Javier Corchero

PHARMACOLOGICAL AND BIOCHEMICAL INTERACTIONS BETWEEN OPIOIDS AND CANNABINOIDS

Opioids and cannabinoids are among the most widely consumed drugs of abuse in humans. A number of studies have shown that both types of drugs share several pharmacological properties, including hypothermia, sedation, hypotension, inhibition of both intestinal motility and locomotor activity and, in particular, antinociception. Moreover, phenomena of cross-tolerance or mutual potentiation of some of these pharmacological effects have been reported. In recent years, these phenomena have supported the possible existence of functional links in the mechanisms of action of both types of drugs. The present review addresses the recent advances in the study of pharmacological interactions between opioids and cannabinoids, focusing on two aspects: antinociception and drug addiction. The potential biochemical mechanisms involved in these pharmacological interactions are also discussed together with possible therapeutic implications of opioid-cannabinoid interactions.

Opioid and cannabinoid receptor-mediated regulation of the increase in adrenocorticotropin hormone and corticosterone plasma concentrations induced by central administration of Δ9-tetrahydrocannabinol in rats

The purpose of this study was to investigate the cannabinoid and opioid mediated regulation on the effects of central Delta(9)-tetrahydrocannabinol (Delta(9)-THC) administration on hypothalamus-pituitary-adrenal (HPA) axis activity in the male rat. Intracerebroventricular (i.c.v.) administration of delta(9)-THC (25, 50, 100 microg/rat) markedly increased plasma adrenocorticotropin hormone (ACTH) and corticosterone concentrations. Time course effect studies revealed that both hormones secretion peaked at 60 min after Delta(9)-THC i.c.v. administration (50 microg/rat), decreased gradually and returned to baseline levels by 480 min. The i.c.v. administration of the specific cannabinoid receptor antagonist SR-141716A (3 microg/rat) significantly attenuated the increase of both hormones secretion induced by Delta(9)-THC (50 microg/rat). Nevertheless, higher doses (12.5 and 50 microg/rat) of this compound increased both ACTH and corticosterone plasma concentrations. Subcutaneous (s.c.) administration with the opiate receptor antagonist naloxone (0.3 mg/kg) was without effect but significantly diminished the increase of both hormones secretion induced by Delta(9)-THC (50 microg/rat). Taken together, these results indicate that opiate and cannabinoid receptors are involved in the activation of the HPA axis induced by Delta(9)-THC. Furthermore, the increase of ACTH and corticosterone secretion after the administration of higher doses of SR-141716A than those required to block such activation, suggests that endogenous cannabinoids are tonically inhibiting the release of both hormones or that this agonist-like activity may be part of an uncharacterized action of this compound not mediated by cannabinoid receptors.

Differential basal proenkephalin gene expression in dorsal striatum and nucleus accumbens, and vulnerability to morphine self-administration in Fischer 344 and Lewis rats

We have previously shown that the acquisition rate of intravenous morphine self-administration under a fixed ratio one (FR1) schedule of reinforcement was greater in Lewis (LEW) than Fischer 344 (F344) rats. The purpose of the present experiment was to examine the relative motivational properties of morphine (1 mg/kg) or food under progressive ratio (PR) schedules of reinforcement in LEW and F344 rats. In addition, by using in situ hybridization histochemistry we have measured in both strains of rats the basal level of proenkephalin (PENK) gene expression in dorsal striatum and nucleus accumbens (NAcc). The results show that LEW rats responded to significantly higher breaking points (BPs) than F344 rats for intravenous morphine self-administration. In contrast, no differences were found in BPs for food pellets. Basal PENK mRNA levels were significantly higher in the dorsal striatum and nucleus accumbens of F344 than in LEW rats. Taken together, these results reveal a strain difference in the reinforcing efficacy of morphine and in the basal PENK gene expression in brain regions involved in the reinforcing actions of opiates. These data also suggest that the strain differences in opiate self-administration behavior found in this and other studies may be related, at least in part, to differences in basal opioid activity between LEW and F344 rats.

