Changes in brain cyclic AMP metabolism and acetylcholine and dopamine during narcotic dependence and withdrawal.

Effects of morphine administration were studied on cyclic AMP metabolism in several regions of rat brain. In the cortex, cerebellum and thalamus-hypothalamus, morphine dependence did not alter the activity of either adenylate cyclase or phosphodiesterase. However, during withdrawal from the opiate treatment, adenylate cyclase activity declined in all three regions studied. In contrast, the striatal cyclic AMP metabolism was enhanced during morphine treatment as reflected by elevated endogenous cyclic AMP and increased adenylate cyclase. Furthermore, narcotic dependence produced significant increases in acetylcholinesterase activity of rat striatum. Whereas morphine withdrawal reversed the changes in striatal acetylcholine levels and acetylcholinesterase activity, the enhanced striatal dopamine remained unaltered. Although the activity of striatal adenylate cyclase was significantly reduced when compared to the morphine-dependent rats, the drop in cyclic AMP levels was not significant. Methadone replacement did not affect the changes in striatal dopamine seen in morphine-withdrawn rats. Whereas dopamine stimulated equally well the striatal adenylate cyclase from control or morphine-dependent animals, it failed to stimulate the striatal enzyme from rats undergoing withdrawal. The crude synaptosomal fraction of the whole brain from morphine-dependent rats exhibited an increase in cyclic AMP which was accompanied by elevated adenylate cyclase and protein kinase activity. Naloxone administration suppressed this rise in cyclic AMP and reversed the morphine-stimulated increases in the activities of adenylate cyclase and protein kinase. Following the withdrawal of morphine treatment, alterations in cyclic AMP metabolism were similar to those noted in morphine-naloxone group. Furthermore, substitution of morphine with methadone antagonized the observed alterations in cyclic nucleotide metabolism during withdrawal.     

Expression changes of hippocampal energy metabolism enzymes contribute to behavioural abnormalities during chronic morphine treatment

Dependence and impairment of learning and memory are two well-established features caused by abused drugs such as opioids. The hippocampus is an important region associated with both drug dependence and learning and memory. However, the molecular events in hippocampus following exposure to abused drugs such as opioids are not well understood. Here we examined the effect of chronic morphine treatment on hippocampal protein expression by proteomic analyses. We found that chronic exposure of mice to morphine for 10 days produced robust morphine withdrawal jumping and memory impairment, and also resulted in a significant downregulation of hippocampal protein levels of three metabolic enzymes, including Fe-S protein 1 of NADH dehydrogenase, dihydrolipoamide acetyltransferase or E2 component of the pyruvate dehydrogenase complex and lactate dehydrogenase 2. Further real-time quantitative PCR analyses confirmed that the levels of the corresponding mRNAs were also remarkably reduced. Consistent with these findings, lower ATP levels and an impaired ability to convert glucose into ATP were also observed in the hippocampus of chronically treated mice. Opioid antagonist naltrexone administrated concomitantly with morphine significantly suppressed morphine withdrawal jumping and reversed the downregulation of these proteins. Acute exposure to morphine also produced robust morphine withdrawal jumping and significant memory impairment, but failed to decrease the expression of these three proteins. Intrahippocampal injection of D-glucose before morphine administration significantly enhanced ATP levels and suppressed morphine withdrawal jumping and memory impairment in acute morphine-treated but not in chronic morphine-treated mice. Intraperitoneal injection of high dose of D-glucose shows a similar effect on morphine-induced withdrawal jumping as the central treatment. Taken together, our results suggest that reduced expression of the three metabolic enzymes in the hippocampus as a result of chronic morphine treatment contributes to the development of drug-induced symptoms such as morphine withdrawal jumping and memory impairment.     

Pharmacological manipulation of gamma-aminobutyric acid (GABA) in morphine analgesia,tolerance and physical dependence

The findings from our laboratory indicated that pharmacological manipulations of GABA system modified morphine analgesia, tolerance and physical dependence. Elevating brain levels of GABA by slowing its destruction with aminooxyacetic acid not only antagonized the analgesic action of morphine in both non-tolerant and tolerant mice, but also enhanced the development of tolerance and physical dependence. On the other hand, blockade of postsynaptic sites of GABA receptors by bicuculline resulted in an inhibition of tolerance and dependence development. Administration of 2,4-diaminobutyric acid, an inhibitor of GABA uptake in the neurons, antagonized morphine analgesia in both non-tolerant and tolerant mice. However, it did not modify naloxone precipitated withdrawal jumping. On the contrary, β-alanine, an inhibitor of the GABA uptake process in glial cells, potentiated naloxone precipitated withdrawal jumping in morphine dependent mice, but it had no effect on morphine antinociception in both non-tolerant and tolerant mice.     

