Anatomy of the CNS opioid systems

Cerebrospinal fluid and plasma beta-endorphin in combat veterans with post-traumatic stress disorder

Opioid-mediated analgesia develops in experimental animals following traumatic stress and increased opioid-mediated analgesia has been observed in combat veterans with post-traumatic stress disorder (PTSD). These observations have led to the hypothesis that increased central nervous system (CNS) opioidergic activity exists in patients with PTSD. However, direct CNS data on opioid peptide concentrations and dynamics in patients with PTSD are lacking. We withdrew cerebrospinal fluid (CSF) via a flexible, indwelling subarachnoid catheter over a 6-h period and determined hourly CSF concentrations of immunoreactive beta-endorphin (ir beta END) in 10 well-characterized combat veterans with PTSD and nine matched normal volunteers. Blood was simultaneously withdrawn to obtain plasma for ir beta END. PTSD symptom clusters, as measured by the CAPS, were correlated with neuroendocrine data. Mean CSF ir beta END was significantly greater in patients with PTSD compared with normals and there was a negative correlation between the ir beta END and PTSD intrusive and avoidant symptoms of PTSD. No intergroup difference between plasma ir beta END was found, nor was there a significant correlation between CSF and plasma ir beta END. Immunoreactive beta-lipotropin (ir beta LPH) and pro-opiomelanocortin (irPOMC), both precursors of beta END, were much more plentiful in human CSF than was beta-endorphin itself, as has been previously reported. It remains to be determined whether the increased CNS opioid concentrations predate traumatic stress, thereby conferring a vulnerability to dissociative states and PTSD itself, or result from the trauma. The negative correlation between CSF ir beta END and avoidant and intrusive symptoms suggests that CNS hypersecretion of opioids might constitute an adaptive response to traumatic experience. Poor correlation between CSF and plasma ir beta END limits use of plasma measures to assess CNS opioid activity.

Differential proopiomelanocortin gene expression in the medial basal hypothalamus of rats during pregnancy and lactation

Hypothalamic proopiomelanocortin (POMC) gene expression was determined using in situ hybridization histochemistry (ISHH) during pregnancy and lactation in rats with and without prior reproduction experience. POMC mRNA levels in the arcuate nucleus were compared between primigravid (first pregnancy) and multigravid (second pregnancy) and primiparous and multiparous lactating rats, and between these groups and age-matched, regularly cycling, nulliparous females in diestrus. Hybridizations were performed using a digoxigenin-labeled riboprobe complementary to 837 bp of the POMC gene. The number of cells expressing POMC mRNA in the arcuate nucleus decreased in primiparous rats on day 12 of lactation when compared with the number of POMC cells in the arcuate nucleus of nulliparous rats in diestrus. In addition, the number of cells expressing POMC mRNA in multigravid animals was significantly less than in the primigravid group on days 7 and 21 of pregnancy, and on day 12 of lactation in primiparous animals. Repeated reproductive experience affected the number of POMC mRNA positive cells; there were fewer cells expressing POMC mRNA in the multigravid females on day 7 of pregnancy and an increase in the number of POMC cells in the multiparous group on day 12 of lactation compared to the primiparous animals. Optical density measurements revealed a significant increase in reaction product in the labeled cells on all days of pregnancy compared with virgin females in diestrus and a significant decrease in reaction product on day 12 of lactation in the multiparous group. The results of the present study indicate that POMC gene expression changes across pregnancy and lactation and that repeated reproductive experience has long-term, possibly permanent, effects on the endogenous opioid system.

Nicotinic control of tuberoinfundibular dopaminergic neuron activity and prolactin secretion: diurnal rhythm and involvement of endogenous opioidergic system

The possible involvement of cholinergic and opioidergic neurons in the control of diurnal changes of tuberoinfundibular dopaminergic (TIDA) neuronal activity was reported. Adult Sprague-Dawley rats ovariectomized and treated with estrogen were used. All drugs were administered centrally through preimplanted intracerebroventricular cannula, and both TIDA neuronal activity and serum prolactin level were determined. Nicotine (10 ng/3 microl/rat) given at 10:00 h significantly inhibited TIDA neuronal activity from 5 to 30 min and stimulated serum PRL levels at 5 and 15 min. Co-administration of either mecamylamine (1 microg) or naloxone (2.5 microg) prevented both nicotine's effects. A dose-related (0.1-100 ng) effect of nicotine on TIDA neuronal activity and serum PRL level was also observed in the morning when TIDA neuronal activity is high and serum PRL level is low, but not in the afternoon when the former activity is low and the latter is high. When atropine (20 microg), naloxone (25 microg) or Nor-BNI (20 microg) was given at 14:00 h all increased the lowered TIDA neuronal activity in the afternoon. When atropine was co-administered with either naloxone or Nor-BNI, however, no additive effect was observed. Submaximal doses of atropine (0.2 microg), mecamylamine (0.1 microg) or naloxone (0.25 microg) was also effective in stimulating the afternoon levels of TIDA neuronal activity and inhibiting serum PRL, and no additive effect was observed either. Moreover, simultaneous injection of morphine (15 microg) prevented atropine's effect in the afternoon. These results indicate that cholinergic neurons may act through activating the endogenous opioidergic neurons to exhibit an inhibitory effect on TIDA neuronal activity and a stimulatory one on prolactin secretion. A diurnal difference in its endogenous activity between morning and afternoon was also implicated.

Opiate receptor knockout mice define mu receptor roles in endogenous nociceptive responses and morphine-induced analgesia

Morphine produces analgesia at opiate receptors expressed in nociceptive circuits. mu, delta, and kappa opiate receptor subtypes are expressed in circuits that can modulate nociception and receive inputs from endogenous opioid neuropeptide ligands. The roles played by each receptor subtype in nociceptive processing in drug-free and morphine-treated states have not been clear, however. We produced homologous, recombinant mu, opiate receptor, heterozygous and homozygous knockout animals that displayed approximately 54% and 0% of wild-type levels of mu receptor expression, respectively. These mice expressed kappa receptors and delta receptors at near wild-type levels. Untreated knockout mice displayed shorter latencies on tail flick and hot plate tests for spinal and supraspinal nociceptive responses than wild-type mice. These findings support a significant role for endogenous opioid-peptide interactions with mu opiate receptors in normal nociceptive processing. Morphine failed to significantly reduce nociceptive responses in hot plate or tail flick tests of homozygous mu receptor knockout mice, and heterozygote mice displayed right and downward shifts in morphine analgesia dose-effect relationships. These results implicate endogenous opioid-peptide actions at mu opiate receptors in several tests of nociceptive responsiveness and support mu receptor mediation of morphine-induced analgesia in tests of spinal and supraspinal analgesia.

Molecular mechanisms underlying the regulation of proenkephalin gene expression in cultured spinal cord cells

The regulation of proenkephalin (proENK) mRNA levels by cAMP and protein kinase C (PKC) pathways was studied in cultured rat spinal cord cells in the present study. Spinal cord cells were cultured from 14 day (E 14) embryos of Sprague-Dawley rats. After 7 days in vitro, the spinal cord cells were incubated with either forskolin (5 microM) or phorbol-13-myristate acetate (PMA; 2.5 microM) for 1, 3, 6, 9, 12 or 24 h and total RNA and proteins were isolated for Northern and Western blot analyses. The proENK mRNA level began to increase within an hour, then reached and remained at a peak 3-12 h after stimulation by both forskolin and PMA. The increased proENK mRNA level in forskolin-treated cells was slightly decreased 24 h after the stimulation, whereas the level of proENK mRNA returned to basal levels in PMA-treated cells. A Western blot assay revealed that the intracellular level of proENK protein was not changed by treatment with either forskolin or PMA. Pretreatment of cells with cycloheximide (a protein synthesis inhibitor; 10 microM) did not affect the forskolin- or PMA-induced increase of proENK mRNA. However, pretreatment with nimodipine (an L-type Ca2+ channel blocker; 2 microM), omega-conotoxin (an N-type Ca2+ channel blocker; 1 microM), calmidazolium (a calmodulin antagonist; 1 microM) or KN-62 (a Ca2+/calmodulin-dependent protein kinase II inhibitor; 5 microM) attenuated the forskolin- or PMA-induced increase of proENK mRNA levels. Dexamethasone (1 microM) did not affect the forskolin-induced increase of proENK mRNA levels. Our results suggest that the elevation of proENK mRNA levels in the spinal cord is regulated by both cAMP and PKC pathways. Calcium influx through both L- and N-type calcium channels, calmodulin and Ca2+/calmodulin-dependent protein kinase II appear to be involved in the increase of proENK mRNA levels induced by either forskolin or PMA. Furthermore, ongoing protein synthesis is not required for forskolin- or PMA-induced alterations in proENK mRNA.

