The endocrinology of the opioids

Oxytocin in the periaqueductal gray participates in pain modulation in the rat by influencing endogenous opiate peptides

Periaqueductal gray (PAG) plays a very important role in pain modulation through endogenous opiate peptides including leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek), β-endorphin (β-Ep) and dynorphin A(1-13) (DynA(1-13)). Our pervious study has demonstrated that intra-PAG injection of oxytocin (OXT) increases the pain threshold, and local administration of OXT receptor antagonist decreases the pain threshold, in which the antinociceptive role of OXT can be reversed by pre-PAG administration of OXT receptor antagonist. The experiment was designed to investigate the effect of OXT on endogenous opiate peptides in the rat PAG during the pain process. The results showed that (1) the concentrations of OXT, L-Ek, M-Ek and β-Ep, not DynA(1-13) in the PAG perfusion liquid were increased after the pain stimulation; (2) the concentrations of L-Ek, M-Ek and β-Ep, not DynA(1-13) in the PAG perfusion liquid were decreased by the OXT receptor antagonist; (3) the increased pain threshold induced by the OXT was attenuated by naloxone, an opiate receptor antagonist; and (4) the concentrations of L-Ek, M-Ek and β-Ep, not DynA(1-13) in the PAG perfusion liquid were increased by exogenous OXT administration. The data suggested that OXT in the PAG could influence the L-Ek, M-Ek and β-Ep rather than DynA(1-13) to participate in pain modulation, i.e. OXT in the PAG participate in pain modulation by influencing the L-Ek, M-Ek and β-Ep rather than DynA(1-13).

Oxytocin in the periaqueductal grey regulates nociception in the rat

Studies have demonstrated that oxytocin (OXT) plays important roles in pain modulation in the central nervous system, and there are OXT receptors in the periaqueductal grey (PAG). The experiment was designed to investigate the effect of OXT in the PAG on antinociception. The results showed that (1) intra-PAG injection of OXT increased the pain threshold, whereas the local administration of the high specific OXT receptor antagonist, desGly-NH(2), d(CH(2))(5)[D-Tyr(2), Thr-sup-4]OVT decreased the pain threshold in a dose-dependent manner; (2) Pain stimulation could elevate OXT concentration in the PAG perfusion liquid. The data suggested that OXT in the PAG was involved in the antinociceptive process through the OXT receptor.

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.

Tibolone rapidly attenuates the GABAB response in hypothalamic neurones

Tibolone is primarily used for the treatment of climacteric symptoms. Tibolone is rapidly converted into three major metabolites: 3 alpha- and 3beta-hydroxy (OH)-tibolone, which have oestrogenic effects, and the Delta 4-isomer (Delta 4-tibolone), which has progestogenic and androgenic effects. Because tibolone is effective in treating climacteric symptoms, the effects on the brain may be explained by the oestrogenic activity of tibolone. Using whole-cell patch clamp recording, we found previously that 17beta-oestradiol (E(2)) rapidly altered gamma-aminobutyric acid (GABA) neurotransmission in hypothalamic neurones through a membrane oestrogen receptor (mER). E(2) reduced the potency of the GABA(B) receptor agonist baclofen to activate G-protein-coupled, inwardly rectifying K(+) (GIRK) channels in hypothalamic neurones. Therefore, we hypothesised that tibolone may have some rapid effects through the mER and sought to elucidate the signalling pathway of tibolone's action using selective inhibitors and whole cell recording in ovariectomised female guinea pigs and mice. A sub-population of neurones was identified post hoc as pro-opiomelanocortin (POMC) neurones by immunocytochemical staining. Similar to E(2), we have found that tibolone and its active metabolite 3 beta OH-tibolone rapidly reduced the potency of the GABA(B) receptor agonist baclofen to activate GIRK channels in POMC neurones. The effects were blocked by the ER antagonist ICI 182 780. Other metabolites of tibolone (3 alpha OH-tibolone and Delta 4-tibolone) had no effect. Furthermore, tibolone (and 3 beta OH-tibolone) was fully efficacious in ER alpha knockout (KO) and ER beta KO mice to attenuate GABA(B) responses. The effects of tibolone were blocked by phospholipase C inhibitor U73122. However, in contrast to E(2), the effects of tibolone were not blocked by protein kinase C inhibitors or protein kinase A inhibitors. It appears that tibolone (and 3 beta OH-tibolone) activates phospholipase C leading to phosphatidylinositol bisphosphate metabolism and direct alteration of GIRK channel function. Therefore, tibolone may enhance synaptic efficacy through the G(q) signalling pathways of mER in brain circuits that are critical for maintaining homeostatic functions.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

Dynorphin knockout reduces fat mass and increases weight loss during fasting in mice

Endogenous opioids, particularly dynorphins, have been implicated in regulation of energy balance, but it is not known how they mediate this in vivo. We investigated energy homeostasis in dynorphin knockout mice (Dyn(-/-) mice) and probed the interactions between dynorphins and the neuropeptide Y (NPY) system. Dyn(-/-) mice were no different from wild types with regards to body weight and basal and fasting-induced food intake, but fecal output was increased, suggesting decreased nutrient absorption, and they had significantly less white fat and lost more weight during a 24-h fast. The neuroendocrine and thermal responses to fasting were at least as pronounced in Dyn(-/-) as in wild types, and there was no stimulatory effect of dynorphin knockout on 24-h energy expenditure (kilocalories of heat produced) or physical activity. However, Dyn(-/-) mice showed increased circulating concentrations of 3,4-dihydroxyphenlacetic acid and 3,4-dihydroxyphenylglycol, suggesting increased activity of the sympathetic nervous system. The respiratory exchange ratio of male but not female Dyn(-/-) mice was reduced, demonstrating increased fat oxidation. Interestingly, expression of the orexigenic acting NPY in the hypothalamic arcuate nucleus was reduced in Dyn(-/-) mice. However, fasting-induced increases in pre-prodynorphin expression in the arcuate nucleus, the paraventricular nucleus, and the ventromedial hypothalamus but not the lateral hypothalamus were abolished by deletion of Y(1) but not Y(2) receptors. Therefore, ablation of dynorphins results in increases in fatty acid oxidation in male mice, reductions in adiposity, and increased weight loss during fasting, possibly via increases in sympathetic activity, decreases in intestinal nutrient absorption, and interactions with the NPYergic system.

