Neurotransmitter involvement in naloxone-induced stimulation of pulsatile LH release on day 8 of pregnancy in the rat

Naloxone, an endogenous opioid peptide (EOP) receptor antagonist, increases LH pulse frequency and amplitude in early gestation in the rat (7). The object of this study was to further explore the suppression of pulsatile LH release by EOPs on day 8 of pregnancy by examining whether inhibition of norepinephrine (NE) or epinephrine (EPIN) synthesis, or stimulation of gamma-aminobutyric acid (GABA)-B receptors, modified the ability of naloxone infusion (0.5 mg/kg/hr for 3.5 hr) to stimulate pulsatile LH secretion. Blood sampling (50 microliters whole blood/5 min) began 0.5 hr after the onset of infusion. Three studies were conducted. 1) LY 134046 (PNMT inhibitor, 50 mg/kg IP), given at -27, -20, and -3 hr relative to the onset of a 3-hr blood sampling period, produced no change in hypothalamic-preoptic area (HPOA) levels of NE, and a 76% decline in HPOA-EPIN levels, as determined by HPLC. Although basal LH pulse frequency was reduced, this treatment had no effect on the stimulatory action of naloxone on pulsatile LH release. 2) FLA-63 (DBH inhibitor, 25 mg/kg IP) given at -3 hr produced a 72% decline in HPOA-NE levels, a 44% decrease in HPOA-EPIN values, and blocked the stimulatory action of naloxone on LH pulse frequency. Since depletion of HPOA-EPIN by LY 134046 did not compromise the LH response to naloxone, this effect of FLA-63 is due to depletion of HPOA-NE levels. 3) While saline had no effect on the increased pulsatile LH release caused by naloxone, administration of baclofen (GABA-B receptor agonist, 6 mg/kg IV) suppressed the pulsatile LH secretory response to naloxone infusion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Medial preoptic-anterior hypothalamic area involvement in the suppression of pulsatile LH release by a mu-opioid agonist in the ovariectomized rat

The objective of this study was to examine whether specific activation of mu-opioid receptors at the level of the medial preoptic-anterior hypothalamic area (MPOA-AHA) could suppress pulsatile LH release. The experiments were done using rats that had been ovariectomized (OVX) 24 hr before on diestrus 2, animals in which we have previously demonstrated an active endogenous opioid peptide suppression of pulsatile LH release (2). DAGO, DPDPE, or U50488H, specific agonists of mu-, delta- and kappa-opioid receptors, respectively, were continuously applied directly to the MPOA-AHA by means of push-pull perfusion. Perfusion of the MPOA-AHA with 0.5 micrograms DAGO/hr suppressed LH pulse amplitude. This effect of DAGO was not due to spread to the third ventricle and subsequent diffusion via the CSF to another CNS site, since push-pull perfusion with this dose of DAGO in the region just dorsal to or in the posterior hypothalamus was ineffective in altering LH pulse amplitude. The response to DAGO was dose-dependent since a higher dose (4.8 micrograms/hr) markedly suppressed both LH pulse amplitude and frequency. The same doses of DPDPE and U50488H (0.5 and 4.8 micrograms/hr) had no effect on pulsatile LH secretion, providing support for mu receptor involvement in the DAGO-induced suppressive action. These data demonstrate MPOA-AHA involvement in the suppression of pulsatile LH release by a mu-opioid agonist in the OVX rat.     

Analysis of estradiol-independent and -dependent endogenous opioid peptide suppression of pulsatile LH release between the mornings of diestrus 2 and proestrus in the rat estrous cycle

