Histidine abolishes the inhibition by zinc of naloxone binding to opioid receptors in rat brain.

Zinc ions inhibited the specific binding of [3H]naloxone to opioid receptors in the hippocampus, cortex, midbrain and striatum of the rat in a dose-dependent manner. Scatchard analysis performed by using a concentration of zinc close to its IC50 (about 30 microM) revealed that inhibition was due to a decrease in receptor affinity, without change in the number of binding sites. Of several compounds tested, only histidine was capable of reversing the inhibition by zinc in these areas of the brain so preventing the zinc-induced increase in the KD of opioid receptors for naloxone. Histidine alone did not affect the KD or Bmax of opioid receptors for [3H] naloxone in the areas of brain studied. The fact that histidine prevented the zinc-induced increase in the KD but had no effect on receptor affinity or the number of binding sites for [3H] naloxone suggests that histidine exerts its effects by complexing with zinc ions and acting as a chelator. In addition to Zn2+, Cu2+, Cd2+ and Hg2+ inhibited the binding of [3H] naloxone to opioid receptors in the cerebral cortex of the rat in a dose-dependent manner. However, histidine was not capable of abolishing the inhibition to the same extent as that of zinc. Thus, among all of the metal ions studied, the most dramatic effect of histidine was observed with zinc ions.

Modulation of mu, delta and kappa opioid receptors in rat brain by metal ions and histidine.

The effect of zinc (Zn2+) and several other trace elements was studied on the binding of the opioid receptor agonists [3H] DAGO [( ([Tyr-D-Ala-Gly-Methyl-Phe-Glyol]-enkephalin)a, [3H] DSTLE ([Tyr-D-Ser-Gly-Phe-Leu-Thr]-enkephalin) and [3H] EKC (ethylketocyclazocine), which are specific for the mu, delta and kappa opioid receptors, respectively, in the cerebral cortex of the rat. Physiological concentrations of zinc were inhibitory to mu receptor binding, whereas the delta and kappa receptors were relatively insensitive to this inhibition. Scatchard analysis, using these opioid agonists, revealed curvilinear plots; concentrations of zinc equal to or less than the IC50 (the concentration of cation which caused 50% inhibition of the binding of opioid ligand to its receptor), increased the KD (the dissociation constant) of all three subtypes of receptor, with no effect on the Bmax (the maximum number of binding sites) and abolished the high affinity sites of the delta and kappa receptors. Copper, cadmium and mercury also inhibited the binding of these ligands to their receptors. Histidine was most effective in preventing the inhibitory effects of zinc and copper, whereas it was less effective on cadmium and without any effect on the inhibition caused by mercury. Magnesium and manganese were stimulatory to opioid receptor binding, whereas cobalt and nickel had dual (stimulatory and inhibitory) effects. Non-inhibitory concentrations of zinc significantly decreased the stimulatory effects of magnesium and manganese on the mu and delta receptors, suggesting that part of the effect of zinc was through prevention of the actions of stimulatory cations.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of mu, kappa and delta opioid receptors in the rat brain by isoflurane and enflurane.

This study was done to investigate whether inhalational anesthetics modulated the binding of specific ligands to opioid receptors in the brain of the rat. The effect of isoflurane and enflurane on the binding of specific ligands to various subtypes of opioid receptors in vitro was studied. Isoflurane inhibited the binding of [3H]naloxone to opioid receptors by 50% in the spinal cord, midbrain and cortex at 22, 49 and 50 mM, respectively. Enflurane was more potent than isoflurane in inhibiting the binding of [3H]naloxone. Scatchard analysis of the binding of [3H]naloxone, done in the presence of therapeutic level (5 mM) of isoflurane, suggested that it did not affect the KD (1.3 nM) but decreased the Bmax by 41% in the cortex. Isoflurane and enflurane, at large doses (30-50 mM), inhibited the binding of [3H]ethylketo-cyclazocine (EKC) to kappa receptors in midbrain, cortex and spinal cord. At a smaller dose (5 mM), they increased the binding of EKC in spinal cord. The binding of the analogs of enkephalin [3H]DSTLE(Tyr-D-Ser-Gly-Phe-Leu-Thr-enkephalin) to delta receptors and [3H]DAGO (Tyr-D-Ala-Gly-Methyl-Phe-Glyol-enkephalin) to mu receptors in the midbrain and cortex was inhibited by isoflurane at a significantly smaller concentration than the binding of [3H]naloxone, indicating that the binding of peptides was more susceptible to the inhibition by inhalational anesthetics than the binding of alkaloids, such as naloxone or EKC. These results suggest that the modulation of opioid receptors by inhalational anesthetics is a function of both the nature of the ligand and the tissue used for the receptor binding.(ABSTRACT TRUNCATED AT 250 WORDS)

GM1 ganglioside enhances cholinergic parameters in the brain of senescent rats.

