial mammals that diverged from eutherian mammals approximately 180 million years ago. They are small in size, lack a marsupial pouch, and may have retained more morphological characteristics of early marsupial neocortex than most other marsupials. In the present study, we used several different histochemical and immunochemical procedures to reveal the architectonic characteristics of cortical areas in short-tailed opossums. Subdivisions of cortex were identified in brain sections cut in the coronal, sagittal, horizontal or tangential planes and processed for a calcium-binding protein, parvalbumin (PV), neurofilament protein epitopes recognized by SMI-32, the vesicle glutamate transporter 2 (VGluT2), myelin, cytochrome oxidase (CO), and Nissl substance. These different procedures revealed similar boundaries among areas, suggesting that functionally relevant borders were detected. The results allowed a fuller description and more precise demarcation of previously identified sensory areas, and the delineation of additional subdivisions of cortex. Area 17 (V1) was especially prominent, with a densely populated layer 4, high myelination levels, and dark staining of PV and VGluT2 immunopositive terminations. These architectonic features were present, albeit less pronounced, in somatosensory and auditory cortex. The major findings support the conclusion that short-tailed opossums have fewer cortical areas and their neocortex is less distinctly laminated than most other mammals.

Thalamic connections of architectonic subdivisions of temporal cortex in grey squirrels (Sciurus carolinensis)

The temporal cortex of grey squirrels contains three architectonically distinct regions. One of these regions, the temporal anterior (Ta) region has been identified in previous physiological and anatomical studies as containing several areas that are largely auditory in function. Consistent with this evidence, Ta has architectonic features that are internally somewhat variable, but overall sensory in nature. In contrast, the caudally adjoining temporal intermediate region (Ti) has architectonic features that suggest higher order and possibly multisensory processing. Finally, the most caudal region, composed of previously defined temporal medial (Tm) and temporal posterior (Tp) fields, again has more of the appearance of sensory cortex. To understand their functional roles better, we injected anatomical tracers into these regions to reveal their thalamic connections. As expected, the dorsal portion of Ta, containing two primary or primary-like auditory areas, received inputs from the ventral and magnocellular divisions of the auditory medial geniculate complex (MGv and MGm). The most caudal region, Tm plus Tp, received inputs from the large visual pulvinar of squirrels, possibly accounting for the sensory architectonic characteristics of this region. However, Tp additionally receives inputs from the magnocellular (MGm) and dorsal (MGd) divisions of the medial geniculate complex, implicating Tp in multisensory processing. Finally, the middle region, Ti, had auditory inputs from MGd and MGm, but not from the visual pulvinar, providing evidence that Ti has higher order auditory functions. The results indicate that the architectonically distinct regions of temporal cortex of squirrels are also functionally distinct. Understanding how temporal cortex is functionally organized in squirrels can guide interpretations of temporal cortex organization in other rodents in which architectonic subdivisions are not as obvious.

Comparative analysis of neurogenesis between the core and shell regions of auditory areas in the chick (Gallus gallus domesticus)

Early embryogenesis can reflect constituting organizations and evolutionary origins of brain areas. To determine whether a clear core-versus-shell distinction of neurogenesis that occurs from the auditory midbrain to the telencephalon in the reptile also appears in the bird, a single dose of [(3)H]-thymidine was injected into chick (Gallus gallus domesticus) eggs at some successive embryonic days (E) (from E3 to E10). Towards the end of hatching, [(3)H]-thymidine labeling was examined, and the results were as follows: 1) Neuronal generation in the nucleus intercollicularis (ICo) (shell region) began at E3, whereas neurogenesis began at E4 in the nucleus mesencephalicus lateralis pars dorsalis (MLd) (core region); 2) Neurogenesis initiated at E3 in the nucleus ovoidalis (Ov) shell, but initiated at E4 in the rostral Ov core. In the medial or caudal Ov core, the percentage of heavily-labeled neurons with [(3)H]-thymidine was significantly lower at E3 age group than that in the Ov shell; 3) In field L1 and L3, two flanking regions of the primary telencephalic auditory area (field L2a), neurogenesis started at E5, but started at E6 in field L2a. These data indicate that the onset of embryogenesis began earlier in the auditory shell areas than in the core areas from the midbrain to the telencephalon. These findings provide insight into the organization of auditory nuclei and their evolution in amniotes.