The aim of this study was to analyze possible estradiol (E2)-independent and -dependent endogenous opioid peptide (EOP) suppression of pulsatile LH release between the mornings of diestrus 2 (D2) and proestrus by examining the LH response to naloxone infusions in the presence or absence of proestrous levels of E2. Pulsatile LH secretion remained unchanged between D2 and proestrus but mean blood LH levels, pulse amplitude and frequency increased within 24 hr following ovariectomy on D2. This increase was due in large part to the loss of E2 negative feedback, since restoration of physiological proestrous E2 levels returned LH pulse frequency to proestrous a.m. levels and greatly reduced pulse amplitude. In ovariectomized rats lacking E2 negative feedback, continuous infusion of the EOP receptor antagonist naloxone (0.5 and 2 mg/kg/hr) caused a further increase in pulse amplitude and frequency. This naloxone-induced increment in pulsatile LH release was exerted via centrally located EOP receptors since naloxone did not alter pituitary responsiveness to LHRH, and its stimulatory action on pulsatile release was diminished by simultaneous infusion with morphine. Naloxone also increased pulsatile LH release in E2-treated animals. The naloxone-induced increments in LH pulse amplitude were the same in the presence or absence of E2 negative feedback. Moreover, the increments in amplitude produced by naloxone in E2-treated rats were significantly less than those resulting from the combination of ovariectomy plus naloxone infusion in empty capsule-implanted rats. These data indicated that naloxone infusion in E2-implanted animals blocked an E2-independent EOP suppression of this parameter of pulsatile release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Possible mechanism of hypothermia induced by intracerebroventricular injection of orphanin FQ/nociceptin

Orphanin/nociceptin (OFQ/N), a 17-amino-acid peptide, is an endogenous peptide, the receptor for which is similar to mu-, delta- and kappa-opioid receptors ( approximately 65% homology). Reports indicate that OFQ/N can block the antinociception induced by mu-, delta- and kappa-opioid agonists in the rat and in the mouse, indicating that there is a functional interaction between opioid receptors and OFQ/N receptors in the nervous system. It is well known that activation of the mu- and kappa-opioid receptors results in hyperthermia and hypothermia, respectively, in Sprague-Dawley rats. The present studies were designed to examine effects of OFQ/N on body temperature (Tb) and explore whether the mechanism of T(b) change induced by OFQ/N involved the opioid system. The results show that (1) i.c.v. injection of a high dose of OFQ/N (9-18 micro g) produces hypothermia in adult rats; (2) OFQ/N (1.8 micro g, i.c.v., t=+30 s after morphine) can decrease morphine-induced hyperthermia; (3) neither the opioid receptor antagonist, naloxone (10 mg/kg, s.c., t=-15 s before OFQ/N) nor the kappa-opioid receptor antagonist nor-BNI (1 micro g/5 microl, i.c.v., t=-30 s before OFQ/N) reduces the hypothermia induced by i.c.v. injection of OFQ/N at dose of 18 micro g (P>0.05); (4) 60 micro g/5 microl AS oligo (i.c.v. treatment on days 1, 3 and 5) against OFQ/N receptors significantly reduces the hypothermia induced by i.c.v. injection of 9 micro g OFQ/N (P<0.01). These results suggest that the hypothermia induced by i.c.v. injection of a high dose of OFQ/N (9 or 18 micro g) is mediated, at least partially, by its own receptor, independent or downstream of opioid receptors in the rat brain and that OFQ/N probably acts as a physiological antagonist to reduce morphine-induced hyperthermia.     

Opioid receptors are involved in an NMDA receptor-independent mechanism of LTP induction at hippocampal mossy fiber-CA3 synapses.

Long-term potentiation (LTP) of mossy fiber responses in area CA3 of the rat hippocampus in vivo is blocked by naloxone, an opioid receptor antagonist, in a stereospecific and dose-dependent manner. LTP of commissural afferents to the same population of CA3 pyramidal cells is not attenuated by naloxone. This suggests that opioid receptors are involved in a mechanism of LTP induction that is specific to mossy fiber synapses, and that endogenous opioid receptors are involved in a mechanism of LTP induction that is specific to mossy fiber synapses, and that endogenous opioid peptides, presumably released as a result of mossy fiber stimulation, may be necessary for the induction of mossy fiber LTP. The naloxone sensitivity is limited to the induction phase of LTP, since naloxone does not reverse previously established LTP. These data suggest that LTP at the mossy fiber-CA3 synapse constitutes an NMDA receptor-independent, opioid receptor-dependent, form of hippocampal synaptic plasticity.     

