Reversal of morphine analgesic tolerance by ethanol in the mouse.

The chronic use of opioids in humans, accompanied by the development of tolerance, is a dangerous phenomenon in its own right. However, chronic opioid use is often made more dangerous by the coconsumption of other substances. It has been observed that the blood level of opioids in postmortem analyses of addicts, who consumed ethanol along with the opioid, was much less than that observed in individuals who died from opioids alone. This relationship between ethanol and opioids led us to investigate the hypothesis that ethanol alters tolerance to opioids. In the present study, we report that ethanol significantly and dose-dependently reduced the antinociceptive tolerance produced by morphine and the cross-tolerance between [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) and morphine in the mouse tail-flick test. The reversal of morphine tolerance was partially blocked by both the gamma receptor blocker bicuculline and by the γ-aminobutyric acid (GABA)(B) receptor blocker phaclofen and the administration of both inhibitors completely reversed the effects of ethanol on morphine tolerance. Diazepam, like ethanol, decreased morphine tolerance. However, this inhibition was reversed by the GABA(A) antagonist bicuculline but not by the GABA(B) antagonist phaclofen. These findings have important implications for individuals who abuse opioids and ethanol as well as suggest a mechanism to reduce the amount of opioid needed in chronic pain treatment.

Opioid-like adverse effects of tianeptine in male rats and mice.

RATIONALE: Tianeptine is a mu-opioid receptor (MOR) agonist with increasing reports of abuse in human populations. Preclinical data regarding the abuse potential and other opioid-like adverse effects of tianeptine at supratherapeutic doses are sparse. OBJECTIVES: The present study evaluated tianeptine in a rat model of abuse potential assessment and in mouse models of motor, gastrointestinal, and respiratory adverse effects. METHODS: Abuse potential was assessed in adult male Sprague-Dawley rats using an intracranial self-stimulation (ICSS) procedure to determine effects of acute and repeated tianeptine on responding for electrical brain stimulation. Male ICR mice were used to determine the effects of tianeptine in assays of locomotor behavior and gastrointestinal motility. Male Swiss-Webster mice were monitored for respiratory changes using whole-body plethysmography. RESULTS: In rats, acute tianeptine produced weak and delayed evidence for abuse-related ICSS facilitation at an intermediate dose (10 mg/kg, IP) and pronounced, naltrexone-preventable ICSS depression at a higher dose (32 mg/kg, IP). Repeated 7-day tianeptine (10 and 32 mg/kg/day, IP) produced no increase in abuse-related ICSS facilitation, only modest tolerance to ICSS depression, and no evidence of physical dependence. In mice, tianeptine produced dose-dependent, naltrexone-preventable locomotor activation. Tianeptine (100 mg/kg, SC) also significantly inhibited gastrointestinal motility and produced naloxone-reversible respiratory depression. CONCLUSIONS: Tianeptine presents as a MOR agonist with resistance to tolerance and dependence in our ICSS assay in rats, and it has lower abuse potential by this metric than many commonly abused opioids. Nonetheless, tianeptine produces MOR agonist-like acute adverse effects that include motor impairment, constipation, and respiratory depression.

The effect of protein kinase C and G protein-coupled receptor kinase inhibition on tolerance induced by mu-opioid agonists of different efficacy.

Differences in the mechanisms underlying tolerance and mu-opioid receptor desensitization resulting from exposure to opioid agonists of different efficacy have been suggested previously. The objective of this study was to determine the effects of protein kinase C (PKC) and G protein-coupled receptor kinase (GRK) inhibition on antinociceptive tolerance in vivo to opioid agonists of different efficacy. A rapid (8-h) tolerance-induction model was used where each opioid was repeatedly administered to naive mice. Animals were then challenged with the opioid after injection of a kinase inhibitor to determine its effects on the level of tolerance. Tolerance to meperidine, morphine, or fentanyl was fully reversed by the PKC inhibitor 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)carbazole (Gö6976). However, in vivo tolerance to [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) was not reversed by PKC inhibition. The novel small-molecule GRK inhibitors beta-adrenergic receptor kinase 1 inhibitor and 2-(8-[(dimethylamino) methyl]-6,7,8,9-tetrahydropyridol[1,2-a]indol-3-yl)-3-(1-methylindol-3-yl)maleimide (Ro 32-0432) did not reverse the tolerance to meperidine, fentanyl, or morphine but did reverse the tolerance to DAMGO. To correlate GRK-dependent DAMGO-induced tolerance with mu-opioid receptor desensitization, we used in vitro whole-cell patch-clamp recording from mouse locus coeruleus neurons and observed that the GRK inhibitors reduced DAMGO-induced desensitization of mu-opioid receptors, whereas the PKC inhibitor had no effect. These results suggest that tolerance induced by low- and moderate-efficacy mu-opioid receptor agonists is dependent on PKC, whereas tolerance induced by the high-efficacy agonist DAMGO is dependent on GRK.