Evidence for nonreciprocal organization of the mouse auditory thalamocortical-corticothalamic projection systems

We tested the hypothesis that information is routed from one area of the auditory cortex (AC) to another via the dorsal division of the medial geniculate body (MGBd) by analyzing the degree of reciprocal connectivity between the auditory thalamus and cortex. Biotinylated dextran amine injected into the primary AC (AI) or anterior auditory field (AAF) of mice produced large, "driver-type" terminals primarily in the MGBd, with essentially no such terminals in the ventral MGB (MGBv). In contrast, small, "modulator-type" terminals were found primarily in the MGBv, and this coincided with areas of retrogradely labeled thalamocortical cell bodies. After MGBv injections, anterograde label was observed in layers 4 and 6 of the AI and AAF, which coincided with retrogradely labeled layer 6 cell bodies. After MGBd injections, thalamocortical terminals were seen in layers 1, 4, and 6 of the secondary AC and dorsoposterior AC, which coincided with labeled layer 6 cell bodies. Notably, after MGBd injection, a substantial number of layer 5 cells were labeled in all AC areas, whereas very few were seen after MGBv injection. Further, the degree of anterograde label in layer 4 of cortical columns containing labeled layer 6 cell bodies was greater than in columns containing labeled layer 5 cell bodies. These data suggest that auditory layer 5 corticothalamic projections are targeted to the MGBd in a nonreciprocal fashion and that the MGBd may route this information to the nonprimary AC.

Neurogenic development of the auditory areas of the midbrain and diencephalon in the Xenopus laevis and evolutionary implications

To study whether the core-versus-shell pattern of neurogenesis occurred in the mesencephalic and diencephalic auditory areas of amniotes also appears in the amphibian, [(3)H]-thymidine was injected into tadpoles at serial developmental stages of Xenopus laevis. Towards the end of metamorphism, [(3)H]-thymidine labeling was examined and led to two main observations: 1) neuron generation in the principal nucleus (Tp) started at stage 50, and peaked at stage 53, whereas it began at stage 48.5, and peaked around stage 49 in the other two mesencephalic auditory areas, the laminar nucleus (Tl) and the magnocellular nucleus (Tmc). 2) Neuron generation appeared at stage 40, and peaked around stage 52 in the posterior thalamic nucleus (P) and the central thalamic nucleus (C). Our study revealed that, like the cores of mesencephalic auditory nuclei in amniotes, Tp showed differences from Tl and Tmc in the onset and the peak of neurogenesis. However, such differences did not occur in the P and C. Our neurogenetic data were consistent with anatomical and physiological reports indicating a clear distinction between the mesencephalic, but not the diencephalic auditory areas of the amphibian. Our data are helpful to get insights into the organization of auditory nuclei and its evolution in vertebrates.

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.

Connections of functional areas in the mustached bat's auditory cortex with the auditory thalamus

The auditory thalamus is the major target of the inferior colliculus and connects in turn with the auditory cortex. In the mustached bat, biosonar information is represented according to frequency in the central nucleus of the inferior colliculus (ICc) but according to response type in the cortex. In addition, the cortex has multiple areas with neurons of similar response type compared to the single tonotopic representation in the ICc. To investigate whether these transformations occur at the level of the thalamus, we injected anatomical tracers into physiologically defined locations in the mustached bat's auditory cortex. Injections in areas used for target ranging labeled contiguous regions of the auditory thalamus rather than separate patches corresponding to regions that respond to the different harmonic frequencies used for ranging. Injections in the two largest ranging areas produced labeling in separate locations. These results indicate that the thalamus is organized according to response type rather than frequency and that multiple mappings of response types exist. Injections in areas used for target detection labeled thalamic regions that were largely separate from those that interconnect with ranging areas. However, injections in an area used for determining target velocity overlapped with the areas connected to ranging areas and areas involved in target detection. Thus, separation by functional type and multiplication of areas with similar response type occurs by the thalamic level, but connections with the cortex segregate the functional types more completely than occurs in the thalamus.