Time-course of the cannabinoid receptor down-regulation in the adult rat brain caused by repeated exposure to ?9-tetrahydrocannabinol

Recent studies have demonstrated that the pharmacological tolerance observed after prolonged exposure to plant or synthetic cannabinoids in adult individuals seems to have a pharmacodynamic rather than pharmacokinetic basis, because down‐regulation of cannabinoid receptors was assessed in the brain of cannabinoid‐tolerant rats. In the present study, we have examined the time‐course of cannabinoid receptor down‐regulation by analyzing cannabinoid receptor binding, using autoradiography, and mRNA expression, using in situ hybridization, in several brain structures of male adult rats daily exposed to Δ9‐tetrahydrocannabinol (Δ9‐THC) for 1, 3, 7, or 14 days. With only the exception of a few number of areas, most of the brain regions exhibited a progressive decrease in cannabinoid receptor binding. Two facts deserve to be mentioned. First, the pattern of this down‐regulation process presented significant regional differences in terms of onset of the decrease and magnitude reached. Second, the loss of cannabinoid receptor binding was usually accompanied by no changes in its mRNA expression. Thus, some structures, such as most of the subfields of the Ammons horn and the dentate gyrus in the hippocampus, exhibited a rapid (it appeared after the first injection) and marked (it reached approximately 30% of decrease after 14 days) reduction of cannabinoid receptor binding as a consequence of the daily Δ9‐THC administration. However, no changes occurred in mRNA levels. Decreased binding was also found in most of the basal ganglia, but the onset of this reduction was slow in the lateral caudate‐putamen and the substantia nigra (it needed at least three days of daily Δ9‐THC administration), and, in particular, in the globus pallidus (more than 3 days). The magnitude of the decrease in binding was also more moderate, with maximal reductions always less than 28%. No changes were seen in the entopeduncular nucleus and only a trend in the medial caudate‐putamen. However, the decrease in binding in some basal ganglia was, in this case, accompanied by a decrease in mRNA levels in the lateral caudate‐putamen, but this appeared after 7 days of daily Δ9‐THC administration and, hence, after the onset of binding decrease. In the limbic structures, cannabinoid receptor binding decreased in the septum nuclei (it needed at least 3 days of daily Δ9‐THC administration), tended to diminish in the nucleus accumbens and was unaltered in the basolateral amygdaloid nucleus, with no changes in mRNA levels in these last two regions. Binding also decreased in the superficial and deep layers of the cerebral cortex, but only accompanied by trends in mRNA expression. The decrease in binding was initiated promptly in the deep layer (after the first injection) and it reached more than 30% of reduction after 14 days of daily Δ9‐THC administration, whereas, in the superficial layer, it needed more than 3 days of daily Δ9‐THC administration and reached less than 30% of reduction. Finally, no changes in binding and mRNA levels were found in the ventromedial hypothalamic nucleus. In summary, the daily administration of Δ9‐THC resulted in a progressive decrease in cannabinoid receptor binding in most of the brain areas studied, and it was a fact that always occurred before the changes in mRNA expression in those areas where these existed. The onset of the decrease in binding exhibited regional differences with areas, such as most of the hippocampal structures and the deep layer of the cerebral cortex, where the decrease occurred after the first administration. Other structures, however, needed at least 3 days or more to initiate receptor binding decrease. Two structures, the entopeduncular nucleus and the ventromedial hypothalamic nucleus, were unresponsive to chronic Δ9‐THC administration, whereas others, the medial caudate‐putamen and the basolateral amygdaloid nucleus, only exhibited trends. Synapse 30:298–308, 1998.