Central effects of opioid agonists and naloxone on blood pressure and heart rate in normotensive and hypertensive rats

1. The central cardiovascular effects of several opioid receptor selective agonists and the nonselective opioid antagonist, naloxone, were studied in anesthetized normotensive control rats, in spontaneously hypertensive rats (SHR), and in foot-shock-stressed rats. 2. Receptor-selective agonists injected into the rostral ventrolateral medulla (RVLM), paraventricular nucleus (PVN), and dorsal hippocampus (dHip) were DAGO (mu), DADLE (delta), and U50,488H (kappa). 3. DAGO and DADLE (3 nM) decreased arterial pressure and heart rate in RVLM and PVN of all rat strains, while U-50,488H (9 nM) had only minimal effects in these areas. 4. In dHip, only DADLE (3 nM) had depressor and bradycardic effects, and then, only in SHR, with DAGO and U50,488H being ineffective in any strain, even at 9 nM. 5. Prior injection of naloxone (10 nM) into the RVLM, PVN and dHip blocked and postinjection reversed the cardiovascular effects of the agonists. Naloxone alone increased blood pressure and heart rate in all three areas, in all rat strains except SHR, suggesting a tonic depressor effect of endogenous opioids. 6. Lack of significant quantitative differences in opioid agonist and antagonist effects between normotensive and hypertensive or stressed rats argues against a role for endogenous brain opioids in experimental hypertension.

Further Characterization of Nociception-Related and Arterial Pressure-Related Neuronal Responses in the Nucleus Reticularis Gigantocellularis of the Rat

The present study was undertaken to further characterize the nucleus reticularis gigantocellularis (NRGC) of the medulla oblongata in the central processing of nociceptive and cardiovascular signals, and its modulation by met-enkephalin. In Sprague-Dawley rats anesthetized with pentobarbital sodium, we found that all 125 spontaneously active NRGC neurons that responded to noxious stimuli (tail clamp) also exhibited arterial pressure-relatedness. Forty neurons additionally manifested cardiac periodicity that persisted even during nociceptive responses. While maintaining their cardiovascular responsive characteristics, the nociception-related NRGC neuronal activity was blocked, naloxone-reversibly (0.5 mg/kg, i.v.), by morphine (5 mg/kg, i.v.). Microiontophoretically applied met-enkephalin suppressed the responsiveness of NRGC neurons to individually delivered tail clamp or transient hypertension induced by phenylephrine (5 &mgr;g/kg, i.v.). Interestingly, in NRGC neurons that manifested both nociception and arterial pressure relatedness, the preferential reduction in the response to noxious stimuli upon simultaneous elevation in systemic arterial pressure was reversed to one that favored nociception in the presence of met-enkephalin. All actions of met-enkephalin were discernibly blocked by the opioid receptor antagonist, naloxone. Our results suggest that individual NRGC neurons may participate in the processing of both nociceptive and cardiovascular information, or in the coordination of the necessary circulatory supports during nociception. In addition, neuropeptides such as met-enkephalin may exert differential modulation on neuronal responsiveness according to the prevailing physiologic status of the animal. They also showed that NRGC may be a central integrator for pain and cardiovascular-related functions. Copyright 1996 S. Karger AG, Basel

Alterations of beta-endorphin-like immunoreactivity in CSF following behavioral training using a passive avoidance procedure

The central opioid system may have an important influence on memory processes. In view of this, the concentration of beta-endorphin-like immunoreactivity (beta-ELIR) in cerebrospinal fluid (CSF) was measured by a radioimmunoassay in rats trained in a passive avoidance procedure. The beta-ELIR in CSF was examined immediately, 2, 5, 10, and 30 min after the learning trial in which rats were exposed to footshock (0, 0.25, or 1.0 mA for 3 s). Avoidance latency and beta-ELIR in CSF were examined 24 and 120 h after the learning trial. The beta-ELIR in CSF was increased at 5 min after the learning trial in rats exposed to footshock of 0.25 mA. The beta-ELIR in CSF was elevated at 5 and 10 min, followed by a significant decrease at 30 min after the learning trial in rats exposed to a footshock of 1.0 mA. Thus, although an increase in beta-ELIR in CSF was not, the duration of the increase was, related to the shock intensity. Interestingly, a decrease followed the increase in beta-ELIR in CSF which was significant only in rats exposed to the high shock intensity. Avoidance latencies were enhanced in a shock intensity-dependent manner at both 24 and 120 h retention tests. No change in beta-ELIR in CSF was found during retention trials. The results suggest that behavioral manipulations alter beta-ELIR in CSF. An increase in beta-ELIR in CSF may be highly associated with stressful and emotional responses during behavioral training.

Differential messenger RNA expression of prodynorphin and proenkephalin in the human brain

The opiate system is involved in a wide variety of neural functions including pain perception, neuroendocrine regulation, memory, drug reward, and tolerance. Such functions imply that endogenous opioid peptides should have anatomical interactions with limbic brain structures believed to be involved in the experience and expression of emotion. Using in situ hybridization histochemistry, the messenger RNA expression of the opioid precursors, prodynorphin and proenkephalin, was studied in whole hemisphere human brain tissue. Different components of the limbic system were found to be characterized by a high gene expression of either prodynorphin or proenkephalin messenger RNA. Brain regions traditionally included within the limbic system (e.g. amygdala, hippocampus, entorhinal cortex and cingulate cortex) as well as limbic-associated regions including the ventromedial prefrontal cortex and patch compartment of the neostriatum showed high prodynorphin messenger RNA expression. In contrast, high levels of proenkephalin messenger RNA were more widely expressed in the hypothalamus, periaqueductal gray, various mesencephalic nuclei, bed nucleus of the stria terminalis, and ventral pallidum; brain regions associated with endocrine-reticular-motor continuum of the limbic system. The marked anatomical dissociation between the expression of these two opioid peptide genes, seen clearly in whole hemisphere sections, indicates that distinct functions must be subserved by the prodynorphin and proenkephalin systems in the human brain.

An immunocytochemical mapping of beta-endorphin (1-27) in the cat diencephalon

The distribution of beta-endorphin (1-27) immunoreactive cell bodies and fibres was studied in the diencephalon of the cat using an indirect immunoperoxidase technique. In the thalamus, almost all the immunoreactive fibres were found in the midline region and in nuclei located near the midline, whereas in the hypothalamus fibres containing beta-endorphin (1-27) were visualized extending by the whole structure. The hypothalamus showed a higher density of beta-endorphin (1-27) immunoreactive fibres than the thalamus, as well as immunoreactive cell bodies, since in the thalamus no beta-endorphin (1-27) immunoreactive neuron was located. The densest network of immunoreactive fibres was observed in the epithalamus (nucleus periventricularis anterior) and in the hypothalamic nuclei arcuatus, hypothalami ventromedialis, suprachiasmaticus, periventricularis hypothalami, hypothalamus dorsomedialis, area hypothalamica dorsalis, hypothalamus anterior, filiformis, hypothalamus posterior and regio praeoptica. In the hypothalamus, a high density of perikarya containing beta-endorphin (1-27) was observed in the nucleus arcuatus and a low density in the nucleus hypothalami ventromedialis. The distribution of beta-endorphin (1-27) immunoreactive fibres and perikarya is compared with the location of other neuropeptides in the cat diencephalon. Our findings reveal that b-endorphin (1-27) immunoreactive structures are widely distributed in the cat diencephalon, suggesting that the peptide might be involved in several physiological functions.

