Topographic organization of convergent projections to the thalamus from the inferior colliculus and spinal cord in the rat

Dissociation of tactile and acoustic components in air puff startle

Tactile (air puff) or acoustic startle stimuli elicit behavioral (motor) and complex cardiovascular responses which include pressor as well as cardiac decelerative and accelerative responses. An acoustic component of the air puff stimulus (12.5 psi) was identified. Studies were conducted to separate the contributions of both stimulus modalities to the observed responses. The acoustic component was approximated with a wide-spectrum 97-dB white-noise stimulus. This acoustic stimulus failed to evoke heart rate responses but did yield motor and pressor responses. In a second approach, tympanic membrane rupture (TMR) was used to interrupt acoustic sensory stimuli. TMR fully abolished the motor and pressor responses to acoustic startle. With air puff startle, while TMR severely attenuated the motor response it only decreased slightly the pressor and cardiac accelerative responses and failed to influence the cardiac decelerative component. Our results indicate that air puff startle contains both tactile and acoustic modalities. Further, the motor response is largely driven by the acoustic modality since TMR abolished this response elicited by either acoustic or tactile stimulation. More importantly, motor and cardiovascular responses to startle may be separated through discrimination of afferent stimuli suggesting either differences in neural pathways for acoustic and tactile stimuli or a differential dependency of the various responses on stimulus characteristics.

Primary auditory cortex in the rat: transient expression of acetylcholinesterase activity in developing geniculocortical projections

A characteristic pattern of acetylcholinesterase (AChE) activity is expressed transiently in primary auditory cortex (cortical area 41) of developing laboratory rats during early postnatal life. This AChE activity occurs as a dense plexus in cortical layer IV and the deep part of layer III. This transient band of AChE activity is first detected by histochemical techniques on postnatal day (P) 3, reaches peak intensity at approximately P8-10, and declines to form the adult pattern by P23. The ventral nucleus of the medial geniculate body of the thalamus also displays prominent, and transient, staining for AChE. This intense staining for AChE, found within neuronal somata and neuropil, is detected at the time of birth, reaches peak intensity around P8, and declines to adult levels by P16. The areal and laminar patterns of the transient band of AChE activity in temporal cortex correspond to the patterns of anterograde transneuronal labeling of geniculocortical terminals following injection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the inferior colliculus. Placement of lesions that include the medial geniculate nucleus or the geniculocortical axons results in a marked decrease in AChE staining in thalamorecipient layers of auditory cortex. Placement of lesions that include the medial globus pallidus reduce AChE staining of some axons in temporal cortex of developing rats, but the dense band of AChE in layers III and IV remains. Placement of lesions in the inferior colliculus in newborn animals results in marked decrease in AChE staining in cells of the ipsilateral ventral medial geniculate nucleus and in ipsilateral auditory cortex of developing pups. These data indicate that transiently expressed AChE activity is characteristic of geniculocortical neurons, including their somata in the medial geniculate body and their terminal axons in primary auditory cortex. This AChE activity is expressed early in postnatal development, probably during the time when thalamocortical axons are proliferating in cortical layer IV and forming synaptic contacts with cortical neurons.

Ventral temporal cortex in the rat: connections of secondary auditory areas Te2 and Te3

The present study in the rat deals with the hodological organization of two cytoarchitectonically distinct areas lying caudoventrally (Te2) or ventrally (Te3) to the primary auditory area (Te1). The afferent and efferent systems of connections were identified by using the properties of retrograde and anterograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP). Large tracer deposits in the ventral temporal cortex involving Te2, Te3, and the dorsal bank of the perirhinal cortex induced a dense retrograde and anterograde pattern of labeling in the following nuclei of the medial geniculate (MG) complex: caudodorsal (MGCD), dorsal (MGD), medial (MGM), suprageniculate (SG), and peripeduncular area (PPA). The ventral nucleus (MGV) was only slightly labeled in its caudal division. Several extrageniculate structures were also labeled. Retrograde cell labeling occurred in centers giving rise to ascending systems of diffuse projections: locus coeruleus (LC), dorsal raphe nucleus (DR), and basal magnocellular nucleus (B). Slight anterograde labeling was present in the dorsal and external cortices of the inferior colliculus (IC), central gray, deep layers of the superior colliculus (SC), reticular thalamic nucleus (RT), and caudate putamen (CPU). Callosal connections were also noted with the contralateral homotopic cortex. In the cases in which there was a notable extension of the zone of diffusion of the tracer into the dorsal bank of the perirhinal cortex, a characteristic pattern of labeling in the subparafascicular, reuniens and paraventricular thalamic nuclei, mammillary complex, lateral and dorsal hypothalamic nuclei, amygdaloid complex, laterodorsal tegmental nucleus, subiculum, and retrosplenial cortex was displayed. Tracer deposits restricted to Te2 induced a dense labeling of the caudal, ventrolateral MGD, lateral PPA and, to a lesser extent, MGCD. The MGM and SG were only slightly labeled. Extrageniculate afferents essentially consist of sparse projections from LC, DR, and B, whereas efferent fibers are directed to the dorsal cortex of the IC, central gray, deep SC layers, and CPU. Callosal connections were also identified. Following tracer deposits restricted to Te3, dense labeling occurred in the MGD, mostly in its medial division, in the caudal MGM, and in the PPA. The MGCD, SG, and MGV were only sparsely labeled. Extrageniculate afferents arise from LC, DR, and B, and efferents are directed to the RT and dorsal cortex of the IC. Contralateral connections with the homotopic cortical area were also noted. Te2 and Te3 share some degree of similitude in their pattern of connections with the MG complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Anatomy of the rat medial geniculate body: II. Dendritic morphology

