α-Melanocyte stimulating hormone discloses a stimulatory effect of β-endorphin on somatostatin release

The influence of alpha-melanocyte stimulating hormone (alpha-MSH) and beta-endorphin (beta-END) on the secretion of somatostatin (SRIF) from the median eminence (ME) was studied using an in vitro incubation system. The MEs from adult male rats were first preincubated at 37 degrees C for 30 min with constant shaking in 0.4 ml of Krebs-Ringer bicarbonate-glucose buffer (pH 7.4) containing bacitracin in an atmosphere of 95% O2/5% CO2. Medium was discarded and replaced by medium containing different doses of alpha-MSH, beta-END, or a fixed dose of alpha-MSH (10(-7) M or 10(-9) M) plus beta-END at various concentrations. By themselves alpha-MSH and beta-END did not alter basal SRIF release, but in the presence of alpha-MSH (10(-7) M) beta-END stimulated somatostatin release. This effect was significant at concentrations of beta-END of 10(-8) M and higher. The permissive effect of alpha-MSH was observed at a concentration as low as 10(-9) M, but in this case the stimulatory effect of beta-END became evident only at higher doses tested (10(-7) M). It is suggested that alpha-MSH and beta-END participate in the modulation of SRIF release. By themselves beta-END and alpha-MSH did not affect basal release of SRIF but in the presence of alpha-MSH, beta-END had a stimulatory effect on SRIF release. The mechanism for this interaction is unknown. The results are consistent with the possibility that beta-END neurons have stimulatory and inhibitory effects on SRIF release and that alpha-MSH, by blocking the inhibitory components, discloses the stimulatory effect of beta-END on SRIF release.     

Presence of strong glucocorticoid receptor immunoreactivity within hypothalamic and hypophyseal cells containing pro-opiomelanocortic peptides.

The presence of nuclear glucocorticoid receptor immunoreactivity (GR IR) was studied in the adrenocorticotropin (ACTH), beta-Endorphin (beta-END) and alpha-melanocyte stimulating hormone (alpha-MSH) IR neuronal populations of the rat hypothalamus and hypophysis using double immunolabelling techniques. All the nuclei of the ACTH/beta-END/alpha-MSH IR neurons of the arcuate and periarcuate nuclei were strongly GR IR in the 48 h colchicine treated animal, but very few alpha-MSH-like IR perikarya located in the dorsal and lateral hypothalamus displayed nuclear GR IR. GR IR was present in the ACTH/beta-END corticotrophs and absent in the intermediate lobe of the hypophysis. The data provide morphological evidence for a glucocorticoid action through a nuclear GR in the arcuate ACTH/beta-END/alpha-MSH IR neurons and the ACTH/beta-END corticotrophs, whereas the alpha-MSH-like IR neurons of the lateral hypothalamus and the melanotropes of the intermediate lobe may not be directly affected by glucocorticoids under normal conditions.     

Distribution of the pro-opiomelanocortin derived peptides, adrenocorticotrope hormone, alpha-melanocyte-stimulating hormone and beta-endorphin (ACTH, alpha-MSH, beta-END) in the rat hypothalamus

Rat hypothalamic nuclei were removed and assayed for adrenocorticotropic hormone (ACTH), beta-endorphin (beta-END) and alpha-melanocyte-stimulating hormone (alpha-MSH) content by radioimmunoassay, from the same samples. We also performed immunostaining for these 3 pro-opiomelanocortin (POMC) derived peptides on paraffin embedded serial sections of the hypothalamus. Areas known to project to the external zone of the median eminence receive a dense POMC innervation while those projecting to the posterior pituitary are not innervated. In addition, hypothalamic areas previously suggested to project to medullary autonomic centers are densely innervated. This innervation pattern may provide the morphological basis for the involvement of POMC peptides in neuroendocrine and autonomic functions. The biochemical data raise the possibility that the POMC precursor is processed differently in various brain regions.     