GM1 ganglioside and nerve growth factor both promote the recovery of injured central cholinergic neurons in young animals. Brain cholinergic activity declines with aging and nerve growth factor has been shown to correct cholinergic deficits in senescent animals. We have administered GM1, to young (three months old) or senescent (22-24 months old) rats and evaluated acetylcholine and choline content, choline acetyltransferase and acetylcholinesterase activity as well as choline uptake in striatum, hippocampus and frontal cortex. For some studies, nerve growth factor was administered alone or together with GM1. Our results indicate that cholinergic neurochemical parameters are decreased in some brain areas of senescent animals and that both GM1 and nerve growth factor can enhance their recovery.

Effect of 7,12-dimethylbenz[a]anthracene-induced mammary carcinogenesis on the opioid peptide levels in the rat central nervous system.

Mammary tumors were induced in female Sprague-Dawley rats by giving a single oral dose of 20 mg 7,12-dimethylbenz[a]anthracene (DMBA). Animals were killed after full development of tumors 4 months after the ingestion of DMBA. Opioid peptides in various tissues were estimated by radioimmunoassay (RIA). Tumor-bearing rats (n = 5) had higher (P less than 0.05) contents of beta-endorphin in pituitary (+60%), striatum (+52%) and midbrain (+85%) compared to animals with no tumors. However, tumor-bearing rats showed a decrease of 35% in striatal met-enkephalin content. Dynorphin level decreased (P less than 0.05) in pituitary (-49%) and hypothalamus (-29%) of tumor-bearing rats. Thus for the first time, we report the alteration in the level of these neuropeptides during the process of chemical carcinogenesis.

Plasma nicotine, plasma beta-endorphin and mood states during periods of chronic smoking, abstinence and nicotine replacement.

Nicotine is known to release neuroendocrine substances which may subsequently reinforce smoking behavior by improving mood states. The purpose of this study was to examine changes in plasma beta-endorphin and mood states during periods of chronic smoking, abstinence from smoking, and abstinence while chewing nicotine gum. A modified A-B-A-C design was used. Normal male volunteers were randomly assigned to an experimental or control group. Over a 12-day protocol, experimental subjects smoked ad libitum for 2 days, were abstinent for 4 days, resumed smoking for 2 days, and then chewed nicotine gum for the final 4 days. Control subjects smoked ad libitum throughout the entire protocol. Results indicated that changes in plasma beta-endorphin levels were not related to changes in the four smoking conditions. Plasma nicotine and mood states were related, such that dysphoric moods increased during abstinence from smoking in comparison to the control group. To investigate further the relationships between nicotine, beta-endorphin and reinforcement for smoking, it may be necessary to characterize endogenous opioid peptide release in the central nervous system during smoking.

Naltrexone administration affects ad libitum smoking behavior.

Endogenous opioid peptides have been implicated in the reinforcement of smoking and opioid antagonists have been examined to determine their role in smoking behavior. To date, the relationship between smoking behavior and chronic opiate antagonist administration during ad libitum smoking has not been investigated. The purpose of this study was to examine the relationships between naltrexone, an opiate antagonist administered orally, and smoking behavior and mood states during ad libitum smoking. A repeated measures experimental design was used. Normal adult male and female volunteers, admitted to the Clinical Research Center, were randomly assigned to naltrexone-treated (n = 22) or placebo-control (n = 21) groups in a double-blind manner. Day 1 was considered acclimation to the unit and day 2 was baseline, or pre-drug administration. On days 3, 4, and 5, subjects received 50 mg naltrexone or a placebo at 0700 and 1600 hours. Plasma nicotine and expired air carbon monoxide levels were measured daily at 1900 hours. Number of cigarettes smoked, mood states, withdrawal symptomatology and self-reported satisfaction with smoking were also quantified daily. Results indicated that plasma nicotine levels (P = 0.005), number of cigarettes smoked daily (P = 0.003) and self-reported satisfaction with smoking (P = 0.043) were significantly lower among those treated with naltrexone, compared to the placebo-control group. Expired air carbon monoxide levels did not differ between the two groups. In addition, mood states and withdrawal symptoms did not differ between groups. These findings suggest that endogenous opioid peptides influence specific smoking behavior variables.

