Morphine addiction and withdrawal alters brain peptide concentrations

The pharmacology of neurokinin receptors in addiction: prospects for therapy

Addiction is a chronic disorder in which consumption of a substance or a habitual behavior becomes compulsive and often recurrent, despite adverse consequences. Substance p (SP) is an undecapeptide and was the first neuropeptide of the neurokinin family to be discovered. The subsequent decades of research after its discovery implicated SP and its neurokinin relatives as neurotransmitters involved in the modulation of the reward pathway. Here, we review the neurokinin literature, giving a brief historical perspective of neurokinin pharmacology, localization in various brain regions involved in addictive behaviors, and the functional aspects of neurokinin pharmacology in relation to reward in preclinical models of addiction that have shaped the rational drug design of neurokinin antagonists that could translate into human research. Finally, we will cover the clinical investigations using neurokinin antagonists and discuss their potential as a therapy for drug abuse.

Altered morphine-induced analgesia in neurotensin type 1 receptor null mice

Both neurotensin (NT) and opioid agonists have been shown to induce antinociception in rodents after central administration. Besides, previous studies have revealed the existence of functional interactions between NT and opioid systems in the regulation of pain processing. We recently demonstrated that NTS1 receptors play a key role in the mediation of the analgesic effects of NT in long-lasting pain. In the present study, we therefore investigated whether NTS1 gene deletion affected the antinociceptive action of mu opioid drugs. To this end, pain behavioral responses to formalin were determined following systemic administration of morphine in both male and female NTS1 knockout mice. Acute injection of morphine (2 or 5 mg/kg) produced strong antinociceptive effects in both male and female wild-type littermates, with no significant sex differences. On the other hand, morphine analgesia was considerably reduced in NTS1-deficient mice of both sexes compared to their respective controls, indicating that the NTS1 receptor actively participates in mu opioid alleviating pain. By examining specifically the flinching, licking and biting nociceptive behaviors, we also showed that the functional crosstalk between NTS1 and mu opioid receptors influences the supraspinally-mediated behaviors. Interestingly, sexual dimorphic action of morphine-induced pain inhibition was found in NTS1 null mice in the formalin test, suggesting that the endogenous NT system interacts differently with the opioid network in male and female mice. Altogether, these results demonstrated that NTS1 receptor activation operates downstream to the opioidergic transmission and that NTS1-selective agonists combined with morphine may act synergistically to reduce persistent pain.

Characterizing intercellular signaling peptides in drug addiction

Intercellular signaling peptides (SPs) coordinate the activity of cells and influence organism behavior. SPs, a chemically and structurally diverse group of compounds responsible for transferring information between neurons, are broadly involved in neural plasticity, learning and memory, as well as in drug addiction phenomena. Historically, SP discovery and characterization has tracked advances in measurement capabilities. Today, a suite of analytical technologies is available to investigate individual SPs, as well as entire intercellular signaling complements, in samples ranging from individual cells to entire organisms. Immunochemistry and in situ hybridization are commonly used for following preselected SPs. Discovery-type investigations targeting the transcriptome and proteome are accomplished using high-throughput characterization technologies such as microarrays and mass spectrometry. By integrating directed approaches with discovery approaches, multiplatform studies fill critical gaps in our knowledge of drug-induced alterations in intercellular signaling. Throughout the past 35 years, the National Institute on Drug Abuse has made significant resources available to scientists that study the mechanisms of drug addiction. The roles of SPs in the addiction process are highlighted, as are the analytical approaches used to detect and characterize them.

Morphine upregulates functional expression of neurokinin-1 receptor in neurons

Neuronkinin-1 receptor (NK-1R), the neuropeptide substance P (SP) preferring receptor, is highly expressed in areas of the central nervous system (CNS) that are especially implicated in depression, anxiety, and stress. Repeated exposure to opioids may sensitize neuronal systems involved in stress response. We examined the effects of morphine, the principal metabolite of heroin, on the functional expression of NK-1R in the cortical neurons. NK-1R and mu-opioid receptor (MOR) are co-expressed in the cortical neurons. Morphine enhanced NK-1R expression in the cortical neurons at both the mRNA and protein levels. The upregulated NK-1R by morphine had functional activity, because morphine-treated cortical neurons had greater SP-induced Ca(2+) mobilization than untreated neurons. Blocking opioid receptors on the cortical neurons by naltrexone or CTAP (a mu-opioid receptor antagonist) abolished the morphine action. Investigation of the mechanism(s) responsible for the morphine action showed that morphine activated NK-1R promoter and induced the phosphorylation of p38 MAPK protein in the cortical neurons. These in vitro data provide a plausible cellular mechanism for opioid-mediated neurological disorders.

