G. Macchi

The thalamo-caudate versus thalamo-cortical projections as studied in the cat with fluorescent retrograde double labeling

SummaryThe distribution of thalamic cells projecting to the head of the caudate and their interrelations with thalamo-cortical cells were studied in the cat with different combinations of fluorescent tracers. Injections in the head of the caudate were combined with the injections in the pericruciate, proreal, suprasylvian, anterior cingulate, occipital and ectosylvian cortices. The following results were obtained: (i) Injections in the head of the caudate resulted in retrograde labeling of thalamic cells medially and laterally to the anteromedial (AM) nucleus, and in the medioventral part of the ventral anterior (VA) nucleus. Further, labeled cells were distributed throughout the anterior intralaminar central medial (CeM), paracentral (Pc) and central lateral (CL) nuclei, and the posterior intralaminar center median-parafascicular complex (CM-Pf). Labeled cells were mainly grouped in the mediodorsal parts of the anterior intralaminar nuclei; they were also found in the more dorsal part of the mediodorsal (MD) nucleus, ventral to the thalamic paraventricular (Pv) nucleus and to the habenular complex, (ii) Thalamo-cortical and thalamo-caudate cells overlapped in the medial part of the VA; in the anterior intralaminar nuclei they were either intermingled or were distributed in separate clusters or longitudinal bands. The two cell populations also overlapped in the posterior intralaminar complex. The greatest overlap occurred with the thalamic cell population projecting to the pericruciate cortex. (iii) Thalamic cells bifurcating to the head of the caudate and to the pericruciate cortex were found lateral to the AM, within the VA, and throughout the anterior intralaminar nuclei, especially in the CeM and in the posterior part of the CL; a few branched cells were also found in the CM. Thalamic cells bifurcating to caudate and anterior suprasylvian cortex were also found in the VA. Very few cells (scattered in the anterior thalamus lateral to the AM, as well as in the CeM, Pc and CL) were found to bifurcate to the head of the caudate and the other cortical fields here examined.

The cortical projections of the thalamic intralaminar nuclei restudied by means of the HRP retrograde axonal transport.

Following multiple injections of HRP in different cortical areas in the cat, labeled cells were, in some cases, found in the thalamic intralaminar nuclei. The following cortical zones were found to constitute the preferential target for the projections from the respective intralaminar nuclei: motor and anterior suprasylvian areas for the nucleus centralis lateralis, cingulate cortex for the nuclei paracentralis and centralis medialis, sensory and motor areas for the nucleus centrum medianum. These data are compared with the results previously obtained by means of the retrograde degeneration technique.

The Thalamic Intralaminar Nuclei and the Cerebral Cortex

The use of the term “intralaminar nuclei” or “nuclei of the internal medullary lamina” dates back to Vogt (1909) and Friedemann (1911). According to its literal definition, the term “intralaminar nuclei” refers to the nuclear structures which lie within the internal medullary lamina of the thalamus. However, in its most common use, this term indicates the nuclear structures which are topographically related to three distinct regions of the thalamus of mammals. The first, the central medial nucleus (CeM), is located at the midline between the internal medullary laminae of the two sides. The second is located laterally in the anterior part of each internal medullary lamina, and includes the paracentral (Pc) and central lateral (CL) nuclei. The third expands posteriorly in a splitting of the internal medullary lamina and includes the posterior intralaminar, centre median (CM), and parafascicular (Pf) nuclei, which are commonly designated as the CM-Pf complex in the species in which they can both be identified (see Section 2).

The cortical projections of the thalamic intralaminar nuclei, as studied in cat and rat with the multiple fluorescent retrograde tracing technique.

Two retrograde fluorescent tracers were injected in two different areas of the cerebral cortex in rats and in cats. In all the experiments many single labeled cells and only some double labeled ones were seen in the thalamic intralaminar nuclei. The present results suggest that the diffusely distributed intralaminar-cortical projections mainly consist of axons of separate cells, and only to a minor extent of axon collaterals of the same cells.