Immunoreactivity for calretinin and calbindin in the vestibular nuclear complex of the monkey

Immunoreactivity to calcium-binding proteins has been a useful extension to cytoarchitectonics in defining the organization of many central nervous system regions. Previously we found subdivisions of the cat medial vestibular nucleus (MVe) defined by immunoreactivity to the calcium-binding proteins, calretinin and calbindin. Here we report similar subdivisions in both the squirrel and the macaque monkey. Calretinin immunoreactivity reveals a small area of cells and processes located dorsally in the MVe. In the anterior-posterior direction these cells extend over less than half of the nucleus. This area is not distinct in Nissl-stained sections. Elsewhere in the vestibular nuclear complex (VNC) and in the nucleus prepositus hypoglossi (PrH) there are scattered labeled cells. Immunoreactivity for calbindin shows a small patch of dense fiber label at the border of MVe and PrH, and a patchy distribution in the rest of the VNC that changes at different anterior-posterior levels. There are also calbindin-labeled cells in the underlying reticular formation over a very restricted anterior-posterior extent in both squirrel and macaque monkey. The dendrites of some of these cells can be followed into PrH, and data from other studies suggests that they may contribute to vestibular-oculomotor function. Scattered cells in the VNC are densely outlined by calbindin-labeled terminals, suggesting a major drive from the calbindin-labeled fiber input. These findings, along with observations from rodents and cats, suggest that there are subdivisions of the MVe defined by calcium-binding proteins that are homologous across rodents, cats, and New World and Old World monkeys.

Immunoreactivity for calcium-binding proteins defines subregions of the vestibular nuclear complex of the cat

The vestibular nuclear complex (VNC) is classically divided into four nuclei on the basis of cytoarchitectonics. However, anatomical data on the distribution of afferents to the VNC and the distribution of cells of origin of different efferent pathways suggest a more complex internal organization. Immunoreactivity for calcium-binding proteins has proven useful in many areas of the brain for revealing structure not visible with cell, fiber or Golgi stains. We have looked at the VNC of the cat using immunoreactivity for the calcium-binding proteins calbindin, calretinin and parvalbumin. Immunoreactivity for calretinin revealed a small, intensely stained region of cell bodies and processes just beneath the fourth ventricle in the medial vestibular nucleus. A presumably homologous region has been described in rodents. The calretinin-immunoreactive cells in this region were also immunoreactive for choline acetyltransferase. Evidence from other studies suggests that the calretinin region contributes to pathways involved in eye movement modulation but not generation. There were focal dense regions of fibers immunoreactive to calbindin in the medial and inferior nuclei, with an especially dense region of label at the border of the medial nucleus and the nucleus prepositus hypoglossi. There is anatomical evidence that suggests that the likely source of these calbindin-immunoreactive fibers is the flocculus of the cerebellum. The distribution of calbindin-immunoreactive fibers in the lateral and superior nuclei was much more uniform. Immunoreactivity to parvalbumin was widespread in fibers distributed throughout the VNC. The results suggest that neurochemical techniques may help to reveal the internal complexity in VNC organization.

Effects of conditioning during amygdalar inactivation on training-induced neuronal plasticity in the medial geniculate nucleus and cingulate cortex in rabbits (Oryctolagus cuniculus)

This study addressed the amygdala's role in avoidance conditioning in rabbits (Oryctolagus cuniculus). Intra-amygdalar muscimol infusion before 60 or 120 conditioning trials blocked training-induced neuronal activity (TIA) in the medial geniculate (MG) nucleus. One hundred twenty trials with muscimol blocked TIA permanently, during conditioning with muscimol and then later without muscimol; 60 trials with muscimol blocked TIA only when muscimol was present. Cingulate cortical TIA was blocked only when muscimol was present. Behavioral learning did not occur with muscimol, but later learning was facilitated (i.e., savings occurred) in rabbits initially given muscimol plus training. These results define the time period wherein amygdalar processes initiate TIA in the MG nucleus and suggest that distinct forms of amygdalar processes induce TIA in the MG nucleus and cingulate cortex.

