Ontogenesis of enkephalinergic systems in rat brain: post-natal changes in enkephalin levels, receptors and degrading enzyme activities

Developmental changes in environmentally induced analgesia

Previous studies have shown that exposure to inescapable, front-paw shock produces an opioid-mediated analgesia. Additionally, research has revealed that the majority of opioid receptors in the central nervous system are formed between birth and adulthood. The purpose of the present experiment was to examine the relationship between the development of the opioid receptors and the function of the endogenous opioid pain-inhibitory system activated by shock. Ten-day-old, 28-day-old and 5-7-month-old rats were exposed to 90 s of front-paw shock (1.6 mA). Results revealed that 10-day-old rats displayed lower levels of shock-induced analgesia than 28-day-old and 5-7-month-old rats. This age-related difference in shock-induced analgesia confirms a parallel in the development of opioid receptors and the function of an endogenous pain-inhibitory system. In addition, injection of naloxone produced an increased analgesia in the 10-day-old rats. In the 28-day-old rats naloxone completely blocked the shock-induced analgesia while in the 5-7-month-old rats naloxone only partially attenuated analgesia. This age-related difference (28-day-old vs 5-7-month-old) in the effectiveness of naloxone in blocking shock-induced analgesia suggests the involvement of a non-opioid analgesia system in the 5-7-month-old rats that is not present in the 28-day-old rats. This last difference led to the speculation that the non-opioid analgesia system develops more slowly than the opioid system.

The effect of perinatal lead administration on the ontogeny of striatal enkephalin levels in the rat

The effect of administration of lead in the maternal drinking water from conception to weaning on enkephalin levels in the developing rat striatum was studied between 10 and 100 days after birth. Concentrations of 300 and 1000 ppm Pb produced no overt toxicity in terms of effects on body and brain weight. However, marked differences in striatal enkephalins were observed. In lead-dosed animals enkephalin content was up to 50% lower at 10 and 21 days postpartum, and in addition lead appeared to delay the attainment of adult levels of this opioid neurotransmitter. Measures at later time points indicated that the effects of lead on enkephalin are reversible. The results suggest that lead depresses and delays the ontogeny of the enkephalinergic system within the striatum.

Development of Met-enkephalin immunoreactivity in organotypic explants of fetal mouse spinal cord and attached dorsal root ganglia

Radioimmunoassays of methionine-enkephalin (Met-Enk) in organotypic cultures of 13-day fetal mouse spinal cord explants with attached dorsal root ganglia (DRG) demonstrate a progressive development of immunoreactivity (IR) during 5 weeks in vitro. Met-Enk IR in these cultures increased to levels observed in adult rodent spinal cord. most of the Met-Enk IR assays were made on cord explants excised from cord-DRG cultures. In smaller numbers of assays performed on entire DRG-cord cultures or on cord cultured in the absence of DRGs, similar levels of Met-Enk IR were obtained. Thus most of the Met-Enk IR appeared to be located within the cord tissue. No Met-Enk IR was detected in DRGs cultured in the absence of cord. In contrast, low levels of Met-Enk IR were present in about 50% of the assays of DRGs cultured attached to the cord. Since these assays included the neuritic outgrowths of the cultures, our data do not preclude possible contamination by Met-Enk immunoreactive cord neurites that may have aberrantly projected into the outgrowth zones. Nevertheless, the data raise the possibility of a trophic influence of cord tissue on the development of Met-Enk IR in DRG neurons. The development of Met-Enk IR in cord regions of cord-DRG explants extends previous binding assays demonstrating development of opiate receptors in these cultures and provides further support to electrophysiological analyses suggesting tonic opioid inhibitory networks in these explants.

