Release of beta-endorphin immunoreactivity from brain by activation of a hypothalamic N-methyl-D-aspartate receptor

Mechanism and Clinical Importance of Respiratory Failure Induced by Anticholinesterases

Respiratory failure is the predominant cause of death in humans and animals poisoned with anticholinesterases. Organophosphorus and carbamate anticholinesterases inhibit acetylcholinesterase irreversibly and reversibly, respectively. Some of them contain a quaternary atom that makes them lipophobic, limiting their action at the periphery, i.e. outside the central nervous system. They impair respiratory function primarily by inducing a desensitization block of nicotinic receptors in the neuromuscular synapse. Lipophilic anticholinesterases inhibit the acetylcholinesterase both in the brain and in other tissues, including respiratory muscles. Their doses needed for cessation of central respiratory drive are significantly less than doses needed for paralysis of the neuromuscular transmission. Antagonist of muscarinic receptors atropine blocks both the central and peripheral muscarinic receptors and effectively antagonizes the central respiratory depression produced by anticholinesterases. To manage the peripheral nicotinic receptor hyperstimulation phenomena, oximes as acetylcholinesterase reactivators are used. Addition of diazepam is useful for treatment of seizures, since they are cholinergic only in their initial phase and can contribute to the occurrence of central respiratory depression. Possible involvement of central nicotinic receptors as well as the other neurotransmitter systems – glutamatergic, opioidergic – necessitates further research of additional antidotes.

Endometriosis Is Associated With a Shift in MU Opioid and NMDA Receptor Expression in the Brain Periaqueductal Gray

Studies have examined how endometriosis interacts with the nervous system, but little attention has been paid to opioidergic systems, which are relevant to pain signaling. We used the autotransplantation rat model of endometriosis and allowed to progress for 60 days. The brain was collected and examined for changes in endogenous opioid peptides, mu opioid receptors (MORs), and the N-methyl-d-aspartate subunit receptor (NR1) in the periaqueductal gray (PAG), since both of these receptors can regulate PAG activity. No changes in endogenous opioid peptides in met- and leu-enkephalin or β-endorphin levels were observed within the PAG. However, MOR immunoreactivity was significantly decreased in the ventral PAG in the endometriosis group. Endometriosis reduced by 20% the number of neuronal profiles expressing MOR and reduced by 40% the NR1 profiles. Our results suggest that endometriosis is associated with subtle variations in opioidergic and glutamatergic activity within the PAG, which may have implications for pain processing.

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

Midazolam attenuates the antinociception induced by d-serine or morphine at the supraspinal level in rats

Our recent study has shown that the intracerebroventricular administration of d-serine, an endogenous and selective agonist for the glycine site of the N-methyl-d-aspartate receptor, alone or in combination with morphine, leads to the potentiation of antinociception on the tail-flick response. Although there is a variety of information concerning the effects of benzodiazepines on opioid-induced antinociception, little is known about the effect of benzodiazepines on the N-methyl-d-aspartate receptor agonist-induced antinociception. To clarify the analgesic interactions among the benzodiazepine/GABA(A), N-methyl-d-aspartate and opioid receptors at the supraspinal level, we investigated the effects of intracerebroventricular administration of midazolam, a benzodiazepine receptor agonist, on the antinociception evoked by the intracerebroventricular application of d-serine or morphine. The intracerebroventricular administration of midazolam alone produced hyperalgesia on the tail-flick response in a benzodiazepine receptor antagonist, flumazenil-reversible manner. The antinociception induced by the intracerebroventricular application of d-serine or morphine was attenuated by the intracerebroventricular administration of midazolam. In addition, this inhibitory effect of midazolam on the antinociception of d-serine or morphine was antagonized by the intracerebroventricular administration of flumazenil. Together with the facts that d-serine and midazolam act as selective agonists for the glycine site of the N-methyl-d-aspartate receptor and benzodiazepine/GABA(A) receptor, respectively, these observations suggest a functional interaction between the NMDA and benzodiazepine/GABA(A) receptors in the regulation of antinociception at the supraspinal level.