Role of protein kinase C and mu-opioid receptor (MOPr) desensitization in tolerance to morphine in rat locus coeruleus neurons.

In morphine tolerance a key question that remains to be answered is whether mu-opioid receptor (MOPr) desensitization contributes to morphine tolerance, and if so by what cellular mechanisms. Here we demonstrate that MOPr desensitization can be observed in single rat brainstem locus coeruleus (LC) neurons following either prolonged (> 4 h) exposure to morphine in vitro or following treatment of animals with morphine in vivo for 3 days. Analysis of receptor function by an operational model indicated that with either treatment morphine could induce a profound degree (70-80%) of loss of receptor function. Ongoing PKC activity in the MOPr-expressing neurons themselves, primarily by PKCalpha, was required to maintain morphine-induced MOPr desensitization, because exposure to PKC inhibitors for only the last 30-50 min of exposure to morphine reduced the MOPr desensitization that was induced both in vitro and in vivo. The presence of morphine was also required for maintenance of desensitization, as washout of morphine for > 2 h reversed MOPr desensitization. MOPr desensitization was homologous, as there was no change in alpha(2)-adrenoceptor or ORL1 receptor function. These results demonstrate that prolonged morphine treatment induces extensive homologous desensitization of MOPrs in mature neurons, that this desensitization has a significant PKC-dependent component and that this desensitization underlies the maintenance of morphine tolerance.

Water-soluble derivatives of 1 -tetrahydrocannabinol.

Delta1-Tetrahydrocannabinol, which is resinous and insoluble in water and therefore difficult to study pharmacologically, can be converted to a watersoluble derivative without loss of its biological activity. This has been achieved by preparing esters bearing a nitrogen moiety with the use of carbodiimide as the condensing agent. The availability of such water-soluble derivatives will allow the evaluation of Delta1-tetrahydrocannabinol in self-administration studies in monkeys for its addiction liability potential in man. This technique of water solubilization is also applicable to other compounds of chemical and biological significance.

Region-dependent attenuation of mu opioid receptor-mediated G-protein activation in mouse CNS as a function of morphine tolerance.

BACKGROUND AND PURPOSE: Chronic morphine administration produces tolerance in vivo and attenuation of mu opioid receptor (MOR)-mediated G-protein activation measured in vitro, but the relationship between these adaptations is not clear. The present study examined MOR-mediated G-protein activation in the CNS of mice with different levels of morphine tolerance. EXPERIMENTAL APPROACH: Mice were implanted with morphine pellets, with or without supplemental morphine injections, to induce differing levels of tolerance as determined by a range of MOR-mediated behaviours. MOR function was measured using agonist-stimulated [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTPgammaS) and receptor binding throughout the CNS. KEY RESULTS: Morphine pellet implantation produced 6-12-fold tolerance in antinociceptive assays, hypothermia and Straub tail, as measured by the ratio of morphine ED(50) values between morphine-treated and control groups. Pellet implantation plus supplemental injections produced 25-50-fold tolerance in these tests. In morphine pellet-implanted mice, MOR-stimulated [(35)S]GTPgammaS binding was significantly reduced only in the nucleus tractus solitarius (NTS) and spinal cord dorsal horn in tissue sections from morphine pellet-implanted mice. In contrast, MOR-stimulated [(35)S]GTPgammaS binding was significantly decreased in most regions examined in morphine pellet+morphine injected mice, including nucleus accumbens, caudate-putamen, periaqueductal gray, parabrachial nucleus, NTS and spinal cord. CONCLUSIONS AND IMPLICATIONS: Tolerance and the regional pattern of apparent MOR desensitization were influenced positively by the level of morphine exposure. These results indicate that desensitization of MOR-mediated G-protein activity is more regionally widespread upon induction of high levels of tolerance, suggesting that this response contributes more to high than low levels of tolerance to CNS-mediated effects of morphine.

