Jamshid Rabii

INHIBITION OF TUBEROINFUNDIBULAR DOPAMINERGIC NEURAL ACTIVITY DURING SUCKLING : INVOLVEMENT OF MU AND KAPPA OPIATE RECEPTOR SUBTYPES

Previous studies have shown that mu (μ) and kappa (κ) opioid antagonists inhibit suckling‐induced prolactin release. Prolactin responses elicited by pup suckling or opioid administration are mediated, at least in part, by suppression of dopamine (DA) release from tuberoinfundibular dopaminergic (TIDA) neurons in the hypothalamus. We examined the effects of the μ opiate receptor antagonist, β‐funaltrexamine (β‐FNA), and the κ opiate receptor antagonist, nor‐binaltorphimine (nor‐BNI) on the activity of TIDA neurons in lactating rats. TIDA neuronal activity was determined by measuring DOPA accumulation in the caudate putamen (CP) and median eminence (ME). The effects of opioid antagonist treatment were determined in pup‐deprived (low circulating prolactin levels) or pup‐suckled rats (high circulating prolactin levels). The accumulation of 5‐hydroxytryptophan (5‐HTP) in the medial preoptic area (MPOA), the anterior hypothalamus (AH) and the median eminence (ME) was quantified as an index of serotonergic activity in the same animals for comparative purposes.

Beta-endorphin regulation of luteinizing hormone-releasing hormone release at the median eminence in ewes : immunocytochemical and physiological evidence

Beta-endorphin (beta-END) is an inhibitory factor in the neuroendocrine control of luteinizing hormone (LH) release and thus, presumably also of hypophysiotropic luteinizing hormone-releasing hormone (LHRH) release. In order to address if the median eminence (ME) is a site of beta-END action, we studied its functional role in ewes by assessing: (a) the hypothalamic distribution of beta-END using immunolabeling and by comparing this distribution with our data on the localization of LHRH; (b) the ME in vivo release of LHRH and beta-END during the luteal (day 12) and the follicular (day 15) phases of the estrous cycle; (c) the in vivo release of LHRH from the posterior-lateral ME, as assessed by push-pull cannula (PPC) sampling, before, during, and after infusion of increasing doses of beta-END or naloxone through the PPC, during the follicular phase; and (d) the in vivo release of ME-LHRH and serum LH, before, during, and after infusion of beta-END or naloxone in luteal and follicular ewes. In the ewe, beta-END-containing perikarya are located in and around the arcuate nucleus. Their processes are also present in the diagonal band, medial septal nucleus, and medial and lateral hypothalamic areas, including the preoptic region and posterior ME. Perikarya containing LHRH are located in the preoptic area and project also to the ME, providing opportunities for synaptic interactions between beta-END and LHRH-containing perikarya and processes at these levels. ME in vivo release of LHRH and beta-END increase from the luteal (low LH/high progesterone, P4) to the follicular phase (high LH/low P4). In follicular ewes, in vivo LHRH and LH release is decreased, in a dose-dependent manner, by beta-END infused through the PPC probe into the posterior-lateral ME. In contrast, infusion of naloxone under similar conditions increases LHRH and LH release, also in a dose-dependent fashion. The inhibitory effect of beta-END on LHRH and LH, as well as the stimulatory effect of naloxone on LHRH and LH, were only marginally apparent in luteal ewes. These results suggest that the ME is a major control site where beta-END exerts its influence on hypophysiotropic LHRH release. The strength of this inhibitory effect apparently increases throughout the follicular phase, and might prevent the premature onset of the preovulatory surge of LHRH and LH.

