Diurnal fluctuations in methionine-enkephalin levels in the hypothalamus and preoptic area of the male rat: effects of pinealectomy

Enkephalinase regulation

After millennia of knowledge of opium, it was only recently that endogenous substances called opioids with similar properties to opium and derivatives were discovered. The first to be discovered were enkephalins. In addition to the regulation of their synthesis and expression of receptors, an important mechanism for the regulation of their functions carried out by multiple proteolytic enzymes acting at all levels of their structure is described. The action of such enzymes, known as enkephalinases, is also regulated by endogenous and exogenous factors which ultimately affect the control of the enkephalins's action. For therapeutic purposes, it is not only necessary to develop specific inhibitors but also to acquire a deep knowledge of the influence that such factors exert on their activities. This knowledge could help us to establish adapted therapeutic strategies in the treatment of pain or other processes in which enkephalinases are involved. In this chapter, some of these regulatory factors are discussed, such as regional and subcellular distribution, developmental changes, diurnal variations, hormonal influences, stress, dietary factors or interactions with other neurotransmitters.

Melatonin for the treatment of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common disorder characterized by recurrent abdominal pain or discomfort, in combination with disturbed bowel habits in the absence of identifiable organic cause. Melatonin (N-acetyl-5-methoxytryptamine) is a hormone produced by the pineal gland and also large number by enterochromaffin cells of the digestive mucosa. Melatonin plays an important part in gastrointestinal physiology which includes regulation of gastrointestinal motility, local anti-inflammatory reaction as well as moderation of visceral sensation. Melatonin is commonly given orally. It is categorized by the United States Food and Drug Administration as a dietary supplement. Melatonin treatment has an extremely wide margin of safety though it may cause minor adverse effects, such as headache, rash and nightmares. Melatonin was touted as a potential effective candidate for IBS treatment. Putative role of melatonin in IBS treatment include analgesic effects, regulator of gastrointestinal motility and sensation to sleep promoter. Placebo-controlled studies in melatonin suffered from heterogeneity in methodology. Most studies utilized 3 mg at bedtime as the standard dose of trial. However, all studies had consistently showed improvement in abdominal pain, some showed improvement in quality of life of IBS patients. Melatonin is a relatively safe drug that possesses potential in treating IBS. Future studies should focus on melatonin effect on gut mobility as well as its central nervous system effect to elucidate its role in IBS patients.

Melatonin reduces hyperalgesia associated with inflammation

The perception of pain is altered by inflammatory processes. Anti-inflammatory drugs block this by raising the pain threshold and by reducing the inflammatory process. Melatonin is claimed to have anti-inflammatory activity in animal models of acute and chronic inflammation. However, little is known whether melatonin can reverse the hyperalgesia that is secondary to the inflammation. This study assessed the effect of melatonin on in a well-established model of hyperalgesia associated with inflammation in rats. Peroxynitrite, as generated by the interaction between superoxide anion radical exogenously supplied (O(2)(˙-) ) and endogenous nitric oxide (NO), led to the development of hyperalgesia. This subplantar injection of O(2)(˙-) into the right hindpaw evoked potent thermal hyperalgesia measured by changes in withdrawal latency. Melatonin (25-100 mg/kg, given ip 30 min prior to O(2)(˙-) ) dose dependently attenuated the hyperalgesic responses to O(2)(˙-) . Moreover, melatonin (100 mg/kg) significantly improved tissue damage and inflammation, blocked protein nitration affecting cyclooxygenase-2 and inducible nitric oxide synthase expression in paw tissue. To investigate the antinociceptive activity of melatonin and characterize the underlying mechanisms involved in this action, mitogen-activated protein kinase and NF-κB pathways were explored. Moreover, antihyperalgesic effect of melatonin derived partly from the inhibition of superoxide-driven PARP activation. These results suggest that melatonin has ameliorative potential in attenuating the hyperalgesia associated with inflammation.

The relationship of pineal calcification and melatonin secretion to the pathophysiology of tardive dyskinesia and tourette's syndrome

Despite current intensive research, the pathophysiology of tardive dyskinesia (TD), a serious neurological side effect of neuroleptic treatment, is poorly understood. Prompted by the observation of an increased incidence and severity of abnormal perioral movements in neuroleptic-treated pinealectomized, as compared to intact rats, we suggested that the pineal gland exerts a protective effect which mitigates against the development of TD and, by inference, that reduced melatonin secretion may be related to the pathophysiology of TD. To investigate this proposition further, we studied the association of TD with pineal calcification (PC) on CT scan in chronic schizophrenic patients. Our findings revealed a significant association between TD and PC and suggest, furthermore, that PC may be a neuroradiological marker of TD. Since PC may reflect diminished secretory activity of the gland, these findings support the hypothesis that the pathophysiology of TD is linked to disturbances of melatonin secretion. The clinical and therapeutic implications of these novel findings are discussed. In the following communication, in which we introduce the hypothesis that disturbances of 5-HT and melatonin secretion are related to the pathophysiology of TD. Subsequently, we present a series of studies which relate to the association of TD with PC. We conclude by presenting the hypothesis that disturbances in melatonin secretion may also be relevant to the pathophysiology of Tourette's syndrome.

Rat brain opioid peptides-circadian rhythm is under control of melatonin

Several experiments have revealed an Endogenous Opioid System (EOS)-circadian rhythm. The brain-borne hormone, melatonin (MEL) has been shown to regulate the organism photoperiodic activity and may be implicated in the EOS-circadian rhythm. To explore this hypothesis, we studied the effect of functional pinealectomy on the EOS-circadian rhythm by measuring the immunoreactive content of Met-Enkephalin, Leu-Enkephalin and Synenkephalin in both hypothalamus and hippocampus of the rat brain, using standard radioimmunoassay procedures. Experimental animals exposed to white fluorescent light (WFL) for 15days (<50lux), displayed a disruption of the EOS-circadian rhythm, showing that absence of MEL induced a significant decrease of tissue content of enkephalin peptides at 01:00h during the dark-phase of the 24-h circadian rhythm, when compared to control rats. Functional pinealectomized rats exposed to 4 or 6h period of darkness (used to revert the effects induced by the absence of melatonin) significantly increased the tissue content of ME-IR and LE-IR, when compared to both controls and non-exposed WFL-treated rats. In addition, subcutaneous administration of exogenous melatonin (10, 100, 150, 300, 600microg/kg), in WFL-treated animals produced significant dose-dependent increases of ME-IR in both brain regions tested. Finally, luzindole (melatonin receptor antagonist) administration, was not able to prevent the enkephalin tissue increase, induced with the MEL administration (150microg/kg). This data suggest that MEL not only regulates the EOS-circadian rhythm, but also appears to modulate their synthesis in the rat brain from their respective neurons.

