SYNAPTIC CONNECTIVITY OF SEROTONIN GRAFT EFFERENTS IN THE SUPRACHIASMATIC AND SUPRAOPTIC NUCLEI OF THE HYPOTHALAMUS

We have previously reported that a cell suspension from the rostral pa rt of the embryonic raphe grafted to the basal hypothalamus of 5,7-dih ydroxytryptamine-denervated rats produced incomplete serotonin (5-HT) re-innervation of the suprachiasmatic nucleus (SCN) as opposed to hype r-innervation of the supraoptic nucleus (SON). We took advantage of th is experimental model to investigate whether the graft-derived, 5-HT f ibres retained normal ultrastructural features, and, particularly, a n ormal density of synaptic junctions, irrespective of the extent of tar get re-innervation. The intrinsic features of immunostained, graft-der ived 5-HT axonal varicosities in both the SCN (ventral portion) and th e SON were essentially similar to those exhibited by the respective en dogenous innervation. Analysis of well-preserved varicosities in unint errupted series of thin sections allowed us to evaluate directly the p roportions of junctional to non-junctional 5-HT varicosities in both r egions. Synaptic incidences were also remarkably conserved after graft ing (45.5% in the SCN versus 38.5% in the SON; 48% and 38% in normal r ats, respectively). Synapses were primarily reestablished on dendritic shafts, which also were identified as the major post-synaptic targets of the normal 5-HT innervations. We noted, however, a tendency toward increased numbers of symmetrical versus asymmetrical synapses in both the SCN and SON of grafted rats. Thus, irrespective of whether hypo- or hyper-innervation patterns developed post-grafting, the transplante d 5-HT neurons essentially retained normal ultrastructural features in their target territories, with a normal incidence of synaptic junctio ns. The data provide further support to the hypothesis that the innerv ation territory is the major determinant of the frequency with which i ngrowing 5-HT fibres make synaptic junctions.