Feed Restriction in Prepubertal Lambs: Effect on Puberty Onset and on in vivo Release of Luteinizing-Hormone-Releasing Hormone, Neuropeptide Y and Beta-Endorphin from the Posterior-Lateral Median Eminence

The exact nature of the interaction between energy balance and reproduction is still elusive. Theoretically, nutrition-related variables must reach the hypothalamic luteinizing-hormone-releasing hormone (LHRH) network and/or its neuronal inputs, to alter plasma luteinizing hormone (LH) and therefore reproductive activity. In an attempt to assess the potential mechanism of such interaction at the median eminence (ME) level, the area of hypophysiotropic LHRH neuronal terminals and release, we used a decreased caloric intake lamb model which delays the onset of puberty. Thus, we determined the in vivo release of neuropeptides, by push-pull cannula (PPC) sampling from the posterior-lateral ME, in feed-restricted (FR) ewe lambs and in full-fed (FF), age-matched, contemporary control animals. Specifically, we assessed: (1) serum LH and ME in vivo release of LHRH, beta-endorphin (beta-END) and neuropeptide Y (NPY); beta-END and NPY are two putative neuronal inputs to LHRH neuronal terminals at the ME, reported to be involved in the control of both reproduction and feed intake; (2) the effect that exogenous infusion of beta-END through the PPC might have on the release of ME LHRH and NPY, and on plasma LH. In contrast to other works, the present results were obtained in lambs with intact ovaries. Furthermore, FR lambs were always compared statistically with FF contemporary paired controls that had attained puberty. Feed restriction decreased ME LHRH release, lowered plasma LH and prevented the onset of puberty. The changes induced by feed restriction in both LHRH and LH release were associated predominantly with decreases in pulse amplitude, rather than alterations in pulse frequency. The decreased LHRH and LH release occurred in the presence of a decreased beta-END but unchanged NPY release from the ME. Exogenous infusion of beta-END into the posterior-lateral ME decreased both LHRH and NPY release from this site and decreased plasma LH. In conclusion, decreased caloric intake lowers LH release and prevents puberty onset by decreasing the amplitude of the LHRH output from the hypothalamic hypophysiotropic network. A compensatory but unsuccessful mechanism for the FR status might be a lower beta-END-inhibitory tone on ME LHRH neuronal terminals. The unchanged release of NPY at this site supports the specificity of the changes induced by feed restriction on LHRH and beta-END in vivo release.

Aminergic involvement in the control of luteinizing hormone secretion in the domestic fowl

Abstract Neuropharmacologic agents have been used, in vivo , to investigate the possible involvement of the biogenic amines in the hypothalamic control of luteinizing hormone (LH) secretion in the domestic fowl. Inhibitors of norepinephrine (NE)/dopamine (DA) synthesis (for DA NE α-methyl- p -tyrosine, α-mpt; for NE diethyldithiocarbamic acid, DDC) and granule reuptake of catecholamines (reserpine) and the α-adrenergic antagonist (phenoxybenzamine) induced reductions in the circulating concentrations of LH. The administration of p -chlorophenylalanine (an inhibitor of 5-hydroxytryptamine (5HT) synthesis) and methysergide (a 5HT antagonist) also depressed plasma levels of LH. Neither β-adrenergic (propranolol) nor cholinergic (atropine) blockers affected circulating LH levels. These data suggest the involvement of a catecholamine (probably acting via α-adrenergic receptors), together with some serotoninergic components in the hypothalamic control of LH in the chicken.

Do pup ultrasonic cries provoke prolactin secretion in lactating rats

Marked prolactin (PRL) secretion in response to the ultrasonic distress vocalizations of rat pups in lactating dams deprived of their pups for 6 hr was reported by others. In two experiments, this phenomenon could not be confirmed under our testing conditions at either 1 or 2 weeks postpartum, although behavioral responses to the ultrasounds were noted. In addition, suckling-induced PRL secretion did not differ consistently as a function of the tape recording (pup ultrasounds, 45 kHz artificially produced ultrasounds, or blank tape) heard prior to the return of pups. The functional significance of rat pup ultrasounds is considered.

