The role of the mu(1) opioid receptor subtype in the regulation of prolactin and growth hormone secretion by Beta-endorphin in female rats: studies with naloxonazine

Neurogenesis and sexual behavior

Different conditions induce proliferation, migration and integration of new neurons in the adult brain. This process of neurogenesis is a clear example of long lasting plastic changes in the brain of different species. Sexual behavior is a motivated behavior that is crucial for the survival of the species, but an individual can spend all his life without displaying sexual behavior. In the present review, we briefly describe some of the effects of pheromones on neurogenesis. We review in detail studies describing the effects of sexual behavior in both males and females on proliferation, migration and integration of new cells and neurons. It will become evident that most of the studies have been done in rodents, assessing the effects of this behavior on neurogenesis within the dentate gyrus of the hippocampus and in the subventricular zone - rostral migratory stream - olfactory bulb system.

Neurogenesis in the olfactory bulb induced by paced mating in the female rat is opioid dependent

The possibility to control the rate of sexual stimulation that the female rat receives during a mating encounter (pacing) increases the number of newborn neurons that reach the granular layer of the accessory olfactory bulb (AOB). If females mate repeatedly, the increase in the number of neurons is observed in other regions of the AOB and in the main olfactory bulb (MOB). It has also been shown that paced mating induces a reward state mediated by opioids. There is also evidence that opioids modulate neurogenesis. In the present study, we evaluated whether the opioid receptor antagonist naloxone (NX) could reduce the increase in neurogenesis in the AOB induced by paced mating. Ovariectomized female rats were randomly divided in 5 different groups: 1) Control (not mated) treated with saline, 2) control (not mated) treated with naloxone, 3) females that mated without controlling the sexual interaction (no-pacing), 4) females injected with saline before pacing the sexual interaction and 5) females injected with NX before a paced mating session. We found, as previously described, that paced mating induced a higher number of new cells in the granular layer of the AOB. The administration of NX before paced mating, blocked the increase in the number of newborn cells and prevented these cells from differentiating into neurons. These data suggest that opioid peptides play a fundamental role in the neurogenesis induced by paced mating in female rats.

Mu opioids and their receptors: evolution of a concept

Opiates are among the oldest medications available to manage a number of medical problems. Although pain is the current focus, early use initially focused upon the treatment of dysentery. Opium contains high concentrations of both morphine and codeine, along with thebaine, which is used in the synthesis of a number of semisynthetic opioid analgesics. Thus, it is not surprising that new agents were initially based upon the morphine scaffold. The concept of multiple opioid receptors was first suggested almost 50 years ago (Martin, 1967), opening the possibility of new classes of drugs, but the morphine-like agents have remained the mainstay in the medical management of pain. Termed mu, our understanding of these morphine-like agents and their receptors has undergone an evolution in thinking over the past 35 years. Early pharmacological studies identified three major classes of receptors, helped by the discovery of endogenous opioid peptides and receptor subtypes-primarily through the synthesis of novel agents. These chemical biologic approaches were then eclipsed by the molecular biology revolution, which now reveals a complexity of the morphine-like agents and their receptors that had not been previously appreciated.

Orphanin FQ/nociceptin is a physiological regulator of prolactin secretion in female rats

Orphanin FQ/nociceptin (OFQ/N), the most recently identified endogenous opioid peptide, stimulates prolactin secretion in both male and female rats. OFQ/N, however, did not elicit this stimulatory effect through the mu-, delta-, or kappa-opiate receptor subtype. The role OFQ/N plays in prolactin regulation under physiological conditions and its mechanism of action are not known. The purpose of these studies was to determine the physiological significance and pharmacological specificity of the prolactin secretory response to OFQ/N. In addition, the role of the tuberoinfundibular dopaminergic (TIDA) neurons in mediating this response was examined. Opioid receptor-like-1 (ORL-1) receptors were blocked by pretreatment with compound B (Comp B), a purported OFQ/N antagonist, or receptor synthesis was disrupted by pretreatment with ORL-1 receptor antisense oligonucleotides. The prolactin secretory response to OFQ/N administration in diestrous females was measured. Furthermore, the suckling-induced prolactin response was also determined after Comp B pretreatment. TIDA neuronal activity was quantified in diestrous female rats to determine whether OFQ/N stimulates prolactin release by inhibiting TIDA neurons. OFQ/N significantly inhibited the TIDA neurons by 1 min, preceding the prolactin secretory response. Both Comp B and antisense pretreatment blocked the stimulatory effects of OFQ/N on prolactin release, and Comp B abolished the suckling-induced prolactin response. These studies indicate that OFQ/N is a potent stimulus for prolactin secretion in female rats and that it mediates this effect by rapid and transient inhibition of TIDA neuronal activity. Furthermore, OFQ/N plays a physiologically significant role in the regulation of prolactin secretion during lactation, and it mediates its effects via actions at the ORL-1 receptor subtype.