Through V2, not V1 receptor relating to endogenous opiate peptides, arginine vasopressin in periaqueductal gray regulates antinociception in the rat

Our previous study has proven that central arginine vasopressin (AVP) plays an important role in antinociception, and pain stimulation raises AVP concentration in the periaqueductal gray (PAG). The nociceptive effect of AVP in PAG was investigated in the rat. The results showed that microinjection of AVP into PAG increased pain threshold, whereas microinjection of V2 receptor antagonist-d(CH2)5[d-Ile2, Ile4, Ala9-NH2]AVP into PAG decreased pain threshold in a dose-dependent manner, but local administration of V1 receptor antagonist-d(CH2)5Tyr(Me)AVP did not change pain threshold; Pain stimulation elevated AVP, Leucine-enkephalin (L-Ek), Methionine-enkephalin (M-Ek) and beta-endorphin (beta-Ep), not dynorphinA(1-13) (DynA(1-13)) concentrations in PAG perfuse liquid; PAG pre-treatment with naloxone, an opiate receptor antagonist or V2 receptor antagonist completely reversed AVP-induced increase in pain threshold, however, PAG pre-treatment with V1 receptor antagonist did not influence this effect of AVP administration. The data suggest that AVP in the PAG, through V2 rather than V1 receptor, regulates antinociception, which progress relates to enkephalin and endorphin.

Arginine vasopressin enhances periaqueductal gray synthesis and secretion of enkephalin and endorphin in the rat

Previous study has proven that arginine vasopressin (AVP) enhances periaqueductal gray (PAG) secreting enkephalin and endorphin in vivo in the rat. Present work investigated that AVP effect on PAG secretion and synthesis of enkephalin and endorphin in vitro. Radioimmunoassy results showed that AVP increased leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek), beta-endorphin (beta-Ep) rather than dynorphin A(1-13) (DynA(1-13)) concentrations in PAG slice culture liquid, and V(2) receptor antagonist: d(CH(2))(5)[D-Ile(2), Ile(4), Ala(9)-NH(2)]AVP decreased L-Ek, M-Ek and beta-Ep, not DynA(1-13) concentrations in PAG slice culture liquid in a dose-dependent manner, but V(1) receptor antagonist: d(CH(2))(5)Tyr(Me)AVP did not change these peptide concentrations in PAG slice culture liquid. RT-PCR data displayed that AVP enhanced proenkephalin and proopiomelanocortin (Pro-beta-Ep) rather than prodynorphin mRNA expressions in culture PAG tissues, and V(2) receptor antagonist weakened proenkephalin and proopiomelanocortin (Pro-beta-Ep), not prodynorphin mRNA expressions in culture PAG tissues, but V(1) receptor antagonist did not influence these mRNA expressions in culture PAG tissues. The data suggest that AVP enhances PAG synthesizing and secreting enkephalin and endorphin rather than dynorphin through V(2) receptors.

Prior activation of kappa opioid receptors by U50,488 mimics repeated forced swim stress to potentiate cocaine place preference conditioning

Repeated forced-swim stress (FSS) produced analgesia, immobility and potentiation of cocaine-conditioned place preference (CPP) in wild-type C57Bl/6 mice, but not in littermates lacking the kappa opioid receptor (KOR) gene. These results were surprising because kappa agonists are known to produce conditioned place aversion and to suppress cocaine-CPP when coadministered with cocaine. The possibility that disruption of the kappa system blocked the stress response by adversely affecting the hypothalamic-pituitary axis was examined by measuring plasma corticosterone levels. However, disruption of the dynorphin/kappa system by gene deletion or receptor antagonism did not reduce the FSS-induced elevation of plasma corticosterone levels. A second explanation for the difference is that kappa receptor activation caused by FSS occurred prior to cocaine conditioning rather than contemporaneously. To test this hypothesis, we measured the effects of the kappa agonist (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide (U50,488) administered to mice at various intervals preceding cocaine conditioning. The results showed that the interaction between the kappa system and cocaine reinforcement depended on the timing of the drug pairing. Mice given U50,488 60 min prior to cocaine showed a robust, nor-BNI-sensitive potentiation of cocaine-CPP, whereas administration 15 min before cocaine significantly suppressed cocaine-CPP. In the absence of cocaine, U50,488 given 60 min prior to saline conditioning produced no place preference, whereas administration 15 min before saline conditioning produced significant place aversion. The results of this study suggest that kappa receptor activation induced by FSS prior to the cocaine-conditioning session may be both necessary and sufficient for potentiation of the reinforcing actions of cocaine.

In vivo evidence for an increase in 5alpha-reductase activity in the rat central nervous system following morphine exposure

In the present study, the effects of acute and chronic morphine exposure on testosterone concentrations in the central nervous system (CNS) and serum were investigated in rats. Acute morphine administration (5 mg/kg, s.c.) reduced significantly testosterone levels in serum and spinal cord but not in the brain. Following chronic morphine administration (orally for 21 days), the brain testosterone was also significantly reduced as well as serum and spinal cord. Since, the decrease in testosterone levels following morphine exposure was more obvious in the CNS than serum, we suggested that it cannot be caused by only a direct decline in testosterone levels in periphery, and an increased local metabolism of testosterone in the CNS might be attributed in these effects. This hypothesis was supported with the findings that pretreatment with finasteride, a 5alpha-reductase inhibitor (5 mg/kg, s.c.) blocked testosterone elimination from the CNS following morphine exposure. Moreover, the serum concentration of 5alpha-reduced metabolites of testosterone, dihydrotestosterone and 3alpha-diol glucuronide was increased significantly following chronic morphine exposure, but not after co-treatment with finasteride. These results suggest that morphine exposure increase the CNS activity of 5alpha-reductase, which is an important metabolizing enzyme for testosterone.

Testosterone Reverses Ethanol-Induced Deficit in Spatial Reference Memory in Castrated Rats

The present study was designed to evaluate the effects of ethanol, testosterone and combination of ethanol and testosterone, on spatial reference memory and beta-endorphin (beta-EN) levels in castrated rats. Male Sprague-Dawley rats (120-150 g) were used in this study, Animals were castrated and ethanol, testosterone or combination of the drugs were administered to rats at 09:00 h. The drugs were administered after a training period of 5 days and spatial reference memory was evaluated for 7 days using the Morris water maze. One hour after the last injection, animals were sacrificed, their brains removed and dissected into cortex, hypothalamus, hippocampus and midbrain. The beta-EN levels in these brain regions were determined by radioimmunoassay. The time to find the platform (latency period) was significantly increased in ethanol-treated rats, indicating that ethanol induces deficit in spatial reference memory. On the other hand, testosterone administration improved spatial reference memory by significantly decreasing the latency period. In addition, there was a significant decrease in latency period in the animals treated with combination of ethanol and testosterone. Results also indicate that administration of ethanol resulted in a significant increase in beta-EN levels in the hippocampus and in the cortex while concurrent administration with testosterone abolished this increase. These findings clearly indicate that administration of testosterone did not only improve memory but also abolished the spatial memory deficit induced by ethanol in castrated rats.