The medial geniculate body (MGB) of the rat was studied with Golgi methods to determine the distribution of neurons identified by dendritic morphology. These findings were compared with major divisions and constituent nuclei established by somatic and fiber architectonics, and by connections with temporal neocortex (Clerici et al.: Society of Neuroscience Abstracts 12:1272, 1986; 13:325, 1987; Anatomical Record 218:23, 1987; Winer and Larue: Journal of Comparative Neurology 257:282-315, 1987; Clerici and Coleman: Journal of Comparative Neurology 297:14-31, 1990). It was found that an elaboration of the prototypical scheme proposed by Morest (Journal of Anatomy 98:611-630, 1964) for partitioning the mammalian MGB is valid for characterizing the rat MGB. Two predominant categories of principal neuron dendritic patterning were identified: a bushy cell having tufted dendritic fields and a stellate cell with a radiate dendritic domain. Tufted neurons have large caliber dendritic trunks that divide profusely into daughter branches close to the soma with intertwining higher order branches that maintain a relatively restricted dendritic field. Stellate neurons typically emit primary dendrites in all directions that then divide dichotomously at wide angles at subsequent orders of branching to produce a somewhat spheroidal dendritic field. In the present study, the rat MGB is found to be a tripartite structure composed of ventral (MGv), dorsal (MGd), and medial (MGm) divisions, each uniquely characterized by constituent dendritic morphology. The paramount neuronal class of the MGv is the tufted principal cell. In the ventral and ovoid nuclei of the MGv the neuronal orientation of highly oriented bitufted cells is in register with afferent brachial axons. In the ventral nucleus, this arrangement approximates vertical with a dorsomedial tilt most prominent rostrally; in the ovoid nucleus, tufted cells adhere to the double spiraled course of afferent axons. The transition zone between ventral and ovoid nuclei contains tufted neurons that align with radially oriented fibers issuing from the junction of the ovoid and midgeniculate bundles. Bitufted neurons of the marginal zone parallel fibers at the lateral margin of the geniculate. Within the MGd the dorsal and caudodorsal nuclei are characterized by stellate cells with extensive dendritic arbors and busy neurons with dendritic branches less tufted than those observed in the MGv. The deep dorsal nucleus contains bitufted neurons that polarize with the long axis of the midgeniculate bundle and intermingle with stellate neurons. The suprageniculate nucleus includes neurons with large somata and long, sparsely branched and dorsoventrally oriented dendrites orthagonal to corticothalamic axons, as well as smaller neurons and classical stellate cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Anatomy of the rat medial geniculate body: I. Cytoarchitecture, myeloarchitecture, and neocortical connectivity

