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A comparative quantitative study of the somatosensory thalamocortical connections in the rat and rabbit, labeled with the fluorescent retrograde tracer Fluoro-Gold (FG), was conducted by means of unbiased stereology. FG was injected into the primary somatosensory cortex of the rat and rabbit in different age groups from P0 to P180 (P-postnatal day). The numerical density of retrogradely labeled the ventroposterolateral (VPL) projection neurons was analyzed. A significant decrease in this parameter was observed during the first two weeks of postnatal life in both studied species. Changes of the neuropil volume and selective elimination of early cortical connections stemming from the VPL may possibly cause this process. A withdrawal of axon collaterals from the expanded cortical sites as well as apoptosis (existing both in the VPL and parietal cortex) contribute to a decrease in the numerical density. Our observations allow us to conclude that the thalamocortical somatosensory connections established before the birth undergo significant quantitative changes in both studied species during the first two weeks of postnatal life and this period seems to be crucial for maturation of the thalamocortical loop.

Retrograde axonal transport method of the fluorescent tracer FluoroGold (FG) was combined with immunocytochemistry to investigate the occurrence of nitric oxide synthase (NOS), somatostatin (SOM), neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) in both intrinsic and cortically projecting neurons of the rat claustrum. Only NOS was detected in both the scattered projecting neurons and internal neurons of the claustrum. Approximately 20% of NOS-immunoreactive neurons in the claustrum were also retrogradely labeled with FG after tracer injections into the frontal cortex. The other substances were exclusively confined to the population of interneurons, which mainly displayed an oval, round or fusiform shape and a medium size. Apart from the neuronal somata, the proximal parts of the dendritic arborization were clearly visible. The immunoreactive neurons were randomly distributed in the claustrum and their neuronal size and shape did not differ in the various parts of the studied structure. Co-localization of NOS and SOM or NOS and NPY was reported. In conclusion, SOM, VIP and NPY do not appear to play a significant role in the claustro-cortical projection but are most probably involved in modulation and information transfer in the claustrum. The appearance of NOS in both cortically projecting and intrinsic neurons of the claustrum may be indicative of a fundamentally different role in the functioning of the claustro-cortical loop.

The quantitative analysis of the claustrocortical connections labeled with the fluorescent retrograde tracer Fluoro-Gold (FG) was conducted on 90 rabbits subdivided into the following age groups (P2, P7, P14, P21, P30, P60, P90, P120, P180). The equal volumes of retrograde fluorescent tracer FluoroGold (FG) were injected into the selected regions of the motor or somatosensory cortices. The volume of the dorsal part of the claustrum, total number of projecting neurons, numerical density and percentage distribution of projecting neurons were estimated by means of the unbiased stereological methods. The claustrocortical connections both with the motor and somatosensory areas in a rabbit are established in the postnatal life. The parts of the claustrum occupied by the motor and somatosensory projection zones as well as the morphology of the cortically projecting neurons do not reveal characteristic changes during the studied period. The significant decrease of the total number and numerical density of cortically projecting neurons as well as the increase of the claustral volume may reflect the process of adjustment of the claustrum to its modulatory function upon corresponding cortical areas. The intensity of the claustral connections with the motor and somatosensory cortices reveals significant difference during the studied period, being higher for the motor projection. It may be assumed that the claustrocortical connections established before birth undergo significant quantitative changes during postnatal development.

The connections of the amygdaloid body with the areas of the limbic cortex in the rat were studied by means of the method on the axonal retrograde transport of the fluorescent tracer FluoroGold. The tracer was injected into the anterior and posterior limbic cortices (cingulate gyrus, granular and agranular retrosplenial area, respectively. The localization of the corresponding amygdalar projection zones was investigated and the semiquantitative analysis of the connections was conducted. The projection zones in the rat amygdaloid body are organized topographically. Administration of the fluorescent tracer to the anterior and posterior part of the limbic cortex in the rat (cingulate gyrus and both retrosplenial areas, respectively) reveals labeling of cells only for injections to the former one. The labeled cells were present only in the major components of the basolateral amygdaloid complex. The main source of this projection was anterior part of basolateral nucleus (BLA). Some labeled neurons were found in the lateral nucleus and few in the ventral part of basolateral nucleus. No labeled cells were found within the basomedial nucleus.

The corticoclaustral connections in the rat were investigated by means of the method of the retrograde axonal transport of the fluorescent tracer (Fluoro-Gold; FG). The material consisted of 20 adult Wistar rats. The fluorescent tracer was injected into the anterior, middle or posterior parts of the claustrum. The retrogradely labeled neurons were detected in the layer VI of the neocortex. Injections of the tracer into the anterior part of the claustrum resulted in labeling of the neurons in the motor cortex. After administration of the tracer into the middle part of the insular claustrum, labeled neurons were present both in the motor and somatosensory cortices, while the injections of the tracer into its posterior part resulted in labeling of neurons in the visual cortex. Administration of the fluorescent tracer into the insular claustrum of the rat resulted in labeling of the cortical neurons of the corresponding areas of both hemispheres, however, the contralateral projections seem to be less numerous than the ipsilateral. Our results confirm the existence of reciprocal connections of the claustrum with the neocortex and suggest its role in integration and modification of information reaching the neocortex.