Role of alpha-MSH and related peptides in the central nervous system

Alpha-melanocyte stimulating hormone (alpha-MSH) is a tridecapeptide secreted by intermediate lobe cells and synthesized in the brain as well. As a hormonal peptide, the physiological function of alpha-MSH consists mainly in the control of pigment movements within dermal melanophores. At the pituitary level, alpha-MSH secretion is under multifactorial control: it is inhibited by dopamine and GABA and stimulated by corticoliberin (CRF), thyroliberin (TRH), beta-adrenergic agonists and (or) serotonin. Identification of alpha-MSH containing neurons in the hypothalamus and other brain regions (septum, thalamus, mid-brain, striatum, hippocampus, cerebral cortex and spinal cord) has been carried out by means of immunological and biochemical techniques combined with bioassays. In the central nervous system (CNS) as in the hypophysis, alpha-MSH is synthesized from a high molecular weight precursor, pro-opiomelanocortin (POMC). Maturation of this protein yield similar end products in the hypothalamus and the intermediate lobe. Several peptides chemically related to alpha-MSH are generated including the desacetyl and monoacetyl (authentic alpha-MSH) forms; the latter has the greatest behavioral activity. The demonstration that alpha-MSH has numerous central nervous system effects has raised the possibility that this neuropeptide acts as a neuromodulator or a neurotransmitter. In the rat, intra-cerebroventricular administration of ACTH/MSH peptides induces the stretching-yawning syndrome (SYS) which is frequently preceded by excessive grooming. This excessive grooming is blocked by neuroleptics indicating that the central dopaminergic neurons are implicated in this behavioral effect of the peptide. alpha-MSH is involved in memory, arousal and attention; in hypophysectomized animals, the learning ability is restored after administration of MSH or related peptides. Injection of alpha-MSH delays also extinction of passive avoidance behavior and affects performances motivated by hunger as well as aggressive behavior. Recent studies concerning the role of alpha-MSH have been undertaken in human beings. The effects of MSH-related peptides favour a role of these peptides in arousal: they maintain a high level of vigilance and improve visual discrimination. These behavioral changes were accompanied by marked changes in CNS electrophysiology. Current studies, which aim at establishing a neurotransmitter function for alpha-MSH, concern the distribution and characterization of alpha-MSH receptors in the central nervous system and the mechanism controlling the release of neuronal alpha-MSH.     

Proopiomelanocortin peptide immunocytochemistry in rhesus monkey brain

The immunocytochemical distribution of proopiomelanocortin (POMC) peptides (beta-endorphin, ACTH, alpha-MSH, 16K fragment) was studied in the brain of the rhesus monkey (Macaca mulatta). Some animals were administered colchicine intracerebroventricularly prior to sacrifice to enhance the visualization of perikaryal immunoreactivity. Immunoreactive perikarya are localized to hypothalamic infundibular nucleus, giving rise to several distinct projections. Rostral projections extend through midline diencephalic and preoptic areas, and enter the telencephalon. Along this course, immunoreactive fibers are seen in midline hypothalamic and preoptic nuclei, nucleus of the diagonal band, olfactory tubercle, nucleus accumbens, bed nucleus of stria terminalis, septum, and other limbic structures in telencephalon. Caudal to the anterior commissure, some fibers ascend dorsally to enter the midline thalamus, which they innervate. Lateral projections of the infundibular perikarya course through the medial-basal hypothalamus, dorsal to the optic tracts, and enter the amygdala region where they innervate more medially situated amygdaloid nuclei. Caudal projections of the POMC neurons also extend through midline diencephalon, some coursing along a periventricular path to innervate midline hypothalamic and thalamic nuclei. This projection extends into the mesencephalic substantia grisea centralis and may also contribute to the innervation of more dorsally situated nuclei in the pons and medulla, such as the parabrachial nuclei and nucleus tractus solitarius. Other caudal projections originating in the hypothalamus course through the ventral tegmentum of mesencephalon and pons and may contribute to the innervation of midline raphe and other ventrally situated nuclei in the pons and medulla. The distribution of immunoreactive perikarya and fibers in the brain of rhesus monkey is strikingly similar to that found in the rat brain. However, subtle differences appear to exist in the innervation patterns of particular brain regions.     

Exercise and mental health

This paper reviews the mood altering properties of exercise and its potential in the prevention and treatment of mental disorders. The role of the brain monoamines, opioid peptides, the sympathetic nervous system, and cognitive behavioural theory as mediating pathways for the psychological benefits of exercise is critically examined. Clinical trials on exercise are reviewed and suggestions are made for future research in this field.     

