Lack of effect of opiates on release of vasopressin from isolated rat neurohypophysis

The effects of opioids and opioid analogs on animal and human endocrine systems

Opioid abuse has increased in the last decade, primarily as a result of increased access to prescription opioids. Physicians are also increasingly administering opioid analgesics for noncancer chronic pain. Thus, knowledge of the long-term consequences of opioid use/abuse has important implications for fully evaluating the clinical usefulness of opioid medications. Many studies have examined the effect of opioids on the endocrine system; however, a systematic review of the endocrine actions of opioids in both humans and animals has, to our knowledge, not been published since 1984. Thus, we reviewed the literature on the effect of opioids on the endocrine system. We included both acute and chronic effects of opioids, with the majority of the studies done on the acute effects although chronic effects are more physiologically relevant. In humans and laboratory animals, opioids generally increase GH and prolactin and decrease LH, testosterone, estradiol, and oxytocin. In humans, opioids increase TSH, whereas in rodents, TSH is decreased. In both rodents and humans, the reports of effects of opioids on arginine vasopressin and ACTH are conflicting. Opioids act preferentially at different receptor sites leading to stimulatory or inhibitory effects on hormone release. Increasing opioid abuse primarily leads to hypogonadism but may also affect the secretion of other pituitary hormones. The potential consequences of hypogonadism include decreased libido and erectile dysfunction in men, oligomenorrhea or amenorrhea in women, and bone loss or infertility in both sexes. Opioids may increase or decrease food intake, depending on the type of opioid and the duration of action. Additionally, opioids may act through the sympathetic nervous system to cause hyperglycemia and impaired insulin secretion. In this review, recent information regarding endocrine disorders among opioid abusers is presented.

Modulation of GABAA receptors and neuropeptide secretion by the neurosteroid allopregnanolone in posterior and intermediate pituitary

A number of neurosteroids bind to GABAA receptors and alter their responsiveness to neurotransmitters. Considerable effort has been devoted to understanding how this form of receptor modulation alters inhibitory synaptic function. Neurosteroid-sensitive GABAA receptors have also been demonstrated in many endocrine cells, but little is known about how neurosteroids modulate the release of hormones. Here, the action of allopregnanolone, a neurosteroid that enhances GABAA receptor-mediated responses, was investigated in posterior pituitary nerve terminals and intermediate pituitary endocrine cells. Patch clamp recordings showed that GABA-evoked currents were enhanced to similar degrees and with similar concentration dependences in both locations. An organ bath preparation of the neurointermediate lobe was used to investigate drug effects on secretion of vasopressin and alpha-melanocyte stimulating hormone. GABA increased the basal release of vasopressin and alpha-melanocyte stimulating hormone from the posterior and intermediate pituitary lobe, respectively, an effect that could be blocked by picrotoxinin. Vasopressin release evoked by electrical stimulation was also examined, and a small statistically significant inhibition by 5 microM GABA was observed. Allopregnanolone increased the basal release of vasopressin, and this effect was blocked by the GABAA receptor antagonist picrotoxinin. Allopregnanolone had no effect in conjunction with GABA. In contrast to the posterior lobe, allopregnanolone had no effect on release from the intermediate lobe. Thus, allopregnanolone in physiological relevant concentrations modulates GABAA receptors in both the posterior and intermediate lobes, but only affects hormone release in the posterior lobe.

kappa-Opioid agonist inhibition of osmotically induced AVP release: preferential action at hypothalamic sites

Although several studies indicate that kappa-opioid agonists induce a water diuresis by inhibiting vasopressin (AVP) secretion, the locus of the kappa-receptors (neurohypophysial vs. hypothalamic) responsible for this effect remains unclear. We have ascertained the effect of the selective kappa-agonist BRL-52656 (BRL) on AVP secretion by using compartmentalized rat hypothalamoneurohypophysial explants in culture. When applied to the hypothalamus, nanomolar concentrations of BRL inhibited osmotically stimulated AVP secretion. This response was blocked by the highly selective kappa-opioid antagonist nor-binaltorphimine (BNI). However, osmotically stimulated AVP release was suppressed at the neurohypophysial site only by 100 nM BRL and was not reversed by BNI but only by naloxone. This dose of BRL, administered to the posterior pituitary compartment, did not appear to act by the agonist gaining access to hypothalamic kappa-opiate receptors, because BNI added to the hypothalamus failed to prevent the inhibition of AVP secretion. The data demonstrate that BRL is a potent inhibitor of osmotically stimulated AVP secretion via activation of kappa-opiate receptors within the hypothalamus, but that higher concentrations of the drug may also stimulate non-kappa-neurohypophysial opiate receptors that suppress AVP release.

