Reinforcing properties of morphine chronically used in aversive life conditions: place-preference paradigm, long-term changes in behavioral reactivity

To substantiate significance of the organism's functional state in the mediation of an opiate's reinforcing activity, the rats' attitude to an initially nonpreferred environment associated with chronic (10) administration of morphine (MOR 0.67 and 2.0 mg/kg) was regularly estimated after drug administrations in conditions of two-hour restraint stress. The absence of any significant changes in the quantity of entries and the time in restraint box in rats received MOR at a low dose and only a slight increase in the quantity of entries found in animals administered MOR at a high dose allowed us to propose a blockade or prominent inhibition of the drug's rewarding efficacy to be typical under these conditions. Inverted changes in movement reactivity (its increase) as response to apomorphine at low "presynaptic" doses found in animals treated by MOR may indicate the involvement of modified presynaptic dopamine receptors in the mediation of revealed changes in an opiate's reinforcing activity.     

Cold-restraint stress reduces [3H]etorphine binding to rat brain membranes: influence of acute and chronic morphine and naloxone

[3H]Etorphine binding was characterized in rat brain homogenates depleted of endogenous opioids from animals acutely and chronically treated with morphine or naloxone and either unstressed or subjected to a 3-h restraint period in the cold. There was significant reduction in the number of high-affinity opiate binding sites in brain tissue from stressed as compared to unstressed animals. Despite the fact that the opiate drug regimens used produce marked behavioral and physiological effects, stress-induced opiate receptor depletion was not influenced by the drugs or by withdrawal. The various drug treatments also failed to produce significant changes in opiate receptor site densities or affinities in either stressed or unstressed animals. We propose that persistent activation of opiate receptors by endogenous opioids released during restraint stress leads to receptor 'down-regulation'.     

Cold-restraint stress reduces [H]etorphine binding to rat brain membranes: Influence of acute and chronic morphine and naloxone

[³H]Etorphine binding was characterized in rat brain homogenates depleted of endogenous opioids from animals acutely and chronically treated with morphine or naloxone and either unstressed or subjected to a 3-h restraint period in the cold. There was significant reduction in the number of high-affinity opiate binding sites in brain tissue from stressed as compared to unstressed animals. Despite the fact that the opiate drug regimens used produce marked behavioral and physiological effects, stress-induced opiate receptor depletion was not influenced by the drugs or by withdrawal. The various drug treatments also failed to produce significant changes in opiate receptor site densities or affinities in either stressed or unstressed animals. We propose that persistent activation of opiate receptors by endogenous opioids released during restraint stress leads to receptor ‘down-regulation’.     

Morphine Analgesia is potentiated in adult rats prenatally exposed to ethanol

Exposure to inescapable, intermittent footshock elicits an opioid-mediated stress-induced analgesia in rats. We have previously shown that this response is markedly potentiated in adult rats, prenatally exposed to ethanol. To further investigate our hypothesis that endogenous opioid pain-inhibitory systems are modified by prenatal ethanol exposure, we have measured the analgesic response to morphine, in vitro brain opiate receptor binding characteristics, and occupation of brain opiate receptors following systemic administration of morphine. Compared to controls, rats prenatally exposed to ethanol had significantly enhanced morphine analgesia. This enhancement, however, does not appear attributable to changes in number or affinity of mu or delta opiate receptors, or to altered occupation of receptors by morphine.     

Anxiolytic-like effect induced by chronic stress is reversed by naloxone pretreatment

The present study assesses the influence of different restraint schedules on behavioral parameters determined by a conflict test, namely the light-dark transitions (LDT) as well as the opiate modulation on the behavioral consequences induced by chronic restraint. Finally, another group of animals that received naloxone (NAL) and/or chronic stress was either exposed to a single foot shock session or administered a single dose of the β-carboline FG 7142 (N′-methyl-β-carboline-3-carboxamide) immediately prior to the LDT test. We observed that a single restraint session (2 h) induced a decrease of LOT and time spent in the lit box, while chronic restraint (2 h per day for up to 7 days) induced a significant increase in both parameters. However, this increasing effect was blocked by a NAL administration (2 mg/kg IP) prior to each of the seven restraint events. A single foot shock or FG administration produced a clear anxiogenic response, an effect that was absent in animals previously submitted to chronic stress. In addition, NAL pretreatment abolished the chronic stress-induced attenuating effect on the behavioral suppression induced after either foot shock or FG administration. Therefore, these findings demonstrate that a previous history of chronic stress, leading to adaptation, induced an anxiolytic-like effect, and attenuated the behavioral supresslon produced by acute stressors. There seems to be an endogenous opiate mechanism involved in the behavioral influence induced by chronic stress.     