The cytoarchitecture, myeloarchitecture, and neocortical connectivity of the rat medial geniculate body (MGB) were comprehensively studied in adult and immature rats to define major anatomical divisions and nuclei. The MGB is a highly intricate structure composed of the ventral (MGv), dorsal (MGd), and medial (MGm) divisions and component nuclei, each having reciprocal connections with auditory neocortex. The MGv lies inferior to the midgeniculate bundle and extends to the rostral, but not caudal MGB tip. The MGv is composed of ventral and ovoid nuclei bounded by a marginal zone, each region containing dark staining small and medium sized, densely packed neurons shown to have tufted dendritic morphology; in contrast to the MGd, but similar to the dorsal lateral geniculate nucleus, only the perikarya of MGv neurons stain for Nissl in early postnatal material. Ventral nucleus cells align with afferent brachial axons, which penetrate the nucleus in a dorsoventral direction, whereas rostrocaudal cellular arrays are retrogradely labeled after injections of horseradish peroxidase (HRP) into auditory cortex. The ovoid nucleus is a double spiraled structure encircled and penetrated by afferent fibers that determine the orientation of constituent perikarya. Neurons in the transition zone align with a spray of axons emanating from the juncture of the ovoid and midgeniculate bundles. Marginal zone neurons are oriented in parallel to the free geniculate wall. The MGd resides within and superior to the midgeniculate bundle, and is composed of several nuclei that stain palely for myelin. In immature material, both dendritic processes and somata in the MGd stain for Nissl with our protocol; many of these cells show a stellate arborization pattern that distinguishes this region from the MGv, but is similar to the staining pattern of immature neurons of the lateral posterior nucleus. The adult dorsal nucleus has medium-sized, loosely packed neurons. The deep dorsal nucleus is situated among the fibers of the midgeniculate bundle and contains loosely packed round and fusiform cells; the latter cell type constitutes a minor proportion of the adult neuronal population but the major cell type in immature animals. The caudodorsal nucleus, which occupies the caudal tip of the MGB and rostrally courses superior to the dorsal nucleus, contains small, dark staining multipolar cells; the ventrolateral nucleus courses inferior to the MGv. The suprageniculate and limitans nuclei are included in the auditory thalamus on the basis of connections with auditory neocortex; the former has medium to dark staining mixed-sized cells, and the latter has densely packed cells which form a vertical column.(ABSTRACT TRUNCATED AT 400 WORDS)

The cells of origin of the spinothalamic tract of the rat: a quantitative reexamination

We quantitatively reinvestigated the cells of origin of the spinothalamic tract (STT) of the rat. Injections of Fluoro-Gold that filled the thalamus on one side labeled large numbers of neurons throughout the length of the spinal cord. In 3 cases, we estimated the total number of STT neurons by counting labeled neurons in 18 of the 34 total segments, applying correction factors to these counts, and estimating the numbers of labeled neurons in the 16 remaining unexamined segments. The accuracy of these estimates was tested in two animals in which labeled neurons were counted in all 34 spinal segments. In both cases, the estimated totals of STT neurons differed from the counted totals by less than 5%. In the most effective case, we estimated that more than 9500 STT neurons were labeled. This study indicates that the number of STT neurons in rats is larger than previously reported and suggests that the STT may play an important role in nociception in rats, as it does in primates including humans.

Sensory properties and afferents of the N. dorsolateralis posterior thalami of the pigeon

According to previous studies, the avian n. dorsolateralis posterior thalami (DLP) receives visual and somatosensory afferents. While some authors (e.g., Gamlin and Cohen: J. Comp. Neurol. 250:296-310, '86) proposed a distinction between a visual caudal (DLPc) and a somatosensory rostral (DLPr) part, other authors (e.g., Wild: Brain Res. 412:205-223, '87) could not confirm such a differentiation. The aim of the present experiment was to study with physiological and anatomical methods the proposed parcellation of the DLP into various components dealing with different modalities. The physiological properties of the DLP of the pigeon were analysed with extracellular single unit recordings. With the same approach, neurons of the n. dorsalis intermedius ventralis anterior (DIVA), a somatosensory relay nucleus in the dorsal thalamus, were also analysed. The afferents of the DLP were studied by using anatomical tract tracing techniques with retrograde and anterograde tracers. The sensory properties of DLP cells revealed that somatosensory, visual, and auditory modalities affect the neuronal firing frequency in this nucleus. All three modalities were present throughout the full caudorostral extent of the DLP. Cells recorded in DIVA responded nearly exclusively to somatosensory stimulation. Unlike the DLP, single units in DIVA generally had smaller receptive fields encompassing only one extremity. The analysis of afferent connections of the DLP by using injections of retrograde and anterograde tracers (HRP, WGA-HRP, Fast Blue, and Rhodamine-beta-isothiocyanate) demonstrated extensive projections from the nuclei gracilis et cuneatus (GC) and more sparse projections from the nucleus tractus descendens trigemini (TTD), and the nucleus cuneatus externus (CE). Brainstem afferents of the DLP came from different vestibular nuclei, various areas of the brainstem reticular formation, and the optic tectum. Prosencephalic afferents originated in the n. posteroventralis thalami (PV), the n. ventromedialis posterior thalami (VMP), the n. dorsalis intermedius ventralis anterior (DIVA), and the nucleus reticularis superior pars dorsalis and ventralis (RSd and RSv). Telencephalic afferents of the DLP came from the hyperstriatum accessorium (HA) and a group of cells at the borderline between the hyperstriatum intercalatus superior (HIS) and the hyperstriatum dorsale (HD). The somatosensory afferents of the DLP probably originate from the GC, TTD, and CE, whereas it is likely that the visual input is mediated by the optic tectum. The anatomical source for the acoustic input is unclear. The very long latencies of auditory DLP neurons make it likely that the acoustic input originates at least partly in the reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Anatomical study of the connections of the primary auditory area in the rat