Decreased neuropeptide content in the spinal cord of aged rats: the effect of GM1 ganglioside.

This study investigated the status of substance P (SP), methionine-enkephalin (Met-Enk) and dynorphin A(1-13) (Dyn A) in the spinal cord of aged Sprague-Dawley rats and the effect of GM1 ganglioside on these neuropeptides. SP and Met-Enk, but not Dyn A, were decreased in both dorsal and ventral horns of the aged spinal cord. Treatment with GM1 ganglioside (30 mg/kg i.p., daily for 30 days) restored, in part, the neuropeptide deficits in the ventral horns, but not in the dorsal horns. This information might be important for understanding the sensory and motor deficits associated with ageing, and how the spinal cord neuropeptides might be amplified in the aged spinal cord.

Effect of oral contraceptives on the rat brain and pituitary opioid peptides.

This study was designed to explore the hormonal regulation of CNS opioid peptide levels in female Sprague Dawley rats. Forty-eight animals were divided into 2 equal groups for acute and chronic studies. Each group was further divided into 4 subgroups, each containing 6 animals. Each rat in the control group received an inert pill (in 0.25 ml corn oil daily by gavage); the second group, 15 micrograms norethindrone (NE, a potent progestin present in the oral contraceptive Micronor); the third group, 15 micrograms NE and 1 microgram ethinyl estradiol, EE2 (present in the oral contraceptive Modicon) and the fourth group, 10 times the dose of the third group. Rats were treated either acutely for 5 days or chronically for 7 weeks. Opioid peptides were estimated by radioimmunoassay. Acute administration of 150 micrograms NE + 10 micrograms EE2 decreased the levels of methionine-enkephalin (ME), leucine-enkephalin (LE), dynorphin (DYN) and beta-endorphin like immunoreactivity (beta-EI) by about 50% in the pituitary. The same dose on chronic administration also decreased DYN, but increased the levels of ME and LE in the pituitary by 331 and 69%, respectively. In the hypothalamus, chronic administration of NE + EE2 increased the level of ME (155%) and LE (87%) as well as of DYN (97%). In the striatum, the levels of LE (33%) and DYN (115%) were elevated during chronic administration. It is concluded that the acute administration of NE + EE2, in general, reduces the levels of ME, LE, DYN and beta-EI.(ABSTRACT TRUNCATED AT 250 WORDS)

The neurotoxic actions of ibotenic acid on cholinergic and opioid peptidergic systems in the central nervous system of the rat.

The neurotoxic effects produced by ibotenic acid (IA) induced chemical lesions of the central nervous system (CNS) cholinergic system were examined on the opioid peptidergic system in adult rats. Forebrain cholinergic systems were bilaterally lesioned by the infusion of IA (1 or 5 micrograms/site) into the nucleus basalis magnocellularis (NBM). One week after the injections, the animals were sacrificed, and activities of acetylcholinesterase (AChE), choline acetyltransferase (ChAT) and concentrations of beta-endorphin (beta-End) and Met-enkephalin (Met-Enk) were measured in different brain regions. Animals treated with IA showed a decrease in the activity of ChAT (-24%), AChE (-36%) and beta-End level (-33%) in the frontoparietal cortex (FC). For the first time we report that these changes were associated with a compensatory increase in the activity of ChAT (+27%), AChE (+25%), beta-End level (+66%) in the remaining part of the cortex, i.e. cortex devoid of frontal cortex (C-FC). Met-enkephalin level increased by 59% in the frontoparietal cortex and did not change in the cortex devoid of frontal cortex upon IA treatment. These results suggest that IA treatment results in changes in the activity of cortical ChAT and AChE, and beta-End level in the same direction. Injection of IA in the NBM did not cause a change in the activity of ChAT or AChE in other brain regions such as hippocampus, striatum or midbrain. In addition to cortex devoid of frontal cortex, midbrain also showed a significant increase in the beta-End level in the IA treated animals. However, pituitary beta-End decreased in the neurotoxin treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of chronic treatment with morphine, midazolam and both together on dynorphin(1-13) levels in the rat.

