Enkephalin may mediate euphoria and drive-reduction reward

Addictive agents and intracranial stimulation (ICS): novel antagonists and agonists of morphine and pressing for ICS

Rats fixed with chronically indwelling bipolar electrodes pressed for intracranial stimulation (ICS) of the lateral hypothalamus during daily sessions. The effects of two antagonists of morphine (Win 44,441 and naloxone) were then assessed. Naloxone (10 mg/kg) produced its characteristic reduction in pressing. Win 44, 441 produced a reliable increase in pressing at doses as small as 1 mg/kg. Large doses of morphine (10 mg/kg) produced its characteristic effects: depression in pressing when given 1 hr before the test session and facilitation when given 3 hr before the test session. Win 44,441 antagonized morphine's depressive effects. Other compounds (Win 44,156, Win 42,156), having similar structure to Win 44,441 but having agonist and mixed agonist-antagonist activity with respect to analgesia, also facilitated pressing for ICS. All three compounds' effects on pressing for ICS were antagonized by naloxone. It is inferred that opioids' facilitatory effects on pressing for ICS are separable from opioids' other capabilities such as production of analgesia.

Deltakephalin, Tyr-D-Thr-Gly-Phe-Leu-Thr: a new highly potent and fully specific agonist for opiate delta-receptors

Deltakephalin, Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET) was rationally designed as pure delta-probe from proposed models of mu and delta opiate receptors. On peripheral organs, deltakephalin displays a 3000 times higher inhibitory potency on the electrically stimulated mouse vas deferens (IC50 = 0.15 nM) as on the guinea pig ileum (IC50 = 460 nM). As expected [3H]deltakephalin interacts at 35 degrees C in rat brain tissue to a single class of binding sites (delta) (Bmax = 0.115 pmole/mg protein) with a high affinity: KD = 1.35 nM from equilibrium measurements and KD = 0.43 nM from kinetic determinations. Deltakephalin occurs as the most specific ligand for delta-binding sites as shown by the following discrimination ratios KI(mu)/KI(delta): 0.31 for D-Ala2-D-Leu5-enkephalin; 0.15 for D-Ser2-Thr6-Leu-enkephalin and 0.05 for deltakephalin.

Minireview. Benzodiazepine-opiate antagonist interactions in relation to feeding and drinking behavior

Benzodiazepines reliably produce overconsumption of food and fluids. Opiate antagonists, naloxone and naltrexone, block the benzodiazepine-induced hyperphagia and hyperdipsia at low doses. Hence, activation of endogenous opioid mechanisms may be closely involved in the benzodiazepine facilitatory effects on ingestional behavior. Evidence is reviewed that opiate antagonists diminish feeding and drinking responses, and may enhance satiety processes in feeding and drinking, in addition to selectively diminishing the palatability of attractive foods and fluids. It is proposed that a single mechanism of action of the opiate antagonists would be sufficient to account for both effects on feeding and drinking. Biochemical data confirm that acute benzodiazepine treatment in vivo is associated with a naloxone-reversible release of striatal enkephalin. It is possible therefore that there is a close association between the behavioral and biochemical data, which both show that acute benzodiazepine effects are reversed by opiate antagonists. The implied relationship between benzodiazepine and endogenous opioid mechanisms may be relevant to the question of concurrent opiate-benzodiazepine abuse.

Discriminating between reward and performance: a critical review of intracranial self-stimulation methodology

Despite numerous pharmacological investigations of intracranial self-stimulation (ICSS), the substrates of this behavior have yet to be completely understood. In view of the likelihood that inadequate methodology has hindered the quest for these substrates, the present review was undertaken. Criteria for ICSS methodology should include not only the ability to discriminate reward from gross performance deficit, but also adequate capacity (ability to generate experimental data at a reasonable rate). For numerous reasons, bar-pressing on a continuous reinforcement schedule fails the first criterion despite its ease and rapidity. The use of partial reinforcement schedules may alleviate some of these shortcomings. Analysis of drug-induced response decrement patterns can discriminate gross motoric incapacity from other variables, although the question of subtle response maintenance deficits remains to be answered. Measurements of response rates using alternative operants do not differentiate reward and performance adequately. More promising, "rate-free" measures using locomotion as an operant include the two-platform method of Valenstein and the "locus of rise" method. Comparison of drug effects on ICSS with those on alternate tasks are fraught with pitfalls including the problems of assuring equivalent rates and patterns of responding. The use of differential electrode placements is ideally suited for neurochemically well-characterized drugs, particularly if "double dissociations" can be established during studies of multiple placements. Presentation of different current intensities or frequencies permits the compilation of rate-intensity functions, and drug-induced shifts in these functions have considerable analytical power. Self-regulation of current intensity constitutes a powerful tool that has yet to realize its full potential in the pharmacological study of ICSS. Extensive studies involving self-regulation of stimulation duration ("shuttlebox" studies) suggest that this method may be highly versatile despite several practical difficulties. It is concluded that at least six of these methods appear to do a reasonable job of excluding gross performance deficit. However, the possible influences of other factors, such as subtle response maintenance deficit, incentive or arousal, remain to be resolved in view of the multifactorial nature of ICSS. Multiple tests for ICSS drug or lesion studies are advocated whenever feasible, as no single test appears capable of resolving all theoretical complexities.