Resistance to extrapyramidal effects of opiates in rats chronically treated with SCH 23390

Rats made tolerant to morphine show neither a change in brain opiate receptor number nor altered sensitivity to the inhibitory effect of opiates on striatal adenylate cyclase (AC) activity. Interestingly, SCH 23390, a selective blocker of D1 dopamine (DA) receptors which, given chronically to rats, induces a 32% increase in D1 receptor number and increases the Vmax of D1-stimulated striatal AC, resulted in marked resistance to acute morphine effects. In particular, rats chronically treated with SCH 23390 failed to show muscular rigidity and increased striatal dihydroxyphenylacetic acid (DOPAC) concentration after morphine. Moreover, basal striatal AC activity in these animals had a significantly reduced sensitivity to opiate inhibition. On the other hand, decreased AC sensitivity to acetylcholine (ACh) inhibition observed in the striatum of rats chronically treated with DFP, an irreversible blocker of acetylcholinesterase, appeared to be secondary to the downregulation of muscarinic receptors and thus did not modify the opiate inhibitory capacity. It was concluded that although a potentiation of striatal AC impairs opiate action, such mechanism is not involved in morphine tolerance.     

Chronic dopamine antagonism facilitates opiate-induced feeding.

Chronic interference with dopamine (DA) transmission has been found to facilitate opiate reward and opiate-induced behavioral activation derived from the nucleus accumbens. This study was aimed at determining the extent to which these effects are generalizable to opiate-induced feeding. Rats were tested for their feeding response to morphine following chronic interference with DAergic transmission with the long-acting neuroleptic, flupenthixol decanoate (FLU). It was found that FLU-treated animals showed an enhanced feeding response to morphine following three and four weeks of DA blockade, but not on weeks 1 and 2. Neither morphine treatment in FLU-control animals nor chronic FLU treatment alone produced any such time-dependent facilitation in feeding. The results indicate that the increased sensitivity to the rewarding effects of opiates following chronic DA blockade is generalizable to opiate-induced feeding.     

Chronic stress sensitizes frontal cortex dopamine release in response to a subsequent novel stressor: reversal by naloxone

The present study examined the influence of an early chronic variable stress procedure with or without concurrent naloxone administration at different doses (1, 2 or 3 mg/kg, i.p.) on stress (restraint)-induced dopamine release in the frontal cortex in vivo. A higher increase in cortical dopamine release in response to a subsequent restraint event was observed in chronically stressed rats as compared with those without chronic stress exposure. Naloxone pretreatment normalized this sensitized response only at the higher dose (3 mg/kg, i.p.). The present results indicate that cortical dopamine response to a novel and uncontrollable stressor sensitizes after exposure to a chronic variable stress procedure and that an endogenous opiate mechanism, presumably activated during chronic stress, may be involved in the development of such a sensitization process.     

Dopamine output in the nucleus accumbens shell is related to the acquisition and the retention of a motivated appetitive behavior in rats

Chronic stress exposure consistently impairs the reactivity to aversive and pleasurable stimuli in rats; these behavioral modifications are associated with a decrease in dopamine output in the nucleus accumbens shell (NAcS). However, when rats that have already acquired an appetitive behavior are exposed to chronic stress, they develop an impaired reactivity to avoidable aversive stimuli while retaining the appetitive behavior. The dissociation between these two behavioral traits was used to study whether the decreased dopaminergic activity in the NAcS was connected to either of the two deficits. Dopamine output was studied through microdialysis as dopamine accumulation following re-uptake inhibition by cocaine. When rats that had previously acquired the appetitive behavior were exposed to chronic stress, they showed a dopaminergic transmission in the NAcS similar to that of controls and significantly higher than that of chronically stressed animals. Thus, dopamine output in the NAcS was consistently associated to the acquisition and maintenance of appetitive behavior, while the expression of a deficit in avoidance appeared to be independent of it.     