Presence of kappa-opioid tone at the onset of the ovulatory luteinizing hormone surge in the proestrous rat

A decrease in endogenous opioid peptide inhibitory tone on the afternoon of proestrus is one event underlying generation of the ovulatory luteinizing hormone (LH) surge. Whether this disinhibition involves a complete loss of opioid suppression at the time of the LH surge is controversial. The objective of the present study was to determine whether a total loss specifically of the kappa-opioid inhibitory component suppressing LH secretion occurs on proestrus at the onset of the LH surge. Proestrous rats were infused intraventricularly with either artificial cerebrospinal fluid (aCSF) or aCSF containing nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist, from 15:30 or 16:30 h (the approximate onset time of the spontaneous LH surge) to 18:50 h. The LH surge in rats treated with nor-BNI beginning at 15:30 h started 0.5 h earlier than the spontaneous surge in aCSF controls, and had significantly higher plasma LH levels from 16:30 to 17:30 h. Nor-BNI administration begun at 16:30 h also produced an LH surge with more elevated plasma LH levels at 17:30 and 18:00 h than in aCSF-treated controls. These results demonstrate that significant amounts of kappa-opioid tone are still present during the hours when the LH surge is initiated. Thus, a complete loss of kappa-opioid inhibition is not required for the onset of the LH surge on proestrus.     

Neural circuits regulating pulsatile luteinizing hormone release in the female guinea-pig: opioid, adrenergic and serotonergic interactions

We studied three neurotransmitters involved in the regulation of pulsatile luteinizing hormone (LH) release: opioid peptides, serotonin and norepinephrine, using the ovariectomized guinea-pig. This is an attractive animal model due to the regularity of its LH pulses, enabling any disruptions to be clearly ascertained. In all experiments, a specific agonist or antagonist was administered, either alone or serially to enable detection of interactions, and effects on mean LH concentrations, pulse amplitude and interpulse interval were determined by PULSAR analysis. In the ovariectomized guinea-pig, catecholamines are stimulatory (acting through the alpha1 and alpha2 but not beta receptors, unlike other species), opioids inhibitory and serotonin permissively stimulatory to pulsatile LH release. Stimulatory effects of the opiate antagonist were not blocked by pretreatment with an alpha1- or alpha2-adrenergic antagonist. Similarly, pretreatment with the opiate antagonist did not prevent the suppression of LH release by alpha1 and alpha2 antagonists. This suggests that, in the guinea-pig, effects of opiates and catecholamines on LH release are exerted by independent pathways to luteinizing hormone releasing hormone (LHRH) neurones. For the opiate-serotonin interactions, pretreatment with the serotonergic antagonist did not block the stimulatory effect of the opiate antagonist on LH release. However, pretreatment with the opiate agonist could not be overcome by the serotonergic agonist. This suggests that the effects of the serotonin system on LHRH release may be indirectly mediated by opioid neurones. Taken together, these studies demonstrate that the three neurotransmitter systems studied are critically involved in normal pulsatile LH release in the female guinea-pig, and demonstrate novel functional relationships between the opioid and the adrenergic and serotonergic systems.     

Involvement of endogenous opioid systems in nociceptin-induced spinal antinociception in rats

The present study investigates the involvement of opioid receptors in the antinociceptive effects of nociceptin in the spinal cord of the rat. Intrathecal administrations of 5 and 10 nmol of nociceptin significantly increase the withdraw response latencies to noxious thermal and mechanical stimulations. This nociceptin-induced antinociceptive effect is significantly attenuated by intrathecal injection of (Nphe(1))nociceptin(1-13)-NH(2), a selective antagonist of the nociceptin receptor (opioid receptor-like receptor ORL1), indicating an ORL1 receptor-mediated mechanism. This antinociceptive effect is also significantly attenuated by intrathecal injections of naloxone (a nonselective opioid receptor antagonist), naltrindole (a selective delta-opioid receptor antagonist), and beta-funaltrexamine (a selective mu-opioid receptor antagonist) in a dose-dependent manner, but not by the selective kappa-opioid receptor antagonist norbinaltorphimine. Since it is unlikely that nociceptin acts by direct binding to opioid receptors, these results suggest a possible interaction between the nociceptin/ORL1 and opioid systems in the dorsal horn of the rat spinal cord.     