Changes of smoking behavior and serum adrenocorticotropic hormone, cortisol, prolactin, and endogenous opioids levels in nicotine dependence after naltrexone treatment

This study was done to evaluate the therapeutic effects of naltrexone on smoking behaviors and to measure the changing of brain substances for elucidating the mode of action by naltrexone. Twenty-five voluntarily participated healthy male smokers were randomly assigned to naltrexone group or placebo group for 2 weeks. In this study, naltrexone group showed significant reduction in daily cigarette consumption amount, the expiratory CO levels, brief questionnaire for smoking urge (B-QSU) score, and FTQ score. However, only 2 subjects in naltrexone group quitted smoking completely at 4th week. Plasma levels of pituitary hormones (ACTH, cortisol, and prolactin) and endogenous opioids (beta-endorphin and dynorphin A) were checked weekly before and after the 'provocation and smoking coupled' stimulus once in a week for 3 weeks. In naltrexone group, pituitary hormones showed upward tendencies even though only the prolactin had statistical significance. However, beta-endorphin and dynorphin A were not significantly different between the two groups. It was suggested that naltrexone made effects on hypothalamo-pituitary-adrenocortical axis activity as well as smoking behavior. However, the meaning of these endocrinal changes by naltrexone is not conclusive, whether it is beneficial or aversive.

Opioid-induced proliferation through the MAPK pathway in cultures of adult hippocampal progenitors

Administration of opioid agonists or antagonists has been reported to regulate proliferation or survival of neural progenitors in vivo. Here we report that beta-endorphin and selective mu-opioid receptor (MOR) and delta-opioid receptor (DOR) agonists stimulate proliferation of isolated rat adult hippocampal progenitors (AHPs). The AHPs were found to express DORs and MORs, but not kappa-opioid receptors. Incubation with beta-endorphin for 48 h increased the number of AHPs found in mitosis, the total DNA content, and the expression of proliferating cell nuclear antigen. This proliferative effect from beta-endorphin on AHPs was antagonized by naloxone. The beta-endorphin-induced proliferation was mediated through phosphorylation of extracellular signal-regulated kinases 1 and 2 and dependent on phosphatidylinositol 3-kinase and both intra- and extracellular calcium. These data suggest a role for the opioid system in the regulation of proliferation in progenitors from the adult hippocampus.

Review article: the concept of entero-colonic encephalopathy, autism and opioid receptor ligands

There is growing awareness that primary gastrointestinal pathology may play an important role in the inception and clinical expression of some childhood developmental disorders, including autism. In addition to frequent gastrointestinal symptoms, children with autism often manifest complex biochemical and immunological abnormalities. The gut-brain axis is central to certain encephalopathies of extra-cranial origin, hepatic encephalopathy being the best characterized. Commonalities in the clinical characteristics of hepatic encephalopathy and a form of autism associated with developmental regression in an apparently previously normal child, accompanied by immune-mediated gastrointestinal pathology, have led to the proposal that there may be analogous mechanisms of toxic encephalopathy in patients with liver failure and some children with autism. Aberrations in opioid biochemistry are common to these two conditions, and there is evidence that opioid peptides may mediate certain aspects of the respective syndromes. The generation of plausible and testable hypotheses in this area may help to identify new treatment options in encephalopathies of extra-cranial origin. Therapeutic targets for this autistic phenotype may include: modification of diet and entero-colonic microbial milieu in order to reduce toxin substrates, improve nutritional status and modify mucosal immunity; anti-inflammatory/immunomodulatory therapy; and specific treatment of dysmotility, focusing, for example, on the pharmacology of local opioid activity in the gut.

Intracerebroventricular met-enkephalin administration modulates adjuvant arthritis

The purpose of this study was to investigate the anti-inflammatory properties of intracerebroventricular met-enkephalin (met-enk) administration in an animal model of arthritis. Adjuvant arthritis was induced in rats by intradermal inoculation of mycobacterium butyricum and the effects of intraventricular met-enk+thiorphan (enkephalinase inhibitor) were studied. Treatment was initiated either simultaneously with the bacterial inoculation (preventive group) or on post-inoculation day 17 after the appearance of inflammation (treatment group). The degree of inflammation was evaluated by measuring the diameter and the circumference of the ankle joint immediately before the sacrifice (day 31) and by histologic examination of ankle joint sections. The results of this study revealed that combined intraventricular injections of met-enk+thiorphan reduced the arthritic-like inflammation in the preventive group as well as in the treatment group. These findings suggest that centrally applied met-enk+thiorphan may suppress the development adjuvant arthritis as well as the symptoms of manifest arthritis. Thus central met-enk may be involved in both hypothalamic pituitary adrenal axis and immune forms of stress-induced modulation.

Opioids intrinsically inhibit the genesis of mouse cerebellar granule neuron precursors in vitro: differential impact of mu and delta receptor activation on proliferation and neurite elongation

Although opioids are known to affect neurogenesis in vivo, it is uncertain the extent to which opioids directly or indirectly affect the proliferation, differentiation or death of neuronal precursors. To address these questions, the intrinsic role of the opioid system in neurogenesis was systematically explored in cerebellar external granular layer (EGL) neuronal precursors isolated from postnatal mice and maintained in vitro. Isolated neuronal precursors expressed proenkephalin-derived peptides, as well as specific mu and delta, but negligible kappa, opioid receptors. The developmental effects of opioids were highly selective. Morphine-induced mu receptor activation inhibited DNA synthesis, while a preferential delta2-receptor agonist ([D-Ala2]-deltorphin II) or Met-enkephalin, but not the delta1 agonist [D-Pen2, D-Pen5]-enkephalin, inhibited differentiation within the same neuronal population. If similar patterns occur in the developing cerebellum, spatiotemporal differences in endogenous mu and delta opioid ligand-receptor interactions may coordinate distinct aspects of granule neuron maturation. The data additionally suggest that perinatal exposure to opiate drugs of abuse directly interfere with cerebellar maturation by disrupting normal opioid signalling and inhibiting the proliferation of granule neuron precursors.