Oral and spinal melatonin reduces tactile allodynia in rats via activation of MT2 and opioid receptors

The antiallodynic effect of melatonin after intrathecal (it) and oral administration as well as the possible participation of MT(2) and opioid receptors in melatonin-induced antiallodynia in neuropathic rats were assessed. Ligation of the L5/L6 spinal nerves produced a clear-cut tactile allodynia in the rats. Intrathecal (3-100 microg) and oral (37.5-300 mg/kg) administration of melatonin decreased tactile allodynia induced by spinal nerve ligation. Intrathecal administration of the preferential MT(2) receptor antagonist luzindole (1-100 microg), but not vehicle, significantly diminished in a dose-dependent manner the antiallodynic effect induced by melatonin (100 microg, it). Oral (0.01-1mg/kg) or intrathecal (0.1-10 microg) administration of the highly selective MT(2) receptor antagonist 4P-PDOT diminished the antiallodynic activity induced by oral (150 mg/kg) or intrathecal (100 microg) administration of melatonin, respectively. Subcutaneous (1mg/kg) or intrathecal (0.5-50 microg) treatment with naltrexone, but not vehicle, significantly diminished the antiallodynic effect induced by oral (150 mg/kg) or intrathecal (100 microg) administration of melatonin. Oral melatonin (150 mg/kg)-induced antiallodynia was partially reduced by the spinal administration of 4P-PDOT (10 microg). Moreover, the spinal effect of melatonin (100 microg) was significantly reduced by the combination 4P-PDOT (0.1 microg)-naltrexone (0.5 microg). At the greatest tested doses, the antagonist drugs did not modify tactile allodynia in neuropathic rats. Melatonin (100 microg or 300 mg/kg) did not affect motor co-ordination in the rotarod test. Results indicate that melatonin reduces tactile allodynia in neuropathic rats after intrathecal and oral administration. Moreover, data suggest the participation of spinal MT(2) and opioid receptors in the melatonin-induced antiallodynic effect in this model.

Enhancement of cellular and humoral immunity following embryonic exposure to melatonin in turkeys (Meleagris gallopavo)

Two experiments were performed to determine the effect of in ovo melatonin supplementation on the ontogeny of immunity in the Large White turkey poult. Different levels of melatonin were injected into the air cell of the egg 4 days prior to hatch. In Experiment 1, turkey embryos received 3 ml of solution containing 200, 100, 50, 25, 10, or 1 microg/ml of melatonin. The hatchability at each dose was determined and compared to vehicle-injected controls. In Experiment 2, only poults from melatonin treatments in Experiment 1 that resulted in normal hatchability (10 and 1 microg/ml) were used. Lymphoproliferative responses to phytohemagglutinin (PHA-P) and primary antibody responses to Chukar red blood cells (CRBC) were determine at five time intervals: 0, 1, 7, 14, and 21 days post-hatch. At each of these times, including 28 days post-hatch, treatment effects on body weights were determined. At 28 days post-hatch, bursal, thymic, and splenic weights were obtained. In ovo melatonin administration significantly accelerated (P0.05) the development of cell-mediated (PHA-P) and humoral (CRBC) immune responses, and these responses were significantly elevated above vehicle-injected controls through 21 days post-hatch. No effect was observed on bursal, thymic, splenic or body weights. These data suggest that embryonic exposure to melatonin enhances post-hatch immune development and responsiveness.

Melatonin enhances cellular and humoral immune responses in the Japanese quail (Coturnix coturnix japonica) via an opiatergic mechanism

It is known that melatonin has important immunomodulatory properties in the Japanese quail. However, the mechanism of melatonin action on the immune system is not clearly understood in avian species. In mammals, the immunostimulatory properties of melatonin are mediated by the release of opioid peptides from activated T-lymphocytes. The present study was performed to determine if these same melatonin-induced opioids (MIO) are involved with the immunoenhancing effects of melatonin in quail. Three treatment groups were given melatonin (50 microg/ml) in the drinking water ad libitum along with naltrexone, a known opioid receptor-blocking agent. Melatonin was administered throughout the 3 week study and each bird received a daily intramuscular injection of naltrexone at a dose of 0.1, 1.0, or 10.0 mg/kg. In addition, three control groups were established that received only melatonin, naltrexone, or diluent. Evaluation of the cellular and humoral immune responses was initiated after 2 weeks of treatments. A cutaneous basophil hypersensitivity reaction to phytohemagglutinin (PHA-P) was measured to evaluate the cellular immune response. To evaluate the humoral immune response, primary antibody titers were determined 7 days post-intravenous injection with a Chukar red blood cell (CRBC) suspension. Both the cellular and humoral immune responses were significantly increased by 22 and 34%, respectively, upon melatonin exposure as compared to quail receiving diluent only. Concomitant administration of naltrexone and melatonin significantly reduced the immunoenhancing effect of melatonin across all naltrexone doses. We conclude that melatonin enhances a cellular and humoral immune response in Japanese quail via an opiatergic mechanism.

The analgesic effects of peripheral and central administration of melatonin in rats

To explore the site and mechanism of the analgesic action of melatonin, the present study was designed to evaluate the analgesic effects of intraperitoneal (i.p.) and intracerebroventricular (i.c.v. ) administration of melatonin, and to investigate the effect of i.c. v. naloxone on the analgesic effect induced by i.p. melatonin in rats. Antinociception was determined by tail-flick latency to hot water at 50 degrees C. On i.p. administration, melatonin (30, 60 and 120 mg/kg) produced the antinociceptive effect in a dose-dependent manner, with an A(50) of 72.8 mg/kg. Administered i.c.v., melatonin (0.25, 0.5 and 1 mg/kg) also resulted in dose-dependent antinociception, with an A(50) of only 0.693 mg/kg. Injected i.c.v. to rats, 10 microg of naloxone antagonized significantly the antinociceptive effect induced by i.p. melatonin. It is concluded that melatonin has an analgesic effect in rats and the central nervous system (CNS) may be the primary site for melatonin to elicit the response, and the effect of melatonin is related to the central opioid system.

Effects of extremely low frequency magnetic fields on pain thresholds in mice: roles of melatonin and opioids

1. We studied the effects of extremely low frequency (ELF, 60 Hz) magnetic fields (MFs) on pain thresholds using the hot plate test. The implication of opioid and benzodiazepine system in the MFs-induced alteration of pain thresholds was also studied. 2. There was an increase at night time and a decrease at daytime of pain thresholds in normal mice. Exposure of MFs (24 h, 20 gauss (G)) inhibited the increase of pain thresholds at night time and even produced hyperalgesia at daytime. 3. The increase of pain thresholds induced by melatonin at daytime was inhibited by exposure to MFs (24 h, 20 G) or opioid antagonist naloxone. The MFs and naloxone synergically inhibited hypoalgesia produced by melatonin. The hyperalgesia at daytime after MFs exposure was potentiated by the benzodiazepine agonist, diazepam, and inhibited by the benzodiazepine antagonist, flumazenil. There was no significant difference in all rotarod performance we tested. 4. From these results, it is suggested that exposure to MFs inhibits the increase of pain thresholds at night time and produces hyperalgesia at daytime with the involvement of opioid and benzodiazepine systems.