Antineoplastic activity of cannabinoids.

Lewis lung adenocarcinoma growth was retarded by the oral administration of delta9-tetrahydrocannabinol (delta9-THC), delta8-tetrahydrocannabinol (delta8-THC), and cannabinol (CBN), but not cannabidiol (CBD). Animals treated for 10 consecutive days with delta9-THC, beginning the day after tumor implantation, demonstrated a dose-dependent action of retarded tumor growth. Mice treated for 20 consecutive days with delta8-THC and CBN had reduced primary tumor size. CBD showed no inhibitory effect on tumor growth at 14, 21, or 28 days. Delta9-THC, delta8-THC, and CBN increased the mean survival time (36% at 100 mg/kg, 25% at 200 mg/kg, and 27% at 50 mg/kg, respectively), whereas CBD did not. Delta9-THC administered orally daily until death in doses of 50, 100, or 200 mg/kg did not increase the life-spans of (C57BL/6 times DBA/2)F1 (BDF1) mice hosting the L1210 murine leukemia. However, delta9-THC administered daily for 10 days significantly inhibited Friend leukemia virus-induced splenomegaly by 71% at 200 mg/kg as compared to 90.2% for actinomycin D. Experiments with bone marrow and isolated Lewis lung cells incubated in vitro with delta9-THC and delta8-THC showed a dose-dependent (10(-4)-10(-7)) inhibition (80-20%, respectively) of tritiated thymidine and 14C-uridine uptake into these cells. CBD was active only in high concentrations (10(-4)).

Effects of glucose and diabetes on binding of naloxone and dihydromorphine to opiate receptors in mouse brain.

The effects of glucose and diabetes on the high-affinity lofentanil-displaceable opiate-receptor binding in mouse brain membranes were studied to determine if the attenuation of opiate actions by hyperglycemia previously observed in our laboratory was due to a modification of receptor affinity or number. With membranes from normal ICR mice, glucose (100-400 mg/dl) caused small but significant concentration-dependent decreases in receptor affinities for [3H]naloxone and [3H]dihydromorphine, both in the absence and presence of 20 mM NaCl, without changing the maximum number of binding sites. Fructose and the nonmetabolizable sugar 3-O-methylglucose had intermediate effects on naloxone affinity in the presence of NaCl that were not significantly different from control or from the effect of glucose. Similar results were obtained with brain membranes from streptozocin-induced diabetic mice. The binding affinity for [3H]naloxone in the presence of NaCl was not affected by the induction of diabetes in ICR mice via streptozocin or in spontaneously diabetic (db/db) C57BL/KsJ mice compared with their nondiabetic (m+/m+) littermates. These results indicate that the previously observed attenuation of opiate effects by glucose may be partly due to a glucose-induced decrease in opiate-receptor affinity. However, the much greater attenuation of morphine by fructose in vivo cannot be explained by this mechanism.

Sensitization of enteric neurons to morphine by HIV-1 Tat protein.

BACKGROUND: Gastrointestinal (GI) dysfunction is a major cause of morbidity in acquired immunodeficiency syndrome (AIDS). HIV-1-induced neuropathogenesis is significantly enhanced by opiate abuse, which increases proinflammatory chemokine/cytokine release, the production of reactive species, glial reactivity, and neuronal injury in the central nervous system. Despite marked interactions in the gut, little is known about the effects of HIV-1 in combination with opiate use on the enteric nervous system. METHODS: To explore HIV-opiate interactions in myenteric neurons, the effects of Tat ± morphine (0.03, 0.3, and 3 µM) were examined in isolated neurons from doxycycline- (DOX-) inducible HIV-1 Tat(1-86) transgenic mice or following in vitro Tat 100 nM exposure (>6 h). KEY RESULTS: Current clamp recordings demonstrated increased neuronal excitability in neurons of inducible Tat(+) mice (Tat+/DOX) compared to control Tat-/DOX mice. In neurons from Tat+/DOX, but not from Tat-/DOX mice, 0.03 µM morphine significantly reduced neuronal excitability, fast transient and late long-lasting sodium currents. There was a significant leftward shift in V(0.5) of inactivation following exposure to 0.03 µM morphine, with a 50% decrease in availability of sodium channels at -100 mV. Similar effects were noted with in vitro Tat exposure in the presence of 0.3 µM morphine. Additionally, GI motility was significantly more sensitive to morphine in Tat(+) mice than Tat(-) mice. CONCLUSIONS & INFERENCES: Overall, these data suggest that the sensitivity of enteric neurons to morphine is enhanced in the presence of Tat. Opiates and HIV-1 may uniquely interact to exacerbate the deleterious effects of HIV-1-infection and opiate exposure on GI function.