An in vitro model of morphine withdrawal manifests the enhancing effect on human immunodeficiency virus infection of human T lymphocytes through the induction of substance P

Opioid withdrawal is a crucial and recurring event during the course of opioid abuse that has a negative impact on the immune system. In this study, we investigated whether abrupt withdrawal (AW) or precipitated withdrawal (PW) potentiates human immunodeficiency virus (HIV) infection of human T lymphocytes. AW and PW enhanced HIV infection of peripheral blood lymphocytes and T-cell lines (Jurkat and CEMX174). In addition, both AW and PW induced HIV replication in a latently HIV-infected human T-cell line (J1.1). The enhancing effect of AW and PW was associated with the induction of neuropeptide substance P in both peripheral blood lymphocytes and the T-cell lines. The substance P receptor antagonist, CP-96,345, not only blocked AW- or PW-induced endogenous substance P expression but also abrogated AW- or PW-induced HIV replication in T cells. These findings provide a cellular mechanism that supports the notion that opioids have a co-factor role in promoting HIV infection of the immune cells.

A non-peptide substance P antagonist (CP-96,345) inhibits morphine-induced NF-kappa B promoter activation in human NT2-N neurons

Opioids and the neuropeptide substance P (SP) modulate the expression of inflammatory cytokines and chemokines, which are under the control of nuclear factor kappaB (NF-kappaB). We investigated whether the neurokinin-1 receptor (SP receptor) pathway is biologically involved in morphine-mediated modulation of NF-kappaB promoter activation in a human neuronal cell line (NT2-N) that expresses both the mu-opioid receptor (MOR) and the SP receptor. Morphine significantly enhanced NF-kappaB promoter-directed luciferase activity in NT2-N neurons. DAMGO, a selective mu-opioid receptor agonist, also induced NF-kappaB promoter activation. The induced activation of NF-kappaB promoter by morphine or DAMGO was abolished not only by naltrexone (a opioid receptor antagonist) and CTAP (a selective, competitive mu-opioid receptor antagonist), but also by CP-96,345, a non-peptide SP receptor antagonist. Investigation of the mechanism responsible for morphine-induced activation of NF-kappaB promoter in NT2-N neurons demonstrated that morphine activates the SP promoter and induces SP expression in these cells. We also observed that SP activated NF-kappaB promoter and that CP-96,345 downregulated the expression of endogenous SP. Furthermore, dual immunofluorescent labeling revealed that there is co-expression of NK-1R and MOR in the processes of NT-2N neurons. These results suggest that morphine, by activating MOR, engages a positive feedback loop between NK-1R and SP. Activation of NK-1R could then impact NF-kappaB expression and therefore may be an important participant in the effect of morphine on immune responses in the central nervous system.

Withdrawal from morphine in mice suppresses splenic macrophage function, cytokine production, and costimulatory molecules

We have previously shown that abstinence from morphine by either abrupt (AW) or precipitated (PW) withdrawal induces greater than 80% suppression in the capacity to mount an in vitro plaque-forming cell (PFC) response to sheep red blood cells at 24-h post withdrawal. Present studies on the mechanisms of immunosuppression showed that addition of normal unfractionated spleen cells, macrophage-enriched adherent cells, or CD11b(+) purified macrophages, to spleen cells taken from withdrawn mice, restored immune responses. Spleen cells from mice undergoing withdrawal also had decreased splenic mRNA and/or protein levels of IL-1beta, IL-1Ra, TNF-alpha, IL-12, and IFN-gamma. Addition of IL-1beta or IFN-gamma to AW cultures was able to reverse their immunosuppression. These results strongly suggest that morphine withdrawal results in a deficit of macrophage function.

The effects of chronic morphine treatment on neurotensin-induced antinociception

Previous studies have shown that the opioid antagonist naloxone does not alter neurotensin (NT)-induced antinociception. In the present studies, tolerance to morphine in mice significantly attenuated NT-induced antinociception, but not NT-induced hypothermia. In addition, centrally administered NT inhibited naloxone-precipitated jumping in morphine-dependent mice. These results indicate complex interactions between NT-induced antinociception and opioid systems.

Effect of chronic morphine administration on glucose tolerance and insulin binding to isolated rat adipocytes

Morphine addiction was induced in six male Wistar rats. Improved glucose tolerance (peak value less by 28%, p<0.01) was observed in chronically morphinized rats as compared to the control rats, injected with saline. An increase in the maximal specific binding of 125I-labeled insulin to unit membrane area of adipocytes was observed in the experimental group (p < 0.01). The changes in insulin receptor number could be responsible for the improved glucose tolerance observed during morphine addiction.