We have recently reported that midazolam, a benzodiazepine receptor agonist that is also a short acting anesthetic and analgesic drug, can produce analgesia and decrease morphine tolerance and dependence in the rat by interacting with the opioidergic system. This study was designed to investigate the chronic effect of midazolam and/or morphine on the levels of dynorphin(1-13) in the pituitary gland, different brain regions, spinal cord and peripheral tissues of the rat. Four sets of animals were used: (I) saline-saline; (II) midazolam (0.03, 0.3 or 3.0 mg/kg, body wt., i.p.)-saline; (III) saline-morphine (10.0 mg/kg, body wt., s.c.); and (IV) midazolam-morphine (0.03, 0.3 or 3.0 mg/kg midazolam + 10.0 mg/kg morphine) groups. The first saline or midazolam injection was given i.p. and after 30 min, the second injection of saline or morphine was given s.c. daily for 11 days. Animals were sacrificed on the 11th day, 60 min after the last injection and dynorphin(1-13) was measured in indicated tissues by radioimmunoassay method. The midazolam treated animals showed a significant decrease in dynorphin(1-13) levels in the cortex, cerebellum, cervical region of spinal cord, heart and adrenals, and a significant increase in the hypothalamus, striatum and lumbar region of the spinal cord. The morphine treated animals showed a significant decrease in dynorphin(1-13) levels in the pituitary gland, hypothalamus, hippocampus, striatum, cerebellum, pons, medulla, kidneys, adrenals and spleen, and a significant increase only in the lumbar region of the spinal cord. When both drugs were injected together there was no effect on pituitary gland, kidneys and spleen. These drugs antagonize each other's effect on dynorphin(1-13) in the hypothalamus, striatum, cerebellum, pons, medulla and heart. However, the midazolam-morphine combination significantly increases dynorphin(1-13) levels in the hippocampus, cortex, midbrain, cervical and lumbar regions of the spinal cord, and adrenals. These results suggest the involvement of dynorphin(1-13) in the inhibition of morphine-induced tolerance and dependence by midazolam in the rat. These results may also help us in understanding the intrinsic mechanisms involved in narcotic tolerance and dependence.

Met-enkephalin alteration in the rat during chronic injection of morphine and/or midazolam.

We have recently reported that the short-acting anesthetic and analgesic drug midazolam can produce analgesia and decrease morphine tolerance and dependence in the rat by interacting with the opioid system. This study was designed to investigate the effect of midazolam, morphine, and both together on met-enkephalin levels in the rat. Male Sprague-Dawley rats were divided into four groups: (1) saline-saline; (2) saline-morphine; (3) midazolam-saline, and (4) midazolam-morphine groups. First, a saline or midazolam injection was given intraperitoneally and after 30 min a second injection of saline or morphine was given subcutaneously once daily for 11 days. Animals were sacrificed on the 11th day 60 min after the last injection to measure met-enkephalin by radioimmunoassay. Morphine tolerant animals showed a significant increase in met-enkephalin levels in the cortex (137%) and midbrain (89%), and a significant decrease in met-enkephalin levels in the pituitary (74%), cerebellum (34%) and medulla (72%). Midazolam treated animals showed a significant decrease in met-enkephalin levels in the pituitary (63%), cortex (39%), medulla (58%), kidneys (36%), heart (36%) and adrenals (43%), and a significant increase in met-enkephalin levels in the striatum (54%) and pons (51%). When morphine and midazolam were injected together, midazolam antagonized the increase in met-enkephalin levels in cortex and midbrain region and the decrease in met-enkephalin level in the medulla region observed in morphine tolerant animals. These results indicate that morphine tolerance and dependence is associated with changes in the concentration of met-enkephalin in the brain. Midazolam may inhibit morphine tolerance and dependence by reversing some of the changes induced in met-enkephalin levels in brain by morphine in morphine tolerant and dependent animals.

Inhibition of morphine tolerance and dependence by diazepam and its relation to the CNS Met-enkephalin levels.

