Effects of morphine on the turnover of brain catecholamines and serotonin in rats-chronic morphine administration

Postmortem serotonin (5-HT) concentrations in the cerebrospinal fluid of medicolegal cases

In a medicolegal study the postmortem serotonin (5-HT) cerebrospinal fluid (CSF) concentrations were determined in routine autopsies using a high performance liquid chromatographic procedure with electrochemical detection. There was no correlation between 5-HT concentrations and age, sex or blood alcohol concentration using a postmortem delay < or = 3 days. In suicides the suboccipital CSF concentrations were significantly decreased compared to the levels measured in the control group (8.55+/-5.99 ng/ml versus 20.15+/-13.56 ng/ml). Additionally, a decrease of 5-HT was found in the suboccipital CSF of opiate fatalities (15.56+/-13.52 ng/ml). The results support the hypothesis that decreased 5-HT concentrations in the CSF are characteristic in suicides. However, due to a rather broad overlapping of values between suicides and controls the results failed to define a possible cut-off level in the 5-HT CSF concentration to distinguish between a suicidal and a non-suicidal incident.

Naloxone-precipitated changes in biogenic amines and their metabolites in various brain regions of butorphanol-dependent rats

Influence of a naloxone (an opioid receptor antagonist) challenge (5 mg/kg, IP) on levels of biogenic amines and their metabolites in various brain regions of rats infused continuously with butorphanol (a mu/delta/kappa mixed opioid receptor agonist; 26 nmol/microliter/h) or morphine (a mu-opioid receptor agonist; 26 nmol/microliter/h) was investigated using high-performance liquid chromatography with electrochemical detection (HPLC-ED). Naloxone precipitated a withdrawal syndrome and decreased the levels of: dopamine (DA) in the cortex and striatum, 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum, homovanilic acid (HVA) in the striatum, limbic, midbrain, and pons/medulla regions in butorphanol-dependent rats. However, the levels of norepinephrine (NE), serotonin (5-hydroxytryptamine; 5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) in the regions studied were not affected by naloxone-precipitated withdrawal. In addition, naloxone increased the HVA/DA ratio in the cortex, while this ratio was reduced in the limbic, midbrain, and pons/medulla. The reduction of 5-HIAA/5-HT ratio was also detected in the limbic area. In the animals rendered dependent on morphine, the results obtained were similar to those of butorphanol-dependent rats except for changes of 5-HIAA levels in some brain regions. These results suggest that an alteration of dopaminergic neuron activity following a reduction of DA and its metabolites in specific brain regions (e.g., striatum, limbic, midbrain, and pons/medulla) play an important role in the expression of the opioid withdrawal syndrome.

Simultaneous measurement of biogenic amines and their metabolites in rat brain regions after acute administration of and abrupt withdrawal from butorphanol or morphine

Changes in levels of biogenic amines and metabolites were measured using high performance liquid chromatography fitted with an electrochemical detection in various rat brain regions after acute administration of and abrupt withdrawal from continuous intracerebroventricular infusion of butorphanol (a mu/delta/kappa mixed opioid receptor agonist) or morphine (a mu-opioid receptor agonist). A single dose of butorphanol (26 nmol/5 microliters) or morphine (26 nmol/5 microliters) increased levels of 3,4-dihydroxyphenylacetic acid in the striatum and limbic region and of homovanilic acid in the cortex, striatum, and limbic region. In animals which had been infused with butorphanol (26 nmol/microliters/hr) or morphine (26 nmol/microliters/hr) for 3 days, an increase in dopamine turnover was observed. The levels of 3,4-dihydroxyphenylacetic acid was decreased and that of homovanilic acid was increased in the striatum, limbic region, and midbrain immediately after termination of opioid infusion. Both dopamine metabolites (in these areas) were decreased at 2 and 6 hr after butorphanol or morphine withdrawal. Changes in norepinephrine, serotonin, and 5-hydroxyindoleacetic acid levels in some brain regions were observed in the morphine-, but not in butorphanol-dependent rats. These data suggest that the increase and the decrease in dopaminergic activity, but not noradrenergic and serotonergic neurons, in the some brain regions are closely associated with the production of antinociception of and the expression of withdrawal syndrome from butorphanol and morphine, respectively.