Role of opioid peptides in pulsatile release of gonadotropins and prolactin in the rat

To determine the role of endogenous opioid peptides in the pulsatile release of gonadotropins and prolactin in the ovariectomized rat, the opiate receptor blocker, naloxone, was administered intravenously, and its effect on plasma FSH, LH and prolactin was determined by multiple sampling prior to and after injection. Naloxone produced a dose-related increase in plasma LH and to a lesser extent FSH and decreased prolactin levels in the experiment in which they were examined. Higher doses of naloxone produced a significant increase in plasma LH pulse amplitude and lengthened the interpulse interval with a consequent decrease in pulse frequency. Minimum values between pulses were also increased. There was no clear effect on FSH pulsations but pulses of prolactin were blocked. Intraventricular (third ventricle) injection of a specific anti beta endorphin antiserum (3 microliter) produced an initial decline followed by an elevation of LH but had no effect on plasma FSH. The normal rabbit serum control injections were without effect. It is hypothesized that initiation of LH pulses in the castrated rat may be related to a periodic removal of tonic beta endorphinergic tone.     

High postovariectomy LH levels are not due to decreased opioid inhibition of GnRH

It has been proposed that endogenous opioid peptide (EOP) inhibition of hypothalamic GnRH secretion mediates and is dependent upon gonadal steroid feedback of LH secretion, although considerable conflicting data have been reported. Accordingly, a well-characterized replacement regimen was used to approximate physiological stimulation by estradiol (E2) and progesterone (P4) in adult rats 10 days after ovariectomy (OVX), followed by in vitro incubation of the isolated median eminence to evaluate the role of E2 and P4 in modifying GnRH release in response to the opiate receptor antagonist, naloxone (NAL). Basal (control) GnRH release from median eminences of OVX, OVX + E2, and OVX + E2 + P4 rats was similar, and NAL treatment elicited a comparable increase in GnRH release under all three gonadal steroid conditions. Thus, EOP suppression of median eminence GnRH secretion does not appear to mediate or be dependent upon negative feedback regulation of LH secretion by physiological concentrations of gonadal steroids.     

Ovarian Steroid Regulation of Pulsatile Luteinizing Hormone Release During Early Gestation in the Rat

The object of this study was to examine ovarian regulation of pulsatile luteinizing hormone (LH) secretion during early gestation. This was done primarily by analyzing pulsatile LH release in rats that were either sham ovariectomized (OVX) on Day 7 of pregnancy, implanted with empty Silastic capsules, and bled on Day 8, or OVX on Day 7, immediately implanted with Silastic capsules producing plasma levels of estradiol and/or progesterone characteristic of Day 7 to 8 of pregnancy, and bled on Day 8. In addition, the role of progesterone in regulating pulsatile LH secretion was also examined by administration of the progesterone receptor antagonist, RU486, on Day 7 and examining pulsatile LH release on Day 8 of pregnancy. OVX caused a marked increase in LH pulse amplitude and frequency within 24 h. Replacement with physiological plasma levels of estradiol or progesterone alone had no suppressive effect on this OVX-induced increase in pulsatile LH secretion. Restoration of physiological plasma levels of both estradiol and progesterone returned LH pulse amplitude to values seen in sham OVX controls, and prevented the OVX-induced increase in LH pulse frequency. The group mean LH pulse frequency tended to be less in estradiol + progesterone-treated rats than in sham OVX controls, but this difference was not statistically significant. RU486 blocked uterine progesterone receptors as evidenced by endometrial hemorrhaging. In agreement with the OVX + steroid replacement data, RU486 administration also resulted in increases in LH pulse amplitude and frequency. These data demonstrate that the frequency and amplitude of LH pulses on Day 8 of gestation are held in check by negative feedback signals coming from the ovary. Neither steroid alone exerts any suppressive influence over pulsatile LH secretion during early gestation, but both steroids acting together exert a prominent negative feedback regulation on the pulsatile LH release process.     

Intravenous injections of nicotine decrease the pulsatile secretion of LH by inhibiting the gonadotropin-releasing hormone (GnRH) pulse generator activity in female rats.

Whether nicotine inhibits the electrical activity of the gonadotropin-releasing hormone (GnRH) pulse generator to suppress pulsatile LH secretion, and whether this suppression of LH secretion by nicotine is mediated by opioid neurons, were studied in ovariectomized rats by examining changes in LH secretion and the multiunit activity (MUA) of the medial basal hypothalamus. Intravenous (i.v.) injection of nicotine (nicotine bitartrate, 100 micrograms) significantly increased the interval between characteristic increases (volleys) in MUA and LH pulses. This inhibitory effect of nicotine on the GnRH pulse generator activity was not blocked by the prior injection of an opiate receptor antagonist naloxone (naloxone hydrochlolide, 2 mg/kg bw), which was effective in significantly decreasing the interval between MUA volleys. The results suggest that nicotine alters the activity of the GnRH pulse generator, and that cholinergic neurons appear to be directly involved in suppressing pulsatile secretion of LH.     

