Opiate dependence and withdrawal--A new synthesis? more

Pharmacology Biochemistry& Behavior, Vol. 23, pp. 863-870, 1985. ©Ankho InternationalInc. Printed in the U.S.A. 0091-3057/85$3.00 + .00 Opiate Dependence and Withdrawal A New Synthesis? C O L I N A. H E N D R I E P h a r m a c o e t h o l o g y Laboratory, School o f Studies in Psychology, University o f Bradford Bradford B D 7 IDP, West Yorkshire, England HENDRIE, C. A. Opiate dependence and withdrawal---A new synthesis? PHARMACOL BIOCHEM BEHAV 23(5) 863-870, 1985.--There is a growing body of evidence to suggest that adrenocorticotropin (ACTH) may have a physiological role as an endogenous contra-opioid agonist. In addition to having appreciable affinity for opiate receptors and inducing many bebavioural and intracellular effects opposite to those observed following opioid administration, ACTH may interact with endorphins in a mutually antagonistic manner. On the basis of these data a model of opiate dependence is proposed whereby several aspects of the opiate abstinence syndrome may be attributed to the excitatory actions of ACTH acting at opiate receptors. Thus, it may be predicted that opiate antagonist administration during primary abstinence should significantly attenuate many aspects of this behavioural syndrome. The present study was conducted in order to investigate this hypothesis. Results inicated that whilst naloxone (1.5 mg/kg) exerted little influence in non-dependent animals, it significantly attenuated abstinence-exacerbated grooming, body shaking, teeth chattering and sneezing, in addition to completely antagonizing withdrawal hyperalgesia in post-dependent animals. These data are consistent with the proposed existence of an endogenous contra-opioid ligand, the antagonism of which markedly reduces the severity of the morphine withdrawal syndrome. Morphine abstinence Naloxone Opiates Opioids Contra-opioids sis. Levels of endorphins are consequently further reduced, and thus a vicious cycle is developed. Upon cessation of drug regimen, the dependent system is temporarily devoid of all opiate or opioid inhibitory influences and thereby enters into a state of withdrawal. Similarly, Gold et al. [22] propose that the functional integrity of endorphinergic mechanisms in the locus coeruleus may be compromised by chronic opiate administration. In this instance, withdrawal is seen as being the result of the release of neurotransmitters more usually under endorphinergic inhibitory control. Thus, on the view that withdrawal is the consequence of the absence of endorphins, it has been suggested that only opioid agonists may supress the abstinence syndrome [44]. However, in contrast to the above view, several lines of evidence now indicate that the opiate withdrawal syndrome may instead be mediated by the presence of an endogenous ligand which competes with and has functional consequences opposite to those normally ascribed to opiates. Evidence further suggests that this "contra-opioid agonist" [27] may be ACTH. Firstly, ACTH induces many behavioural alterations that are in direct contrast to those induced by fl-endorphin. For example, ACTH stimulates sexual behaviour [5,7] and induces hyperalgesia [6] whilst fl-endorphin inhibits sexual responding [46,47] and is a powerful analgesic [9, 68, 70]. Similarly, at the intracellular level, ACTHI_._,~ stimulates adenylate cyclase activity leading to increased levels of cyclic 3', 5'-adenosine monophosphate (cAMP) [57, 83, 84] whilst opiates inhibit adenylate cyclase and prostaglandin-E stimulated cAMP formation [13, 26, 49, 59, 88]. Secondly, in addition to the well documented relationship between ACTH and fl-endorphin, both in terms of synthesis and aspects of their IN view of the high abuse potential of opiates such as morphine [23], coupled with the difficulty in weaning addicts off such drugs, the isolation and identification of the endogenous opioid peptides [14,33] heralded great hopes for the development of a new class of powerful non-addictive analgesics. However, even before the end of the 1970's, it had become apparent that this was not to be the case, with several groups reporting the develpment of tolerance and dependence following chronic administration of both fl-endorphin [69, 73, 74] and synthetic enkephalin analogues [8, 15, 80, 81], in addition to their cross-tolerance with morphine [45, 55, 77]. Further, tolerance has been shown to develop following even a single injection of fl-endorphin [35] whilst naloxone is able to precipitate withdrawal after only 24 hours of repeated peptide administration [32, 60, 86, 87]. Thus, as it has become increasingly unlikely that synthetic opioids will be of clinical