IDENTIFICATION OF ENDOCANNABINOIDS AND CANNABINOID CB1 RECEPTOR MRNA IN THE PITUITARY GLAND

Most data on effects of natural and synthetic cannabinoids on anterior pituitary hormone secretion point out to a primary impact on the hypothalamus. There is also some evidence, however, of possible direct actions of these compounds on the anterior pituitary, although the presence of cannabinoid receptors in the pituitary has not been documented as yet. In the present study, we evaluated the presence of cannabinoid CB1 receptor-mRNA transcripts in the pituitary gland by in situ hybridization. We observed CB1 receptor-mRNA transcripts in the anterior pituitary and to a lesser extent in the intermediate lobe whereas they were absent in the neural lobe. We then examined whether CB1 receptor-mRNA levels in both pituitary lobes responded to chronic activation by a specific agonist, as did receptors located in adjacent hypothalamic nuclei and in other brain regions. Daily administration of CP-55,940 for 18 days produced a small, but statistically significant paradoxical increase in CB1 receptor-mRNA levels in the anterior pituitary, with no changes in the intermediate lobe, in contrast to reduced CB1 receptor-mRNA levels observed in the ventromedial hypothalamic nucleus (VMN), and to decreased CB1 receptor binding in the VMN and the arcuate nucleus. The time-course of up-regulation of CB1 receptor-mRNA transcripts in the anterior lobe was biphasic; daily administration of Δ9-tetrahydrocannabinol produced an early and marked decrease in CB1 receptor-mRNA levels after 1 and 3 days, followed by normalization after 7 days and by a small increase after 14 days. We also checked whether endogenous cannabinoid ligands are present in the anterior pituitary and the hypothalamus. Although anandamide itself was detected only in trace amounts, concentrations of its precursor N-arachidonoyl-phosphatidyl-ethanolamine and of 2-arachidonoyl-glycerol were found in both tissues, suggesting that endocannabinoids may be synthetized in the anterior pituitary. In summary, CB1 receptors and corresponding ligands seem to be expressed in cells of the anterior and intermediate lobes of the pituitary, but the response of CB1 receptor-mRNA transcripts in the anterior lobe to chronic agonist activation is different than the desensitization observed in hypothalamic nuclei.

Chronic administration of cannabinoids regulates proenkephalin mRNA levels in selected regions of the rat brain

This study was designed to examine the interactions between the cannabinoid and enkephalinergic systems in the rat brain. To this aim, we have examined the effects of subchronic (5 days) administration (10 mg.kg-1.day-1; i.p.) of delta 9 -tetrahydrocannabinol (THC) or R-methanandamide (AM356) and chronic (18 days) administration with the synthetic cannabinoid receptor agonist CP-55,940 (1 mg.kg-1.day-1; i.p) on proenkephalin (PENK) mRNA levels in several brain regions of the rat. Twenty micrometer brain sections from striatum, nucleus accumbens, paraventricular nucleus, ventromedial nucleus, periaqueductal grey matter and mammillary nucleus were hybridized with an oligonucleotide probe complementary to PENK using in situ hybridization technique. Subchronic administration of THC or AM356 increased PENK mRNA levels in the ventromedial nucleus of the hypothalamus, (82%) and (39%), in the periaqueductal grey matter, (97%) and (49%), and mammillary nucleus, (43%) and (9%), respectively. In contrast, both drugs were without effect in the striatum and nucleus accumbens. On the other hand, chronic administration of CP-55,940 increased PENK mRNA levels in the striatum (44%), nucleus accumbens (25%), paraventricular (31%) and ventromedial nuclei of the hypothalamus (41%). These results revealed that chronic cannabinoid administration increases opioid gene expression in the rat central nervous system and suggest an interaction between the cannabinoid and enkephalinergic systems that may be part of a molecular integrative response to behavioral and neurochemical alterations that occur in cannabinoid drug abuse.