Dihydropyridine ligands influence the evoked release of oxytocin and vasopressin dependent on stimulation conditions

The effects of dihydropyridine ligands on the electrically evoked release of neurohypophysial hormones from isolated, rat neurointermediate lobes were investigated as a function of all combinations of two pulse widths (0.2 and 2 ms) and three stimulation frequencies (6.5, 13 and 30 Hz). The dihydropyridine agonist (S)-(+)-202-791 potentiated concentration dependently the release of both oxytocin and vasopressin at a pulse width of 2 ms and a frequency of 6.5 Hz. This effect of (S)-(+)-202-791 was abolished by the antagonist (-)-nitrendipine and stereospecifically by (R)-(-)-202-791 (only vasopressin). The antagonist (R)-(-)-202-791 alone inhibited the release of oxytocin at 13 Hz and 2 ms. The results presented show that the effects of the dihydropyridine ligands are dependent on the stimulation conditions, and thus demonstrate that the entry of Ca2+ through the dihydropyridine sensitive L-type Ca2+ channel is associated with electrically evoked release of neurohypophysial hormones under certain conditions.

Intracellular calcium and hormone release from nerve endings of the neurohypophysis in the presence of opioid agonists and antagonists

Rat neural lobes and isolated nerve terminals from the neurohypophysis were stimulated in the presence of different opioid agonists and antagonists. The secretion of arginine vasopressin and oxytocin and rise in cytoplasmic calcium induced by depolarization were analyzed by radioimmunoassay and the fluorescent probe fura-2, respectively. The kappa-agonists dynorphin A(1-13) and dynorphin A(1-8) did not affect electrically evoked release of vasopressin, although oxytocin release was slightly reduced. U-50 488, a relatively specific kappa-receptor agonist, had no effect on the amount of vasopressin or oxytocin secreted, although it significantly reduced K(+)-evoked changes in [Ca2+]i in isolated nerve endings. Two kappa-receptor antagonists, MR 2266 and diprenorphin, alone had no effect on vasopressin and oxytocin secretion from isolated nerve endings depolarized with potassium. Opioid agonists less selective for the kappa receptors, etorphin and ethylketocyclazocin, were found to inhibit the release of both vasopressin and oxytocin significantly. Naloxone, a nonselective opiate receptor antagonist, alone had no effect on vasopressin release but potentiated the electrically evoked release of oxytocin. Naloxone also could overcome the inhibitory effect of etorphin on oxytocin and vasopressin release observed after electrical stimulation of the neural lobe. A number of inconsistencies therefore exist between the effects of opioid agonists and antagonists on neuropeptide release and on the evoked changes in [Ca2+]i. In view of these inconsistencies and the high concentrations of opioid agonists and antagonists necessary to modify release, we conclude that it is doubtful that opioid molecules have a physiological role in controlling neurohypophysial secretion.

Influence of interleukin-1 beta on the secretion of oxytocin and vasopressin from the isolated rat neurohypophysis

The effect of the cytokine interleukin-1 beta on the secretion of oxytocin and vasopressin from electrically stimulated rat neurohypophysis was examined in vitro. The release of oxytocin and vasopressin was concentration-dependently increased by interleukin-1 beta in the concentration range from 4.4 pM to 440 pM. The effect of interleukin-1 beta on oxytocin secretion was less intense as compared to vasopressin. After 440 pM interleukin-1 beta the electrically evoked release of oxytocin was increased about 22% and had not reached its maximum. The vasopressin response was maximal after 44 pM interleukin-1 beta, the response being increased 43% compared to control. No trace of interleukin-1 beta was found in the posterior pituitary (less than 350 pmol/lobe, radioimmunoassay). The results indicate that interleukin-1 beta might be involved the regulation of oxytocin and vasopressin at the pituitary level.

Kappa-opioid receptor agonists differentially affect the release of neurohypophysial hormones

The influence of kappa-opioid receptor agonists and antagonists on release of oxytocin and vasopressin was examined in isolated rat neurointermediate lobes. Electrically evoked release of oxytocin and vasopressin was concentration-dependently inhibited by the specific kappa-receptor agonist U69593, whereas bremazocine only inhibited the secretion of oxytocin markedly. Treatment with naloxone enhanced the evoked release of oxytocin significantly without effect on vasopressin secretion. The U69593-mediated inhibition of oxytocin release was abolished by naloxone, whereas that of vasopression was unaffected. Naloxone did not reverse the bremazocine-induced inhibition of hormone release. The data support the theory of an inhibiting endogenous control over oxytocin secretion and show that the release of oxytocin and vasopresin is differentially affected by the two K-receptor agonists.

Interleukin-1 beta stimulates the release of vasopressin from rat neurohypophysis

The release of vasopressin from the isolated superfused rat neurohypophysis was measured. The electrically evoked release of vasopressin after phasic submaximal stimulation was increased on exposure to the cytokine, interleukin-1 beta (44 pM). The release returned to its control level when the peptide was withdrawn. The results indicates a permissive role of interleukin-1 beta in the release of vasopressin.

GABA receptor stimulation increases the release of vasopressin and oxytocin in vitro

The release of oxytocin and vasopressin from rat neurointermediate lobes was determined in vitro. The electrically evoked release of posterior pituitary hormones was markedly potentiated by the GABA receptor agonist, isogauvacin, an effect which was abolished by the GABAA receptor antagonist bicuculline. Spontaneous hormone outflow was not affected by the substances tested. The results suggest the existence of a GABA receptor on the terminal fibres in the pituitary, facilitating the release of oxytocin and vasopressin.