Ontogeny of enkephalin-like immunoreactivity in the rat hippocampus

The postnatal development of leucine5-enkephalin-like immunoreactivity within the hippocampal formation of the rat has been analyzed using immunocytochemical techniques. From the day of birth to postnatal day three, no intrinsic hippocampal elements exhibit immunoreactivity although labeled axons are found within the fimbria, within the alveus, and in the vicinity of the angular bundle. On postnatal day 4, a few immunoreactive hippocampal neurons can be seen in stratum radiatum of the region CA3 and by postnatal day 8, within the hilus, strata pyramidale and oriens of regio superior, and the subiculum. There is a dramatic increase in the incidence of immunoreactive perikarya between postnatal days 8 and 10 in all fields as well as the appearance of labeled neurons in CA1 stratum pyramidale and stratum granulosum of the dentate gyrus. Two days after the first appearance of immunoreactive perikarya, intensely immunoreactive neurons, labeled much more extensively than is ever seen in the adult, are encountered in each subfield of the hippocampus. The spatio-temporal order in both the emergence of perikaryal immunoreactivity and the transient appearance of intensely immunoreactive neurons follows that of neurogenesis, with immunoreactivity developing 12-14 days after the peak period of last cell division for a given hippocampal region. The incidence of immunoreactive perikarya in the dentate gyrus was quantified in rat pups ranging from postnatal days 8 to 19. The appearance of labeled neurons followed the spatio-temporal gradients that have been described for neurogenesis in this region as well. Immunoreactive perikarya emerged in the suprapyramidal stratum granulosum prior to their emergence in the infrapyramidal zone and in the temporal pole of the dentate earlier than in the mid-dorsoventral dentate. The lateral perforant path and mossy fiber axons, seen to exhibit enkephalin-like immunoreactivity in the adult hippocampal formation, differ in their relative maturity at the age immunoreactivity first appears. Immunoreactivity appears as early as postnatal day 4 in the lateral perforant path, an age at which these axons are just growing into their target field while it is not found within the mossy fibers until after postnatal day 10, an age at which mossy fiber bouton elaboration is well advanced and physiologically competent mossy fiber synapses with the regio inferior pyramidal cells have been established. The latter observation indicates that enkephalin is not necessary for synaptic transmission at the mossy fiber synapse.

Enkephalin receptor changes in rat brain during aging

In experiments on 2-, 10- and 22-month-old rats, it was found that the number (Bmax) of brain enkephalin (Enk) receptors decreased with aging. The Bmax values in the cortex of 22-month-old rats decreased by 57%, in the striatum by 33% and in the hypothalamus by 84% as compared to those in 2-month-old rats. The Enk receptor sensitivity in the brain of aged rats also decreased (increased Kd). It is suggested that the changes in opiate receptors play a role in the age-related reduction of the adaptive capacity of the organism. The studies with repeated administration of L-dopa or haloperidol provided further evidence for interactions between opiate and dopaminergic systems.

Developmental changes in the effect of morphine, epinephrine and carbachol on plasma cyclic nucleotide level in rats

The developmental changes in the effects of morphine, FK 33-824, a synthetic opioid peptide, epinephrine and carbachol on the plasma cyclic AMP and GMP level were investigated in rats ranging in age from 1 to 49 days. The effect of morphine in elevating plasma cyclic AMP and GMP level increased with age with a peak at day 21. The effect of FK 33-824 followed a similar developmental course of that of morphine except at day 49. Epinephrine and carbachol increased the plasma cyclic AMP level even in one-day old rats. The increase in plasma cyclic GMP induced by carbachol increased with age, peaked at day 14, then declined until day 49. These results suggest that the delay in advent of the effect of opioids on plasma cyclic nucleotides in the early neonatal period is mainly due to the slow development of opioid receptors in the brain.

Development of cerebral methionine-enkephalinergic neurons in rats: some difference in Wistar-Kyoto rats and spontaneously hypertensive rats

Development of methionine-enkephalin (ME) and ME receptor binding in the embryonic and neonatal rat cerebral nuclei was defined quantitatively by immunocytofluorescent and microautoradiographic methods. On embryonic days 14 and 18, ME was localized in 10 of 83 nuclei particularly in the n. amygdaloideus centralis, n. periventricularis, n. supraopticus, n. interpeduncularis, n. suprachiasmaticus, n. arcuatus and n. ambiguus. ME receptor binding was distributed in the former 4 nuclei on embryonic day 14 and additional 5 nuclei on embryonic day 18. At the day of birth both levels markedly increased in these nuclei (2-3 times) and abruptly emerged in 65 out of 73 nuclei in which ME neurons were not detectable in embryonic nuclei. The abrupt appearance in most nuclei at birth probably regulates nociceptive and non-nociceptive stimuli at birth and in the neonatal period. Both levels attained their maximum at the postnatal day 7 and gradually declined thereafter. Among the perinatal period examined, ME immunoreactivity in the tractus spinalis nervi trigemini and ME receptor binding in the n. tractus spinalis nervi trigemini were markedly lower in both SHR neonates than in corresponding Wistar-Kyoto rats. Lowered levels in both areas of neonatal SHR may be involved in central hyperreactivity and preganglionic sympathetic activation of young SHR.