Production of antinociception by peripheral administration of [Lys7]dermorphin, a naturally occurring peptide with high affinity for mu-opioid receptors

1. The opioid activity of the amphibian peptide, [Lys7]dermorphin, was studied in rats and mice. When administered intracerebroventricularly (i.c.v.), intravenously (i.v.) or subcutaneously (s.c.) it produced a long lasting analgesia. Its antinociceptive potency exceeded that of morphine 290 times by i.c.v. injection, and 25-30 times by peripheral administration. 2. The dose-response curves of [Lys7]dermorphin antinociception were shifted to the right by the pretreatment with naloxone (0.1 mg kg-1, s.c.) or with the mu 1-selective antagonist, naloxonazine (10 mg kg-1, i.v. 24 h before peptide injection). 3. The peptide also displayed potent antinociceptive effects in a chronic inflammatory pain model (rat Freund's adjuvant arthritis). In this pain model, systemic administration of the peptide raised the nociceptive threshold more in inflamed than in healthy paw. 4. High central and peripheral doses of [Lys7]dermorphin in rats produced catalepsy. The cataleptic response was antagonized by naloxone but left unchanged by naloxonazine pretreatment. 5. In rats and mice, central or peripheral administration of [Lys7]dermorphin induced a significantly slower development of tolerance to the antinociceptive effect than did morphine. 6. Upon naloxone precipitation of the withdrawal syndrome, [Lys7]dermorphin-dependent mice made fewer jumps and lost less weight than the morphine-dependent animals. Withdrawal hyperalgesia did not develop in [Lys7]dermorphin-dependent mice. 7. In conclusion, [Lys7]dermorphin seems to be a unique opioid peptide having a high penetration into the blood-brain barrier despite its low lipid solubility. This peptide causes fewer side-effects than other opioids and appears less likely than morphine to cause physical dependence in rats and mice.

Brain and adrenal monoamines and neuropeptide Y in codeine tolerant rats

1. Monoamine turnover and neuropeptide Y (NPY) levels were investigated in the central and peripheral nervous systems in adult male rats chronically treated with codeine. 2. An increase in the dopaminergic turnover was observed in the striatum and cortex. The norepinephrine levels and the serotoninergic turnover were unchanged in all the brain areas. 3. Epinephrine levels were decreased in the adrenal glands. 4. In addition, we observed a significant decrease of NPY levels in the hypothalamus, the striatum and the adrenal glands. These observed changes were not found when assessing NPY level in plasma fluid. 5. The implication of these modifications in the induction of codeine dependence are discussed in view of previous results obtained with morphine.

The effects of chronic administration of morphine on the levels of brain and adrenal catecholamines and neuropeptide Y in rats

1. Monoamine turnover and neuropeptide Y (NPY) levels were investigated in the central and peripheral nervous systems in adult male rats chronically treated with morphine. 2. The well-recognized biochemical alterations (serotoninergic turnover increased in the hypothalamus, hippocampus and striatum; dopaminergic turnover increased in the striatum and cortex; adrenaline levels decreased in the adrenal glands) were observed. 3. In addition, we observed a significant decrease of the NPY levels in the hypothalamus, the striatum and the adrenal glands. The observed changes were not reflected in plasma. 4. Our results contribute to the evidence that brain and adrenal monoamines and NPY could be involved in the mechanism of morphine tolerance and/or dependence.

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.

Analysis of the role of behavioural factors in the development of tolerance to the benzodiazepine midazolam

Two experiments were conducted on the acute and chronic effects of the short-acting benzodiazepine midazolam on fixed ratio schedule-maintained operant responding in rats. Acute administration of midazolam produced suppression of responding in large doses but elevation of responding in small doses. Following intermittent (every third day) chronic treatment tolerance developed rapidly to the rate-suppressant effects of large doses of midazolam but did not develop to the rate-elevating effects of small doses, even after chronic treatment with large doses of the drug. Thus, when tolerance was measured in terms of shifts in dose-effect curves, it was manifested as a non-parallel shift in the curve after chronic treatment. Since tolerance developed to the effects of large but not small doses, the observed tolerance could not be attributed to changes in disposition of the drug. The development of tolerance did not depend on whether the drug was given before or after behavioural testing. These findings contrast with data reported in behavioural studies with other sedative-hypnotics (ethanol, barbiturates). Animals tolerant to midazolam showed no cross-tolerance to ethanol, a drug for which there is reliable evidence indicating that behavioural factors play a role in acquisition of tolerance. Tolerance to midazolam cannot therefore be explained in terms of learned strategies acquired as a result of drug-induced loss of rewarding stimuli. This conclusion contrasts with recent suggestions (File, 1985; File and Pellow, 1985) that tolerance to benzodiazepines may be mediated by instrumental conditioning processes.

