Michaelidou, Irene; (1996) Ontogeny of neuropeptide m-RNA containing neurons in the visual cortex of normal and 6-hydroxydopamine treated rats. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The neuronal distribution of neuropeptides: somatostatin (SOM), neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP) in the rat visual cortex was studied using the technique of in situ hybridization histochemistry. Given the sensitivity of this method and using probes of the same length and G-C ratio, a comparative analysis of the three neuronal categories was attempted during the postnatal development of the rat. Along with a qualitative, a quantitative approach has been used in order to reveal any differences in neuronal distribution among the primary and secondary visual cortex or among supra- and infra-granular layers of the same area. Each peptide was found to exhibit a distinct distribution pattern, both in the adult rat visual cortex and during development. In the adult, neurons producing SOM-mRNA were the most abundant of the three. They were present m all layers except layer I, but were mainly located in lower layers V and VI, of which layer V exhibited predominantly lightly labelled cells. Area 18 was shown to contain the highest density of SOM-neurons. Cells expressing NPY-mRNA appeared in all layers II-VI, but were encountered in supragranular layers II/III and in layer VI. The latter was characterised by the presence of many strongly labelled cells through its whole depth and of a number of mildly labelled neurons at the border between this layer and the white matter. All three visual areas showed a similar neuronal distribution. Neurons positive for VIP-mRNA were present in all layers with a preference for upper layers II/III, and were distributed according to a similar pattern in both primary and secondary visual cortex. During the rat postnatal life, neurons positive for SOM-mRNA were present in the visual cortex as early as postnatal day 0 (P0), occuping layer VI of the cortex and progressively appearing in upper layers. During the third postnatal week they acquired the distribution pattern of the adult. A transient population of NPY-producing neurons was present in the first week of postnatal life, slowly disappearing during the second and replaced by a neuronal distribution similar to that of the adult by the third postnatal week. Neurons expressing VIP-mRNA appeared as late as P7 and their numbers were progressively increased till they reached an adult like distribution at the end of the second postnatal week. Animals pretreated with 6-OHDA from day 1 to day 4 postnatal were shown, by HPLC with electrochemical detection, to have a 93% depletion of cortical noradrenaline (NA). The neuronal distribution of the three peptides remained unaffected by the NA-depletion. Furthermore, the level of peptidergic expression in individual neurons is not altered in the absence of NA. Thus, this study suggests that the early and widespread NA-ergic projection does not regulate the expression of SOM, NPY or VIP in the visual cortex of the rat.

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