Morphine Up-regulates expression of substance P and its receptor in human blood mononuclear phagocytes and lymphocytes

In vitro and in vivo studies have indicated that there is an important relationship between morphine and neuropeptide substance P (SP). We therefore investigated the interaction of morphine and cultured human immune cells on the expression of SP, a neuropeptide which we have recently demonstrated to be produced by human monocytes and lymphocytes. Morphine up-regulated SP production in human mononuclear phagocytes and lymphocytes at both the mRNA and the protein level. In addition, morphine induced SP receptor (NK-1R) expression in human lymphocytes. The specific morphine receptor antagonist (naltrexone) blocked morphine-induced SP expression in human mononuclear phagocytes, supporting the concept of authentic morphine receptor-mediated regulation. Since SP modulates neurogenic inflammation and immunologic events, these data suggest that morphine-induced SP expression in cells of the immune system may be of importance in the pathogenesis of immune-mediated diseases, including neuroimmunologic diseases and AIDS.

The neurochemistry of morphine addiction in the neocortex

Different strategies have been used in an attempt to understand the neurobiology of opioid addiction. Here, Michéle Simonato initially discusses the identification of key anatomical areas involved in the phenomenon and purposes an explanation of opioid addiction based on the theory of complexity. The variable importance of direct and indirect effects in phenotypically different neuronal populations can imply differences in the adaptive changes that occur with chronic morphine exposure. Opioid addiction is therefore proposed as a complex multicellular event, where individual neurones differentially adapt both on the basis of the signals they receive and of their second messengers and genetic programmes.

Morphine withdrawal alters anterior pituitary hormone secretion, brain endopeptidase activity and brain monoamine metabolism in the rat

Rats were made tolerant to morphine by a 5-day regimen with increasing doses. The time course of changes in serum anterior pituitary hormone levels, brain endo- and exopeptidase activity, levels of brain biogenic amines and body weight were studied during abrupt morphine withdrawal. Cold stimulated secretion of thyrotropin and the secretion of growth hormone were both decreased whereas that of prolactin was increased. In the hypothalamus both prolyl endopeptidase and dipeptidyl peptidase IV activities were concomitantly increased. The hypothalamic 5 hydroxyindole acetic acid levels were also increased. Changes in hormone secretion, peptidase activity and monoamine turnover had returned to baseline levels by 92 hr. Our results indicate that morphine withdrawal and the associated stress produce alterations in anterior pituitary thyrotropin and growth hormone secretion. Concomitant increases in hypothalamic prolyl endopeptidase and dipeptidyl peptidase activities may contribute to these changes.

RP 67580, a selective antagonist of neurokinin-1 receptors, modifies some of the naloxone-precipitated morphine withdrawal signs in rats

In order to clarify the participation of substance P in the expression of opiate withdrawal, we have investigated the effects induced by the new selective neurokinin-1 antagonist RP 67580 on naloxone-induced morphine withdrawal syndrome in rats. Intracerebroventricular administration of RP 67580 elicited a decrease in 7 of the 13 withdrawal signs evaluated. Mastication, salivation and signs related to the motor component of withdrawal (jumping, rearing and locomotor activity) were particularly reduced. One sign, wet dog shakes, was increased, but it was also enhanced by the inactive enantiomer RP 68651. Our results indicate that blockade of NK1 receptors induces a decrease in the expression of naloxone-precipitated morphine abstinence in rats, and support the participation of substance P in the opiate withdrawal response.

Sub-chronic exposure to opiates in the rat: effects on brain levels of substance P and calcitonin gene-related peptide during dependence and withdrawal

Substance P (SP) and calcitonin gene-related peptide (CGRP) are putative transmitters in the central and peripheral (sensory) nervous systems. In this study, we examined the effects of dependence on and withdrawal from morphine and methadone on brain SP and CGRP content. Female Long Evans rats (70-100 g) were provided with plain drinking water or solutions containing opiate. No choice of drinking fluid was allowed. The maintenance level of each opiate (0.8 and 0.4 mg/ml for morphine and methadone, respectively) was continued for 4 days. Following an injection with naloxone (10 mg/kg i.p.) or saline, animals were decapitated 0, 20, or 60 min later and regional brain peptide content was measured by specific radioimmunoassays. SP and CGRP content in opiate-maintained and naive animals were similar following saline injection. However, following naloxone injection in morphine-maintained animals, SP content was elevated in the hypothalamus and midbrain at 20 min, but by 60 min was no longer distinguishable from basal (0 min) level. CGRP content was increased in the medulla oblongata and followed a comparable time course but, unlike SP, was not altered in the hypothalamus or midbrain. No alterations were observed in methadone-maintained animals. These results correlated with the peak of the behavioral morphine withdrawal syndrome and were consistent with the comparatively milder abstinence encountered in methadone medication.