Differential role of delta-opioid receptors in the development and expression of behavioral sensitization to cocaine

The present study was designed to determine whether the selective delta-opioid receptor antagonist naltrindole hydrochloride can prevent the expression and development of sensitization to the locomotor-activating effects of cocaine. Rats were sensitized to the motor stimulant effects of cocaine (20 mg/kg i.p. x 3 days). 48 h after withdrawal of pretreatment, rats were pretreated with naltrindole (0.1-1.0 mg/kg s.c.) or its vehicle and 15 min later challenged with either saline or the sensitizing dose of cocaine. In a second set of experiments, naltrindole (0.1-1.0 mg/kg s.c.) or its vehicle were given in combination with either saline or cocaine (20 mg/kg i.p.) for 3 days. Activity in response to saline and to cocaine (20 mg/kg i.p.) was assessed on days 4 and 5, respectively. Additional experiments determined whether naltrindole prevents the development of sensitization to the locomotor-activating effects of nicotine: naltrindole (0.3, 1.0 mg/kg s.c.) or its vehicle were given in combination with nicotine (0.6 mg/kg s.c.) for 3 days. Naltrindole blocked the development but not expression of sensitization to the locomotor-activating effects of cocaine. In contrast, naltrindole failed to modify nicotine-induced sensitization in nicotine-treated animals. These data suggest that delta-opioid receptors are involved in the development but not expression of behavioral sensitization to cocaine.

The PC12 rat pheochromocytoma cell line expresses the prodynorphin gene and secretes the 8 kDa dynorphin product

Most adrenal chromaffin cells synthesize opioids derived from proenkephalin but not from prodynorphin. However, human pheochromocytomas and the PC12 rat pheochromocytoma cell line synthesize dynorphins. The aim of this study was to confirm the presence of the authentic prodynorphin transcript and its dynorphin product in PC12 cells. We have found that the sequence of a 458 bp cDNA fragment derived from RT-PCR amplification of total PC12 RNA was in complete accordance with the published sequence of the equivalent region of the prodynorphin gene. It encodes the potent endogenous kappa opioid agonists alpha-neo-endorphin, dynorphin A and dynorphin B. Furthermore, immunoaffinity-purified PC12 cell extracts were subjected to RP-HPLC. Most of its IR-dynorphin eluted on a peak exhibiting the retention time of similarly treated rat anterior pituitary. The expression of the prodynorphin gene in pheochromocytomas can be explained as either the result of (a) the process of dedifferentiation of chromaffin cells to pheochromocytoma which may thus cause the expression of a previously unexpressed prodynorphin or that (b) those pheochromocytomas expressing the prodynorphin gene derive from the few, centrally located chromaffin cells, which express this gene even under normal conditions.

State-dependent changes of brain endogenous opioids in mammalian hibernation

Endogenous opioids belonging to three opioid families were measured in different states of the hibernation cycle in brain of the Columbian ground squirrels. Using high-performance liquid chromatography-EC detection, the hypothalamic and septal concentrations of met-enkephalin were found to be significantly higher (p < 0.05) in the hibernating state than that in the nonhibernating state. In contrast, met-enkephalin content in the medulla decreased significantly during hibernation. Leuenkephalin content was only increased in the hypothalamus of hibernating animals. Using radioimmunoassay, dynorphin A-like immunoreactivity was observed to increase in the claustrum and striatum, whereas beta-endorphin-like peptides showed a significant increase in the hypothalamus during hibernation. It is evident that the changes in endogenous opioids in brain during hibernation are state dependent, type specific and region specific. These specific alterations of various endogenous opioids may imply their different roles in hibernation.

Effects of morphine, naloxone and their interaction in the learned-helplessness paradigm in rats

The aim of this study was to investigate the possible involvement of the mu-opioid system on the learned-helplessness paradigm, an experimental model of depression, in rats. In this test, rats were first exposed to inescapable foot-shocks (IS); 48 h later, they were submitted to a daily shuttle-box session (30 trials) for 3 consecutive days. Avoidance responses, escape failures and animal activity during each intertrial interval were recorded. Twice daily injections of morphine (0.25-8 mg/kg per day, SC), a mu-opioid agonist, reduced the increased escape failures induced by IS, as did tricyclic antidepressants. Significantly higher intertrial activity was observed in rats treated with morphine (2-8 mg/kg per day) compared with their associated control groups. Naloxone (1 and 2 mg/kg, IP), a mu-opioid antagonist, injected 10 min before each shuttle-box session impaired escape behavior in non-stressed rats and worsened the escape deficit induced by IS. Morphine-induced improvement of escape behavior and increase in intertrial activity were clearly reversed by a low inactive dose of naloxone (0.5 mg/kg). These results suggest that mu-opioid receptor mediation is involved in the deleterious effects of uncontrollable stress.

Neuropeptide Y stimulates beta-endorphin release in the basal hypothalamus: role of gonadal steroids

Recent pharmacological and morphological studies have raised the likelihood of a regulatory link between the neuropeptide Y (NPY) and beta-endorphin (beta-END) networks in the hypothalamus. To evaluate this link we have examined the effects of NPY administration on the in vivo release of beta-END by push-pull perfusion of the basal hypothalamus. Results showed that centrally injected NPY stimulated the in vivo release of beta-END. This effect was evident only in the absence of testicular steroids since NPY failed to alter beta-END release in intact rats. These data suggest that the well-documented effects of NPY such as inhibition of pituitary gonadotropin release in gonadectomized rats and stimulation of feeding may be mediated, in part, by stimulation of beta-END.

Recent advances in molecular recognition and signal transduction of active peptides: receptors for opioid peptides

1. Opioid peptides are a family of structurally related neuromodulators which play a major role in the control of nociceptive pathways. These peptides act through membrane receptors of the nervous system, defined as mu, delta and kappa and endowed with overlapping but distinct pharmacological, anatomical and functional properties. 2. Recent cloning of an opioid receptor gene family has opened the way to the use of recombinant DNA technology at the receptor level. 3. This review focuses on the molecular cloning and functional characterization of opioid receptors and provides first insights into molecular aspects of opioid peptide recognition and signal transduction mechanisms, using the cloned receptors as investigation tools.

Opioid and anti-opioid peptides

The numerous endogenous opioid peptides (beta-endorphin, enkephalins, dynorphins ... ) and the exogenous opioids (such as morphine) exert their effects through the activation of receptors belonging to four main types, mu, delta, kappa and epsilon. Opioidergic neurones and opioid receptors are largely distributed centrally and peripherally. It is thus not surprising that opioids have numerous pharmacological effects and that endogenous opioids are thought to be involved in the physiological control of various functions, among which nociception is particularly emphasized. Some opioid targets may be components of homeostatic systems tending to reduce the effects of opioids. "Anti-opioid" properties have been attributed to various peptides, especially cholecystokinin (CCK), neuropeptide FF (NPFF) and melanocyte inhibiting factor (MIF)-related peptides. In addition, a particular place should be attributed, paradoxically, to opioid peptides themselves among the anti-opioid peptides. These peptides can oppose some of the acute effects of opioids, and a hyperactivation of anti-opioid peptidergic neurones due to the chronic administration of opioids may be involved in the development of opioid tolerance and/or dependence. In fact, CCK, NPFF and the MIF family of peptides have complex properties and can act as opioid-like as well as anti-opioid peptides. Thus, "opioid modulating peptides" would be a better term to designate these peptides, which probably participate, together with the opioid systems, in multiple feed-back loops for the maintenance of homeostasis. "Opioid modulating peptides" have generally been shown to act through the activation of their own receptors. For example, CCK appears to exert its anti-opioid actions mainly through the activation of CCK-B receptors, whereas its opioid-like effects seem to result from the stimulation of CCK-A receptors. However, the partial agonistic properties at opioid receptors of some MIF-related peptides very likely contribute to their ability to modulate the effects of opioids. CCK- and NPFF-related drugs have potential therapeutic interest as adjuncts to opioids for alleviating pain and/or for the treatment of opioid abuse.

