Comparative study on analgesic effect of Met5-enkephalin and related lipotropin fragments

beta-endorphin: synthesis and morphine-like activity of analogs with D-amino acid residues in positions 1, 2, 4, and 5

The solid-phase syntheses of [D-Tyr1]-, [D-Ala2]-, [D-Phe4]- and [D-Met5]-beta c-endorphins are described. A comparison of certain methods of purification and criteria of homogeneity is made with the use of these compounds. Bioassay of these synthetic analogs both in vitro and in vivo show that [D-Ala2]-beta c-endorphin possesses significant opiate activity whereas the other analogs have minimal activity.

Synthesis of biological activity of human beta-endorphin analogs with disulfide bridges

Three analogs of human beta-endorphin containing internal cystine bridges have been synthesized and their biological activities have been determined. It was found that the analogs with disulfide bridges between positions 17 and 26 or 11 and 26 retained full opiate activity as estimated by guinea ileum assay, whereas the analog with a cystine bridge between positions 7 and 26 showed reduced opiate activity. None of the analogs retained significant immunoactivity as revealed by radioimmunoassay.

Analgesic effects of different acupoint stimulation frequencies in humans

Transcutaneous electrical acupoint stimulation (TEAS) provides a convenient and standardized technique for pain treatment. The cold-pressor test is a simple and reliable model in humans for the induction of tonic pain. In this controlled study, the effects of TEAS on cold pressor-induced pain were evaluated in 22 healthy human subjects. Electrical stimulation at 4 Hz and 32 Hz was applied to He-Gu (LI 4) and Nui-Guan (P 6) acupoints for 15 minutes. Pain score ratings were evaluated at four time points from 30-170 seconds during the cold-pressor test. We observed an analgesic effect at both 4 Hz and 32 Hz of stimulation, and pain score rating reductions were statistically significant compared to control (p < 0.01). Our data support the efficacy of TEAS analgesia. However, there was no significant difference between pain scores at 4 Hz and 32 Hz stimulation.

Transcutaneous electrical acupoint stimulation potentiates analgesic effect of morphine

Pain is the major complaint of patients who choose acupuncture treatment. Transcutaneous electrical acupoint stimulation (TEAS) provides a safe, standardized technique without needle insertion. TEAS can be tested with the cold-pressor test, a simple, reliable, and widely used model in humansfor the induction of tonic pain. In this controlled study, the effects of TEAS on cold-pressor-induced pain were evaluated in 20 healthy human subjects. Electrical stimulation electrodes were applied to He-Gu (LI 4) and Nui-Guan (P 6) acupoints. The effects of saline plus no TEAS, 15-minute TEAS alone, 0.05 mg/kg morphine alone, and 15-minute TEAS plus morphine were assessed. Pain score ratings were evaluated at four time points from 30 to 170 seconds during the cold-pressor test. The authors observed analgesic effects in both TEAS-alone and morphine-alone sessions, and pain score rating reductions were statistically significant compared to unstimulated control (both p < 0.01). The degree of TEAS analgesia combined with 0.05 mg/kg morphine was significantly higher than TEAS alone (p < 0.01). The results support the efficacy of TEAS analgesia and suggest that combination of TEAS with low-dose morphine can achieve better pain control in a variety of clinical settings.

Diprotin A, an inhibitor of dipeptidyl aminopeptidase IV(EC 3.4.14.5) produces naloxone-reversible analgesia in rats

The dipeptidyl aminopeptidase IV (DP IV) inhibitor Diprotin A produces a full, dose-dependent, short-lasting and naloxone-reversible analgesia in the rat tail-flick test when given intracerebroventricularly, with an ED50 of 295 nmol/rat but it has no direct opioid agonist activity in the longitudinal muscle strip of guinea-pig ileum bioassay. Two of the potential DP IV substrates, morphiceptin and endomorphin 1, identified recently in bovine brain were also analgesic given by similar route. The action of endomorphin 1 was more potent (ED50 = 7.9 nmol/rat) and slightly but significantly more sustained than that of Diprotin A. Diprotin A neither potentiated nor prolonged the effect of a marginally analgesic dose of endomorphin 1. The distinct time course and the lack of potentiation indicate that in the analgesic effect of Diprotin A in rats the protection of a brain Tyr-Pro-peptide other than endomorphin 1 is involved.

