Article1983

Hypothalamic and Pituitary Sites of Action of Oxytocin to Alter Prolactin Secretion in the Rat*

Michael D. Lumpkin · Willis K. Samson · Samuel M. McCann

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Neuroendocrine regulation and behavior Growth Hormone and Insulin-like Growth Factors Hypothalamic control of reproductive hormones

Abstract

To determine whether oxytocin (OT) could alter the release of PRL and other hormones from the anterior pituitary gland, the effects of OT were examined in two in vitro and two in vivo test systems. Cells dispersed from anterior pituitary glands of intact adult male rats were incubated in medium containing OT at doses of 10(-8), 10(-7), 10(-6), and 10(-5) M in two trials. OT stimulated PRL release 1.5-fold (P less than 0.01) and 2- to 3-fold (P less than 0.001) above control levels at 10(-8) and 10(-7) M doses, respectively, thus indicating a dose-dependent relationship. Higher doses did not produce a further elevation above that obtained with 10(-7) M OT. Arginine vasopressin (AVP) caused a slight decrease in PRL release from dispersed cells while TRH produced a small (25%), significant, but nondose-related increase in PRL release. Hemipituitary glands from adult male rats, incubated with 10(-6) and 10(-5) M OT, released twice as much PRL (P less than 0.01) into the medium as paired controls, but 10(-7) M OT was ineffective. The iv injection of 1 or 10 micrograms OT into conscious male rats elevated plasma PRL by 50% (P less than 0.05) or 500% (P less than 0.001), respectively, above basal values at 5 min only. Vehicle or 0.1 microgram OT were without effect. When 0.1 microgram OT was microinjected into the third ventricle (3V) of conscious male rats, it paradoxically reduced plasma PRL by 40% at 30 min (P less than 0.05), whereas 1 microgram OT significantly lowered PRL at 5-60 min, with the maximum suppression (60%, P less than 0.001) occurring at 30 min. These latter findings may indicate that an ultrashort loop feedback mechanism exists whereby exogenous OT decreases hypothalamic OT secretion, thereby reducing the OT stimulus for PRL release. The specificity of the OT effect on PRL was attested to by the failure of OT to alter significantly FSH, LH, and TSH in each system. GH was unchanged except that 3V-injected OT (1 microgram only) elevated (P less than 0.001) plasma GH at 15-30 min. These results support the view that OT acts directly on the cells of the anterior pituitary gland at low to high doses to release PRL specifically and in a dose-related fashion. In contrast, 3V injection of OT reduces PRL secretion, thereby suggesting that OT may decrease its own neurosecretion by ultrashort loop feedback and thus reduce an OT stimulus for PRL release.

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References

1977The origin of the vasopressinergic and oxytocinergic fibres of the external region of the median eminence of the rat hypophysis 1981Connections of the hypothalamic paraventricular necleus with the neurohypophysis, median eminence, amygdala, lateral septum and midbrain periaqueductal gray: An electrophysiological study in the rat

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1987Oxytocin Gene Expression in Discrete Hypothalamic Magnocellular Cell Groups Is Stimulated by Prolonged Salt Loading 2001Vasopressin- and oxytocin-induced activity in the central nervous system: electrophysiological studies using in-vitro systems 1995Prolactin Release After Mating and Genitosensory Stimulation in Females*1995Oxytocin receptor messenger ribonucleic acid: characterization, regulation, and cellular localization in the rat pituitary gland 1987Posterior Pituitary Lobectomy Abolishes the Suckling-Induced Rise in Prolactin (PRL): Evidence for a PRLReleasing Factor in the Posterior Pituitary*2006Expression of the long form of the prolactin receptor in magnocellular oxytocin neurons is associated with specific prolactin regulation of oxytocin neurons 1988Role of Oxytocin on Prolactin Secretion during Proestrus and in Different Physiological or Pharmacological Paradigms 1984Oxytocin potentiates the ACTH-releasing activity of CRF(41) but not vasopressin

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