Trigeminal and dorsal column nuclei projections to the anterior pretectal nucleus in the rat

The projections of the trigeminal (V) sensory nuclei (VSN) and the dorsal column nuclei (DCN) to the anterior pretectal nucleus (APT) of the rat were investigated by the use of anterograde and retrograde transport of wheat-germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Injections of WGA-HRP into the APT retrogradely labeled neurons in the contralateral VSN and DCN. The labeled neurons in the VSN were most concentrated in the rostral V subnucleus interpolaris (Vi), but were also found in caudal V subnucleus oralis (Vo). No labeled neurons were seen in V subnucleus caudalis. In the DCN, retrogradely labeled neurons were observed in rostral portions of both the cuneate (Cu) and gracile (Gr) nuclei. Injections of WGA-HRP into the rostral Vi or caudal Vo resulted in dense anterograde terminal labeling in the ventral two-thirds of the APT; the labeling was maximal in the ventromedial part of the caudal half of the APT and did not extend into its most rostral portion. Labeling resulting from injections of tracer into Cu or Gr was located primarily in the ventral half of the APT, was maximal in the mid-levels of the nucleus and extended into its rostral portions. These results indicate the existence of prominent somatosensory projections to the APT and are consistent with recent findings suggesting a role for the APT in sensorimotor integration.     

Synaptic relationships between corticocuneate terminals and glycine-immunoreactive neurons in the rat cuneate nucleus

The present study describes the ultrastructural synaptic relationships between corticocuneate terminals (CCTs) and glycine-immunoreactive (glycine-IR) neurons in the cuneate nucleus of rats using anterograde tract-tracing of wheatgerm agglutinin conjugated with horseradish peroxidase (WGA–HRP) and anti-glycine immunoperoxidase labeling methods. The HRP-labeled CCTs made axodendritic synapses preferentially in the ventral part of the cuneate nucleus near the obex. In a total of 182 CCTs surveyed, 14 of them made direct synaptic contacts with immunoperoxidase-labeled glycine-IR dendrites. The present results suggest that cortical modulation on the sensory transmission of cuneate nucleus may be mediated through glycine-IR neurons.     

Direct vagal input to neurons in the area postrema which project to the parabrachial nucleus: An electron microscopic-HRP study in the cat

This study in cat examines the synaptic relationship of vagal afferents to parabrachial projecting neurons in the area postrema (AP) using anterograde and retrograde transport of horseradish peroxidase (HRP). Wheat germ agglutinin-HRP injected into the parabrachial nucleus (PBN) produced retrograde neuronal labeling in the AP and in the nucleus of the tractus solitarius bilaterally, but with an ipsilateral predominance. Labeled neurons were confined mainly to the caudal 2/3's of the AP. Following injection of WGA-HRP into the PBN and HRP into the nodose ganglion in the same animal, examination of sections of the AP with the electron microscope revealed anterogradely labeled axon terminals in apposition to retrogradely labeled somata and dendrites. In some instances, labeled terminals were observed to form synaptic contacts with retrogradely labeled neurons. We conclude that in the cat a vagal input to neurons in the AP is monosynaptically relayed to the PBN.     

Localization of preganglionic neurons of the accessory ciliary ganglion in the midbrain: HRP and WGA-HRP studies in the cat

Localization of preganglionic neurons of the accessory ciliary ganglion (ACG), including ectopic intraocular ganglion cells, was investigated in the cat with the aid of horseradish peroxidase (HRP) and HRP-conjugated wheat germ agglutinin (WGA-HRP) methods. When HRP or WGA-HRP was injected into the anterior and posterior chambers of the eye, no retrogradely labeled cells were found in the visceral oculomotor nuclei, although most neurons of the ACG and the main ciliary ganglion (CG) were intensely labeled. When a microsyringe needle was inserted into the ciliary body, the tracer diffused into the suprachoroid lamina and the intraocular ganglion cells, and a small number of labeled neurons appeared in the midplane between each side of the somatic oculomotor nuclei. After injection into the ACG, many labeled neurons were observed in the anteromedian nucleus, Edinger-Westphal nucleus, and midplane between the somatic oculomotor nuclei, their ventral continuations of the ventral tegmental area, and the periaqueductal gray. HRP /WGA-HRP injection into the CG labeled cells in all these areas and in the lateral border zones of the anteromedian, Edinger-Westphal and somatic oculomotor nuclei, and their ventral continuations of the ventral tegmental area. These findings indicate that the visceral oculomotor neurons which project to the ACG tend to be located more medially than those to the CG. © Wiley-Liss, Inc.     