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.

Parvalbumin-like immunostaining in the cat inferior colliculus. Light and electron microscopic investigation

The presence of the calcium-binding protein parvalbumin (PV) was studied in neuronal elements of the cat's inferior colliculus (IC) by means of light and electron microscopic immunocytochemistry. Immunostaining of PV was detected in all three main parts of the IC. Several subtypes of large neurons that differed in size and shape were immunostained, comprising approx. 15% of the total number of PV-containing neurons. Approx. half of the labeled neurons were medium sized. Two types of small neurons were found to be PV synthesizing, and comprised approx. 35% of the total PV-containing population. Ultrastructurally, many dendrites were heavily immunolabeled, and the reaction product was present in dendritic spines as well. Several types of synaptic boutons contained reaction product, and terminated on both labeled and unlabeled postsynaptic targets forming asymmetric and symmetric synapses. Approx. 70% of all PV-immunolabeled terminals contained round synaptic vesicles and formed asymmetric synapses. The majority of these boutons were of the "large round" type and corresponded to the terminals of cochlear nuclei. A lower number were of the "small round" type, and were probably corticotectal terminals. The remaining 30% of PV-containing terminals contained pleomorphic or elongated vesicles and formed symmetric synapses. These terminals corresponded with "P" and "F1" bouton types. Part of these boutons appeared to arise from nuclei of the lateral lemniscus and the superior olive, and a certain percentage likely represented endings of inhibitory interneurons.

Auditory thalamic nuclei projections to the temporal cortex in the rat

Thalamocortical projections from the auditory thalamic nuclei were examined systematically in the rat, including those from the dorsal division (MGD) of the medial geniculate body (MG), which were less clearly determined in previous studies. Injections of biocytin confined in each thalamic nucleus revealed characteristic features of projections in terms of cortical areas and layers of termination. In contrast to exclusively selective projections to cortical area Te1 from the ventral division (MGV) of the MG, diffuse and selective terminations were observed in the projections from the dorsal (MGD) and medial divisions (MGM) of the MG and the suprageniculate nucleus (SG). Diffuse termination was continuous in layer I or VI of the temporal cortex, while selective termination was in layers III and IV of discrete cortical areas. In addition to diffuse termination in the upper half of layer I of cortical areas Te1, Te2d and Te3v, the MGD and SG projections formed plexuses of axons selectively in lower layer III and layer IV of Te2d and Te3v. The SG projections targeted further the dorsal bank of the perirhinal cortex (PRh), while the MGD projections targeted in part the ventral fringe of Te1. The MGM projections terminated diffusely in layer VI of Te1 and Te3v, and selectively in lower layer III and layer IV of the rostral part of Te3v. Diffuse projections to layers I and VI from the SG and MGM extended in cortical regions over the dorsal fringe of Te1. Selective dense projections to middle cortical layers of Te2d and Te3v (especially its rostral part) indicate the existence of auditory areas, which could be involved in cross-modal interaction with visual and somatosensory system, respectively. Diffuse projections are supposed to bind information processings in these areas and the primary auditory area (Te1).

Functional subregions in primary auditory cortex defined by thalamocortical terminal arbors: an electrophysiological and anterograde labeling study

Several functional maps have been described in primary auditory cortex, including those related to frequency, tuning, latency, binaurality, and intensity. Many of these maps are arranged in a discontinuous or patchy manner. Similarly, thalamocortical projections arising from the ventral division of the medial geniculate body to the primary auditory cortex are also patchy. We used anterograde labeling and electrophysiological methods to examine the relationship between thalamocortical patches and auditory cortical maps. Biotinylated dextran-amine was deposited into physiologically characterized sites in the ventral division of the medial geniculate body of New Zealand white rabbits. Approximately 7 d later, the animal was again anesthetized and the ipsilateral auditory cortex was mapped with tungsten microelectrodes. Multi-unit physiological data were obtained for the following characteristics: best frequency (BF), binaurality, response type, latency, sharpness of tuning, and threshold. Immunocytochemical methods were used to reveal the injection site in the ventral division of the medial geniculate body as well as the anterogradely labeled thalamocortical afferents in the auditory cortex. In 86% of the cases (12 of 14), entry into a thalamocortical patch was associated with a marked change in physiological responses. A consistent BF and binaural class were usually observed within a patch. The patches appear to innervate distinct functional regions coding frequency and binaurality. A model is presented showing how patchy thalamocortical projections participate in the formation of tonotopic and binaural maps in primary auditory cortex.