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

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

Androgenic-anabolic steroids modify beta-endorphin immunoreactivity in the rat brain

Immunocytochemical localization of beta-endorphin in the brains of intact and castrated male rats was conducted after the administration of high levels of androgenic-anabolic steroids (AAS; 14 daily injections of sesame oil or a cocktail of 2 mg/kg testosterone cypionate, 2 mg/kg nandrolone decanoate, and 1 mg/kg boldenone undecylenate) at doses commonly self-administered by athletes who are considered 'heavy abusers'. In normal intact oil-treated males, cytoplasmic immunoreactivity was prevalent throughout the arcuate nucleus while intense fiber tract immunoreactivity was most prevalent in the bed nucleus of the stria terminalis and the paraventricular hypothalamic nucleus. Administration of AAS significantly decreased the number of neurons exhibiting cytoplasmic immunoreactivity only in the rostral region of the arcuate nucleus. AAS treatment had no effect on beta-endorphin immunoreactivity in the middle or caudal aspects of the arcuate nucleus.

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.

The role of endogenous peptides in the action of opioid analgesics

The observation that the narcotic antagonist naloxone could inhibit analgesia produced by electrical stimulation of the brain indicated the involvement of an endogenous chemical in the relief of pain. Multiple endogenous opioid peptides have been identified that have similar pharmacological properties to known narcotic analgesics. The biosynthesis, release, and degradation of opioid peptides have been studied in order to better understand how the manipulation of endogenous opioid systems can be used to produce or augment analgesia. The results of our studies reveal that various conditions and manipulations, such as electrical brain stimulation, acupuncture, stress, and the administration of opioid analgesics, can cause the release of endogenous opioid peptides and possibly endogenous nonpeptide substances. It has also been discovered that nonopioid peptides, such as cholecystokinin, calcitonin, and angiotensin II, can alter the action of opioid analgesics by antagonizing or potentiating their effects. An understanding of the role of endogenous peptides in endogenous opioid mechanisms is necessary for the development of new ways to treat pain and such other disorders as sleep apnea in children (sudden infant death syndrome), head injury, and opioid addiction that involve the activation or alteration of endogenous opioid systems.

Electroacupuncture and morphine analgesia potentiated by bestatin and thiorphan administered to the nucleus accumbens of the rabbit

The endogenous opioid peptide enkephalin (EK) is known to be degraded mainly by two enzymes, the dipeptidyl carboxypeptidase 'enkephalinase' and aminopeptidase. Microinjection of the enkephalinase inhibitor thiorphan or the aminopeptidase inhibitor bestatin into the nucleus accumbens of the rabbit produced a dose-dependent analgesic effect. This analgesic effect was totally reversed by the narcotic antagonist naloxone or by antibodies against [Met5]enkephalin (MEK) administered to the same site. Antibodies against [Leu5]enkephalin were not effective. Moreover, microinjection of thiorphan or bestatin into the nucleus accumbens resulted in a marked potentiation of the aftereffect of electroacupuncture (EA) produced analgesia, as well as the analgesia induced by a small dose of morphine. It is concluded that the analgesic effect elicited by EA and morphine is mediated, at least in part, by MEK-like immunoreactive substance(s) in the nucleus accumbens.

An immunohistochemical study of methionine-enkephalin-Arg6-Gly7-Leu8-like immunoreactivity-containing liquor-contacting neurons (LCNs) in the rat spinal cord

The localization of methionine-enkephalin-Arg6-Gly7-Leu8 (Met-Enk-Arg-Gly-Leu)-like immunoreactivity in the medullospinal liquor-contacting neurons (LCNs) of the rat was immunohistochemically investigated using a modified Sternberger's PAP method. In this study, we also examined the distribution of Met-Enk-Arg-Gly-Leu-like immunoreactive LCNs in the entire length of the medulla oblongata and spinal cord. The immunoreactive LCN possessed an oval, polygonal or rod-shaped perikaryon which protruded an axon-like process and a CSF-contacting process. In the entire length of the medulla oblongata and spinal cord, 60-70 immunoreactive LCNs were observed. They were sparsely distributed along the central canal. The ratio of the number of immunoreactive LCNs in each segment/total number of immunoreactive LCNs in the medulla oblongata and whole spinal cord was as follows: medulla oblongata, 0%; cervical segments, 28.1%; thoracic segments, 56.8%; lumbar segments, 12.0%; sacral segments, 3.0%; and coccygeal segments, 0%. When the immunoreactivity of perikarya of LCNs was weak, a few nerve terminals with a strong Met-Enk-Arg-Gly-Leu-like immunoreactivity were noticed on them. These findings suggest that preproenkephalin A-related opioid peptides including Met-Enk-Arg-Gly-Leu may be secreted into the cerebrospinal fluid (CSF) through the terminal portions of axon-like processes and/or CSF-contacting processes of LCNs. Enkephalin-containing nerve terminals on the LCNs may represent modulating afferents aiding the receptive and/or secretory functions of the neurons.