Inhibition of suckling-induced prolactin release by μ- and κ-opioid antagonists

Abstract Evidence suggests that endogenous opioid peptides (EOP) are involved in the hyperprolactinemia and suppression of leuteinizing hormone (LH) release associated with lactation. To address this hypothesis, we investigated the effects of various opioid receptor antagonists on suckling-induced prolactin (PRL) and LH responses in primiparous, lactating rats. All animals were fitted with indwelling jugular catheters to allow serial blood sampling, and some rats received intracerebroventricular (i.c.v.) cannulae for central drug injection. Naloxone (2.0 mg/kg, i.v.) was employed as a broad spectrum opioid antagonist, whereas β-funaltrexamine (β-FNA, 1.0–5.0 μ, i.c.v.), naloxonazine (NAZ, 20 mg/kg, i.v.) and nor-binaltorphimine (nor-BNI, 4.0–16.0 μg, i.c.v.) were used to block μ, μ 1 and κ receptor sites, respectively. In vehicle-treated rats, pup suckling evoked a dramatic increase in plasma PRL and a concurrent decrease in circulating LH. Naloxone caused a modest, though significant, attenuation of the PRL surge during nursing. β-FNA and nor-BNI inhibited suckling-induced PRL release in a dose-related fashion, and at sufficient doses, both antagonists abolished the PRL response. Conversely, the suckling-induced rise in plasma PRL was not affected by NAZ. Naloxone, β-FNA, and NAZ did not alter the profile of circulating LH in suckled rats, but the highest dose nor-BNI (16 μg, i.c.v.) produced a significant elevation in plasma LH. However, even in rats treated with 16.0μg of nor-BNI, plasma LH levels declined in response to the nursing stimulus. Taken together, our results indicate that (1) EOP are important modulators of physiological PRL release in lactating rats, (2) both μ- and κ-opoid receptors are involved in the suckling-induced PRL response, and (3) while a κ-opioid mechanism may inhibit LH secretion during lactation, EOP cannot fully account for suckling-induced LH suppression.

Morphine does not stimulate prolactin release during lactation

The ability of morphine to stimulate prolactin and growth hormone (GH) release was investigated in male rats and in female rats during diestrus, proestrus and lactation. In agreement with previous reports, acute morphine administration produced an increase in circulating levels of prolactin in male and in diestrous and proestrous female rats. In contrast to these results, morphine administration (10 or 15 mg/kg, s.c.; 5 mg/kg, i.v.; 5 or 10 micrograms, i.c.v.) did not produce an increase in prolactin levels in lactating dams. Morphine stimulates prolactin release in part by decreasing dopamine turnover in the tuberoinfundibular neurons in the median eminence. In order to assess the functional activity of these neurons during lactation, haloperidol (0.1 or 0.5 mg/kg, i.v.) was given to lactating dams. There was a significant increase in prolactin levels following haloperidol administration, suggesting that these dopaminergic neurons are participating in the modulation of prolactin release during lactation. In contrast to the insensitivity of the lactating rat to morphine stimulation of prolactin release, the intraventricular administration of two other opiate receptor agonists, beta-endorphin (10 or 20 micrograms) and [D-Ala-D-Leu]enkephalin (DADLE; 5 or 10 micrograms), produced significant increases in circulating levels of this hormone. The GH response to morphine, beta-endorphin and DADLE was also measured in these same rats. All these opiate receptor agonists stimulated GH release in male rats and in female rats during diestrus and proestrus as well as during lactation. These observations suggest that the suckling stimulus during lactation renders the rat refractory to morphine stimulation of prolactin release, possibly as a result of down-regulation of the mu-opiate receptor subtype.

Mu-selective opioid peptides stimulate prolactin release in lactating rats.