PP/PYY cells from endocrine pancreas of the scincid lizard Eumeces inexpectatus synthesize ACTH- and alpha-MSH-like molecules

The endocrine pancreas of the scincid lizard Eumeces inexpectatus secretes four major hormones, insulin, glucagon, somatostatin, and pancreatic polypeptide (PP); in addition, other peptides and neuropeptides, often colocalized in one of the principal cell types (A, B, D, and PP), were detected by light and ultrastructural immunocytochemistry. In particular, the pancreas is rich in peptide tyrosine tyrosine (PYY), ACTH, and alpha-MSH immunoreactivity. When single- and double-immunolabeled serial sections were compared for immunostaining for PP, PYY, ACTH, and alpha-MSH, there was broad coincidence with PP, termed PP/PYY, cells in view of the extensive colocalization of these two peptides. Furthermore, ultrastructural morphometric studies revealed similar secretory granules for PP immunoreactive (ir) and ACTH ir cells, while the endocrine cells express pro-opiomelanocortin (POMC) mRNA, indicating an active, extrapituitary synthesis of the POMC-derived peptides in these cells. In conclusion, the presence of POMC-derived peptides in the endocrine pancreatic cells suggests that they may regulate insulin secretion.

Combination treatment of hepatic encephalopathy due to thioacetamide-induced fulminant hepatic failure in the rat with benzodiazepine and opioid receptor antagonists

BACKGROUND/AIMS

Treatment of hepatic encephalopathy with drugs acting on the target organ of this syndrome, the brain, is unsatisfactory. Combination treatment with different neurotransmitter receptor antagonists may be a rational option to optimize treatment.

METHODS

The effects of various doses of the benzodiazepine receptor antagonist Ro 15-3505 and the opioid receptor antagonist naloxone, alone or in combination, were tested on hepatic encephalopathy in rats with thioacetamide-induced hepatic failure in an open-field activity meter. Comparison of single and combination treatment was also done using a neurological test battery. In addition, we compared survival of treatment-responder rats with treatment non-responders.

RESULTS

Naloxone dose dependently increased ambulatory activity and improved neurological score. Ro 15-3505 also improved ambulatory activity and neurological score; however, the improvement was less evident at higher doses. Combination treatment was not superior to single treatment. Survival was increased in treatment-responder rats.

CONCLUSIONS

The failure of combination treatment with Ro 15-3505 and naloxone to further improve hepatic encephalopathy may suggest that the two neurotransmitter systems are interrelated or that hepatic encephalopathy may not be further improved by drugs acting on the brain.

Opioids modulate cell division in the germinal zone of the late embryonic neocortex

Opioid effects on cell division in the embryonic cerebral cortex were examined using two experimental approaches: (i) the presence of opioid receptors in the embryonic day 16 mouse neocortex was tested using immunohistochemical techniques; (ii) the values of the indices of [3H]thymidine pulse labelled cells and mitotic indices were estimated in the ventricular zone of the embryonic day 16 mouse neocortex 2.5, 4.5 and 8.5 h after administration to pregnant females of selected opioid receptor agonists or the opioid antagonist naloxone. The immunohistochemical study demonstrated that distinct subpopulations of the ventricular zone cells express mu, delta or kappa opioid receptors. Acute exposure of mouse embryos to mu, delta and kappa opioid receptor agonists or naloxone differentially affects the indices of [3H] thymidine pulse labelled cells and mitotic indices indicating changes in the cell cycle composition. Treatment with the mu opioid receptor agonist D-Ala2-MePhe4, Gly-ol5-enkephalin (DAGO), or the partially selective kappa opioid receptor agonist bremazocine, increased the [3H]thymidine labelling and mitotic indices. In contrast, the delta receptor agonist (D-Ser8)-leucine enkephalin-Thr (DSLET) produced a decrease in the labelled cell indices and mitotic indices. Naloxone provided a biphasic effect: a decrease in the values of labelled cell indices 2.5 h after naloxone administration, followed by an increase in the values of the indices at 4.5 and 8.5 h. These results suggest that the endogenous embryonic/maternal opioid systems are involved in the regulation of cell division in the ventricular zone of the late embryonic cortex.

Centrally applied NPY mimics immunoactivation induced by non-analgesic doses of met-enkephalin

Neuropeptide Y (NPY) and endogenous opioids (EOPs) such as methionine-enkephalin (Met-enk) regulate similar physiological responses, but it is not known whether nociceptive and immune responses also show analogy after intracerebroventricular (i.c.v.) application. Dose-response studies show that Met-enk stimulates the blood granulocyte and splenic natural killer (NK) cell function of Lewis rats at a low dose (10(2) ng/kg, i.c.v.), whereas a high dose (10(5) ng/kg) causes suppression of innate immune functions associated with analgesia in the hot-plate test. At 15 min, 1 h and 24 h after i.c.v. application, both Met-enk (10(2) ng/kg) and NPY (1 ng/kg) produced similar effects: An initial suppression of innate immune function was followed by a long lasting stimulatory action on cell functions and serum interleukin-6 (sIL-6) levels. Thus, central NPY application resembles Met-enk-induced immunostimulation at doses not affecting nociception, suggesting an involvement of both peptides in shaping stress-induced immunomodulation of the non-analgetic form, possibly via activation of a common immunomodulatory effector mechanism.

Opioid receptor ligands in human hepatic encephalopathy

BACKGROUND/AIMS

Opioid peptides may contribute to some of the manifestations of hepatic encephalopathy. To address the role of the opioid system in the pathogenesis of hepatic encephalopathy, three representative opioid ligands were measured in plasma and cerebrospinal fluid of patients with hepatic encephalopathy.

METHODS

Plasma and cerebrospinal fluid were obtained in three groups of patients: group 1: patients with hepatic encephalopathy; group 2: patients with lumbar back pain; group 3: healthy controls. Met-enkephalin, leu-enkephalin and beta-endorphin levels were measured in extracted plasma and cerebrospinal fluid samples by radioimmunoassay.

RESULTS

Plasma met-enkephalin levels were 656% (p<0.05) and 301% (p<0.05) and cerebrospinal fluid met-enkephalin levels were 1481% (p<0.01) and 645% (p<0.05) higher when compared to healthy control and pain control patients, respectively. Although plasma and cerebrospinal leu-enkephalin levels were elevated in patients with hepatic encephalopathy, the increases were not statistically significant. Plasma and cerebrospinal beta-endorphin levels were similar in the three study groups.

CONCLUSIONS

The results of this study support accumulating data on the role of the delta opioid receptor ligand met-enkephalin in the pathogenesis of hepatic encephalopathy, and provide a rationale for the use of opioid receptor antagonists in the treatment of hepatic encephalopathy.