Possible role of cyclic AMP and dopamine in morphine tolerance and physical dependence.

In chronically morphinized rats undergoing naloxone induced withdrawal the cerebellar Cyclic 3′, 5′ adenosine monophosphate (Cyclic AMP) was significantly higher than the controls. The cerebellar dopamine (DA) and norepinephrine (NE) were decreased, elevated or unchanged depending on the duration of morphine treatment. The corpus striatal DA levels during withdrawal were markedly elevated and the striatal cyclic AMP levels were unchanged. The NE levels in the striatal tissue were either elevated or unchanged depending upon the duration of morphine administration. In sharp contrast to the chronically morphinized rats undergoing naloxone induced withdrawal, the rats made morphine dependent over a period of eight weeks showed quite moderate changes in the striatal and cerebellar cyclic AMP and DA levels. Thus alterations in the DA and the cyclic AMP levels in the central nervous system (CNS) may play an important role in the naloxone induced stereotyped morphine withdrawal behavior.     

A comparison of morphine-induced locomotor activity and mesolimbic dopamine release in C57BL6, 129Sv and DBA2 mice

Inbred mouse strains show marked variations in morphine-induced locomotion and reward behaviors. As increases in mesolimbic dopamine release and locomotion have been implicated as being critical aspects of drug-seeking and reward-related behaviors, the present study sought to determine the relationship between morphine-induced changes in locomotion and mesolimbic dopamine release. Freely moving microdialysis of the ventral striatum was performed in mouse strains chosen on the basis of their documented differences in locomotor and reward response to morphine (C57BL6 and DBA2) and use in the production of genetically modified mice (129Sv). Both C57BL6 and 129Sv mice showed significant increases in locomotion and ventral striatal extracellular dopamine levels following subcutaneous morphine administration (3 mg/kg), with the former strain showing the largest increase in both parameters. Ventral striatal extracellular DA levels increased in DBA2 mice to a similar extent as 129Sv mice following morphine administration, despite this strain showing no locomotor response. Intra-strain analysis found no correlation between morphine-induced locomotion and mesolimbic dopamine release in any of the strains studied. Thus, no universal relationship between morphine-induced mesolimbic dopamine release and locomotion exists between, and particularly within, inbred mouse strains. Furthermore, morphine-induced increases in mesolimbic activity correlate negatively with the rewarding potential of morphine described in previously reported conditioned place preference studies.     

Narcotic dependence, narcotic action and dopamine receptors.

Acute systematic administration of narcotic analgesics increases the firing rate of nerve cells in the zona compacta of the substantia nigra, causes an increase in the rate of dopamine turnover in striatal and mesolimbic areas of the brain, stimulates prolactin release, inhibits brain self-stimulation and discriminated shock-avoidance, blocks cardiovascular effects of systemically injected dopamine, blocks aggression as well as compulsive jumping in mice treated with DOPA and amphetamine, antagonizes stereotypy induced by apomorphine or amphetamine, and blocks apomorphine-induced vomiting in dogs. Chronic administration of narcotic analgesics results in withdrawal signs upon the cessation of the drug administration. These signs include, tolerance to the increase in striatal dopamine turnover caused by narcotic analgesics or haloperidol, aggressive behaviors which are further stimulated by directly or indirectly acting dopamine-receptor agonists and are blocked by dopamine-receptor blockers, facilitation of recovery from the “lateral hypothalamic syndrome”, an increase in basal levels of striatal adenylate cyclase which shows greater sensitivity to dopamine, and, an enhanced sensitivity to apomorphine-induced reduction of dopamine turnover. It is therefore, concluded that acute administration of narcotic drugs results in an inhibition of dopamine-receptor activity while chronic administration of these drugs results in an increased response of these dopamine receptors to dopamine agonists. Recent experiments on the interaction of other drugs with narcotic analgesics suggest that, unlike the direct action of neuroleptics on the dopamine receptors, the narcotic action on dopamine receptors is indirect.     