The aim of the present study was to identify in the rat the overall input-output pattern of connections of the primary auditory field, with special attention to the topographical organization of the geniculocortical auditory projection. By using cytoarchitectural criteria, three temporal cortical fields were distinguished in the rat: Te1, Te2, and Te3. The primary auditory field Te1 is characterized by a relatively specific differentiation of its layers when compared with other temporal fields. The afferent and efferent connections of Te1 were identified by using the retrograde and anterograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP). The results indicate that Te1 is connected by a dense and reciprocal system of fibers with the auditory thalamus. Based on the nomenclature of Morest ('64) in the cat, five cytoarchitectural subdivisions of the medial geniculate complex (MG) were identified in the rat: ventral (MGv), dorsal (MGd), medial (MGm), suprageniculate (Sg), and peripeduncular (PPA). The major rostrocaudal extent of the MGv is connected to Te1. The surrounding cortical fields Te2 and Te3 do not receive a projection from the MGv, except from its most caudal pole. The MGv projection is topographically organized. When the deposit area of the tracer is shifted from dorsal to ventral upon Te1, the corresponding labeled zone within the MGv moves from rostral to caudal, whereas a cortical displacement of the deposit area of the tracer from rostrodorsal to caudoventral leads to a medial to lateral shift of the labeled zone in the MGv. In addition, more dorsal parts of the MGv project on more dorsal sectors of Te1. Te1 receives a sparser, topographically organized projection from the deep dorsal subdivision of the MGd. The MGm and the lateral part of the posterior group of thalamic nuclei (Pol) also distribute fibers to the primary auditory field. Te1 is reciprocally connected by a system of callosal fibers with the contralateral homotypic cortex. Finally, Te1 sends fibers to the dorsal and, to a lesser extent, external cortices of the inferior colliculus, caudomedial caudate-putamen complex, and caudoventral thalamic reticular nucleus.

Development of the rat thalamus: V. The posterior lobule of the thalamic neuroepithelium and the time and site of origin and settling pattern of neurons of the medial geniculate body

Long-survival, sequential, and short-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to examine the time of origin, site of origin, migratory route, and settling pattern of neurons of the medial geniculate body (MG). Quantitative evaluation of long-survival radiograms established that the bulk of MG neurons are generated between embryonic (E) days E13 and E15, with a pronounced peak on day E14. There is an overall lateral-to-medial and caudal-to-rostral chronological gradient in MG neurogenesis. On the basis of significant regional differences in the birth dates of neurons, the MG was divided into several chronoarchitectonic areas. The earliest-generated neurons (with close to 20% of the cells produced on day E13 and a negligible proportion on day E15) form the dorsal and ventral clusters far laterally. Next in sequential order are the neurons of the lateral shell, intermediate shell, and medial shell of the MG. The medial shell with it latest-generated neurons (with over 30% produced rostrally on day E15) corresponds to the medial (magnocellular) subnucleus of the MG. There were no neurogenetic differences between the traditional dorsal and ventral divisions of the MG. Examination of sequential radiograms in rats labeled with 3H-thymidine on day E14 or E15 and killed on successive days brought supportive evidence for our earlier identification, in short-survival radiograms, of a posteroventral thalamic neuroepithelial evagination as the putative source, or committed cell line, of MG neurons. Wave fronts of apparently migrating unlabeled and labeled cells could be traced from this sublobule in a posterolateral direction to the future site of the MG.

