Phelorphan, an inhibitor of enzymes involved in the biodegradation of enkephalins, affected the withdrawal symptoms in chronic morphine-dependent rats

Acute morphine alters GABAergic transmission in the central amygdala during naloxone-precipitated morphine withdrawal: role of cyclic AMP

The central amygdala (CeA) plays an important role in opioid addiction. Therefore, we examined the effects of naloxone-precipitated morphine withdrawal (WD) on GABAergic transmission in rat CeA neurons using whole-cell recordings with naloxone in the bath. The basal frequency of miniature inhibitory postsynaptic currents (mIPSCs) increased in CeA neurons from WD compared to placebo rats. Acute morphine (10 μ M) had mixed effects (≥20% change from baseline) on mIPSCs in placebo and WD rats. In most CeA neurons (64%) from placebo rats, morphine significantly decreased mIPSC frequency and amplitude. In 32% of placebo neurons, morphine significantly increased mIPSC amplitudes but had no effect on mIPSC frequency. In WD rats, acute morphine significantly increased mIPSC frequency but had no effect on mIPSC amplitude in 41% of CeA neurons. In 45% of cells, acute morphine significantly decreased mIPSC frequency and amplitude. Pre-treatment with the cyclic AMP inhibitor (R)-adenosine, cyclic 3',5'-(hydrogenphosphorothioate) triethylammonium (RP), prevented acute morphine-induced potentiation of mIPSCs. Pre-treatment of slices with the Gi/o G-protein subunit inhibitor pertussis toxin (PTX) did not prevent the acute morphine-induced enhancement or inhibition of mIPSCs. PTX and RP decreased basal mIPSC frequencies and amplitudes only in WD rats. The results suggest that inhibition of GABAergic transmission in the CeA by acute morphine is mediated by PTX-insensitive mechanisms, although PTX-sensitive mechanisms cannot be ruled out for non-morphine responsive cells; by contrast, potentiation of GABAergic transmission is mediated by activated cAMP signaling that also mediates the increased basal GABAergic transmission in WD rats. Our data indicate that during the acute phase of WD, the CeA opioid and GABAergic systems undergo neuroadaptative changes conditioned by a previous chronic morphine exposure and dependence.

Transgene-mediated enkephalin expression attenuates signs of naloxone-precipitated morphine withdrawal in rats with neuropathic pain

Chronic morphine exposure induces physical dependence and tolerance. Previous studies have shown that there is a decrease in met-enkephalin levels in states of morphine physical dependence, and that increasing enkephalin during opiate physical withdrawal ameliorates the severity of the morphine withdrawal syndrome. In order to investigate the role of spinal opioid peptide in the phenomenon of naloxone-precipitated withdrawal we examined the effect of herpes simplex virus vector-mediated overexpression of proenkephalin in lumbar dorsal root ganglia in rats with neuropathic pain treated with morphine. The morphine physical dependence was induced by chronic administration of intraperitoneal (IP) morphine for 2 weeks. Rats with neuropathic pain inoculated subcutaneously with the vector-mediated overexpression of proenkephalin showed a significant reduction in jumps, 'wet-dog' shakes, diarrhea and ptosis precipitated by naloxone after 2 weeks of morphine treatment. The global withdrawal score was also reduced significantly by vector-mediated overexpression of proenkephalin. These studies demonstrate a role for opioid peptide in the spinal cord in mediating some of the withdrawal response.

Identification of bikunin as an endogenous inhibitor of dynorphin convertase in human cerebrospinal fluid

Dynorphin-converting enzymes constitute a group of peptidases capable of converting dynorphins to enkephalins. Through the action of these enzymes, the dynorphin-related peptides bind to delta-opioid instead of kappa-opioid receptors, leading to a change in the biological function of the neuropeptides. In this article, we describe the identification of the protein bikunin as an endogenous, competitive inhibitor of a dynorphin-converting enzyme in human cerebrospinal fluid. This protein is present together with its target enzyme in the same body fluids. The K(M) value of the convertase was found to be 9 microm, and the K(i) value of the inhibitor was 1.7 nm. The finding indicates that bikunin may play a significant role as a regulatory mechanism of neuropeptides, where one bioactive peptide is converted to a shorter sequence, which in turn, can affect the action of its longer form.

Aminopeptidase activity in the postmortem brain of human heroin addicts

Several studies have reported that the chronic administration of opioids induces changes in the biosynthesis of endogenous opioid peptides or their precursors in specific brain regions of the adult central nervous system. However, little is known about the catabolic regulation of opioid peptides and its contribution to neuroadaptative changes underlying drug addiction. In the present study, we have analyzed the activity of two enkephalin-degrading enzymes (puromycin-sensitive aminopeptidase or PSA and aminopeptidase N or APN) and two functionally different, soluble aminopeptidases (aminopeptidase B and aspartyl-aminopeptidase) in postmortem samples of prefrontal cortex and caudate nucleus of eight human heroin addict brains and eight matched-controls. Enzyme activities were fluorimetrically measured using beta-naphthylamide derivatives. An increase in the activity of soluble PSA in the prefrontal cortex of heroin abusers was observed (heroin addict group: 51,452+/-3892 UAP/mg protein versus control group: 42,003+/-2597 UAP/mg protein; P<0.05), while the activity of the other peptidases in both brain regions remained unaltered. This result agrees with previous findings in morphine-tolerant rats, and indicates that soluble PSA may be involved in neurobiological processes which underlie heroin addiction.