Ligand‐induced μ opioid receptor internalization in enteric neurons following chronic treatment with the opiate fentanyl

Morphine differs from most opiates its poor ability to internalize μ opioid receptors (μORs). However, chronic treatment with morphine produces adaptational changes at the dynamin level, which enhance the efficiency of acute morphine stimulation to promote μOR internalization in enteric neurons. This study tested the effect of chronic treatment with fentanyl, a μOR‐internalizing agonist, on ligand‐induced endocytosis and the expression of the intracellular trafficking proteins, dynamin and β‐arrestin, in enteric neurons using organotypic cultures of the guinea pig ileum. In enteric neurons from guinea pigs chronically treated with fentanyl, μOR immunoreactivity was predominantly at the cell surface after acute exposure to morphine with a low level of μOR translocation, slightly higher than in neurons from naïve animals. This internalization was not due to morphine's direct effect, because it was also observed in neurons exposed to medium alone. By contrast, D‐Ala2‐N‐Me‐Phe4‐Gly‐ol5‐enkephalin (DAMGO), a potent μOR‐internalizing agonist, induced pronounced and rapid μOR endocytosis in enteric neurons from animals chronically treated with fentanyl or from naïve animals. Chronic fentanyl treatment did not alter dynamin or β‐arrestin expression. These findings indicate that prolonged activation of μORs with an internalizing agonist such as fentanyl does not enhance the ability of acute morphine to trigger μOR endocytosis or induce changes in intracellular trafficking proteins, as observed with prolonged activation of μORs with a poorly internalizing agonist such as morphine. Cellular adaptations induced by chronic opiate treatment might be ligand dependent and vary with the agonist efficiency to induce receptor internalization. © 2013 Wiley Periodicals, Inc.     

Functional recovery of supersensitive dopamine receptors after intrastriatal grafts of fetal substantia nigra

Interruption of the ascending dopamine neurons of the nigrostriatal pathway, by 6-hydroxydopamine (6-OHDA) lesion in rats, produced a significant loss of the dopamine transport complexes labeled with the phencyclidine derivative [3H]BTCP. This loss of dopamine innervation in the striatum was present at least 12 to 14 months after lesioning and was functionally manifested by ipsilateral rotation of the animals in response to amphetamine. In these same animals, in comparison to controls, there was a significant increase in the number (Bmax) of [3H]SCH 23390-labeled D-1 receptors in the striatum (36.7%) and the substantia nigra (35.1%) and a 54.4% increase in the number (Bmax) of [3H]sulpiride-labeled striatal D-2 receptors without an apparent change in affinity (Kd). Ten to twelve months after the transplantation of homologous fetal substantia nigra into the denervated striatum, there was a significant decrease in amphetamine-induced turning behavior. In these animals, there was an ingrowth of dopamine nerve terminals in the striatum as demonstrated by a return of [3H]BTCP binding. Accompanying this reinnervation was the normalization of D-1 and D-2 receptors to control values in the striatum as well as the return of D-1 receptors to prelesion densities in the substantia nigra. In a subgroup of transplanted rats, amphetamine continued to induce ipsilateral turning. In these animals both D-1 and D-2 receptors remained supersensitive. These results support the hypothesis that the functional recovery of transplanted animals is due, in part, to reinnervation of the striatum. In addition, long-term alterations in receptor density may be related to the behavioral deficits that are associated with the 6-OHDA-lesioned rat. Furthermore, dopamine receptor plasticity may play a role in the functional recovery of substantia nigra transplanted animals and graft viability seems to be a prerequisite for behavioral recovery as well as receptor normalization.     

The effects of prior chronic stress on cardiovascular responses to acute restraint and formalin injection