The effect of diazepam on the development of morphine tolerance and dependence was investigated. Male Sprague-Dawley rats were rendered tolerant and dependent by subcutaneous implantation of six morphine pellets. Diazepam (0.025, 0.25 or 2.5 mg/kg body weight) was once daily injected intraperitoneally into rats starting on the first day of implantation. Antinociception was measured by tail-flick (TF) and hot plate (HP) tests, and the extent of sedation determined by a rotarod test before and one hour after diazepam injections everyday for 5 days. Physical dependence on morphine was assessed by an antagonist-precipitated abstinence syndrome on the fifth day of treatment by injecting naloxone 10 mg/kg subcutaneously. Diazepam (0.025-2.5 mg/kg body weight) did not produce significant antinociception or sedation (sensorimotor impairment) in rats implanted with placebo pellets. Diazepam (0.25 and 2.5 mg/kg) inhibited tolerance to TF antinociception in rats implanted with morphine pellets. Sedation as evidenced by sensorimotor impairment induced by morphine pellet implantation was not influenced by diazepam (0.025-2.5 mg/kg). Diazepam administration (0.25 mg/kg) also decreased the degree of jumping behavior observed following naloxone injection in morphine pellet implanted rats. Serum morphine concentration in morphine-diazepam treated rats was not significantly different from that in morphine-saline treated rats. Finally, a decrease in the Met-enkephalin levels observed in the hypothalamus, hippocampus, cortex and spinal cord of morphine dependent rats was reversed by injecting diazepam along with morphine pellet implantation. These results suggest that diazepam inhibits morphine tolerance and dependence, and also prevents morphine-induced decrease in the CNS Met-enkephalin levels in morphine dependent rats.

Opioid peptides in pituitary gland, brain regions and peripheral tissues of spontaneously hypertensive and Wistar-Kyoto normotensive rats.

The concentrations of beta-endorphin (beta-END), dynorphin (DYN) and methionine-enkephalin (MEK) in pituitary, brain regions, heart, kidney and adrenal of 8 week old male spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive rats were determined by radioimmunoassay and compared. The brain regions examined were hypothalamus, striatum, pons + medulla, midbrain and cortex. The concentration of beta-END in pituitary of SHR rats was 49% higher than those of WKY rats. The concentration of beta-END in the striatum of SHR rats was 71% lower as compared to WKY rats. The concentration of beta-END in the heart, adrenals and kidney of SHR rats was significantly lower (92, 48 and 57%, respectively), than those of WKY rat tissues. The concentration of DYN in pituitary, striatum and heart were lower by 38, 55 and 46%, respectively, in SHR compared to WKY rats, but in hypothalamus it was greater (33%) than in WKY rats. The concentration of DYN in other brain areas and in kidney and adrenal did not differ. The tissues of SHR and WKY rats which showed significant difference in the concentration of MEK were pituitary, pons + medulla, cerebral cortex and adrenals. The concentration of MEK was greater in SHR rats with pons + medulla, cortex and adrenals showing 33, 40, 268% higher levels, respectively, over the WKY rat tissues. However, the concentration of MEK in pituitary of SHR rats was 40% lower than that of WKY rats. These studies suggest that the endogenous opioid peptides of both central and peripheral tissues may be important in the regulation of blood pressure in SHR rats.

Beta-endorphin and methionine-enkephalin levels in discrete brain regions, spinal cord, pituitary gland and plasma of morphine tolerant-dependent and abstinent rats.

The effect of morphine tolerance-dependence, protracted and naloxone-precipitated abstinence on the levels of beta-endorphin and methionine-enkephalin in discrete brain regions, spinal cord, pituitary gland and plasma was determined in the male Sprague-Dawley rats. Among the brain regions examined, the levels of beta-endorphin in descending order were: hypothalamus, amygdala, midbrain, hippocampus corpus striatum, pons and medulla and cortex. The levels of beta-endorphin in midbrain, hypothalamus, and pituitary of morphine tolerant-dependent rats were decreased significantly. During protracted withdrawal beta-endorphin levels were decreased in amygdala, spinal cord and pituitary. During naloxone-precipitated abstinence beta-endorphin levels were increased in corpus striatum, midbrain and cortex. In addition, in naloxone-precipitated abstinence beta-endorphin levels were decreased in pituitary gland and hippocampus but increased in plasma. The levels of methionine-enkephalin in brain regions in decreasing order were: corpus striatum, pons and medulla, amygdala, hypothalamus, midbrain, hippocampus and cortex. The levels of methionine-enkephalin in pons and medulla, amygdala, hippocampus and pituitary gland were decreased in morphine tolerant-dependent rats. During protracted abstinence from morphine, methionine-enkephalin levels in spinal cord, amygdala, pons and medulla, midbrain, cortex, corpus striatum and pituitary gland were decreased. The levels of methionine-enkephalin in hypothalamus and corpus striatum of naloxone-precipitated abstinent rats were increased but were decreased in amygdala and pituitary gland. These results suggest that during morphine tolerance-dependence and during protracted abstinence beta-endorphin and methionine-enkephalin levels in discrete brain regions and pituitary gland are decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of morphine tolerance and dependence by diazepam and its relation to mu-opioid receptors in the rat brain and spinal cord.