Diurnal variations of opioid peptides and synenkephalin in vitro release in the amygdala of kindled rats

Pentylenetetrazol (PTZ) kindling was induced in male Wistar rats (250-300 g) by daily intraperitoneal injections of 35 mg/kg of the convulsant agent. Immunoreactive (IR)-Met-enkephalin (IR-ME), IR-Leu-enkephalin (IR-LE), IR-heptapeptide (IR-HE), IR-octapeptide (IR-OC) and IR-synenkephalin (IR-Syn) in vitro release was measured from amygdala slices 24 h after the last stimulus, in groups of eight rats, every 4 h beginning at 08:00 h. Opioid peptides in vitro release displayed diurnal variations. IR-ME and IR-Syn showed maximal levels before the onset of darkness (16:00 h). IR-LE and IR-OC release was enhanced 4 h later (20:00 h), no changes were detected for IR-HE. These results show that endogenous opioid system (EOS) release displays diurnal variations. The peak for the analysed peptides was reached before and during the dark phase. It is suggested that EOS release enhancement in PTZ-kindled rats, seems to be due to a compensatory mechanism against the excitation induced by the blockade of the GABAergic transmission.

Circadian neuroendocrine functions in disorders of eating behavior

Eating behavior is a complex function determined by regulatory mechanisms characterized by bioperiodic fluctuations. It involves the hypothalamus as well as the related higher centers in the central nervous system (CNS). Many hormones, neurotransmitters and neuropeptides play an important role in the synchronization of food intake. Our study therefore sets out to evaluate the circadian rhythms of several endocrine functions in women with eating disorders, to clarify the pathophysiology of the limbic-hypothalamic system. We measured the circadian rhythms of plasma melatonin, serum cortisol, growth hormone (GH) and prolactin (PRL) in 26 patients with anorexia nervosa (AN), 27 with primary obesity (OB) and 7 with bulimia nervosa (BN). Simultaneous evaluation of different neuroendocrine rhythms in these three groups revealed similar circadian abnormalities, (namely daytime persistence of melatonin secretion in AN and OB, and similar cortisol profile changes in AN and BN), together with evidence of internal desynchronization among the different bioperiodic functions. These findings suggest that some changes of the central pathways involved in the control of eating, mood and endocrine functions are common to dissimilar kinds of eating disorders.

The lateral hypothalamic area revisited: ingestive behavior

This article discusses the role of the lateral hypothalamic area (LHA) in feeding and drinking and draws on data obtained from lesion and stimulation studies and neurochemical and electrophysiological manipulations of the area. The LHA is involved in catecholaminergic and serotonergic feeding systems and plays a role in circadian feeding, sex differences in feeding and spontaneous activity. This article discusses the LHA regarding dietary self-selection, responses to high-protein diets, amino acid imbalances, liquid and cafeteria diets, placentophagia, "stress eating," finickiness, diet texture, consistency and taste, aversion learning, olfaction and the effects of post-operative period manipulations by hormonal and other means. Glucose-sensitive neurons have been identified in the LHA and their manipulation by insulin and 2-deoxy-D-glucose is discussed. The effects on feeding of numerous transmitters, hormones and appetite depressants are described, as is the role of the LHA in salivation, lacrimation, gastric motility and secretion, and sensorimotor deficits. The LHA is also illuminated as regards temperature and feeding, circumventricular organs and thirst and electrolyte dynamics. A discussion of its role in the ischymetric hypothesis as an integrative Gestalt concept concludes the review.

Melatonin effects on behavior: possible mediation by the central GABAergic system

The best described function of the pineal hormone melatonin is to regulate seasonal reproduction, with its daily production and secretion varying throughout the seasons or the photoperiod. Additionally, a number of behavioral effects of the hormone have been found. This review describes the effects of melatonin in rodent behavior. We focus on: (a) inhibitory effects (sedation, hypnotic activity, pain perception threshold elevation, anti-convulsive activity, anti-anxiety effects); and (b) direct effects on circadian rhythmicity (entrainment, resynchronization, alleviation of jet-lag symptoms, phase-shifting). Most of these effects are clearly time-dependent, with a peak of melatonin activity during the night. One of the possible mechanisms of action for melatonin in the brain is the interaction with the GABAergic system, as suggested by neurochemical and behavioral data. Finally, some pineal hormone effects might be candidates as putative therapies for several human disorders.

Differential effects of intrahypothalamic administration of opioids on food intake in naive and tolerant rats

We investigated the effects of intrahypothalamic administrations of the opioid agonists morphine (MOR) and ketocyclazocine (KCZ) and antagonists naltrexone (NALTX) and Mr2266 on food intake (FI) during light and dark phases of the diurnal cycle, after acute or chronic administration in rats. Acute intralateral hypothalamic (LH) administration of MOR or KCZ (1 microgram/rat) enhanced FI during dark and light phases, respectively, whereas intraventromedial hypothalamic (VMH) injections resulted in moderate hyperphagia during dark phases by both mu and kappa agonists. The receptor specificity was evident from blockade of the responses to MOR or KCZ by the respective antagonists NALTX and Mr2266. After repeated administrations of MOR and KCZ, FI responses to the test dose of these agonists injected in LH were modulated in opposite directions. However, the adaptative changes in FI after intra-VMH injection of KCZ were similar to those seen with MOR. These results are discussed in light of a differential opioid receptor involvement and their possible functional interactions within the hypothalamus during food intake.

Diurnal variations in vision and relations to circadian melatonin secretion in multiple sclerosis

It has long been recognized that symptoms of multiple sclerosis (MS) wax and wane with fluctuations occurring on an hour to-hour basis throughout a 24 hour period. It has been proposed that changes in circadian core body temperature, which alter axonal conductivity, may account for the fluctuations of symptoms in MS. A 51-year-old man with MS is reported in whom visual acuity deteriorated throughout the course of the day only to improve again at night between 10:00 p.m. and 2:00 a.m. These changes in vision were unrelated to rest or physical activity, but appeared to coincide with the circadian secretion of melatonin which is coupled to the circadian temperature rhythms. Since melatonin lowers body temperature, it is hypothesized that the nocturnal rise in melatonin secretion was related to improvement in vision in this patient. This hypothesis is supported by the observation that administration of melatonin (3 mg, orally) at 2:00 p.m., when the patient experienced severe blurring of vision, resulted within 15 minutes in a dramatic improvement in visual acuity and in normalization of the visual evoked potential latency after stimulation of the left eye. Moreover, since the pineal gland is a thermoregulatory organ which functions to prevent excessive rise of body temperature, it is possible that since MS is associated with dysfunction of the pineal gland, these patients may experience diminished capacity to eliminate heat at rest or during physical activity with resultant elevation of body temperatures which may further compromise neurologic functions by causing failure of axonal conduction.