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.

Parvalbumin and calbindin are differentially distributed within primary and secondary subregions of the mouse auditory forebrain

The calcium binding proteins parvalbumin and calbindin are thought to differentially regulate physiological functions and often show complementary distributions in the CNS. Our goal was to determine parvalbumin and calbindin distributions in the different subdivisions of mouse auditory thalamus and auditory cortex. Following fixation, FVB mouse brains (postnatal days 38-80) were sectioned along coronal and horizontal planes, then processed for parvalbumin and calbindin immunohistochemistry (antibodies: parvalbumin pa-235, calbindin-d-28k cl-300). Strong complementary differences in calcium binding protein distributions were found in mouse auditory thalamus. The ventral division of the medial geniculate, which is the principal relay to primary auditory cortex, exhibited dense parvalbumin but weak calbindin immunoreactivity. In contrast, most of the 'secondary' auditory thalamic regions surrounding the ventral division showed strong calbindin and lighter parvalbumin levels. Thus, the mouse auditory thalamus is composed of a parvalbumin positive 'core' surrounded by a calbindin positive 'shell'. Parvalbumin immunoreactivity was also more prominent in the primary auditory cortex than in the secondary belt auditory cortex. Calbindin immunoreactivity in auditory cortex was less clearly divided along primary/secondary lines, especially in supragranular layers. However, within infragranular layers, there was heavier staining in belt areas than in primary auditory cortex. In auditory thalamus, parvalbumin labeling was largely confined to the neuropil, whereas calbindin labeling involved somata and neuropil. In auditory cortex, somata and neuropil were positive for both proteins.In summary, the calcium binding proteins parvalbumin and calbindin were found to be differentially distributed within the primary and non-primary regions of mouse auditory forebrain. These differences in protein distribution may contribute to the distinct types of physiological responses that occur in the primary vs. non-primary areas.

The total number of neurons and calcium binding protein positive neurons during aging in the cochlear nucleus of CBA/CaJ mice: a quantitative study

The quantitative stereological method, the optical fractionator, was used for determining the total number of neurons and the total number of neurons immunostained with parvalbumin, calbindin-D28k (calbindin), and calretinin in the dorsal and posteroventral cochlear nucleus (DCN and PVCN) in CBA/CaJ (CBA) mice during aging (1-39 months old). CBA mice have only a modest sensorineural pathology late in life. An age-related decrease of the total number of neurons was demonstrated in the DCN (r=-0.54, P<0.03), while the total number of neurons in the PVCN did not show any significant age-related differences (r=0.16, P=0.57). In the DCN 5.5% of neurons were parvalbumin positive in the very old (30-39 months) mice, vs. 2.2% in the 1 month old mice. In the DCN 3% of the neurons were calbindin immunopositive in the 30-39 months mice compared to 1.9% in the 1 month old group. In the PVCN, 20% of the neurons in the very old mice were parvalbumin immunopositive, compared to 12% in the young mice. Calbindin did not show any significant age-related differences in the PVCN. The total number of calretinin immunopositive neurons both in the DCN and PVCN did not show any significant change with increasing age. In conclusion, the total neuronal number in the DCN and PVCN was age-related and region-specific. While the neuronal number in the DCN and PVCN was decreased or unchanged, respectively, the calcium binding protein positive neuronal number showed a graded increase during aging in a region-specific and protein-specific manner.

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.

Amygdalar efferents initiate auditory thalamic discriminative training-induced neuronal activity

It is well known that neurons of the medial geniculate (MG) nucleus of the thalamus send axonal projections to the amygdala. It has been proposed that these projections supply information that supports amygdalar associative processes underlying acquisition of acoustically cued conditioning and learning. Here we demonstrate the reverse direction of influence. Temporary inactivation of the amygdala using the GABA(A) receptor agonist muscimol just before the onset of discriminative avoidance conditioning permanently blocked the development of training-induced discriminative neuronal activity in the MG nucleus of rabbits. No discriminative activity developed when the amygdala was inactivated or during later training to criterion without muscimol. Thus, amygdalar processing at the outset of training is necessary for the development of training-induced discriminative activity of neurons in the MG nucleus.