Development of tolerance to the hormonal effects of morphine without changes in the aminergic functions in the brain of the rat

The effect of morphine on cold-stimulated secretion of TSH and prolactin was studied in male rats, both in acute studies and after the chronic administration of morphine for 14 days (twice a day with increasing doses). The duration of the stimulatory effect of a single dose of morphine on secretion of prolactin was shorter (less than 2 hr) than its inhibitory effect on cold-stimulated secretion of TSH (over 2 hr). In the rats pretreated with morphine, a tolerance to the depressant effect of TSH of the challenge dose of morphine was seen at 2 hr but not at 1 hr after the injection. In contrast, a tolerance to the stimulatory effect of morphine on prolactin was seen at 1 hr after the acute dose of morphine. The minor alterations of the hypothalamic amine neurotransmitters and their metabolites did not correlate with the hormonal responses or to the development of tolerance.

Cholecystokinin (CCK)-like material and CCK mRNA levels in the rat brain and spinal cord after acute or repeated morphine treatment

The effects of a single or repeated administrations of morphine on the tissue levels of cholecystokinin-like material (CCKLM) and pre pro cholecystokinin mRNA (CCK mRNA) were examined in various brain and spinal cord regions (cerebral cortex, cerebellum, hippocampus, septum, substantia nigra, lumbar enlargement) in adult rats using a specific radioimmunoassay and 'Northern blot' analysis, respectively. Although a clear parallelism existed between the regional distribution of CCKLM (septum greater than cerebral cortex greater than or equal to hippocampus much greater than lumbar enlargement, dorsal zone greater than substantia nigra greater than lumbar enlargement, ventral zone much much greater than cerebellum) and that of CCK mRNA, some mismatch was found notably in the septum where CCK mRNA levels were less than in other regions except the cerebellum. Neither CCKLM nor CCK mRNA levels were altered one hour after an acute administration of morphine (5 mg/kg i.p.). Similarly, morphine addiction after a four-day treatment with this drug was not associated with any change in the tissue levels of CCKLM and CCK mRNA. These data indicate that the previously reported modulatory action of opioids on central CCKergic systems could occur without affecting the preproCCK gene transcription and the tissue peptide concentrations.

Down-regulation of central receptors for thyrotropin-releasing hormone in kappa opiate agonist-induced abstinence in the rat

The effect of U-50,488H, a selective kappa opiate agonist, on tolerance-dependence and abstinence on the TRH receptors of the spinal cord and discrete regions of the brain of male Sprague-Dawley rats was determined. Rats were injected intraperitoneally twice daily with 25 mg/kg of U-50,488H for 4 days. Rats serving as controls were injected with the vehicle. On day 5, rats which were labeled as tolerant to U-50,488H were injected with U-50,488H (25 mg/kg) and sacrificed 1 hr later, whereas those labeled as abstinent were sacrificed without any injection. The above procedure has been previously shown to produce a high degree of tolerance to the analgesic and hypothermic effects of U-50,488H. The spinal cord and regions of the brain (hippocampus, cortex, midbrain, hypothalamus, corpus striatum, pons and medulla, and amygdala) were isolated for binding studies. The ligand [3H]MeTRH was used for TRH receptors. The binding constants, Bmax and Kd values, of [3H]MeTRH to bind to membranes prepared from various regions of the brain and spinal cord of rats tolerant-dependent on U-50,488H were unaffected. However, in rats abstinent to U-50,488H, the binding of [3H]MeTRH to membranes of the hypothalamus, and pons and medulla, was decreased. The decreased binding of [3H]MeTRH to hypothalamic membranes was due to changes in Bmax value, while in pons and medulla it was due to an increase in the Kd value.(ABSTRACT TRUNCATED AT 250 WORDS)

Intraspinal release of substance P and calcitonin gene-related peptide during opiate dependence and withdrawal

The antibody microprobe technique was used to study the release of immunoreactive substance P and immunoreactive calcitonin gene-related peptide within the lower lumbar spinal cord of anaesthetized spinalized cats pretreated twice daily for 3.5 days with increasing doses of morphine hydrochloride (2-20 mg/kg, i.p.). Both peptides were released in the region of the substantia gelatinosa during noxious cutaneous thermal stimulation or high-intensity electrical stimulation of a hind limb nerve. Intravenous administration of naloxone increased the nociceptive excitation of lumbar dorsal horn neurons, but did not alter the evoked release of immunoreactive substance P or immunoreactive calcitonin gene-related peptide in the superficial gray matter dorsal to these neurons. In addition, the release of both peptides was not significantly different to that detected under similar experimental conditions in opioid-naive cats. The results suggest that alterations in neuropeptide release from the central terminals of nociceptive primary afferent neurons do not occur during states of opiate dependence and withdrawal, and thus do not contribute to the characteristic signs of these phenomena in dependent animals.