Long term beneficial effects of weak electromagnetic fields in multiple sclerosis

A 39 year-old severely disabled woman with a 19 year history of chronic relapsing-remitting multiple sclerosis (MS) began to experience improvement in symptoms within 24 hours after she received experimental treatment with picotesla electromagnetic fields (EMFs). Pattern reversal visual evoked potential (VEP) study obtained three weeks after the initiation of the first magnetic treatment showed a return to normal of the P100 latencies in each eye. The patient continued to receive 1-2 EMFs treatments per week and during the following 32 months she made a dramatic recovery with resolution of diplopia, blurring of vision, dysarthria, ataxia of gait, and bladder dysfunction as well as improvement in fatigue, heat tolerance, mood, sleep, libido, and cognitive functions. VEP studies, which were repeated in April of 1995 more than 2 1/2 years after the initiation of magnetic treatment, showed that P100 latencies remained normal in each eye providing objective documentation that continued application of these EMFs may sustain normal conduction in the damaged optic pathways over a long period of time. This is the first case report documenting the dramatic long term beneficial effects of treatment with picotesla range EMFs in a patient with MS.

Resolution of Lhermitte's sign in multiple sclerosis by treatment with weak electromagnetic fields

Lhermitte's sign, the occurrence of an electrical sensation passing down the back to the legs on flexion of the neck is a common and characteristic feature of multiple sclerosis (MS) which is related to spinal cord lesions affecting the posterior columns and cervical nerve roots. The Lhermitte's sign, which has been reported to occur at some time in up to 25% of MS patients, is seldom painful but is often a cause of distress to the patient and usually a marker of increased disease activity. Treatment with extracranial picotesla range pulsed electromagnetic fields (EMFs) has been found efficacious in the management of various MS symptoms including pain syndromes. The present communication concerns three MS patients in whom two brief applications of EMFs resulted in resolution of the Lhermitte's sign which emerged during a period of exacerbation of symptoms in one patient and during a prolonged phase of symptom deterioration in the other two patients. As the cause of the Lhermitte's sign is thought to result from the spread of ectopic excitation in demyelinated plaques in the cervical and thoracic regions of the spinal cord, it is hypothesized that the effects of EMFs are related to the reduction of axonal excitability via a mechanism involving changes in ionic membrane permeability. A systemic effect on pain control systems is also postulated to occur secondary to the effects of EMFs on neurotransmitter activity and pineal melatonin functions. This report underscores the efficacy of picotesla EMFs in the management of paroxysmal pain symptoms in MS.

The effects of melatonin on isolation distress in chickens

Melatonin (0.1-1.0 mg/kg) reduced isolation-induced distress vocalizations (DVs) in young domestic chickens in a dose-dependent manner. This effect was unaffected by the administration of d-amphetamine (1.0 mg/kg) suggesting that melatonin's effects were not merely due to fatigue. The melatonin reduction in DVs was not naloxone reversible, indicating an action independent of the endogenous opioid system. However, chronic pretreatment with naltrexone facilitated the melatonin effect, suggesting a complex relationship between melatonin and the endogenous opioids in regulating distress vocalizations. Chickens exhibited a marked reduction in DVs when isolation chambers were darkened, suggesting endogenous, as well as exogenous, melatonin mediation of isolation distress; however; pinealectomy only partially reversed the darkness effect. Pinealectomized animals, like control animals, exhibited a reduction in DVs following melatonin treatment; however, the melatonin effect was shorter lasting. The implications that these results may have for socialization and emotional distress are discussed.

Melatonin replacement nullifies the effect of light-induced functional pinealectomy on nociceptive rhythm in the rat

Rats maintained on a 12 h daily photoperiod (12:12 LD cycle), exhibited a diurnal variation in sensitivity to both heat-elicited and pressure-elicited pain, with low sensitivity at 2 h before the end of the scotophase and higher at 4 h after the onset of photophase. Functional pinealectomy induced by a single LL day effaced the baseline diurnal rhythm of sensitivity to pressure-elicited pain, and reversed that to heat-elicited pain. Oral administration of physiological doses of melatonin into functionally pinealectomized rats, nullified the effect of functional pinealectomy, restoring the normal baseline rhythms of both pressure-elicited and heat-elicited nociceptive responses. The role of melatonin in modulating nociception is discussed in light of an indoleaminergic-opioid system.

Endogenous opioids and sexual motivation and performance during the light phase of the diurnal cycle

The sexual motivation and performance of sexually experienced male rats were tested during the light phase of the diurnal cycle after treatment with saline or 1 mg.kg-1 naloxone in a bilevel testing box. The sexual motivation during the light phase, as assessed by the increase in anticipatory level changes prior to introduction of a receptive female on subsequent weekly sessions, was comparable to that during the dark phase. Opioid blockade reduced the increase of level changes, suggesting that endogenous opioids are involved in sexual motivation. The sexual performance was impaired during the light phase. Naloxone treatment failed to affect the sexual performance, other than that the post ejaculatory refractory period was increased. This increased latency to re-initiate copulation may be an expression of the reduced sexual motivation. It is concluded that endogenous opioids are not involved in the regulation of the impaired sexual performance during the light phase of the diurnal cycle. In contrast, the sexual motivation, which displays no marked diurnal variation, may be stimulated by endogenous opioids.

Melatonin counteracts pinealectomy-dependent decreases in rat brain [3H]flunitrazepam binding through an opioid mechanism

The effect of intracerebroventricular (i.c.v.) injection of melatonin and/or beta-endorphin on the [3H]flunitrazepam binding sites in the cerebral cortex of pinealectomized or superior cervical ganglionectomized rats was studied. Pinealectomy decreased the maximum concentration of benzodiazepine receptors (Bmax) without affecting the dissociation constant (KD), while melatonin, ineffective in control animals, counteracted the effect of pinealectomy. Intracerebroventricular injection of beta-endorphin increases Bmax in both control and pinealectomized animals, the effect being significantly higher in the latter. Simultaneous i.c.v. injection of melatonin + beta-endorphin did not further increase Bmax in any group, whereas i.c.v. injection of naloxone significantly blocked the effects of melatonin and/or beta-endorphin administration. Pineal sympathetic denervation produced a significant increase in Bmax and KD, whereas i.c.v. injection of melatonin further increased the former, restoring KD to control values. Neither i.c.v. administration of beta-endorphin or melatonin + beta-endorphin significantly modified the ganglionectomy-dependent increase in Bmax, although both treatments restored KD to control values. Naloxone administration had no effect on beta-endorphin- and melatonin + beta-endorphin-treated ganglionectomized groups, but counteracted the increased effect of melatonin on Bmax in ganglionectomized animals.