Elevated CSF beta-endorphin immunoreactivity in Rett's syndrome: report of 158 cases and comparison with leukemic children.

Because some symptoms of Rett's syndrome are suggestive of excessive endogenous opioid activity, we measured the levels of beta-endorphin-like immunoreactivity in lumbar CSF from 158 affected female patients and from 13 female controls. The mean (+/- SE) control level of beta-endorphin immunoreactivity in CSF was 35.3 +/- 2.8 pg/ml (range, 23 to 48 pg/ml), whereas those with Rett's syndrome had a mean level of 95.3 +/- 3.6 pg/ml (range, 31 to 293 pg/ml). The levels of beta-endorphin immunoreactivity in initial CSF samples exceeded the control range in 90% of the patients with Rett's syndrome. The mean beta-endorphin immunoreactivity was also elevated in CSF from leukemic children (119.2 +/- 16.9 pg/ml; range, 40 to 159 pg/ml), relative to the control group. These results are consistent with the hypothesis that some symptoms of Rett's syndrome may be associated with excessive endogenous opioid levels in the CNS.

The spinal cord as a major site for the antinociceptive action of nicotine in the rat.

These studies were conducted to localize the antinociceptive action of nicotine within the CNS. Antinociceptive and biodispositional studies were carried out after the injection of [3H]nicotine subcutaneously and intracerebroventricularly into the common carotid and vertebral arteries and into the subarachnoid space. The data indicated that [3H]nicotine was most potent when given into the subarachnoid space than by any of the other route of administration. Further, the disposition studies showed that [3H]nicotine was almost entirely contained in the thoracic and lumbar areas. These results are consistent with the hypothesis that the spinal cord is an important site for antinociception induced by nicotine.

Some 11-substituted tetrahydrocannabinols. Synthesis and comparison with the potent cannabinoid metabolites 11-hydroxytetrahydrocannabinols.

A series of compounds was prepared in which the 11-hydroxyl of 11-hydroxy-delta8-THC, the potent metabolite of delta8-THC, was replaced by a methyl, methyoxy, amino, or acetamido group. All of the compounds tested produced behavioral changes in dogs, but only the methoxy compound has analgesic properties in mice. An isosteric oxime was inactive in mice.

Some 9-hydroxycannabinoid-like compounds. Synthesis and evaluation of analgesic and behavioral properties.

A series of 9-hydroxylated cannabinoid-like compounds was prepared and tested for analgesic properties in mice and behavioral properties in dogs. Although the prototype compound, 9-nor-9-hydroxyhexahydrocannabinol, has potent antinociceptive activity in laboratory animals, the new analogues were relatively inactive. All of the compounds produced an alteration of behavior in unanesthetized dogs. Two of the compounds produced cannabinoid-like effects and the other two produced general CNS depression.

Hashish: synthesis and central nervous system activity of some novel analogues of cannabidiol and oxepin derivatives of delta 9-tetrahydrocannabinol.