Methionine enkephalin as a possible neuromodulator of regional cerebral blood flow

In swine, cerebral blood flow was documented by a left ventricular injection of radiolabeled 15-micron spheres. Utilizing this procedure, the effect of the putative neurotransmitter methionine-enkephalin on regional cerebral blood flow was systemically evaluated. Our results revealed that a peripheral infusion of methionine enkephalin into miniature swine significantly increased cerebral blood flow in the basal ganglia, cerebellum, pons, inferior parietal cortex and frontal cortex. Non-significant increases were observed in the hippocampus, occipital cortex and medulla oblongata while no effect on blood flow was observed in the pituitary gland. Significance of these results in the potential role of methionine enkephalin as a modulator of blood flow to the brain.

A study of intracerebroventricular self-administration of leucine or methionine enkephalin by rats in response to intermittent electric shocks

Rats were taught to self-administer Leu-E (10, 25 and 100 micrograms/microliters) or Met-E (0.5, 10 and 100 micrograms/microliters) through a cannula implanted in the lateral cerebral ventricle (i.c.v.). Their self-injection behaviour was studied before, during and after nociceptive stimulation. In the course of the control period of the experiment, the rats rapidly learned lever pressing for self-injection of enkephalin but they did not increase their self-administration of Leu-E or Met-E during the nociceptive electrical stimulation period. Also studied were the acute effect of i.c.v. enkephalin and morphine on tail-flick latency (sec) and electrical threshold vocalization (mA). The analgesic effect of Leu-E and Met-E was of short duration (less than 2-6 min). The mean rise (i.e., analgesia) of the tail-flick threshold showed a significant difference after i.c.v. Met-E only. The acute i.c.v. effect of 20 or 30 micrograms of morphine induced a long-lasting analgesia, greater than 40 min. These results show that Leu-E and Met-E are not rewarding during a nociceptive stimulus. This may be due to the short and inconstant analgesic action of i.c.v. enkephalins.

Pain threshold changes in rats following central injection of beta-endorphin, met-enkephalin, vasopressin or oxytocin antisera

Both opioid peptides such as beta-endorphin and met-enkephalin and nonopioid peptides such as vasopressin and oxytocin increase pain thresholds in rodents. Antisera raised against each of these peptides have been developed for use in immunocytochemical and radioimmunoassay procedures. The present study assessed whether central administration of antisera raised against beta-endorphin (ABE), met-enkephalin (AME), arginine, vasopressin (AAVP) or oxytocin (AOT) altered tail-flick latencies elicited by three different levels of radiant heat, jump thresholds, core body temperatures and locomotor activity. ABE induced a transient hyperalgesia on the tail-flick test at thermal levels at which beta-endorphin administration would elicit an analgesic effect. While met-enkephalin increases tail-flick latencies elicited by high thermal levels, AME failed to alter latencies at this level, but rather induced a short-acting hyperalgesia at a low thermal level. While vasopressin increased tail-flick latencies at high thermal levels, AAVP produced reciprocal decreases. Yet AAVP inexplicably induced analgesic effects at moderate and low thermal levels. Finally, while oxytocin increased latencies at high thermal levels, AOT failed to alter latencies. Rather, it decreased latencies at the moderate thermal level and increased latencies at the low thermal level. Neither jump thresholds nor core body temperatures were affected by any antiserum pretreatment. While activity levels were unaffected by either ABE, AME or AAVP pretreatment, AOT decreased activity in a fashion complementry to oxytocin-induced hyperactivity and seizures. There data are discussed in terms of tonic versus phasic influences of these peptides upon pain perception.

beta-Endorphin-induced analgesia is inhibited by synthetic analogs of beta-endorphin

Competitive antagonism of human beta-endorphin (beta h-EP)-induced analgesia by synthetic beta h-EP analogs with high in vitro opiate receptor binding to in vivo analgesic potency ratio has been demonstrated. A parallel shift of the dose-response curve for analgesia to the right was observed when either beta h-EP or [ Trp27 ] -beta h-EP was coinjected with various doses of [Gln8, Gly31 ]-beta h-EP-Gly-Gly-NH2, [Arg9,19,24,28,29]-beta h-EP, or [ Cys11 ,26, Phe27 , Gly31 ]-beta h-EP. It was estimated that the most potent antagonist, [Gln8, Gly31 ]-beta h-EP-Gly-NH2, is at least 200 times more potent than naloxone.