Functional organization of auditory cortex in the Mongolian gerbil (Meriones unguiculatus). IV. Connections with anatomically characterized subcortical structures

The subcortical connections of the four tonotopically organized fields of the auditory cortex of the Mongolian gerbil, namely the primary (AI), the anterior (AAF), the dorsoposterior (DP) and the ventroposterior field (VP), were studied predominantly by anterograde transport of biocytin injected into these fields. In order to allow the localization of connections with respect to subdivisions of subcortical auditory structures, their cyto-, fibre- and chemoarchitecture was characterized using staining methods for cell bodies, myelin and the calcium-binding protein parvalbumin. Each injected auditory cortical field has substantial and reciprocal connections with each of the three subdivision of the medial geniculate body (MGB), namely the ventral (MGv), dorsal (MGd) and medial division (MGm). However, the relative strengths of these connections vary: AI is predominantly connected with MGv, AAF with MGm and MGv, and DP and VP with MGd and MGv. The connections of at least AI and MGv are topographic: injections into caudal low-frequency AI label laterorostral portions of MGv, whereas injections into rostral high-frequency AI label mediocaudal portions of MGv. All investigated auditory fields send axons to the suprageniculate, posterior limitans, laterodorsal and lateral posterior thalamic nuclei, with strongest projections from DP and VP, as well as to the reticular and subgeniculate thalamic nuclei. AI, AAF, DP and VP project to all three subdivisions of the inferior colliculus, namely the dorsal cortex, external cortex and central nucleus ipsilaterally and to the dorsal and external cortex contralaterally. They also project to the deep and intermediate layers of the ipsilateral superior colliculus, with strongest projections from DP and VP to the lateral and basolateral amygdaloid nuclei, the caudate putamen, globus pallidus and the pontine nuclei. In addition, AAF and particularly DP and VP project to paralemniscal regions around the dorsal nucleus of the lateral lemniscus (DNLL), to the DNLL itself and to the rostroventral aspect of the superior olivary complex. Moreover, DP and VP send axons to the dorsal lateral geniculate nucleus. The differences with respect to the existence and/or relative strengths of subcortical connections of the examined auditory cortical fields suggest a somewhat different function of each of these fields in auditory processing.

Distribution of parvalbumin immunoreactivity in the brain of the tench (Tinca tinca L., 1758)

The distribution of parvalbumin (PV) immunoreactivity in the tench brain was examined by using the avidin-biotin-peroxidase immunocytochemical method. This protein was detected in neuronal populations throughout all main divisions of the tench brain. In the telencephalic hemispheres, PV-immunopositive neurons were distributed in both the dorsal and ventral areas, being more abundant in the area ventralis telencephali, nucleus ventralis. In the diencephalon, the scarce distribution of PV-containing cells followed a rostrocaudal gradient, and the most evident staining was observed in the nucleus periventricularis tuberculi posterioris and in a few nuclei of the area praetectalis. In the mesencephalon, abundant PV-immunoreactive elements were found in the tectum opticum, torus semicircularis, and tegmentum. In the tectum opticum, PV-immunoreactivity presented a laminar distribution. Three PV-containing neuronal populations were described in the torus semicircularis, whereas in the tegmentum, the PV staining was mainly located in the nucleus tegmentalis rostralis and in the nucleus nervi oculomotorii. In the metencephalon, Purkinje cells were PV-immunopositive in the valvula cerebelli, lobus caudalis cerebelli, and in the corpus cerebelli. In the myelencephalon, PV immunoreactivity was abundant in the nucleus lateralis valvulae, in the nucleus nervi trochlearis, nucleus nervi trigemini, nucleus nervi abducentis, nucleus nervi glossopharyngei, and in the formatio reticularis. Mauthner cells were also PV immunostained. By contrast to other vertebrate groups, only a restricted population of PV-containing neurons was GABA-immunoreactive in the tench, demonstrating that this calcium-binding protein cannot be considered a marker for GABAergic elements in the teleost brain. This study demonstrates a low phylogenetic conservation of the distribution of PV comparing teleosts and tetrapods.

