Serotonergic innervation of the pituitary pars intermedia of xenopus laevis

At this point three brain centres are thought to be involved in the regulation of the melanotrope cells of the pituitary pars intermedia of Xenopus laevis: the magnocellular nucleus, the suprachiasmatic nucleus and the locus coeruleus. This study aims to investigate the existence of a fourth, serotonergic, centre controlling the melanotrope cells. In-vitro superfusion studies 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 cells. Retrograde neuronal tract tracing experiments, with the membrane probe FAST Dil applied to the pars intermedia, reveals retrogradely labelled neurones in the magnocellular nucleus, the suprachiasmatic nucleus, the locus coeruleus and the raphe nucleus. Of these brain centres, after immunocytochemistry only the raphe nucleus revealed serotonin-immunoreactive cell bodies. In addition, serotonin-immunoreactive cell bodies were found in the nucleus of the paraventricular organ, the posteroventral tegmental nucleus and the reticular istmic nucleus. In the pituitary, the pars nervosa, pars intermedia and pars distalis all reveal serotonin-immunoreactive nerve fibres. With immunocytochemical double-labelling for tyrosine hydroxylase and serotonin 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 neurointermediate lobes with serotonin, tyrosine hydroxylase and serotonin appear to 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 are innervated by a 5-HT network originating in the raphe nucleus; this network represents the first identified stimulatory input to the pars intermedia of this species.