Cellular distribution of the mRNA for the kappa-opioid receptor in the human neocortex: a non-isotopic in situ hybridization study

Opioid receptors (OR) provide primary interaction sites of the human brain with opiates. Presently kappa-OR mRNA expression was studied in different cortical areas (A4, A10, A17) by in situ hybridization using digoxigenin-labeled oligonucleotides and an alkaline phosphatase-mediated color reaction. kappa-OR mRNA was expressed mainly in layers II/III and V pyramidal and layer VI multiform neurons. A4 giant pyramidal and A17 giant stellate neurons stood out labeled. These findings fit in with our data on kappa-OR protein distribution. Combined cellular assessment of protein and mRNA will enable the study kappa-OR expression under physiological and pathological conditions.

Occurrence of secretogranin II in the prolactin-immunoreactive neurons of the rat lateral hypothalamus: an in situ hybridization and immunocytochemical study

The occurrence of secretogranin II in a neuron population of the rat lateral hypothalamus specifically detected by an anti-serum to ovine prolactin was examined. As this population was previously reported to synthesize dynorphin, the distribution of neurons recognized by ovine prolactin-, dynorphin B- and secretogranin II anti-sera was investigated on adjacent sections of hypothalami. The prolactin immunoreactive neurons were the only cells in the lateral hypothalamus to be stained by secretogranin II anti-serum. Moreover, coupling immunocytochemical detection and in situ hybridization with an oligonucleotide probe complementary to secretogranin II mRNA showed that these neurons expressed the secretogranin II gene. These new findings should help to study the physiological role of the prolactin immunoreactive neurons of the lateral hypothalamus.

Met-enkephalin-like immunoreactivity in microdialysates from nucleus tractus solitarii in piglets during normoxia and hypoxia

Met-enkephalin-like immunoreactivity in microdialysates from the respiratory-related nucleus tractus solitarii was determined simultaneously with ventilatory responses in seven, spontaneously breathing, developing swine under conditions of normoxia, hypoxia and recovery from hypoxia for 30 min each. Assayed levels of Met-enkephalin-like immunoreactivity in normoxia were 0.89 +/- 0.23 pg/microliters. These levels increased to 203.6 +/- 32.2% and 283.1 +/- 55.8% of control during hypoxia and recovery, respectively. Hyperventilation during hypoxia was not sustained, comprising brief stimulation followed by return to near-control level. Taken together, these results provide further evidence that opioid release may contribute to the suppression of ventilation in hypoxia during development.

Distribution of natriuretic peptide precursor mRNAs in the rat brain

Atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) represent members of a recently discovered neuropeptide family involved in central regulation of endocrine and autonomic functions. The present study employed an in situ hybridization approach to provide the first detailed comparative mapping of ANP, BNP, and CNP mRNAs in brain. Results indicate that ANP mRNA is highly expressed in anterior olfactory nuclei, limbic cortices, dorsal endopiriform nucleus, hippocampal subfield CA1, cortical amygdaloid nuclei, medial habenula, anteroventral periventricular and arcuate nuclei, periventricular stratum, zona incerta, mammillary nuclei, inferior olive, nucleus ambiguus, and pontine paragigantocellular nuclei. CNP mRNA is expressed at highest levels in olfactory nuclei, limbic cortices, dorsal endopiriform nucleus, hippocampal subfields CA1-3, anteroventral periventricular and arcuate nuclei, and numerous brainstem regions (including the pontine, lateral reticular, solitary tract, prepositus hypoglossal, and spinal trigeminal nuclei). Positive labeling for BNP mRNA was not observed in brain. The presence of both ANP and CNP mRNA in the same regions of distinct nuclei (e.g., the anteroventral periventricular and arcuate nuclei) suggests the potential for coexpression. Overall, the present data are consistent with a prominent role for both ANP and CNP in neuroendocrine regulation and central cardiovascular integration. The extensive localization of ANP and/or CNP mRNA in olfactory nuclei, limbic cortex, hippocampus, amygdala and diencephalic limbic relays further indicate a putative role for ANP and CNP as neuromodulators of olfactory/limbic information processing.

Social play alters regional brain opioid receptor binding in juvenile rats

An in vivo autoradiographic procedure was employed to visualize local changes in brain opioid receptor occupancy in juvenile rats. This procedure is based on the assumption that released endogenous ligand will exclude exogenously applied tracer, in this case [3H]diprenorphine, from opioid receptors. Increases in availability of opioid peptides will then result in decreased opioid receptor binding. From behavioral studies there is ample evidence that opioid systems are involved in the regulation of social play behavior in juvenile rats. In the present study, changes in regional brain opioid activity as a result of social isolation-induced social play behavior were monitored. Twenty-one-day-old rats were socially isolated for 0, 3.5 or 24 h prior to testing, and tested alone or in a dyadic encounter. After behavioral testing, [3H]diprenorphine was administered and the brain was prepared for autoradiography. Social isolation caused increases in social behavior (dyadic encounters) but not in non-social behavior (singly tested animals). Modest differences in brain opioid receptor binding due to social isolation, social play behavior, or an interaction of the two, were found in claustrum, nucleus accumbens, globus pallidus, paraventricular and arcuate nuclei of the hypothalamus, and the dorsolateral and paratenial thalamic nuclei. These results support the notion that opioid systems are involved in the regulation of social play behavior. In addition, the observation of changes in opioid binding in areas involved in reward processes, adds evidence to the hypothesis that opioid systems are involved in the regulation of the rewarding aspects of social play in juvenile rats.

BDNF produces analgesia in the formalin test and modifies neuropeptide levels in rat brain and spinal cord areas associated with nociception

Previous studies have demonstrated an antinociceptive effect of brain-derived neurotrophic factor (BDNF) following infusion into the midbrain, near the periaqueductal grey and dorsal raphe nuclei. BDNF administration attenuated the behavioural response in the tail-flick and hot-plate tests, two models employing a phasic, thermal high-intensity nociceptive stimulus; the present studies extend our previous findings to include a model of moderate, continuous pain resulting from a chemical stimulus, the formalin test. Midbrain infusion of BDNF decreased the behavioural paw flinch response to subcutaneous formalin injection in both the early and late phases of the test. As our previous studies showed that BDNF-induced analgesia was reversible by naloxone, we have examined the effects of BDNF administration on brain and spinal cord levels of neuropeptides involved in the modulation of nociceptive information, including the endogenous opioid peptides, met-enkephalin and beta-endorphin, as well as substance P and neuropeptide Y (NPY). At the site of infusion, within the PAG and dorsal raphe, BDNF increased the level of beta-endorphin by 63%, but had no effect on substance P, metenkephalin or NPY levels. In the dorsal spinal cord, substance P (113% increase), beta-endorphin (97% increase) and NPY (64% increase) were elevated, although ventral spinal cord levels of these peptides remained unchanged. These studies demonstrate a modulatory effect of BDNF on relevant neuropeptides within areas of the brain and spinal cord involved in the processing of nociceptive information.