Naloxone attenuates self-abusive behavior in developmentally disabled clients

The opiate antagonist naloxone was effective in reducing self-abusive behavior in two mentally retarded clients with an extensive history of such behavior. Three doses of naloxone (0.1, 0.2, 0.4 mg) were compared with a vehicle solution in a double-blind, crossover design. Naloxone greatly attenuated self-abusive episodes in one client and eliminated them entirely in the second client. In addition, use of self-restraining behavior by one client was reduced. The findings suggested that some clients with self-injurious behavior may have disturbances of the endogenous opiate system. Maintenance of self-abuse by tonically elevated pain threshold and/or by the putative addictive characteristics of such behavior was discussed.

Naloxone modifies sugar-water intake in rats drinking with open gastric fistulas

The effects of the opiate antagonist naloxone (NX) on fluid preference and intake were determined in rats drinking with chronically indwelling gastric fistulas. The subjects were tested both after 22.5 hr fluid deprivation, and no deprivation, with open fistulas (sham drinking), as well as with closed fistulas. Following an injection of either saline or NX (0.5-10.0 mg/kg, administered SC), or no injection, the subjects were given the choice to drink water or 10% sucrose, in a two-bottle test, for 1 hr/day. With open fistulas, and following fluid deprivation, the animals sham drank both sucrose and water, but had a strong preference for sucrose. When not fluid deprived, the same animals sham drank sucrose almost exclusively. NX significantly reduced sucrose intake by the sham drinking animals, in both the deprived and not deprived conditions, but did not modify fluid preference. These data support the idea that NX modifies affective reactivity to palatable solutions, and that NX's antidipsogenic actions are not due to feedback from post-absorptional events.

Endorphins in the cerebrospinal fluid of psychiatric patients

In this paper we have reported the results of studies in psychiatric patient groups using the strategy of measuring opioid activity and beta-endorphin (ir) in CSF. Our findings do not lend support to the notion of excess endorphin activity in schizophrenia, but rather suggest the possibility of a decrease in endogenous opioid activity in some schizophrenic patients. In affectively ill patients our data suggest that there may be a relative change in endogenous opioid system activity across state change in manic-depressive illness. Who also found a relationship between nurses' ratings of anxiety and CSF opioid activity in depressed patients, although it is unknown whether this directly relates to the pathophysiology of this symptom, or is related to stress response. The relationship between CSF opioid activity and HPA axis activity, as reflected by urinary free cortisol excretion, supports the notion of important physiologic relationships between these systems and raises the issue of a role for the endogenous opioid system in the abnormal activation of this system in depression. Finally, the finding of increased CSF opioid activity in anorexia nervosa patients when a minimum weight coupled with data relating endogenous opioids to eating behavior raises interesting questions regarding a possible involvement of the endogenous opioid system involvement in this illness.

Intraventricular self-administration of leucine-enkephalin by laboratory rats

Naive laboratory rats, without pre-exposure to operant training procedures or to opioids, were shown to self-administer directly into their cerebral ventricles the endogenous opiate peptide, leucine-enkephalin. They were shown to self-administer the peptide consistently for six consecutive days with no indication of the development of tolerance. The results indicate that leucine-enkephalin may possess potent reinforcing properties and suggests that it may play a role as an endogenous reward transmitter.