Spinal cord thyrotropin releasing hormone receptors of morphine tolerant-dependent and abstinent rats

The effect of chronic administration of morphine and its withdrawal on the binding of 3H-[3-MeHis2]thyrotropin releasing hormone (3H-MeTRH) to membranes of the spinal cord of the rat was determined. Male Sprague-Dawley rats were implanted with either 6 placebo or 6 morphine pellets (each containing 75-mg morphine base) during a 7-day period. Two sets of animals were used. In one, the pellets were left intact at the time of sacrificing (tolerant-dependent) and in the other, the pellets were removed 16 hours prior to sacrificing (abstinent rats). In placebo-pellet-implanted rats, 3H-MeTRH bound to the spinal cord membranes at a single high affinity binding site with a Bmax of 21.3 +/- 1.6 fmol/mg protein, and an apparent dissociation constant Kd of 4.7 +/- 0.8 nM. In morphine tolerant-dependent or abstinent rats, the binding constants of 3H-MeTRH to spinal cord membranes were unaffected. Previous studies from this laboratory indicate that TRH can inhibit morphine tolerance-dependence and abstinence processes without modifying brain TRH receptors. Together with the present results, it appears that the inhibitory effect of TRH on morphine tolerance-dependence and abstinence is probably not mediated via central TRH receptors but may be due to its interaction with other neurotransmitter systems.

The binding of 3H-(3-MeHis2) thyrotropin releasing hormone to brain and pituitary membranes of morphine tolerant-dependent and abstinent rats

The effect of chronic administration of morphine and subsequent withdrawal on brain and pituitary receptors for thyrotropin releasing hormone (TRH) was investigated in Sprague-Dawley rats. The rats were implanted subcutaneously with four morphine pellets (each containing 75 mg of morphine free base) during a 3-day period. Placebo pellets, which contained all the excipients of morphine pellets except the morphine, were implanted in rats which served as controls. Both tolerance and physical dependence on morphine have been shown to develop as a result of this procedure. TRH receptors were labeled with 3H-(3-MeHis2) TRH (MeTRH). 3H-MeTRH bound to brain membranes at a single high affinity site with Bmax (receptor density) value of 24.6 +/- 2.2 fmol/mg protein and Kd (apparent dissociation constant) value of 3.7 +/- 0.4 nM. The binding of 3H-MeTRH to five regions of the brain namely, hypothalamus, cortex, striatum, midbrain and pons + medulla, as well as pituitary was also investigated. The binding of 3H-MeTRH to pituitary membranes was increased during the development of tolerance, whereas the binding to membranes prepared from different brain regions was unaffected. Serum concentration of triiodothyronine (T3) and thyroxine (T4) were found to be lower in chronic morphine-treated rats when compared to placebo-treated rats, however, serum TSH level remained unaltered. Twenty-four hours after the removal of morphine pellets (natural withdrawal), the binding of 3H-MeTRH to pons + medulla membranes was greater than in placebo control group. Naloxone-precipitated withdrawal produced results which were qualitatively similar to those obtained in rats from which pellets had been removed. The results suggest that the development of tolerance to morphine may be associated with changes in the pituitary-thyroid axis.

Hyperglycemic suppression of morphine withdrawal signs in the rat

Male Sprague-Dawley rats maintained under controlled lighting and temperature conditions were used in this experiment. Morphine dependency was induced by giving increasing doses of morphine by intraperitoneal injection (IP group) or by the ingestion of morphine through drinking water (PO group). Animals were injected with 10, 20, 30 and 50 mg/kg morphine sulfate at days 1, 2, 3 and 4, respectively. Another group of animals received increasing concentrations of morphine through drinking water from 0.1, 0.2, 0.3 to 0.4 mg/ml at 48 h intervals. Morphine dependent animals were given naloxone by the intraperitoneal route to precipitate withdrawal. Glucose (3 g/kg or 10 g/kg) was given 10 min prior to the administration of naloxone to the respective groups. Another two groups of animals were made diabetic by the administration of streptozotocin. In one group, animals received increasing concentrations of 10, 20, 30 and 50 mg/kg morphine sulfate by the IP route at days 1, 2, 3 and 4, while the other group was not treated with morphine but was assessed for withdrawal signs to serve as the control. Withdrawal signs were assessed by observing the presence of diarrhea, tremor, piloerection, hunchbacked posture, teeth chattering, salivation, erection, restless activity, territorial exploring, irritability to handling, vocalization and jumping. Results obtained indicate that glucose administration at 10 g/kg abolished most of the withdrawal signs, and we were unable to induce the same degree of morphine dependency in diabetic animals as compared to the non-diabetic groups. It was concluded from this study that hyperglycemia could suppress morphine withdrawal signs.

