'Non-specific' cholinesterase-containing neurons of the dorsal thalamus project to medial limbic cortex

Thalamocortical neurons that contain 'non-specific' cholinesterase (ChE) were studied with cholinesterase histochemistry and experimental axonal tracing techniques in adult rats. In addition to the presence of ChE that is ubiquitous in capillary endothelium, neurons that contain ChE are found in 3 distinct regions of the dorsal thalamus, the thalamic reuniens nucleus (Re), the anterior dorsal nucleus (AD) and a region that includes the lateral part of the central lateral nucleus (CL) and the ventral portion of the lateral dorsal nucleus (LD). ChE activity appears light in cerebral cortex in general but histochemical staining is slightly greater in neuropil of the cingulate gyrus. Anterograde transport techniques with autoradiography demonstrated that neurons in the LD-CL region project to anterior cingulate cortex and the dorsal retrosplenial area. Anterograde degeneration techniques demonstrated that AD projects primarily to ventral retrosplenial cortex. Injections of horseradish peroxidase (HRP) in the anterior cingulate cortex resulted in double labeled cells (cells containing both ChE and HRP reaction products) primarily in LD and CL. HRP injections into ventral retrosplenial cortex resulted in double labeled cells in AD and Re. HRP injections in the subiculum resulted in double labeled cells in Re. Lesions placed in the region of thalamocortical projections resulted in a loss of ChE in the ipsilateral cingulate gyrus, as measured both histochemically and enzymatically. The finding that neurons containing ChE project to medial limbic cortex suggests that the ChE may be involved in the function of the thalamocortical component of the limbic system.     

Cortical and thalamic afferent connections of the insular and adjacent cortex of the rat

Thalamic and cortical afferents to the insular and perirhinal cortex of the rat were investigated. Unilateral injections of horseradish peroxidase (HRP) were made iontophoretically along the rhinal sulcus. HRP injections covered or invaded areas along the rhinal fissure from about the level of the middle cerebral artery to the posterior end of the fissure. The most anterior injection labeled a few cells in the mediodorsal nucleus. More posterior injections labeled neurons in the basal portion of the nucleus ventralis medialis, thus suggesting that this cortical region constitutes the rat's gustatory (insular) cortex. We consider the cortex situated posterior to the gustatory cortex in and above the rhinal sulcus as the core region of the rat's (associative) insular cortex, as this cortex receives afferents from the regions of and between the nuclei suprageniculatus and geniculatus medialis, pars magnocellularis. It includes parts of the cortex termed perirhinal in other studies. The cortex dorsal and posterior to the insular cortex we consider auditory cortex, as it receives afferents from the principal part of the medial geniculate nucleus, and the cortex ventral to the insular cortex (below the fundus of the rhinal sulcus) we consider to constitute the prepiriform cortex, which is athalamic. The posterior part of the perirhinal cortex (area 35) receives afferents from nonspecific thalamic nuclei (midline nuclei). Cortical afferents to the injection loci arise from a number of regions, above all from regions of the medial and sulcal prefrontal cortex. Those injections confined to the projection cortex of the suprageniculate-magnocellular medial geniculate nuclear complex also led to labeling in contralateral prefrontal regions, particularly in area 25 (infralimbic region). A comparison of our results with those on the insular cortex of cats and monkeys suggests that on the basis of thalamocortical connections, topographical relations, and involvements of neurons in information processing and overt behavior, the insular cortex has to be regarded as a heterogeneous region which may be separated into prefrontal insular, gustatory (somatosensory) insular, and associative insular portions.     

Frontal projections to the region of the oculomotor complex in the rat: a retrograde and anterograde HRP study

The retrograde and anterograde capabilities of the horseradish peroxidase (HRP) technique were employed to study frontal projections to the perioculomotor region in the rat. Following HRP microinjections or transcannular HRP gel implants into the oculomotor complex (OMC), the majority of retrogradely labeled pyramidal cells were located in lamina V of the dorsomedial frontal shoulder cortex, i.e., medial precentral and anterior cingulate (PrCm/AC) cortices, the proposed frontal eye field (FEF) in the rat. A smaller number of labeled cells were present in the frontal polar cortex, agranular insular (AI), and lateral precentral (PrCl) cortices. Following HRP gel implants into the PrCM/Ac, anterogradely labeled projections were observed to the dorsal medial subthalamic region (nucleus campi Foreli, NCF) and medial accessory nucleus of Bechterew (MAB), and to other subcortical nuclei known to receive inputs from cortical area 8 in the monkey. These results, taken together with previous anatomical and physiological studies, support the conclusion that the PrCm/AC cortex contains the rat FEF. Its homology with the primate FEF is discussed.     

