TABULACIÓN:

PRIMARY AND SECONDARY VISUAL CORTICES

One of the aims of this study has been to compare the distribution of peptidergic neurons between the different

visual areas. According to the findings reviwed in the introduction it is clear that primary and secondary visual cortices are differentially innervated in respect to both the specific retinotopic thalamic afferents, since the primary visual cortex is mainly innervated by dLGN and the s e c o n d a r y by LPN, and the w i d e s p r e a d s u b c o r t i c a l monoaminergic and cholinergic projections: In the rat dopaminergic fibers are considerably richer in area 18 t han in e i t h e r 17 or 18a, w h ile c h o l i n e r g i c fibers i n n e r v a t i n g area 18 and the me d i a l part of area 17 originate in a different part of the basal forebrain than fibers innervating area 18a and the lateral part of area 17. Furthermore, in the primate there is a regional specificity concerning noradrenergic and serotonergic fibers, w i t h area 17 r e c e i v i n g d e n s e s e r o t o n e r g i c projection and area 18 dense noradrenergic projection.

Additional anatomical evidence, already mentioned in the i n t r o d u c t i o n , s h o w i n g d i f f e r e n c e s in the cytoarchitecture of the three visual areas such as the thickness of various layers as well as evidence showing an areal hierarchy in the processing of visual information, further emphasize the distinct morphology and function of these areas, rising the question of possible differences between the organization of the neural circuits in each area.

The findings of this study show that while NPY- and VIP- positive neurons exhibit roughly equal densities amongst the three visual areas, cells expressing SOM-mRNA are significantly less numerous in area 18a. This is a feature present in early stages of development (first

noticed at PIO) as well as in later d e v e l o p m e n t and adulthood. A p r e v i o u s i m m u n o c y t o c h e m i c a l s t u d y by Papadopoulos et al (92) also reported on a differential distribution of SOM-neurons between the p r i m a r y and s e c o n d a r y vi s u a l areas s h o w i n g that area 17 and in particular the upper layers of it display a lower density of positive neurons than the neighboring areas 18 and 18a. Although both studies confirm the non-uniformity of the SOM-neuronal distribution and agree that area 18 exhibits a high neuronal density they fail, probably because of the different techniques used, to come to an agreement as to which of the two remaining areas exhibit the lowest density of SOM-neurons.

Apart from the areal d i versity in respect to SOM- neuronal densities there is an additional d i versity concerning the distribution pattern of SOM- neurons in the different areas: Area 18 displays a distinct distribution pattern exhibiting a much greater proportion of the SOM- population in the suprgranular layers. Only one in three neurons of areas 17 or 18a is present in the supragranular layers whereas approximately two in five neurons of area

18 are present in the same layers.

Another interesting feature differentiating the SOM- population of the supragranular layers of area 18, is the presence at an early stage of development (P14), of positive neurons expressing a heavier autoradiographic signal than their counterparts of areas 17 and 18a. Since this difference is not observed in later development or adulthood, where the proportion of heavily versus lightly labelled cells in the supragranular layers of all areas is

virtually the same, and since the equivalent cells in areas 17 and 18a display at P14 a lighter hybridization signal than they do in the adult, it is obvious that there is an earlier upregulation of the SOM transcription in the supragranular layers of area 18.

In the case of NPY no difference in neuronal densities or laminar distribution patterns are obvious between the visual areas; yet there is a feature discriminating one area (area 18a) from the rest : the layer VI of this area is particularly rich in hybridization signal displaying a greater proportion of heavily labelled cells. While 55% of the layer VI population in this area is heavily labelled, heavily labelled cells comprise 45% of the same layer in the other two areas. In addition, as can be seen in Fig 2, the most heavily labelled NPY-cells of the visual cortex are almost always to be found in the lower part of this area.

Contrary to both peptides discussed above VIP-neuronal p o p u l a t i o n is u n i f o r m l y d i s t r i b u t e d in p r i m a r y and secondary visual cortex both during development and in the adult.

T h e s e f i n d i n g s s u g g e s t that t h e r e c o u l d be a differential distribution

or a differential density of peptidergic neurons or even a different regulation of peptidergic expression between the three visual areas. It is clear however, that not every peptidergic interneuronal population displays all or any of these particularities. Furthermore it is not known if the p o p u l a t i o n of i n t e r n e u r o n s as a w h o l e s h o w s any diversity in either distribution or density between the

p r i m a r y a n d s e c o n d a r y v i s u a l c o r t i c e s , s i n c e no comparative quantitative analysis of GABA distribution has been yet attempted.