Why Are Several Inhibitory Transmitters Present in the Innervation of Pituitary Melanotrophs?

Abstract

The aim of the present experiments was to determine whether some rationale for the presence of the several inhibitory neurotransmitters in the innervation to the toad melanotroph might be found in differences in their individual effects or in possible cooperative interactions affecting secretion. Measurements of peptide release from isolated, perifused neurointemediate lobes of the toad Xenopus laevis showed that each of the three identified inhibitory transmitters, dopamine, GABA and NPY, was able to inhibit secretion profoundly and no less effectively than omission of Ca. Moreover, the inhibitory effects were rather similar in onset, duration and recovery. Furthermore, there was no evidence of any cooperative interactions when the transmitters were given in various combinations. And finally, the inhibitory response to each of the transmitters was abolished by pretreatment with pertussis toxin. While not excluding differential postsynaptic effects on other parameters of melanotroph function, the similarities observed have encouraged alternative speculations on the significance of the apparent redundancy of inhibitory transmitters.

Dual Action of GABA_A Receptors on the Secretory Process of Melanotrophs of <i>Xenopus laevis</i>

“…Besides its central function, the GABAa receptor has been shown to have important neuroendocrine actions in the regulation of pituitary hormones [4,5] and is for instance directly involved in the regulation of a-melanocyte-stimulating hormone (a-MSH) secretion from mela notrophs of the pituitary pars intermedia, both in mam mals [6][7][8] and amphibians [9][10][11][12]. Secretion of a-MSH from superfused intermediate lobes can be easily mea sured.…”

The Significance of Multiple Inhibitory Mechanisms Converging on the Melanotrope Cell of Xenopus laevisa

“…1,3 Efforts to attach functional significance to these multiple inhibitory receptor mechanisms, by looking for potentiating interactions among them, showed in fact that the inhibitions induced by these receptors are strictly additive. 4,5…”

“…1,3 Efforts to attach functional significance to these multiple inhibitory receptor mechanisms, by looking for potentiating interactions among them, showed in fact that the inhibitions induced by these receptors are strictly additive. 4,5 The adenylyl cyclase system plays a central role in the regulation of α-MSH secretion from the Xenopus melanotrope cell. The D 2 , Y 1 , and GABA B receptor mechanisms have all been shown to inhibit cyclic adenosine monophosphate (cAMP) production in this cell while the release-stimulating neuropeptide corticotropin-releasing hormone (CRH) stimulates production of the cyclic nucleotide.…”

Cholinergic Regulation of the Pituitary: Autoexcitatory Control by Acetylcholine of Melanotrope Cell Activity in Xenopus laevis

“…A number of neuronal factors with both inhibitory and stimulatory capacity are known to act on the pituitary to regulate the secretion of pituitary hormones from both the intermediate and anterior lobe of the pituitary. 1,2 In addition to these neurotransmitters/peptides, intercellular interactions in the anterior pituitary control the secretion of hormones from corticotropes, lactotropes, gonadotropes and growth hormone‐producing cells (for review see Refs. 3 and 4).…”