Distribution and size of thalamic neurons projecting to layer I of the auditory cortical fields of the cat compared to those projecting to layer IV

The distribution of thalamocortical neurons projecting to layer I of the cat auditory cortical fields was examined by the horseradish peroxidase (HRP) method. After HRP injection into layer I of the primary auditory cortex (AI), HRP-labeled neuronal cell bodies were distributed mainly in the medial, dorsal, and ventrolateral divisions of the medial geniculate nucleus (MGN) and suprageniculate nucleus (Sg), and additionally in the lateral and medial divisions of the posterior group of the thalamus (Pol and Pom), lateroposterior thalamic nucleus (Lp), and nucleus of the brachium of the inferior colliculus (BIN). After HRP injection into layer I of the second auditory cortex (AII), labeled neurons were seen mainly in the medial, dorsal, and ventrolateral divisions of the MGN and Sg and additionally in the Pom, Lp, and BIN. After HRP injection into layer I of the anterior auditory field (AAF), labeled neurons were located mainly in the medial and dorsal divisions of the MGN, Sg, Pol, and BIN, and additionally in the ventrolateral divisions of the MGN, Pom, and Lp. After HRP injection into layer I of the dorsal part of the posterior ectosylvian gyrus (Epd), labeled neurons were observed chiefly in the medial and dorsal divisions of the MGN, Sg, and Lp and additionally in the ventrolateral division of the MGN, Pom, and BIN. After HRP injection into layer I of the ventral part of the posterior ectosylvian gyrus (Epv), labeled neurons were distributed chiefly in the medial and dorsal divisions of the MGN and Pol and additionally in the ventrolateral division of the MGN, Sg, and BIN. Thus no labeled neurons were found in the ventral division of the MGN after HRP injection into layer I of all auditory cortical fields examined in the present study. The average soma diameters of neurons that were labeled after HRP injection into layer I were statistically smaller than those of neurons that were labeled after HRP injection into layer IV.     

Neurons of origin and fiber trajectory of amygdalofugal projections to the medial preoptic area in Syrian hamsters

The amygdaloid neurons of origin and the trajectory of amygdaloid fibers to the medial preoptic area of the adult male Syrian hamster were identified by using horseradish peroxidase (HRP) histochemistry. After iontophoresis of HRP into the medial preoptic area, retrogradely labeled amygdaloid neurons were located in the dorsal and caudal parts of the medial amygdaloid nucleus and throughout the amygdalohippocampal area. No amygdaloid neurons were labeled after HRP applications confined to the most rostral portion of the medial preoptic area (anterior to the body of the anterior commissure). Following more caudal medial preoptic area injections (body of the anterior commissure to the suprachiasmatic nucleus) the distribution of retrogradely labeled cells in the medial amygdaloid nucleus and the amygdalohippocampal area revealed no topographic organization of the amygdalopreoptic connections. When amygdaloid neurons were labeled, the amygdalohippocampal area contained two to five times as many HRP-filled cells as the medial amygdaloid nucleus. Retrogradely transported HRP could be followed from the medial preoptic area to the amygdala through fibers in the dorsomedial quadrant of the stria terminalis. In addition, electrolytic lesions of the stria terminalis prior to iontophoresis of HRP into the medial preoptic area prevented retrograde transport to neurons in both the dorsocaudal medial amygdaloid nucleus and the amygdalohippocampal area. These results confirm earlier observations describing the location of autoradiographically labeled efferents from the medial amygdaloid nucleus to the medial preoptic area and provide new information about the restricted region within the medial amygdaloid nucleus from which these projections arise. They also suggest that, unlike the projections from the medial amygdaloid nucleus to the bed nucleus of the stria terminalis, the efferents to the medial preoptic area travel entirely in the stria terminalis.     

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

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

The medial dorsal nucleus is one of the thalamic relays of the cerebellocerebral responses to the frontal association cortex in the monkey: horseradish peroxidase and fluorescent dye double staining study.