Several C-10 substituted cannabidiol (CBD) derivatives and novel oxepin derivatives of delta 9-tetrahydrocannabinol (delta 9-THC) were synthesized and evaluated for biological activity in mice and dogs. Treatment of 10-bromocannabidiol diacetate (3) with various amines in Me2SO gave the corresponding 10-aminocannabidiol derivatives 4-6. Similarly, treatment of 3 with NaN3 gave the azido compound 7, which with LiA1H4 afforded the 10-aminocannabidiol 9. However, reduction of 7 with CrCl2 formed the amide 8, which on further reduction with LiA1H4 gave the N-ethyl analogue 10. Coupling of 9 with 11 in the presence of dicyclohexylcarbodiimide formed 12, which was then deprotected with HCl to give the analogue 13. The oxepin analogue 14a was synthesized from 3 by treatment with Na2CO3 in CH3OH/H2O at room temperature. The dimethylheptyl analogue 14b was similarly prepared. Incorporation of N-ethyl (10), N-methyl-N-propargyl (6), and morpholino (4) groups in CBD at position 10 resulted in analogues that were more potent than CBD in producing hypoactivity in mice. These analogues had relatively little effect on rectal temperature. Selected substitutions at C-10 also resulted in analogues that were partially effective in blocking delta 9-THC antinociceptive activity. This blockade was observed particularly in compound 10, which also showed unusually toxic properties. Incorporation of a seven-membered oxepin in the delta 9-THC structure eliminated cannabinoid activity although substitution of the pentyl side chain with a 1,2-dimethylheptyl in the oxepin 14b resulted in CNS depression in mice.

Synthesis and pharmacological activity of some 9-substituted delta 8-tetrahydrocannabinol analogues.

Several 9-substituted delta 8-tetrahydrocannabinol (delta 8-THC) analogues were synthesized and evaluated for biological activity in mice. Compounds with phenyl (2b) and butyl (2c) substituents were prepared by the addition of phenyllithium and n-butyllithium, respectively, to (-)-9-nor-9-oxohexahydrocannabinol (1), followed by dehydration, whereas, isopropyl (2d), PhCH2 (2e), and Ph(CH2)2 (2f) derivatives were synthesized via the Grignard reaction with subsequent dehydration. Compounds with C2H5CH(OH) (2g) and CH3CH(OH) (2h) substituents at C-9 were prepared from (-)-9-nor-9-formyl-delta 8-tetrahydrocannabinol acetate (3) by the reaction of ethyl and methyl Grignard reagents, respectively. Biological activity indicated that a methyl group at the C-9 position is, thus far, optimum for producing hypoactivity and hypothermia in mice. In addition, hydroxyethyl substitution at position 9 reduced and antinociceptive activity of delta 8-THC, in contrast to the increased activity reported for hydroxymethyl substitution.

3'-Hydroxy- and (+/-)-3',11-dihydroxy-delta 9-tetrahydrocannabinol: biologically active metabolites of delta 9-tetrahydrocannabinol.

Synthesis of 3'-hydroxy- (7) and (+/-)-3',11-dihydroxy-delta 9-tetrahydrocannabinol (11), metabolites of delta 9-THC, is described. Condensation of the monoterpene (+/-)-cis-p-menth-2-ene-1,8-diol (1) and (+/-)-3'-acetoxyolivetol (2) in the presence of fused ZnCl2 in CH2Cl2 gave a mixture from which the delta 9-THC derivative 3, containing small amounts of the delta 8-isomer 4, was isolated after column chromatography. This mixture was separated as their diacetates 5 and 6 by high-pressure liquid chromatography. Alkaline hydrolysis (5% KOH in MeOH) of 5 furnished the metabolite 7. Condensation of (+/-)-8 with 2 in the presence of p-toluenesulfonic acid gave 9a, which was acetylated to 9b. Treatment with HgO/BF3 X Et2O in wet THF gave the aldehyde 10. Reduction with LiAlH4 furnished the metabolite 11. These metabolites were compared with delta 9-THC for their ability to depress spontaneous activity and rectal temperature in mice and for their effects on overt behavior in dogs. 3'-Hydroxy-delta 9-THC was also compared to delta 9-THC in the mouse sympatomatology test and cardiovascular system in dogs. The metabolites produced pharmacological effects similar to those of delta 9-THC in all tests. 3'-Hydroxy-delta 9-THC was 2-3 times more effective than delta 9-THC in the behavioral tests, whereas (+/-)-3',11-dihydroxy-delta 9-THC was approximately 3 times less active than delta 9-THC.

Optically pure (+)-nicotine from (+/-)-nicotine and biological comparisons with (-)-nicotine.

Optically pure (+)-nicotine has been obtained from (+/-)-nicotine using a combination of d-tartaric acid and di-p-toluoyl-l-tartaric acid. As the di-d-tartrate salt, (+)-nicotine is less potent than (-)-nicotine di-l-tartrate in producing lethality in mice, on blood pressure in anesthetized rats, and in the isolated guinea-pig ileum, indicating substantial stereospecificity for nicotine receptors. Potency ratios are 0.14, 0.06, and 0.019, respectively.