Acute Immobilization Stress and Intraventricular Injection of CRF Suppress Naloxone-Induced LH Release in Ovariectomized Estrogen-Primed Rats

The present study was undertaken to evaluate the role and possible interaction of the endogenous opioid peptide (EOP) and corticotropin-releasing factor (CRF) in the acute stress-induced suppression of gonadotropin secretion in ovariectomized estrogen-primed rats. An intravenous (i.v.) injection of naloxone (10 or 20  mg/kg), an EOP antagonist, significantly elevated serum luteinizing hormone (LH) levels within 10  min in non-stressed animals. The naloxone-induced LH release was completely eliminated when tested 30  min after the onset of acute immobilization. In a subsequent study, it was found that suppression of the naloxone-induced LH release occurred as early as 5  min after the stress onset, and was still evident 60  min after the end of a 30-min period of immobilization. The effect of naloxone was restored 3  h after liberation of the animal from the 30-min immobilization. An intraventricular (i.c.v.) injection of CRF (1 or 5  μg) also significantly suppressed, in a dose-related manner, the effect of a subsequent i.v. injection of naloxone. However, an i.c.v. injection of α -helical CRF(9-41) (25 or 50  μg), a CRF antagonist, prior to immobilization, could not interfere with the suppressive effect of stress on naloxone-induced LH release. These results suggest that both acute immobilization stress and CRF can inhibit the LH secretory activity without mediation by EOP neurons. However, the stress-related suppression may involve non-CRF mechanism(s).     

Mu-, delta-, kappa- and epsilon-opioid receptor modulation of the hypothalamic-pituitary-adrenocortical (HPA) axis: subchronic tolerance studies of endogenous opioid peptides

In opiate-naive rats, the endogenous opioid peptides, beta-endorphin, dynorphin(1-13) and Met-Enk-Arg-Phe (MEAP) and the synthetic enkephalin analogue D-Ala2-D-Leu5-Enk (DADLE) potently stimulated plasma corticosterone in a dose-dependent, naloxone reversible manner. To characterize their in vivo affinities, the effects of these peptides on plasma corticosterone release were tested in rats made tolerant to morphine, U50488H, DADLE/morphine or beta-endorphin. These cross-tolerance studies showed that dynorphin and MEAP exerted their action on plasma corticosterone release at kappa-opioid receptors. The action of DADLE occurred at delta-opioid receptors, while the action of beta-endorphin occurred principally at another receptor site. These results indicate that there is independent modulation of the hypothalamic-pituitary-adrenal axis by endogenous opioid peptides at mu-, delta- and kappa-opioid receptors. In addition there may be modulation by beta-endorphin at a separate site that we suggest could be a central epsilon-receptor site. This cross-tolerance paradigm, using a neuroendocrine model, provides in vivo evidence for the action of centrally active endogenous opioid peptides at multiple and independent opioid receptors.     

Opiate antagonists do not alter neuronal responses to stimulation of opioid-containing pathways in rat hippocampus

The opioid receptor antagonists, naloxone and beta-chlornaltrexamine, were used to determine whether activation of endogenous opioid peptide containing pathways produced pharmacologically reversible opioid actions. Extracellularly recorded responses of the hippocampal CA3 pyramidal cells were evoked by stimulation of the dynorphin-containing mossy fiber pathway. Neither naloxone nor beta-chlornaltrexamine pretreatment significantly changed the evoked response. However, both antagonists blocked the effect of applied dynorphin-A(1-17) on CA1 pyramidal cell evoked responses. Thus, our data demonstrate that if endogenous opioids are released from this pathway, the peptides cannot be responsible for the evoked response measured in hippocampal CA3 cellular field. With no direct evidence for endogenous opioid peptides acting through opioid receptors, the neurotransmitter role of dynorphins in rat hippocampus remains obscure.     

