Sonsoles Martı́n

Maternal exposure to Δ9-tetrahydrocannabinol facilitates morphine self-administration behavior and changes regional binding to central μ opioid receptors in adult offspring female rats

Opiates and cannabinoids are among the most widely consumed habit-forming drugs in humans. Several studies have demonstrated the existence of interactions between both kind of drugs in a variety of effects and experimental models. The present study has been focused to determine whether perinatal delta9-tetrahydrocannabinol (Delta9-THC) exposure affects the susceptibility to reinforcing effects of morphine in adulthood and whether these potential changes were accompanied by variations in mu opioid receptor binding in brain regions related to drug reinforcement. Adult female rats born from mothers that were daily treated with delta9-THC during gestation and lactation periods, exhibited a statistically significant increase in the rate of acquisition of intravenous morphine self-administration behavior when compared with females born from vehicle-exposed mothers, an effect that did not exist in delta9-THC-exposed male offspring. This increase was significantly greater on the last day of acquisition period. There were not significant differences when the subjects were lever pressing for food. In parallel, we have also examined the density of mu opioid receptors in the brain of adult male and female offspring that were exposed to Delta9-THC during the perinatal period. Collectively, perinatal exposure to delta9-THC produced changes in mu opioid receptor binding that differed regionally and that were mostly different as a function of sex. Thus, delta9-THC-exposed males exhibited a lower density for these receptors than their respective oil-exposed controls in the caudate-putamen area as well as in the amygdala (posteromedial cortical nucleus). On the contrary, delta9-THC-exposed females exhibited higher density of these receptors than their respective oil-exposed controls in the prefrontal cortex, the hippocampus (CA3 area), the amygdala (posteromedial cortical nucleus), the ventral tegmental area and the periaqueductal grey matter, whereas the binding was lower than control females only in the lateral amygdala. These results support the notion that perinatal delta9-THC exposure alters the susceptibility to morphine reinforcing effects in adult female offspring, in parallel with changes in mu opioid receptor binding in several brain regions.

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.

Modulation of the endogenous opioid system after morphine self-administration and during its extinction: a study in Lewis and Fischer 344 rats.

Lewis (LEW) and Fischer 344 (F344) rats show differential morphine self-administration rates. In this study, after animals of both strains self-administered morphine (1mg/kg) or extinguished this behaviour for 3, 7 or 15days, we measured the binding to, and functional state of mu opioid receptors (MORs) as well as proenkephalin (PENK) mRNA content in several brain regions. The results showed that in most brain areas: 1) LEW rats had less binding to MORs in basal conditions than F344 rats; 2) after morphine self-administration, either one of the strains or both (depending on the brain area) showed increased levels of binding to MORs as compared to basal groups; and 3) these binding levels in morphine self-administration animals came down in each extinction group. Moreover, F344 rats exhibited, in general, an increased functionality of MORs after morphine self-administration, as compared to basal groups, which also went down during extinction. Finally, the basal content of PENK mRNA was lower in LEW rats than in F344 rats and it decreased more after self-administration; during extinction, the levels of PENK mRNA got normalized in this strain. This differential modulation of the endogenous opioid system might be related to the different rates of morphine self-administration behavior exhibited by both inbred rat strains.

Genetic differences in NMDA and D1 receptor levels, and operant responding for food and morphine in Lewis and Fischer 344 rats

Previously, we have shown that Lewis (LEW) rats acquire faster than Fischer 344 (F344) rats operant food- and morphine-reinforced tasks under fixed-ratio schedules of reinforcement. The first purpose of the present work has been to study if differences in operant responding behavior may participate in the reported differences in morphine self-administration behavior between both inbred rat strains. To this end, we have analyzed the microstructure of responding obtained under a variable-interval (VI) of food reinforcement by calculating the inter-response time (IRT) for each rat strain. LEW rats exhibited shorter IRTs than F344 rats, suggesting that LEW rats may have an inherent high or compulsive operant responding activity. When subjects of both inbred rat strains were submitted to a schedule of morphine reinforcement of high responding requirements such as progressive ratio schedules, LEW rats also reached significantly higher breaking points and final response ratio than F344 rats for i.v. morphine self-administration. Given that there are neurochemical differences between both rat strains and that glutamatergic N-methyl-D-aspartate (NMDA) and dopaminergic D(1) receptors have been involved in operant responding behavior, a second purpose of this work has been to measure basal NMDA and D(1) receptor levels in these rat strains by quantitative receptor autoradiography. Compared to F344 rats, LEW rats showed higher basal NMDA receptor levels in frontal and cingulate cortex, caudate putamen, central amygdaloid nuclei, and intermediate white layer of superior colliculus, and higher basal D(1) receptor levels in several areas of hippocampus and thalamus, and substantia nigra pars reticulata. Taken together, these results suggest that an inherent high operant responding activity of LEW rats may have a role in the previous reported faster acquisition of opiate-reinforced behavior in operant self-administration paradigms under fixed-ratio schedules of reinforcement. In addition, a basal higher NMDA and D(1) receptor levels of LEW rats compared to F344 rats may participate in the neurochemical background that mediates the behavioral differences between both inbred rat strains.

