Serotonergic innervation of the pituitary pars intermedia of Xenopus laevis

At this point three brain centres are thought to be involved in the regulat ion of the melanotrope cells of the pituitary pars intermedia of Xenopus la evis: the magnocellular nucleus, the suprachiasmatic nucleus and the locus coeruleus. This study aims to investigate the existence of a fourth, seroto nergic, centre controlling the melanotrope cells. In-vitro superfusion stud ies show that serotonin has a dose-dependent stimulatory effect on peptide release (1.6 x basal level at 10(-6) M serotonin) from single melanotrope c ells. Retrograde neuronal tract tracing experiments, with the membrane prob e FAST Dir applied to the pars intermedia, reveals retrogradely labelled ne urones in the magnocellular nucleus, the suprachiasmatic nucleus, the locus coeruleus and the raphe nucleus. Of these brain centres, after immunocytoc hemistry only the raphe nucleus revealed serotonin-immunoreactive cell bodi es. In addition, serotonin-immunoreactive cell bodies were found in the nuc leus of the paraventricular organ, the posteroventral tegmental nucleus and the reticular istmic nucleus. In the pituitary, the pars nervosa, pars int ermedia and pars distalis all reveal serotonin-immunoreactive nerve fibres. With immunocytochemical double-labelling for tyrosine hydroxylase and sero tonin no colocalization of serotonin and tyrosine hydroxylase was observed in cell bodies in the brain, and in the pituitary hardly any colocalization was found in the nerve fibres, However, after in-vitro loading of neuroint ermediate robes with serotonin, tyrosine hydroxylase and serotonin appear t o coexist in a fibre network in the pars intermedia. On the basis of these data we propose that the melanotrope cells in the Xenopus pars intermedia a re innervated by a 5-HT network originating in the raphe nucleus; this netw ork represents the first identified stimulatory input to the pars intermedi a of this species.