An analysis of physiological mechanisms underlying the antigonadotropic action of intracranial prolactin in ring doves

Intracerebroventricular (ICV) injections of prolactin (PRL) exert potent an tigonadal and antigonadotropic effects in ring doves (Streptopelia risoria) at doses that are insufficient to stimulate prolactin-dependent crop growt h. To explore the physiological basis of these effects, we tested the abili ty of ICV-injected PRL to influence pituitary responsiveness to chicken gon adotropin-releasing hormone-I (cGnRH-I) and to alter GnRH content and conce ntration in the preoptic area (POA) and median eminence (ME). cGnRH-I-induc ed changes in plasma LH were monitored by radioimmunoassay (RIA) in photost imulated male doves after they received five daily ICV injections of ovine PRL (1 mu g/2 mu l) or saline vehicle. Although PRL treatment reduced basal plasma LH levels and testes weight, it did not reduce the amount or alter the pattern of LH released in response to a bolus injection of cGnRH-I. Thi s suggests that ICV PRL does not suppress LH by reducing pituitary responsi veness to GnRH, In two subsequent studies, GnRH content (ng/region) and con centration (pg/mu g protein) in the POA and ME were measured in male doves by RIA and by competitive enzyme immunoassay after 5 days of ICV PRL or veh icle treatment. Although ICV PRL reduced plasma LH levels in both studies, no significant PRL-induced alterations in GnRH content or concentration wer e apparent. In a final study, PRL-treated female doves had lower plasma LH levels than vehicle-treated control females at 12 and 24 h after a single I CV injection. GnRH content of the POA was also lower in PRL-treated females than in controls at 24 h. However, the two treatment groups did not differ in POA or ME GnRH content at earlier postinjection sampling intervals. Ana lysis of GnRH concentration data revealed no treatment group differences in either region at any sampling interval (1, 6, 12, or 24 h post-PRL. inject ion), Collectively, these results are consistent with the idea that ICV-inj ected PRL acts at the level of the CNS to inhibit the reproductive system. However, the nature of the alterations involved remains to be clarified. Pl ausible hypotheses are (1) that ICV PRL suppresses the gonadal axis by infl uencing the activity of GnRH neurons at brain sites other than the POA or M E or (2) that PRL alters the synthesis, storage, degradation, and/or releas e of GnRH in the POA or ME, but the dynamic changes involved are not reflec ted in integrated, steady-state measures such as peptide content or concent ration in tissue. (C) 1999 Academic Press.