Opioid precursor gene expression in the human hypothalamus

Using in situ hybridization histochemistry, we studied the distribution of neurons that express preproopiomelanocortin (pre-POMC), preprodynorphin (pre-PDYN), and preproenkephalin (pre-PENK) gene transcripts within the human hypothalamus and surrounding structures. Of the three opioid systems, pre-POMC neurons have the most restricted distribution. Pre-POMC cells are most numerous in the infundibular nucleus and retrochiasmatic area of the mediobasal hypothalamus; a few labeled cells are present within the boundaries of the ventromedial nucleus and infundibular stalk. Pre-POMC message was not found in the limited samples of structures adjacent to the hypothalamus. In contrast to neurons that express pre-POMC, neurons expressing pre-PDYN and pre-PENK are more widely represented throughout the hypothalamus and extrahypothalamic structures. However, pre-PDYN and pre-PENK cells differ from one another in distribution. Pre-PDYN message is especially abundant in neurons of the tuberal and mammillary regions, with a distinct population of labeled cells in the premammillary nucleus and dorsal posterior hypothalamus. Pre-PDYN gene expression also is found in neurons of the dorsomedial nucleus, ventromedial nucleus, caudal magnocellular portion of the paraventricular nucleus, dorsolateral supraoptic nucleus, tuberomammillary nucleus, caudal lateral hypothalamus, and retrochiasmatic area. In structures immediately adjacent to the hypothalamus, pre-PDYN neurons were observed in the caudate nucleus, putamen, cortical nucleus of the amygdala, and bed nucleus of the stria terminalis. Pre-PENK neurons occur in varying numbers in all hypothalamic nuclei except the mammillary bodies. The chiasmatic region is particularly rich in pre-PENK neurons, with the highest packing density in the intermediate nucleus [the intermediate nucleus (Braak and Braak [1987] Anat. Embryol. 176:315-330) has also been termed the sexually dimorphic nucleus of the preoptic area (SDA-POA; Swaab and Fliers [1985] Science 228:1112-1115) or the interstitial nucleus of the anterior hypothalamus 1 (Allen et al. [1989] J. Neurosci. 9:497-506)], dorsal suprachiasmatic nucleus, medial preoptic area, and rostral lateral hypothalamic area. Pre-PENK neurons are numerous in the infundibular nucleus, ventromedial nucleus, dorsomedial nucleus, caudal parvicellular portion of the paraventricular nucleus, tuberomammillary nucleus, lateral hypothalamus, and retrochiasmatic area. Only a few lightly labeled cells were found in the periphery of the supraoptic nucleus and lateral tuberal nucleus. In areas adjacent to the hypothalamus, cells that contain pre-PENK message occur in the nucleus basalis of Meynert, central nucleus of amygdala, bed nucleus of the stria terminalis, caudate nucleus, and putamen.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of endogenous opioid peptides in central glucocorticoid receptor (GR)-induced decreases in circulating LH in the male rat

While it has been shown that intracerebral administration of exogenous glucocorticoids diminishes pituitary luteinizing hormone (LH) release, it is not known if these hormones act directly on gonadotropin-releasing hormone (GnRH)-synthesizing neurons, or if their central inhibitory effects are mediated by specific neural substrates. In the present study, we examined whether the opioid receptor antagonist, naltrexone (NALT), alters patterns of LH release elicited by either systemic or intracerebroventricular (i.c.v.) delivery of GR agonists. Subcutaneous (s.c.) injection of the GR agonist, RU362 (2.5 mg/kg), promoted a significant reduction in circulating LH levels; pretreatment by i.c.v. injection of 1.0 microgram NALT, however, attenuated this inhibitory hormonal response. It was also found that rats treated sequentially with NALT and RU362 exhibited significantly lower plasma LH levels compared to rats injected with NALT alone. In other experiments, intracranial delivery of the synthetic glucocorticoid, dexamethasone (DEX), into either the ventricular system or the hypothalamic ARC resulted in significantly decreased plasma LH concentrations; the central inhibitory effects of DEX on peripheral LH release were reversed, however, by i.c.v. pretreatment with NALT. In summary, the present studies show that opioid receptor blockade attenuates systemic, as well as intracerebral inhibitory effects of GR agonists on the GnRH-pituitary LH axis, suggesting that circulating glucocorticoids inhibit LH, in part, through central actions involving endogenous opioid receptors. The observed decline in peripheral plasma LH following intra-ARC injection of DEX suggests that local GR may be functional target sites for glucocorticoid effects on LH.(ABSTRACT TRUNCATED AT 250 WORDS)

Posthatching development of preproenkephalin mRNA-expressing cell populations in the pigeon telencephalon

Enkephalin peptides are highly expressed in the vertebrate telencephalon. Our previous investigations in the pigeon and in the chicken [26] suggested that the cellular distribution of these peptides is conserved in phylogenetically 'old' telencephalic regions (e.g. the basal ganglia), while it has species-specific organizations in areas (e.g. dorsomedial forebrain and bulbus olfactorius) that are likely to play important roles in species-specific behaviors. In the present study, we investigated the posthatching development of preproenkephalin (PPE) mRNA-containing cells in the pigeon forebrain using in situ hybridization histochemistry. These cells are densely distributed in the paleostriatal complex (corresponding to the mammalian caudate-putamen) at hatching, and their density progressively decreases during the first 9 days posthatching, when it is similar to that of adult pigeons. In the dorsomedial forebrain (corresponding to the mammalian hippocampus), PPE mRNA-expressing cells are present at hatching, and their density reaches a peak around the 6th day posthatching. In the bulbus olfactorius, the first PPE mRNA-containing cells are observed after 9 days posthatching. The developmental profile of PPE mRNA expression in these areas of the pigeon telencephalon shows remarkable similarities with the development of enkephalinergic cells in corresponding brain areas of mammals. As in the mammalian caudate-putamen, the developmental expression of enkephalin peptides in the paleostriatal complex is likely to be related to neuronal withdrawal from the mitotic cycle. The developmental pattern of expression of PPE mRNA in the dorsomedial forebrain suggests that enkephalin peptides contribute to the maturation of the behavioral functions of this area.

Effects of endogenous opioid peptides and their analogs on the activities of hypothalamic arcuate neurons in brain slices from diestrous and ovariectomized rats

Various endogenous opioid peptides and some of their analogs were used in this study to test their effects on the membrane activities of hypothalamic arcuate neurons in brain slices. Both ovariectomized and diestrous rats were used in the study, and freshly prepared brain slices from these animals were used for extracellular single-unit recording studies. All of the opioids exhibited potent inhibitory effects on the firing of arcuate neurons, viz., beta-endorphin inhibited 55% (n = 33), DAGO 62% (n = 21), dynorphin A 55% (n = 11), U50,488 36% (n = 39), Met-enkephalin 35% (n = 54), and DPDPE 50% (n = 8) of tested arcuate neurons from ovariectomized rats. Significantly higher percentage of inhibition was observed in slice preparations from diestrous rats for DAGO 86% (n = 22), and slightly higher for dynorphin A 59% (n = 22) and U50,488 53% (n = 15). Pretreatment with naloxone prevented most of the actions by beta-endorphin and DAGO, and nor-binaltorphimine prevented those by dynorphin A and U50,488. Most of the effects of Met-enkephalin could also be blocked by nor-binaltorphimine (67%, n = 6), but less by naltrindole (25%, n = 8). Naltrindole, however, seemed to be more effective in blocking the action of [D-Pen2,5]-enkephalin (100%, n = 2). In summary, all opioids tested exerted potent inhibitory effects upon the firing of arcuate neurons possibly through multiple opioid receptors, and the presence of ovarian hormones may have an effect on the neuron's responsiveness to opioid acting on mu type receptors.

c-fos expression in the rat brain following central administration of neuropeptide Y and effects of food consumption