Inhibition of release of neurohypophysial hormones by endogenous opioid peptides in pregnant and parurient rats

Naloxone, an opiate receptor antagonist, was used to determine whether opioid peptides modulate release of oxytocin (OT) or vasopressin (AVP) in the rat after expulsion of the fetus, i.e. parturition. We measured the concentrations of AVP and OT in plasma and in the neurointermediate lobe of the pituitary of pregnant rats given naloxone (5 mg/kg, s.c.) or saline on day 20 of gestation, and on day 21 either before or during the expulsive stage of labor. Non-pregnant rats in diestrus were giben naloxone for comparison. On days 20 and 21 of gestation, before the onset of parturition, plasma [AVP] but not [OT] was elevated, compared to the non-pregnant controls. After delivery of the first two pups, plasma [OT] approximatelyy doubled, whereas plasma [AVP] remained unchanged. Blocking the action of endogenous opioid peptides with naloxone caused an elevation of plasma [OT] in pregnant animals on days 20 and 21 of gestation and during parturition. Naloxone, however, did not alter plasma [AVP] in either parturient or preparturient animals. In contrast, [AVP], but not [OT], was increased in plasma of non-pregnant rats given naloxone. The content of OT in the neuro-intermediate lobe was similar in pregnant and non-pregnant rats and was unaffected delivery of the first two pups. However, AVP content and the ratio of AVP/OT in the pituitary were lower in pregnant animals before during delivery than in the non-pregnant controls. The content of neither hormone was altered by naloxone. Thus, AVP release apparently increase and pituitary stores of this peptide are decreased by day 20 gestation, when labor has not yet begun. In contrast, OT secretion becomes elevated only during delivery. Inhibition of OT release by opioid peptides may: (1) allow preferential release of AVP during pregnancy; and (2) prevent depletion of pituitary stores of OT and neuronal fatigue during the 1–2 h period of parturition in the rat.     

Morphine produces a multiphasic effect on the release of substance P from rat trigeminal nucleus slices by activating different opioid receptor subtypes.

Morphine (MOR) produces a concentration-dependent multiphasic effect (inhibitions and facilitations) on K(+)-evoked substance P (SP) release from rat trigeminal nucleus slices. In this study, we tested the action of selective opioid receptor antagonists on this multiphasic effect of MOR. 1 nM MOR produced an inhibition of K(+)-evoked release of SP that was affected only by the selective mu 1-opioid receptor antagonist naloxonazine (1 nM). MOR at 100 nM elicited an increase in SP release which was abolished selectively by the mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA; 20 nM) and attenuated by the delta-opioid receptor antagonist, ICI 174,864 (0.3 microM). 3 microM MOR produced an inhibition of SP release that was reversed only by ICI 174,864 (0.3 microM). MOR at even higher concentrations (30 microM) produced an enhancement of SP release that was reversed selectively by 3 nM n-binaltorphimine (n-BNI; 3 nM), a kappa-opioid receptor antagonist. In slices pretreated with 20 nM beta-FNA and in the presence of 0.3 microM ICI 174,864 (mu- and delta-opioid receptor blockade), both 100 nM and 3 microM MOR elicited a strong facilitation of K(+)-evoked SP release which was sensitive to 3 nM n-BNI. Thus, the increase in SP release produced by 100 nM may be mediated by the simultaneous stimulation of beta-FNA-sensitive mu- and excitatory delta-opioid receptors whereas the facilitation of SP release induced by 30 microM MOR could be due to the activation of kappa-opioid receptors. 1 nM and 3 microM MOR may inhibit SP release by stimulating naloxonazine-sensitive mu 1- and inhibitory delta-opioid receptors, respectively.     

Effects of Different Opioid Receptor Antagonists on the Electrically-Evoked Release of Endogenous Dopamine from the Isolated Neural Lobe of the Rat Pituitary Gland in vitro

Isolated neural lobes of the rat pituitary gland were incubated in Krebs-HEPES solution which contained the dopamine uptake inhibitor GBR 12921 and in some experiments additionally pargyline. The release of endogenous dopamine evoked by electrical stimulation of the pituitary stalk was determined by high-performance liquid chromatography with electrochemical detection. (±)- Naloxone increased the evoked dopamine release maximally by 440% (EC₅₀ 209 nM). The (+)-enantiomer of naloxone (up to 10 μM) did not affect the release of dopamine. The preferential κ-opioid receptor antagonist MR 2266 increased the evoked dopamine release maximally by 135% (EC₅₀ 7 nM). MR 2267, the inactive (+)-enantiomer of MR 2266, had no effect on dopamine release. The δ-opioid receptor selective antagonist ICI 174864 increased the release of dopamine maximally by 120% (EC₅₀ 10 nM). The non-selective opioid receptor agonist etorphine up to 10 μM had no effect on the evoked dopamine release. In conclusion, endogenous opioids in the neurohypophysis strongly inhibit the release of endogenous dopamine from this gland. Activation of κ- and δ-opioid receptors appears to be involved in the inhibitory action of the endogenous opioids on the neurohypophysial release of dopamine.     