Expression of c-Fos in arcuate nucleus induced by electroacupuncture: relations to neurons containing opioids and glutamate

Electroacupuncture (EA) at the Neiguan-Jianshi acupoints (P5-P6, overlying the median nerve) attenuates sympathoexcitatory reflexes probably through the opioid system. The arcuate nucleus (ARC) within hypothalamus is an important brain area that produces opioid peptides. Physiological studies have demonstrated that the predominant response to EA is excitation in the ARC and that excitatory projections from the ARC to the ventrolateral periaqueductal gray during EA at P5-P6 contribute to inhibition of sympathoexcitatory cardiovascular reflexes. These data imply that ARC neurons activated by EA also may contain excitatory neurotransmitters. Thus, the present study evaluated activation of the ARC induced by EA at P5-P6, in relation to the opioid system and glutamate, by detecting c-Fos, an immediate early gene, opioid peptides and vesicular glutamate transporter 3 (VGLUT3). To enhance detection of perikarya containing the opioid peptides, colchicine (90-100 microg/kg) was administered in cats 28-30 h before EA or the sham-operated control. EA was performed at P5-P6 for 30 min. Compared to controls (n=5), c-Fos-positive cells and neurons double-labeled with c-Fos and beta-endorphin, enkephalin or VGLUT3 in the ARC were significantly increased in EA-treated cats (n=6; all P<0.05). Moreover, neurons triple-labeled with c-Fos, beta-endorphin and VGLUT3 were noted in this region following EA stimulation, but not in controls. Thus, EA at P5-P6 activates neurons in the ARC, some of which contain opioids as well as glutamate or both. The results imply that EA at P5-P6 has the potential to influence ARC neurons containing multiple neuronal substances that subsequently modulate cardiovascular function.

Activation of supraspinal NMDA receptors by both D-serine alone or in combination with morphine leads to the potentiation of antinociception in tail-flick test of rats

Although there is a variety of information concerning the effects of the N-methyl-D-aspartate (NMDA) receptor on opioid-induced antinociception at the spinal level, little is known about the effects at the supraspinal level. To clarify the role of the NMDA receptor on the morphine-induced antinociception at the supraspinal level, we investigated the effects of the intracerebroventricular (i.c.v.) administration of D-serine, a selective agonist for the glycine site of the NMDA receptors, alone or in combination with morphine using the tail-flick test. The i.c.v. administration of D-serine, but not L-serine, alone produced a dose-dependent antinociception in the tail-flick response. D-Serine also dose-dependently potentiated the antinociceptive effect induced by the i.c.v. administration of morphine and the simultaneous administration produced an additive effect. The potentiation of the antinociception produced by both D-serine alone or in combination with morphine was dose-dependently attenuated by the i.c.v. administration of L-701,324, a selective antagonist for the glycine site of the NMDA receptors. In addition, the potentiation of the D-serine-induced antinociception was antagonized by the i.c.v. administration of naloxone, a nonselective opioid receptor antagonist. These observations, together with the fact that D-serine is an endogenous and selective co-agonist for the glycine site of the NMDA receptors, strongly suggested that the activation of the supraspinal NMDA receptors by D-serine leads to the potentiation of the antinociception in the tail-flick test and that endogenous D-serine could modulate the mu-opioid receptor mediated antinociception via the glycine site of the NMDA receptors at the supraspinal level.

Glutamatergic innervation of neuropeptide Y and pro-opiomelanocortin-containing neurons in the hypothalamic arcuate nucleus of the rat

Abstract The hypothalamic arcuate nucleus contains a number of neurochemically different cell populations, among others neuropeptide Y (NPY)- and pro-opiomelanocortin (POMC)-derived peptide-expressing neurons; both are involved in the regulation of feeding and energy homeostasis, NPY neurons also in the release of hypophysiotropic hormones, sexual behaviour and thermogenesis. Recent observations indicate that there is a dense plexus of glutamatergic fibres in the arcuate nucleus. The aim of the present studies was to examine the relationship of these fibres to the NPY and POMC neurons in the arcuate nucleus. Double-label immunoelectron microscopy was used. Glutamatergic elements were identified by the presence of vesicular glutamate transporter 1 (VGluT1) or 2 (VGluT2) (selective markers of glutamatergic elements) immunoreactivity. A significant number of VGluT2-immunoreactive terminals was observed to make asymmetric type of synapses with NPY and with beta-endorphin (a marker of POMC neurons)-immunostained nerve cells of the arcuate nucleus. About 15% of VGluT2 synapsing terminals established asymmetric synapses with NPY-positive cells and more than 40% of VGlut2-positive terminals formed synapse on beta-endorphin-positive neurons. VGluT2-positive perikarya were also observed, part of them also contained beta-endorphin. Nerve terminals containing both VGluT2 and beta-endorphin were demonstrated in the cell group. Only very few VGluT1 fibres were detected. Our observations provide the first direct neuromorphological evidence for the existence of glutamatergic innervation of NPY and POMC neurons of the arcuate nucleus.