usefulness as non-addictive analgesics, the need to develop a greater understanding of the mechanisms of tolerance and dependence per se remains undiminished. The finding that naloxone can precipitate withdrawal in dependent animals [82], and inhibit the development of tolerance and dependence when administered concomitantly with opiate agonists [10,34], strongly indicates the involvement of opiate receptors in these phenomena. Therefore several authors have postulated that exogenous opiates induce tolerance and dependence through an interaction with endogenous opioidergic mechanisms [23, 31, 39]. For example, Kosterlitz and Hughes [39] have suggested that exogenous opiates inhibit the synthesis and release of endorphins. The resultant decrease in opioid production induces a need for greater dosages of opiate in order to maintain homeosta- 863 864 distribution (for review see [51]), many effects of ACTH have been found to be naloxone-reversible [21,38]. These data suggest an action of ACTH at opiate receptors, a view supported by demonstrations that ACTHI_~, ACTH1 2, and ACTH4 ,~ have appreciable affinity for opiate binding sites I 1, 64, 66]. Thirdly, /3-endorphin has been found to antagonise ACTH~ .,~induced yawning/stretching syndrome, spontaneous penile erections and hyperalgesia [6,18]. Conversely. ACTH1 24 attenuates /3-endorphin-induced analgesia [2,31, catalepsy [18] and elevated prolactin levels [16] in addition to curtailing opioid-mediated environmentally-induced alterations in nociception [24]. In view of these findings, it has been suggested that ACTH may have a physiological role as an endogenous opioid antagonist [2, 6, 65]. In accordance with this hypothesis ACTH has been shown to competitively inhibit opiate binding in vitro [40, 66, 85] whilst ACTH1 24 selectively displaces/3-endorphin from opiate receptors in a dose dependent manner [1]. Finally, whilst observations of parallel increases in stress-induced ACTH and/3-endorphin secretion have led to the conclusion that these peptides are released concomitantly [17, 25, 42, 75], there is now evidence to suggest that they be under mutual physiological control, each influencing the release of the other. Opiates or opioids stimulate ACTH activity [37, 43, 48, 72, 75] whilst ACTH or corticosteroids inhibit/3-endorphin output [56,61]. Thus, it is postulated that a feedback mechanism exists between these peptides./3-endorphin inducing ACTH release, which in turn (perhaps via corticosteroids) inhibits /3-endorphin. On this view, whilst there would be a short time-lag between stimulated/3-endorphin and ACTH output (i.e. [52]), these peptides could appear to be increasing and decreasing in parallel. In summary, ACTH has affinity for opiate receptors, interacts with/3-endorphin in a mutually antagonistic manner and induces many behavioural and cellular effects opposite to those observed following opioid administration. Together, these data suggest a physiological role for A C T H as an endogenous contra-opioid agonist (i.e., a ligand interacting with opiate receptors but producing detectable effects opposite to those induced by the traditional receptor agonist). Further, these findings may also indicate a significant influence of ACTH in the mediation of opiate tolerance and dependence. On the basis of these data, a model of dependence is proposed whereby many aspects of the opiate abstinence syndrome may be attributed to the excitatory actions of ACTH. A schematic representation of this model is presented in Fig. 1. TOLERANCE/DEPENDENCE In the first instance, it is assumed that under baseline conditions in opiate naive animals there is a functional balance between excitatory ACTH influences and/3-endorphin mediated inhibition. In behaving organisms, this balance may be tipped in either direction by environmental circumstance. Thus, certain situations may stimulate ACTH release thereby inducing hyperalgesia and increased grooming behavior [6,20] whilst other situations (e.g., uncontrollable noxious stimuli) activate endorphinergic mechanisms such that analgesia becomes the salient reaction [53,54]. Acute opiate administration to a large extent mimics the actions of endogenously released opioids [62]. Thus, potency of analgesic reaction is dependent on the degree of imbalance in the ACTH/opioid ratio, whilst duration is controlled by (i) rate of opiate elimination and (ii) rate of opioid-induced ACTH re- HENDRIE I Acute d Tolerant Dependent Withdrawn FIG. 1. Model of opiate tolerance, dependence and withdrawal. Schematic representation of proposed mechanism for opiate tolerance, dependence and withdrawal. (White semi circle=opiate receptor; black arrows=opioid; hatched