Exposure to acute stressors activates both the hypothalamic-pituitary–adrenal (HPA) and cardiovascular systems. Prior chronic stress enhances HPA responses to novel, acute stressors, but whether it alters cardiovascular responsivity to novel, acute stress is unknown. In the present study, we examined mean arterial blood pressure (MAP) and heart rate (HR) to two distinct stimuli, restraint and formalin, following prior exposure to 7 days of intermittent cold. In two sets of control and chronically stressed animals, we measured MAP and HR for 60 min following onset of 30 min restraint and MAP, HR and behavioral responses to intraplantar injection of formalin. Chronic stress raised MAP and HR under resting conditions and elevated HR during, but not following termination of, restraint. These increases in HR during restraint were due to the differences in resting levels of HR, since both control and chronically stressed animals exhibited similar increases from resting levels in HR during restraint. Conversely, chronically stressed animals exhibited lower changes in MAP and HR from resting levels following termination of restraint. Formalin produced the characteristic biphasic pattern of cardiovascular and behavioral responses. Prior chronic stress did not alter behavior, but increased MAP and HR in Interphase and only MAP in Phase 2. The increases in MAP during Interphase and Phase 2 were a result of the elevations in resting levels of MAP, but even when differences in resting levels were taken into account, HR remained elevated in the Interphase in chronically stressed animals. Together, these data demonstrate that prior chronic intermittent cold stress modifies cardiovascular function both under resting conditions and, in very specific ways, under stimulated conditions produced by restraint and formalin. We propose that these modifications are produced by brain regions that are known to regulate cardiovascular function and which are activated by chronic stress.     

Effect of different restraint schedules on the immobility in the forced swim test: modulation by an opiate mechanism

The present research was conducted to evaluate the influence of different stress schedules on behaviors displayed during both phases of the forced swim test (FST). In addition, the involvement of an opiate mechanism in the behavioral consequences of chronic restraint was investigated. Exposure to a single, but not to chronic, restraint event induced an increase in the immobility score obtained during the 10-min initial swimming exposure (initial test) of the FST. Animals submitted to a previous regime of repeated restraint showed a significant increase in immobility during the 5-min second swimming exposure (retest period) of this behavioral task. However, naloxone (NAL) administered before each of the seven restraint events, blocked the higher immobility observed in chronically stressed rats during the retest period suggesting the involvement of an opiate mechanism. Results concerning the effect of chronic stress on the behavior displayed during the FST were discussed with reference to previous reports which have proposed that immobility performed during the retest period of the FST represents an efficient adaptive response in this inescapable aversive experience.     

Altered subcellular signaling in murine peritoneal macrophages upon chronic morphine exposure

Alterations in opioid signaling that take place in murine peritoneal macrophages in vitro are variably dependent on opiate exposure conditions. Acute exposure to morphine inhibits Fc-mediated phagocytosis by a pertussis toxin (PT)-sensitive mechanism, but has no effect on cAMP levels. In contrast, chronic exposure to morphine results in a "tolerant" state, wherein test and control values for both phagocytosis and cAMP are equivalent. However, drug withdrawal after chronic exposure to morphine results in inhibition of phagocytosis and a concomitant 4-fold increase in cAMP by a PT-insensitive mechanism. This increase is causally related to inhibition of phagocytosis since an artificial increase in cAMP inhibits phagocytosis in non-withdrawn cells exposed chronically to morphine. We suggest that macrophage opioid receptors signaling switches from a Gi/o-mediated mechanism that does not involve adenylate cyclase in acute exposure to a non-Gi/o-mediated adenylate cyclase superactivation during chronic exposure.     

Can the supersensitivity of rodents to dopamine be regarded as a model of tardive dyskinesia?

1. The paper presents arguments derived from both, clinical work and animal experiments, for or against the traditional hypothesis suggesting that tardive dyskinesia (TD) is caused by supersensitivity to dopamine. The main aim of this study was to answer the question posed in the title - whether the supersensitivity to dopamine evoked in rodents by neuroleptics can be regarded as an adequate pharmacological model of TD. 2. The data presented here prove that chronic administration of neuroleptics to schizophrenic patients cannot be the only factor inducing TD; furthermore, symptoms similar or identical to those of TD are also observed in the course of other disorders, not connected with neuroleptics, e.g. aging or schizophrenia itself. 3. Clinical data offer no clear evidence for the existence of a direct cause-effect relationship between super-sensitivity to dopamine and occurrence of TD. 4. The role of brain degeneration, caused by different factors but in particular by the process of aging, in the pathogenesis of dyskinetic disorders, including TD, has been stressed. 5. Pharmacological and biochemical data show that chronic administration of classic neuroleptics to animals induces an increase in the density of dopamine D-2 receptors (Bmax). It seems that this receptor-mediated supersensitivity may concern both the postsynaptic and the presynaptic D-2 dopamine receptors. On the other hand, it is not clear enough whether a dopamine D-1 receptor-mediated supersensitivity might also be a causal factor of TD. 6. The analysis in animals, of biochemical and pharmacological effects of neuroleptics which do not induce TD showed that in some situations these drugs may also evoke the receptor-mediated supersensitivity concerning dopamine D-2 receptors. 7. The method of a prolonged (approx. 1 year) oral administration of neuroleptics seems to differentiate those which induce TD from those which do not, at least regarding the induction of an increase of Bmax for butyrophenone neuroleptics and an increase of apomorphine-induced stereotypy, however, some exceptions are noted. 8. The above analysis of clinical and experimental data suggests that the supersensitivity to dopamine in rats treated chronically with neuroleptics cannot be accepted as a model which reflects the etiopathogenesis of TD. Neither a positive nor a negative result obtained in this test is reliable enough, and either depends on the tested parameters (apomorphine stereotypy and [3H]spiperon binding seem to be the most reliable), route of neuroleptic administration, duration of treatment and, probably, a number of other, still unknown factors.(ABSTRACT TRUNCATED AT 400 WORDS)     