Enhanced morphine withdrawal and micro -opioid receptor G-protein coupling in A2A adenosine receptor knockout mice

Much evidence supports the hypothesis that A2A adenosine receptors play an important role in the expression of morphine withdrawal and that the dopaminergic system might also be involved. We have evaluated morphine withdrawal signs in wild-type and A2A receptor knockout mice and shown a significant enhancement in some withdrawal signs in the knockout mice. In addition, micro -opioid and dopamine D2 receptor autoradiography, as well as micro -opioid receptor-stimulated guanylyl 5'-[gamma-[35S]thio]-triphosphate ([35S]GTPgammaS) autoradiography was carried out in brain sections of withdrawn wild-type and knockout mice. No significant changes in D2 and micro -opioid receptor binding were observed in any of the brain regions analysed. However, a significant increase in the level of micro receptor-stimulated [35S]GTPgammaS binding was observed in the nucleus accumbens of withdrawn knockout mice. These data indicate that the A2A receptor plays a role in opioid withdrawal related to functional receptor activation.     

Opiate Modulation of Striatal Dopamine and Hippocampal Norepinephrine Release Following Morphine Withdrawal

When opiates are abruptly withdrawn after chronic treatment, increases in hippocampal noradre-nergic function are observed which are accompanied by decreases in striatal dopamine release. The latter effects have to shown to persist for several weeks following the onset of opiate withdrawal. We examined the long-term effects of opiate withdrawal on 4-aminopyridine and potassium stimulated release of striatal dopamine and hippocampal norepinephrine. Tissue samples were obtained either from rats that had been exposed to opiate withdrawal following a seven day morphine infusion or sham treated control subjects. At 48 hours after the onset of withdrawal (cessation of morphine infusions), slices were loaded with [³H] neurotransmitter, washed extensively, and exposed to different drug treatments. 4-aminopyridine induced concentration related increases in striatal dopamine release, which was 36% calcium independent. Similar values for fractional release of striatal dopamine were obtained in morphine withdrawn and control subjects, for both potassium and 4-aminopyridine induced release. In addition, thresholds for 4-aminopyridine or potassium induced release of striatal dopamine did not differ between control and morphine withdrawn subjects. Treatment with 1.0 M morphine sulfate potentiated potassium evoked release of norepinephrine to an equal extent in both morphine withdrawn and sham treated hippocampal tissue. Exposure to a threshold concentration of potassium (8.0 mM), stimulated increased release of hippocampal norepinephrine in a significantly greater fraction of tissue samples obtained from morphine withdrawn animals. Although these results do not support changes in striatal dopamine release following opiate withdrawal, opiate mechanisms appear to be important determinants of in vitro hippocampal norepinephrine release.     

Dopamine receptor sensitivity after repeated morphine administrations to rats.

It was studied in rats, if chronic morphine treatment induces a supersensitivity of dopamine receptors in brain. The rats were treated twice daily for 8–11 days with single doses of morphine, increasing from 10 to 20 mg/kg i.p. The experiments were carried out 16–20 hours after the last injection of morphine. After chronic morphine treatment, the potency of apomorphine in lowering the striatal dopamine turnover was increased. On the other hand, apomorphine was not more potent in inducing stereotypies (sniffing, licking, gnawing) after chronic morphine administration than in saline controls. Finally, dopamine activated the adenylate cyclase in striatal homogenates of rats after chronic morphine treatment to a similar extent as in homogenates of control rats. The results suggest that a supersensitivity of dopamine receptors in brain is not necessarily involved in symptoms of an increased dopaminergic activity after chronic morphine application.     

M₅ muscarinic receptors mediate striatal dopamine activation by ventral tegmental morphine and pedunculopontine stimulation in mice