Age-related changes in the contents of neuropeptides in the rat brain and pituitary

beta-Endorphin, Leu-enkephalin, Met-enkephalin, substance P, somatostatin, and cholecystokinin were measured in the brain and the pituitary of male Sprague-Dawley rats aged 3 months, 12 months, and 22 months. beta-Endorphin, Met-enkephalin and Leu-enkephalin contents in the neurointermediate lobe, and the enkephalin levels in the anterior lobe of the pituitary increased with age. The increases in contents were both in the day and at night for beta-endorphin and Met-enkephalin. However, the increase for Leu-enkephalin content was in the day only. Hypothalamic beta-endorphin content decreased with age only in the day. beta-Endorphin and Leu-enkephalin contents in the brain stem, and Leu-enkephalin levels contents in the cortex decreased with age at night. Leu-enkephalin in the striatum decreased with age in the day. There was also an age-related decrease for somatostatin and substance P contents in the striatum and the hypothalamus in the day, and in cholecystokinin levels in the hippocampus, and the hypothalamus at night. It is concluded that there are age differences in neuropeptide levels, and that these changes may differ according to diurnal rhythms.

The immunoneuroendocrine role of melatonin

A tight, physiological link between the pineal gland and the immune system is emerging from a series of experimental studies. This link might reflect the evolutionary connection between self-recognition and reproduction. Pinealectomy or other experimental methods which inhibit melatonin synthesis and secretion induce a state of immunodepression which is counteracted by melatonin. In general, melatonin seems to have an immunoenhancing effect that is particularly apparent in immunodepressive states. The negative effect of acute stress or immunosuppressive pharmacological treatments on various immune parameters are counteracted by melatonin. It seems important to note that one of the main targets of melatonin is the thymus, i.e., the central organ of the immune system. The clinical use of melatonin as an immunotherapeutic agent seems promising in primary and secondary immunodeficiencies as well as in cancer immunotherapy. The immunoenhancing action of melatonin seems to be mediated by T-helper cell-derived opioid peptides as well as by lymphokines and, perhaps, by pituitary hormones. Melatonin-induced-immuno-opioids (MIIO) and lymphokines imply the presence of specific binding sites or melatonin receptors on cells of the immune system. On the other hand, lymphokines such as gamma-interferon and interleukin-2 as well as thymic hormones can modulate the synthesis of melatonin in the pineal gland. The pineal gland might thus be viewed as the crux of a sophisticated immunoneuroendocrine network which functions as an unconscious, diffuse sensory organ.

Methionine enkephalin-like, substance P-like, and beta-endorphin-like immunoreactivity in human parotid saliva

These three neuropeptides were measured at daily baseline values by radioimmunoassay. Stimulated parotid saliva was collected from 31 subjects using a modified Carlson-Crittenden device affixed over Stenson's duct. Methionine enkephalin-like immunoreactivity ranged from 6.6 to 11.7 fmol/ml, with a mean of 9.3 fmol/ml. Substance P-like immunoreactivity ranged from 6.1 to 12.6 fmol/ml, with a mean of 9.3 fmol/ml. beta-Endorphin-like immunoreactivity ranged from 1.2 to 3.6 fmol/ml, with a mean of 2.6 fmol/ml. This is believed to be the first documentation of methionine enkephalin- and substance P-like activities in human parotid saliva and the first demonstration of beta-endorphin-like activity in any type of human saliva. Substance P-like activity was significantly higher in morning than evening samples; beta-endorphin-like activity also tended to be higher in the morning samples. Substance P and beta-endorphin-like immunoreactivities covaried in a significant positive manner, suggesting either common control mechanisms or similar responses to physiological variables.

Melatonin and petit-mal epilepsy: an hypothesis

Intraventricular administration of the opioid peptide leucine-enkephalin has been reported to induce petit-mal-like seizures in rats. These seizures have been found to be an age-dependent phenomenon. In rats, the full manifestation of these seizures develops after 4 weeks of age during which time ethosuximide was effective in aborting these seizures, while phenytoin and phenobarbital were ineffective. The period associated with the development of enkephalin-induced seizures in rats coincides with an important milestone in pineal chronobiology. In rats, melatonin plasma levels peak at 3 weeks of age, a period which also corresponds with the emergence of melatonin circadian rhythms. It is proposed that melatonin mediates the anticonvulsant action of drugs effective for petit-mal (absence) epilepsy and that the pineal gland is implicated in the pathogenesis of this form of childhood epilepsy.

Effect of the pineal gland on circadian rhythmicity of colony forming units for granulocytes and macrophages (CFU-GM) from rat bone marrow cell cultures

The present study provides evidence that the pineal gland has a physiological role in the proliferation of colony forming units for granulocytes and macrophages (CFU-GM). A biphasic circadian rhythm of CFU-GM proliferation in rat bone marrow cell cultures (BMC) from intact animals peaking at 0600 and 1800 was observed. Pinealectomy (Px) at 1600 obliterated the circadian rhythm patterns of CFU-GM. Afternoon injections of melatonin (1630, 20 micrograms/per day for 10 days) to Px animals restored the rhythmicity. When pinealectomy was done at 0800, the morning peak remained unaltered and the colony number at 1800 was higher than that found in the afternoon Px animals. In conclusion, the pineal gland or its main hormone melatonin seems to have a regulatory role in the proliferation of CFU-GM in rat BMC. Further, the expression of the activity of CFU-GM in rat BMC depends on the time when pinealectomy is done or melatonin is substituted.

Role of diurnal variation and receptor specificity in the opioidergic regulation of food intake in free-fed and food-deprived rats

The effects of opioid agonists, morphine (MOR) and ketocyclazocine (KCZ), and antagonists, naltrexone (NALTX) and Mr2266, were investigated on food intake under various conditions, i.e., during light and dark phases of diurnal cycle and free-fed and fasting states in rats. NALTX showed a greater anorexic effect during dark phase, whereas Mr2266 produced such effect during light phase. This suggests that mu-receptors play a major role during dark phase while kappa-receptors are more important in light phase. The comparison of effects of different opioidergic drugs in fasted and free-fed rats showed that NALTX and Mr2266 reduced the elevated basal food intake in 18-h fasted rats to free-fed control levels. Therefore, it appears that enhanced endogenous mu- and kappa-directed neural mechanisms are one of the factors responsible for enhancing food intake in fasted rats. Differential role of MOR and KCZ on food intake in free-fed and fasted rats is also indicated in our study. Both agonists produced a biphasic response in fasted rats, i.e., hyperphagia (0-1 h) followed by hypophagia (1-6 h). However, a generalized hyperphagic effect is observed in free-fed rats (except during 3-6 h by MOR). The initial hyperphagic effect is more prominent in fasted rats which may be due to additive effects of endopioid mechanisms. Specificity of the response at various intervals is confirmed by blockade with NALTX and Mr2266. NALTX appears more potent than Mr2266 in antagonising the effects of MOR but markedly less potent than Mr2266 in inhibiting the effects of KCZ. This suggests that both MOR and KCZ have a mu as well as kappa component in food intake response.