Administration of neuropeptide Y (NPY) intracerebroventricularly (i.c.v.) results in the release of a number of hypothalamic and pituitary hormones and stimulation of feeding and suppression of sexual behavior. In this study, we sought to identify cellular sites of NPY action by evaluating perikaryal Fos-like immunoreactivity (FLI), a marker of cellular activation, in those hypothalamic and extrahypothalamic sites previously implicated in the control of neuroendocrine function and feeding behavior. Additionally, we compared the topography of FLI in these brain sites when food was either available ad libitum or withheld after NPY injection (1 nmol/3 microliters, i.c.v.). The results showed that one hour after NPY injection a larger number of cells in the parvocellular region of the paraventricular nucleus (PVN) were FLI-positive in the absence of food consumption. However, in association with food intake, a significant number of cells were intensely stained in the magnocellular region of the PVN. An analogous increase in FLI in association with feeding was apparent in the supraoptic nucleus (SON), the dorsomedial nucleus and the bed nucleus of the stria terminalis in the hypothalamus. Among the extrahypothalamic sites, feeding facilitated FLI in a large number of cells located in the lateral subdivision of the central amygdaloid nucleus and the lateral subdivision of the solitary tract. FLI was observed in a moderate number of cells in the hypothalamic arcuate nucleus (ARC) and ventromedial nucleus, and this response was not changed by feeding.(ABSTRACT TRUNCATED AT 250 WORDS)

Spontaneous and ethanol-stimulated in vitro release of beta-endorphin by the hypothalamus of AA and ANA rats

Previous studies demonstrated that both the spontaneous and ethanol-stimulated release of beta-endorphin (beta-EP) like-peptides (beta-EPLPs) by the hypothalami of the ethanol-preferring C57BL/6 mice is more pronounced than by the hypothalami of the ethanol-avoiding DBA/2 mice. The objective of the present studies was to investigate the effects of various concentrations of ethanol on the in vitro release of beta-EP peptides by the hypothalami of the ethanol-preferring Alko-Alcohol (AA) and ethanol-avoiding Alko Non-Alcohol (ANA) lines of rats. Results indicated that although the spontaneous release of hypothalamic beta-EPLPs was higher by the ANA than by the AA rats, the percentage increase following exposure to various concentrations of ethanol was similar in both lines of rats. Furthermore, the release of hypothalamic beta-EPLPs following exposure to 30 mM ethanol was significantly higher than the release following exposure to 10 mM ethanol in the AA, but not the ANA, rats. Analysis of the released beta-EPLPs with Sephadex G-75 and reversed phase HPLC indicated that the nonacetylated beta-EP 1-31 was the major component in the hypothalamic perifusates of the AA rats, whereas the shorter and acetylated forms of beta-EP were the predominant components in the hypothalamic perifusates of the ANA rats. Because the shorter and acetylated forms of beta-EP are devoid of opioid activity, their pronounced release by the hypothalami of the ANA rats may be important in maintaining their low ethanol consumption, even after long-term access to ethanol solutions.

Release of alpha-neoendorphin from the anterior pituitary gland of conscious rats

alpha-Neoendorphin (alpha-NEO) is a proenkephalin B-derived opioid peptide and kappa type opioid receptor agonist. In the present study, we used a combination of microdialysis and a highly sensitive radioimmunoassay to measure the extracellular levels of immunoreactive (ir)-alpha-NEO from the anterior pituitary gland of conscious free-moving rats. When rats were given water ad libitum under a 12:12 h light-dark cycle with lighting from 06.00 to 18.00 h, ir-alpha-NEO showed circadian rhythmicity that peaked at 00.00-03.00 h and reached a minimum at 12.00-15.00 h. Furthermore, we investigated whether naloxone (10(-6) to 10(-8) M) affected ir-alpha-NEO level. The alpha-NEO release induced by naloxone was optimum at 10(-7) M, and 10(-6) M naloxone seemed to induce alpha-NEO release to a lesser degree. These results suggested that alpha-NEO release by naloxone might be mediated via anterior pituitary cell auto-receptor (kappa type) inhibition.

Distribution of preproenkephalin mRNA in the chicken and pigeon telencephalon

Bioassay and immunological studies have detected the presence of opioid peptides in the nervous system of representatives of all classes of vertebrates. The present study evaluates the expression and localization of preproenkephalin (PPE) mRNA to determine the sites of synthesis of the enkephalin peptides in the adult chicken and pigeon telencephalon using in situ hybridization histochemistry. We used a 500-base-pair chicken RNA probe corresponding to chicken PPE cDNA. In both the chicken and the pigeon telencephalon, the highest concentration of PPE mRNA-containing cells was observed in the lobus parolfactorius, paleostriatum augmentatum, nucleus accumbens, and septum. Distinct populations of labeled cells were also detected in the hyperstriatum accessorium, hippocampus, area parahippocampalis, nucleus of the diagonal band, cortex dorsolateralis, and cortex piriformis. Differences in PPE mRNA expression between chicken and pigeon were observed in several telencephalic regions. For instance, the bulbus olfactorius was heavily labeled in the pigeon, but was not labeled in the chicken, and numerous PPE mRNA-containing cells were present in the area parahippocampalis of pigeons but not of chickens. In contrast, in the hyperstriatum dorsale and hyperstriatum ventrale, numerous PPE mRNA-expressing cells were detected in the chicken but not in the pigeon. Overall, PPE mRNA-expressing cells were more numerous than enkephalin-immunoreactive cells described in previous studies. In addition, our results suggest that the general pattern of enkephalin expression in the avian telencephalon is similar to that found in other vertebrates. Finally, the results of the present study illustrate some differences in the pattern of PPE mRNA distribution between closely related species, indicating the existence of species-specific neurochemical pathways, which may influence and perhaps mediate different behaviors characteristics of these species.

Enhanced activity of the brain beta-endorphin system by free-choice ethanol drinking in C57BL/6 but not DBA/2 mice

The objective of the present studies was to investigate the effect of voluntary ethanol consumption for 21 days on the brain beta-endorphin system of C57BL/6 (alcohol-preferring) and DBA/2 (alcohol-avoiding) strains of mice. As expected, C57BL/6 mice consumed a significantly higher quantity of the 10% ethanol solution than the DBA/2 mice. Under basal conditions the content of beta-endorphin like peptides differed only in the nucleus accumbens, higher levels being found in the DBA/2 mice. Voluntary ethanol consumption induced an increase in the hypothalamic content of mRNA coding for proopiomelanocortin, associated with a significant increase in the tissue content of beta-endorphin-like peptides in the arcuate nucleus and septum of the C57BL/6 mice, but did not alter the activity of the brain beta-endorphin system of the DBA/2 mice. Since voluntary ethanol consumption was not associated with nutritional deficits and stress, the ethanol-induced enhanced activity of the brain beta-endorphin system of the C57BL/6 mice must be a direct effect of ethanol and may be important in controlling the voluntary ethanol consumption by this strain of mice.

Proopiomelanocortin gene expression during pig pituitary and brain development

The expression of proopiomelanocortin (POMC) mRNA, which plays an important role in neural, endocrine, neuroendocrine and immune systems, was studied by in situ hybridization during the development of pituitary in the domestic pig. The POMC gene was activated as early as fetal day 30 (E30). The signal for POMC mRNA in the anterior lobe progressively increased from E30 to E80 and then remained at relatively constant level. In contrast, POMC transcripts in the intermediate lobe first appeared at E40 and steadily increased during development. POMC transcripts in the brain were first found at E40 and were scattered in the arcuate nucleus (AN) and nucleus medialis thalami (NMT). At E50 the extra-pituitary POMC mRNA was located not only in the AN and NMT but also in the fasciculus tegmenti and entorhinal cortex. In the posterior lobe, no signal was detected. The specific pattern of expression of the pig POMC gene in the pituitary and in specific regions of the central nervous system suggests important roles for POMC in fetal development. These results also suggest that POMC is excellent for studying the expression and regulation of pituitary hormone genes because of its tissue-specific regulation and developmental pattern.