To reveal the thalamic relay nucleus of the cerebellocerebral responses in the frontal association cortex, simultaneous labeling of the cerebellothalamic (C-T) terminals and the thalamocortical (T-Cx) neurons was performed in three monkeys. Horseradish peroxidase (HRP) was injected into the deep cerebellar nuclei and small doses of HRP or fluorescent dye were injected into the prefrontal cortex. The distribution of anterogradely labeled C-T terminals and retrogradely labeled T-Cx neurons was examined in the same sections. In addition to being distributed in the ventral thalamic nuclei and nucleus X, as previously reported, anterogradely labeled terminals were distributed in the ventrolateral part of the medial dorsal (MD) nucleus where retrogradely labeled thalamo-frontal projection neurons were localized. This study revealed that the ventrolateral parts of the MD together (MDmf, MDpc and MDdc) form one of the thalamic relays of the cerebelloprefrontal responses.     

The origin of the afferent supply to the mediodorsal thalamic nucleus: enhancement of HRP transport by selective lesions.

This experiment attempted to identify the cell bodies of origin of axons in the forebrain which supply the mediodorsal thalamic nucleus (MD). Injections of horseradish peroxidase (HRP) were placed into MD in 60 rats. Survival times varied from 1–4 days. Following injections placed into different portions of MD, HRP positively labeled cells were observed in a variety of forebrain structures which lie rostral to the injection sites. Discrete injections of HRP placed into the midline of MD labeled cells situated exclusively in the extreme ventromedial aspect of central levels of the reticular nucleus. HRP injections which included anterior levels of MD labeled cells situated principally in layers V–VI of the sulcal prefrontal cortex, while injections which involved more posterior and lateral portions of MD labeled cells in the deepest layers of the dorsomedial prefrontal cortex. Injections of this region of MD also labeled cells in adjacent portions of the deepest layers of anterior cingulate gyrus, polymorphic cell layer of prepyriform cortex and olfactory tubercle and sites immediately lateral to the vertical limb of the diagonal band and dorsal to its horizontal limb. Positively labeled cells were observed in the cortical, medial, basolateral and basomedial amygdaloid nuclei and adjacent pyriform cortex only when ablations of prefrontal cortex preceded HRP injections of MD. This findings indicates that lesions of the prefrontal cortex dramatically enhance the HRP labeling process in the amygdala and suggests the possibility that the technique involving the placement of selective lesions may be used to advangate in other anatomical systems which receive multiple inputs from widely distributed sources.     

Studies on the cerebellocerebral and thalamocortical projections in squirrel monkeys (Saimiri sciureus)

Laminar field potential analysis of the cerebellocerebral and thalamocortical (T-C) responses was carried out in the cerebral cortex of squirrel monkeys (Saimiri sciureus) under pentobarbital anesthesia. Anatomical studies using horseradish peroxidase (HRP) were combined with electrophysiological studies. Stimulation of the fastigial nucleus induced bilaterally surface positive-depth negative potentials, i.e., deep T-C responses, in the medial part of the motor cortex and in the parietal cortex (mainly area 5) with a latency of 4 to 5 ms. Stimulation of the interpositus and dentate nucleus evoked contralaterally surface negative-depth positive potentials (superficial T-C responses) in the intermediate and lateral part of the motor cortex and in the premotor cortex (area 6) with a latency of 3 to 4 ms. Stimulation of the thalamic nuclei as well as the HRP study revealed that the medial part of V.o.p. (posterior basal part of VL) and Z.o (upper part of VL) and the intralaminar nucleus (CL) mediate the interpositus- and dentate-induced responses to the motor and premotor cortex, and that the ventrolateral part of V.o.p. and Z.im (upper part of VPLo) relays the fastigial-induced responses to the motor cortex.     