Δ-9-Tetrahydrocannabinol increases prodynorphin and proenkephalin gene expression in the spinal cord of the rat

Hypoalgesia induced by cannabinoid drugs has been found to implicate the opioid system. The effect of five days treatment with delta-9-tetrahydrocannabinol (THC) was examined on prodynorphin (PDYN) and proenkephalin (PENK) gene expression in the spinal cord of male rats. PDYN and PENK gene expression was estimated measuring by northern blot analysis mRNA levels in the whole spinal cord, containing perikarya of these neurons. The subchronic treatment with THC (5 mg/kg/day; 5 days; i.p.) produced an increase in PDYN (39%) and PENK (34%) gene expression when compared with the vehicle treated group. These results suggest that the effects of THC in the spinal cord involve an increase in opioid activity, and therefore sustain the hypothesis of an interaction between the cannabinoid and opioid systems in this region.

Cannabinoids as potential new analgesics.

Among other pharmacological properties analgesia is one of the important features of cannabinoids with therapeutical prospects. Cannabinoids have been shown to produce antinociception in experimental animals and humans. Recently a new system of neuromodulation based upon the existence of cannabinoid receptors and their endogenous agonists has emerged. This has been proposed as another of the endogenous pain control systems. Current evidence indicate an interaction between cannabinoid and opioid systems, the latter being of known relevance in nociception. The fact that either exogenous or endogenous opioids enhanced cannabinoid-induced antinociception suggests simultaneous activation of both opioid and cannabinoid receptors by drugs as a new analgesic strategy.

Δ9-Tetrahydrocannabinol increases proopiomelanocortin gene expression in the arcuate nucleus of the rat hypothalamus

delta 9-Tetrahydrocannabinol, the main psychoactive component of cannabis, produces a large spectrum of pharmacological effects, many of which have been linked to interaction with the opioid system. The aim of this study was to examine the effects of delta 9-tetrahydrocannabinol on proopiomelanocortin (POMC) gene expression in the arcuate nucleus of the hypothalamus and anterior lobe of the pituitary. We report, for the first time, that a 5-day treatment with delta 9-tetrahydrocannabinol (5 mg/kg per day, i.p.) increased (38%) POMC mRNA levels in the arcuate nucleus of the hypothalamus but was without effect in the anterior lobe of the pituitary. These data indicate that delta 9-tetrahydrocannabinol stimulates opioid gene expression and regulates distinctively POMC in the hypothalamus and the anterior lobe of the pituitary in the rat.

Time-dependent differences of repeated administration with Δ9-tetrahydrocannabinol in proenkephalin and cannabinoid receptor gene expression and G-protein activation by μ-opioid and CB1-cannabinoid receptors in the caudate–putamen

The purpose of the present study was to examine the time-related effects of repeated administration of Delta9-tetrahydrocannabinol during 1, 3, 7 and 14 days on cannabinoid and mu-opioid receptor agonist-stimulated [35S]GTPgammaS binding, and CB1 cannabinoid receptor and proenkephalin gene expression in the caudate-putamen. Repeated administration with Delta9-tetrahydrocannabinol produced a time-related reduction in cannabinoid receptor synthesis and activation of signal transduction mechanisms in the caudate-putamen. Indeed, WIN-55,212-2-stimulated [35S]GTPgammaS binding decreased 24% on day 1 and then progressively decreased finding a 42% decrease on day 14. Similarly, CB1 cannabinoid receptor mRNA levels decreased (22%) on day 3, reaching 50% reduction on day 7. In contrast, a pronounced increase is detected in DAMGO-stimulated [35S]GTPgammaS binding and proenkephalin mRNA levels in the caudate-putamen. The highest degree of increase was reached on day 7 of the treatment (35% of proenkephalin mRNA levels and 62% of DAMGO-stimulated [35S]GTPgammaS binding) and then values slightly decreased on day 14. Taken together, the results of the present study indicate that, in the caudate-putamen, repeated administration with Delta9-tetrahydrocannabinol produces a time-related increase in proenkephalin gene expression and mu-opioid receptor activation of G-proteins, and a time-related decrease in CB1 cannabinoid receptor gene expression and reduction in CB1 cannabinoid receptor activation of G-proteins. These results also suggest a possible interaction between the cannabinoid and opioid systems in the caudate-putamen which may be potentially relevant in the understanding of the alterations of motor behavior that occur after prolonged exposure to cannabinoids.