Opiates, like other addictive drugs, elevate forebrain dopamine levels and are thought to do so mainly by inhibiting GABA neurons near the ventral tegmental area (VTA), in turn leading to a disinhibition of dopamine neurons. However, cholinergic inputs from the laterodorsal (LDT) and pedunculopontine (PPT) tegmental nucleus to the VTA and substantia nigra (SN) importantly contribute, as either LDT or PPT lesions strongly attenuate morphine-induced forebrain dopamine elevations. Pharmacological blockade of muscarinic acetylcholine receptors in the VTA or SN has similar effects. M₅ muscarinic receptors are the only muscarinic receptor subtype associated with VTA and SN dopamine neurons. Here we tested the contribution of M₅ muscarinic receptors to morphine-induced dopamine elevations by measuring nucleus accumbens dopamine efflux in response to intra-VTA morphine infusion using in vivo chronoamperometry. Intra-VTA morphine increased nucleus accumbens dopamine efflux in urethane-anesthetized wildtype mice starting at 10 min after infusion. These increases were absent in M₅ knockout mice and were similarly blocked by pre-treatment with VTA scopolamine in wildtype mice. Furthermore, in wildtype mice electrical stimulation of the PPT evoked an initial, short-lasting increase in striatal dopamine efflux, followed 5 min later by a second prolonged increase in dopamine efflux. In M₅ knockout mice, or following systemic pre-treatment with scopolamine in wildtype mice, the prolonged increase in striatal dopamine efflux was absent. The time course of increased accumbal dopamine efflux in wildtype mice following VTA morphine was consistent with both the prolonged M₅-mediated excitation of striatal dopamine efflux following PPT electrical stimulation and accumbal dopamine efflux following LDT electrical stimulation. Therefore, M₅ receptors appear critical for prolonged PPT excitation of dopamine efflux and for dopamine efflux induced by intra-VTA morphine.     

Mapping of a quantitative trait locus for morphine withdrawal severity

Chronic morphine exposure results in physical dependence, manifested by physical symptoms during naloxone-precipitated withdrawal. Jumping frequency is widely considered the most sensitive and reliable index of withdrawal intensity in mice. Inbred mouse strains surveyed for naloxone-precipitated withdrawal display large and significant strain differences in jumping frequency, including an approximately tenfold difference between C57BL/6 and 129P3 mice. In the present study, (B6 × 129)F₂ hybrid mice were given daily morphine injections for four days using an escalating dosing schedule, and naloxone-precipitated withdrawal on day 5 was measured. A full-genome scan for linkage to phenotypic data was performed using polymorphic microsatellite markers. Significant linkage was observed between withdrawal jumping frequencies and a 28 cM-wide region of Chromosome 1 (32–60 cM; peak at 51 cM), accounting for 20% of the overall phenotypic variance. Two other suggestive QTLs were found, on Chromosomes 5 and 10, and an additive model fitting all three loci accounted for 43% of the total variance. F₂ mice were also assessed for changes in morphine analgesic potency using the tail-withdrawal test in dose–response studies on days 1 and 4. No linkage was observed between Chromosomes 1, 5, and 10 and morphine analgesic tolerance, suggestive of genetic dissociation of naloxone-precipitated withdrawal from morphine and chronic morphine intake per se. The significant quantitative trait locus for naloxone-precipitated withdrawal severity in morphine-dependent mice, which we name Depmq1, may prove to be of considerable heuristic value once the underlying gene or genes are identified.     

Abnormal behavior,striatal dopamine turnover and opioid peptide gene expression in histamine‐deficient mice

Hypothalamic histaminergic neurons regulate a variety of homeostatic, metabolic and cognitive functions. Recent data have suggested a modulatory role of histamine and histamine receptors in shaping striatal activity and connected the histaminergic system to neuropsychiatric disorders. We characterized exploratory behavior and striatal neurotransmission in mice lacking the histamine producing enzyme histidine decarboxylase (Hdc). The mutant mice showed a distinct behavioral pattern during exploration of novel environment, specifically, increased frequency of rearing seated against the wall, jumping and head/body shakes. This behavioral phenotype was associated with decreased levels of striatal dopamine and serotonin and increased level of dopamine metabolite DOPAC. Gene expression levels of dynorphin and enkephalin, opioids released by medium spiny neurons of striatal direct and indirect pathways respectively, were lower in Hdc mutant mice than in control animals. A low dose of amphetamine led to similar behavioral and biochemical outcomes in both genotypes. Increased striatal dopamine turnover was observed in Hdc KO mice after treatment with dopamine precursor l ‐Dopa. Overall, our study suggests a role for striatal dopamine and opioid peptides in formation of distinct behavioral phenotype of Hdc KO mice.     