Orphanin FQ stimulates prolactin and growth hormone release in male and female rats

Intracerebroventricular administration of Orphanin FQ (5.5, 55 or 550 pmol) caused a dose-related increase in prolactin secretion in both male and female rats and stimulated GH secretion in males. The magnitude of the prolactin secretory response was greater in females than in males. These effects of OFQ on prolactin and growth hormone release are the same as the stimulatory effects of the endogenous opioid peptides.

mu-Opioid agonist-stimulated [35S]GTPgammaS binding in guinea pig hypothalamus: effects of estrogen

mu-Opioid receptors play a critical role in the regulation of the female reproductive cycle, and estrogen modulates the coupling of mu-opioid receptors to a potassium channel in the basal hypothalamus (BH) of the female guinea pig. Therefore, we ascertained the distribution of mu-opioid receptors in the BH with autoradiography using the mu-opioid selective agonist [3H]DAMGO. In addition, we investigated the effects of estrogen on DAMGO- or the GABAB receptor agonist baclofen-stimulated [35S]GTPgammaS binding in the BH. Based on the high density of mu-opioid receptors, but the lack of effects of estrogen on [35S]GTPgammaS binding, we conclude that mu-opioid receptor interaction with its G-protein is not the target of estrogen's actions.

Effects of central injection of kyotorphin and L-arginine on oxytocin and vasopressin release and blood pressure in conscious rats

Intracerebroventricular (I.C.V.) administration of an inhibitor of nitric oxide synthase (NOS) increases oxytocin but not vasopressin secretion, in dehydrated rats [38]. Surprisingly, central injection of L-arginine, the substrate for NOS, caused a similar effect. Kyotorphin (L-tyrosyl-L-arginine), a dipeptide formed from L-arginine by kyotorphin synthetase in the brain may mediate this magnocellular response. Therefore, the dose and time responses of hormone release were compared following I.C.V. injection of kyotorphin and L-arginine to conscious rats that were normally hydrated or deprived of water for 24 h. In water-sated rats, both L-arginine and kyotorphin increased blood pressure and plasma glucose levels coincident with elevating circulating levels of oxytocin, but not vasopressin. In dehydrated animals, both L-arginine and kyotorphin increased plasma oxytocin levels with a similar time course but only kyotorphin decreased vasopressin release. D-arginine, like L-arginine, stimulated secretion of oxytocin, indicating a nonstereospecific effect. A kyotorphin receptor antagonist (L-leucyl-L-arginine) given I.C.V. to dehydrated animals elevated plasma oxytocin and prevented the decrease in vasopressin levels after kyotorphin. Thus, kyotorphin, but not L-arginine, appears to attenuate release of vasopressin either directly from magnocellular neurons or indirectly via modulating compensatory reflexes activated by the pressor response. On the other hand, an excess of L-arginine and kyotorphin within the CNS may mimic the stress response by augmenting release of oxytocin and activating the sympathetic nervous system to increase blood pressure and plasma glucose levels.

Effect of chronic naloxone and morphine treatments on testicular, body weight, and plumage pigmentation cycle of lal munia, Estrilda amandava

At Imphal (24 degrees 44' N) testes of lal munia, Estrilda amandava, began in June/July, peaked in September/ October, and thereafter declined to a minimum in December/January. Daily im treatments of 2.5-10 mg/kg/ bird/30 days of naloxone during progressive phase suppressed testicular growth, but without effects during quiescent, peak, and regression phases. Daily morphine (5 mg/kg/bird) during progressive and peak phases stimulated testicular growth, but without effects during quiescent and regression phases. Daily morphine (10 mg/kg/bird) during progressive phase stimulated the testes, an effect reversed by daily im treatments of an equivalent dose of naloxone. Seasonal changes in body weight closely correlated with testicular size. Daily im naloxone (5 and 10 mg/kg/bird/30 days) during progressive phase inhibited the increased body weight, but had no effects during quiescent, peak, and regression phases. Morphine (5 mg/kg/bird/day) during progressive and peak phases increased body weight, but had no effects during quiescent and regression phases. Morphine (10 mg/kg/bird/day) during progressive phase increased body weight, an effect which was reversed by equivalent dose of naloxone. Plumage color increased progressively between May and August/September, was maintained during October, and thereafter declined to reach dull-brown henny feathers by December. Daily im naloxone (2.5-10 mg/kg/bird/30 days) regardless of the reproductive states did not affect plumage color cycle. Morphine (5 mg/kg/ bird/day) accelerated plumage pigmentation between June and August, but had no effect during progressive or peak phases. Postnuptial decline in plumage color was inhibited by morphine (5 and 10 mg/kg/bird/day) and naloxone failed to reverse this effect. It is concluded that in the lal munia, endogenous opioid peptides are important constituents of the neuroendocrine mechanisms that influence development of the testes and body weight.

Evidence for an interaction between alpha-MSH and opioids in the regulation of gonadotropin secretion in man

Gonadotropin secretion is inhibited by the endogenous opioids and stimulated by their antagonist naloxone. LH secretion is stimulated by alpha-MSH, a tridecapeptide derived from the post-translational processing of POMC. The possibility that alpha-MSH interacts with the opioids, as suggested by the experimental evidence, was investigated in 7 normal males aged 24-29 through the performance of seven tests: naloxone (0.8 mg i.v. bolus, followed by infusion of 1.6 mg/h for 120'); alpha-MSH (2.5 mg i.v. bolus); naloxone + alpha-MSH (2.5 mg i.v. 15' after commencement of the naloxone infusion); naloxone + GnRH (100 micrograms i.v. 15' after commencement of the naloxone infusion); alpha-MSH + GnRH (respectively 2.5 mg and 100 micrograms at time 0), GnRH alone (100 micrograms at time 0), placebo (150 nmol/l NaCl solution). The LH AUCs during both naloxone (30.3 +/- 2.7 mIU/ml.min-1) and alpha-MSH test (32.9 +/- 4.6 mIU/ml.min-1) were significantly greater (p < 0.005) than that observed during placebo (16.9 +/- 3.6 mIU/ml.min-1). The LH AUC during alpha-MSH + naloxone (37.6 +/- 2.6 mIU/ml.min-1) was not significantly different from that recorded during their separate administration. GnRH injected alone, during the naloxone infusion and with alpha-MSH produced similar increases in LH, that were significantly higher than that observed during the other tests (AUCs: GnRH 89.4 +/- 10.6, GnRH + naloxone 100.5 +/- 9.1, GnRH + alpha-MSH 94.6 +/- 7.9 mIU/ml.min-1, p < 0.001). Significant increase in FSH (p < 0.001) was only observed during GnRH, GnRH + naloxone and GnRH + aMSH tests (AUCs: placebo 13.3 +/- 1.7; naloxone 14.7 +/- 2.5; alpha-MSH 15.5 +/- 2.3; alpha-MSH + naloxone 16.9 +/- 1.9; GnRH 19.1 +/- 1.1; GnRH + alpha-MSH 20.7 +/- 1.3; GnRH + naloxone 21.2 +/- 1.8 mIU/ml.min-1). These results are in line with the possibility of an interaction between alpha-MSH and the opioids in the regulation of gonadotropin secretion, perhaps with opposing effects on a final common pathway.