Ontogeny of endocrine responses to methadone in rats

The effects of methadone (METH) on serum levels of prolactin (PRL), growth hormone (GH), corticosterone (CS) and TSH were determined in developing rats. METH increased PRL, GH and CS and decreased TSH at all ages tested, but the time course and magnitude of these effects changed during ontogeny. METH effects on day 10 were lower in magnitude than those observed in adults. In 20 day old pups, METH effects on GH and CS were comparable to those of adults, but TSH effects were still blunted. METH effects on hormone secretion in both 10 and 20 day old pups lasted longer than those observed in adults. Naloxone blocked all hormonal responses in adults, but did not completely block METH effects on CS secretion in 10 day old pups.

Age-related differences in the effect of chronic administration of naloxone on opiate binding in rat brain

Infant and adult rats were injected chronically with either naloxone or saline for 21 consecutive days. At various intervals after cessation of the pretreatment with naloxone, animals were sacrificed and assessed for specific binding of [3H]naloxone in different regions of the CNS. Infants displayed an increase in opiate binding in the spinal cord, hypothalamus, striatum and cortex one day after cessation of the pretreatment with naloxone, but the increase in opiate binding was dissipated within one week after cessation of the pretreatment. The increase in opiate binding in infants was accompanied by an increase in the antinociceptive efficacy of morphine. In contrast to infants, adults failed to display any alteration in opiate binding following the chronic pretreatment with naloxone. Infants may be especially susceptible to naloxone-induced receptor supersensitivity because infants excrete naloxone more slowly than adults, and thus their opiate receptors may be blocked for a longer duration following an injection of naloxone.

A study of the endogenous opioid system in the sudden infant death syndrome

To investigate the possible role of the endogenous opioid system in the pathogenesis of the sudden infant death syndrome (SIDS), we measured met-enkephalin immunoreactivity by radioimmunoassay in brain, lung, and adrenal glands of SIDS victims and of infants (controls) dying of nonneurologic causes. Met-enkephalin was stable in brain tissue up to 24 hours after death. On inspection, met-enkephalin levels in the cerebral cortex of SIDS victims were similar to those in controls. Levels in the caudate nucleus were lower in infants than in adults. In the medulla, the levels in SIDS cases and controls were not found to differ significantly. The linear relationship between the levels in the medulla and age was not detectably different in SIDS and controls. However, as a subset, levels in the control group significantly decreased with increasing age (P = 0.005), whereas levels in the SIDS group showed no correlation with age (P = 0.33). Levels of met-enkephalin in the adrenal gland of SIDS victims were similar to those in controls and were considerably lower than adult values. Lung tissue was assayed for beta-endorphin immunoreactivity and met-enkephalin: for both peptides the values in SIDS cases were low (femtomolar range) and similar to those in controls. These data suggest that met-enkephalin is not markedly overproduced in brains of SIDS victims. Future postmortem studies should focus on more subtle evidence of endogenous opioid overactivity such as differences in age related changes, receptor number, and levels of other endogenous opioid peptides.

Effects of chronic antenatal and postnatal administration of narcotics on naloxone-induced anorexia in preweanling rats

Previous studies have reported age-related changes in opiate receptors and in their response to narcotics during the process of normal growth of the brain. By inducing alterations in this developmental sequence, the present study attempted to provide correlates of the opiate receptor system with naloxone-induced anorexia. Offspring of mothers treated with morphine (7.5 mg/kg twice daily, s.c.) or saline during pregnancy, and infants from untreated mothers given morphine (5 mg/kg), naltrexone (10 mg/kg) or saline subcutaneously on postnatal days 1-5, were tested at days 10, 12 and 14 for deprivation-induced milk consumption following an acute dose of naloxone (1 or 5 mg/kg, i.p.). Naloxone reduced the food intake of 10- and 12-day old infants chronically treated with morphine postnatally. At age 14, naloxone reduced the food consumed by all the pretreatment groups, and pretreatment with morphine altered the dose-response curves for feeding modulation induced by naloxone. Naloxone had no effect on the food consumed by 10- or 12-day old offspring of mothers treated with morphine or saline, or on those age-groups that received naltrexone chronically or saline pretreatments postnatally. The observed changes occurred in the absence of gross malformations, drug-withdrawal symptoms and differences in activity. These results demonstrate that opiate receptors may participate in feeding.