The effect of methadone on the behavioral and neuroendocrine responses of manic patients

In a double-blind, crossover design, 10 mg of methadone or an inert placebo were administered intramuscularly to nine acutely manic patients in identical experimental sessions separated by 1 day. Standardized behavioral observations, self-ratings of mood and affect, and blood samples for prolactin and cortisol were obtained at baseline and 30, 60, and 120 minutes after the administration of methadone or placebo. Methadone decreased symptoms of euphoria, elation, and grandiosity in these manics. There was a significant rise in prolactin, peaking at 60 minutes, and a significant decrease in cortisol, maximal at 120 minutes following methadone administration. There appeared to be a correlational consistency between the subduing effects of methadone on behavior and its effect upon prolactin and cortisol secretion. These findings, together with data from previous studies of our own and others, suggest that a potentially important interrelationship may exist between the central endogenous opioid peptide systems, various neuroendocrine regulatory factors, and the pathophysiology of affective disorder.

Circulating opioids: possible physiological roles in central nervous function

Evidence is reviewed regarding the release of endorphins by such diverse conditions as stress, long distance running, acupuncture, sexual activity, suggestion and ritualistic dancing ceremonies. Additional evidence is cited regarding possible physiological roles of endorphins in antinociception, socialization, euphoria, some mental disorders, drive states and vegetative functions. The concentration of this latter type of evidence is on conditions during which endorphins seem to be exerting effects on a number of different systems together (for example, euphoria is almost always accompanied by analgesia), and the possibility is suggested that the activation of a number of functions together may be due to a global activation of opiate receptors throughout the CNS. A possible basis for this global activation arises from results from this laboratory indicating the presence of a blood-borne opioid hormone, secreted by the pituitary or by an endocrine gland under pituitary control, which is capable of passing from the blood into the CNS. This diffuse endorphinergic system, which is complementary to the well-established endorphinergic neuronal systems in the CNS, thus derives its property of global action on opiate receptors by the diffuse means by which the hormone reaches its target sites, i.e., by passing through the blood brain barrier. Thus, while each specific endorphin-mediated function can be activated by the activation of its respective neural pathway, it is proposed that the hormonal endorphinergic mechanism is activated to produce a global response provoked by conditions to which a more generalized response, including physiological and behavioural changes, is most appropriate.

Intracerebroventricular self-injection of morphine in response to pain in the rat

Rats were chronically implanted with a lateral cerebral ventricular guide cannula. After recovery they were trained to self-inject morphine sulfate. A week after this training period, self-injection behavior was studied during nociceptive stimulation. In response to a nociceptive stimulation rats increased their intracerebroventricular self-administration of morphine. This effect is specific for morphine since under identical conditions rats did not increase NaCl administration. Naloxone pretreatment inhibits self-administration of morphine in 5 rats. However, two rats increased morphine self-intake. These results show that rats can directly perceive a decrease of pain sensation induced by the morphine self-administration. This experimental situation is therefore similar to the therapeutic situation in humans, in which the criterion of efficacy of an analgesic is a reduction of the conscious pain sensation. Experimental self-administration during painful stimulation might therefore be used for preclinical evaluation of the efficacy of an analgesic drug.

Actions of drugs of abuse on brain reward systems: a reconsideration with specific attention to alcohol

Research in the areas of intracranial self-stimulation and drug self-administration has provided a substantial data base that has contributed to our understanding of brain reward mechanisms. In a recent article, Wise [83] argued that dopamine is the catecholamine critically involved in the central mediation of reward. The present paper attempts to examine the available data with particular reference to alcohol, but also with reference to opiates, and argues that the reinforcing effects of at least these drugs are primarily and directly mediated by noradrenergic rather than dopaminergic systems in the brain. It also argues, in direct contrast to Wise, that in the context of these drugs, dopamine seems to play a minor if not negligible role.

Reinforcing effects of brain microinjections of morphine revealed by conditioned place preference

The previously documented place conditioning paradigm was used to study the reinforcing effects of cerebral microinjections of morphine. Rats with implanted cannulae experienced place conditioning procedures, involving morphine administration into the IV (0.5 or 10 microgram), III (10 microgram) or lateral (0.5-10 microgram) ventricles. Positive reinforcement, indicated by a significant preference for the place paired with morphine compared to the place similarly paired with control treatment, was seen in rats given 10 microgram morphine into the lateral ventricle. The rats given 10 microgram into the III ventricle also showed a preference, but the effect was not statistically significant. Positive reinforcement was subsequently demonstrated with morphine microinjections (10 microgram) into the lateral hypothalamus, periaqueductal gray or nucleus accumbens. No clear preferences were produced by morphine injections into the caudate-putamen, amygdala or nucleus ambiguus. Following the final place conditioning test, rats were re-administered the treatment dose and analgesia and body temperature were measured. All three sites associated with reinforcement evidenced hyperthermia, but only the periaqueductal gray evidenced a short-latency analgesia. Sites with null place conditioning were not associated with any major behavioral effects. Using (+) and (-)-morphine (10 microgram), it was demonstrated that only the active (-)-stereoisomer was effective in producing place preferences after injection into the periaqueductal gray. It was concluded that morphine administered directly into parts of the rat brain can produce place conditioning similar to that seen after systematically administered morphine. Morphine-produced place preference is not related to the acute depressant aspects of morphine, but may be related to the stimulant aspects.