arrows=ACTH). In situations where the ACTH/opioid balance is shifted in either direction the effects of that peptide become salient. Thus, opiate receptor blockade may (i) attenuate opiate analgesia (Acute), (ii) precipitate withdrawal (Tolerant/Dependent) or (iii) reduce the severity of the opiate abstinence syndrome by blocking the unattenuated actions of ACTH (Withdrawn), depending on timing of antagonist administration. See text for details. lease. Eventually, the ACTH/opioid ratio equalizes, at which point analgesia is no longer detectable. ACTH levels are then gradually reduced as the opiate continues to be eliminated. In situations where opiate administration is continued at a constant dosage, tolerance is induced. Essentially, in a chronic drug regimen, each successive administration of opiate acts to maintain ACTH concentrations at excessively high levels. Subsequent doses of opiate fail to induce analgesia as the ACTH/opioid ratio is not significantly shifted toward inhibition. In constant release preparations [78], tolerance occurs when ACTH levels are increased to counterbalance functionally equivalent opiate concentrations. As above, when the ACTH/opioid ratio has balanced, analgesia and other behavioural influences are not detected. Dependence occurs when each successive dose of opioid is sufficiently greater than the previous treatment. In the same manner as previously described, the opioid induces a corresponding increase in the levels of ACTH. However, the following larger dose of opioid enables a significant shift in the ACTH/opioid ratio to occur such that opioid-induced behavioural changes may be observed. The greater concentrations of opioid introduced into the system in chronic dependence-inducing regimens stimulate the release of more and more ACTH. Eventually, considerable amounts of opioid are required simply to counteract the residual effects of the high levels of ACTH produced as the result of the previous administration of increasing doses of opioid. Therefore, tolerance and dependence are induced by the same mechanisms. The former is achieved when successive doses of opioid are insufficient to overcome the excitatory effects of increased ACTH activity, whilst the latter ensues when the available concentrations of opioid are significantly greater than previous doses. In both instances, the N A L O X O N E A N T A G O N I S M O F M O R P H I N E A B S T I N E N C E SYNDROME 865 100 60 v/v V nxmSv mS6x 60 50 ~t I Maintenance Sleeping ** p< 0.01 0 Ambulation ,k It 10 Rearing -Or 10 Investigation ~t "Jr 0 10 10 20 - 250 0 III Body Shake 0 ,T t. ,A- -h 0 I I 0 Teeth Chatter Sneezing FIG. 2. Effects of abstinence and naloxone ( 1.5 mg/kg) on behaviours displayed by morphine dependent rats. Frequency data are presented as medians (Lower-Upper quartiles). Statistical analysis revealed significant abstinence-induced exacerbations of maintenance, body shake, teeth chatter and sneezing (MS/V) and their marked attenuation by naloxone (MS/NX). This antagonist further induced significant increases in sleeping time (seconds) and consequently decreases in ambulation, rearing and investigatory behaviours. See text for details. ACTH/opioid ratio eventually stabilizes but, in the case of dependency, at higher and higher levels. WITHDRAWAL Thus, it can be seen that upon cessation of drug regimen opioid elimination continues, producing a shift in the ACTH/opioid ratio towards excitation, with ACTH effects becoming increasingly more pronounced as the level of opioid continues to fall. Therefore, it may be predicted that opiate withdrawal syndrome is mediated by the unattenuated influence of ACTH. This view has been proposed by other workers [7,36] and is consistent with observations that (i) ACTH~ ._,4 precipitates withdrawal in morphine dependent rats [7] and (ii) opiate withdrawal is correlated with a significant increase in ACTH levels [50]. In this context it is noteworthy that central injections of ACTH, e, may induce many of the behavioural effects observed during the opiate withdrawal syndrome [36]. Whilst the postulated involvement of ACTH in the mediation of opiate withdrawal may provide an explanation for the mechanisms of the abstinence-induced withdrawal syndrome, it does not account for the phenomenon of naloxone-precipitated withdrawal. The current model may however provide such an explanation. As has been previously stated, ACTH and /3-endorphin are maintained in functional equilibrium under normal circumstances, with a shift in either direction rendering the effects of that peptide salient. This balance has two components, ACTH-induced excitation and/3-endorphin mediated inhibition. In tolerant and