Changes in anterior pituitary hormone secretion and hypothalamic catecholamine metabolism during morphine withdrawal in the female rat

Studies were undertaken to evaluate the acute responses of hypothalamic noradrenergic and dopaminergic neurons and anterior pituitary hormones to naloxone (NAL)-precipitated morphine (MOR) withdrawal in the rat. Ovariectomized female rats were rendered MOR-dependent and injected with NAL (1 mg/kg b.w., s.c.). During precipitated MOR withdrawal, a decline in norepinephrine (NE) concentrations was preceded by an increase in the level of its metabolite normetanephrine (NME) in the medial basal hypothalamus (MBH) as well as the preoptic area-anterior hypothalamus (POA-AH). Both dopamine (DA) and its major acid metabolite, dihydroxyphenylacetic acid (DOPAC), showed increased concentrations in these two hypothalamic regions within 30 min of NAL administration. Elevated luteinizing hormone (LH) and beta-endorphin secretion was evident within 5 min of NAL injection to MOR-dependent rats, while serum prolactin (PRL) increased 15 min into MOR withdrawal. Both growth hormone (GH) and thyroid-stimulating hormone (TSH) were depressed over the course of MOR withdrawal. Although a cause and effect relationship cannot be established, during NAL-precipitated MOR withdrawal, a heightened hypothalamic monoaminergic neuronal activity is accompanied by a differential response of anterior pituitary hormones. The observed responses, which are similar to those seen during acute stress, indicate that MOR withdrawal may activate the same mechanisms which mediate the neuroendocrine response to stress.

Short-term effects of ovariectomy: the opioid control of LH secretion in fertile climacteric and postmenopausal women

The aim of this study was to evaluate the activity of opiate receptors involved in the regulation of LH secretion in relationship to ovariectomy. Menstruating fertile (n = 5) and climacteric (n = 7) patients and postmenopausal (n = 5) women who underwent therapeutical bilateral ovariectomy were studied in the first week postsurgery and LH plasma levels were evaluated after naloxone (4 mg in bolus plus 4 mg infusion/90 min), LHRH (10 micrograms + 10 micrograms iv) and saline administration. Two groups of fertile (n = 6) and postmenopausal (n = 6) subjects were studied as controls. Since the LH responsiveness to naloxone was impaired in climacteric patients after ovariectomy, the test was repeated in 5 of them after 1 and 6 months of estrogen-gestagen treatment (conjugated estradiol + noretisterone acetate), showing a significant increase in all patients in both cases. In four subjects treated with only gestagen, naloxone was still unable to significantly modify LH plasma levels. These results indicate that ovariectomy affects the activity of opiate receptors, resulting in the first week postsurgery LH rise inversely related to basal LH levels. Furthermore, these results indicate that one or six cycles of estrogen-gestagen treatment in ovariectomized patients similarly induces a restoration of the opiate receptors neuroendocrine activity.

Comparative effects of thyrotropin releasing hormone, MK-771 and DN-1417 on morphine abstinence syndrome

The effects of thyrotropin releasing hormone (TRH) were compared with two of its analogs, L-N-(2-oxopiperidine-6-yl-carbonyl)-L-histidyl-L-thiazolidine-4-carbo xam ide (MK-771) and gamma-butyrolactone-4-carboxyl-histidyl-prolineamide (DN-1417) on the abrupt and naloxone-precipitated abstinence symptoms in morphine-dependent male Swiss-Wester mice. Mice were made physically dependent on morphine by subcutaneous implantation for 3 days of a pellet containing 75 mg morphine free base. Control mice were implanted with placebo pellets. Intracerebral administration of TRH (10 ng-10 micrograms per mouse) immediately after removal of placebo pellets had no effect on the basal temperature of mice. Mice implanted with morphine pellets exhibited a characteristic hypothermic response following the removal of the pellets. TRH at all doses employed prevented the hypothermia observed during abrupt withdrawal of morphine (pellet removal). DN-1417 and MK-771 (10 ng-10 micrograms per mouse) on the other hand produced a short lived hyperthermic response in mice from which placebo pellets had been removed. However, both TRH analogs produced long-lasting antagonism of withdrawal hypothermia in mice from which morphine pellets had been removed. TRH and its analogs had no effect on the body weight loss observed during abrupt withdrawal of morphine. Intracerebral administration of 10 micrograms TRH and its analogs inhibited the naloxone-induced jumping response as evidenced by increases in naloxone ED50 values to elicit this response. It is concluded that TRH and its analogs may be useful in combating some of the withdrawal symptoms in opiate-dependent subjects.

Substance P levels in various regions of the rat central nervous system after acute and chronic morphine treatment

Substance P levels were measured in various CNS regions from rats treated acutely and chronically with morphine. There was no observable effect in the group treated with an acute dose of morphine (10 mg/kg) and sacrificed after 2 h. After 35 days chronic treatment with increasing doses of the drug, the rats were divided into three groups and sacrificed 2 h, 24 h and 7 days after the last injection. The substance P level was increased in the corpus striatum 2 h and 24 h and in the medulla oblongata and dorsal part of the spinal cord 2 h after withdrawal. Seven days after the last injection the levels had returned to normal in these areas. No effects were observed in the cerebral cortex, the hypothalamus or the ventral spinal cord at any time of measurement. Earlier studies have demonstrated that morphine inhibits release of substance P. The observed increase in tissue levels after long-term treatment is therefore interpreted as an accumulation of substance P in the neurons.

