Peptide inhibition of morphine-induced dopaminergic supersensitivity

Inhibition of muscimol on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

This study was performed to investigate the effect of muscimol on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity in mice. A single administration of morphine induced hyperactivity as measured in mice, and the morphine-induced hyperactivity was inhibited dose-dependently by the administration of the GABA(A) agonist, muscimol (0.3, 0.5 and 1.0 mg kg(-1) i.p.). However, daily repeated administration of morphine caused the development of reverse tolerance against morphine hyperactivity (10 mg kg(-1) s.c.). The administration of muscimol inhibited the development of reverse tolerance against morphine hyperactivity (10 mg kg(-1) s.c.) in mice that had received chronic administration of morphine. Postsynaptic dopamine receptor supersensitivity, as shown by the enhanced ambulatory activity after administration of apomorphine (2 mg kg(-1) s.c.), also developed in reverse-tolerant mice. Muscimol also inhibited the development of postsynaptic dopamine receptor supersensitivity induced by the chronic administration of morphine. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine can be inhibited via the activation of GABA(A) receptors.

Inhibition of THIP on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

This study was performed to investigate the effect of tetrahydroisoxazolopyridine (THIP), a GABAA agonist, on the morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity in mice. A single administration of morphine induced hyperactivity in mice. However, the morphine-induced hyperactivity was inhibited dose-dependently by the administration of THIP (0.2, 0.4 and 0.8 mg/kg, i.p.). In contrast, daily administration of morphine resulted in a reverse tolerance to the hyperactivity caused by morphine (10 mg/kg, s.c.). THIP inhibited the development of reverse tolerance in the mice that had received the repeated same morphine (10 mg/kg, s.c.) doses. The postsynaptic dopamine receptor super-sensitivity, which was evidenced by the enhanced ambulatory activity after the administration of apomorphine (2 mg/kg, s.c.), also developed in the reverse tolerant mice. THIP also inhibited the development of the postsynaptic dopamine receptor supersensitivity induced by the chronic morphine administration. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine can be inhibited activating the GABAA receptors.

Inhibition of diazepam on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

The effects of diazepam on the development of reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine were examined in mice. A single administration of morphine induced hyperactivity and the morphine-induced hyperactivity was inhibited dose-dependently by the administration of diazepam (1, 2 and 4 mg x kg(-1), i.p.), an agonist for benzodiazepine receptor linked to the GABA(A) receptor. Daily repeated administration of morphine developed reverse tolerance to the hyperactivity of morphine. The concomitant administration of diazepam inhibited the morphine-induced hyperactivity and the diazepam administration prior to and during the chronic administration of morphine in mice inhibited the development of reverse tolerance to the hyperactivity of morphine (10 mg x kg(-1), s.c.). Postsynaptic dopamine receptor supersensitivity was also developed in reverse tolerant mice that had received the same morphine. The development of postsynaptic dopamine receptor supersensitivity was evidenced by the enhanced ambulatory activity of apomorphine (2 mg x kg(-1), s.c.). Diazepam also inhibited the development of postsynaptic dopamine receptor supersensitivity induced by the chronic administration of morphine. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine may be modulated via the activation of the GABA(A) receptor induced by diazepam.

Inhibition of Baclofen on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

The effects of baclofen on the development of reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine were examined in mice. A single administration of morphine induced hyperactivity and the morphine-induced hyperactivity was inhibited dose dependently by the administration of a GABA(B)receptor agonist, baclofen (1.25, 2.5 and 5 mg kg(-1), i.p.). Daily repeated administration of morphine developed reverse tolerance to the hyperactivity of morphine. The concomitant administration of baclofen inhibited the morphine-induced hyperactivity and the baclofen administration prior to and during the chronic administration of morphine in mice inhibited the development of reverse tolerance to the hyperactivity of morphine (10 mg kg(-1), s.c.). Postsynaptic dopamine receptor supersensitivity was also developed in reverse-tolerant mice that had received the same morphine. The development of postsynaptic dopamine receptor supersensitivity was evidenced by the enhanced ambulatory activity of apomorphine (2 mg kg(-1), s.c.). Baclofen also inhibited the development of postsynaptic dopamine receptor supersensitivity induced by the chronic administration of morphine. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine may be modulated via the activation of GABA(B)receptors induced by baclofen.