Conserved neurochemical pathways involved in hypothalamic control of energy homeostasis

The melanocortin system, which includes alpha-melanocyte-stimulating hormone (alpha-MSH) and its endogenous antagonist, agouti-related protein (AgRP), is fundamental for the central control of energy homeostasis in mammals. Recent studies have demonstrated that many neuropeptides involved in the control of ingestive behavior and energy expenditure, including melanocortins, are also expressed and functional in teleost fishes. To test the hypothesis that the underlying neural pathways involved in energy homeostasis are conserved throughout vertebrate evolution, the neuroanatomical distribution of alpha-MSH in relation to AgRP was mapped in a teleost (zebrafish, Danio rerio) by double-label immunocytochemistry. Zebrafish alpha-MSH- and AgRP-immunoreactive (ir) cells are found in discrete populations in the ventral periventricular hypothalamus, the proposed arcuate homologue in teleosts. Major ascending projections are similar for both peptides, and dense ir-fibers innervate preoptic and ventral telencephalic nuclei homologous to paraventricular, lateral septal, and amygdala nuclei in mammals. Furthermore, alpha-MSH and AgRP-ir somata and fibers are pronounced at 5 days post fertilization when yolk reserves are depleted and larvae begin to feed actively, which supports the functional significance of these peptides for feeding behavior. The conservation of melanocortin peptide function and projection pathways further support zebrafish as an excellent genetic model system to investigate basic mechanisms involved in the central regulation of energy homeostasis.     

Injection of alpha-MSH, but not beta-endorphin, into the PVN decreases POMC gene expression in the ARC

beta-Endorphin (beta-END) and alpha-melanocyte stimulating hormone (alpha-MSH), neuropeptides derived from proopiomelanocortin (POMC), have opposite effects on eating behavior. We injected rats with alpha-MSH (0.6 nmol) or beta-END (1 nmol) into the PVN (three times in a 26 h period). These doses of alpha-MSH and beta-END decreased and increased feeding respectively. Following alpha-MSH administration into the PVN, mRNA levels of POMC decreased by 17%, whereas there was no significant change in gene expression of either proDynorphin or proEnkephalin. PVN injection of beta-END failed to alter gene expression of POMC, proDynorphin or proEnkephalin. These data suggest that a feedback pathway exists between the PVN and ARC for alpha-MSH and POMC, but not for beta-END and POMC.     

Urocortin expression in rat brain: evidence against a pervasive relationship of urocortin-containing projections with targets bearing type 2 CRF receptors.

Histochemical and axonal transport methods were used to clarify the central organization of cells and fibers that express urocortin (UCN), a recently discovered corticotropin-releasing factor (CRF)-related neuropeptide, which has been proposed as an endogenous ligand for type 2 CRF receptors (CRF-R2). Neurons that display both UCN mRNA and peptide expression were found to be centered in the Edinger-Westphal (EW), lateral superior olivary (LSO), and supraoptic nuclei; lower levels of expression are seen in certain cranial nerve and spinal motoneurons and in small populations of neurons in the forebrain. Additional sites of UCN mRNA and peptide expression detected only in colchicine-treated rats are considered to be minor ones. UCN-immunoreactive projections in brain are predominantly descending and largely consistent with central projections attributed to the EW and LSO, targeting principally accessory optic, precerebellar, and auditory structures, as well as the spinal intermediate gray. Although neither the EW nor LSO are known to project to the forebrain, UCN-ir neurons in the EW were identified that project to the lateral septal nucleus, which houses a prominent UCN-ir terminal field. Although substantial UCN-ir projections were observed to several brainstem cell groups that express CRF-R2, including the dorsal raphe and interpeduncular nuclei and the nucleus of the solitary tract (NTS), most prominent seats of CRF-R2 expression were found to contain inputs immunopositive for piscine urotensin I, but not rat UCN. The results define a central UCN system whose organization suggests a principal involvement in motor control and sensorimotor integration; its participation in stress-related mechanisms would appear to derive principally by virtue of projections to the spinal intermediolateral column, the NTS, and the paraventricular nucleus. Several observations, including the lack of a pervasive relationship of UCN-ir projections with CRF-R2-expressing targets, support the existence of still additional CRF-related peptides in mammalian brain.     