Anatomy of the auditory thalamocortical system of the guinea pig

We investigated the projection from the medial geniculate body (MG) to the tonotopic fields (the anterior field A, the dorsocaudal field DC, the small field S) and to the nontonotopic ventrocaudal belt in the auditory cortex of the guinea pig. The auditory fields were first delimited in electrophysiological experiments with microelectrode mapping techniques. Then, small quantities of horseradish peroxidase (HRP) and/or fluorescent retrograde tracers were injected into the sites of interest, and the thalamus was checked for labeled cells. The anterior field A receives its main thalamic input from the ventral nucleus of the MG (MGv). The projection is topographically organized. Roughly, the caudal part of the MGv innervates the rostral part of field A and vice versa. After injection of tracer into low or medium best-frequency sites in A, we also found a topographic gradient along the isofrequency contours: the dorsal (ventral) part of a cortical isofrequency strip receives afferents from the rostral (caudal) portions of the corresponding thalamic isofrequency band. However, it is not so obvious whether such a gradient exists also in the high-frequency part of the projection. A second, weaker projection to field A originates in a magnocellular nucleus that is situated caudomedially in the MG and was therefore named the caudomedial nucleus. The dorsocaudal field DC receives input from the same nuclei as the anterior field, but the location of the labeled cells in the MGv is different. This was demonstrated by injection of different tracers into sites with like best frequencies in fields A and DC, respectively. After injection of HRP into the 1-2-kHz isofrequency strip in field A and injection of Nuclear Yellow (NY) into the 1-2-kHz site in field DC, the labeled cells in the MGv form one continuous array that runs from caudal to rostral over the whole extent of the MGv. The anterior part of this array consists of NY-labeled cells; i.e., it projects to field DC. The caudal part is formed by HRP-labeled cells; i.e., it innervates field A. These findings indicate that there is only one continuous tonotopic map in the MGv. This map is split when projected onto the cortex so that two adjacent tonotopic fields (A and DC) result. The cortical maps are rotated relative to the thalamic map in that rostral portions of the MGv project to caudal parts of the tonotopic cortex and vice versa.(ABSTRACT TRUNCATED AT 400 WORDS)

Auditory forebrain organization of an Australian marsupial, the northern native cat (Dasyurus hallucatus)

Structures and connections of auditory forebrain regions of the Northern native cat, a member of one of the most primitive families among Australian marsupials, have been examined anatomically by using anterograde and retrograde tracing techniques with wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) after defining the acoustically responsive neocortical area physiologically. The structure of the medial geniculate body (MG) was similar to that described in other species. The results obtained from a case with a WGA-HRP injection into the MG showed that the MG strongly projects to the lateral amygdaloid nucleus (LAmy) and the putamen as well as the auditory neocortex (ACx). Results obtained from other cases with WGA-HRP injections into the physiologically defined ACx show also that the ACx is connected not only with the ipsilateral MG and the contralateral ACx but also with the LAmy both bilaterally and reciprocally. The regions within the LAmy in which the MG-LAmy projection fibers terminate largely overlap with those in which the ACx-LAmy fibers terminate and the LAmy-ACx pathway originates. The connectional relationships revealed in the present study--that the LAmy receives auditory information from the MG and reciprocates auditory information with the ACx bilaterally--strongly suggest that, in some primitive mammals with small neocortical areas, a specific portion of noncortical telencephalon functions as an auditory center and occupies a relatively large volume of space in the forebrain. It is possible that the auditory sector of noncortical telencephalon in some primitive mammals such as the American Didelphidae and the Australian Dasyuridae is homologous with part of the auditory sector of the dorsal ventricular ridge (DVR) in reptiles and birds and also may have functions shared with the auditory primary and association neocortex in advanced mammals such as the domestic cat and the monkey (Kudo et al., '86a).

Topographic organization of the auditory thalamocortical system in the albino rat

The organization of the auditory thalamocortical connections was studied in rats. Retrograde transport of horseradish peroxidase conjugated to wheat germ agglutinin following injections into parietal, occipital and temporal cortex was used. The medial geniculate body, the suprageniculate, the lateral part of the nucleus posterior thalami, the posterior part of the nucleus lateralis thalami, and the nucleus ventroposterior project to the investigated part of the neocortex. Corresponding to different patterns of labeling, five areas of auditory neocortex were distinguished: 1. The rostral area is innervated by neurons of the nucleus ventroposterior, the lateral part of the nucleus posterior thalami, and the medial division of the medial geniculate body. 2. The dorsal area is innervated by neurons of the suprageniculate, the posterior part of the nucleus lateralis thalami and the rostral region of the dorsal division of the medial geniculate body. 3. The caudal area is innervated by neurons of the posterior part of the nucleus lateralis thalami, the suprageniculate, the medial division, the caudal region of the dorsal division and the ventrolateral nucleus of the medial geniculate body. 4. The ventral area is innervated by neurons of the suprageniculate, the medial division, the caudal region of the dorsal division, and the ventrolateral nucleus of the medial geniculate body. 5. The core area of the temporal cortex is exclusively connected to the caudal region of the medial division and the ventral division of the medial geniculate body. The findings of the present study indicate topographic organizations of the ventral division of the medial geniculate body and of the corea area. Four segments (a-d) of the ventral division each show a different set of topographic axes. They correspond to sets of topographic axes in the core area of the auditory cortex. These topographies characterize the segments which are each exclusively connected to one of the four fields of the core area.