Species differences in the relative analgesic potencies of some classical opiates and opioid peptides

The analgesic ED50 values of some classical morphine congeners (morphine, methadone, fentanyl, azidomorphine) in the rat and mouse tail-flick tests were found to be similar. However, several synthetic derivatives of the natural enkephalins were more potent in mice than in rats. (These analogs contain D-amino acid in position 2 and D- or L-sulfonic (or phosphonic) acid residue in position 5). beta-Endorphin, D-Met2, Pro5-enkephalinamide and two partial agonists showed intermediate interspecies relative potencies. According to the data obtained, similar opiate receptors might mediate the analgesic action of classical opiates in rats and in mice. However, the opiate receptors responsible for the antinociceptive effects of the above mentioned enkephalin analogues must be dissimilar in the two species examined. The results are discussed in terms of the role of mu- and delta-receptors in mediation of the analgesic effect induced by different types of opioids.

Complete structure of the porcine pro-opiomelanocortin mRNA derived from the nucleotide sequence of cloned cDNA

Polyadenylated RNA isolated from porcine pituitary neurointermediate lobes was used to construct a cDNA library. The library was screened with a rat genomic DNA fragment specific for pro-opiomelanocortin sequences. Two positive clones, pJA-19 and pJA-20, containing respectively 850 bp and 550 bp were characterized. Sequence analysis of the cDNA inserts revealed the complete structure of the porcine pro-opiomelanocortin mRNA. This mRNA would include 129 5'-untranslated nucleotides, 801 nucleotides coding for the 267 amino acids precursor and 162 3'-untranslated nucleotides. Comparison with pro-opiomelanocortin mRNA sequences from other species shows regions of high homology not only in the coding sequences but also in the 5'untranslated region where the first 50 nucleotides are over 80% purines.

Multi-dimensional analyses of behavior in mice treated with morphine, endorphins and [des-tyrosine1]-gamma-endorphin

An investigation was made as to the effects of an intracerebral injection of morphine, endorphins and [des-tyrosine1]-gamma-endorphin (DT gamma E) on spontaneous locomotor activity in mice. This was done by exploiting a newly devised multi-dimensional behavioral analyser with a capacitance system. This apparatus simultaneously recorded nine different degrees of behavior (1/1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128 and 1/256) according to the movement sizes in mice. Within 15 min after injection, gamma-endorphin (5 and 10 microgram), leucine-enkephalin (200 micrograms) and methionine-enkephalin (100 micrograms) produced a significant increase in the 1/1 size of movement. Fifteen to 30 min after injection, the movement patterns induced by morphine (40 micrograms) and DT gamma E (40 micrograms) became similar, although morphine (40 micrograms) caused a significant decrease in the 1/1 and 1/2 sizes of movement. beta-Endorphin (2 micrograms) significantly decreased most of the movement sizes for 30 min compared with saline-treated group. [D-alanine2]-methionine-enkephalinamide (20 micrograms) significantly decreased almost all the movement sizes within 15 min after injection. The significant alteration in the movement sizes induced by morphine and endorphins except for DT gamma E was antagonized by pretreatment with naloxone (1 mg/kg). These results strongly suggest the qualitative difference in the behavioral effects of each of the opioids and nonopioid.