Involvement of nociceptin/orphanin FQ in release of hypothalamic GnRH mediated by ORL1 receptor in ovariectomized rats

AIM

To investigate effect of the nociceptin/orphanin FQ (OFQ) on hypothalamus gonadotropin-releasing hormone (GnRH) release in ovariectomized (OVX) rats.

METHODS

GnRH radioimmunoassay (RIA) was used to study the effect of OFQ on GnRH release in hypothalamus slices in vitro. Push-pull perfusion and intracerebroventicular (icv) injection were used to examine the effect of OFQ on GnRH release in the hypothalamus medial preoptic area (POA) in vivo. Ovariectomies were performed on female Sprague-Dawley rats, and their plasma luteinizing hormone (LH) levels were measured after icv injection of OFQ with or without [Nphe1]NC(1-13)NH2, a competitive antagonist of opioid receptor-like1 receptor (ORL1 receptor). Reverse transcription-polymerase chain reaction (RT-PCR) was used to investigate the expression of the ORL1 receptor in rat pituitary.

RESULTS

GnRH release from hypothalamus slices was inhibited 90 min after the administration of 2 mmol/L and 20 mmol/L OFQ (P<0.05). Accordingly, GnRH release from hypothalamus POA was also significantly reduced by the injection of 0.2 mmol/L and 2 mmol/L OFQ. Plasma LH levels were also decreased significantly 2 h after icv injection of 20 nmol OFQ in OVX rats (P<0.05) and this effect could be abolished by pretreatment with 20 nmol [Nphe1]NC(1-13)NH2, that is, NC13. More interestingly, plasma LH levels in OVX rats increased markedly 2 h after icv injection of 100 nmol and 200 nmol NC13. RT-PCR analysis further revealed that the ORL1 receptor was not expressed in the pituitary of OVX rats.

CONCLUSION

Central administration of nociceptin/orphanin FQ might inhibit the release of hypothalamic GnRH and decrease the plasma LH levels through ORL1 receptors in OVX rats.

[Phe(1)Psi(CH(2)-NH)Gly(2)]NC(1-13)NH(2) does not antagonize orphaninFQ/nociceptin-induced prolactin release

The specificity of the orphaninFQ (OFQ)/nociceptin (N)-induced prolactin increase was determined in male and female rats by pretreating animals with different doses of [Phe(1)Psi(CH(2)-NH)Gly(2)]NC(1-13)NH(2), a compound originally reported to be a specific OFQ/N antagonist. In addition, the effect of naloxone pretreatment on OFQ/N-induced prolactin release was examined to determine if OFQ/N's effects were mediated by opiate receptors. Furthermore, dose response studies using [Phe(1)Psi(CH(2)-NH)Gly(2)]NC(1-13)NH(2) only were performed to determine potential agonist activity of this drug. Finally, growth hormone (GH) levels were determined as an index of specificity of the prolactin response. Our results confirm previous findings that OFQ/N potently stimulates prolactin release and that a gender difference exists in the magnitude of the response, with females showing a much greater response than male rats. The endocrine response is specific because OFQ/N potently stimulated prolactin, but not GH secretion. The prolactin response is not mediated by actions at opiate receptors because naloxone did not inhibit OFQ/N's effects on prolactin release. However, [Phe(1)Psi(CH(2)-NH) Gly2]NC(1-13) NH(2) did not antagonize OFQ/N's effects on prolactin release. Indeed, this drug acted as a potent agonist. Demonstrating pharmacological specificity of OFQ/N's effects on prolactin release awaits the development of more selective, specific antagonists.

Immunoneutralization of beta-endorphin blocks prolactin release during suckling without affecting tuberoinfundibular dopaminergic neural activity

The effect of immunoneutralization of beta-endorphin on the suckling-induced prolactin increase and on the activity of the tuberoinfundibular dopaminergic (TIDA) neurons was determined in lactating female rats between days 8 - 12 post-partum. Two antisera were used in the immunoneutralization studies. Both were specific for beta-endorphin, exhibiting little cross reactivity with met- or leu-enkephalin or dynorphin. Antisera to beta-endorphin completely abolished the suckling-induced prolactin increase indicating that this endogenous opioid peptide is involved in this response. Suckling significantly inhibited DOPA accumulation in the median eminence and antiserum to beta-endorphin did not prevent this inhibition. Additionally, 5-endorphin antiserum significantly reduced TIDA neural activity even in pup-deprived dams. These results indicate that beta-endorphin is involved in the prolactin secretory response to suckling but that inhibition of TIDA neuronal activity is not its mechanism of action. Other possible mechanisms are discussed.