Glutamatergic Agents for Cocaine Dependence

Effective medications for cocaine dependence are needed to improve outcome in this chronic, relapsing disorder. Medications affecting glutamate function are reasonable candidates for investigation, given the involvement of glutamate circuits in reward-related brain regions and evidence of cocaine-induced glutamatergic dysregulation. In addition, it is increasingly apparent that glutamatergic mechanisms underlie several clinical aspects of cocaine dependence, including euphoria, withdrawal, craving, and hedonic dysfunction. Even denial, traditionally viewed as purely psychological, may result, in part, from dysfunctional glutamate-rich cortical regions. We review the involvement of glutamate in reward-related circuits, the acute and chronic effects of cocaine on these pathways, and glutamatergic mechanisms that contribute to the neurobiology of cocaine dependence. We also present preliminary data from our research of modafinil, a glutamate-enhancing agent with promise in the treatment of cocaine-addicted individuals.

Independent time courses of supraspinal nociceptive activity and spinally mediated behavior during tonic pain

The behavioral response to acute tissue injury is usually characterized by different phases, but the brain mechanisms underlying changes in pain-related behavior over time are still poorly understood. We aimed to analyze time-dependent changes in metabolic activity levels of 49 forebrain structures in the formalin pain model, using the autoradiographic 2-deoxyglucose method in unanesthetized, freely moving rats. We examined rats during the first phase of pain-related reactions ('early' groups), or during the third recovery phase, 60 min later, when the supraspinally mediated behavioral responses were reduced ('late' group). In the early groups, metabolic rates were bilaterally increased over control values in the periaqueductal gray, zona incerta and in several thalamic nuclei (anteroventral, centrolateral, lateral dorsal, parafascicular, posteromedial, submedius, ventromedial, and ventrobasal complex), as well as in the habenulae and in the parietal, cingulate, antero-dorsal insular, and anterior piriform cortex. A contralateral, somatotopically specific activation was found in the putative hindlimb representation area of the somatosensory cortex. In the late group, noxious-induced activation declined in most structures. However, metabolic rates were higher than controls in the periaqueductal gray and zona incerta and in two other structures not previously active: the prerubral area/field of Forel and the arcuate hypothalamic nucleus. These findings provide a time-dependent functional map of nociceptive and anti-nociceptive forebrain circuits during tonic pain. The parallel decrease in licking behavior and forebrain activity, at times when spinally mediated limb flexion responses were still present, suggests that endogenous antinociceptive systems may differently modulate spinal and supraspinal nociceptive networks following acute tissue injury.

Fear and power-dominance motivation: proposed contributions of peptide hormones present in cerebrospinal fluid and plasma

We propose that fear and power-dominance drive motivation are generated by the presence of elevated plasma and cerebrospinal fluid (CSF) levels of certain peptide hormones. For the fear drive, the controlling hormone is corticotropin releasing factor, and we argue that elevated CSF and plasma levels of this peptide which occur as a result of fear-evoking and other stressful experiences in the recent past are detected and transduced into neuronal activities by neurons in the vicinity of the third ventricle, primarily in the periventricular and arcuate hypothalamic nuclei. For the power-dominance drive, we propose that the primary signal is the CSF concentration of vasopressin, which is detected in two circumventricular organs, the subfornical organ and organum vasculosum of the lamina terminalis. We suggest that the peptide-generated signals detected in periventricular structures are transmitted to four areas in which neuronal activities represent fear and power-dominance: one in the medial hypothalamus, one in the dorsolateral quadrant of the periaqueductal gray matter, a third in the midline thalamic nuclei, and the fourth within medial prefrontal cortex. The probable purpose of this system is to maintain a state of fear or anger and consequent vigilant or aggressive behavior after the initial fear- or anger-inducing stimulus is no longer perceptible. We further propose that all the motivational drives, including thirst, hunger and sexual desire are generated in part by non-steroidal hormonal signals, and that the unstimulated motivational status of an individual is determined by the relative CSF and plasma levels of several peptide hormones.