The role of the endogenous opiates in zinc deficiency anorexia

Anorexia is a major symptom of zinc deficiency, but the mechanism(s) for this anorexia are poorly defined. Recent studies have suggested an integral role for endogenous opiate peptides in appetite regulation. Dynorphin, a leucine-enkephalin containing opiate peptide, is a potent inducer of spontaneous feeding. In this study we showed that zinc deficient animals were relatively resistant to dynorphin-induced feeding. Measurement of dynorphin levels using a highly sensitive radioimmunoassay showed that zinc deficient animals had lower levels of dynorphin in the hypothalamus than did ad lib fed animals, with weight restricted animals having intermediate values. [3H]-naloxone binding was significantly increased to isolated brain membranes from zinc deficient animals using 1 nM unlabeled naloxone when compared to ad lib fed controls with the weight restricted animals again having intermediate values. These data suggest that abnormalities in endogenous opiate regulation of appetite may well play a role in the anorexia of zinc deficiency. The effects of zinc deficiency on endogenous opiate action appear to include alterations in receptor affinity, a post-receptor defect and alterations in the synthesis and/or release of dynorphin.

Evidence for the neuropeptide cholecystokinin as an antagonist of opiate analgesia

The endogenous neuropeptide cholecystokinin, when administered systemically or perispinally, potently antagonizes opiate analgesia produced by foot shock and morphine. Nonopiate foot-shock analgesia is not reduced by this neuropeptide. The spinal cord appears to be a critical site of cholecystokinin action. These experiments suggest a physiological role for cholecystokinin as a specific opiate antagonist in analgesia-mediating systems. A similar mode of action may explain other behavioral effects of cholecystokinin, such as suppression of food intake.

Interactions of thyrotropin releasing hormone, its metabolites and analogues with endogenous and exogenous opiates

The interactions of thyrotropin releasing hormone, its metabolites and synthetic analogues with acute and chronic effects of endogenous and exogenous opiates have been described. The endogenous and exogenous opiates are represented by beta-endorphin and morphine, respectively. The pharmacological effects of opiates include analgesia, temperature effects, respiratory depression, catalepsy, locomotor activity, opiate receptor binding, tolerance, and physical dependence. Thyrotropin releasing hormone and related compounds appear to (a) antagonize hypothermia, respiratory depression, locomotor depression and catalepsy but not the analgesia induced by opiates, (b) inhibit the development of tolerance to the analgesic effect but not to the hypothermic effect of opiates, (c) inhibit the development of physical dependence on opiates as evidenced by the inhibition of development of certain withdrawal symptoms, and (d) suppress the abstinence syndrome in opiate dependent rodents. Thyrotropin releasing hormone does not interact with the opiate receptors in the brain. Potential therapeutic applications of thyrotropin releasing hormone and its synthetic analogues in counteracting some of the undesirable effects of opiates are discussed.

Neuroendocrine effects of endogenous opioid peptides in human subjects: a review

In the decade since the discovery of specific opioid receptors in the brain, there have been rapid advances in our understanding of the physiological and pathological roles of the endogenous opioid systems in humans. Endogenous opioid peptides have been demonstrated to play a role as modulators of a number of hormonal functions in humans. In particular they appear to inhibit luteinizing hormone and ACTH release, and the response of arginine vasopressin to osmotic stimuli. They appear to participate in the modulation of carbohydrate homeostasis. In pathophysiological states, they appear to play a role in the decreased pulsatile luteinizing hormone release seen in patients with prolactinomas. Circulating beta-endorphin appears to be an important regulator of immune function. Preliminary studies in humans have suggested a role for endogenous opioid peptides in appetite regulation. In the last few years, a few case reports have suggested the possibility of a series of syndromes due to endogenous opioid excess. Within the next decade, we can expect to see the routine use of opioid antagonists in a variety of pathophysiological states.

The effects of stress on central nervous system concentrations of the opioid peptide, dynorphin

Dynorphin is an opioid peptide distributed throughout the central nervous system. Using a highly specific and sensitive radioimmunoassay for dynorphin we have examined the effect of stress on ir-dynorphin levels in the cortex and hypothalamus of the rat. Stresses related to food ingestion, i.e. starvation (72 hr), mild tail-pinch and insulin (10 U/kg) induced hypoglycemia all produced alterations in ir-dynorphin levels in the cortex. In contrast, restraint stress and 10-minute swim stress produced no changes in ir-dynorphin levels in either the hypothalamus or the cortex. Two hour exposure at 4 degrees C resulted in a fall in ir-dynorphin levels in the hypothalamus. Taken together with previously reported pharmacological effects of dynorphin-(1-13), these results suggest a possible physiological role for dynorphin in appetite and temperature regulation.