The role of hypophyseal and adrenal mechanisms in the hypoalgesic response to non-contingent food delivery in the rat

Exposure to non-contingent food delivery has been shown to elicit an increase in nociceptive thresholds in rats. This hypoalgesia has previously been shown to be reversible by the opiate antagonist naloxone. Given that most opioid forms of SIA are found to rely on an intact pituitary-adrenal axis, this research examined the possibility that the hypoalgesic response to non-contingent food delivery is also hormonally mediated. Hypophysectomy (Expt. 1) but not bilateral adrenalectomy (Expt. 2) was found to completely attenuate the hypoalgesic response to non-contingent food delivery. Preliminary data from ongoing research is presented that points to the possible involvement of the hypophyseal peptides beta-endorphin and adrenocorticotropin hormone (ACTH).

Effect of central administration of the kappa-opiate receptor agonist U 69.593 on neurohypophyseal hormone levels in blood and cerebrospinal fluid

Intracerebroventricular (i.c.v.) administration of the kappa-opiate receptor agonist U 69.593 induces a rapid and short lasting suppression of oxytocin (OXT) levels in plasma of water deprived rats, whereas only a tendency towards a suppression of vasopressin (AVP) levels in plasma is observed. No change in neurohypophyseal hormone levels in CSF occurs following i.c.v. administration of U 69.593 at the various times points studied. It is concluded that, upon i.c.v. administration, the suppressive influence of U 69.593 is much weaker than that of the dynorphins and that neurophypophyseal hormone levels in CSF behave differently from those in the peripheral circulation.

Distribution and changes in mu- and kappa-opiate receptors during the midlife neurodevelopmental period of Coho salmon, Oncorhynchus kisutch

Parr-smolt transformation (PST) in coho salmon is associated with a plasma thyroid hormone (PT4) surge and a critical period of neural development that includes axonal sprouting, neurogenesis, and surges of select neurotransmitters. Here we provide a description of the selectivity, distribution, and the changes in the density of mu- and kappa-opiate receptors during PST, as revealed by quantitative in vitro autoradiography of [3H]Tyr-D-Ala-Gly-NMe-Phe-Gly-ol ([3H]DAMGO) and [3H]ethylketocyclazocine ([3H]EKC), respectively. The concentration of mu-receptors increased significantly in select cell groups in the early stages of parr-smolt transformation, until a peak was reached at the time coinciding with the peak of the PT4 surge. In other cell groups, the peaks occurred 1 or 2 weeks later. With one exception, this increase was followed by a decrease in concentration. The brain areas showing the highest concentrations are the dorsal nucleus of the ventral telencephalic area, the glomerular region, the granular layer of the valvula cerebelli, the nucleus diffuses of the inferior lobe, and the nucleus diffuses of the torus lateralis. Other regions with distinctly elevated mu-receptor concentrations are the stratum griseum centrale of the optic tectum and the preoptic area. The distribution of kappa-receptors is more diffuse, and the densities are considerably lower. The overlap in distribution of mu- and kappa-receptors is considerable, but significant exceptions are noted. For example, the dorsomedial nucleus of the dorsal telencephalic area, the habenular nucleus, and the dorsomedial nucleus of the thalamus exhibit a surge in density of kappa-receptors at the time of the PT4 surge, while the density of mu-receptors in these nuclei remain very low throughout parr-smolt transformation. The kappa-receptor containing cell groups are not identifiable until 3 weeks before the PT4 surge because of low densities. The most prominently labeled kappa-receptor regions are the ventral and dorsal nuclei of the ventral telencephalic area, the medial and dorsal zones of the dorsal telencephalic area, the optic tectum (all layers), the dorsomedial nucleus of the thalamus, the torus lateralis of the ventral hypothalamus, and the preoptic area. An increase of mu- and kappa-opiate receptor densities in specific brain regions may reflect roles in the alteration of brain organization, olfactory imprinting, neuroendocrine activity or other physiological activities. The overall distribution of these receptors are relatively more extensive in salmon than in other vertebrates so far studied.

Effects of melatonin in vivo upon luteinizing hormone and prolactin releases induced by opiate receptor antagonists in adult male rats

The effects of melatonin on LH and PRL releases induced by treatment with naloxone, naloxone methyliodide and nalmefene were studied in adult male rats. Subcutaneous melatonin injection (1.4 mg/Kg) had no effect on LH secretion, but caused an inhibition effect (84%) on LH release induced by naloxone (2.4 mg/Kg). Melatonin too totally inhibited LH secretion induced by naloxone methyliodide (2.8 mg/Kg) and nalmefene (2 mg/Kg) when it was simultaneously administered with each opioid receptor antagonist. Melatonin alone had no significant effect on serum PRL levels, but decreased by 25.5% the inhibitory effect potency of nalmefene on PRL secretion after simultaneous injections. The inhibitory effect potency of naloxone on PRL release increased (16%) when it was administered with melatonin. Simultaneous injection of melatonin with naloxone methyliodide inhibited PRL release (78%) while naloxone methyliodide alone did not modify this secretion. The results obtained with a quaternary opioid antagonist indicate that the opioid receptor type which mediates LH and PRL responses is located respectively outside and inside the blood-brain barrier. Our findings show that opiate antagonists and their quaternary ammonium salts affect secretion of LH and PRL through different mechanisms susceptible to the influence of melatonin.

Peptide quantification by tandem mass spectrometry

This manuscript reviews the literature on the mass spectrometry (MS) and tandem mass spectrometry (MS/MS) quantification of biologically important peptides that have been extracted from tissues. The most important aspect of this quantification process is the use of MS/MS to link the protonated molecule ion, (M + H)(+) , of the peptide with one or more of its amino acid sequence-determining fragment ions. The actual name of a peptide cannot be used in any study until the amino acid sequence of that peptide has been firmly established. This article reviews the analytical data obtained from the measurement of opioid peptides in human pituitary tissues. For example, the proopiomelanocortin (POMC)-derived beta-endorphin (BE) and the proenkephalin-derived methionine enkephalin (ME) opioid peptides have been quantified. The biogenesis of opioid neuropeptides is briefly reviewed; critical aspects of pituitary neuropeptides are discussed, including their localization and regulation, and their role in tumor formation; other analytical methods used to detect and measure neuropeptides are mentioned, including radioimmunoassay (RIA), radioreceptorassay (RRA), in situ hybridization, mRNA, and cDNA methods; and the MS and MS/MS methods are described. The use of stable isotope-incorporated synthetic peptide internal standards is described. Data are presented on the measurement of BE and ME in control pituitaries and in pituitary tumors (PRL-secreting and nonsecreting tumors). A significant alteration in the POMC peptide BE was found between the control and tumor tissues. That difference suggests that the POMC neuropeptidergic system had been down-regulated in those tumors. © 1997 John Wiley & Sons, Inc.