The effects of chronic administration of naltrexone on appetite and water exchange in rats

The effects of chronic administration of naltrexone (200 microgram/kg/hr) on appetitive behaviors and renal water and electrolyte excretions were studied in rats. Naltrexone reduced food and water intake, the renal excretions of water and electrolyte excretions were studied in rats. Naltrexone reduced food and water intake, the renal excretions of water and electrolytes, and osmolar clearance. No changes in plasma levels of electrolytes, plasma and urine Na+-K+ ratios, hematocrit ratio, plasma osmolality, the clearances of K+ and Na+, and the reabsorption of solute free water were found. The changes in appetite were compensated for by appropriate changes in renal excretions, resulting in no change in electrolyte balance or water exchange. These observations are discussed in relation to current theories of the role of endorphins in appetite control.

A comparison of the effects of opiate antagonists on operant and ingestive behavior

Previous studies have found that naloxone and other opiate antagonists will decrease the food and water intake of experimental animals. The present study investigated the possibility that these effects may be due to a generalized action of opiate antagonists to block an endogenous reward system. A direct comparison was made between the effects of naloxone and naltrexone on FR responding maintained by small quantities of milk and on the consumption of milk when it was freely available. Both drugs reduced milk consumption at all doses (0.3-30 mg/kg) but produced only small decreases in FR response rates at the highest doses. These results do not support the view that the actions of opiate antagonists on eating and drinking represent an inhibitory action on central reward mechanisms.

Reinforcing effects of morphine in the nucleus accumbens

The possibility that morphine exerts in reinforcing effects via an action in the nucleus accumbens was investigated in the rat using the self-administration model and the method of intracerebral injection. Male, adult rats implanted with a chronic cannula aimed at this nucleus were tested for self-administration in a rectangular chamber equipped with two levers, one at each end. Each subject was given several sessions in blocks of two sessions for a given substance, the first session with the active lever at one end of the chamber, the next with the active lever at the other end. During a session, responses on the inactive lever were without consequences. Depression of the active lever was coupled with the presentation of a tone to facilitate discrimination between the two levers. All subjects received a sequence of 5 sessions, the first for habituation, the next two to test for effects of artificial cerebrospinal fluid, and the next two to test for effects of morphine. A few subjects in which the cannula had not been dislodged or plugged were further tested for effects of morphine and then for effects of morphine mixed with naloxone. Each response on the active lever resulted in an injection of 0.02 microliter. During the morphine sessions, this volume contained 0.2 microgram of the opiate; during the sessions of morphine and naloxone, this volume contained 0.2 microgram morphine mixed with 0.2 microgram naloxone. The results show that self-administration was induced and maintained when morphine was injected inside the nucleus accumbens, not in an area outside this region but at the same brain level. The rate or responding on the active lever for morphine was higher than for artificial cerebrospinal fluid and it showed a pattern of gradual increase over time. In contrast, the rate of responding on the inactive lever during the morphine sessions showed a decline. This pattern was not evident in these animals during control sessions and in the animals in which the injections were outside the n. accumbens. These results are interpreted to indicate that the n. accumbens may be part of a system of structures mediating the reinforcing effects of opiates.

Effects and interactions of naloxone and amphetamine on self-stimulation of the prefrontal cortex and dorsal tegmentum

Naloxone (0.1 to 10 mg/kg) caused a dose-dependent depression of self-stimulation of the medial prefrontal cortex (PFC) lateral hypothalamus (LH) and a region in the dorsal tegmentum lateral to the central gray (DT). The DT contains many enkephalin fibers and is a site for stimulation-produced analgesia, while the PFC contains little enkephalin and does not support stimulation-produced analgesia. However, self-stimulation rates of the PFC, DT and LH were all equally depressed by naloxone. In order to study possible opiate-dopamine interactions, we examined the effects of naloxone on the facilitatory effects of D- and L-amphetamine (1.0 mg/kg) on self-stimulation of the DT and PFC. If amphetamine mildly facilitated self-stimulation (L-amphetamine on DT self-stimulation and D-amphetamine on PFC self-stimulation) then the addition of naloxone was without effect. If amphetamine greatly increased self-stimulation (D-amphetamine on DT self-stimulation), naloxone caused a depression of the amphetamine effect. It is argued that naloxone's effects in this and other reports reviewed is related to the level of self-stimulation performance, and not to the level of enkephalin at the self-stimulation site, nor to amphetamine's effects on dopamine activity.