Presynaptic Dopaminergic Function in the Nucleus Accumbens Following Chronic Opiate Treatment and Precipitated Withdrawal

Naloxone treatment at three days following implantation of pellets containing morphine base increased uptake of tritiated dopamine by the nucleus accumbens but did not alter efflux of tritiated dopamine by the nucleus accumbens or tritiated norepinephrine by the hippocampus. At six days following placement of pellets containing morphine base, withdrawal score was increased after treatment with either saline or naloxone, indicating that animals were undergoing spontaneous opiate withdrawal. Fractional efflux of tritiated dopamine was decreased at this time point following intermittent stimulation with 317 and 1000 M 4-aminopyridine, for striatal slices obtained from animals pretreated with either saline or naloxone. For the nucleus accumbens at six days after placement of morphine pellets, similar decreases in the efflux of tritiated dopamine were only observed in slices obtained from naloxone treated animals. Fractional dopamine efflux was also diminished after in vitro exposure to rising concentrations of 4-aminopyridine, amphetamine, or cocaine for tissue obtained from the nucleus accumbens, but not for slices from the striatum at six days following morphine pellet implantation. In conclusion, deficits in dopamine efflux by the nucleus accumbens occur at a time when animals are undergoing spontaneous opiate withdrawal at six days following morphine pellet implantation, but do not occur at an earlier time point when withdrawal is precipitated by naloxone treatment. These deficits are apparent for brain slices obtained from the striatum or nucleus accumbens after exposure to rising concentrations of different in vitro treatments, with tissue obtained from the nucleus accumbens being more sensitive than the striatum to dopamine efflux produced by a wider range of treatments.     

Alcohol withdrawal-induced changes in brain biogenic amines in mice: Influence of the genotype

Alterations in striatal and hippocampal dopamine (DA) and serotonin (5HT) activities were investigated in two inbred strains of mice (C57B1 and Balb/c) after 3 withdrawal periods following 5 months chronic ethanol administration. Two groups of animals with different levels of ethanol administration (15% and 30%, v/v) were examined. A striking strain dependency has been noted. Striatal dopaminergic mechanisms of the Balb/c strain are profoundly disturbed in both groups. In contrast no changes were noted for either transmitter activities in C57B1 mice at any withdrawal time studied. Strain dependency has also been noted for hippocampal serotonin neurotransmission, since only Balb/c mice showed a progressive decrease in 5HT levels. These impairments observed in striatum and hippocampus could be involved in motor incoordinations and convulsions often associated with the withdrawal syndrome. The differences in withdrawal effects we noted between the two strains may be linked to the specific chemical neuroanatomy of the strains. Such specificities could be implied in the well known variability of withdrawal induced behavior in man.     

The role of mu1 receptor in physical dependence on morphine using the mu receptor deficient CXBK mouse.

It is known that the CXBK inbred strain of mouse is deficient in mu1 opioid receptors, whereas the strain has a delta opioid receptor population that is less consistently altered. In the present study, we compared physical dependence on morphine between CXBK and C57BL/6 mice. Both strains of mice were treated with morphine-admixed food for 5 days. During the treatment, the two strains of mice showed no signs of toxicity. There was no significant difference in morphine intake during the treatment between CXBK and C57BL/6 mice. After the treatment, the withdrawal was precipitated by injecting naloxone (0.01-30 mg/kg, s.c.). CXBK mice showed weight loss, diarrhea and ptosis, but not jumping and body shakes after low dose of naloxone. Whereas, C57BL/6 mice showed weight loss, diarrhea, ptosis, body shakes and jumping. These results suggest that naloxone-precipitated weight loss, diarrhea and ptosis may be mediated by mu2 and/or delta opioid receptor, while naloxone-precipitated jumping and body shakes may be mediated by mu1 opioid receptors.     

两种吗啡依赖大鼠模型脑内单胺神经递质水平的比较

目的通过对吗啡诱导的躯体依赖与精神依赖两种大鼠模型脑内单胺类递质水平的比较,探讨其在吗啡依赖形成中的作用。方法采用剂量递增法复制吗啡依赖大鼠模型,然后用纳洛酮催促,引起躯体戒断症状。连续给予吗啡（5mg／kg,ip）6d,引起大鼠产生显著的条件性位置偏爱效应。脑组织去甲肾上腺素（NE）、5-羟色胺（5-HT）和多巴胺（DA）含量采用荧光分光光度法测定。结果吗啡依赖大鼠催促戒断后脑内NE和5-HT水平明显升高,DA水平下降。吗啡在引起大鼠明显位置偏爱的同时,使大鼠脑内DA和5-HT水平显著升高,NE无明显改变。结论吗啡依赖的形成和戒断与脑内单胺神经递质有密切关系,吗啡依赖的躯体戒断症状与NE升高有关,而吗啡诱导的精神依赖则与脑内DA水平升高有关。     