Organization of the projections of a circumventricular organ: the area postrema in the rat

The projections of the rat area postrema were analysed using anterograde and retrograde axonal transport techniques. Discrete injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the area postrema produced anterograde labeling in specific medullary and pontine nuclei. In the medulla, anterograde labeling was present in the internal solitary zone and dorsal division of the medial solitary nucleus, both of which also contained a small number of retrogradely labeled perikarya. Prominent projections to the dorsal motor nucleus of the vagus were seen only if the WGA-HRP injections in the area postrema invaded dorsal solitary nuclei. In the pons, anterograde labeling was present in the parabrachial nuclei, the dorsolateral tegmental nucleus, and the pericentral division of the dorsal tegmental nucleus. By far the major pontine projection was to the dorsolateral region of the middle one-third of the rostrocaudal extent of the parabrachial nuclei. Retrograde fluorescent tracing studies indicated that most area postrema neurons take part in this parabrachial projection. The area postrema projection to the parabrachial nuclei was bilaterally distributed, whereas that from the dorsal solitary nuclei was primarily ipsilateral. The external solitary zone, immediately subadjacent to the area postrema, neither received area postrema projections nor participated in the projections to the parabrachial nuclei. Fluorescent retrograde double labeling studies confirmed the bilateral nature of the area postrema projection to the parabrachial nuclei. In addition, because no doubly labeled neurons were observed it appears that individual area postrema neurons project to either side but not both sides of the dorsal pons. Thus, numerous neuronal pathways exist for the transfer of blood-borne information (that cannot cross the blood-brain barrier) from the area postrema to other brain regions.     

Projections of thalamic gustatory and lingual areas in the monkey, Macaca fascicularis

The efferent projections of the parvicellular division of the ventroposteromedial nucleus of the thalamus (VMPpc; thalamic taste area) were traced to cortex in Macaca fascicularis by using tritiated amino acid autoradiography. Labeled fascicles could be traced from VPMpc to two discrete regions of cortex. The primary efferent projection was located on ipsilateral insular-opercular cortex adjacent to the superior limiting sulcus and extended as far rostrally as the posterior lateral orbitofrontal cortex. An additional projection was located within primary somatosensory (SI) cortex subjacent to the anterior subcentral sulcus. Following autoradiographic injections in VPM, the trigeminal somatosensory relay, a dense terminal plexus was labeled on SI cortex of both pre- and postcentral gyri, but not within insular-opercular cortex. The autoradiographic data were verified by injecting each cortical projection area with horseradish peroxidase (HRP) and observing the pattern of retrogradely labeled somata within the thalamus. Injections in the precentral gyrus near the anterior subcentral sulcus retrogradely labeled neurons within VPMpc, whereas injections further caudally near the floor of the central sulcus labeled neurons within VPM. Injections of HRP within opercular, insular, or posterior lateral orbitofrontal cortex retrogradely labeled neurons within VPMpc.     

Topography and synaptology of mamillary body projections to the mesencephalon and pons in the rat.