dependent animals, the exogenous opiate acts synergistically with the endogenous opioid and hence the equation is held at a higher level than in drug naive animals. Under these circumstances therefore, behavioural and cellular excitations are held in check by opioid-mediated inhibitory influences. However, upon administration of an opiate antagonist both ACTH and opioid effects are blocked. Thus, in the abLwnce of any inhibitory influences withdrawal ensues. Since this antagonist-induced disinhibition is (by contrast to endogenous ACTH-mediated excitation) very rapidly produced, precipitated withdrawal may be expected to be of greater intensity than abstinence-induced withdrawal. Further, the effects of opiate receptor blockade should be directly related to the level at which the ACTH/opioid ratio is held. In support of this suggestion it has been found that the withdrawal syndrome in animals rendered dependent to high doses of morphine is significantly more severe than in rats chronically exposed to lower levels [67]. Therefore, precipiated withdrawal appears to be the result of the antagonism of opioid-induced inhibition, whilst abstinence-induced withdrawal is mediated by the excitatory influences of ACTH acting at opiate receptors. On this view it may be predicted that opiate antagonist administration during primary abstinence should significantly attenuate ACTH 866 HENDRIE ,o I sI 0 Ambulation Rea ring Investigation Maintenance 1 * 1 0 ..... Body S h a k e TT 0 Ii ii4 * T T Teeth Chatter Sneezing FIG. 3. Effects of abstinence and naloxone (1.5 mg/kg) on behaviours displayed by morphine dependent rats. Analysis excluding periods of somnolence. Data are presented as medians (Lower-Upper quartiles) of mean behavioural responding per minute active. Column (C)I =V/V; C2=V/NX; C3=MS/V; C4=MS/NX. Whilst naloxone had no effect on measures of ambulation, rearing or investigation in post-dependent animals, this antagonist induced marked decreases in maintenance, body shake, teeth chatter and sneezing behaviours. See text for details. mediated excitation and thereby reduce the severity of the opiate withdrawal syndrome. The present study was conducted in order to investigate this hypothesis. METHOD PROCEDURE Following 3 days habituation to sweetened milk solutions, rats were assigned to drug condition in randomised counterbalanced order, with 20 animals exposed to 0.5 mg/ml morphine sulphate solutions available ad lib for a period of 9 days (mean intake=74.6 ml/day). The remaining 20 animals were exposed to milk alone for the same period (mean intake=98.8 ml/day). At the end of the 9th day both milk and morphine sulphate solutions were withdrawn. Twenty-four hours later, animals were transported to the laboratory, habituated for two hours and randomly assigned to receive either 1.5 mg/kg naloxone hydrochloride or 0.9% saline. In all, four experimental conditions (n= 10/group), milk/saline (V/V), milk/naloxone (V/NX), morphine sulphate/saline (MS/V) and morphine sulphate/naloxone (MS/NX) were used. Ten minutes post-injection animals were introduced into an observation cage (45 x 28 x 20 cm) and their behaviour recorded on videotape over the next 15 minutes. Immediately following this period, hot-plate tests were carried out, after which temperatures were recorded. The experimenter remained blind to drug condition throughout, with codes only being broken after complete analysis. Subjects Forty adult male Sprague-Dawley rats (350-450 g) from Bradford University Breeding Unit were used. Animals were individually-housed for 3 weeks prior to the start of experimentation (cage size: 45 x 28 x 30 cm) and maintained on a 12 hour light/dark cycle (lights on: 0800 hr) in a temperature controlled room ( 2 4+ I°C). Food and water were available ad lib throughout the study. Apparatus Analgesia was assessed by measuring the latency to hind-paw lick in response to hot-plate stimulation, the temperature of which was controlled at 5 5 - I ° C . A cut-off of 120 sec was used to prevent tissue damage. Colonic temperature was measured in tube-restrained animals by means of an ITT u23uS thermistor probe inserted 3 cm into the rectum for 20 seconds. Behaviour was recorded on videotape for subsequent detailed analysis. Only the camera was present in the testing laboratory with ancillary equipment being housed in an adjacent room. Data Analysis Analysis of behavioural data was performed utilising computer-aided data-logging techniques [28,29] to record frequency (f) and duration (d) of behavioural elements. In view of their non-parametric nature, data for each behavioural element were analysed by Kruskal-Wallis one-way analysis of variance in cases of