Behavioral steering in dual and social states of conation by the amygdala, hypothalamus, ventral striatum, and thalamus

Conative information relating to goal states is proposed as regulating columnar information structures in the neocortex. These structures may handle a spatiotemporal hierarchy of sensorimotor information and conative information. In basic behavioral states, a subcortical system selects a dominant modality of conation (corresponding to a specific goal state) from cortical and subcortical sources, and enhances or suppresses activity in neocortical information structures, yielding the selected modality. These adjustments steer behavior relative to the goal state, allowing assembly of new behavior patterns. For opposing or cooperative modalities of conation, a dual-state system is proposed that modulates the basic-state system. For more extended combinations of conative modalities, a further function is needed that interrelates the basic-state system and the cortex to provide hierarchical conation. Basic-state system control may be exercised by the hypothalamus, influencing the cortex via the thalamus. Other conative control functions may be implemented by the amygdala and the ventral striatum. Opiatergic circuits may be involved in modifying conative components of information.

D-Ala2,D-Leu5-enkephalin generalizes to a discriminative stimulus produced by fentanyl but not ethylketocyclazocine

Male Sprague-Dawley rats were trained to discriminate between the effects of saline and either fentanyl (0.04 mg/kg) or ethylketocyclazocine (0.32 mg/kg) by responding on a FR 10 schedule of food reinforcement with a lever on one side of a food cup following a subcutaneous saline injection and responding with a lever on the alternate side following the subcutaneous injection of one of those drugs. The effects of an intracerebroventricular injection of either fentanyl (2.89-14.45 nmol) or D-Ala2,D-Leu5-enkephalin (0.172-1.72 nmol) dose-dependently generalized to the discriminative stimulus produced by fentanyl injected subcutaneously. Intracerebroventricular injection of ethylketocyclazocine (15.7-125.8 nmol) but not D-Ala2,D-Leu5-enkephalin (1.72-13.76 nmol) dose-dependently generalized to the discriminative stimulus produced by ethylketocyclazocine injected subcutaneously. These data demonstrate similarities in the discriminative stimulus properties of proposed micron and delta but not kappa and delta opiate receptor agonists.

Opiate receptors and endogenous opiates: panorama of opiate research

1. In the last ten years basic research on the mechanism of action of opiates has led to the clearcut demonstration of the existence of opiate receptors--possibly several slightly different kinds--in the nervous system. 2. A number of endogenous ligands also called endorphins or enkephalins for these receptors have been discovered that proved to be peptides with opiate-like pharmacological activity and were shown to be localized in strategic neuronal pathways in the brain, spinal cord and pituitary gland. 3. Clinical researchers are beginning to explore the possible role of these opiate-like peptides in a variety of clinical situations such as: pain and analgesia, tolerance and dependence, reinforcement mechanisms, memory processes and learning, and mental illness such as schizophrenia. 4. The discovery of biologically active opioid and other peptides coexisting with more traditional neurotransmitters in the same neurons may lead to a reevaluation of our fundamental notions of how the brain operates. This is viewed as a major advancement not only to our understanding of the theoretical basis of drug actions but also as a first step towards the development of new, practically useful methods for treating the clinical problems associated with drug abuse.

Possible involvement of endorphin withdrawal or imbalance in specific premenstrual syndromes and postpartum depression

Premenstrual and postpartum dysphoric changes are very prevalent. However, their etiology is still obscure. The authors hypothesize that changes in levels of endorphins may be involved in the pathophysiology of these changes. Studies of various endorphins indicate a possible relationship between levels of endorphins and depressive symptoms. In addition, some studies of naloxone and naltrexone suggest a relationship between a blockage in the action of endorphins and the development of a syndrome of dysphoric symptoms similar to the depressive features manifested premenstrually and postpartum by many women and frequently seen in some depressed outpatients. There is also some evidence that there may be a relationship between elevated levels of endorphins and other subtypes of depressive syndromes. Endorphins and estrogen levels have been shown to covary. During the postpartum and the premenstrual period, levels of both change rapidly and substantially. Therefore the link between changes in levels of endorphins and the dysphoric changes during the periods in focus is supported from three complementary directions: (1) the characteristic psychiatric symptomatology, (2) the reported hormonal changes, and (3) the possible involvement of endorphins in neuroendocrine regulation.