The anterograde and retrograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) was used to study the anatomical organization of descending projections from the mamillary body (MB) to the mesencephalon and pons at light and electron microscopic levels. Injections of WGA-HRP into the medial mamillary nucleus resulted in dense anterograde and retrograde labeling in the ventral tegmental nucleus, while injections in the lateral mamillary nucleus resulted in dense anterograde labeling in the dorsal tegmental nucleus pars dorsalis and dense anterograde and retrograde labeling in the pars ventralis of the dorsal tegmental nucleus. Anterogradely labeled fibers in the mamillotegmental tract diverged from the principal mamillary tract in an extensive dorsocaudally oriented swath of axons which extended to the dorsal and ventral tegmental nuclei, and numerous axons turned sharply ventrally and rostrally to terminate topographically in the dorsomedial nucleus reticularis tegmenti pontis and rostromedial pontine nuclei. The anterograde labeling in these two precerebellar relay nuclei was distributed near the midline such that projections from the lateral mamillary nucleus terminated mainly dorsomedial to the terminal fields of projections from the medial mamillary nucleus. In the dorsal and ventral tegmental nuclei, labeled axon terminals contained round synaptic vesicles and formed asymmetric synaptic junctions primarily with small diameter dendrites and to a lesser extent with neuronal somata. A few labeled terminals contained pleomorphic vesicles and formed symmetric synaptic junctions with dendrites and neuronal somata. Labeled axon terminals were also frequently found in synaptic contact with retrogradely labeled dendrites and neuronal somata in the dorsal and ventral tegmental nuclei. These findings indicate that neurons in the dorsal and ventral tegmental nuclei are reciprocally connected with MB projection neurons. In the nucleus reticularis tegmenti pontis and medial pontine nuclei, labeled axon terminals contained round synaptic vesicles and formed asymmetric synaptic junctions primarily with small diameter dendrites. The present study demonstrates that projections from the medial and lateral nuclei of the MB are topographically organized in the mesencephalon and pons. The synaptic morphology of mamillotegmental projections suggests that they may have excitatory influences primarily on the distal dendrites of neurons in these brain regions.     

Topography of the corticofugal projection to the lateral reticular nucleus in the monkey

The cortical projection to the lateral reticular nucleus (LRN) was explored in monkeys prepared for autoradiography and horseradish peroxidase (HRP) histochemistry. An unambiguous projection was revealed only in cases with injections of the precentral forelimb and hindlimb areas. The forelimb area projection occupied centromedial segments, the hindlimb area projection occupied ventrolateral segments of the LRN with very little overlap. Some sparse labeling was also seen with injections of the supplementary motor area (SMA), but only when the lectin-bound tracer HRP was injected and not when autoradiography was used. Retrogradely labeled cortical cells occupied a larger cortical area in one case with injection of free HRP into the LRN. Since the additional expanse of cortex, however, was not examined in anterograde cases, and since the injected marker substance had diffused to neighboring structures, the significance of the labeled cells outside the precentral motor cortex is questionable. There was no evidence for a projection from the precentral face area with either anterograde tracing method. The corticoreticular projection was bilateral and only slightly more marked contralateral to the injection. The labeling was largely confined to the magnocellular division with minor amounts in the parvicellular division (especially in the hindlimb cases). The subtrigeminal portion was spared in all cases. It is concluded that the LRN constitutes another somatotopically organized precerebellar nucleus relaying signals from the motor cortex to the cerebellum. Compared with the corticopontocerebellar pathway in monkeys, however, the LRN is only a minor component of the corticocerebellar transmission system.     

Electron microscopic study of the rubrocerebellar projection in the cat

Rubral neurons sending axons to the cerebellar anterior interpositus nucleus (AIN) in the cat were identified light microscopically by labeling them with horseradish peroxidase (HRP). The synaptic organization of these rubral neurons and of their afferents from the cerebral motor cortex and the AIN was also analyzed electron microscopically by combined anterograde degeneration and retrograde HRP-labeling techniques. In the light microscopic study, either HRP or a mixture of HRP and kainic acid was injected into the AIN. Both of the injections resulted in retrograde labeling of rubrocerebellar projection neurons in the red nucleus on the contralateral side. The labeled neurons were distributed throughout the rostrocaudal extent of the red nucleus: some lay in clusters. Most labeled neurons were small to medium-sized, although some were large. The injection of HRP into the AIN also resulted in anterograde labeling of cerebellorubral projection fibers terminating in a wider area of the red nucleus on the contralateral side of the injection, whereas the injection of a mixture of HRP and kainic acid showed no anterograde labeling of fibers or terminals. In one set of electron microscopic observations, HRP injections into the AIN were combined with ablation of the motor cortex. Degenerating axon terminals were occasionally found to synapse with both dendrites and neuronal somata labeled with HRP retrogradely. In another set of electron microscopic observations, a mixture of HRP and kainic acid was injected into the AIN in order to label rubrocerebellar projection neurons retrogradely and to bring about degeneration in the cerebellorubral projection fibers anterogradely. Abundant degenerating axon terminals were observed to make axosomatic synaptic contacts with rubral neurons labeled with HRP retrogradely and also with unlabeled rubral neurons. These results indicate that cerebrorubrocerebellar and rubrocerebellorubral monosynaptic circuitries exist which constitute one of the cerebrocerebellar linkages, as well as those linkages via the inferior olivary complex and the pontine nuclei.     