Estrogen modulation of two subpopulations of β-endorphin neurons in ovariectomized guinea pigs distinguished by peripherally injected fluorogold

β-endorphin released by neurons in the arcuate nucleus affects the output of several neuroendocrine systems and estrogen levels modulate the production and secretion of β-endorphin. We used intraperitoneal injection of fluorogold to retrogradely label the cell bodies of neurons that project outside the blood-brain-barrier in conjunction with immunohistochemistry for β-endorphin to dual label the subpopulation of β-endorphin neurons that project to the median eminence or other sites of access to the peripheral circulation. We found that some identified β-endorphin neurons in the arcuate nucleus of ovariectomized guinea pigs sequestered fluorogold. Approximately 7% of β-endorphin-containing cells co-localized with fluorogold. The effect of estrogen on the number of identified β-endorphin cells was examined. A single estradiol benzoate injection to ovariectomized guinea pigs 24 h prior to sacrifice dramatically decreased the total number of β-endorphin cells identified in the rostral, medial and the caudal portions of the arcuate nucleus. Also, a significantly smaller percentage of fluorogold-filled cells was found to contain β-endorphin immunoreactivity in the estrogen-treated group. These data suggest that a subpopulation of β-endorphin neurons has access to the peripheral circulation and may alter the output of neurosecretory terminals at the level of the median eminence. Furthermore, estrogen affects this subpopulation and the general population of β-endorphin neurons in the arcuate nucleus in a similar manner.

Melatonin modifies prolactin release induced by opiate antagonists in male rats

The effect of melatonin on the prolactin (PRL) release induced by treatment with naloxone, naloxone methyliodide, naltrexone and nalmefene were studied in adult male rats. Subcutaneous melatonin injection (1.4 mg/Kg) had no significant effect on serum PRL levels, but decreased by 29% and 26% respectively the inhibitory effect potency of naltrexone (2.5 mg/Kg) and nalmefene (2 mg/Kg) on PRL secretion after simultaneous injections. The inhibitory effect potency of naloxone on PRL release increased (16%) when it was administered with melatonin. Simultaneous injection of melatonin with naloxone methyliodide (2.8 mg/Kg) inhibited PRL release (77.5%) while naloxone methyliodide alone did not modify this secretion. The results obtained with a quaternary opioid antagonist indicate that the opioid receptor type which mediates PRL response is located inside the blood-brain barrier. Our findings show that opiate antagonists and their quaternary ammonium salts affect secretion of PRL through mechanism susceptible to the influence of melatonin.

Beta-endorphin inhibits prostaglandin synthesis in rat ovaries and blocks induced ovulation

The aim of the present study was to explore the action of exogenous beta-endorphin on the number of oocytes ovulated and on prostaglandin (PG) production in ovaries isolated from pregnant mare's serum gonadotropin/human chorionic gonadotropin(PMSG/hCG)-primed immature rats. An intrabursal injection of the opioid (0.084 microgram) was given 4 hours after hCG and the number of oocytes within the oviducts on the following morning was reduced (P < 0.05). The same effect was also attained with an intraperitoneal (IP) injection (0.5 microgram). The time course of PG synthesis was quantified in ovaries of rats treated with an IP injection. Eight hours after hCG, prostaglandin content increased (P < 0.01) and remained high until 12 hours after hCG (P < 0.001). This increase was inhibited by the in vivo treatment with beta-endorphin. On isolated ovaries, beta-endorphin (10(-8) M) had a clear inhibitory action on prostaglandin production. beta-Endorphin's effect on prostaglandin synthesis in the ovaries is of importance in the ovulatory process. The possible physiological role of beta-endorphin merits further investigation.

Repeated morphine administration down-regulates glucocorticoid, but not mineralocorticoid, receptors in the rat hippocampus

Activation of the hypothalamic-pituitary-adrenal axis (HPA) is known to be an important component of a neuroendocrine response to opiates. Since this axis is under inhibitory control of hippocampal corticosteroid receptors, the present study has been designed to determine the effects of single (20 mg/kg IP) and repeated (from 20 to 100 mg/kg/day, IP, twice a day for 10 days) morphine administration on binding parameters of these receptors. Glucocorticoid (GR) and mineralocorticoid (MR) receptors were examined by an in vitro [3H]-corticosterone binding in the cytosol from the rat hippocampus, using the selective GR agonist RU 28362 to discriminate between MR and GR. Repeated, but not single, treatment with morphine significantly (by approximately 22%) reduced the density of GR at 2 h and tended to decrease it at 72 h after the last drug injection. The density of MR was unchanged at those time points. No changes in dissociation constants of either type of the receptors were found following single or repeated morphine administration. The plasma corticosterone level was significantly increased 1 h after acute and chronic administration of morphine (382 and 527%, respectively). These results indicate that repeated morphine administration downregulates GR receptors, which may impair the feedback control mechanism of the HPA activity.

Morphine inhibits Purkinje cell survival and dendritic differentiation in organotypic cultures of the mouse cerebellum

The effects of morphine on the morphogenesis and survival of calbindin-D28k-immunoreactive Purkinje cells were studied in organotypic explant cultures isolated from 1- or 7-day-old mouse cerebella. To reduce experimental variability, bilaterally matched pairs of organotypic cultures were used to compare the effects of opiate drug treatment. One explant within each pair was untreated, while the remaining explant was continuously treated for 7 to 10 days with morphine, morphine plus naloxone, or naloxone alone. In explants derived from 1-day-old mice, morphine treatment significantly reduced Purkinje cell dendritic length compared to symmetrically matched untreated control explants. The concentration of morphine estimated to cause a half-maximal reduction (EC50) in dendritic length was 4.9 x 10(-8) M. At higher concentrations (EC50 = 3.6 x 10(-6) M), morphine also significantly decreased the number of Purkinje cells in explants from 1-day-old mice compared to untreated explants. Electron microscopy identified increased numbers of degenerating Purkinje cells in explants derived from 1-day-old mice. This showed that high concentrations (10(-5) M) of morphine reduced Purkinje cell numbers by decreasing their rate of survival. In explants derived from 7-day-old mice, morphine (10(-5) M) neither affected Purkinje cell dendritic length nor cell numbers compared to symmetrically matched untreated (control) explants. Collectively, these findings suggest that morphine per se, through a direct action on the cerebellum, can affect Purkinje cell differentiation and survival. The results additionally suggest that there is a critical period during development when Purkinje cells are especially vulnerable to the effects of morphine.