Involvement of phospholipid signal transduction pathways in morphine tolerance in mice.

1. Opioid tolerance involves an alteration in the activity of intracellular kinases such as cyclic AMP-dependent protein kinase (PKA). Drugs that inhibit PKA reverse morphine antinociceptive tolerance. The hypothesis was tested that phospholipid pathways are also altered in morphine tolerance. Inhibitors of the phosphatidylinositol and phosphatidylcholine pathways were injected i.c.v. in an attempt to acutely reverse morphine antinociceptive tolerance. 2. Seventy-two hours after implantation of placebo or 75 mg morphine pellets, mice injected i.c.v. with inhibitor drug were challenged with morphine s.c. for generation of dose-response curves in the tail-flick test. Placebo pellet-implanted mice received doses of inhibitor drug having no effect on morphine's potency, in order to test for tolerance reversal in morphine pellet-implanted mice. Injection of the phosphatidylinositol-specific phospholipase C inhibitor ET-18-OCH3 significantly reversed tolerance, indicating a potential role for inositol 1,4,5-trisphosphate (IP3) and protein kinase C (PKC) in tolerance. Alternatively, phosphatidylcholine-specific phospholipase C increases the production of diacylglycerol and activation of PKC, without concomitant production of IP3. D609, an inhibitor of phosphatidylserine-specific phospholipase C, also reversed tolerance. Heparin is an IP3 receptor antagonist. Injection of low molecular weight heparin also reversed tolerance. PKC was also examined with three structurally dissimilar inhibitors. Bisindolylmaleimide I, Go-7874, and sangivamycin significantly reversed tolerance. 3. Chronic opioid exposure leads to changes in phospholipid metabolism that have a direct role in maintaining a state of tolerance. Evidence is accumulating that opioid tolerance disrupts the homeostatic balance of several important signal transduction pathways.

Effect of intrathecal morphine on the fate of glucose. Comparison with effects of insulin and xanthan gum in mice.

The hypoglycemic effect of morphine (40 micrograms) injected intrathecally (i.t.) was studied with regard to disposition of i.v. [14C]glucose and [3H]2-deoxyglucose and was compared with the effects of two other hypoglycemic agents, insulin (1 IU/kg, s.c.) and xanthan gum (50 mg/kg, i.p.). Mice given i.t. morphine or s.c. insulin exhaled a greater amount of 14CO2 from i.v. [14C]glucose than did control mice given i.t. saline, whereas there was less 14CO2 expiration in xanthan-treated mice. In morphine-treated mice there was less 14C in liver, brain and blood, and more 3H in kidney and hindleg muscle than in control mice. Insulin-treated mice had more 14C in muscle, less 14C in liver, brain, kidney and blood, and less 3H in liver and blood. In xanthan-treated mice, levels of both radiolabels were higher in liver, brain and kidney. Much lower glycogen content in muscle and depletion of liver glycogen occurred in morphine-treated mice, compared with control mice. Spinal transection completely inhibited the hypoglycemic effect of morphine, whereas adrenalectomy caused no inhibition. Morphine, insulin and xanthan appear to be acting by different mechanisms, although the hypoglycemic effects of both morphine and insulin appear to be due largely to an increased glucose uptake by muscle.

A comparison of some pharmacological actions of morphine and delta9-tetrahydrocannabinol in the mouse.

The effects of morphine and delta9-tetrahydrocannabinol(THC)on the tail-flick reflex, body temperature, and catecholamine synthesis were examined in the mouse in order to compare their effects in a single species and strain under uniform conditions. Naloxone antagonism of THC and cross-tolerance between morphine and THC were also studied. Both morphine and THC produced antinociception, hypothermia, and increased catecholamine synthesis at 30 min after s.c. injection. Morphine produced greater increases in dopamine synthesis and was a more potent antinociceptive agent, while THC produced greater increases in norepinephrine synthesis and was a more potent hypothermic agent. Naloxone pretreatment (1 mg/kg) partially antagonized the hypothermia and increase in catecholamine synthesis produced by THC. There was also cross-tolerance between morphine and THC, but it was asymmetric in that THC-tolerant animals were cross-tolerant to only the hypothermic action of morphine and morphine-tolerant animals cross-tolerant to only the antinociceptive action of THC.