Colocalization of fixative-modified glutamate and glutaminase but not GAD in rubrospinal neurons

In an attempt to identify putative neurotransmitters of rubrospinal neurons, immunocytochemical procedures were utilized in combination with retrograde tracing techniques in 15 adult male rats. Following injections of horseradish peroxidase (HRP) or wheat germ agglutinin conjugated to HRP (WGA-HRP) into the spinal cord, midbrain sections were processed with a combined procedure that allowed visualization of both the retrograde tracer and one or more antigens including glutamate, glutaminase, and glutamatic acid decarboxylase (GAD). Initial colocalization studies demonstrated that glutamatelike and glutaminaselike immunoreactivities were cocontained within the same neurons. Following injections of HRP or WGA-HRP into the spinal cord approximately 53% of retrogradely labeled neurons contained glutamate immunoreactivity. Triple-labeling experiments indicated that glutamatelike immunoreactivity was colocalized with glutaminase immunoreactivity in retrogradely labeled rubrospinal neurons. Retrogradely labeled neurons did not contain GAD immunoreactivity. Moreover, triple labeling experiments verified that glutamatelike immunoreactive retrogradely labeled cells did not cocontain GAD immunoreactivity. These studies demonstrate that glutamate and its synthesizing enzyme, glutaminase, are present in some rubrospinal neurons and raise the possibility that a component of the rubrospinal projection may be glutamatergic. GAD, on the other hand, is not present in rubrospinal neurons. This finding supports the hypothesis that GABAergic neurons play a role as interneurons in the red nucleus.     

A catecholaminergic projection from the ventral tegmental area to the diagonal band of broca: Modulation by neurotensin

The ventral tegmental area contains a high density of dopaminergic perikarya having ascending projections to a number of limbic forebrain regions. In this study, we use combined retrograde labeling with horseradish peroxidase (HRP) and immunohistochemical staining for tyrosine hydroxylase to examine the catecholaminergic projection from the ventral tegmental area to the diagonal band of Broca. When injection of HRP was restricted to the diagonal band, only neurons in the nucleus linearis, nucleus interfascicularis and ventromedial portion of the nucleus paranigralis were labeled. In contrast, HRP injection into the adjacent nucleus accumbens labeled neurons throughout these nuclei, plus the nucleus parabrachialis pigmentosus, nucleus retroruber and substantia nigra, pars compacta. Approximately 60% of neurons in the ventral tegmental area labeled from the diagonal band contained tyrosine hydroxylase, compared with 79% of the neurons labeled from the nucleus accumbens. Neurotensin is a tridecapeptide found in the ventral tegmental area which has been shown to activate dopamine neurons projecting to the nucleus accumbens. In this study, microinjection of neurotensin into ventral tegmental nuclei which contained neurons retrogradely labeled from the diagonal band significantly elevated the levels of dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, in the diagonal band. The results of this study demonstrate that a catecholaminergic projection exists from the ventral tegmental area to the diagonal band of Broca, and that this pathway can be stimulated by intra-ventral tegmental injection with neurotensin.     

The cerebral and cerebellar connections of pretecto-thalamic and pretecto-olivary neurons in the anterior pretectal nucleus of the cat

The neuronal connections of the anterior pretectal nucleus (PTA) were investigated in the cat. For the light microscopy, the retrograde double-labeling technique by means of Fluoro-Gold (FG) and horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) was used. Following injections of one tracer into the central lateral nucleus of the thalamus (CL) and the other into the dorsal accessory olivary nucleus (DAO), distributions of labeled neurons in the PTA were observed. Most of the labeled neurons were single-labeled either with FG or with WGA-HRP. The result indicated that pretecto-thalamic projection neurons were distributed throughout the whole extent of the PTA, whereas pretecto-olivary projection neurons were located in a restricted area of the ventral part of the PTA. Only a very small number of double-labeled neurons were found in the PTA. These two efferent projections thus seemed to be derived from different populations of PTA neurons. For the electron microscopy, a combination of retrograde transport of horseradish peroxidase (HRP) and anterograde degeneration technique was used. After HRP injections into the CL combined with lesions either in the motor cortex (MCx) or in the anterior interpositus nucleus of the cerebellum (Cbl), some degenerating axon terminals originating from the cerebrum or cerebellum were found to synapse with retrogradely labeled pretecto-thalamic projection neurons. We have already observed direct cerebral and cerebellar projections to the pretecto-olivary projection neurons (J. Comp. Neurol., 259 (1987) 348-363). We conclude that the two different populations of PTA neurons comprise two different kinds of neuronal circuitries, i.e. MCx-PTA-CL-MCx and Cbl-PTA-DAO-Cbl, and that these two circuitries might interrelate with each other in the PTA at the cellular level.     