Enkephalinase inhibition facilitates sexual behavior in the male rat but does not produce conditioned place preference

The effects of two enkephalinase inhibitors, SCH 34826 and phospho-leu-phe, on male rat sexual behavior and conditioned place preference were evaluated. SCH 34826, administered intraperitoneally, reduced the ejaculation latency to both the first and second ejaculation at a dose of 30 mg/kg. This dose also reduced the first postejaculatory interval. No other effect was obtained with this drug. Phospho-leu-phe, administered intracerebroventricularly, increased mount and intromission latency at doses of 50 and 100 micrograms. A dose of 25 micrograms reduced the latency to the first ejaculation as well as the number of preejaculatory intromissions. The postejaculatory interval was also reduced at this dose. SCH 34826, 100 and 30 mg/kg, and phospho-leu-phe, 25 micrograms, had no effect in the conditioned place preference procedure. These observations seem to suggest that there is no functionally relevant tonic release of enkephalins. Therefore, the effects obtained on sexual behavior may indicate that enkephalins are released before and during the course of sexual activity. The function of such a release could be to facilitate ejaculatory mechanisms in the way found in the present studies. Previous work has shown that ejaculation-induced reward is opioid dependent, further supporting the hypothesis of opioid release during sexual activity. Taken together, these data suggest an important role for opioids, probably enkephalins, in the physiological control of sexual behavior.

Effects of estrogen on the number of neurons expressing beta-endorphin in the medial basal hypothalamus of the female guinea pig

The distribution pattern of immunoreactive beta-endorphin neurons was studied in female guinea pigs that were ovariectomized, and one week later were injected with 25 micrograms estradiol benzoate or oil. The animals (5 from each group) were perfused after 24 hours with 4% paraformaldehyde. The locations of beta-endorphin cells and fibers were determined using avidin-biotin immunohistochemistry on free-floating vibratome sections. beta-endorphin-immunoreactive fibers were distributed widely throughout specific regions of the rostral forebrain, similar to what has been described in other species. beta-endorphin cell bodies were found in the arcuate nucleus and in adjacent ventrolateral areas throughout the rostrocaudal extent of the basal hypothalamus. Cells immunoreactive to beta-endorphin were also present in the caudal part of the ventromedial nucleus of the hypothalamus. The number of beta-endorphin neurons was quantified in anatomically matched sections through the rostral, medial and caudal basal hypothalamus of estradiol benzoate- and oil-treated guinea pigs. Analysis of variance revealed that the number of immunoreactive beta-endorphin cells was significantly increased in all regions of the basal hypothalamus of estrogen-treated guinea pigs as compared to vehicle-treated animals (P < 0.01). These data indicate that in the guinea pig, the number of neurons expressing beta-endorphin is increased in the arcuate nucleus 24 hours after estrogen treatment.

Ovarian melanotropic peptides and adaptation in two teleostean species: Sparus aurata L. and Dicentrarchus labrax L

The ovarian tissue of Dicentrarchus labrax and Sparus aurata displays two immunoreactive peaks that correspond to the elution time of human des-acetyl alpha-MSH [ACTH(1-13)-amide] and human alpha-MSH. In view of the close identity between the primary structure of fish and human alpha-MSH, these data demonstrate that two MSH-related peptides are present both in sea bream and sea bass ovary. alpha-MSH-like immunoreactivity was found within both granulosa and thecal layers of mature follicles, as well as in the cytoplasm of oogonia of sea bream and sea bass ovary. Gonadal content of ACTH(1-13)-amide and alpha-MSH display differences with regard to season, showing the highest peptide levels in reproductive animals. Moreover, the alpha-MSH content is significantly higher in the ovary of fish farm animals, whereas that of ACTH(1-13)-amide prevails in wild fish ovary.

Effects of space flight stress on proopiomelanocortin, proenkephalin A, and tachykinin neuropeptidergic systems in the rat posterior pituitary

beta-endorphin-like immunoreactivity (BE-li), methionine enkephalin-like immunoreactivity (ME-li), and substance P-like immunoreactivity (SP-li) were measured in the posterior pituitary of rats that experienced a 5-day space flight in a Space Shuttle. ME-li and SP-li were both significantly lower compared to the control rats. However, there was no difference in BE-li between flight and control rats. These data suggest that the space flight stress diminished the methionine enkephalin (ME) and substance P (SP) concentrations in the posterior pituitary without affecting the beta-endorphin (BE) concentration. Thus, the proenkephalin A and tachykinin, but not proopiomelanocortin, neuropeptidergic systems in the posterior pituitary may respond to this type of unique stress.

The delta-opioid signal transduction on the gonadotropin-releasing hormone release is eicosanoid dependent

In static incubations, the K+ induced release of gonadotropin-releasing hormone (GnRH) and of prostaglandin (PG) E2, was 2-3 times higher in the isolated median eminence (ME) compared to the hypothalamic area containing the arcuate nucleus (ARN) plus the ME. The delta-opioid agonist DTLET, induced a parallel, dose-dependent reduction of GnRH and PGE2 release in the ARN plus ME. Both effects of DTLET were blocked by the delta-opioid antagonist Diallyl-G. In the isolated ME, DTLET reduced the secretion of PGE2 but enhanced the release of GnRH. In this area Diallyl-G had no effect on the PGE2 release but blocked the GnRH secretion. When the PGE2 production was blocked by indomethacin in the ARN plus ME preparation, DTLET increased the release of GnRH and induced the production of leukotrienes (LTs). On the other hand, DTLET decreased the release of both GnRH and PGE2 in the presence of nordihydroguaiaretic acid (NDGA), an inhibitor of the production of LTs. The above results suggest that: (a) the delta-opioid agonist DTLET modulates GnRH release differentially in the hypothalamic areas examined; and (b) the arachidonic acid metabolites are involved in the mode of action of DTLET on the release of GnRH in the ARN plus ME hypothalamic fragment.

Serotonergic lesions decrease mu- and delta-opiate receptor binding in discrete areas of the hypothalamus and in the midbrain central gray

Serotonergic nerve terminals in the brain were lesioned by intraventricular infusion of the selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) and levels of mu- and delta-opiate binding were measured in brain areas implicated in reproductive behavior and gonadotropin secretion. The lesion decreased mu-receptor binding in the preoptic area (mPOA) and the midbrain central gray, while delta-receptor binding was decreased in the mPOA and the dorsomedial nucleus of the hypothalamus. Hypothalamic serotonergic lesions also attenuated morphine inhibition of female sexual behavior. These results indicate the existence of serotonergic-opiate interactions in select regions of the brain and suggest that these interactions may be important in the regulation of lordosis behavior.