Strain differences in the dose–response relationship for morphine self-administration and impulsive choice between Lewis and Fischer 344 rats

Dose–response studies are thought to be a valuable tool to predict the most genetically drug-vulnerable individuals. However, dose–response curves for morphine self-administration have not yet been examined and nor strain differences might be evident. Therefore, this study aimed to define the dose–response curve for morphine self-administration (0.25, 0.5, 1 and 2 mg/kg) in Lewis (LEW) rats and their histocompatible Fischer-344 (F344) rats. In addition, impulsivity has been suggested as one of the genetic factors contributing most to the initiation of drug use. Therefore, the impulsive choice of both rat strains in the presence or absence of the same morphine doses was also analysed. LEW rats self-administered significantly more morphine whatever the dose tested and they exhibited greater basal impulsive choice compared with F344 rats. The F344 strain showed a preference for the dose of 0.5 mg/kg, while any of the doses used had a differential reinforcing effect in the LEW strain. The basal pattern of strain differences in impulsive choice was not affected by morphine administration. These data suggest that the LEW strain has a highly drug-vulnerable phenotype and they point to the strength of impulsivity as a pre-existing behavioural trait that might make this rat strain more vulnerable to the reinforcing effects of drugs and, therefore, to develop addiction.

Effects of perinatal exposure to Δ9-tetrahydrocannabinol on operant morphine-reinforced behavior

The present study examined the effects of Delta(9)-tetrahydrocannabinol (Delta(9)-THC) when administered during the perinatal period on morphine self-administration in adulthood. To this end, pregnant Wistar rats were daily exposed to Delta(9)-THC from the fifth day of gestation up to pup weaning, when they were separated by gender and left to mature to be used for analyses of operant food- and morphine-reinforced behavior in a progressive ratio (PR) schedule. We also analyzed dopaminergic activity (DOPAC/DA) in reward-related structures during specific phases of the behavioral study. In both reinforcement paradigms, food and morphine, females always reached higher patterns of self-administration than males, but this occurred for the two treatment groups, Delta(9)-THC or vehicle. These higher patterns measured in females corresponded with a higher DOPAC/DA in the nucleus accumbens prior to the onset of morphine self-administration in comparison to males. Interestingly, DOPAC/DA was lower in Delta(9)-THC-exposed females compared to oil-exposed females and similar to oil- and Delta(9)-THC-exposed males. In addition, Delta(9)-THC-exposed females also exhibited a reduction in DOPAC/DA in the ventral tegmental area, which did not exist in males. All these changes, however, disappeared after 15 days of morphine self-administration and they did not reappear after 15 additional days of extinction of this response. Our data suggest that females are more vulnerable than males in a PR schedule for operant food and morphine self-administration; perinatal Delta(9)-THC exposure is not a factor influencing this vulnerability. The neurochemical analysis revealed that the activity of limbic dopaminergic neurons prior to morphine self-administration was higher in females than males, as well as that the perinatal Delta(9)-THC treatment reduced the activity of these neurons only in females, although this had no influence on morphine vulnerability in these animals.

Role of Corticotropin-Releasing Hormone in Gastrin-Releasing Peptide-Mediated Regulation of Corticotropin and Corticosterone Secretion in Male Rats