Demonstration of a pallidostriatal pathway by retrograde transport of HRP-labeled lectin

A projection from the globus pallidus (GP) to the striatum in the rat was demonstrated by a retrograde tracer technique. After injections of lectin-conjugated horseradish peroxidase (WGA-HRP) which were wholly confined to the striatum, the GP exhibited reaction product characteristic of both retrograde and anterograde transport of marker. Electron microscopic examination of the GP confirmed the presence of both perikaryal and terminal labeling. Coinjection of kainic acid and WGA-HRP into the striatum blocked the appearance of anterograde label in the GP but the retrogradely labeled pallidal neurons remained visible.     

Cholinergic projections from the laterodorsal and pedunculopontine tegmental nuclei to the pontine gigantocellular tegmental field in the cat

This study demonstrates that the laterodorsal tegmental nucleus (LDT) and pedunculopontine tegmental nucleus (PPT) are sources of cholinergic projections to the cat pontine reticular formation gigantocellular tegmental field (PFTG). Neurons of the LDT and PPT were double-labeled utilizing choline acetyltransferase immunohistochemistry combined with retrograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP). In the LDT the percentage of cholinergic neurons retrogradely labeled from PFTG was 10.2% ipsilaterally and 3.7% contralaterally, while in the PPT the percentages were 5.2% ipsilaterally and 1.3% contralaterally. These projections from the LDT and PPT to the PFTG were confirmed and their course delineated with anterograde labeling utilizing Phaseolus vulgaris leucoagglutinin (PHA-L) anterograde transport.     

Efferent connections of the caudate nucleus, including cortical projections of the striatum and other basal ganglia: an autoradiographic and horseradish peroxidase investigation in the cat

Tritiated tracer was injected into the head of the caudate nucleus in cats. Following such injections, labeling is present within extensive regions of both the globus pallidus and entopeduncular nucleus, where it presents a mottled or meshlike appearance. These projections are topographically organized in that there is simple correspondence between the mediolateral, dorsoventral, and rostrocaudal origin of the caudate projection and its input to the globus pallidus and entopeduncular nucleus. Transported tracer is also present within the substantia nigra, where it is most abundant within the pars reticularis. However, distinct labeling also overlies cells of the pars compacta, and lesser amounts of labeling are present within the pars lateralis and within the retrorubral area. Following injections of horseradish peroxidase into the caudate nucleus, and subsequent tissue processing by the tetramethylbenzidine (TMB) method of Mesulam ('78), labeled anterograde fibers are present in abundance within the globus pallidus, entopeduncular nucleus, and all subdivisions of the substantia nigra, thus confirming the autoradiographic findings. Also, it is especially obvious in this HRP material that, contrary to previous degeneration studies, both the rostromedial and caudolateral parts of the pars lateralis of the substantia nigra contain numerous anterogradely labeled fibers. Retrogradely labeled neurons are also present within the substantia nigra of these same tissue sections, where they are most abundant within the pars compacta, but lesser numbers of labeled neurons are also present within the pars reticularis, pars lateralis, retrorubral area, and ventral tegmental area on the ipsilateral side, and all of these same subdivisions of the substantia nigra on the contralateral side. Also, within the subthalamic nucleus in these experiments, there are anterogradely labeled fibers, as well as retrogradely labeled neurons, which are interpreted to represent a reciprocal connection between the subthalamic nucleus and the striatum. In a separate series of experiments, horseradish peroxidase was injected into the motor cortex-specifically into the anterior sigmoidal gyrus. Following such injections, labeled neurons representing afferents to the motor cortex are found in all subcortical nuclei commonly known as the "basal ganglia," including the caudate nucleus, putamen, globus pallidus, entopeduncular nucleus, substantia innominata, nucleus of the diagonal band of Broca, medial septal nucleus, claustrum, and basolateral amygdaloid nucleus.     