Effects of photoperiod and androgen on proopiomelanocortin gene expression in the arcuate nucleus of golden hamsters

In golden hamsters, seasonal changes in day length act via a pineal-dependent mechanism to regulate feedback and behavioral effects of androgen. Endogenous opiates participate in photoperiodically regulated neuroendocrine functions, but the effects of androgen on expression of the gene encoding POMC, the precursor of beta-endorphin, have been controversial. We used quantitative in situ hybridization to examine regulation of POMC messenger RNA (mRNA) by testosterone and to test the hypothesis that short day lengths act through the pineal gland to amplify POMC mRNA expression. We studied intact hamsters and castrates with or without androgen treatment held in long (14 h of light, 10 h of darkness) or short (5 h of light, 19 h of darkness) days for 10 weeks. POMC gene expression differed with rostral-caudal plane, photoperiod, and surgical treatment (castration and testosterone administration). Testosterone increased the number of silver grains in labeled cells throughout the arcuate nucleus, and short day castrates given androgen consistently had more silver grains per labeled cell than did their long day counterparts. Testosterone exerted an inhibitory effect, however, on the number of POMC mRNA-positive cells, and more POMC mRNA-labeled cells were found in the arcuate nucleus of long than short day castrates treated with testosterone. Photoperiod had no significant influence in castrates not receiving androgen. Testosterone treatment had generally similar effects whether it was begun at the time of castration or 5 weeks later. Pinealectomy blocked the influence of photoperiod on both the mean number of silver grains per labeled cell and the number of labeled cells. The results indicate that day length regulates POMC gene expression when androgen levels are held constant, but that androgen is necessary for photoperiod effects to be expressed.

Aspartic acid in the arcuate nucleus attenuates the depressive effects of naloxone on ventilation

Ventilation, oxygen consumption, the ventilatory equivalent for oxygen, and ventilatory responses to hypoxia and to hypercapnia were evaluated in conscious male rats who received each of four treatments: (1) microinjection of artificial cerebrospinal fluid (aCSF) into the arcuate nucleus and subcutaneously saline (CS); (2) aspartic acid into the arcuate nucleus and saline subcutaneously (AS); (3) aCSF into the arcuate nucleus and naloxone subcutaneously (CN); and (4) aspartic acid into the arcuate nucleus and naloxone subcutaneously (AN). Rats treated with CN exhibited a depression of ventilation, ventilatory equivalent, ventilatory response to hypercapnia, and tidal volume response to hypoxia and to hypercapnia. AS had no effect on any parameters. Administration of both aspartic acid and naloxone attenuated all the effects of CN except the depression of minute ventilation in response to hypercapnia. Therefore the naloxone (a mu opioid receptor antagonist) induced a depression of ventilation that was attenuated by aspartic acid acting on N-methyl-D-aspartic acid receptors in the arcuate nucleus.

The release of beta-endorphin and the neuropeptide-receptor mismatch in the brain

Microprobes bearing immobilized antibodies to the carboxy-terminus of beta-endorphin were used to study the release of beta-endorphin in the urethane anaesthetized rat following electrical stimulation of the ipsilateral arcuate nucleus. The microprobes were inserted through the cerebral hemisphere, the superior colliculus and the midbrain periaqueductal grey. Since such microprobes detect extracellular molecules along their entire length they give information on the persistence and spread of compounds following release. Little immunoreactive-beta-endorphin was detected in the areas of brain sampled during electrical stimulation of arcuate nucleus but a remarkable spread throughout the midbrain and cerebral cortex occurred within 30 min of the cessation of stimulation. The results suggest that although beta-endorphin-containing fibres are absent in many parts of the brain, this neuropeptide can access receptors in these sites and it is not necessary for release to be directly adjacent to opiate receptors. As such it is important evidence supporting the hypothesis of volume transmission as a means of neuronal communication. The results also suggest that an important mechanism of the transport of beta-endorphin is the cerebrospinal fluid.