We have recently observed that concomitant administration of diazepam to morphine pellet implanted rats results in the inhibition of the development of morphine tolerance and dependence. We have now analyzed mu-opioid receptors in rats treated with morphine and diazepam for 5 days by using [3H]-DAMGO for binding studies. Male Sprague-Dawley rats were made tolerant and dependent by subcutaneous (s.c.) implantation of six morphine pellets (two pellets on the first day, and four on the second day). Diazepam (0.25 mg/kg b.wt) was injected once daily intraperitoneally (i.p.) for 5 days. Control rats were implanted with placebo pellets and injected once daily with saline or diazepam (i.p.). Animals were administered s.c. naloxone (10 mg/kg) to induce naloxone-precipitated withdrawal syndrome on the final day of the experiment (day 5). There was an up-regulation of mu-receptor (Bmax increased) in the spinal cord of morphine tolerant (+139%) and dependent (+155%) rats compared to saline treated animals. Diazepam treatment abolished the up-regulation of mu-receptors in spinal cord of morphine treated rats. In the cortex, Bmax was not affected in morphine tolerant or dependent rats but it decreased by 38% in morphine tolerant and 65% in morphine dependent rats treated with diazepam. The Kd of mu-receptors increased in the cortex, striatum and hypothalamus of morphine dependent rats. Diazepam treatment decreased the Kd of mu-receptors in the cortex of morphine tolerant and hypothalamus of morphine-dependent rats. These results suggest that diazepam treatment antagonizes the up-regulation of CNS mu-receptors observed in morphine tolerant rats. In addition, morphine tolerance and dependence may be associated with conversion of mu-opioid receptors to mu-constitutive opioid receptors that are less active, and this conversion is prevented in the brain of animals treated with diazepam.

Inhibition of morphine tolerance and dependence by diazepam and its relation to cyclic AMP levels in discrete rat brain regions and spinal cord.

Diazepam inhibits morphine tolerance and dependence and reverses a decrease in the met-enkephalin level in brain induced by morphine. In this study, we investigated whether inhibition of morphine-induced tolerance and dependence by diazepam involved a change in cyclic AMP levels in discrete rat brain regions and spinal cord. Male Sprague-Dawley rats were made tolerant and dependent by subcutaneous (s.c.) implantation of six morphine pellets (two pellets on the first day, and four on the second day). Diazepam (0.25 mg/kg b. wt) was injected once daily intraperitoneally (i.p.) for 5 days. Control rats were implanted with placebo pellets and injected once daily with saline or diazepam (i.p.). Tail-flick antinociception was measured 1 h after injections everyday. Animals were administered s.c. naloxone (10 mg/kg) to induce naloxone-precipitated withdrawal syndrome on the final day of the experiment (day 5), and the jumping behavior was observed for 30 min. Concomitant treatment with diazepam (0.25 mg/kg) significantly decreased the development of morphine tolerance and dependence. Diazepam (0.25 mg/kg) treated rats also showed a significant decrease in the jumping behavior compared to animals treated with morphine alone. Rats were sacrificed 2 h after the injection of saline or diazepam (0.25 mg/kg) on the fifth day. Cyclic AMP was estimated by RIA. In the control rats, the concentration of cyclic AMP in cortex was > hippocampus > cerebellum > hypothalamus > striatum > midbrain > pituitary > pons/medulla > spinal cord. There was no change in the concentration of cyclic AMP in any of the brain regions examined from morphine tolerant animals.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of U-50,488H tolerance-dependence and abstinence on the levels of dynorphin (1-13) in brain regions, spinal cord, pituitary gland and peripheral tissues of the rat.