Distribution of substance P, GnRH, Met-enkephalin in the central nervous system of the pig

The distribution of substance P (SP), gonadotropin releasing hormone (GnRH) and Met-enkephalin in the brain and spinal cord of the domestic pig is described for the first time. The levels of SP, GnRH and Met-enkephalin were measured by specific radioimmunoassays in various regions of the brain and spinal cord of the pig. Substance P and Met-enkephalin are widely distributed within the central nervous system of the pig. High levels of SP were found in the preoptic area (POA), suprachiasmatic area (SCA), medial basal hypothalamus (MBH) and brain stem while moderate amounts of SP were found in olfactory bulb (OB). High levels of Met-enkephalin were found in POA, SCA and MBH, and moderate levels of Met-enkephalin in OB and brain stem. Both SP and Met-enkephalin levels were higher in the dorsal spinal cord in comparison with the levels of these peptides in the ventral spinal cord. This finding is in agreement with the predominant role played by these neural systems in primary afferent mediation of nociceptive impulses. The POA and SCA contained only low levels of GnRH while the MBH contained high levels of GnRH. Finally, some differences in the quantitative distribution of these peptides in the pig and rat are discussed.

Time-dependent melatonin analgesia in mice: inhibition by opiate or benzodiazepine antagonism

The aim of this study was to determine whether melatonin-induced analgesia in mice exhibits the time dependency known to occur for several other effects of the hormone, and to analyze to what extent the activity of melatonin can be inhibited by the opiate antagonist naloxone or the central-type benzodiazepine (BZP) antagonist Ro 15-1788. Analgesia was assessed with the hot plate procedure. There was a significant diurnal variation in the pain threshold, with an increase in latency during the dark phase of the daily photo period. Melatonin (20-40 mg/kg i.p.) exhibited maximal analgesic effects at late evening (20:00 h). The administration of naloxone or Ro 15-1788 at 20:00 h, although unable by themselves to modify pain threshold, blunted the analgesic response to melatonin. Significant increases in the latency of the hot plate response were found after diazepam injection, an effect blocked by Ro 15-1788 or naloxone. These results indicate that time-dependent melatonin analgesia is sensitive to opioid or central-type BZP antagonism.

Effect of early exposure to delta-9-tetrahydrocannabinol on the levels of opioid peptides, gonadotropin-releasing hormone and substance P in the adult male rat brain

The effects of neonatal exposure to delta-9-tetrahydrocannabinol (THC) on the adult animal brain neurochemistry and pain perception were evaluated. Newborn rat pups were culled to a litter size of 8 (males and females) and treated either with THC (2 mg/kg) or oil (control) daily, during days 1-4 after birth. After weaning, the THC-treated males were housed 4 per cage. During the juvenile period (day 50), the THC-treated animals exhibited significantly lower baseline tail-flick values (a measure of pain perception) than the control. However, as adults, the THC-treated animals exhibited significantly higher sensitivity to pain following 5 mg/kg morphine challenge. Furthermore, the THC-treated animals had significantly elevated beta-endorphin and methionine-enkephalin levels in almost all the brain areas sampled for the study. In addition, the neonatally THC-treated rats exhibited significantly higher levels of substance P (SP) and significantly lower levels of gonadotropin releasing hormone (GnRH) in the anterior hypothalamus-preoptic area. The SP and GnRH levels did not differ among the THC-treated and control animals in the medial basal hypothalamus. The results of this study indicate that even a very low dose of THC administered during the neonatal period has a long-lasting effect on the brain neurochemistry. In particular, neonatal administration of THC appears to alter functioning of the endogenous opioid system.

Day-night rhythms of shoaling behavior in goldfish: opioid and pineal involvement

Intracerebroventricular (ICV) administration of the opioid peptide, beta-endorphin (5.0 pg/g) to goldfish, Carassius auratus, significantly increased the cohesiveness and duration of shoaling ('bout' length) in shoals of five fish, as well as decreasing the latency of shoal formation in response to an external disturbance, while a higher dose of beta-endorphin (15 pg/g) decreased shoaling. There were day-night rhythms in shoaling and in the extent of the facilitatory effects of beta-endorphin (5.0 pg/g) on shoaling behavior, the fish displaying significantly greater shoaling responses in the day than during the night. The facilitatory effects of the low dose of beta-endorphin were blocked by systemic administration of naloxone (1.0 mg/kg), while ICV administrations of naloxone (1.0 pg/g) decreased daytime shoaling behavior. Removal of the pineal gland disrupted the day-night rhythm of shoaling, reducing daytime levels of shoaling. In addition, pinealectomy reduced the stimulatory effects of beta-endorphin (5.0 pg/g) on shoaling, and attenuated the day-night rhythms in the effects on beta-endorphin on shoaling. These results suggest that both opioid systems and the pineal gland influence shoaling behavior and the expression of its day-night rhythm in goldfish.

Pineal melatonin and sensory symptoms in Parkinson disease

Sensory symptoms have been reported in 40-60% of patients with Parkinson's disease, and in at least 10% of patients these symptoms precede the onset of the motor disorder. The pathophysiology of these symptoms remains unknown. Diminished brain serotonin concentration has been reported to be associated with sensory symptoms. Serotonin metabolism is regulated by pineal melatonin. The secretory activity of the pineal gland may be diminished in Parkinson's disease. In experimental animals pineal melatonin has been shown to exert analgesic effects by interacting with opiate receptors. In addition, since opioid peptides mediate the analgesic effects of melatonin, decreased opioid peptide functions in Parkinson's disease may be associated with disruption of the "fine-tuning" pain modulatory functions of melatonin and possibly indirectly facilitate the emergence of sensory symptoms.

Ethanol ingestive behavior as a function of central neurotransmission

Uncontrollable alcohol ingestive behavior has been linked to deficits of central neurotransmission. The pineal gland plays an important role in modulating ethanol intake in numerous animal species. The opioidergic (i.e. beta-endorphin, enkephalin, and dynorphin) system is involved in both the actions of alcohol and opiates, as well as craving and/or genetic predisposition towards abuse of these two agents. Furthermore, there is significant evidence to link ingestive behaviors with the ventral tegmental accumbens-hypothalamic axis, whereby the biogenic amines dopamine and serotonin are reciprocally involved. Evidence is presented which implicates the striatum and the hypothalamus as possible specific loci for regional differences between alcohol-preferring and alcohol-nonpreferring mice. We believe that photoperiod-induced alcohol ingestive behavior may involve alterations in both pineal and hypothalamic opioid peptides.