Ontogeny, regional and cellular distribution of the novel metalloprotease neprilysin 2 in the rat: a comparison with neprilysin and endothelin-converting enzyme-1

The localisation of the gene transcripts of a recently discovered peptidase, neprilysin 2 (NEP2), was established by in situ hybridisation in rat tissues during development and adulthood. It was compared with those of neprilysin (NEP), a closely related enzyme in terms of sequence homology or substrate specificity, and of endothelin-converting enzyme 1 (ECE-1) which, like the other two, belongs to the M-13 sub-family of zinc-dependent metallopeptidases. The ontogeny of the three enzymes differed markedly, the expression of NEP2 being restricted to developing and differentiating fields of the CNS, whereas NEP and ECE-1 genes were broadly expressed early on in the CNS and periphery. In contrast to the wide expression of NEP and ECE-1 in peripheral adult tissues and in CNS, NEP2 was almost exclusively expressed in selected neuronal populations of the brain and spinal cord. The only exceptions were the intermediate and anterior lobes of the pituitary as well as the choroid plexuses, where NEP2 was also strongly expressed. These localisations as well as those in the hypothalamic nuclei, together with the previously established pattern of cleaved peptides, suggest the involvement of NEP2 in the metabolism of neurohormones of the hypothalamo-pituitary axis.Complementary distributions of NEP and NEP2 mRNAs were observed in a large number of brain areas with, for instance the former being highly expressed in the striatum in which NEP2 transcripts were almost undetectable. In contrast, NEP2 was highly expressed in numerous thalamic, hypothalamic and brainstem nuclei from which NEP was absent. Since both peptidases are able to cleave the same neuropeptides, this pattern may suggest a complementary role in their peptide inactivation functions in the CNS. Finally, ECE-1 mRNAs were generally observed in neuronal populations known to express the pre-proendothelin-1 gene, confirming the function of the metallopeptidase in endothelin-1 generation.

Enkephalinergic neurons in the periaqueductal gray and morphine withdrawal

The effects of opioid (e.g., morphine) withdrawal on levels of endogenous opioid peptides and their mRNA in the various brain regions have been studied. However, the role of this opioidergic mechanism in the mediation of opioid withdrawal is not fully understood. Preproenkephalin (PPE) mRNA in the caudal periaqueductal gray (cPAG), an important brain region in opioid withdrawal, is increased by both opioid antagonist (naloxone)-precipitated and spontaneous morphine withdrawal, but not by various other stresses in rats, indicating a role of endogenous enkephalins in the cPAG in morphine withdrawal. In addition, PPE mRNA levels in the cPAG increase in the course of the dissipation of morphine withdrawal, and they are returned to the control levels after disappearance of morphine withdrawal signs. Local administration of an enkephalin analog or peptidase inhibitors into the cPAG suppresses morphine withdrawal signs. These facts suggest that enkephalinergic neurons in the PAG may have a critical role in the recovery phase of morphine withdrawal. Recently, an involvement of transcription factors in morphine withdrawal has been suggested. Thus, the possible role of transcription factors in the regulation of PPE gene expression in the cPAG during morphine withdrawal is also discussed.

Detailed immunoautoradiographic mapping of enkephalinase (EC 3.4.24.11) in rat central nervous system: comparison with enkephalins and substance P

The metallopeptidase enkephalinase known to participate in the inactivation of endogenous enkephalins and, possibly, other neuropeptides such as tachykinins, was visualized by autoradiography using a [125I]iodinated monoclonal antibody. A detailed mapping of the enzyme in rat brain and spinal cord was established on 10-micron serial sections prepared in a frontal plane as well as a few sections in a sagittal plane. On adjacent sections, and for the purpose of comparison, substance P-like and enkephalin-like immunoreactivities were also visualized by autoradiography using a 125I-monoclonal antibody and a polyclonal antibody detected by a secondary 125I-anti-rabbit antibody respectively. Histological structures were identified on adjacent Nissl-stained sections. Using the highly sensitive 125I-probe, enkephalinase immunoreactivity was found to be distributed in a markedly heterogeneous manner in all areas of the central nervous system. Immunoreactivity was undetectable in white matter areas, for example the corpus callosum or fornix, and had a laminar pattern in, for example, the cerebral cortex or hippocampal formation. Hence, although immunodetection was not performed at the cellular level, a major neuronal localization of the peptidase is suggested. The latter is consistent with the detection of a strong immunoreactivity in a pathway linking the striatum to the globus pallidum, the entopeduncular nucleus and the substantia nigra, as well as with a series of biochemical and lesion data. The strong immunoreactivity also present in choroid plexuses and ependymal cells as well as in the intermediate lobe and in scattered cells of the anterior lobe of the pituitary suggests that populations of glial and endocrine cells also express the peptidase. The highest density of enkephalinase immunoreactivity was observed in basal ganglia and limbic areas (caudate putamen, globus pallidus, nucleus accumbens, olfactory tubercles) as well as in areas involved in pain control mechanisms (superficial layers of the spinal nucleus of the trigeminal nerve or of the dorsal horn of the spinal cord) which also display the highest immunoreactivities for both enkephalins and substance P (except in globus pallidus for the latter). These localizations account for the opioid-like analgesic and motor effects of enkephalinase inhibitors inasmuch as a selective or predominant participation of the peptidase in enkephalin inactivation is assumed. A number of other areas appear richly endowed in both enkephalinase and enkephalins whereas substance P is hardly detectable. This is particularly the case for the olfactory bulb, bed nucleus of the accessory olfactory tract, the cerebellum (where enkephalinase mainly occurs in the molecular layer) and the hippocampal formation (namely in the molecular layer of the dentate gyrus).(ABSTRACT TRUNCATED AT 400 WORDS)

Endogenous opiates: 1987

This paper is the tenth installment of our annual review of the research during the past year involving the endogenous opiate system. It covers the nonanalgesia and behavioral studies of the opiate peptides published in 1987. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal activity; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.