The organization of the ipsi- and contralateral claustrocortical system in rat with notes on the bilateral claustrocortical projections in cat

The organization of the claustrocortical system was investigated in rat by means of cortical injections of either lectin-conjugated horseradish peroxidase or retrograde fluorescent tracers. The latter were also employed in cat. Evans Blue, Fast Blue, True Blue, Nuclear Yellow and Diamidino Yellow were used in different combinations and were injected, uni- or bilaterally, in different cortical fields. Cells retrogradely labeled from each cortical injection were observed in the ipsi- and contralateral claustrum. Anterogradely labeled terminals were also seen in the claustra of both sides in the horseradish peroxidase experiments. The topographic and quantitative study of the distribution of labeled neurons showed a topographic organization of the rats claustrocortical system, although a certain degree of overlap of the cell populations projecting to frontal and occipital fields was also evident. Four types of branched claustrocortical neurons were observed in the double labeling experiments: neurons branching ipsilaterally (A) or contralaterally (B) to anterior and posterior cortical fields; neurons branching bilaterally to homotopic (C) or heterotopic (D) cortical fields. Each population of branched neurons was equivalent to a different percent value of the total labeled cell populations; the percent value decreased from type A to type D. Type C branched neurons were also identified in the claustrofrontal system of the cat. The intricate organization of the claustral-ascending projections suggests that the nucleus is involved in different cortical activities and that its efferents may also provide the substrate of a powerful subcortical mechanism of interhemispheric communication.

Efferent fibers from the motor cortex terminate bilaterally in the thalamus of rats and cats

SummaryThe anterograde transport of lectin-conjugated horseradish peroxidase (WGA-HRP) was here employed in order to visualize crossed cortico-thalamic efferents of the motor cortex in rats and cats. After WGA-HRP cortical injections in the rat retrogradely labeled cells were observed in the ipsilateral thalamus, and heavy anterograde labeling was observed both in the ipsi-and contralateral thalamus. The contralateral anterograde labeling was less intense than the ipsilateral one and it was distributed in the anterior intralaminar structures, in the parafascicular nucleus, in the ventromedial, ventrolateral and ventrobasal nuclei and in the posterior complex, symmetrically to the labeling observed on the ipsilateral side. Further experiments were made in the rat in order to ascertain that the bilateral anterograde labeling in the thalamus derived unilaterally from the cortex. To this purpose, kainic acid was injected unilaterally either into the frontal cortex or into the thalamus, and WGA-HRP was later injected on the same side in the frontal cortex. Moreover, WGA-HRP was injected into the frontal cortex after splitting of the corpus callosum. The results obtained in these experiments confirmed that cortical neurons projected bilaterally upon the thalamus. Further, these experiments indicated that at least the majority of the contralateral fronto-thalamic fibers crossed the midline in the thalamic massa intermedia. WGA-HRP injections into the pericruciate cortex in the cat confirmed the presence of anterogradely labeled terminals in the contralateral anterior and posterior intralaminar, ventral anterior, ventromedial and ventrolateral nuclei. The labeling was in all cases heavier in the intralaminar nuclei than in the other structures, but it was less intense than that observed in the rat. The labeling in the contralateral principal nuclei was heavier in one case in which the injection area diffused into the proreal cortex than in the other cats. Thus, the present results indicate that frontal efferents terminate more densely in the rats than in the cats contralateral thalamus. In the latter species frontal fibers are mainly distributed upon the contralateral intralaminar nuclei, whereas in the rat frontal efferents are also heavily distributed upon the ventral nuclear complex.

The basolateral amygdaloid projections to the neocortex in the cat.

Retrogradely labeled cells have been found in the ipsilateral basolateral nucleus of the amygdala (BL) following injections of horseradish peroxidase (HRP) in various neocortical areas in the cat. The number of labeled neurons was small in the majority of the cases; however the amygdaloid projections to the prefrontal, anterior cingulate, insular and posterior sylvian cortices appeared heavier. HRP injections in the parietal associative and visual areas did not result in labeled cells in the amygdala. The present data suggest that the cortical projections arising from BL are more widely distributed than previously reported.