Characterization of opioid binding sites in the neural and intermediate lobe of the rat pituitary gland by quantitative receptor autoradiography

Previous studies have suggested an involvement of enkephalins in regulation of oxytocin (OXT) and vasopressin (AVP) release, which seems to disagree with the very low affinities of Met- and Leu-enkephalin for the kappa opioid receptor. As opioid receptors in the neural lobe exclusively exist of kappa receptors, we studied the binding characteristics of larger pro-enkephalin derived peptides for opioid binding sites in the neural lobe by means of light microscopic receptor autoradiography. In addition, the pharmacological characteristics of opioid binding sites in the neural lobe were compared with those in other parts of the pituitary. In the neural as well as the intermediate lobe both high and low affinity 3H-bremazocine binding sites were present. Binding to these sites was completely displaceable by both naloxone and nor-binaltorphimine suggesting that these sites represent kappa opioid receptors. Also with regard to selectivity and affinity characteristics to other ligands, opioid binding sites in the neural and intermediate lobe were quite similar. In the anterior lobe a very low level of bremazocine binding was present, which could not be displaced by nor-binaltorphimine. Displacement studies with pro-enkephalin and pro-dynorphin derived peptides showed that both groups of peptides could bind to opioid binding sites in the neural and intermediate lobe. Especially the relatively large pro-dynorphin and pro-enkephalin derived peptides, such as dynorphin 1-17 and BAM22, appeared to be very potent ligands for these opioid binding sites and were much more potent than smaller fragments, such as dynorphin 1-8, and Met- and Leu-enkephalin. These results contradict the existence of a mismatch in the neural (and intermediate) lobe with regard to the local type of opioid peptides and receptors present.

Comparative study of the effects of mu, delta and kappa opioid agonists on 3H-dopamine uptake in rat striatum and nucleus accumbens

The effect of the mu opioid agonist DAGO, delta opioid agonist DPDPE and kappa opioid agonist U50,488H on 3H-dopamine (3H-DA) uptake was studied in synaptosomes prepared from rat striatum and nucleus accumbens. Over the range of concentrations tested (1 nM-10 microM) DAGO and DPDPE were devoid of effects on 3H-DA uptake in the striatum and the nucleus acumbens. In contrast, U50,488H significantly decreased 3H-DA uptake in both structures. The inhibition of uptake induced by the kappa agonist was not reversed in the presence of the opiate antagonists naloxone (10 microM) or nor-binaltorphimine (0.1 microM). Dynorphin A (1-13) also induced a significant reduction in 3H-DA uptake in both structures at the concentrations of 10 and 30 microM. This inhibitory effect was not reversed by naloxone (10 microM). These data suggest that kappa opioid agonists modulate dopamine uptake in the striatum and the nucleus accumbens and their effects may not be due to an activation of opioid receptors.

Influence of halothane and methoxyflurane on regional brain and spinal cord concentrations of methionine-enkephalin in the rat

Rats were exposed to either oxygen (controls), 1.5% halothane in oxygen, or methoxyflurane (0.5%) in oxygen over a period of 2 h, then sacrificed at the end of exposure (2-h group), 4 h after removal from environmental chamber (4-h group), or at 24 h following anesthetic exposure (24-h group). Pituitary (excluding the neural lobe, Pit), brain, and spinal cord areas were isolated and processed with Met-enkephalin tissue concentrations determined. In halothane-exposed animals, Met-enkephalin concentrations in pit and across CNS areas studied were significantly lower at 2 h following anesthetic exposure than in control animals. Concentrations of Met-enkephalin in many areas of CNS and Pit of 4-h group approached control levels. Concentrations of Met-enkephalin in all areas studied except spinal cord returned to basal levels by 24 h following halothane exposure. Exposure to methoxyflurane resulted in less dramatic changes in Met-enkephalin concentrations across CNS regions examined. Exposure to methoxyflurane resulted in significant decreases in Met-enkephalin levels in olfactory bulb, thalamus, and hippocampus only. Met-Enkephalin levels did not change significantly in other areas of the central nervous system following methoxyflurane exposure. These results indicate that halothane and methoxyflurane may have differential effects on the endogenous opioid system.

Opioid peptide gene expression in the nucleus tractus solitarius of rat brain and increases induced by unilateral cervical vagotomy: implications for role of opioid neurons in respiratory control mechanisms

Neurons expressing messenger RNA encoding the opioid peptide precursors, preproenkephalin and preprodynorphin were localized in the medulla oblongata of the rat by in situ hybridization of specific DNA oligonucleotide probes. Neurons containing preproenkephalin messenger RNA were found throughout the medullary reticular formation in the gigantocellular and paragigantocellular reticular nuclei, the parvicellular and lateral reticular nuclei; commissural, medial and ventrolateral subnuclei in the nucleus tractus solitarius and the nucleus of the spinal trigeminal tract. Labelled cells were also concentrated in the more medial regions of the area postrema. In contrast, neurons containing preprodynorphin messenger RNA had a more restricted distribution and were detected in the commissural and ventrolateral nucleus tractus solitarius and nucleus of the spinal trigeminal tract, especially in the more dorsal regions. Expression of preproenkephalin and preprodynorphin messenger RNA was also examined in the dorsal vagal complex of rats that had undergone a unilateral nodose ganglionectomy or cervical vagotomy. Twenty-four hours after both cervical vagotomy and nodose ganglionectomy, there was a specific 1.5-2-fold elevation in preproenkephalin and preprodynorphin messenger RNA levels in the ventrolateral subnucleus of the contralateral nucleus tractus solitarius relative to levels in the ipsilateral nucleus tractus solitarius and in the nucleus tractus solitarius of sham-operated animals. Previous immunohistochemical studies demonstrating the co-localization of enkephalin and dynorphin in the ventrolateral nucleus tractus solitarius suggest that these changes occurred in the same population of neurons. In light of the suggested role of the ventrolateral nucleus tractus solitarius as a central respiratory centre and the activation of the intact pulmonary afferents that innervate this area following a unilateral vagotomy (which increases inspiration volume and expiratory time by affecting the Hering-Breuer reflex), our results suggest a specific involvement of enkephalin- and dynorphin-containing neurons in the ventrolateral nucleus tractus solitarius in central respiratory control mechanisms.

Postexercise hypotension. Key features, mechanisms, and clinical significance

Recent investigations have demonstrated that there is a sustained reduction in arterial blood pressure after a single bout of exercise, ie, postexercise hypotension (PEH). The purpose of this discussion is to integrate the available information on this topic and to review studies using sustained stimulation of somatic afferents in experimental rats as a model to study the role of somatic afferents in PEH. PEH occurs in response to several types of large-muscle dynamic exercise (ie, walking, running, leg cycling, and swimming) at submaximal intensities greater than 40% of peak aerobic capacity and exercise durations generally between 20 and 60 minutes. PEH is observed in both normotensive and hypertensive humans and in spontaneously hypertensive rats but is generally greater in magnitude in hypertensive subjects. The maximal exercise-induced reductions in systolic and diastolic arterial blood pressures have been on average 18 to 20 and 7 to 9 mm Hg, respectively, in hypertensive humans and 8 to 10 and 3 to 5 mm Hg, respectively, in normotensive humans. PEH has been reported to persist for 2 to 4 hours under laboratory conditions. Whether PEH is sustained for a prolonged period of time under free-living conditions remains controversial, although the results of one study indicate that PEH can persist for up to 13 hours. Possible mechanisms involved in mediating postexercise and poststimulation reductions in arterial blood pressure include decreased stroke volume and cardiac output; reductions in limb vascular resistance, total peripheral resistance, and muscle sympathetic nerve discharge; group III somatic afferent activation; altered baroreceptor reflex circulatory control; reduced vascular responsiveness to alpha-adrenergic receptor-mediated stimulation; and activation of endogenous opioid and serotonergic systems. It appears that the magnitude of PEH in hypertensive subjects is clinically significant; however, more investigation is required to determine if the duration is sufficient under real-life conditions to contribute to the reduction in blood pressure observed with chronic exercise conditioning.