Thalamocortical afferents of Lorente de Nó: medial geniculate axons that project to primary auditory cortex have collateral branches to layer I

The injection of anterograde tracers into the ventral division of the medial geniculate body (MGV) of both rats and rabbits labels terminal axons in layer I of auditory cortex as well as the more conventional terminal arbors in layers III/IV. Whether these layer I projections represent a separate lemniscal pathway to the molecular layer or arise as collaterals of axons terminating in III/IV has not been addressed. Focal injections of the anterograde tracers biocytin or biotinylated dextran amine were made into the MGV of young rabbits. Serial section reconstruction of single MGV axons in auditory cortex revealed that layer I axons were collaterals of thalamocortical afferents that formed multiple divergent patches within III/IV. MGV collaterals to layer I often coursed tangentially for several millimeters before terminating. In some cases, the layer I collaterals descended to arborize within a thalamocortical patch in layers III/IV. These results suggest considerable radial and tangential divergence in the auditory thalamocortical pathway and argue for an expanded role for layer I in the processing of specific sensory stimuli.

Do auditory responses recorded from awake animals reflect the anatomical parcellation of the auditory thalamus?

Previous studies performed in anesthetized animals have shown differences between the acoustic responses of neurons recorded from the different divisions of the medial geniculate body (MGB). This study aimed at determining whether or not such differences are also expressed when neurons are recorded from awake animals. The auditory responses of 130 neurons of the auditory thalamus were determined in awake, restrained guinea pigs while the state of vigilance of the animals was continuously monitored. There were significantly more 'on' phasic evoked responses and significantly fewer 'non-responsive' or 'labile' cells in the ventral division of the MGB (MGv) than in the other divisions. The response latencies and the variability of the latencies were smaller in the MGv than in the other divisions. The tuning of the neurons obtained from MGv and from the lateral part of the posterior complex were significantly sharper than those coming from the dorsal division of the MGB and the medial division. The mean threshold and the percentage of monotonic vs. non-monotonic intensity functions were not different in the subdivisions of the auditory thalamus. When compared with previous studies, the quantifications of the acoustic responses obtained in the present study gave values that differed from those reported under deep anesthesia, but were close to those reported under light anesthesia. Lastly, even if none of the physiological characteristic makes it possible, by itself, to determine the locus of recordings in the auditory thalamus, we conclude that the physiological characteristics of the evoked responses obtained in MGv differ from those of other divisions.

Thalamocortical connections of the parabelt auditory cortex in macaque monkeys

The auditory cortex of macaque monkeys contains a core of primary-like areas surrounded by a narrow belt of associated fields that encompass much of the superior temporal plane in these animals. Adjacent to the lateral belt on the superior temporal gyrus is a parabelt region that contains at least two subdivisions (rostral and caudal). In a previous study (Hackett et al. [1998] J. Comp. Neurol. 394:475-495), we determined that the parabelt has topographic connections with the belt areas surrounding the core, but minimal connections with the core itself. In this study, we describe the thalamocortical connections of the parabelt auditory cortex based on multiple injections of neuronal tracers into this region in each of five macaque monkeys. Injections confined to the parabelt labeled large numbers of neurons in the dorsal (MGd) and magnocellular (MGm) divisions of the medial geniculate complex (MGC), suprageniculate (Sg), limitans (Lim), and medial pulvinar (PM) nuclei. Only when injections encroached on the lateral belt cortex were substantial numbers of labeled neurons found in the ventral (MGv) division of the MGC, consistent with the absence of significant connections between the parabelt and core fields. The rostrocaudal topography of the parabelt region was maintained in the thalamocortical connections, supporting the parcellation of this region of cortex. The results suggest that the parabelt region represents a third level of auditory cortical processing, which is not influenced by direct inputs from primary cortical or subcortical auditory structures.