Zimeldine: a review of its effects on ethanol consumption

This review evaluates the literature and describes an extensive series of experiments which examined the effects of zimeldine , its metabolite norzimeldine and other serotonin and norepinephrine reuptake inhibitors on voluntary ethanol consumption in rats. The results of these experiments indicate that drugs which specifically inhibit serotonin reuptake are capable of decreasing voluntary ethanol consumption. The behavioral mechanism through which these drugs exert their effects seems to be extinction of the primary reinforcing properties of alcohol. These effects seem to be partially attenuated both by drugs which modulate the norepinephrine system as well as by the serotonin postsynaptic receptor blocker methergoline. The data presented in this review are discussed in terms of the involvement of the serotonin and norepinephrine systems in the mechanism of action of these drugs. In addition, several alternative hypotheses concerning the nature of the phenomenon are offered. Finally, the implications of these data for the possible development of a treatment procedure for problem drinkers is discussed.

Analgesia, development of tolerance, and 5-hydroxytryptamine turnover in the rat after cerebral and systemic administration of morphine

Morphine HCl (10 micrograms/0.5 microliter) was injected into the right striatum, the caudal aqueduct and the region of the nucleus raphe magnus of the rat. Turnover of 5-hydroxytryptamine (5-HT) in the brain was assessed by fluorimetric estimation of 5-hydroxyindol-3-ylacetic acid following the administration of probenecid. Injection into the right striatum (a region containing 5-HT terminals) increased 5-HT turnover in the right, but not in the left striatum or in the anterior medulla. The pain threshold was unaltered. Injection into the aqueduct accelerated 5-HT turnover in the anterior medulla, but the striata and spinal cord showed no such change. Analgesia was pronounced. Injection of morphine into the region of the nucleus raphe magnus analgesia and increased 5-HT turnover in the posterior medulla and the spinal cord. The action on the cord must have been the result of the stimulation of cells in the raphe. The effects of the local injections of morphine on 5-HT turnover were antagonized by systemic naloxone (1-2 mg/kg) in all the regions studied. When morphine was administered subcutaneously three times a day for five days, tolerance developed to the analgesic effect of morphine (7mg/kg). However, tolerance to its acceleration of 5-HT turnover was only seen in the spinal cord, not in striatum or anterior and posterior medulla. When morphine was withdrawn, its effects on analgesia and 5-HT turnover in the spinal cord recovered simultaneously. The results emphasize the likely part played by the descending serotoninergic pathway in the analgesic effect of morphine.

The relationship between morphine analgesia and the activity of bulbo-spinal serotonergic system as studied by tolerance phenomenon

The effect of various doses of acute morphine on both analgesia and 5-hydroxytryptamine (5-HT) synthesis in the brain and the spinal cord has been studied in rats rendered tolerant by chronic administration of the analgesic. In morphine-tolerant rats, the incorporation of tritiated-L-tryptophan (TRP) in the brain and the spinal cord was higher than in non-tolerant rats, but there was no significant difference in the synthesis rate of the newly formed 5-HT between the two groups. An acute dose of morphine (10 mg/kg) which induced a powerful analgesia and a large increase in 5-HT synthesis in non-tolerant rats, did not produce analgesia nor changes in 5-HT synthesis in tolerant rats. Higher acute doses of morphine which restored analgesia in tolerant rats, induced a discrete increase in [3H]TRP incorporation and a marked increase in 5-HT synthesis in the spinal cord of these animals. The same doses significantly increased [3H]TRP incorporation in the forebrain but did not modify 5-HT synthesis. These results show that tolerance to morphine is associated with a decrease in the effects of the drug on 5-HT synthesis in the spinal cord and the brain and tend further support to the hypothesis that an enhancement of 5-HT synthesis in the spinal cord, induced independently of modifications of the availability of TRP, is associated with the analgesic effect of morphine.

The effects of acute and chronic administration of morphine on the turnover of brain and adrenal catecholamines in rats

Brain and adrenal catecholamine turnover in adult female rats treated with morphine was investigated. A different time course response of brain and adrenal catecholamines to alpha-methyl-p-tyrosine methyl-ester (AMT) administration in normal rats was observed; the catecholamine turnover rate in adrenal glands appeared to be much slower than in the brain. Acute morphine increased the turnover of brain dopamine and noradrenaline as well as of adrenal catecholamines, whereas chronic morphine treatment induced a decrease in the turnover of brain noradrenaline. Withdrawal induced by nalorphine produced an increase in the utilization of brain noradrenaline and adrenal catecholamines; this effect could be related to the withdrawl stress situation induced by the opiate antagonist. Although the mechanism of morphine action may implicate other neurotransmitters besides catecholamines, our results contribute to evidence that brain and adrenal catecholamines could be involved in the mechanism of morphine tolerance and/or dependence.