Inhibition of release of neurohypophysial hormones by endogenous opioid peptides in pregnant and parturient rats

Naloxone, an opiate receptor antagonist, was used to determine whether opioid peptides modulate release of oxytocin (OT) or vasopressin (AVP) in the rat after expulsion of the fetus, i.e. parturition. We measured the concentrations of AVP and OT in plasma and in the neurointermediate lobe of the pituitary of pregnant rats given naloxone (5 mg/kg, s.c.) or saline on day 20 of gestation, and on day 21 either before or during the expulsive stage of labor. Non-pregnant rats in diestrus were given naloxone for comparison. On days 20 and 21 of gestation, before the onset of parturition, plasma [AVP] but not [OT] was elevated, compared to the non-pregnant controls. After delivery of the first two pups, plasma [OT] approximately doubled, whereas plasma [AVP] remained unchanged. Blocking the action of endogenous opioid peptides with naloxone caused an elevation of plasma [OT] in pregnant animals on days 20 and 21 of gestation and during parturition. Naloxone, however, did not alter plasma [AVP] in either parturient or preparturient animals. In contrast, [AVP], but not [OT], was increased in plasma of non-pregnant rats given naloxone. The content of OT in the neuro-intermediate lobe was similar in pregnant and non-pregnant rats and was unaffected by delivery of the first two pups. However, AVP content and the ratio of AVP/OT in the pituitary were lower in pregnant animals before and during delivery than in the non-pregnant controls. The content of neither hormone was altered by naloxone. Thus, AVP release apparently increases and pituitary stores of this peptide are decreased by day 20 of gestation, when labor has not yet begun.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacological characterization of the ameliorating effect on learning and memory impairment and antinociceptive effect of KT-95 in mice

3-Acetoxy-6beta-acetylthio-10-oxo-N-cyclopropylmethyl-dihydronormorphine (KT-95) is a synthesized compound that binds to mu-, delta- and kappa-opioid receptors in vitro. KT-95 induces analgesia and this effect is antagonized by nor-BNI, a selective kappa-opioid receptor antagonist. We have reported that kappa-opioid receptor agonists improve impairment of learning and memory in mice and/or rats. In this study, the effects of KT-95 were investigated in an acetic acid-induced writhing test and scopolamine-induced memory impairment test using spontaneous alternation performance in a Y-maze and a step-down type passive avoidance test. Male ddY mice were treated with KT-95 (0.24-2.35 micromol/kg, s.c.) 30 min before the behavioral test. In the writhing test, the antinociceptive effect of KT-95 (2.35 micromol/kg) was completely antagonized by nor-BNI (4.9 nmol/mouse, i.c.v.), but not by naloxone (3.05 micromol/kg, s.c.). KT-95 significantly improved the impairment of spontaneous alternation induced by scopolamine (1.65 micromol/kg, s.c.). The ameliorating effect of KT-95 was not antagonized by nor-BNI, but was almost completely antagonized by a selective sigma-receptor antagonist, N,N-dipropyl-2-[4-methoxy-3-(2-phenylenoxy)-phenyl]-ethylamine monohydrochloride (NE-100, 2.6 micromol/kg, i.p.). KT-95 also significantly improved scopolamine-induced learning and memory impairment in the passive avoidance test, although the effect was partial. Administration of KT-95 itself induced impairment of learning and memory. KT-95-induced impairment was not antagonized by naloxone, naltrindole, nor-BNI or NE-100 indicating that this impairment was not because of opioid receptor stimulation. These results suggested that although the KT-95-induced antinociceptive effect was mediated by kappa-opioid receptors, the KT-95-induced improvement in scopolamine-induced impairment of memory was mediated mainly via sigma-receptors and partially by kappa-opioid receptors.