A possible involvement of the central endorphin system in the autoanalgesia induced by chronic administration of Freund's adjuvant solution in rats

Chronic pain was induced in rats by daily injections of complete Freund's adjuvant into hind paws. Daily changes of pain threshold and endorphin (ED) content and their receptors in 4 divided parts: cortex, diencephalon-mesencephalon containing striatum (D-M), pons-medulla (P-M) and spinal cord, were measured. Decrease in pain responsiveness was observed in the adjuvant-injected group with concomitant increase of ED content in P-M and spinal cord. This decrease in pain responsiveness in the adjuvant-injected group was significantly different from that in the non-treated control group being partially reversed by naloxone. Furthermore, [3H]met-enkephalin binding sites increase in number in P-M of the adjuvant-injected group when maximal decrease of pain responsiveness was observed, returning to control level thereafter. Scatchard analysis revealed the increase of the low affinity binding site in P-M of the adjuvant-injected group. In cortex and D-M, on the other hand, ED content tended to decrease and no change was observed in number of [3H]met-enkephalin binding sites. These results indicate that the ED system in P-M and spinal cord may be more substantially involved in autoanalgesia than in cortex and D-M.

Bacitracin produces analgesia by increasing brain immunoreactive beta-endorphin (beta-E) content

The effects of protease inhibitor bacitracin on brain beta-endorphin content and analgesia, were examined in vivo. Male Sprague-Dawley rats were injected with bacitracin intracerebroventricularly and sacrificed by microwave irradiation 15 and 30 min after injection. Brain beta-endorphin levels were 27% higher in bacitracin treated rats than in controls. A second group of bacitracin injected rats was subjected to continuous intermittent 55 degrees C hot plate exposure. Bacitracin-injected rats exhibited total analgesia 15 min after bacitracin injection. At 30 min, this analgesic effect subsided. Control rats exhibited no analgesia. Bacitracin induced analgesia was naloxone reversible at a low dose of naloxone (1 mg/kg). At a higher dose of naloxone (10mg/kg), bacitracin induced analgesia was only partially antagonized. These results suggest that bacitracin induced analgesia might be due to the elevated levels of brain beta-endorphin caused by a decrease in its breakdown by bacitracin.

beta-Endorphin: demonstration of a tertiary structure in aqueous solution

Difference spectra generated during thermolysin digestion of camel beta-endorphin at pH 8.2 or at pH 6.5 indicate rapid blue shifting of the near-UV absorption bands of the NH2-terminal tyrosine. A similar spectral change is not observed for the NH2-terminal tyrosine in [Met]enkephalin when it is digested under similar conditions. These results suggest that enzymatic digestion destroys or alters some structural interaction between the NH2-terminal tyrosyl residue of the endorphin and a residue(s) within the COOH-terminal segment of the molecule. Peptide mapping of the digest as a function of time suggests that cleavage of the bond linking the alanine-21 and isoleucine-22 residues produces most of the observed effect. These data provide evidence for the existence of a tertiary structure for beta-endorphin in aqueous solutions.

Influence of beta-lipotropin fragments on responsiveness of rats to electric footshock

Subcutaneous administration of graded doses of neuropeptides related to lipotropin (beta LPH) changed responsiveness of rats to electric footshock. gamma-Endorphin and related peptides increased the susceptibility of rats, whereas beta-endorphin induced a reversed effect. The enhanced responsiveness induced by DT gamma E, persisted for more than 24 hr and appeared to be of a central origin. Structure-activity relationship studies revealed that the sequence LPH 65-69 may contain the active core in this respect. An increased sensitivity to electric shock within one test procedure was observed with DT gamma E and alpha-endorphin. Prior experience with the test procedure diminished or abolished this effect of alpha-endorphin. It is suggested that beta-endorphin and its fragments play a physiological role in adaptive behavioral changes induced by peripheral stimuli.

beta-Endorphin: possible involvement in the antihypertensive effect of central alpha-receptor activation

Clonidine and L-alpha-methylnoradrenaline (but not D-alpha-methylnoradrenaline) increase the release of a substance with beta-endorphin immunoreactivity from slices of brainstem of spontaneously hypertensive rats, but not that of normotensive rats. It was reported earlier that opiate antagonists inhibit the hypotensive action of clonidine and alpha-methyldopa in spontaneously hypertensive but not in normotensive rats and that beta-endorphin has hypotensive effects of its own. Together, these findings indicate that release of beta-endorphin by central alpha-receptor agonists may contribute to the antihypertensive action of these drugs.

Further evidence for the neuroleptic-like activity of gamma-endorphin

gamma-Endorphin inhibits the amphetamine-induced stereotypy in rats with an approximate ED50 of 0.3 mg/kg. SC. The anti-amphetamine effect of gamma-endorphin can be antagonized to a certain extent by both the dopamine receptor stimulant 2-bromo-ergocryptine and the opiate receptor antagonist naloxone.