Association and commissural fiber systems of the olfactory cortex of the rat II. Systems originating in the olfactory peduncle

The structure and connections of areas within the olfactory peduncle (anterior olfactory nucleus and tenia tecta) have been examined. The anterior olfactory nucleus has been divided into external, lateral, dorsal, medial, and ventro-posterior parts. In spite of the term nucleus which is applied to these areas, all of them contain pyramidal-type cells with apical and basal dendrites oriented normal to the surface, and are essentially cortical in organization. Experiments utilizing retrograde and anterograde axonal transport of horseradish peroxidase (HRP) have demonstrated that each of these parts of the anterior olfactory nucleus possesses a unique pattern of afferent and efferent connections with other olfactory areas. All subdivisions have projections to both the ipsilateral and contralateral sides, although the ipsilateral projection of the pars externa (to the olfactory bulb) is extremely light. Interestingly, crossed projections are in each case directed predominantly to areas adjacent to the homotopic areas. Two primary subdivisions may also be distinguished in the tenia tecta: a dorsal part composed largely of tightly packed neurons which closely resemble the granule cells of the dentate gyrus (bushy apical but no basal dendrites) and a ventral part which contains predominantly pyramidal-type cells. The connections of these two parts are also very different. The ventral tenia tecta receives substantial projections from the olfactory bulb, pars lateralis of the anterior olfactory nucleus, piriform cortex and lateral entorhinal area. It gives off a heavy return projection to the pars lateralis and lighter projections to the olfactory bulb, piriform cortex and olfactory tubercle. The dorsal tenia tecta receives a heavy projection from the piriform cortex, but none from the olfactory bulb. A few cells in the dorsal tenia tecta are retrogradely labeled from HRP injections into the medial aspect of the olfactory peduncle (involving the ventral tenia tecta and adjacent areas), but none are labeled from the other olfactory areas that have been injected. An area on the dorsal aspect of the olfactory peduncle that differs significantly from the anterior olfactory nucleus, tenia tecta and piriform cortex in terms of its connections and cytoarchitecture has been termed the dorsal peduncular cortex. The most striking feature of this area is its very heavy reciprocal connection with the entorhinal cortex, although it is also reciprocally connected with the olfactory bulb and piriform cortex and projects to the olfactory tubercle. Cells in layer I of the medial and ventral aspects of the olfactory peduncle have been retrogradely labeled from HRP injections into the olfactory tubercle and lateral hypothalamic area. These cells overlie the ventral tenia tecta, medial part of the anterior piriform cortex and pars ventro-posterior and pars lateralis of the anterior olfactory nucleus, but do not appear to be distributed in relation to the cytoarchitectonic boundaries. Possible functional roles of the areas within the olfactory peduncle have been discussed.     

Afferent projections to the thalamic mediodorsal nucleus in the cat studied by retrograde and anterograde axonal transport of horseradish peroxidase

The afferent pathways to the thalamic mediodorsal nucleus (MD) in the cat were studied using the methods of anterograde and retrograde axonal transport of horseradish peroxidase (HRP) and wheat germ agglutinin conjugated to HRP (WGA-HRP). The MD receives fibers from the prefrontal cortex in a topically organized manner in accordance with the thalamocortical projections. The medial or ventral portion of the MD receives afferents from the islands of Calleja of the olfactory tubercle, the nucleus of the diagonal band, the amygdala and the claustrum. The lateral hypothalamic nucleus sends a moderate number of fibers to the medial MD, but other hypothalamic nuclei send only a few fibers to the MD. The lateral or dorsal portion of the MD receives fibers from the nucleus of the diagonal band, the ventral pallidum and the entopeduncular nucleus, but only few from the olfactory tubercle and the amygdala. The thalamic reticular nucleus sends many fibers to the MD without showing any topography. The MD, particularly its lateral part, receives afferents from brainstem structures, such as the substantia nigra, superior colliculus, reticular formation, raphe nuclei and nucleus loci coerulei. Only the interpeduncular nucleus sends fibers mainly to the medial part of the MD. The cerebellar nuclei send only a few fibers to the lateral part of the MD at posterior levels.     

Visual thalamocortical projections in normal and enucleated rats: HRP and fluorescent dye studies