Opioid modulation of appetite

The discovery of opiate receptors and endogenous opioid peptides within the central nervous system has resulted in a number of speculations concerning the physiological significance of these peptides. In the present article, we review the evidence suggesting a primary role for some of the opioid peptides as regulators of ingestive behavior. In particular, we elaborate a hypothesis in which we suggest that in some species opioid peptides may play a role as a tonic inducer of ingestive behaviors, held in check by a variety of neuropeptides and monoamines. This review explores in detail the role of the opioid peptides as major mediators of the reward system and as a link between reward and feeding behaviors. Finally, a teleological role for opioid peptides in species preservation, which may explain the discrepancies in the role of the opioid peptides in feeding behavior in different species is proposed. It is suggested that the feeding profile of the animal provides important clues as to whether or not the animal has an opiate-sensitive feeding system. We stress that interactions with ingested nutrients and the milieu interieur provide an important means by which animals modulate the opiate-entrained feeding drives.     

Opioid-induced feeding: Localization of sensitive brain sites

These experiments were designed to identify brain sites at which opioids might act to influence ingestive behavior and to determine which opioid receptor types are involved. After food deprivation, rats were given microinjections of naloxone into several brain regions and food intake was measured. Injections into or near the paraventricular (PVN) or ventromedial (VMH) hypothalamic nuclei or the globus pallidus (GP) reduced food intake; injections into the striatum or lateral hypothalamus (LH) were ineffective. A second study examined the ingestive effects of roughly equimolar doses (1.43-1.75 nmol) of dynorphin A (DYN), beta-endorphin (beta-END), and D-Ala2,D-Leu5-enkephalin (DADLE) when injected into 4 different brain regions. Only DYN significantly increased food intake, and this effect was seen only with injections into the PVN and VMH. Beta-END stimulated water intake when injected into the PVN, VMH and GP but not the LH. Further studies indicated that with PVN injections, DYN was effective at a dose as low as 0.47 nmol, and that a higher dose of DADLE (4.39 nmol) did stimulate food intake. These studies support an important role for dynorphin and the kappa opioid receptor in the regulation of feeding and suggest that the opioid regulation of food and water intake can be differentiated both by sites of action and by effective agonists.     

Characterization of alpha-melanocyte-stimulating hormone (alpha-MSH)-like peptides in discrete regions of the rat brain. In vitro release of alpha-MSH from perifused hypothalamus and amygdala

The neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) is synthesized by discrete populations of hypothalamic neurons which project in different brain regions including the cerebral cortex, hippocampus and amygdala nuclei. The purpose of the present study was to identify the alpha-MSH-immunoreactive species contained in these different structures and to compare the ionic mechanisms underlaying alpha-MSH release at the proximal and distal levels, i.e. within the hypothalamus and amygdala nuclei, respectively. The molecular forms of alpha-MSH-related peptides stored in discrete areas of the brain were characterized by combining high-performance liquid chromatography (HPLC) separation and radioimmunoassay detection. In mediobasal and dorsolateral hypothalamic extracts, HPLC analysis confirmed the existence of a major immunoreactive peak which co-eluted with the synthetic des-N alpha-acetyl alpha-MSH standard. In contrast, 3 distinct forms of immunoreactive alpha-MSH, which exhibited the same retention times as synthetic des-, mono- and di-acetyl alpha-MSH, were resolved in amygdala nuclei, hippocampus, cortex and medulla oblongata extracts. The proportions of acetylated alpha-MSH (authentic alpha-MSH plus diacetyl alpha-MSH) contained in these extrahypothalamic structures were, respectively, 78, 80, 60 and 92% of the total alpha-MSH immunoreactivity. In order to compare the ionic mechanisms underlaying alpha-MSH release from hypothalamic and extrahypothalamic tissues, we have investigated in vitro the secretion of alpha-MSH by perifused slices of hypothalamus and amygdala nuclei. High potassium concentrations induced a marked increase of alpha-MSH release from both tissue preparations. However, a higher concentration of KCl was required to obtain maximal stimulation of amygdala nuclei (90 mM) than hypothalamic tissue (50 mM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification, characterization and stereotaxic mapping of intraneuronal alpha-melanocyte stimulating hormone-like immunoreactive peptides in discrete regions of the rat brain.