Male Sprague-Dawley rats were rendered tolerant to and physically dependent on U-50,488H, a kappa-opiate agonist, by injecting 25 mg/kg of the drug intraperitoneally twice a day for 4 days. Two sets of rats were used. Rats labeled as tolerant-dependent were injected with U-50,488H (25 mg/kg) 1 h before sacrificing on day 5, whereas the abstinent rats were sacrificed on day 5 without the injection of U-50,488H. Of all the tissues on day 5 without the injection of U-50,488H. Of all the tissues examined, the pituitary gland had the highest level of dynorphin (1-13), whereas the heart had the lowest level. The levels of dynorphin (1-13) increased in the hypothalamus, hippocampus and pons/medulla of U-50,488H tolerant-dependent rats, whereas in abstinent rats the levels of dynorphin (1-13) were elevated only in the midbrain. The levels of dynorphin (1-13) in the pituitary gland of U-50,488H tolerant-dependent or abstinent rats were unchanged. In peripheral tissues, the levels of dynorphin (1-13) in the heart of U-50,488H tolerant-dependent rats were increased. In the abstinent rats they were elevated in the adrenals, spleen, and the heart but were decreased in the kidneys. Compared to morphine tolerant-dependent and abstinent rats, significant differences in the levels of dynorphin (1-13) in tissues of 50,488H tolerant-dependent and abstinent rats were observed and may explain many pharmacological differences in the mu- and kappa-opiate induced tolerance-dependence and abstinence processes.

The effect of chronic administration of nicotine on antinociception, opioid receptor binding and met-enkelphalin levels in rats.

The effect of chronic nicotine administration on (1) antinociception; (2) opioid receptor binding; and (3) met-enkelphalin levels in discrete brain regions in rats was investigated. Male and female Sprague-Dawley rats were treated with nicotine 0.3 mg/kg, 0.1 mg/kg, or saline three times a day subcutaneously during a 14-day protocol. Antinociception was measured by hotplate (HP) test on days 1, 2, 7, 10 and 14. After completion of the protocol, mu-opioid receptors were analyzed by [3H]-DAMGO binding studies and met-enkelphalin levels were determined by radioimmunoassay. Results indicated that hot-plate latency increased during the first 2 days of nicotine administration for male and female rats who were treated with 0.3 mg/kg nicotine. There was an up-regulation of mu-receptors (increased Bmax) in the striatum of rats treated with 0.3 mg/kg nicotine, compared to 0. 1 mg/kg nicotine and saline groups. An interaction effect of group by gender was noted. After 14 days of chronic nicotine administration, met-enkelphalin levels were significantly lower in striatum and midbrain of animals treated with 0.3 mg/kg nicotine, as compared to controls. These results suggest that chronic nicotine administration, in doses representative of human smoking, produces antinociception initially, and is accompanied by an upregulation of micro-opioid receptors in the striatum of rats. In addition, nicotine-induced tolerance to antinociception may be associated with a decrease in met-enkelphalin level over a period of time.

The effect of morphine tolerance dependence and abstinence on immunoreactive dynorphin (1-13) levels in discrete brain regions, spinal cord, pituitary gland and peripheral tissues of the rat.

The effect of morphine tolerance dependence and protracted abstinence on the levels of dynorphin (1-13) in discrete brain regions, spinal cord, pituitary gland and peripheral tissues was determined in male Sprague-Dawley rats. Of all the tissues examined, the highest level of dynorphin (1-13) was found to be in the pituitary gland. Among the brain regions and spinal cord examined, the levels of dynorphin (1-13) in descending order were: hypothalamus, spinal cord, midbrain, pons and medulla, hippocampus, cortex, amygdala and striatum. The descending order for the levels of dynorphin (1-13) in peripheral tissues was: adrenals, heart and kidneys. In morphine tolerant rats, the levels of dynorphin (1-13) increased in amygdala but were decreased in pons and medulla. In morphine abstinent rats, the levels of dynorphin (1-13) were increased in amygdala, hypothalamus and hippocampus. The levels of dynorphin (1-13) were increased in pituitary but decreased in spinal cord and remained so even during protracted abstinence. The levels of dynorphin (1-13) in the peripheral tissues of morphine tolerant rats were unaffected. However, in the heart and kidneys of morphine abstinent rats, the levels of dynorphin (1-13) were increased significantly. It is concluded that both morphine tolerance and abstinence modify the levels of dynorphin (1-13) in pituitary, central and peripheral tissues. Morphine abstinence differed from non-abstinence process in that there were additional changes (increases) in the levels of dynorphin (1-13) in brain regions (hypothalamus and hippocampus) and peripheral tissues (heart and kidneys) and may contribute to the symptoms of the morphine abstinence syndrome.(ABSTRACT TRUNCATED AT 250 WORDS)