Ontogeny of benzomorphan-selective (kappa) sites: a computerized analysis

In an investigation of the postnatal development of kappa opiate receptors, the affinity and capacity of 0.5 nM [3H]-ethylketocyclazocine (EKC) binding in crude rat brain homogenates was measured by displacement with unlabeled EKC, morphine, or D-ala2-D-leu5-enkephalin (DADL). Displacement curves were analyzed using a weighted, non-linear regression, curve fitting computer program. At all stages of development, [3H]-EKC binding fit a two site model significantly better than a one site model. Affinities of EKC, morphine, or DADL for the high affinity [3H]-EKC binding site did not change during the postnatal period. The density of the high affinity [3H]-EKC binding site increased linearly with age, whereas the levels of the low affinity site rose more rapidly during the second week.

Opiates, endorphins and the developing organism: a comprehensive bibliography

A comprehensive bibliography of the literature concerned with opiates, endorphins, and the developing organism is presented. A total of 1378 clinical and laboratory references, with citations beginning in 1875, are recorded. A series of indexed accompanies the citations in order to make the literature more accessible. These indexes are divided into clinical and laboratory topics. The clinical section is subdivided into: age of subject examined; maternal aspects; effects on the fetus; pharmacology, physiology, and the withdrawal syndrome; and "other" effects on the offspring. The laboratory section is subdivided into: type of opiate/endorphin studied; species utilized; and major subject areas explored.

Ontogenesis of opiate binding sites and radioimmunoassayable beta-endorphin and enkephalin in regions of rat brain

The postnatal changes in the levels of radioimmunoassayable enkephalin and beta-endorphin, as well as the densities of [3H]methionine-enkephalin and [3H]naloxone binding sites in rat cerebellum, brainstem and whole forebrain were determined. The opiate peptides and the opiate binding sites reached their highest levels at the first week postpartum in the cerebellum, at the second week in the brainstem and at the third week in the whole forebrain. This finding is in line with the developmental profiles of other well-established neuronal pathways which also showed a caudal-to-rostral sequence of development. Moreover, there was a close relationship between the elevation and decline in the amounts of opiate binding sites and in the levels of opiate peptides in each brain region. These observations are consistent with other evidence which suggests that enkephalin and beta-endorphin are functioning as neurotransmitters or neuromodulators in the central nervous system.

Multiple immunoreactive forms of met- and leu-enkephalin in fetal and neonatal rat brain and in rat gut

We have used radioimmunoassays with carboxyl-specific antisera to study the development of met- and leu-enkephalin in rat brain and gut from 13-days of fetal age through adulthood. Fractionation of HCl extracts of fetal and neonatal brain tissues by HPLC revealed the presence of immunoreactive forms other than met- and leu-enkephalin. For example, HPLC separation of extracts of 16-day fetal brain yielded two peaks of leu-enkephalin-like immunoreactivity. One emerged at the position of leu-enkephalin, the other eluted with about one-third the retention time. There were four peaks of met-enkephalin-like immunoreactivity, one with the retention time characteristic of met-enkephalin, the others with shorter retention times. In contrast, all of the met-enkephalin-like immunoreactivity in adult brain eluted with the retention time characteristic of authentic met-enkephalin and all of the leu-enkephalin-like immunoreactivity eluted in the position of authentic leu-enkephalin. Multiple immunoreactive forms of met- and leu-enkephalin were found in extracts of both fetal and adult gut tissues.

Postnatal change in receptivity for methionine5-enkephalin in rat duodenum: transition from neurogenic to myogenic receptivity

The study concerned the postnatal ontogenesis of the response to methionine5-enkephalin (ENK) in the duodenum of developing rats. Sustained relaxation was produced by ENK in the isolated duodenum of 8 day old rats. The response was blocked by tetrodotoxin or naloxone. The response increased by day 18, decreased thereafter and was extinguished after 40 days. On the other hand, a tetrodotoxin resistant response to ENK, transient relaxation, appeared at day 20 and was augmented thereafter. The development of neurogenic receptivity for ENK preceded the appearance of myogenic receptivity in the rat duodenum.