Visual thalamocortical projections of neonatally enucleated and control rats were studied after tracer injections into the striate and peristriate areas of adult pigmented rats. The distribution of retrogradely labeled neurons in the visual thalamic nuclei was mapped after (a) small localized injections of horseradish peroxidase into either area 17, 18, or 18a and (b) simultaneous injections of three different retrograde tracers (fast blue, HRP, and diamidino yellow) into the anterior, medial, and posterior regions of area 17. It was shown in both normal and neonatally enucleated rats, that the dorsal lateral geniculate nucleus projects to the striate cortex (area 17), whereas the laterodorsal thalamic nucleus of the lateral thalamus projects to the medial peristriate area 18, and the lateral posterior thalamic nucleus has a projection to the lateral peristriate area 18a. Additionally, both extrageniculate visual thalamic nuclei project to area 17. Neurons in the dorsoanterior region of the dorsal lateral geniculate nucleus project to the posterior part of area 17, while neurons in the ventroposterior region of the nucleus send their axons to the anterior part of area 17. A similarly inverted projection of anterior and posterior divisions of the lateral posterior thalamic nucleus to visual area 18a was detected. In enucleated rats, the general topography of the projections from the thalamic neurons to the striate and peristriate cortices was indistinguishable from that in the controls. Nonetheless, there was noticeable shrinkage of the dorsal lateral geniculate nucleus and lateral thalamus and a significant decrease in the size of the somata of projecting neurons. Mean somal area of the HRP-labeled neurons in the dorsal lateral geniculate nucleus of enucleated rats was reduced by 19.0% and the mean maximum cell diameter by 14.3% compared with controls.     

Brainstem innervation of prefrontal and anterior cingulate cortex in the rhesus monkey revealed by retrograde transport of HRP

The cells of origin of projections from the brainstem to the dorsolateral and orbital prefrontal granular cortex and to the anterior cingulate cortex of the rhesus monkey were analyzed by means of retrograde axonal transport of the enzyme horseradish peroxidase (HRP). Following injections in various portions of the dorsolateral prefrontal and in the cingulate cortex, HRP-positive neurons were found in three main locations: (1) the ventral midbrain including the anterior ventral tegmental area, the medial one-third of the substantia nigra pars compacta, and the retrorubral nucleus; (2) the central superior nucleus and the dorsal raphe nucleus, primarily in its caudal subdivision; and (3) the locus coeruleus and adjacent medial parabrachial nucleus. Labeled neurons in the raphe nuclei and locus coeruleus were distributed bilaterally. A basically similar pattern of labeled somata was found in the brainstem with HRP injections in the orbital prefrontal cortex. Scattered HRP-positive cells were found throughout the ipsilateral ventral tegmental area and in ventromedial portions of the retrorubral nucleus, and a large number of HRP-positive cells were distributed bilaterally in the dorsal raphe and central superior nuclei as well as the dorsolateral pontine tegmentum. However, in contrast to the results obtained with injections on the dorsolateral and medial aspects of the hemisphere, labeled neurons were not found in any portion of the substantia nigra. The neurons labeled retrogradely after injection of HRP in these various regions of the frontal lobe in rhesus monkey correspond both in location and morphology to the monoamine-containing neurons of the brainstem and are thus very likely the source of dopamine, norepinephrine, and serotonin found in the frontal cortex of the same species.     

Connections of the amygdala of the rat. IV: Corticoamygdaloid and intraamygdaloid connections as studied with axonal transport of horseradish peroxidase

The corticoamygdaloid and intraamygdaloid projections of the rat were studied by the use of retrograde transport of horseradish peroxidase (HRP). Observations based on anterograde transport of the enzyme were exploited to determine the course of the intrinsic connections. The HRP was injected stereotactically by means of iontophoresis. Most of the amygdaloid nuclei were selectively injected, and all but a few were reached by more than one approach. The vast majority of corticoamygdaloid fibers was found to originate in cortical areas defined as allocortical (Stephan, 1975). From the medial frontal cortex the central amygdaloid nucleus (AC) receives a hitherto undescribed projection originating in the tenia tecta; and both the AC and the lateral amygdaloid nucleus (AL) receive fibers from the prelimbic and infralimbic areas. The anterior cingulate area entertains a weak connection with the basolateral amygdaloid nucleus (BL). As to the insular cortex, the posterior agranular insular area projects to all amygdaloid subdivisions; the BL, AC, and the anterior cortical nucleus (COa) receive, in addition, fibers from the ventral agranular area. The prepyriform cortex connects with the entire amygdala except the medial nucleus (Am) The amygdala receives afferents from a transitional area between the amygdala and the entorhinal area. The entorhinal area proper is related to the amygdala via projections from the ventral part of the lateral entorhinal area to the AL and from the dorsal part of the lateral entorhinal area to the BL. The former nucleus also receives fibers from the perirhinal region. Additional amygdalopetal connections from the hippocampal region include a previously undescribed projection from the temporal two-thirds of CA1 to the AL and BL and to the posterior cortical nucleus (COp) with the adjacent periamygdaloid cortex (PAC). The subiculum projects to the AL, and more modestly to other amygdaloid nuclei There is an extensive network of intraamygdaloid connections, the Am and AC being the only nuclei not giving rise to intrinsic fibers.     