Role of Corticotropin-Releasing Factor (CRF) Receptor-1 on the Catecholaminergic Response to Morphine Withdrawal in the Nucleus Accumbens (NAc)

Stress induces the release of the peptide corticotropin-releasing factor (CRF) into the ventral tegmental area (VTA), and also increases dopamine (DA) levels in brain regions receiving dense VTA input. Since the role of stress in drug addiction is well established, the present study examined the possible involvement of CRF1 receptor in the interaction between morphine withdrawal and catecholaminergic pathways in the reward system. The effects of naloxone-precipitated morphine withdrawal on signs of withdrawal, hypothalamo-pituitary-adrenocortical (HPA) axis activity, dopamine (DA) and noradrenaline (NA) turnover in the nucleus accumbens (NAc) and activation of VTA dopaminergic neurons, were investigated in rats pretreated with vehicle or CP-154,526 (selective CRF1R antagonist). CP-154,526 attenuated the increases in body weight loss and suppressed some of withdrawal signs. Pretreatment with CRF1 receptor antagonist resulted in no significant modification of the increased NA turnover at NAc or plasma corticosterone levels that were seen during morphine withdrawal. However, blockade of CRF1 receptor significantly reduced morphine withdrawal-induced increases in plasma adrenocorticotropin (ACTH) levels, DA turnover and TH phosphorylation at Ser40 in the NAc. In addition, CP-154,526 reduced the number of TH containing neurons expressing c-Fos in the VTA after naloxone-precipitated morphine withdrawal. Altogether, these results support the idea that VTA dopaminergic neurons are activated in response to naloxone-precipitated morphine withdrawal and suggest that CRF1 receptors are involved in the activation of dopaminergic pathways which project to NAc.     

Dissociation of morphine withdrawal diarrhea and jumping in mice by the peripherally selective opioid antagonist SR 58002 C

Mice were rendered physically dependent by repeated administration of morphine, 25 mg/kg s.c., 5 times daily for 4 days, and on the 5th day, 2 h after the last morphine dose, they were challenged with a s.c. injection of either naloxone, 25 mg/kg, or the peripherally selective opioid antagonist SR 58002 C (N-methyl levallorphan mesilate), 75 mg/kg. Naloxone provoked jumping and diarrhea in all the animals; mice challenged with SR 58002 C presented no significant jumping but a high frequency of withdrawal diarrhea. When naloxone, 12 mg/kg, or SR 58002 C, 50 mg/kg, were given s.c. in combination with repeated morphine as above, mice which had received naloxone with morphine presented virtually no diarrhea or jumping upon naloxone challenge; those repeatedly treated with morphine plus SR 58002 C were substantially protected from naloxone-precipitated diarrhea, but not jumping. These results further support the remarkable selectivity for peripheral opioid receptors of SR 58002 C, even after repeated high-dose treatment, and are strongly consistent with the primary role of a local intestinal mechanism in the development and expression of opioid withdrawal diarrhea in mice. The in vivo dissociation of central and peripheral components of dependence on morphine is illustrated, apparently for the first time.     

Dopamine is necessary to endogenous morphine formation in mammalian brain in vivo

Morphine, the most used compound among narcotic analgesics, has been shown to be endogenously present in different mammalian/invertebrate normal tissues. In this study, we used mice that cannot make dopamine due to a genetic deletion of tyrosine hydroxylase specifically in dopaminergic neurons, to test the hypothesis that endogenous dopamine is necessary to endogenous morphine formation in vivo in mammalian brain. When dopamine was lacking in brain neurons, endogenous morphine was missing in brain mouse whereas it could be detected in brain from wild type rodent at a picogram range. Our data prove for the first time that endogenous dopamine is necessary to endogenous morphine formation in normal mammalian brain. Morphine synthesis appears to be originated from dopamine through L-tyrosine in normal brain tissue. Morphine synthesis is not considered to occur inside the same neuron in normal tissue; released dopamine might be transported into morphinergic neuron and further transformed into morphine. A physiological role for endogenous morphine is suggested considering that dopamine could modulate thermal threshold through endogenous morphine formation in vivo. Thus, dopamine and endogenous opiates/opioid peptides may be interconnected in the physiological processes; yet, endogenous morphine may represent a basic link of this chain.