Effects of morphine dependence, withdrawal and tolerance on prolactin and growth hormone secretion in the rat

The effects of morphine dependence and withdrawal on prolactin (Prl) and growth hormone (GH) secretion were examined in the rat. Morphine dependence, induced by morphine pellet implantation, had no effect on nonstress concentrations of plasma Prl or GH, but it potentiated the response of Prl secretion to the stress associated with blood collection + injection of saline. Naloxone-induced withdrawal had no demonstrable effect on the changes in Prl and GH secretion produced by stress. In addition, signs of tolerance to both the Prl- and GH -stimulating effects of morphine injection were observed in morphine-dependent rats.

Levels of Immunoreactive dynorphin in brain and pituitary of hyperthyroid and hypothyroid rats

Dynorphin is a potent opioid peptide. Using a highly sensitive radioimmunoassay we have measured the levels of immunoreactive (ir) dynorphin in the cortex, hypothalamus, anterior and posterior pituitary in hyperthyroid and hypothyroid rats. Ir dynorphin levels were increased in the cortex of hypothyroid animals and decreased in the hypothalamus of hyperthyroid rats.

Flavour modulates the antidipsogenic effect of substance P

Substance P reduces water intake in a dose-dependent manner. In this report we show that the antidipsogenic effect of substance P is markedly reduced when sucrose (2%), saccharine (0.2%) or quinine (0.1%) are added to the water. Addition of saline (0.8%) has no effect on the antidipsogenic effect of substance P. These findings indicate that flavour modulates the antidipsogenic effect of parenterally administered substance P.

Evidence for an endorphin dysfunction in methadone addicts: lack of ACTH response to naloxone

Chronic exogenous opiate administration might be responsible for the acute and protracted abstinence syndrome by producing a prolonged decrease in the availability of endogenous opioids (endorphins). However, the hypothesis that potent exogenous opiates may have anti-endorphin effects has been difficult to test. We have been investigating this hypothesis with neuroendocrine test paradigms which have provided preliminary evidence of anti-endorphin effects for chronic methadone. Naloxone-induced ACTH response data from chronic methadone addicts offers preliminary support for the hypothesis that chronic exogenous opiate administration has anti-endorphin effects. The subjects were 7 male methadone addicts who had been addicted to greater than or equal to 40 mg of methadone and 7 male healthy opiate-naive volunteers. Naloxone failed to produce a significant increase in ACTH in methadone addicts while opiate-naive normal volunteers demonstrated a significant naloxone-induced release of ACTH. Five of the seven methadone addicts ahd no demonstrable ACTH response to naloxone. These impaired naloxone response data reported here for recently detoxified addicts suggest that chronic methadone administration comprises the functional integrity of the endorphin system. Prolonged abstinence, post-detoxification depression and other affective symptoms which contribute to relapse may result from a prolonged endorphin derangement.

The endocrinology of the opiates and opioid peptides

Since the isolation of the enkephalins five yr ago, there has been an explosive increase in knowledge concerning the effects of the opiates and opioid peptides. This review deals with the interactions of opiates with the endocrine system in rat and man. The opioid peptides have been demonstrated to exert a variety of effects on pituitary hormone secretion in rat and man. In the rat, opiates stimulate growth hormone, prolactin and ACTH release and inhibit the release of the glycoprotein hormones. In man, the physiologic role of the endogenous opiates appears to be involved predominantly in ACTH and gonadotrophin regulation. Opiate effects are mainly exerted at the level of the hypothalamus but further modulating effects may occur at the pituitary and at end-organs. Opiate-induced hormonal effects appear to be mediated through dopaminergic and/or serotonergic mechanisms. Recent studies have also suggested a possible local neuromodulatory role for the opioid peptides in the control of carbohydrate metabolism and reproductive processes.

Endogenous opiates: 1980

Vast amounts of research have been done that have attempted to delineate the pharmacological and physiological effects of the endogenous opiate peptides. A great deal of knowledge has also been accumulated in a limited time span concerning the types and locations of the opiate receptors and peptides, as well as their functions. In 1980, reports were made concerning the effects of these peptides on analgesia, on tolerance and dependence, on activity, on learning and memory, on schizophrenia and other types of emotional disturbances, and on physiological responses such as eating and drinking, cardiovascular responses, and sexual function. Additional understanding was also gained concerning their interactions with neurotransmitters, other neuropeptides, and hormones. These and other studies published only in 1980 are reviewed in this paper, which is the third of an annual series.

Substance P suppresses stress-induced eating

Parenterally administered substance P suppressed stress-induced eating (resulting from mild tail pinch) in a dose-related manner, whereas at similar or higher doses of substance P starvation-induced eating was uneffected. This specific effect of substance P on stress-induced eating is possibly associated with alterations of exogenous substance P of the normal modulation of pain transmission.