Dopamine D2(High) receptors moderately elevated by sertindole

Because long-term administration of antipsychotics can cause behavioral dopamine supersensitivity, this study examined whether the antipsychotic sertindole could elicit biochemical changes indicative of dopamine supersensitivity. In rats, behavioral dopamine supersensitivity is consistently associated with an increased proportion of dopamine receptors that have high affinity for dopamine, namely D2(High), in homogenized striata. Nine days of subcutaneously injected sertindole (1.25 mg/kg/day) increased the proportion of D2(High) receptors between 186% and 215%, although the total population of D2 receptors did not change. Although the findings suggest that rats or patients treated with sertindole might exhibit behavioral dopamine supersensitivity, the drug-induced increase in D2(High) receptors was less than that previously found with haloperidol.     

Chronic clozapine, but not haloperidol, alters the response of mesoprefrontal dopamine neurons to stress and clozapine challenges in rats.

Previously, we demonstrated that serotonin-lesioned rats had an enhanced mesoprefrontal dopaminergic response to restraint stress. This study attempted to extend our knowledge regarding this serotonin/dopamine interaction by seeing if suppression of serotonin metabolism by chronic administration of the atypical antipsychotic, clozapine, would have similar effects. Both typical and atypical neuroleptics require chronic administration in humans before antipsychotic activity is seen. Rats treated for 21 days with clozapine or haloperidol, a typical antipsychotic without significant binding affinity for serotonergic receptors, showed lowered basal dopamine metabolism in the medial prefrontal cortex, the nucleus accumbens, and the striatum, as expected. Basal serotonin metabolism in the prefrontal cortex was also lowered by clozapine treatment, but not haloperidol. One of two challenges were given to chronically treated rats: 30 min of restraint stress or an acute challenge of clozapine. When corrected for baseline differences, both challenges significantly elevated dopamine metabolism in the prefrontal cortex of the clozapine group more than the saline or haloperidol groups. No hyperresponsiveness was seen with serotonin metabolism in the prefrontal cortex or either dopamine or serotonin metabolism in the nucleus accumbens in clozapine-treated, challenged rats. Additionally, this augmentation of the dopaminergic stress response was not seen with a single, acute administration of clozapine. The significance of the clozapine-induced hyperresponsiveness of the mesoprefrontal dopamine system is discussed with regard to clinical efficacy of clozapine.     

Dopaminergic neurons: Effect of acute and chronic morphine administration on single cell activity and transmitter metabolism

Summary At various time points following acute and chronic administration of morphine to rats, dopamine transmitter metabolism and neuronal activity were determined. Following acute injection of morphine (20 mg/kg intraperitoneally), dopamine cell firing rates increased slowly and steadily. This slow increase was accompanied by a similar slow increase in the accumulation of the dopamine metabolite, dihydroxyphenylacetic acid (DOPAC). Apparentin vivo tyrosine hydroxylase activity, measured by dopa accumulation following inhibition of dopa decarboxylase, also increased. In chronically treated animals the average firing rate of dopamine cells was measured two hours after the last injection of morphine. The distribution of dopamine cell firing rates was significantly higher than in controls. DOPAC levels andin vivo tyrosine hydroxylase activity were also increased at this time. When morphine (100 mg/kg intraperitoneally) was administered to chronically treated animals 12 hours after the last injection a slow increase of firing rates was observed similar to that seen in naive animals after an acute morphine injection. In chronically morphine treated animals naloxone caused a rapid dose-dependent decrease in firing rates and DOPAC levels.In vivo tyrosine hydroxylase activity was not changed.This work is derived in part from a dissertation presented to the Yale Graduate School by M. C. Nowycky in partial fulfillment of the requirements for the degree of Doctor of Philosophy and was performed during tenure of a United States Public Health Service Predoctoral Fellowship under Training Grant CA 09085.     