Increased toxicity of methamphetamine in morphine-dependent mice

1. The effect of methamphetamine on morphine-dependent mice was investigated by calculating the LD50 (i.p.), measuring motor activity, anorectic actions, and body temperature. 2. Methamphetamine was more toxic in morphine-dependent mice (LD50 = 20.6 mg/kg) than in normal mice (LD50 = 43.2 mg/kg). 3. Methamphetamine-induced locomotor activity was greater in morphinized than in nonmorphinized mice at doses of 2.5 and 5 mg/kg i.p. 4. Methamphetamine also increased the body temperature of morphinized mice more than that of normal mice (P < 0.05). 5. These findings suggest that methamphetamine is more toxic in morphine-dependent than in nondependent mice.

Dopamine and opioid regulation of the memory retrieval recovery in mice

The reactivation effects of the delta-opioid receptor blockade and D2 dopamine receptor activation on the detention-induced memory deficit in mice were investigated, in order to study possible interactions between opioid and dopamine systems in memory retrieval. Animals were trained in a one-trial passive-avoidance task. Pretesting treatment with ICI 174,864 (1, 3 or 5 mg/kg, i.p.) or quinpirole (0.5, 1 or 2 mg/kg, i.p.) facilitated retrieval of memory trace in saline-pretreated mice. Pretraining injection of the dopamine autoreceptor agonist, (+)-3PPP (2 mg/kg), having no effect alone in learning, prevented the ability of ICI 174,864 to produce the memory-enhancing effect. It is suggested that the normal functioning of the dopamine system was critical for the facilitation of retrieval by delta-antagonist. Quinpirole-induced reactivátion of memory retrieval was enhanced by pretreatment with Leu-enkephalin (0.2 mg/kg), inducing increased retention. We discuss these results in the context of an important interactions between D2 dopamine and delta-opioid receptors.

Prevention and reversal of dopamine receptor supersensitivity by cyclo(leucyl-glycyl) (CLG): biphasic dose-response curves

Chronic administration (21 days) of haloperidol (HAL) (IP, 1.0 mg/kg/day) induced a behavioral supersensitivity (stereotypic sniffing) to dopamine (DA) agonists (apomorphine) and upregulation (increased Bmax for sulpiride-inhibitable [3H]spiroperidol binding) of striatal and limbic D2 DA receptors (DAr). Coadministration of cyclo(leucyl-glycyl) (CLG; 8mg/kg, SC; every third day, every other day, but not every day) with HAL attenuated the behavioral supersensitivity. D2-DAr binding assays showed 1) that CLG-induced changes in Bmax parallel these behavioral changes and 2) that the biphasic CLG dose-response curve may involve CLG failure at high cumulative doses to lower Bmax. CLG also reversed an already established D2 DAr supersensitivity/upregulation (i.e., when CLG was injected daily for four days after the withdrawal of HAL). CLG alone did not alter behavior or binding. CLG's ability to both prevent and reverse D2 DAr upregulation/supersensitivity in animal models suggests that CLG may be useful, within a therapeutic window, in clinical disorders that are thought to involve upregulated DAr (e.g., tardive dyskinesia, L-DOPA-induced dyskinesias, and schizophrenia).