Degradation of enkephalins: the search for an enkephalinase

The opioid peptides methionine-enkephalin and leucine-enkephalin appear to exert their biological effects through a receptor mediated mechanism. There appears to be three potential mechanisms for enkephalin degradation which could serve to control enkephalin levels in the vicinity of enkephalin receptors. These are, 1) cleavage of the tyrosyl-glycine bond by aminopeptidases, 2) cleavage of the glycl-glycine bond by a dipeptidyl aminopeptidase, and 3) cleavage of the glycyl-phenylalanine bond by a dipeptidyl carboxypeptidase. In this review the biochemical properties of these potential enkephalinases are described, and the evidence for each acting as an enkephalinase is reviewed.

Effect of naloxone and D-met2-pro5-enkephalinamide treatment on the DNA synthesis in the developing rat brain

Effects of a single dose of naloxone and of D-Met2-Pro5-enkephalinamide on the DNA synthesis in the forebrain, hypothalamus and cerebellum of 11 day old female rats were studied. As an index of DNA synthesis the rate of incorporation of 3H-thymidine into DNA was measured 30 min after a sc. injection of 40 muCi/100 g b.w.. A time dependent effect of naloxone administration on cerebral DNA synthesis was observed. In the forebrain at 1 and 3 hrs after naloxone injection an increased rate of 3H-thymidine incorporation into DNA was found followed by a marked decrease at 9 and 12 hrs. The effect in the hypothalamus was similar but the initial increase at 1 hr was absent. On cerebellar DNA synthesis naloxone had no effect. The administration of D-Met2-Pro5-enkephalinamide resulted in a marked reduction in the labelling of cerebral and hypothalamic DNA between 1 to 12 hrs. Except a decrease at 1 hr no effect was found in the cerebellum.

Differential postnatal development of mu and delta opiate receptors

We found a differential postnatal development of mu and delta opiate receptors. Mu receptors labelled with low concentrations of [3H]naloxone appeared to develop earlier than did delta receptors labelled with [3H]D-Ala2-D-Leu5-enkephalin (0.5 nM). Competition binding studies also revealed a delayed appearance of delta receptors (day 12 postnatal).

Postsynaptic inhibitory effects of Met- and Leu-enkephalin on endocrine and adjacent neurones in the preoptic-septal region of the guinea pig

Iontophoretic application of enkephalins induced inhibitory effects on unit activity of endocrine and adjacent neurones in the preoptic-septal region. Antagonism or lack of antagonism of these effects by naloxone indicated an action of enkephalins through different opiate receptors. Inhibitory effects of enkephalins were obtained during iontophoretic application of Mg2+, showing that these opioid peptides acted postsynaptically. Because enkephalin acts on endocrine neurones, these opioid peptides might be involved in the control of gonadotrophic hormone release by acting on cell bodies of LH-RH neurones.

Degradation of met-enkephalin by extracts of various regions of the human brain: effects of antipsychotics and narcotics in vitro

Antischizophrenic drugs, reduced in a concentration-dependent fashion enkephalin degradation by the soluble and particulate fractions of the human cerebral cortex and cerebellum. The order of potency is as follows: thioridazine greater than chlorpromazine greater than fluphenazine greater than haloperidol greater than or equal to promazine with IC50 of 50, 80, 120, 200-250 micro M, respectively. Kinetic studies revealed non-competitive and competitive inhibition by thioridazine and chlorpromizine, respectively. Narcotics, were weak inhibitors of enkephalin degradation. For dl-, d-, l-methadone and l-alpha-acetylmethadol, IC50 was about 500 micro M, and 1000 micro M for heroin and morphine. It is suggested that inhibition of the degradation of endogenous morphinomimetic peptides in the CNS may be a crucial factor governing the pharmacology of some neuroleptics and other psychoactive drugs. Enkephalin-hydrolyzing activity was ubiquitous and exhibited considerable regional differences in the normal human and in Huntington's chorea brains. The rate of enkephalin degradation is generally higher in the subcortical nuclei than in the cortex and cerebellum. The highest hydrolytic activity was found in the substantia nigra, anterior thalamus, septal area, globus pallidus and caudate nucleus, in this decreasing order.

