RECIPROCAL SYNAPTIC CONNECTIONS BETWEEN NEUROTENSIN-IMMUNOREACTIVE AND TYROSINE-HYDROXYLASE-IMMUNOREACTIVE NEURONS IN THE MEDIOBASAL HYPOTHALAMUS OF THE GUINEA-PIG

Neurotensin (NT) and dopamine are two neurotransmitters which are pres ent in the hypothalamus of mammals and are often distributed in identi cal areas. In particular, in the periventricular anterior hypothalamus and in the arcuate nucleus, images of apposition between perikarya an d fibers containing dopamine or neurotensin have frequently been obser ved at the light microscope level. The aim of this study was to answer , at the ultrastructural level in the A12 and A14 catecholaminergic ce ll groups, the question as to the existence of the possible synaptic n ature of such contacts. To this end, NT and tyrosine hydroxylase (TH) were simultaneously visualized using double pre-embedding immunocytoch emical methods. In the A12 arcuate area, synaptic contacts were demons trated between TH-immunoreactive terminals and NT-labeled perikarya an d dendrites. The opposite pattern, i.e., NT-stained terminals synapsin g onto TH-positive neurons, was also observed. in contrast, only NT sy naptic inputs onto TH-stained cell bodies could be demonstrated in the hypothalamic periventricular nucleus. In addition, immunoreactive ter minals stained for NT or TH were observed to make synaptic contacts wi th perikaryal profiles stained for the same antigen. These results dem onstrate a strong synaptic NT input onto the dopaminergic neurons of t he mediobasal hypothalamus and suggest a reciprocal influence, at leas t in part, of catecholaminergic terminals on arcuate NT-containing neu rons.