A study on the relationship between the pineal gland and the opioid system in patients with cancer. Preliminary considerations

Recent studies showed that both the pineal gland and the endogenous opioid system are involved in the modulation of the immune system and in the regulation of tumor growth. Moreover, a relationship between pineal and opioid system has been demonstrated. In order get an overall view of the psychoneuroendocrine interactions in cancer patients, the levels of melatonin, the most important pineal hormone, and of beta-endorphin have been measured on blood samples collected during the morning. The study was carried out on 54 patients, 42 healthy subjects, and in 34 patients having illnesses other than cancer. Breast cancer, lung carcinoma, and colorectum cancer were the three neoplasms detected in the patients investigated. Growth hormone (GH), somatomedin-C and prolactin (PRL) levels were also determined. beta-endorphin levels were found to be substantially within the normal range in patients with cancer, whereas those of melatonin were raised in several cases. The beta-endorphin/melatonin ratio was higher than 2 in normal subjects, in non-neoplastic patients and in most cancer patients without metastases, whereas this ratio was lower than 2 in almost all patients in a metastatic stage of the disease. Neither melatonin levels nor those of beta-endorphin appeared to be significantly correlated with GH, somatomedin-C, and PRL concentrations. The low beta-endorphin/melatonin ratio observed in metastatic patients suggests the presence of an unbalanced relation between the pineal and the opioid system in those subjects. Therefore, an anomalous relationship between pineal function and opioid activity might play a role in the clinical course of neoplastic disease.

CSF-endorphines in acute and chronic brain lesions

In order to answer the question of an opioid influence on consciousness, a radio-immuno-assay (n = 852) of beta-endorphin and beta-LPH (beta-lipotropic hormone) in both ventricular CSF and blood plasma was carried out in 101 neurosurgical patients. The following results were obtained: I) beta-END and beta-LPH levels were found to be lower in the CSF than in blood plasma. II) beta-END and beta-LPH in the CSF was the same in both sexes. III) beta-END levels in the CSF decreased with age. IV) beta-END and beta-LPH levels showed a diurnal rhythm with a maximum in the late a. m. hours. V) beta-END levels in the ventricular CSF tend to decrease parallel to a drop in conciousness as well as with longlasting comatous states. VI) beta-END in ventricular CSF becomes higher with increasing systolic arterial blood pressure. VII) beta-END and beta-LPH levels in ventricular CSF are not correlated with the type of the disease, CSF pressure, body temperature or respiratory changes.

Immunohistochemical evidence for the presence of peptides derived from proenkephalin, prodynorphin and proopiomelanocortin in the guinea pig pineal gland

By using a plethora of region-specific antisera, this light microscopic immunohistochemical study revealed that derivatives from the three opioid precursors, i.e. proenkephalin, prodynorphin and proopiomelanocortin are differentially distributed in the pineal gland of guinea pig. Various molecular forms of immunoreactive opioid peptides derived from proenkephalin or prodynorphin were present in a minority of pinealocytes as well as in nerves. In contrast to this dual distribution pattern of opioid-active peptides, the opioid-inactive derivative from proopiomelanocortin, alpha-melanocyte stimulating hormone, was exclusively present in a large proportion of pinealocytes. A multiple and differential origin and function of opioidergic pineal innervation involving sympathetic, parasympathetic and sensory components is suggested. alpha-MSH is proposed as a pineal hormone which may act in concert with melatonin to regulate pineal rhythms or may function like MSH of pituitary origin.

The pineal gland-opioid system relation: melatonin-naloxone interactions in regulating GH and LH releases in man

Several observations have demonstrated that the opioid system can modulate melatonin secretion from the pineal gland and that the effects of opioids may require a pineal participation. In contrast, the role played by the pineal gland in regulating the synthesis and secretion of endorphins and enkephalins in still obscure. To establish whether the neuroendocrine activity of melatonin are mediated by endogenous opioids and to demonstrate a possible action exerted by the pineal gland on opioid peptides, GH and LH serum mean levels were evaluated by RIA in 12 healthy subjects of both sexes after melatonin injection alone (0.4 mg/kg bw im at 09:00 h) and on a separate occasion after a simultaneous administration of melatonin and naloxone (1.2 mg iv as a bolus, followed by an iv infusion of 1.6 mg/h for 3h). On an other occasion, the study was performed during saline or naloxone infusion alone. In each test, venous blood samples were collected at -20, 0, 30, 60, 90, 120 and 180 min. A significant rise of GH was observed after melatonin injection alone. The simultaneous infusion of naloxone blocked melatonin-induced GH rise. Melatonin did not affected LH serum levels, while it was able to reduce LH increase induced by naloxone. These preliminary results suggest that some neuroendocrine effects of melatonin might be mediated by a modulation on the opioid tone.

Melatonin: a principal neuroimmunoregulatory and anti-stress hormone: its anti-aging effects

Major environmental variables such as daily and seasonal changes of light and temperature regulate the daily circadian variations of synthesis and release of the pineal neurohormone N-acetyl-5-methoxytryptamine (melatonin). Melatonin has now been shown to be a potent immunoregulatory agent, and to be able to antagonize the immunosuppressive effects of acute anxiety stress in mice, as measured by antibody production, by thymus weight, and by the capacity of stressed- and evening-melatonin-treated mice to react against a lethal virus. Both psychogenic factors and infectious agents such as viruses can act as "stressors" and induce an immunosuppression. Their combination is a determinant for the course of infectious diseases and, perhaps, cancer. Circadian (evening) melatonin possesses thus the singular ability to up-regulate the immunosuppression of stressed mice. This effect of melatonin is not exerted directly on immunocompetent cells, but mediated via the endogenous opioid system upon antigen-activation of T cells. Melatonin being a short-lived hormone with negligible side-effects which is rapidly degraded and eliminated by the body, the use of melatonin offers a new approach to the physiological control of stress and stress-related infectious diseases. In addition, melatonin could be used for the potentiation of primary immunization (vaccination) against antigens of the most varied nature which do not evoke a robust or longlasting secondary (memory) response. The regulatory function of pineal melatonin is discussed also in relation to hematopoiesis, to its oncostatic effects, and to its possible dual role in reproduction physiology and generation of immunocompetence and tolerance during ontogeny.

Nicotine-induced alterations in brain regional concentrations of native and cryptic Met- and Leu-enkephalin

The distribution of cryptic forms (larger enkephalin-containing peptides) in neostriatum, hypothalamus, spinal cord T3-L1 and neurointermediate lobe of pituitary were determined by radioimmunoassay. Optimal conditions for enzymic hydrolysis of the cryptic enkephalins by trypsin and carboxypeptidase B were established. The proportion of total Met- and Leu-enkephalin represented by native pentapeptide varied markedly among these central nervous system regions. Also, the distributions of native and cryptic Met-enkephalin were distinct from that of Leu-enkephalin. Chromatographic separation by HPLC of immunoreactive Met-enkephalin peptides revealed only two peaks corresponding to Met-enkephalin and Met-enkephalin sulfoxide in rather equal amounts. Hydrolysis of cryptic Met-enkephalin also produced only two HPLC-separable peaks of immunoreactive Met-enkephalin, again corresponding to Met-enkephalin and Met-enkephalin sulfoxide. Bioactivity of cryptic striatal Met-enkephalin after hydrolysis was demonstrated by antinociception and catalepsy in rats following its intracerebroventricular injection. Repeated short-term administration of nicotine, 0.1 mg/kg IP six times at 30 min intervals, produced significant increases in native and cryptic Met-enkephalin in striatum, consistent with an increase in neuronal release of Met-enkephalin together with increases in synthesis and processing of proenkephalin A in this brain region. This regimen of nicotine also decreased levels of native Met-enkephalin and of both native and cryptic Leu-enkephalin in neurointermediate lobe, consistent with nicotine-induced release of both proenkephalin A- and prodynorphin-derived peptides from neurointermediate lobe.