Metabotropic glutamate receptor in GHRH and beta-endorphin neurones of the hypothalamic arcuate nucleus

Growth hormone-releasing hormone (GHRH) and beta-endorphin are mainly synthesized in neurones of the hypothalamic arcuate nucleus. Arcuate neurones also contain both ionotropic and metabotropic glutamate receptors. The aim of present study was to investigate whether glutamate receptors are present in GHRH and beta-endorphin containing nerve cells of this hypothalamic area. Using double-label immunocytochemistry as well as the mirror technique, we found that almost all GHRH and beta-endorphin immunoreactive arcuate neurones contain the metabotropic glutamate receptor la. The observations provide morphological evidence for the view that glutamate, which appears to be a major excitatory neurotransmitter in the hypothalamus, may directly stimulate GHRH and beta-endorphin neurones of the medial hypothalamus.

Local synaptic release of glutamate from neurons in the rat hypothalamic arcuate nucleus

1. The hypothalamic arcuate nucleus (ARC) contains neuroendocrine neurons that regulate endocrine secretions by releasing substances which control anterior pituitary hormonal release into the portal blood stream. Many neuroactive substances have been identified in the ARC, but the existence of excitatory neurons in the ARC and the identity of an excitatory transmitter have not been investigated physiologically. 2. In the present experiments using whole-cell current- and voltage-clamp recording of neurons from cultures and slices of the ARC, we demonstrate for the first time that some of the neurons in the ARC secrete glutamate as their transmitter. 3. Using microdrop stimulation of presynaptic neurons in ARC slices, we found that local axons from these glutamatergic neurons make local synaptic contact with other neurons in the ARC and that all evoked excitatory postsynaptic potentials could be blocked by the selective ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) and D,L-2-amino-5-phosphonovalerate (AP5; 100 microM). To determine the identity of ARC neurons postsynaptic to local glutamatergic neurons, we used antidromic stimulation to reveal that many of these cells were neuroendocrine neurons by virtue of their maintaining axon terminals in the median eminence. 4. In ARC cultures, postsynaptic potentials, both excitatory and inhibitory, were virtually eliminated by the glutamate receptor antagonists AP5 and CNQX, underlining the functional importance of glutamate within this part of the neuroendocrine brain. 5. GABA was secreted by a subset of ARC neurons from local axons. The GABAA receptor antagonist bicuculline released glutamatergic neurons from chronic inhibition mediated by synaptically released GABA, resulting in further depolarization and an increase in the amplitude and frequency of glutamate-mediated excitatory postsynaptic potentials.

Beta-endorphin in the brain. A role in nociception

We have known the endogenous opioid peptide beta-endorphin for 20 years. Surprisingly, our knowledge of the physiological role of this peptide and its receptors in modulation of pain perception is still fragmentary. Whereas most studies have tried to elucidate the physiological role of beta-endorphin by reversing evoked responses by the opioid antagonist naloxone, this review focuses on quantification of release of beta-endorphin in the brain as the approach to define physiological and pathophysiological roles of beta-endorphin in relation to nociception. Using a lateral ventricle-cisterna magna perfusion model in the anesthetized rat, it was shown that depolarization of neurons in the arcuate nucleus of the hypothalamus, where beta-endorphin in produced, was followed by release of beta-endorphin to the cerebrospinal fluid compartment. Intense activation of spinal nociceptive pathways by intrathecal capsaicin injections also led to beta-endorphin release. It is concluded that there may still be good reason to quantify beta-endorphin in human cerebrospinal fluid to elucidate the role of beta-endorphin in pain perception.

Release of beta-endorphin immunoreactivity into ventriculo-cisternal perfusate by lumbar intrathecal capsaicin in the rat