A highly specific antibody to alpha-melanocyte stimulating hormone (alpha-MSH) was used to histochemically localize and biochemically identify and quantitate alpha-MSH immunoreactivity in nerve fibers and cell bodies of the rat brain. alpha-MSH-like immunoreactivity was contained in fibers throughout the brain. The distribution of alpha-MSH was determined by immunocytochemistry as well as by radioimmunoassay combined with microdissection techniques. High concentrations of alpha-MSH were contained in the nucleus interstitialis stria terminalis, the median eminence and the medial preoptic, anterior hypothalamic, periventricular, paraventricular, arcuate, dorsomedial, and posterior hypothalamic nuclei. Moderate alpha-MSH concentrations were noted in the amygdala, septum, central gray, dorsal raphe, and the nucleus tractus solitarius. Cell bodies containing alpha-MSH were observed only in the arcuate nucleus. The alpha-MSH-like compound in brain had similar immunochemical and electrophoretic properties of standard alpha-MSH but high pressure liquid chromatographic analysis demonstrated that the alpha-MSH-like immunoreactivity was comprised of one major and two minor components. The major immunoreactive peak had an identical retention time as alpha-MSH and therefore may be chemically identical to alpha-MSH. The similar retention times and immunoreactivity of the other two compounds suggest a similarity in size and structure to alpha-MSH. These observations demonstrate that fibers containing alpha-MSH emanate from the arcuate nucleus to innervate many other regions of the rat brain.     

Topographical distribution of pro-opiomelanocortin-derived peptides (ACTH/β-END/α-MSH) in the rat median eminence

The detailed distribution of adrenocorticotropin (ACTH), beta-endorphin (beta-END) and alpha-melanotropin (alpha-MSH) immunoreactivity was examined in the rat median eminence (ME) and pituitary stalk using light microscopic immunocytochemistry and radioimmunoassay (RIA). Nerve fibers and varicosities immunoreactive for ACTH/beta-END/alpha-MSH had identical distributions in the ME suggesting that they are part of the same arcuate proopiomelanocortin neuronal (POMC) system. The quantitative image analysis of POMC immunoreactive varicosities in the ME indicates no significant differences between the various rostro-caudal segments. In the main (preinfundibular) portion of the ME, a moderate density of immunoreactive elements was located in the lateral part of the internal zone and throughout the postinfundibular ME. Very few scattered varicosities were observed in the neurohemal (external) zone and in the pituitary stalk. By RIA, alpha-MSH is present in a substantially higher concentration than ACTH and beta-END throughout the ME. Knife cuts between the arcuate nucleus and ME indicate that proopiomelanocortin (POMC) fibers enter the ME in its whole rostro-caudal extent. Thus POMC neurons seem to provide innervation of structures in the internal zone but not in the neurohemal/external/zone where the portal capillary system is located. Moreover, the observation that the density of immunoreactive elements is substantially lower in the pituitary stalk than in the ME, suggests that the majority of immunoreactive fibers in the internal zone are not fibers of passage directed towards the neurohypophysis.     

Variations in opioid peptide levels during the estrous cycle in Sprague-Dawley rats

The estrous cycle, with its various hormonal conditions, may provide us with the means of understanding how endocrine states relate to opioid mechanisms. There has been increasing experimental support for interaction between sex steroids and opioid peptides in the central nervous system. Here, we describe fluctuations in endogenous brain immunoreactive (ir) peptide levels during various phases of the estrous cycle in the female Sprague-Dawley rat. Ir levels of dynorphin A, dynorphin B, Leu-enkephalin-Arg(6), Met-enkephalin-Arg(6)Phe(7) and nociceptin/orphanin FQ were measured in the pituitary gland and in 10 areas of the brain during the diestrus, proestrus and estrus phase. In several areas of the brain, basal levels of endogenous opioid peptides showed variation during the course of the estrous cycle. Significant differences were found between the diestrus state and the proestrus and/or estrus conditions, particularly in the nucleus accumbens, caudate putamen and the substantia nigra. The ir levels of the endogenous peptide nociceptin/orphanin FQ became altered in only one of the areas measured, indicating less variance during the estrous cycle. Correlation analyses revealed that significant associations between dynorphin A or dynorphin B and Leu-enkephalin-Arg(6) were found more often during estrus than during the diestrus and proestrus conditions. The ratio between the ir levels of Leu-enkephalin-Arg(6), a cleavage product of the enzymatic conversion of dynorphin peptides into shorter peptides in vivo, and dynorphin peptides was calculated. The significantly lower ratio between Leu-enkephalin-Arg(6) and dynorphin B in diestrus than in proestrus and estrus also indicates cyclic fluctuations in the enzymatic cleavage of dynorphin. These findings are discussed in relation to the possible role of interactions between sex steroids and opioid peptide mechanisms during the normal estrous cycle.     