POMC-derived peptide immunoreactivity in neural lobe axons of the human pituitary

The efferent projections of proopiomelanocortin (POMC) neurons in the arcuate nucleus and nucleus of the solitary tract have been extensively characterized in the rat, but are less well understood in the human brain. We report here that ACTH, alpha-MSH, beta-endorphin, and N-acetyl-beta-endorphin immunoreactive axons are localized in the neural lobe of the human pituitary gland, in congruence with prior evidence that beta-endorphin and other POMC-derived peptides modulate vasopressin and oxytocin secretion.

Opioids in neural and nonneural tissues

The endogenous opioid peptides (EOP) are grouped in three families, each deriving from the posttranslational processing of a distinct precursor molecule and exhibiting high affinity for a specific opioid receptor. The genes of EOPs are expressed in a wide variety of sites, including many nerve, neurosecretory, and endocrine cells. In reviewing the vast literature on this subject, a few patterns begin to emerge. First, the distribution of EOPs in tissues appears to be a distinct characteristic of each family of opioids. Second, the EOP producing cells can be grouped into two broad categories: those expressing only one and those expressing multiple EOP genes. Most EOP-producing nerve and neurosecretory cells fall into the first category, that is, they express one EOP gene, whereas most nonneural cells fall into the second category, that is, they express multiple EOP genes. Third, it appears that there is a relationship between opioids, proliferation rate, and state of differentiation of cells, since it has been shown that (a) mitogenic factors may change the EOP profile of a cell, and that (b) opioids may inhibit the proliferation rate of normal or neoplastic cells. The physiologic implication of these observations is briefly discussed.

Leucine5-enkephalin afferents to midbrain dopaminergic neurons: light and electron microscopic examination

The relationship between leucine5-enkephalin-containing nerve terminals and midbrain dopaminergic neurons was studied in the adult rat by light and electron microscopy. For light microscopy, alternate midbrain sections were immunostained with rabbit polyclonal antibodies against leucine5-enkephalin and tyrosine hydroxylase, by means of the peroxidase antiperoxidase technique. Leucine5-enkephalin stained fibers and terminals were observed with varying density in the retrorubral field (dopaminergic nucleus A8 region), substantia nigra pars compacta (dopaminergic nucleus A9 region), and ventral tegmental area and related nuclei (dopaminergic nucleus A10 region). For electron microscopy, midbrain sections were immunostained with a mouse monoclonal antibody against leucine5-enkephalin and a rabbit polyclonal antibody against tyrosine hydroxylase, by means of the peroxidase antiperoxidase technique and silver-intensified colloidal gold reactions, respectively. The nucleus A10 area was examined at the electron microscopic level, and there were a) both symmetric (75%) and asymmetric (25%) synapses made between leucine5-enkephalin axon terminals and dopaminergic dendrites, and also synaptic contacts with unlabeled dendrites; b) leucine5-enkephalin synaptic contacts with dopaminergic dendrites that were covered with astrocytic membranes; and c) leucine5-enkephalin appositions with unlabeled nerve terminals that made synaptic contacts with dopaminergic dendrites, suggestive of axo-axonic connections. These findings provide the structural basis for both direct and indirect control of A10 dopaminergic neurons by enkephalin-containing nerve terminals.

Sexual dimorphism in the distribution of alpha-neoendorphin-like immunoreactivity in the anterior pituitary of the rat

The localization of the opioid peptide alpha-neoendorphin (alpha-Neo-E) was studied in the anterior pituitary of normal and castrated male and normal female rats. Immunoreactive (ir) cells were noted in both sexes. These alpha-Neo-E-ir cells were further characterized using double immunostaining with an elution-restaining procedure. It was seen that in males, alpha-Neo-E-ir cells corresponded mainly to luteinizing hormone/follicle-stimulating hormone cells and a few thyroid-stimulating hormone (TSH) cells, whereas in females, virtually all alpha-Neo-E-ir cells corresponded to TSH cells. Castration of male rats caused, within 3 to 5 days a dramatic decrease in the number of alpha-Neo-E-ir gonadotrophs, whereas the number of alpha-Neo-E-ir TSH cells tended to increase. Two weeks after castration, however, most alpha-Neo-E-ir cells were also follicle-stimulating hormone-ir. This study demonstrates that in the anterior lobe of the rat, alpha-Neo-E-ir is located within gonadotrophs and/or thyrotrophs, depending on the sex. In addition, results obtained following castration suggest that the expression of this peptide in the anterior pituitary depends upon the steroid environment, possibly indicating that alpha-Neo-E is implicated in the regulation of the pituitary-gonadal axis.

In vitro study of the interaction of salmon calcitonin with mu, delta and kappa opioid agonists

A possible interaction of salmon-calcitonin with opioid systems was studied in isolated tissues. Neurogenic contractions were elicited by electrical stimulation in guinea-pig ileum myenteric plexus-longitudinal muscle strips, rabbit vas deferens and mouse vas deferens. Bremazocine inhibited neurogenic contractions in all three tissues (presumably through kappa-receptors) [D-Pen2, D-Pen5]enkephalin and [Met5]enkephalin inhibited contractions in mouse vas deferens (presumably through delta-receptors), and [D-Ala2, N-Me-Phe4, Gly5-ol]enkephalin (DAMGO) inhibited contractions in guinea-pig ileum and mouse vas deferens (presumably through mu-receptors). All inhibitory effects were concentration-dependent. Salmon-calcitonin 0.1 IU/ml increased the effect of bremazocine in guinea-pig ileum and rabbit vas deferens and also increased the effects of [D-Pen2, D-Pen5]enkephalin and [Met5]enkephalin in mouse vas deferens. In contrast, salmon-calcitonin up to 0.4 IU/ml did not change the effect of bremazocine in mouse vas deferens and the effect of DAMGO in guinea-pig ileum and mouse vas deferens. It is concluded that salmon-calcitonin enhances agonist effects at opioid kappa- and delta- but not at opioid mu-receptors. The level of this interaction remains to be elucidated. The interaction may be the basis of the analgesic effect of salmon-calcitonin in vivo.

Proopiomelanocortin gene expression as a neural marker during the embryonic development of Xenopus laevis

Proopomelanocortin (POMC) is the precursor protein for a number of peptide hormones and neuropeptides, and the POMC gene is transcriptionally very active in the pars intermedia of the pituitary of the amphibian Xenopus laevis (Xenopus). We analysed the expression of this gene during Xenopus embryogenesis, in order to examine whether it can function as a (novel) neural marker. We investigated the spatio-temporal distribution of POMC mRNA, using a single-stranded probe that corresponds to the 3' untranslated region of Xenopus POMC gene B mRNA. Gene transcripts were first detected at stage 25 of development via RNase protection assays. In situ hybridization analysis performed at stage 46 showed clearly that these transcripts are localised in a region representing the future pars intermedia of the pituitary. Experiments using Xenopus explants indicate that the POMC gene can be used successfully as an indirect marker in studies on neural induction: in the absence of interactions with mesoderm, ectoderm fails to express the POMC gene, whereas POMC transcripts are readily detectable in conjugates of ectoderm and mesoderm. Artificial application of two different signals, which are likely to be relevant for neural differentiation (namely retinoic acid and the activation of protein kinase C via phorbol ester), was not effective in evoking POMC gene expression in cultured ectoderm explants. However, retinoic acid treatment of conjugates of Xenopus ectoderm and mesoderm successfully prevented POMC expression. We conclude that POMC gene expression can be used as an indirect marker for anterior neural differentiation in Xenopus.