Direct cortical projections to the parabrachial nucleus in the cat

Direct projections from the cerebral cortex to the parabrachial nucleus in the cat were examined by the horseradish peroxidase (HRP)method. When HRP was injected into the parabrachial nucleus, retrogradely labeled neuronal cell bodies were seen, bilaterally with an ipsilateral predominance, mainly in the orbital gyrus, the lateral bank of the presylvian sulcus, and a restricted region in the infralimbic cortex on the medial surface of the frontal lobe (stereotaxic coordinates; Fr: 22, L: 1, H: -1); all labeled neurons were in deep pyramidal cell layer. After injecting HRP conjugated to wheat germ agglutinin (WGA-HRP) into the cortical regions where retrogradely labeled neurons were found after injecting HRP into the parabrachial nucleus, anterogradely labeled cortical fibers were traced to the parabrachial nucleus. Corticoparabrachial fibers originating from the orbital gyrus and the lateral bank of the presylvian sulcus ran ipsilaterally through the internal capsule and the cerebral peduncle down to the lower brainstem, whereas those from the infralimbic cortex coursed down ipsilaterally through the medial forebrain bundle. These cortical fibers to the parabrachial nucleus were distributed bilaterally with an ipsilateral predominance. Cortical fiber terminals in the parabrachial nucleus were topographically arranged: Corticoparabrachial fibers from the lateral bank of the presylvian sulcus ended most massively in the dorsal part of the lateral parabrachial nucleus. Corticoparabrachial fibers from the orbital gyrus ended most heavily in the medial parabrachial nucleus and less heavily in the lateral parabrachial nucleus. Corticoparabrachial fibers from the infralimbic cortex ended mostly in the parabrachial regions surrounding the brachium conjunctivum.     

Single intralaminar thalamic neurons project to cerebral cortex, striatum and nucleus reticularis thalami. A retrograde anatomical tracing study in the rat

The hypothesis of triple axonal branching to the cortex, striatum and nucleus reticularis thalami (RT) of the forebrain projecting thalamic intralaminar neurons (TIN) was studied by retrograde axonal transport of horseradish peroxidase (HRP), iron-dextran complex and [3H]wheat germ agglutinin [3H]WGA. The best combination of tracers for this purpose was demonstrated to be: HRP-pellet implantation in the rostral cortex, iron-dextran injections into the striatum and [3H]WGA injections into the rostral RT. Prussian blue labeled neurons were observed in the ipsilateral TIN, substantia nigra, mediodorsal nucleus, medial part of the ventral anterior-ventral lateral complex, anterior medial and anterior ventral nuclei. HRP labeled neurons were observed in ipsilateral ventral nuclei, mediodorsal nucleus and the TIN. Radiolabeled neurons were located only in the TIN. HRP-Prussian blue labeled neurons (cortex-striatum branched neurons) were scattered in the TIN. Prussian blue radiolabeled neurons (striatum-RT branched neurons) could be observed in the TIN, as well as a few HRP radiolabeled neurons (cortex-RT branched neurons). Triply labeled neurons were scattered throughout the TIN until the rostral part of the centre median nucleus. These results demonstrate the existence of triple axonal branching on TIN efferent axons directed to the cerebral cortex, striatum and RT. The RT directed branch provides an anatomical basis to describe an intrathalamic regulatory loop well suited to control ascending messages arising from the TIN.     

Thalamic projections to S-I in macaque monkey.

The organization of thalamic input to functionally characterized zones in primary somatosensory cerebral cortex (S-I) of macaque monkeys (Macaca mulatta) was investigated using the method of labelling by retrograde transport of horseradish peroxidase (HRP). It was found that the cell columns positioned at the posterior margin of the band of cortex representing a given body region receive thalamic input from a posterior level of the ventroposterior thalamic nucleus (VP), and that cell columns at successively more anterior positions within that band receive input from successively more anterior levels of VP. The extreme posterior and anterior margins of the S-I hand, foot and face areas receive input from neuron populations which are not as widely separated in the anteroposterior dimension of VP as the neurons projecting to the extreme anterior and posterior margins of the proximal limb and trunk representations in S-I. These characteristics of the organization of the projections from VP to S-I are consistent with the view that the body representations in VP and S-I have the same connectivity and differential submodality distribution; and with the idea that thalamocortical conncetions only exist between functionally equivalent neuron populations in VP and S-I.     