significance, further comparisons were performed using Mann-Whitney tests. Hot-plate latency data were initially subjected to a twofactor analysis of variance (ANOVA: Factor A, chronic milk or morphine; Factor B, acute saline or naloxone). Further Drugs Morphine sulphate was dissolved in 0.4 g/ml sweetened milk, solutions of which were freshly prepared twice daily. Naloxone hydrochloride was dissolved in physiological saline (0.9%). All injections were performed intraperitoneally in a volume of 1 ml/kg. NALOXONE ANTAGONISM OF MORPHINE ABSTINENCE SYNDROME 12 39' 867 10 8 o 0 38m ® ¢,1 -k 6 I E 0 ÷ 37" 4 36 V/V V/NX MS/V MS/NX FIG. 5. Effects of morphine abstinence and naloxone on rectal temperature. Data are presented as means (_+SEM). Statistical analysis revealed significant abstinence-induced hypothermia (MS/V) and the lack of effect of naloxone in antagonizing this response (MS/NX). *p<0.05, **p<0.01. v,/ V v/ Nx M~ V M~, Nx FIG. 4. Effects of morphine abstinence and naloxone on hot-plate latencies. Data are expressed as means (_+SEM). Statistical analysis revealed significant abstinence-induced hyperalgesia (MS/V) and its complete attenuation by naloxone (MS/NX). *p<0.05. analytical comparisons, which were required in view of significant interactions, were performed using Dunnett's procedure for comparing all treatments with control. Temperature data were also subjected to 2-factor A N O V A and subsequent Dunnett's tests. RESULTS Behavioural data are summarized in Fig. 2. Results indicate firstly, a significantly greater incidence of maintenance grooming, body shaking, teeth chattering and sternutation in morphine sulphate/saline (MS/V) treated animals as compared with milk/saline (V/V) controls, data which strongly suggest the successful induction of dependence. Secondly, as the only measure altered in the milk/naloxone (V/NX) condition was a slight, but significant, decrease in investigatory behaviour, naloxone would appear to exert very little influence in non-dependent rats. Thirdly, and in direct contrast to the above, naloxone induced marked changes in the behaviour of post-dependent animals, with significant attenuation of abstinence-exacerbated maintenance, body shaking, teeth chattering and sneezing behaviours being coupled with a considerable increase in sleeping time (p<0.01). As a consequence of the latter effect, measures of ambulation, rearing and investigation were all significantly reduced (p<0.01). Finally, whilst maintenance behaviours and sneezing were not appreciably different from milk/saline controls, body shaking and teeth chattering behaviours in the morphine sulphate/naloxone (MS/NX) treatment group were increased (p<0.01). Importantly, however, the incidence of both body shaking and teeth chattering in these animals were significantly lower (p<0.01) than those observed in the morphine sulphate/saline condition. In view of the marked increase in sleeping time observed in the morphine sulphate/naloxone treatment condition, it was possible that the apparent antagonism of the morphine withdrawal syndrome was produced as the result of a previously unreported sedative action of naloxone under these conditions. Although decreases in body tone and reactivity (consistent with this hypothesis) were not observed, data were re-analysed excluding periods of somnolence. Standardised behavioural data, expressed as mean behavioural responding per minute active, are presented in Fig. 3. This analysis revealed that whilst naloxone had no significant influence on measures of ambulation, rearing or investigation, marked decreases (as compared to morphine sulphate/saline control) in abstinence-induced behavioural exacerbations were still observed. These data are indicative of a significant naloxone-induced attenuation of behaviours usually associated with the morphine abstinence syndrome. Hot-plate latency data are summarized in Fig. 4. A N O V A revealed a significant chronic-× acute-treatment interaction, F(1,36)=4.4, p<0.05, which further detailed analysis indicated to be due to significant reductions in pain latencies under MS/V, t(36)=2.82, p<0.05. These data demonstrate profound abstinence-induced hyperalgesia (MS/V) and, as 868 V/V and MS/NX were not significantly different, t(36)= 1.18, ns, its complete attenuation by 1.5 mg/kg naloxone. Temperature data are summarized in Fig. 5. ANOVA applied to these data also indicated a significant chronic-× acute-treatment interaction, F(1,36)=4.21, p<0.05, which follow-up tests revealed to be due to an influence of differences between V/V-MS/V, t(36)=3.77, p<0.01, and V/VMS/NX, t(36)=2.95, p<0.05. These data are indicative of naloxone-insensitive hypothermia during primary