Morphine-induced reciprocal alterations in G alpha s and opioid peptide mRNA levels in discrete brain regions

The mechanisms involved in the development of morphine tolerance and dependence are still unknown. Recently much attention has been directed toward the changes in post receptor events. Opiate receptors, like other hormone and neurotransmitter receptors, have been shown to mediate their effects through guanine nucleotide binding proteins (G-proteins). This, in turn, may cause alterations in intracellular events, one of which is transcription of specific genes. We investigated the changes in the levels of mRNA of proenkephalin (PPE) and prodynorphin (DYN) and the stimulatory G protein alpha subunit (G alpha s) in adult morphine tolerant rats. Chronic morphine treatment induced reciprocal alterations in the levels of opioid peptide mRNA and G alpha s mRNA in discrete brain regions. In striatum, PPE mRNA decreased by 49% (P < .01) and in hypothalamus, DYN mRNA showed a decrease of 21% (P < .01). In contrast, G alpha s mRNA increased 20% (P < .01) in striatum and 97% (P < .01), in hypothalamus. In hippocampus the changes were reversed: PPE mRNA increased (55%, P < .05) and G alpha s mRNA decreased (33%, P < .01). Frontal cortex exhibited a small decrease in PPE (11.5%, P < .05) without any change on G alpha s or DYN mRNA levels. These reciprocal alterations suggest an opposing mode of regulation of G alpha s and PPE/DYN gene expression in morphine tolerant animals.

The effects of cyclo(leucyl-glycyl) on nigrostriatal dopaminergic supersensitivity--inhibition of apomorphine-induced climbing

In a previous study we showed that cyclo(leu-gly) (CLG) prevents the behavioural supersensitivity induced in the mesolimbic dopamine (DA) tract (in mice) by chronic haloperidol (HAL). In the current study, we evaluated the effects of CLG on supersensitivity to DA agonists in the nigrostriatal DA tract induced by chronic HAL (1.0 mg/kg, i.p. x 21 days--Experiment 1) or by acute injection of a high dose of apomorphine (APO) (Experiment 2). In Experiment 1 CLG was given at doses of either (a) 0 mg/kg/day (b) 1 mg/kg every third day (30 minutes prior to HAL), (c) 1 mg/kg every day, or (d) 8 mg/kg every third day. In Experiment 2 the dose of CLG was 8 mg/kg, s.c., given 24h after APO. Co-administration of CLG with HAL attenuated the development of HAL-induced supersensitivity in both paradigms (b) and (c) above, although the attenuation was significantly greater in (c) compared to (b). This biphasic dose response (D-R) curve for CLG in Experiment 1 indicates that a therapeutic window exists for CLG (bell-shaped D-R curve) and is similar to previous curves for CLG effects on the mesolimbic DA tract. In Experiment 2, CLG attenuated the DA receptor supersensitivity caused by acute high dose APO. The capacity of CLG to down-regulate DA receptors and attenuate dopaminergic supersensitivity in these experiments suggests a potential therapeutic use in the prevention of tardive and/or L-dopa-induced dyskinesias.

Modification of morphine-induced analgesia, tolerance and dependence by bromocriptine

The effect of two doses of bromocriptine, a dopamine agonist, on morphine-induced analgesia, tolerance and dependence was investigated in mice. Bromocriptine at doses of 0.04 and 0.08 mg/kg did not affect the baseline tail flick latency of mice but potentiated the morphine analgesia. Pretreatment of mice with 5 mg/kg of sulpiride, a D-2 antagonist, not only blocked the effect of 0.08 mg/kg of bromocriptine but also antagonized the morphine analgesia. Control animals given daily injections of 10 mg/kg of morphine rapidly developed tolerance to the analgesic effect. A combined treatment of bromocriptine with morphine given daily suppressed the development of tolerance to morphine analgesia. However, development of tolerance to morphine analgesia was not significantly modified in the animals treated daily with bromocriptine (0.08 mg/kg) plus sulpiride (5 mg/kg). Acute dependence was induced by the administration of 100 mg/kg of morphine. The administration of bromocriptine 30 min before naloxone significantly decreased the ED50 value for naloxone for inducing jumping in mice. Coadministration of sulpiride and bromocriptine attenuated the ability of bromocriptine to potentiate the withdrawal syndrome of morphine dependence. The results indicate that bromocriptine potentiates morphine analgesia, suppresses the development of tolerance to morphine analgesia but exacerbates opiate withdrawal signs in morphine-dependent mice. These effects of bromocriptine appear to be mediated via D-2 receptors.

