Mapping and Analysis of Receptors for Neurohypophyseal Peptides Present in the Brain
Vasopressin (AVP) and oxytocin (OT) meet most of the criteria which are usually applied to assign a role as a neurotransmitter or neuromodulator to a given compound. 1) In addition to their presence in the hypothalamo-hypophysial system, they are also found in neural pathways that project to various areas of the central nervous system (Sofroniew, 1981). 2) They can be released in a calcium dependent manner from several of these extra-hypothalamic areas (Buijs, 1983). 3) They affect the electrical activity of single neurones (Miihlethaler et al., 1985), as well as integrative processes such as temperature control, blood pressure regulation, avoidance behavior, etc. (Kovacs et al., 1987). The commercial availability of tritium-labelled AVP and OT has allowed the detection of high affinity binding sites for these peptides in the brain (Audigier and Barberis, 1985; Jard et al., 1987; Van Leeuwen, 1987). The experimental data presented in this chapter show the type of information which can be obtained by in vitro light microscopic autoradiography. Four points are considered. First, the distribution of high affinity binding sites is described for the rat and the guinea-pig brain; surprisingly, marked differences in their location apparently exist between these two species. Second, brain AVP and OT binding sites in the rat brain are compared to peripheral receptors, using synthetic structural analogues with enhanced selectivity for OT receptors and for the known subtypes of AVP receptors. Third, a case of long term receptor regulation is described, namely the effects of castration and of gonadal steroids on the density of [H]OT binding in the rat brain. Finally, preliminary results obtained with a newly synthetized and iodinated OT receptor ligand are presented and compared with those obtained with tritiated oxytocin.
KeywordsStria Terminalis Amygdaloid Nucleus High Affinity Binding Site Anterior Olfactory Nucleus External Plexiform Layer
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