Ontogeny of opiate receptors in rat forebrain: visualization by in vitro autoradiography

The embryonic and postnatal ontogeny of opiate receptors in rat telencephalon was mapped by in vitro autoradiographic localization of [3H]naloxone and [3H]enkephalin binding. Opiate receptors marked by naloxone binding first appear at embryonic day 14 in the striatum, rapidly proliferate to adult densities and at the time of birth, gradually become reorganized into the adult heterogeneous pattern of receptor-rich patches surrounded by sparse, diffuse labeling in the rest of the striatum. The enkephalin binding in the striatum appears later in embryonic development and gradually increases in density to form the rather homogeneous adult pattern. Naloxone binding in the paleocortical olfactory areas appears early also, densely within the molecular layer as soon as it is formed at E16. This density is only temporary, as labeling just after birth falls to low adult levels in all areas except portions of the amygdala. Receptors disappear also in the islands of Calleja and the pallidum. Naloxone binding in the septum and neocortex appears gradually in development. The early appearance of striatal and paleocortical [3H]naloxone-labeled opiate receptors and their localization within the subependymal zones suggest that receptors appear on immature neurons before and during migration and, therefore, may influence the intricate patterns of connections that later form. The delayed appearance of the [3H]enkephalin-labeled receptors may reflect the dependence of the peptide binding on later developing molecules of adenylate cyclase.

Opiate receptor ontogeny and morphine-induced effects: influence of chronic footshock stress in preweanling rats

The ontogeny of opiate receptors was examined in various CNS regions of preweanling rats which received either daily inescapable footshock stress, exposure to a footshock apparatus without shock, or no handling from birth to 21 days of age. At 21 days of age, each of these treatment groups was also assessed for morphine-induced changes in activity, hot-plate paw-lick latency, and core body temperature. Marked regional differences in [3H]naloxone binding capacity were observed from 7 to 21 days of age in spinal cord, medulla-pons, midbrain, hypothalamus, striatum, and cortex. Caudal regions approached adult-like [3H]naloxone binding before rostral regions. The normal ontogeny of opiate receptors was not significantly influenced by the chronic footshock treatment. However, footshock treatment significantly reduced the efficacy of morphine (2 mg/kg) in producing hypoactive and antinociceptive effects, but not in producing a hyperthermic effect. These results demonstrate that stress-related changes in the behavioral efficacy of morphine do not necessarily depend upon changes in those opiate receptor populations that bind naloxone.

Selective protection of methionine enkephalin released from brain slices by enkephalinase inhibition

Methionine enkephalin release was evoked by depolarization of slices from rat striatum with potassium. In the presence of 0.1 microM thiorphan [(N(R,S)-3-mercapto-2-benzylpropionyl)glycine], a potent inhibitor of enkephalin dipeptidyl carboxypeptidase (enkephalinase), the recovery of the pentapeptide in the incubation medium was increased by about 100 percent. A similar effect was observed with the dipeptide phenylalanylalanine, a selective although less potent enkephalinase inhibitor. Inhibition of other known enkephalin-hydrolyzing enzymes--aminopeptidase by 0.1 mM puromycin or angiotensin-converting enzyme by 1 microM captopril--did not significantly enhance the recovery of released methionine enkephalin. These data indicate that enkephalinase is critically involved in the inactivation of the endogenous opioid peptide released from striatal neurons.

The enkephalinase inhibitor thiorphan shows antinociceptive activity in mice

There is both theoretical and therapeutic interest in establishing whether the signals conveyed by the enkephalins are turned off under the action of a specific peptidase which might, in this case, represent a target for a new class of psychoactive agents. Enkephalinase, a dipeptidyl carboxypeptidase cleaving the Gly3-Phe4 bond of enkephalins and distinct fropm angiotensin coverting enzyme (ACE), might be selectively involved in enkephalinergic transmission. It is a membrane-bound enzyme whose localization in the vicinity of opiate receptors in the central nervous system is suggested by parallel regional and subcellular distributions as well as by the effects of lesions. Such a role is further supported by the ontogenetic development of enkephalinase, its substrate specificity accounting for the increased biological activity of several enkephalin analogues and its adaptive increase following chronic treatment with morphine. To investigate the functional role of this enzyme further, we have designed a potent and specific enkephalinase inhibitor. We report here that this compound, thiorphan [(DL-3-mercapto-2-benzylpropanoyl)-glycine; patent no. 8008601] protects the enkephalins from the action of enkephalinase in vitro in nanomolar concentration and in vivo after either intracerebroventricular or systemic administration. In addition, thiorphan itself displays antinociceptive activity which is blocked by naloxone, an antagonist of opiate receptors.