Origin of the stimulation of food intake by oral administration of enkephalinase inhibitors in sheep

The influence of two enkephalinase inhibitors (thiorphan and acetorphan) orally, parenterally and centrally administered on food intake was tested in hay-fed ewes. When orally administered at a dose of 1 mg/kg, acetorphan, but not thiorphan, produced a biphasic increase in food intake corresponding to a 17.0% increase of daily food intake. Similarly thiorphan (0.1 mg X kg-1) IV administered increased by 19.3% the daily food intake; in contrast acetorphan IV administered produced a early (0-2 h) decrease followed by a late increase in hay consumption without significant (P greater than 0.05) change in the daily food intake. When ICV administered (10 micrograms X kg-1) thiorphan but not acetorphan at the same dose depressed the early (0-2 h) and daily food intake by 43.2% and 25.4% respectively. Pretreatment with naltrexone (0.1 mg X kg-1 IV) blocked the increased food intake induced by oral acetorphan or IV acetorphan and thiorphan but did not affect the anorectic effects of ICV thiorphan. We conclude that enkephalinase inhibitors like thiorphan and acetorphan increase daily food intake in sheep probably by increasing enkephalin levels in peripheral tissues.

Circadian rhythm of feeding induced changes in hypothalamic Met-enkephalin concentrations

Accumulating evidence suggests that opioid peptides play an important role in the hunger component of the control of food intake. The enkephalins, one of the opioid peptide families, stimulate feeding when injected into specific hypothalamic areas and endogenous concentrations change with the fed/fasted condition of rats and sheep and with phase of circadian cycle. To demonstrate a possible circadian rhythm in feeding-induced changes in Met-enkephalin (MEK), 54 male rats initially weighing 255 +/- 3 g were adapted to a 12-hr fast during the light (light-fasted) or dark (dark-fasted) phase of the circadian cycle, then sacrificed before (non-fed) or after (fed) being allowed to eat a meal. In non-fed compared with fed rats, MEK concentrations were higher in the paraventricular nucleus (PVN, 170 vs. 109 pg/mg tissue, p less than 0.05) and ventromedial hypothalamus (VMH, 209 vs. 161 pg/mg tissue, p less than 0.05) in the dark (light-fasted) but not light (dark-fasted), even though rats ate a larger meal in the light (8.6 vs. 5.0 g, p less than 0.01). In rats fed the same amount of food in the light (dark-fasted) as ad lib fed rats in the dark (light-fasted), MEK concentrations did not differ in the PVN or VMH, suggesting that circadian rhythm is more important than meal size. Rats gavaged with an amount of milk equal in calories to dark ad lib-fed rats (light-fasted) had MEK concentrations not different from light-fasted non-fed rats (216 vs. 209 pg/mg tissue, NS) suggesting that feeding behavior, pregastric stimuli and/or form of diet is important for influencing MEK concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Diurnal rhythm of the in vivo release of enkephalin from the globus pallidus of the rat

The in vivo spontaneous release of enkephalin in the globus pallidus of the rat increases from noon to evening by 100%; during this period the local release of exogenous gamma-aminobutyric acid (GABA) decreases by 60%. These diurnal rhythms are more marked in the K+-stimulated release: enkephalin-induced output increases 6-fold while GABA decreases 10-fold during the afternoon and evening hours. Since pallidal enkephalin and GABA are involved in the control of locomotor activity we suggest that these rhythms may be linked to the circadian changes of activity in the rat.

Met-enkephalin-like immunoreactivity in neurointermediate pituitary is decreased by DA receptor stimulation

The effect of dopamine receptor stimulation by administration of the dopamine analogue bromocriptine on Met-enkephalin-LI was examined in rat hypothalamus, and neurointermediate and anterior lobes of pituitary. Bromocriptine treatment resulted in a dramatic decline of Met-enkephalin-LI in neurointermediate pituitary which was significant by 3 days of treatment. Maximal reduction of Met-enkephalin-LI ranged between 60-70% of pretreatment values and was maintained as long as bromocriptine was administered (4 weeks), with no evidence of desensitization or "escape." The effects of bromocriptine on neurointermediate lobe were of long duration and persisted for at least 4 days after discontinuation of treatment. No significant effects of bromocriptine were detected on Met-enkephalin-LI in hypothalamus or anterior pituitary. Whether these differences represent truly different regional regulation of Met-enkephalin-LI or whether the changes are more sensitively reflected in an area such as neurointermediate lobe that largely consists of nerve terminals, remains to be shown.

Changes in brain met-enkephalin concentrations with peripheral CCK injections in Zucker rats

There is increasing evidence that peptides in the brain are important in the control of food intake. Administration of opioid and CCK peptides have elicited hunger and satiety, respectively. To evaluate the interaction of these peptides and their role in the central nervous system, concentrations of met-enkephalin were measured in the hypothalamus of rats following peripheral administration of CCK; in addition, effects of feeding and fasting and obesity were studied. In CCK- vs. saline-injected rats met-enkephalin concentrations were decreased in the paraventricular nucleus (PVN), suprachiasmatic nucleus (SC), supraoptic nucleus (SON), dorsomedial hypothalamus (DMH) and ventromedial hypothalamus (VMH). In fed compared with fasted rats met-enkephalin concentrations were higher in the anterior hypothalamus (AH) and lower in the SC; in obese compared with lean rats, concentrations were higher in the AH, PVN, SC, SON, DMH, lateral hypothalamus and VMH. These results show that peripheral injections of CCK can decrease concentrations of met-enkephalin in the brain and suggest a mechanism by which these peptides may interact to influence behavior. In addition, the findings support the hypothesis that the hyperphagia which is typical of obese rats may be due to increased concentrations of met-enkephalin.

Nocturnal feeding in the mouse--opiate and pineal influences

Mice displayed daily rhythms in their basal and morphine-induced food intake, consuming significantly greater amounts of food at night. Non-invasive inhibition of the activity of the pineal gland by either exposure to a bright pulse of light or treatment with the L-amino-acid decarboxylase inhibitor, benserazide, reduced the elevated night-time food intakes. These effects on feeding were most evident on the first night the activity of the pineal was reduced. On subsequent nights light pulses had a diminished effect on basal and morphine-induced food intake. These results suggest that although the enhanced nocturnal food intake of mice may be modulated by pineal and opioid sensitive mechanisms, pineal activity is not essential for the expression of opioid-mediated feeding.