Antagonism of stress-induced analgesia by D-phenylalanine, an anti-enkephalinase

Methionine- and leucine-enkephalin produce mild and transient analgesic effects, presumably because of enzymatic degradation. Administration of high (250 mg/kg) doses of D-phenylalanine retards the degradation process and elicits analgesia which is reversed by naloxone and which summates with electroacupuncture analgesia. The present study evaluated D-phenylalanine's dose-dependent effects upon a non-opioid analgesic treatment, cold-water swims (CWS), and compared this with morphine. following determination of flinch-jump baselines, three groups of rats received respectively either 25, 50 or 100 mg/kg of D-phenylalanine intraperitoneally in three conditions: alone, with CWS (2 degrees C for 3.5 min), and with morphine (5 mg/kg, SC). Parallel controls with saline were also tested. Simultaneous exposure with each minimally analgesic dose of D-phenylalanine reduced significantly the analgesic, but not hypothermic effects of CWS. By contrast, morphine analgesia was unaffected by D-phenylalanine. These data provide further support that different pain-inhibitory systems mediate CWS and morphine analgesia and suggest that activation of one system is capable of exerting collateral inhibition upon the other.

Stress-induced analgesia: neural and hormonal determinants

Extensive evidence has indicated that distinct neural systems specifically designed to inhibit sensitivity to painful stimuli exist. Recent advances suggest that the endorphins, enkephalins and the opiate receptor interact with a descending serotonergic bulbospinal system to mediate the analgesic responses to opiates and electrical stimulation. In assessing the evolutionary and behavioral significance of this pain-inhibitory system, several laboratories discovered that acute exposure to a wide variety of stressful events results in a transient analgesia. Chronic exposure to a number of these stressors results in adaptation of the analgesic response. The purpose of this review is to identify and characterize the mechanisms by which these stressors activate pain-inhibition. The relationship of stress-induced analgesia to each of the following is reviewed: (a) the role of endorphins, enkephalins and the opiate receptor; (b) the role of the descending serotonergic bulbospinal system; (c) the role of the pituitary gland; and (d) the role of hypothalamic mechanisms. Data will be discussed in terms of "opiate" and "non-opiate" pain-inhibitory mechanisms, in which some stressors act through the former and other stressors act through the latter.

Endogenous opiates: through 1978

The discovery of receptors in the brain for opiates and the structure of the endogenous peptides for these receptors has led to an explosion of interest in this field. The present review is the first of an annual series. It summarizes many of the highlights of research with opiate peptides published with a date of 1978 or earlier.

Analgesia induced by cold-water stress: attenuation following hypophysectomy

In addition to the well-known activation of the pituitary-adrenal axis, acute exposure to severe stressors includes a temporary analgesia in rats. Thus, the present study investigates whether the pituitary was involved in the mediation of analgesia induced by severe cold-water swim (CWS) stress. Flinch-jump thresholds were measured 30 min following 3.5-min swims in water temperatures ranging from 2-35 degrees C. Compared with untreated normal rats, hypophysectomized rats, receiving corticosterone and thyroxin, displayed significantly less CWS-induced analgesia, while similarly-supplemented normal rats exhibited significantly more CWS-induced analgesia. In a second experiment, operant liminal escape pain thresholds were determined following acute and chronic CWS. Whereas normal rats exhibited profound analgesia following the initial swims, the hypophysectomized rats never displayed any CWS-induced operant escape shifts. Stress-induced alterations in general activity levels and/or thermoregulation were shown to be unrelated to the diminished effectiveness of CNS to produce analgesia in hypophysectomized rats. These data imply that the pituitary is involved in the mediation of CWS-induced analgesia.

Comparison of the analgesic effects of various opioid peptides by a newly devised intracisternal injection technique in conscious mice

To determine accurately the central analgesic effects of opioid peptides, we devised a special technique for intracisternal administration of drugs to conscious mice. When this method was utilized we found that a series of endogenous opioid peptides and enkephalin analogues, (D-Ala2,Met5)-enkephalinamide and (D-Met2,Pro5)-enkephalinamide produced dose-related analgesic effects, as determined by the tail pinch test. These effects were antagonized by pretreatment with naloxone 0.5 mg/kg s.c. This technique should be a most accurate one for the determination of central analgesic effects of various drugs in conscious mice.