Naloxone reduces fluid intake in rats with open gastric fistulas

Water-deprived rats, fixed with chronically indwelling gastric fistulas, drank with the fistulas open (sham drinking) for 20 min. The subjects were given three doses of naloxone (0.0, 1.0, and 10 mg/kg, SC) 15 min before the opportunity to sham drink. The mean water intakes on days of 0.0, 1.0 and 10 mg/kg of naloxone were 43.3, 36.3 and 23.1 g, respectively. Naloxone clearly reduced fluid intake in rats engaged in sham drinking. This finding, that naloxone reduces fluid intake when post-ingestional absorption is prevented, lends support to the idea that naloxone modified central neural regulatory processes. The results of these experiments do not support the hypothesis that the endorphins are involved exclusively with drive-reduction.

Identification of a novel class of central reward sites showing a delayed and cumulative response to opiate blockade

Heretofore it has been assumed that the brain's reward system shows a response to opiate receptor blockade that is (a) immediate in nature and (b) close to asymptote after the initial dose. On theoretical grounds a graded and gradual response to blockade might also be predicted. To test this, rats were implanted with subcortical electrodes for self-stimulation and trained to panel-press for contingent intracranial reinforcement. Chronic blockade of the opiate system was produced by repeated injections of naltrexone HCl. A small percentage of self-stimulation sites proved refractory to opiate blockade both acutely and chronically. Also, some sites showed an immediate decrease in response level, as previously reported. As was predicted, a number of sites originally showing no response alteration showed a graded response decrease over a 4-day test period. These findings indicate the existence of a novel class of opiate sensitive reward sites and suggest (a) some apparently insensitive sites may become sensitive to opiate blockade under appropriate testing circumstances and (b) opioid neuropeptides may control certain tonic as well as more immediate aspects of motivation.

The immunocytochemical localization of enkephalin in the central nervous system of the rat

The immunocytochemical localization of enkephalin-like immunoreactivity (ELI) throughout the rat central nervous system (CNS) was investigated. The detection of ELI-containing structures was facilitated through the use of (1) brains from colchicine-treated rats, (2) the proteolytic pretreatment of sections with pronase and (3) the "double-bridge" staining technique. Our findings confirm the presence of ELI in perikarya, neuronal processes and terminals in many areas of the CNS. In addition, the localization of ELI-containing perikarya is reported for the first time in the following areas: the olfactory bulb, the olfactory tubercle, the lateral preoptic nucleus, several nuclei within the amygdaloid nuclear complex, the hippocampus, the neocortex, the cingulate cortex, the posterior mammillary nucleus, the medial nucleus of the optic tract, the brachium of the inferior colliculus, the ventral tegmental nucleus, the locus ceruleus, the sub-ceruleal region, the lateral trapezoid nucleus, the nucleus reticularis lateralis, and lamina VII of the cervical spinal cord. Our results demonstrate ELI in neurons which are heterogeneous in size, some probably functioning as interneurons and others as projection neurons in different areas of the CNS. The location of these neurons within the brain suggests that these pentapeptides serve diverse functions which include, in addition to nociception, the regulation of neuroendocrine, respiratory, auditory, vestibular, and olfactory functions.

Cross tolerance between morphine and the long-term analgesic reaction to inescapable shock

Animals exposed to a variety of stressors display a temporary analgesic reaction. This short-term analgesia has been shown to be reversible by opiate antagonists and cross-tolerant with morphine following some stress conditions, but not following others. It has recently been shown that inescapable shock parameters which produce behavioral '"learned helplessness" effects also produce a short-term analgesic reaction, and that this reaction can be re-aroused by a brief exposure to shock 24 hours later. Further, both the immediate and long-term antinociceptive reaction which follow shocks of this type have been shown to be reversible by opiate antagonists. Here it is shown that the long-term analgesic reaction is completely cross tolerant with morphine. Implications of these results for opioid mediation of learned helplessness and opioid versus nonopioid mediation of stress-induced analgesia are discussed.