ULTRASTRUCTURAL FEATURES OF SEROTONIN NEURONS GRAFTED TO ADULT-RAT HIPPOCAMPUS - AN IMMUNOCYTOCHEMICAL ANALYSIS OF THEIR CELL-BODIES AND AXON TERMINALS

Serotonin (5-HT) immunocytochemistry was used at the electron microscopic level to examine 5-HT neurons reinnervating and hyperinnervating the hippocampus of adult rat, three to four months after a total 5-HT denervation and subsequent graft of embryonic raphe cells. The study focused on immunostained nerve cell bodies, dendrites and axon terminals (varicosities) in the core of grafts, and on a large single section sampling of axon terminals from a CA3 and a dentate gyrus sector of the outgrowth, which were systematically compared to the endogenous 5-HT innervation of the same regions described in a companion paper. The shape, size and synaptic investment of the grafted 5-HT somata and their dendrites resembled those of in situ 5-HT neurons. Clusters of small, clear vesicles were sometimes seen along these 5-HT dendrites. 5-HT axonal varicosities were fairly numerous in the core. A few were directly apposed to, or made asymmetrical synaptic contact with the immunostained dendrites and perikarya, but the vast majority showed no indication of junctional specialization (synaptic incidence of 19%, as stereologically extrapolated for whole varicosities). Occasional myelinated 5-HT axons were also present in the core of grafts. In the two outgrowth sectors, the graft-borne 5-HT varicosities were similar in size, content, frequency of synaptic contact and identity of junctional and appositional elements, irrespective of their laminar location. Moreover, none of these parameters were significantly different from those of the endogenous innervation. Notably, in spite of their excessive number, the synaptic incidence of the outgrowth 5-HT varicosities remained inferior to 20%. The similarity between the respective microenvironments of the supernumerary, graft-borne 5-HT terminals and of their normal counterparts could only be explained by a random intratissular distribution of these varicosities in both the normal and the grafted hippocampus. Thus, in spite of their transplantation and growth into an abnormal milieu, and the fact that they hyperinnervated the host tissue, the grafted embryonic 5-HT neurons appeared committed to express a particular set of intrinsic and relational morphological features corresponding to their normal adult characteristics.