MIF-1 attenuates spiroperidol alteration of striatal dopamine D2 receptor ontogeny

Long-term postnatal treatment of rats with the dopamine D2 receptor antagonist, spiroperidol, results in the impaired development of striatal D2 receptors. Because the tripeptide prolyl-leucyl-glycinamide (MIF-1) attenuates haloperidol-induced up-regulation of striatal dopamine D2 receptors in adult rats, we studied the effect of MIF-1 on the spiroperidol-induced alteration of striatal D2 ontogeny. Postnatal treatment of rats with spiroperidol (1.0 mg/kg/day, IP, x32 days from birth) resulted in a 74% decrease in the Bmax for [3H]spiroperidol binding with no change in the Kd at 5 weeks. When rats were studied at 8 weeks, in the absence of additional treatment, total specific [3H]spiroperidol binding was reduced by 59%. While MIF-1 alone (1.0 mg/kg/day, IP, x32 days from birth) had no effect on [3H]spiroperidol binding, MIF-1 completely attenuated the ontogenic impairment of striatal D2 receptors that was produced by spiroperidol treatment. At 5 weeks the Bmax for [3H]spiroperidol binding was at the saline control level in the group of rats cotreated with spiroperidol and MIF-1. At 8 weeks, with no additional treatments, the specific binding of [3H]spiroperidol to striatum was also at control levels in the group cotreated with spiroperidol and MIF-1. These findings demonstrate that MIF-1 attenuates spiroperidol-induced impairment of development of striatal dopamine D2 receptors in rats.

Dopamine receptor subsensitivity in the substantia nigra after chronic morphine treatment in rats

Several classes of 5-HT and dopamine (DA) receptor binding sites, and the levels of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), DA and dihydroxyphenylacetic acid (DOPAC) were examined in various brain regions 24 h after a 10-day treatment with morphine (2 X 15 mg/kg s.c. daily) in adult rats. Quantitative autoradiography of receptor binding sites revealed only a significant reduction of [3H]SCH-23390 and [3H]spiperone binding to D-1 and D-2 sites, respectively, in the substantia nigra pars compacta of morphine-treated rats. 5-HT and 5-HIAA levels remained unchanged in the substantia nigra and striatum, but the nigral levels of DA and DOPAC and the levels of DOPAC in the striatum were reduced significantly by morphine treatment. Apomorphine, at a dose (0.05 mg/kg s.c.) known to stimulate DA autoreceptors, decreased nigral and striatal DOPAC levels in controls but not in morphine-treated rats. It is concluded that chronic morphine treatment probably induces a down regulation of nigral D-1 and D-2 binding sites and reduces the negative feed-back mechanisms triggered by DA autoreceptors.

Cyclo (Leu-Gly) + haloperidol: effects on dopamine receptors and conditioned avoidance responding

Behavioral effects of cyclo (Leu-Gly) (cLG), administered either acutely or chronically, were assessed in combination with haloperidol in the rat. cLG administered chronically, produced a significant reduction in the increase in apomorphine-induced stereotypy produced by chronic haloperidol infusion. On the other hand, the same dose of cLG which reduced this induction of dopamine receptor supersensitivity due to chronic haloperidol treatment, failed to produce a change in the potency of haloperidol in blocking conditioned avoidance responding in the rat. Furthermore, degeneration-induced supersensitivity of dopamine neurons, produced by unilateral destruction of the nigrostriatal pathway, was not reduced by acute or chronic treatment with cLG as measured by apomorphine-induced rotation. These data suggest that cLG may decrease motor system side effects thought to be caused by chronic antipsychotic administration without affecting the therapeutic efficacy of the antipsychotic agent.