Log dose/response curve flattening in rats after daily injection of opiates

Rats injected (IP) daily with 0, 20, and 200 mg/kg morphine-SO4 for 25-49 days experienced log dose/response (LDR) curve flattening (decrease in slope and/or maximum response) for analgesia (tail immersion test) produced by etorphine-HCl injected IP or intracerebroventricularly (ICV), and for latency to maximum rectal temperature increase produced by IP etorphine. Rats treated similarly with 0, 50, and 500 micrograms/kg etorphine-HCl for 32 days exhibited LDR-curve flattening for analgesia produced by etorphine and morphine (IP). In addition, a profound body weight loss produced by high-dose morphine treatment (200 mg/kg) was found not to be involved in flattening, since similar body weight decreases produced by food restriction in 0 and 20 mg/kg rats did not have this effect. Flattening, however, may be due to a rapidly acquired and rapidly lost within-session (acute) tolerance. When flattening was not seen at short intervals after IP or ICV test etorphine doses, flattening was seen when rats were retested at longer test intervals. Forty-eight hours after cessation of chronic etorphine treatment, flattening of the etorphine analgesia LDR curve was lost, but parallel shift was unaffected. Similarly, 200 mg/kg morphine-treated rats lost morphine tolerance more rapidly than 20 mg/kg-treated rats during the first 12 days after the last treatment injection. Subsequently, however, levels of the analgesia and the amounts of tolerance loss were comparable in both chronically treated groups. The data support the notion that chronic tolerance reflects an enhancement or prolongation of acute tolerance.

The effect of morphine applied locally to mesencephalic dopamine cell bodies on spontaneous motor activity in the rat

The effect of local application of morphine (5 microgram) into the ventral tegmental area (VTA) or substantia nigra, pars compacta (SNc) on spontaneous activity was studied in the rat. Morphine in the VTA but not SNc produced an enhancement of locomotor activity which became progressively augmented with repeated injections. This effect could be blocked by systemic injections of naloxone or haloperidol. It is suggested that stimulation of opiate receptors in the vicinity of dopamine cell bodies can increase the activity of ascending mesencephalic dopamine neurones.

The synthesis rate and turnover time of 5-hydroxy-tryptamine in brains of rats treated chronically with morphine

1. Four schedules of subcutaneous pellet implantation were used to induce tolerance to and physical dependence on morphine in Sprague-Dawley rats. 2. The schedules included implantation of four morphine pellets (each containing 75 mg of morphine free base) during a 3 day period (schedule 1); six pellets during 3 days (schedule 2); six pellets during 7 days (schedule 3) and ten pellets during a 10 day period (schedule 4). 3. A high degree of tolerance and dependence on morphine, comparable to that induced in mouse by implantation of a single morphine pellet for 3 days, was produced with schedule 4. 4. Brain 5-hydroxytryptamine (5-HT) turnover rates as measured by rate of accumulation of 5-HT after monoamine oxidase inhibition by pargyline were not different in rats rendered tolerant to and dependent on morphine according to schedules 1 to 4 when compared with corresponding placebo pellet-implanted rats. 5. The turnover rates of 5-HT in brain of morphine-and placebo pellet-implanted rats (schedule 4) from which the pellets had been removed for 24 h were also similar. 6. It is concluded that tolerance to, and physical dependence upon morphine in the rat is not associated with changes in brain 5-HT dynamics.

The functional pool of brain catecholamines: its size and turnover rate

Behavioral data are reviewed that give evidence for an indiscriminate involvement of brain catecholamines (CA), especially dopamine (DA), in nerve function, regardless of the time elapsed from their synthesis. Critical analysis of biochemical and pharmacological studies shows that a clear-cut distribution of brain catecholamines in two compartments ['newly synthesized' (NS) and 'main storage'] is not at all established, and moreover that there is no adequate proof that the difference in turnover rates attributed to these two supposed pools is due to a preferential extraneuronal release of NS-CA during nerve function rather than to a preferential (nonfunctional) intraneuronal deamination of NS-CA, or at least of NS-DA.