Thalamo-telencephalic connections: new insights on the cortical organization in reptiles

Tracer injections into the dorsal tier of the lacertilian dorsal thalamus revealed an extensive innervation of the cerebral cortex. The medial cortex, the dorsomedial cortex, and the medial part of the dorsal cortex received a bilateral projection, whereas the lateral part of dorsal cortex and the dorsal part of the lateral cortex received only an ipsilateral thalamic projection. Thalamocortical fibers were found superficially in all cortical regions, but in the dorsal part of the lateral cortex, varicose axons within the cellular layer were also observed. The bilateral thalamocortical projection originates from a cell population located throughout the dorsolateral anterior nucleus, whereas the ipsilateral input originates mainly from a rostral neuronal subpopulation of the nucleus. This feature suggests that the dorsolateral anterior nucleus consists of various parts with different projections. The dorsal subdivision of the lateral cortex displayed hodological and topological (radial glia processes) features of a dorsal pallium derivative. After tracer injections into the dorsal cortex of lizards, we found long descending projections that reached the striatum, the diencephalic basal plate, and the mesencephalic tegmentum, which suggests that it may represent a sensorimotor cortex.     

Origin and topography of fibers contributing to the fornix in macaque monkeys

The distribution of neurons contributing to the fornix was mapped by placing the retrograde tracer horseradish peroxidase (HRP) in polyacrylamide gels in different medial to lateral locations within the fornix of three rhesus monkeys (Macaca mulatta). The HRP was placed from 3 to 5 mm caudal to the descending columns of the fornix. Additional information came from a series of rhesus and cynomolgus monkeys (Macaca fasciculata) with anterograde tracer injections in the medial temporal lobe. The hippocampal formation, including the subiculum and presubiculum, together with the entorhinal cortex (EC) and perirhinal cortex (area 35) contribute numerous axons to the fornix in a topographical manner. In contrast, the lateral perirhinal cortex (area 36) and parahippocampal cortical areas TF and TH only contained a handful of cells labeled via the fornix. The medial fornix originates from cells in the caudal half of the subiculum, the lamina principalis interna of the caudal half of the presubiculum, and from the perirhinal cortex (area 35). The intermediate portion of the fornix (i.e., that part midway between the midline and most lateral parts of the fornix) originates from cells in the rostral half of the subiculum and prosubiculum, the anterior presubiculum (only from the lamina principalis externa), the caudal presubiculum (primarily from lamina principalis interna), the rostral half of CA3, the EC (primarily 28I and 28M), and the perirhinal cortex (area 35). The lateral parts of the fornix arise from the rostral EC (28L only) and the most rostral portion of CA3. Subcortically, the medial septum, nucleus of the diagonal band, supramammillary nucleus, lateral hypothalamus, dorsal raphe nucleus, and the thalamic nucleus reuniens all send projections through the fornix, which presumably terminate in the hippocampus and adjacent parahippocampal region. These results not only help to define those regions that project via the fornix, but also reveal those subcortical projections to the hippocampal formation most likely to rely entirely on nonfornical pathways.     

An HRP study of the afferent connections to rat medial hypothalamic region

Afferent connections to the medial hypothalamic region in the rat were studied using horseradish peroxidase (HRP). HRP was injected iontophoretically by a parapharyngeal approach. After HRP injections into the ventromedial hypothalamic nucleus, labeled cells were found mainly in the medial and basolateral amygdaloid nuclei, subiculum, peripeduncular nucleus and the parabrachial area. Labeled cells following HRP injections into the dorsomedial hypothalamic nucleus were found mainly in the lateral septal nucleus, nucleus accumbens, bed nucleus of the stria terminalis, pontine central gray and the parabrachial area. HRP-labeled cells following the medial preoptic area injections were found mainly in the infralimbic cortex, lateral and medial septal nuclei, nucleus accumbens, diagonal band, bed nucleus of the stria terminalis, medial amygdaloid nucleus, subiculum, peripedunclar nucleus and the parabrachial area. The intrahypothalamic connections were also discussed.