Cytoarchitectonic heterogeneities in the thalamic reticular nucleus of cats and ferrets.

The thalamic reticular nucleus has been classically defined as a group of cells surrounding most of the rostral and lateral surfaces of the dorsal thalamus, lateral to the fibres of the external medullary lamina and medial to those of the internal capsule. With the use of Nissl staining and antibodies to gamma-aminobutyric acid (GABA), somatostatin, and parvalbumin, this study describes the cytoarchitecture of the thalamic reticular nucleus of cats and ferrets. In cats, three subdivisions of the nucleus are distinguished, two of which are distinct in ferrets also. First, the main body of the reticular nucleus lies lateral to the fibres of the external medullary lamina (except ventrally) and medial to those of the internal capsule. In both cats and ferrets, this structure is heterogeneous, consisting of distinct layers, the details of which vary along the dorsoventral axis. A prominent rostroventral portion of comparatively small rounded cells is also apparent within the main body. Most reticular cells in all areas of the main body are labelled with all of the above mentioned antibodies. Second, the inner small-celled region is a group of small cells located between the external medullary lamina (ventrally) and the medial margin of the ventral regions of the main body of the reticular nucleus: the inner small-celled region is clearly differentiated in cats only. Previous studies have referred to this area as being part of the main body of the reticular nucleus, but we suggest that it may form a separate subnucleus. For example, the inner small-celled region stands in striking contrast to the main body of the reticular nucleus in that none of its cells are GABA immunoreactive and only a small caudal subpopulation are parvalbumin immunoreactive. A very similar pattern of immunostaining is apparent for the cells in the zona incerta, although the latter contains a small rostral subpopulation of GABA immunoreactive cells. Furthermore, although morphologically distinct from the zona incerta, the inner small-celled region fuses with it ventrocaudally. We suggest that the inner small-celled region may constitute a previously undescribed dorsal extension of the zona incerta, rather than a subdivision of the reticular nucleus. Third, the perireticular nucleus, hitherto unidentified, is a discrete group of small cells lateral to the main body of the reticular nucleus and medial to the corpus striatum (globus pallidus and caudate-putamen). It is apparent throughout most of the dorsoventral extent of the main body of the reticular nucleus of cats and ferrets.(ABSTRACT TRUNCATED AT 400 WORDS)