GABAergic interneurons and neuropil of the intralaminar thalamus: an immunohistochemical study in the rat and the cat, with notes in the monkey

SummaryImmunohistochemistry using antibodies to glutamic acid decarboxylase (GAD) was used to investigate the intralaminar nuclei of the thalamus in rat, cat and monkey. Antibodies to gamma aminobutyric acid (GABA) were also used in the cat. Intralaminar immunoreactive cell bodies were not detected in the rat, but were clearly present in cat and monkey. In the latter species, GABA- or GAD-immunopositive perikarya were distributed throughout the anterior intralaminar nuclei, whereas in the posterior intralaminar complex they prevailed in the lateral part of the centre median nucleus and around the fasciculus retrofiexus. Measurements of the area of immunostained intralaminar cell bodies in cat and monkey indicated that they are represented by small neurons. Experiments in the cat, based on retrograde tracers injections involving large sectors of the frontal and parietal cortices and the head of the caudate nucleus, revealed that the GABA- or GAD-immunoreactive cells and the retrogradely labeled projection neurons represented two separate intralaminar cell populations, although the latter also included small cells. Considerable differences were observed in the immunoreactive GABAergic neuropil of the anterior and posterior intralaminar nuclei. Clusters of densely packed bouton-like immunoreactive elements were detected in the former structures in the rat, cat and monkey, and were especially evident in the central lateral nucleus; immunopositive varicose fibers and puncta were diffusely distributed in the posterior intralaminar structures. Taken together with data from the literature, the present findings indicate that in cat and monkey local circuit inhibitory cells regulate not only the activity of principal thalamic nuclei which project densely upon restricted cortical fields, but also of the intralaminar structures which are widely connected with the cerebral cortex and the striatum. Regional variations in the distribution of GABAergic fibers and terminals suggest major differences in the organization of inhibitory circuits and synaptic arrangements of the anterior and posterior intralaminar thalamus.

Spinal afferents and cortical efferents of the anterior intralaminar nuclei: an anterograde-retrograde tracing study

The topographical relations among the terminal field of spinothalamic fibers and the cells projecting upon areas 4 and 5 were studied in the anterior intralaminar nuclei of the cat. Terminals anterogradely labeled from the spinal cord and cell populations retrogradely labeled from the lateral pericruciate and anterior suprasylvian cortex were simultaneously observed by means of a multiple fluorescent tracing strategy. The present findings confirm that spinal afferents in the central lateral and paracentral nuclei overlap with the cells projecting to area 4. Further, the present data demonstrate that spinal terminals are largely segregated from the intralaminar cell population projecting to area 5.

Neuronal cell types in the thalamic intralaminar central lateral nucleus of the cat

SummaryThe existence of Golgi type II neurons was verified in the anterior intralaminar central lateral (CL) nucleus of the cat thalamus, and its projection cell types were identified, by means of Golgi impregnation. CL principal neurons were found to display a large- or medium-sized cell body and a radiate dendritic pattern. Their primary dendrites were limited in number, and had a rather long course; they were poorly ramified. The axons of principal neurons were impregnated only occasionally and for a short distance. Projection neurons of the ‘bushy’ or tufted type, described in the main thalamic sensory nuclei, were not identified in the CL in the present study. Typical Golgi type II neurons were found throughout CL. They were mainly small-sized, and displayed a rich dendritic arborization characterized by dendritic appendages. The axons of Golgi type II neurons were seen to give rise to extensive local arborizations. The present findings indicate that in the cat CL, principal cells are mainly represented by radiate neurons. Typical local circuit neurons also are evident in CL, suggesting that the activity of anterior intralaminar structures is regulated by intrinsic mechanisms similar to those operating in the main thalamic relay nuclei.