abstinence. DISCUSSION HENDRIE ACTH, naloxone antagonises these effects. Inhibition is thereby reinstated and a marked reduction in the severity of the morphine withdrawal syndrome ensues. Whilst firm conclusions concerning the role of ACTH in the mediation of opiate withdrawal cannot be made on the basis of present data, it is noteworthy that in addition to the in vitro and in vivo evidence suggesting a role for ACTH as an endogenous contra-opioid agonist, many of the biochemical correlates of withdrawal are also consistent with the proposed involvement of ACTH in this phenomenon. Acute administration of opiates decrease intracellular concentrations of cAMP and CA-", whilst chronic exposure increases these levels. In abstinent systems both cAMP and cytosol Ca e~ concentrations are raised to supranormal levels. Thus, it has been postulated that these changes in the adenylate cyclase-cAMP "second messenger" system (for review see [11]) and Ca e` (for review see [79]) may reflect cellular hyperirritability that is more usually inhibited by the actions of opioids. However, as ACTH stimulates adenylate cyclase and cAMP activity in a calcium dependent manner [19, 57, 83, 84] situations where ACTH activity is increased should be correlated with raised intracellular concentrations of adenylate cyclase, cAMP and Ca'-". Thus, the increases in these cellular parameters observed during opiate withdrawal are in accordance with the proposed mediation of this syndrome by the disinhibitory influences of opiate antagonists or the excitatory actions of ACTH. Interestingly, brain homogenates of post-dependent rats have been found to contain a peptide fraction which specifically inhibits morphine analgesia [71] thereby further suggesting that opiate withdrawal is correlated with the presence of an endogenous contra-opioid agonist rather than the absence of opioids per SO. Current data confirm that oral self-administration is an effective method for the induction of morphine dependence in rats [41], as indicated by observed behavioural [12], nociceptive [67,76] and thermic [44] responses during withdrawal. Further, in the absence of major effects in nondependent animals, naloxone markedly attenuated all abstinence-induced behavioural exacerbation whilst significantly increasing sleeping time. Measures of ambulation, rearing and investigation were not significantly different from milk/saline controls when data were re-analysed to allow for the possible confounding influence of this reaction. Therefore, the observed increase in sleeping time does not account for the pronounced antagonism of abstinence-induced increases in grooming, body shaking, teeth-chattering and sneezing produced by naloxone in post-dependent animals. Finally, withdrawal-induced hypothermia was not influenced by naloxone administration, perhaps indicating its greater resistance to attenuation by opiate antagonists or its mediation by non-opioid mechanisms. By contrast however, naloxone completely attenuated abstinence-induced hyperalgesia, a finding entirely consistent with previous demonstrations of naloxone-sensitive environmentallyinduced hyperalgesia [30,53]. Together, these data suggest a significant involvement of opioidergic mechanisms in the mediation of abstinence-induced behavioural exacerbations and withdrawal hyperalgesia. Patently, current findings are at variance with the wealth of literature concerning naloxone-precipitated withdrawal. Whilst it is possible that this compound has agonist-like properties in post-dependent animals, this interpretation would seem unlikely in view of evidence indicating naloxone to have relatively pure antagonist actions in rats at doses of less than 5 mg/kg [58]. However, although more extensive experimentation is needed to further clarify this point, it is believed that the present discrepancy may relate to timing of antagonist administration. On the basis of the current model it is envisaged that naloxone in dependent animals would block the actions of both opioids and ACTH. Withdrawal is therefore the result of naloxone-induced disinhibition. Conversely, in abstinent animals where it is postulated that withdrawal is mediated by the unattenuated excitatory actions of Thus, in conclusion, a model of opiate tolerance and dependence is proposed whereby withdrawal is a consequence of the disruption of a functional balance held between ACTH-excitatory and opioid-mediated inhibitory influences. Data indicating an ability of naloxone to markedly attenuate abstinence-induced behavioural exacerbations are consistent with this view and the hypothesis first proposed by Tatum et al. 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