Erratum

Tritiated tracer was injected into the head of the caudate nucleus in cats. Following such injections, labeling is present within extensive regions of both the globus pallidus and entopeduncular nucleus, where it presents a mottled or meshlike appearance. These projections are topographically organized in that there is simple correspondence between the mediolateral, dorsoventral, and rostrocaudal origin of the caudate projection and its input to the globus pallidus and entopeduncular nucleus. Transported tracer is also present within the substantia nigra, where it is most abundant within the pars reticularis. However, distinct labeling also overlies cells of the pars corapacta, and lesser amounts of labeling are present within the pars lateralis and within the retrorubral area. Following injections of horseradish peroxidase into the caudate nucleus, and subsequent tissue processing by the tetramethylbenzidine (TMB) method of Mesulam (1978), labeled anterograde fibers are present in abundance within the globus pallidus, entopeduncular nucleus, and all subdivisions of the substantia nigra, thus confirming the autoradiographic findings. Also, it is especially obvious in this HRP material that, contrary to previous degeneration studied, both the rostromedial and caudolateral parts of the pars reticularis of the substantia nigra contain numerous anterogradely labeled fibers. Retrogradely labeled neurons are also present within the substantia nigra of these same tissue sections, where they are most abundant within the pars compacta, but lesser numbers of labeled neurons are also present within the pars reticularis, pars lateralis, retrorubral area, and ventral tegmental area on the ipsilateral side, and all of these same subdivisions of the substantia nigra on the contralateral side. Also, within the subthalamic nucleus in these experiments, there are anterogradely labeled fibers, as well as retrogradely labeled neurons, which are interpreted to represent a reciprocal connection between the subthalamic nucleus and the striatum. In a separate series of experiments, horseradish peroxidase was injected into the motor cortex–specifically into the anterior sigmoidal gyrus. Following such injections, labeled neurons representing afferents to the motor cortex are found in all subcortical nuclei commonly known as the “basal ganglia,” including the caudate nucleus, putamen, globus pallidus, entopeduncular nucleus, substantia innominata, nucleus of the diagonal band of Broca, medial septal nucleus, claustrum, and basolateral amygdaloid nucleus.     

Description of a large projection from the mesodiencephalic junction to the rostral red nucleus. An anatomical study in the rat and in the cat

Horseradish peroxidase (HRP) or wheat germ agglutinin conjugated to HRP (WGA-HRP) were deposited in the rostral pole (Rpc) of the red nucleus in 6 rats and 5 cats. In the rat, a zone of dense retrograde cell labeling and of diffuse axonal labeling occurred in the sub-pretectal area, a region which corresponds to the posterior thalamic nucleus (PT) of Bold et al. (Brain Research, 12 (1984) 521-527). In the cat, a similar retrograde and anterograde pattern of labeling was observed. The labeled area extended from the deep pretectum up to the diencephalon above the dorsomedial aspect of the ventral posterior thalamic nucleus. In a second set of experiments, 14 rats received a tracer injection (HRP, WGA-HRP or Phaseolus vulgaris-leucoagglutinin) in the sub-pretectal area. The resultant pattern of labeling consisted in a heavy anterograde terminal labeling within the Rpc of the red nucleus. Sparse retrograde cell labeling also occurred in the Rpc.     

Direct amygdaloid projections to the superior salivatory nucleus: a light and electron microscopic study in the cat.

Amygdaloid projections to the superior salivatory nucleus (SSN) were investigated in the cat by using the anterograde and retrograde tracing techniques of horseradish peroxidase (HRP). After HRP injections were made into the lingual nerve, retrogradely labeled SSN neurons were located in the lateral tegmental field medial to the spinal trigeminal nucleus from the middle level of the superior olivary nucleus to the caudal level of the facial nucleus. These labeled neurons, triangular, oval or polygonal in shape, were small to medium-sized (12-29 microns) and formed loosely packed clusters. In further HRP studies, HRP injections were made into the amygdala and in the reticular formation containing the SSN neurons. The results suggested that the SSN receives direct afferents from the central nucleus of the amygdala with ipsilateral predominance. Final proof of such direct connections from amygdala to the SSN can be obtained only by electron microscopic study. Therefore, HRP injections were made into the lingual nerve and in the amygdala in the same animal and electron microscopic observations were carried out on the SSN. It appeared that anterogradely labeled amygdalo-tegmental fibers formed axosomatic and axodendritic synaptic contacts with retrogradely labeled SSN neurons.     