Viewpoint: the core and matrix of thalamic organization

The integration of the whole cerebral cortex and thalamus during forebrain activities that underlie different states of consciousness, requires pathways for the dispersion of thalamic activity across many cortical areas. Past theories have relied on the intralaminar nuclei as the sources of diffuse thalamocortical projections that could facilitate spread of activity across the cortex. A case is made for the presence of a matrix of superficially-projecting cells, not confined to the intralaminar nuclei but extending throughout the whole thalamus. These cells are distinguished by immunoreactivity for the calcium-binding protein, D28K calbindin, are found in all thalamic nuclei of primates and have increased numbers in some nuclei. They project to superficial layers of the cerebral cortex over relatively wide areas, unconstrained by architectonic boundaries. They generally receive subcortical inputs that lack the topographic order and physiological precision of the principal sensory pathways. Superimposed upon the matrix in certain nuclei only, is a core of cells distinguished by immunoreactivity for another calcium-binding protein, parvalbumin, These project in highly ordered fashion to middle layers of the cortex in an area-specific manner. They are innervated by subcortical inputs that are topographically precise and have readily identifiable physiological properties. The parvalbumin cells form the basis for sensory and other inputs that are to be used as a basis for perception. The calbindin cells, especially when recruited by corticothalamic connections, can form a basis for the engagement of multiple cortical areas and thalamic nuclei that is essential for the binding of multiple aspects of sensory experience into a single framework of consciousness.

Compartmentalization of calbindin and parvalbumin in different parts of rat rubrospinal neurons

The distribution of calbindin-immunoreactive neurons in the red nucleus and the subcellular distribution of the calbindin and parvalbumin in tracer-identified rubrospinal neurons of the rat were studied. Only a fraction of the retrogradely labelled rubrospinal neurons was found to contain calbindin. These neurons filled the caudal part of the red nucleus and also appeared sporadically along the ventromedial border of the middle segment of the red nucleus. In addition to the somata, calbindin was found in the dendritic arbors of tracer-identified rubrospinal neurons, revealed by injecting the fluorescent dye Lucifer Yellow into their cell bodies. The axons of rubrospinal neurons located in the caudal red nucleus were marked by performing anterograde tracing with fluorescent dextran tracer in freshly prepared brainstem slices. Parvalbumin was found to locate in swellings along these tracer-identified axons as well as at their cut ends. The results indicate that calbindin and parvalbumin are segregated to the somadendritic and axonal compartments of the rat rubrospinal neurons, respectively. This anatomical segregation suggests that they may have different functions in neurons.

Differential distribution of calcium-binding proteins in the dorsal column nuclei of rats: a combined immunohistochemical and retrograde tract tracing study

This study aimed to investigate whether different calcium-binding proteins are present in morphologically and functionally separate cell groups in the dorsal column nuclei of rats. Thalamic-projecting neurons were identified by iontophoretic injection of an intraaxonal tracer substance, choleragenoid, into the ventroposterolateral thalamic nucleus, which was localized by extracellular recordings of the responses to natural peripheral stimulation. The presence of the calcium-binding proteins calbindin and paravalbumin in the projection neurons was detected by a double-labelling immunofluorescent method. The vast majority of the thalamic-projecting neurons contained paravalbumin, but not all parvalbumin-immunoreactive cells were retrogradely labelled. Calbindin-immunoreactive neurons were also found in the dorsal column nuclei, but only a small minority of these neurons projected to the thalamus. These findings are generally consistent with the notion that the different calcium-binding proteins represent functionally separate neuronal populations. Taken together with previous observations that parvalbumin is present in large dorsal root ganglion cells, which project to the dorsal column nuclei, and in the thalamocortical relay cells that receive dorsal column nuclear input, the present findings suggest that parvalbumin is associated with neurons that transmit modality-specific low-threshold mechanoreceptive information from the periphery to the somatosensory cortex. However, the presence of parvalbumin-immunoreactive cells that appeared not to project to the thalamus, as well as the occurrence of thalamic-projecting calbindin-immunoreactive neurons, indicate that parvalbumin and calbindin are present within several, functionally different, groups of neurons in the dorsal column nuclei.