Dissociation of increased 5-hydroxyindoleacetic acid levels and physical dependence: the effects of naloxone

1. A slow release emulsion of naloxone (naloxone SR) was administered subcutaneously to rats in an attempt to induce physical dependence of the morphine type. 2. Naltrexone (2.5 mg/kg), injected i.p., failed to elicit an abstinence syndrome in rats treated with 75, 100 or 150 mg/kg naloxone SR for 24, 48, or 72 h. 3. Naloxone SR had no effect on the whole brain levels of noradrenaline, dopamine, homovanillic acid or serotonin. 4. Naloxone SR caused an apparent dose-related increase in the brain levels of 5-hydroxyindoleacetic acid. 5. These results show that while naloxone does not induce physical dependence of the morphine type, it may, like morphine, increase the brain serotonin turnover rate. 6. It is proposed that the increase in brain serotonin turnover rate may not be causally related to physical dependence on morphine-like drugs but may be a property of drugs containing the basic opiate molecular structure.

The antagonizing effect of aspartic acid on the brain levels of monoamines and free amino acids during the development of tolerance to the physical dependence on morphine

Since it has been shown in previous study that aspartic acid prevents the development of physical dependence on and tolerance to morphine and antagonizes the abstinence syndrom signs, the biochemical bases of that prevention were investigated in the present study. The brain contents of serotonin, DA, NA, and free amino acids of the rats given aspartic acid and morphine separately and in combination were determined. It has been observed that most of the morphine-induced changes in the brain were normalized in the group given aspartic acid and morphine together. The relative ineffectiveness of aspartic acid in normalizing some amino acid levels decreased by morphine was discussed and some logical explanations were found.

Changes in striatal dopamine metabolism during precipitated morphine withdrawal

Precipitation of withdrawal in morphine tolerant/dependent rats by either naloxone or the partial agonist ZK 48491 caused a significant increase in the contration of striatal DA, which persisted for at least 1 h. During the same time the probenecid-induced accumulation of HVA and DOPAC was reduced in the striatum in relation to probenecid-treated tolerant/dependent controls. 20 min after precipitation of withdrawal by naloxone, the striatal concentration of 3-methoxytyramine was decreased by about 40%, while the activity of the DA metabolizing enzymes, MAO and COMT, remained unchanged. Naloxone-precipitated withdrawal was, further, found to delay the depletion of striatal DA caused by inhibition of synthesis 90 min after alpha-methyl-p-tyrosine treatment. All these results provide evidence for a decreased release of DA from the striatum during precipitated morphine withdrawal.

Brain serotonin turnover and morphine tolerance-dependence induced by multiple injections in the rat

Tolerance to and physical dependence on morphine in the rat was induced by injecting increasing doses of morphine sulfate (M.S.) administered i.p. twice daily for 14 days. The last dose of M.S. was 200 mg/kg. This procedure produced a 4-fold tolerance to morphine as evidence by the increased dose of morphine required to produce analgesia. The degree of dependence was quantified by determining the naloxone ED50 for the stereotyped withdrawal jumping response. Body weight loss and hypothermic responses during abrupt and naloxone-induced withdrawal were also measured. The degree of tolerance and dependence produced by multiple injection procedure was comparable to that produced by 2 pellets containing 75 mg of morphine base implanted for 3 days. The level and turnover of brain serotonin, determined 6 or 12 h after the last morphine sulfate injection did not differ significantly from that of saline injected control animals. These data indicate that multiple injection technique produces a mild degree of tolerance to, and physical dependence on, morphine which was not related to changes in brain serotonin level or turnover.

Physical dependence in the rat induced by slow release morphine: dose-response, time course and brain biogenic amines

1. Physical dependence was induced in rats by administration of a slow release morphine emulsion (morphine SR), and assessed by scoring abstinence signs and temperature changes after i.p. administration of naloxone (5 mg/kg). Three groups of rats received doses of 75, 100 or 150 mg/kg of morphine SR. Dependence was evaluated in each of these groups after 24, 48 and 72 h. 2. The effect of these treatments at the different times on brain levels of serotonin, 5-hydroxyindoleacetic acid, noradrenaline and dopamine was determined. 3. A ceiling level of dependence was reached 24 h after 75 and 100 mg/kg and 48 h after 150 mg/kg of morphine SR. 4. These different treatments produced no significant effect on the brain levels of noradrenaline, dopamine or serotonin. The levels of 5-hydroxyindoleacetic acid were significantly raised in morphine-dependent rats and the changes correlated well with the changes in abstinence behaviour and temperature after naloxone. 5. The results suggest that a relationship exists between serotonin turnover and physical dependence on morphine.