Radio-frequency analysis of the effect of haloperidol and cyclo (leucyl-glycyl) on apomorphine-induced stereotypy

Our previous studies indicated that the peptide cyclo(leucyl-glycyl) (cLG) prevents the development of supersensitivity to dopamine in several animal models at both biochemical and behavioral levels. We therefore tested cLG in a paradigm more commonly used to model tardive dyskinesia, namely chronic haloperidol administration to rats. We found that cLG administered subcutaneously at a dose of 8 mg/kg, blocked about 50% of the supersensitizing effects of of haloperidol on apomorphine-induced stereotypic behaviors. Further, we used a novel method, radio-frequency analysis, that quantifies sniffing and other stereotypic movements. Unlike methods that rely on visual observation of stereotypy and utilize an ordinal scale, these measurements are rated by an automatic motility monitor and utilize a ratio scale. Unlike other automated motility monitors, this device can distinguish between various forms of stereotypic behaviors. Since parametric statistics can be used, there is a significant improvement in the efficiency of the task of rating and comparing stereotypy scores.

Chronically administered morphine increases dopamine receptor sensitivity in mice

Chronic morphine treatment has been suggested to cause the development of supersensitive dopamine receptors. This increase in sensitivity was detected as a hypersensitivity in direct-acting dopamine agonists and as an increase in the affinity of dopamine receptors. However, these binding studies were performed in animals which had been withdrawn from morphine for a period of 24-48 h prior to killing. In the present study mice were implanted with pellets containing 75 mg of morphine free base. The pellets were left in situ in all experiments. One group of mice exhibited an increased sensitivity to apomorphine 72 h following pellet implantation as evidenced by a decrease in the ED50 of apomorphine for inducing cage climbing behavior. A second matched group of mice was found to have a significant increase in whole brain [3H]spiroperidol binding sites. These results suggest that chronic morphine treatment can cause the development of central supersensitive dopamine receptors. Lithium administered concurrently with the morphine attenuated the increased sensitivity to apomorphine and the increase in the number of [3H]spiroperidol binding sites. Concurrent lithium treatment also facilitated the degree of analgesic tolerance, and naloxone-induced withdrawal hypothermia. The ability of lithium to enhance analgesic tolerance while simultaneously attenuating the increase in dopamine receptors suggests that alterations in dopamine receptors might modify the degree of analgesic tolerance which develops to chronic morphine administration, or might modify the animal's response to thermal stimuli. The mechanism by which lithium enhanced naloxone-induced hypothermia is presently unknown.

In vivo analysis of dopamine and its metabolites in the caudate nucleus during euthermia and hibernation

Golden-mantled ground squirrels (Citellus lateralis) were chronically implanted with a unilateral push-pull cannula in the caudate nucleus. Perfusates obtained in these unanesthetized, unrestrained animals during the euthermic (non-hibernating) and hibernating states were analyzed for dopamine (DA) and its metabolites (homovanillic acid (HVA), 3,4-dihydroxyphenylacetic acid (DOPAC), and 3-methoxy-4-hydroxyphenethanol (MOPET) using high performance liquid chromatography with electrochemical detection. The data revealed clear differences in the performance of the caudate DA system in the two states. During the euthermic state, DA metabolism was indicative of a constant and high turnover rate. Free DA was not detectable in the majority of samples, HVA was detected at consistently high levels, and DOPAC and conjugated DA were present at low levels. By contrast, DA metabolism was sharply altered during hibernation. Free DA was present at high concentrations and HVA concentrations were low. DOPAC was not detected in any sample whereas MOPET was present in all samples. Conjugated DA was present at high concentrations during the second half of the hibernation bout. The shift in the post-release disposition of DA could enhance the stability of DA receptors (i.e. prevent supersensitivity) during the prolonged periods of reduced neural activity typical of hibernation.