The fact that opiates elicit prolactin secretion is well known. However, we have recently discovered that morphine does not stimulate prolactin release in lactating rats. The physiological basis for this alteration in opiate sensitivity during lactation is not known. Since morphine‐induced prolactin secretion in male rats is mediated via the mu opioid receptor subtype, one possible explanation is that mu receptors are down‐regulated during lactation. To address this possibility, the effects of mu opioid peptides on prolactin secretion were examined in lactating rats. The presumed mu‐selective peptides DAGO ([D‐Ala2, Me‐Phe4, Gly‐ol5]‐enkephalin) and PLO‐17 ([NMe‐Phe3, D‐Pro4]‐morphiceptin) were administered to primiparous lactating rats and the resulting hormone responses measured. Both DAGO and PLO‐17 caused a rapid and significant rise in plasma prolactin during lactation. The prolactin‐releasing effects of both peptides were naloxone reversible, suggesting involvement of opioid receptors. Moreover, the DAGO‐induced secretion of prolactin could be completely abolished by pretreatment with the irreversible mu antagonist β‐funaltrexamine. In lactating rats, DAGO and PLO‐17 were poor growth hormone‐releasing agents, providing further evidence for the mu specificity of these peptides. These results imply that during lactation, as in other reproductive states, mu opioid receptor sites are positively coupled to the prolactin secretory mechanism. Thus, the previously observed inability of morphine to elicit prolactin release in lactating rats cannot be explained on the basis of down‐regulation of mu opioid receptors.

Role of serotonin in estrogen-progesterone induced luteinizing hormone release in ovariectomized rats

Pharmacological agents were used to manipulate the surge of luteinizing hormone (LH) induced by progesterone in ovariectomized rats primed with estradiol benzoate. The LH surge was abolished with p-chlorophenylalanine (PCPA), an inhibitor of tryptophan hydroxylase, and restored by 5-hydroxytryptophan, a serotonin precursor. Serotonin receptor agonists, quipazine and N-N-dimethyl-5-methoxytryptamine, were also capable of inducing an LH surge in rats pretreated with PCPA. The serotonin reuptake blocker chlorimipramine was ineffective in stimulating LH release in PCPA blocked animals. Another reuptake blocker, zimelidine was only partially effective in this regard. These two reuptake blockers, as well as amitriptyline, when injected to non-PCPA treated rats led to the reduction or inhibition of the expected LH surge. Four serotonin receptor antagonists, cyproheptadine, methysergide, cinanserin and SQ-10,631, were each able to reduce or abolish the progesterone induced surge of LH. These results suggest that some of the reuptake blockers of serotonin are also capable of inhibiting receptor binding for this neurotransmitter and strongly indicate that serotonin has a stimulatory role in the steroid induced release of LH in castrated rats.

The Role of the Mu1 Opioid Receptor Subtype in the Regulation of Prolactin and Growth Hormone Secretion by Beta‐Endorphin in Female Rats: Studies with Naloxonazine

The μ opioid receptor subtype has been reported to mediate the prolactin secretory response to opioids. This receptor subtype has been implicated in the morphine‐induced prolactin increase, as well as the prolactin response to μ‐specific opioid peptides. Subtypes of the μ receptor have been proposed and the μ1, site has been postulated as the receptor subtype involved in the morphine‐induced prolactin secretory response. However, the role of this receptor subtype in mediating the endocrine effects of the endogenous opioid peptides has not been characterized. In order to determine the physiological significance of this receptor subtype, animals were pretreated with saline, WIN 44,441–3 (a μ, δ and κ antagonist) or naloxonazine (a μ1 antagonist) followed by a stimulatory dose of morphine or β‐endorphin. A dose response study for β‐endorphin was conducted to determine the minimal stimulatory dose of β‐endorphin on the prolactin and growth hormone (GH) secretory response. The dose response study indicated that β‐endorphin is a more potent stimulus for prolactin release than for GH. A dose as low as 25 ng increased prolactin levels as much as 100‐fold in both lactating and diestrous female rats. In contrast, 2.5 μg β‐endorphin was required in order to consistently and significantly increase circulating levels of GH by 2‐ to 3‐fold. WIN 44,441–3 antagonized the stimulatory effects of β‐endorphin on both prolactin and GH secretion. Naloxonazine pretreatment abolished the morphine‐induced prolactin secretory response, without affecting the GH increase in diestrous females. Naloxonazine also antagonized the prolactin response to β‐endorphin in both lactating and diestrous females. In addition, it attenuated the GH secretory response but did not totally abolish it. These data indicate that β‐endorphin elicits an increase in prolactin release through an opioid specific receptor which appears to be the μ1 opioid receptor subtype. They further suggest that β‐endorphin may increase GH levels, at least partially, via its action at this μ1 site.