Morphine antidiuresis in conscious rats: contribution of vasopressin and blood pressure

Intracerebroventricular injections of morphine (5-20 micrograms) produced a dose dependent antidiuresis in the conscious hydrated rat. Naloxone pretreatment completely abolished this antidiuretic effect, but pretreatment with a specific antidiuretic vasopressin antagonist did not change the morphine antidiuresis. The vasopressin concentration in the first voided urine after antidiuresis from morphine treated rats was found to be in the same range as the vasopressin concentration in urine from saline treated rats. Injections of 20 micrograms morphine intracerebroventricularly in conscious hydrated rats gave a short decrease in heart rate but not in mean arterial blood pressure. This indicates, that either liberation of vasopressin nor a fall in systemic blood pressure contribute to the morphine antidiuresis.

Effects of morphine and D-Ala, D-Leu enkephalin on the electrical activity of supraoptic neurosecretory cells in vitro

Experiments were performed to investigate the effects of morphine and [D-Ala2, D-Leu5]enkephalin on supraoptic cells in hypothalamic slices in vitro. To ensure the presence of a steady background activity, the cells were recorded with glutamate-filled glass microelectrodes and the level of activity was controlled by selecting a suitable retaining current (0.1-9.8 nA). Under these conditions, supraoptic cells showed either the non-phasic (65%) or phasic (35%) firing pattern previously associated with oxytocin or vasopressin cells, respectively. During perifusion of the slice with morphine (10 microM), 10 out of 17 non-phasic supraoptic cells were profoundly inhibited, five cells showed no response and the remaining 2 cells were excited. Similarly with [D-Ala2, D-Leu5]enkephalin (10 microM), 11 out of 15 non-phasic cells were inhibited, 3 cells showed no response and 1 cell was excited. The inhibition produced by morphine or [D-Ala2, D-Leu5]enkephalin could be reversed by concomitant application of naloxone (10 microM). In contrast to the profound effects seen in the non-phasic cells, only 1 out of 13 phasic cells tested with either morphine or [D-Ala2, D-Leu5]enkephalin was inhibited. The remaining 12 phasic cells showed no change in either their overall firing rate or pattern of activity during opiate perifusion. These results provide further evidence that, in addition to their inhibitory effects within the posterior pituitary, opiates can directly suppress the electrical activity of magnocellular neurosecretory cells at the level of the hypothalamus. However, the absence of an opiate effect on the phasic cells might suggest that the action of opioid peptides within the hypothalamus would be exerted predominantly on the oxytocin, rather than the vasopressin cells.

Differential inhibitory action by morphine on the release of oxytocin and vasopressin from the isolated neural lobe

Simultaneous release of both oxytocin and vasopressin was evoked by electrically stimulating the isolated neural lobe. Morphine inhibited the electrically evoked release of both oxytocin and vasopressin, but whereas the opiate antagonist naloxone reversed the suppression of oxytocin release it was without effect against the opiate block of vasopressin secretion. Thus, although the secretion of both neurohypophysial hormones can be suppressed by opiates, the mechanisms may involve different types of receptors. We also provide evidence that, at least for the oxytocin neurones, the presence of endogenous opioid peptides in the neural lobe limits the amount of hormone released by electrical stimulation.

Naloxone blocks exercise-stimulated water intake in the rat

To examine whether opiate receptors modulate exercise-induced water intake, we measured water intake during four consecutive hours after a one-hour swim stress in male, Sprague-Dawley rats. Increased cumulative water intake was found four hours following exercise and this response was naloxone-reversible (P = 0.06). Suppression of water intake in the naloxone-treated, exercised group was most marked in the first two hours after exercise (P less than 0.05). Non-exercised rats consumed water at a constant, linear rate (P less than 0.05) whether treated with naloxone or saline. These results indicate an endogenous opioid role in regulating exercise-induced water intake in the rat, but do not delineate whether this role reflects a non-specific stress behavior or specific physiological processes related to thirst.

Naloxone increases vasopressin secretion from the neurointermediate lobe of the hypophysis of the rat: search for the endogenous agonist

Naloxone increases the electrically induced vasopressin release from rat pituitary neurointermediate lobes under appropriate stimulation conditions. In order to examine a possible role of hypophyseal opioid peptides we studied in vitro the effect of opioid peptides and of naloxone on the electrically induced vasopressin release from the rat neurointermediate lobe or isolated neural lobe of the hypophysis. (D-Ala2,D-Leu5)-enkephalin (5 microM), dynorphin-(1-13) (Dyn; 0.2 microM), beta-endorphin (beta-End; 0.02 and 0.2 microM) and also naloxone (1 or 10 microM) increased the evoked vasopressin release from the neurointermediate lobe, but in higher concentrations (2 microM) Dyn or beta-End had no effect. After removal of the intermediate lobe, beta-End 2 microM inhibited, while naloxone 10 microM did not change the evoked vasopressin release from the isolated neural lobe. These results demonstrate that hypophyseal opioid peptides can influence vasopressin release in several ways and suggest that endogenous opioids predominantly provide inhibitory influences which depend on the presence of the intermediate lobe.