The thalamic connectivity of the primary motor cortex (MI) in the raccoon

The purpose of this study was to determine the pattern of thalamic projections of the primary motor cortex (MI) in the raccoon, a carnivore species noted for neural specialization of sensorimotor function. Following electrophysiological identification of circumscribed regions of MI, injections of horseradish peroxidase (HRP) or HRP combined with tritiated amino acids were made in 15 animals. Labeled thalamic cells were found predominantly in the ventral lateral nucleus (VL). For a given cortical injection site within MI, labeled neurons in VL formed a crescent-shaped band which extended in a dorsoventral direction when viewed in coronal sections. These bands were topographically organized. Following an injection of the MI hindlimb area in the medial part of the posterior cruciate gyrus, both retrogradely labeled neurons and anterograde label formed a thin band at the lateral edge of VL while an injection of the MI face area in the lateral part of the anterior cruciate gyrus resulted in both anterograde and retrograde label in medial VL and the principal division of the ventromedial nucleus (VMp). An injection of the MI forepaw area localized to the rostral and central part of the posterior cruciate gyrus resulted in a band of labeled neurons occupying the dorsal extent of VL and continuing into the ventrolateral aspect of the ventral anterior nucleus (VA). In contrast, an injection of the MI forepaw area which was localized to the caudal extent of the posterior cruciate gyrus resulted in a wide and diffuse band of labeled neurons and anterograde label in the ventral portion of VL. All injections of MI produced cell labeling in the paracentral nucleus (PC) and the central lateral nucleus (CL) of the intralaminar group. These results demonstrate that VL is the primary thalamic dependency of MI in the raccoon. Labeled cells were not observed in the ventrobasal complex. The MI pattern of thalamic connectivity observed in the present study suggests that while differences exist in the regional specialization of sensorimotor structures among species, there appears to be little variation in the overall organization of thalamocortical relations.     

Ontogenetic change in the distribution of callosal projection neurons in the postcentral gyrus of the fetal rhesus monkey

In the postcentral gyrus of the mature rhesus monkey the distribution of callosal projection neurons is discontinuous. The density of callosal projection neurons, which are mainly located in the supragranular layers, varies both within and across cytoarchitectonic areas (Killackey et al., '83). In the present study, we investigated the ontogeny of corpus callosum projections of the postcentral gyrus in five fetal rhesus monkeys, ranging in age from embryonic day (E) 108 to E 133. Multiple large injections of horseradish peroxidase that involved the underlying white matter were made into the postcentral gyrus of one hemisphere and the distribution of labeled neurons in the ipsilateral thalamus and the other hemisphere was determined. The pattern of thalamic label indicated that the tracer was effectively transported from all portions of the postcentral gyrus. We found that the areal distribution pattern of labeled callosal projection neurons varied at the different fetal ages. At early fetal ages (E 108, E 111, and E 119) callosal projection neurons were continuously distributed throughout the postcentral gyrus. As in the adult animal, the vast majority of labeled callosal projection neurons were found in the supragranular layers, although a few labeled cells were located in the infragranular layers. From the earliest age, there was regional variation in the width of the band of labeled supragranular callosal projection neurons. The difference between the precentral and postcentral gyrus was most obvious, but there was also a difference between anterior and posterior portions of the postcentral gyrus. The first indication of some discontinuity in the distribution of callosal projection neurons was noted at E 126. By E 133, approximately 1 month before birth, the distribution of callosal projection neurons appeared remarkably mature. On E 119 aggregations of anterograde label could be detected in restricted portions of the posterior postcentral gyrus beneath the cortical layers. By E 133 anterograde label was found within the cortical layers (most densely in layer IV) in these regions of the postcentral gyrus. Thus, the emergence of the discrete pattern of callosal projection neurons appears to be temporally correlated with the ingrowth of callosal afferents. On the basis of these observations, as well as those of others (discussed in the text), we propose that the ontogenetic changes in the distribution of callosal projection neurons reflect the unique strategy employed by cortical projection neurons in establishing their patterns of connectivity. It is hypothesized that this strategy may involve multiple processes.