A model employing perfusion of artificial cerebrospinal fluid from the lateral ventricle to the cisterna magna in the halothane anesthetized rat was used to study beta-endorphin release in the brain. Injection of 75 micrograms capsaicin into the lumbar intrathecal space released beta-endorphin immunoreactivity into perfusate. The release was blocked by intrathecal pretreatment with 1.25 mg lidocaine and the capsaicin receptor antagonist capsazepine (92 micrograms), showing that the release is caused by binding of capsaicin to a spinal receptor. The release was also blocked by intrathecal pretreatment with the NMDA antagonist MK-801 (3 micrograms) and the NK-1 receptor antagonist CP96,345 (200 micrograms), whereas the AMPA receptor antagonist NBQX (6 micrograms) yielded no significant inhibition. Surprisingly, morphine (30 micrograms) and sufentanil (1.5 micrograms) did not prevent release of beta-endorphin immunoreactivity, although blocking the cardiovascular responses to a noxious heat stimulus. High performance liquid chromatography characterization of perfusates collected after capsaicin injection showed that all beta-endorphin immunoreactivity coeluted with authentic beta-endorphin1-31. beta-Endorphin immunoreactivity in plasma was increased 10 min, but not 25 min, after capsaicin injection. Capsaicin injection abolished the motor and cardiovascular responses to tail immersion in 52.5 degrees C water. Addition of MK-801 (10(-4) mol/l) to the lateral ventricle-cisterna magna perfusate blocked the capsaicin-induced beta-endorphin release, showing that our previous demonstration of an NMDA receptor regulating arcuate nucleus beta-endorphin neuron activity has functional significance. We conclude that in this in vivo, anesthetized preparation including three hot water tail immersions, beta-endorphin can be released into a ventriculo-cisternal perfusate, by activation of the central axons of small primary afferent neurons by capsaicin. These data support the idea that central beta-endorphin may be released in response to prolonged, intense noxious stimulation.

Cerebrospinal fluid beta-endorphin in models of hyperalgesia in the rat

Cerebrospinal fluid (CSF) obtained by acute percutaneous puncture of the cisternal membrane of the halothane anesthetized rat has low but measurable concentrations of beta-endorphin-like immunoreactivity (beta-EPir: 32.8 +/- 3.0 pmol/l). Chromatographic separation of beta-EPir showed that authentic beta-endorphin1-31 was the main component of beta-EPir in cisternal CSF. Subcutaneous injection of 5% formalin in the hind paws did not increase beta-EPir in cisternal CSF. Rats with tactile paw hyperalgesia evoked by unilateral ligation of the L5/6 nerve roots 2 weeks earlier had beta-EPir concentrations that did not differ from sham operated or unoperated control animals. In contrast, capsaicin injected in the hindpaws increased the mean beta-EPir concentration compared to saline injections (P = 0.006) 45 min after emerging from anesthesia following injection. These results show that acute activation of C fibers (by capsaicin) will evoke the release of beta-endorphin into the CSF, suggesting activation of the beta-endorphin terminal systems in the brain/midbrain. The failure of formalin injections to release beta-EPir to CSF may be due to specificity of the afferent stimulus evoking beta-EPir release, a lower stimulus intensity, and/or the duration of the stimulus generated by formalin. The normal concentrations of beta-EPir found in the hyperalgesic state following nerve injury suggest that the supraspinal beta-endorphin system does not display tonic changes under such conditions.

Release into ventriculo-cisternal perfusate of beta-endorphin- and Met-enkephalin-immunoreactivity: effects of electrical stimulation in the arcuate nucleus and periaqueductal gray of the rat

To examine the resting and evoked release of the endogenous opioid peptides beta-endorphin and Met-enkephalin from brain, we examined the levels of the respective immunoreactivities in the lateral ventricle-cisterna magna perfusate of the halothane-anesthetized rat. Ten Hz but not 100 Hz stimulation in the arcuate nucleus (ARC) of the hypothalamus released beta-endorphin immunoreactivity (beta-EPir) to the perfusate, whereas 100 Hz but not 10 Hz stimulation in the periaqueductal gray (PAG) of the mid brain released Met-enkephalin immunoreactivity (MEir). MEir was not released by stimulation in ARC and beta-EPir was not released by stimulation in PAG. Characterization of the released beta-EPir and MEir by high performance liquid chromatography showed that authentic beta-endorphin and Met-enkephalin were the major constituents of beta-EPir and MEir, respectively. Systemic administration of the dopaminergic antagonist haloperidol increased plasma, but not perfusate levels of beta-EPir. Both the opioid antagonist naloxone and the NMDA antagonist MK-801 failed to affect beta-EPir or MEir release. ARC and PAG stimulated inhibited a nociceptive reflex (tail-dip in 52.5 degrees C water), and naloxone did not reliably reverse this inhibition. These data support the previously suggested possibility of opioid mediation of stimulation induced analgesia, although we were unable to confirm the theory by naloxone reversibility in this study. Furthermore, the data support the assumption that measurement of opioid peptides in cerebrospinal fluid is a relevant approach in research aimed at elucidating the physiological and pathophysiological roles of endogenous opioid peptides.