Acute and chronic morphine modifies the in vivo release of methionine enkephalin-like immunoreactivity from the cat spinal cord and brain

The effect of acute and chronic morphine treatment was investigated on the spontaneous and evoked release of methionine enkephalin-like immunoreactivity (MELI) in vivo using the technique of cat spinal superfusion and ventriculocisternal perfusion. Stimulation of sciatic nerve, at intensities known to activate small-diameter nerve fibers, resulted in a consistent release of MELI from the spinal cord and the brain. Local application of morphine (5 X 10(-4)M) to the spinal cord resulted in a significant decrease in the evoked release of MELI. Naloxone (2 mg/kg i.v.), administered during morphine treatment, produced an increase in the spontaneous and greatly augmented the evoked release of spinal and ventricular MELI. In morphine-naive animals, naloxone did not affect the spontaneous or evoked MELI release. In cats chronically exposed to parenteral morphine by implantation of morphine pellets (2 X 75 mg), the spontaneous release of spinal and ventricular MELI was significantly greater than this release in control animals. Stimulation of sciatic nerves evoked a normal release of MELI in morphine-pelleted animals. Administration of naloxone to these animals resulted in a large and sustained increase in the spontaneous release of MELI from the spinal cord and brain. The material released by stimulation was identified as methionine enkephalin-like on the basis of similar results with two different antisera, parallel displacement curves with serial dilutions of spinal and ventricular perfusates and comigration with methionine enkephalin on a Sephadex G-25 column. These results suggest that if there is a tonic suppression of enkephalin release mediated by opiate receptors, these receptors display a tolerance development also. The facilitated release by naloxone in the chronic morphine-treated animals may indicate that reversal of the ongoing opioid inhibition results either in an excessive drive of the enkephalinergic neuron by other excitatory systems or the loss of a tonic auto-inhibition, which is not present in the non-tolerant animal.     

Blockade of D-2 dopamine receptors strongly enhances the potency of enkephalins to inhibit dopamine-sensitive adenylate cyclase in rat neostriatum: involvement of delta- and mu-opioid receptors

The interactions between dopamine receptors and opioid receptors coupled to adenylate cyclase in rat neostriatum were investigated. cAMP efflux from neostriatal slices induced by simultaneous activation of (stimulatory) D-1 and (inhibitory) D-2 dopamine receptors with 30 microM dopamine was inhibited by the preferential delta-opioid receptor agonist [D-Ala2-D-Leu5] enkephalin (DADLE) and the mu-opioid receptor agonist morphine with an EC50 of 100 and 800 nM, respectively. On selective D-1 receptor activation (i.e., with D-2 receptors blocked by 10 microM (-)sulpiride), the EC50 of DADLE was strongly reduced to 3 nM, whereas that of morphine was unaffected. When D-1 and D-2 receptors were activated simultaneously, the inhibitory effects of DADLE (0.3 microM) and morphine (3 microM) on cAMP efflux were antagonized equally well by naloxone, a mu-opioid receptor antagonist. In contrast, on selective D-1 receptor activation, naloxone was about 20 times more potent in antagonizing the inhibitory effect of morphine than DADLE. Moreover, the delta-opioid receptor antagonist ICI 174864 (0.75 microM) did not affect the inhibitory effect of morphine but antagonized that of DADLE, provided that D-2 receptors were blocked. The highly selective delta-opioid receptor agonist [D-Pen2-D-Pen5] enkephalin (DPDPE) inhibited dopamine-stimulated cAMP efflux only when D-2 receptors were blocked. Similar results were obtained when the agonists SKF 38393 and LY 141865 were used to activate D-1 and D-2 receptors, respectively. These data indicate that blockade of D-2 receptors in the neostriatum elicits the coupling of delta-opioid receptors to dopamine-sensitive adenylate cyclase, thereby making it considerably more sensitive to inhibition by the enkephalins.