Recent developments in opiate research and their implications for psychiatry

Considerable progress in opiate research has been made during the last few years regarding the identification and localization of opiate receptors in vitro and in vivo, the analysis of drug-receptor interactions and the characterization of an endogenous ligand of the opiate receptor. There is little evidence that effects induced by chronic exposure to opiates - development of tolerance and dependence -are due to changes in opiate receptor mechanisms; it is supposed that the adaptive changes occur mainly in the chain of events triggered by the drug-receptor interaction. Such changes may be directly or indirectly related to the metabolism of neurotransmitters and/or cyclic nucleotides. The obvious links between physical and psychic equivalents of opiate dependence are discussed. Present data points to the significance of brain stem and limbic structures in both these processes, monoamines probably playing an important role. Relations between psychic manifestations of opiate addiction and mental disorders are pointed out.

AMT catalepsy and hypokinesia: interaction with morphine and cocaine

Acute morphine induced a dose-dependent hypokinesia and rigidity, but only mild and non-dose-dependent catalepsy. AMT, injected 1/2 h after morphine, slightly potentiated catalepsy but not hypokinesia during 3 h after morphine; in contrast, rigidity was decreased. The behavioral changes induced by AMT were accelerated in onset and reached their usual development, although AMT toxicity and hypothermia were completely antagonized; thus, it would appear that AMT hypokinesia/catalepsy are not the consequence of toxicity. When morphine was injected 4 h after AMT, a mutual potentiation of the two drugs on hypokinesia and catalepsy was observed, although previous biochemical measurements had shown no effect of morphine on CA depletion under these conditions. Rigidity appeared to be antagonized. After 17 days of repeated injections, morphine no longer elicited hypokinesia and catalepsy, but no cross-tolerance developed to the AMT behavioral changes. A similar lack of cross-tolerance to the effects of AMT or haloperidol was observed when morphine tolerance was induced by pellet implantation. Catalepsy and hypokinesia developed in a much more pronounced way after two large i.p. doses than after small, multiple administration of AMT; this difference was accompanied by a significantly lower concentration of brain DA, but not NA in the former group. The hyperthermic response observed after a 40 mg/kg s.c. injection of morphine was reversed to hypothermia when the same dose was given 4 or 10 h after CA synthesis inhibition. Cocaine strongly antagonized AMT hypokinesia and catalepsy when given 8 1/2 h after AMT, and, although to a lesser extent, even when injected 12 1/2 h after AMT.

Central serotonergic mechanisms and development of morphine dependence

The effects of different manipulations of brain serotonin (5-HT) content on the development of morphine dependence were investigated in rats, which were implanted with morphine pellets for 40 days. Serotonin content was decreased by (a) short or long term inhibition of tryptophan hydroxylase with para-chlorophenylalanine (PCPA), (b) by short or long term degeneration of 5-HT containing nerve terminals with 5,6-dihydroxytryptamine or (c) by degeneration of 5-HT containing nerve terminals by lesioning of midbrain raphe nuclei. With all methods used, the frequency of withdrawal jumping was significantly reduced, while other withdrawal signs remained more or less unchanged. Additional administration of 5-HTP to chronically PCPA treated rats did not reverse the PCPA effect. Since chronic reduction of 5-HT level during the whole time of morphine exposure changed withdrawal symptomatology in nearly the same way as did a decrease in 5-HT level during the time of withdrawal only, it is suggested that serotonergic mechanisms are not linked to the basic processes underlying dependence development but that they are only involved in the nervous pathways mediating the expression of some withdrawal signs.

Effects of morphine on the turnover of brain catecholamines and serotonin in rats-acute morphine administration

Morphine increased the rate of brain dopamine (DA) depletion when given before alpha-methyl-p-tyrosine (AMT) or alpha-propyl-dopacetamide, but not when given after AMT. No effect of morphine was found on the rate of depletion of brain noradrenaline (NA) or serotonin (5-HT) after the two synthesis inhibitors. The accumulation of homovanillic acid and 5-hydroxy-indoleacetic acid induced by probenecid was significantly increased by morphine pretreatment, whereas the accumulation of 3-methoxy-4-hydroxy-phenylglycol sulphate was not changed. These findings can be best explained by the hypothesis that morphine increases the non-functional intraneuronal catabolism of newly synthesized DA and 5-HT, without much effect on the monoamines already taken up in the synaptic vesicles. NA turnover does not seem to be changed by acute morphine administration.