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 part of the embryonic raphe grafted to the basal hypothalamus of 5,7-dihydroxytryptamine-denervated rats produced incomplete serotonin (5-HT) re-innervation of the suprachiasmatic nucleus (SCN) as opposed to hyper-innervation of the supraoptic nucleus (SON). We took advantage of this experimental model to investigate whether the graft-derived, 5-HT fibres retained normal ultrastructural features, and, particularly, a normal density of synaptic junctions, irrespective of the extent of target re-innervation. The intrinsic features of immunostained, graft-derived 5-HT axonal varicosities in both the SCN (ventral portion) and the SON were essentially similar to those exhibited by the respective endogenous innervation. Analysis of well-preserved varicosities in uninterrupted series of thin sections allowed us to evaluate directly the proportions of junctional to non-junctional 5-HT varicosities in both regions. Synaptic incidences were also remarkably conserved after grafting (45.5% in the SCN versus 38.5% in the SON; 48% and 38% in normal rats, 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 transplanted 5-HT neurons essentially retained normal ultrastructural features in their target territories, with a normal incidence of synaptic junctions. The data provide further support to the hypothesis that the innervation territory is the major determinant of the frequency with which ingrowing 5-HT fibres make synaptic junctions.
Boulaïch
S
S
INSERM U297, Institut Fédératif Jean Roche, Faculté de médecine, Marseille, France.
Daszuta
A
A
Geffard
M
M
Bosler
O
O
D015116:5,7-Dihydroxytryptamine; D000818:Animals; D001369:Axons / Q000502:physiology / Q000648:ultrastructure; D016380:Brain Tissue Transplantation / Q000502:physiology*; D003712:Dendrites / Q000502:physiology / Q000648:ultrastructure; D003714:Denervation; D004525:Efferent Pathways / Q000502:physiology*; D016332:Fetal Tissue Transplantation / Q000502:physiology; D007150:Immunohistochemistry; D008297:Male; D008854:Microscopy, Electron; D009474:Neurons / Q000502:physiology*; D011903:Raphe Nuclei / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000032:analysis / Q000378:metabolism*; D013493:Suprachiasmatic Nucleus / Q000502:physiology*; D013495:Supraoptic Nucleus / Q000502:physiology*; D013569:Synapses / Q000502:physiology* / Q000648:ultrastructure
D015116:5,7-Dihydroxytryptamine; D012701:Serotonin