The distinct roles of monoamines in multiple sclerosis: A bridge between the immune and nervous systems?

The monoaminergic neurotransmitters dopamine, noradrenaline, and serotonin are pivotal actors of the interplay between the nervous and the immune system due to their ability of binding to cell-receptors of both systems, crucially regulating their function within the central nervous system and the periphery. As monoamines are dysfunctional in many neurological and psychiatric diseases, they have been successfully used as pharmacological targets. Multiple sclerosis (MS) is one of the best examples of neurological disease caused by an altered interaction between the nervous and immune system and emerging evidence supports a dysregulation of monoaminergic systems in the pathogenesis of MS, secondary to both inflammation-induced reduction of monoamines’ synthesis and structural damage to monoaminergic pathways within the brain. Here we review the evidence for monoamines being key mediators of neuroimmune interaction, affecting MS pathogenesis and course. Moreover, we discuss how the reduction/dysfunction of monoamines in MS may contribute to some clinical features typical of the disease, particularly fatigue and depression. Finally, we summarize different drugs targeting monoamines that are currently under evaluation for their potential efficacy to treat MS, as well as to alleviate fatigue and depression in MS.     

Modulation of Endogenous Opioid Systems by Electroconvulsive Shock

Since the discovery of opioid peptides, the brain opioid system has been implicated in the pathophysiology of a spectrum of mental disorders, including depression and epilepsy. For example, a growing body of evidence demonstrates that these neuropeptides are activated by seizures. Specifically, using electroconvulsive shock, it has been possible to describe an array of antinociceptive, autonomic, behavioral, biochemical, and electroencephalographic responses that appear to be mediated by endogenously activated opioids. A primary role for opioid peptides as neuromodulators of postictal seizure arrest and refractoriness is now recognized, and the existence in the central nervous system (CNS) of an endogenous anticonvulsant substance activated by electroconvulsive shock (ECS) has been determined. This review focuses on the more recent developments regarding ECS-induced modulation of brain opioid systems. The ability of ECS to alter opioid receptors, to influence the release and biosynthesis of the various opioid peptides, and to activate endogenous anticonvulsant mechanisms will be addressed.     

alpha-Melanocyte-stimulating hormone and oxytocin: a peptide signalling cascade in the hypothalamus

alpha-Melanocyte-stimulating hormone (alpha-MSH) and oxytocin share remarkable similarities of effects on behaviour in rats; in particular, they both inhibit feeding behaviour and stimulate sexual behaviour. Recently, we showed that alpha-MSH interacts with the magnocellular oxytocin system in the supraoptic nucleus; alpha-MSH induces the release of oxytocin from the dendrites of magnocellular neurones but it inhibits the secretion of oxytocin from their nerve terminals in the posterior pituitary. This effect of alpha-MSH on supraoptic nucleus oxytocin neurones is remarkable for two reasons. First, it illustrates the capacity of magnocellular neurones to differentially regulate peptide release from dendrites and axons and, second, it emphasises the putative role of magnocellular neurones as a major source of central oxytocin release, and as a likely substrate of some oxytocin-mediated behaviours. The ability of peptides to differentially control secretion from different compartments of their targets indicates one way by which peptide signals might have a particularly significant effect on neuronal circuitry. This suggests a possible explanation for the striking way in which some peptides can influence specific, complex behaviours.     

Cultured astrocytes release proenkephalin.

Astrocytes as well as neurons express the mRNA encoding the opioid peptide precursor, proenkephalin. In neurons proenkephalin is cleaved intracellularly to yield smaller, bioactive peptides such as Met-enkephalin and Leu-enkephalin. By contrast, utilizing a combination of radioimmunoassay and chromatographic analysis, we report here that astrocytes cultured from neonatal rat brain contain primarily unprocessed proenkephalin and only small amounts of Met-enkephalin. Further, similar experiments with and without the inclusion of several peptidase inhibitors indicate that cultured astrocytes release proenkephalin itself into the medium where it may be subsequently cleaved to smaller peptide products. The release of intact proenkephalin by astrocytes suggests that the glial propeptide subserves a different function than neuronal proenkephalin and that opioid peptides may play novel roles in the central nervous system.