Gastrin-releasing peptide (GRP) exerts several functions within the hypothalamus and may be involved in the regulation of pituitary hormone secretion. The purpose of this study was to investigate the central effect of GRP on hypothalamic-pituitary-adrenal axis activity in the male rat. Intracerebroventricular (i.c.v.) but not intravenous administration of GRP (1, 10, 100 ng/rat) increased plasma ACTH and corticosterone concentrations in a dose-dependent manner. The highest dose (100 ng/rat) of GRP increased plasma ACTH and corticosterone 4- and 14-fold, respectively. This increase peaked at 30–60 min after i.c.v. injection, decreased gradually and returned to baseline levels 240 min after GRP administration. The i.c.v. administration of (Leu13-ψ-CH2NH-Leu14) bombesin, a competitive and specific GRP receptor antagonist, had no effect on ACTH and corticosterone secretion; however, a dose of 1 µg/rat completely blocked the increase of both hormones induced by GRP (10 ng). By using α-helical (9–41) corticotropin-releasing factor (CRF), a competitive antagonist of CRF, the role of CRF on GRP-induced ACTH and corticosterone secretion was also explored. α-Helical (9–41) CRF (10 µg/rat) blocked the increase in ACTH and corticosterone secretion induced by GRP (10 ng). The results obtained in this study suggest that GRP increases the secretion of ACTH and corticosterone in the plasma by acting centrally on GRP receptors, and that endogenous GRP receptor ligands do not tonically regulate ACTH and corticosterone secretion. Furthermore, the hypothalamus-pituitary-adrenal-activating effects induced by GRP appear to be mediated, at least in part, by CRF.

Repeated administration with δ9- tetrahydrocannabinol regulates µ-opioid receptor density in the rat brain

Several studies have demonstrated reciprocal, as well as synergistic interactions between cannabinoid and opioid systems. The aim of this study was to explore the time-related effects of repeated administration of δ9-tetrahydrocannabinol on µ-opioid receptor autoradiography in various brain regions of the rat. To this aim, the effects of δ9-tetrahydrocannabinol (δ9-THC, 5 mg/kg/day; i.p.) were examined after 1, 3, 7 and 14 days of repeated administration on regions containing µ-opioid receptors: (i) forebrain [caudate-putamen, nucleus accumbens (core and shell) and piriform cortex]; (ii) amygdala (medial pars and cortical posteromedial pars), hypothalamus (ventromedial and dorsomedial nuclei, zona incerta), hippocampal regions (CA1, CA2, CA3, dentate girus), hindbrain (substantia nigra and ventral tegmental area); and (iii) thalamus, including 12 thalamic nuclei. In most of these regions, repeated cannabinoid administration increases µ-opioid receptor density; however, the onset, degree of magnitude reached and time-related effects produced by administration with δ9-tetrahydrocannabinol are dependent upon the brain region examined. It appears that the major increase in µ-opioid receptor density occurs 1 and 3 days after δ9-THC administration. In some regions, this increase is maintained and, for most of the brain areas examined, this effect is no longer significant by 14 days of administration, suggesting tolerance to cannabinoid treatment. Taken together, the results of this study suggest that cannabinoids produce a time-related differential responsiveness in µ-opioid receptor density in several brain areas that may be relevant to an understanding of the alterations associated with cannabinoid exposure.

The neurobiology of cannabinoid dependence: Sex differences and potential interactions between cannabinoid and opioid systems

Cannabis is the most widely used illicit drug in many western countries. Its psychoactive ingredient, delta9-tetrahydrocannabinol (THC), produces a variety of effects in animals and humans that are probably mediated by specific cannabinoid receptors in the brain and interactions with several neurotransmitter and neuromodulator systems. For instance, recent research has revealed an important mutual functional relationship between cannabinoids and endogenous opioid systems in mediating the pharmacological and behavioral actions produced by these agents, including their reinforcing effects. Perinatal exposure to and interactions between cannabinoids and opioids might also have long-term behavioral consequences lasting into adulthood. In this work, we present preliminary evidence examining the potential effects of maternal exposure to THC on the motivational properties of morphine in male and female adult rats, as measured by an intravenous opiate self-administration paradigm.

Strain differences between Lewis and Fischer 344 rats in the modulation of dopaminergic receptors after morphine self-administration and during extinction.

The Lewis (LEW) and Fischer 344 (F344) rat strains have been used as a model to study genetic vulnerability to drug addiction and they differ in their dopaminergic systems. We have studied the variation in the D1-like and D2-like receptors in distinct brain regions of LEW and F344 rats that self-administered morphine (1 mg/kg) for 15 days and also after different extinction periods (3, 7 and 15 days). Under basal conditions, binding to D1-like receptors in the olfactory tubercle and substantia nigra, and to D2-like receptors in the Pyriform cortex and hippocampal-CA1 was lower in LEW rats than in F344 rats. Conversely, the LEW rats exhibited stronger D2-like binding in the caudate-putamen. In most brain regions there was a decrease in D1-like binding in LEW rats after self-administration while the F344 animals displayed an increment. Additionally, D2 receptors of LEW rats were down-regulated after self-administration in the caudate-putamen and in the nucleus accumbens (shell and core divisions). Binding to D1-like receptors increased in both strains in the early phases of extinction, while in the later stages a differential regulation was observed between both strains. During the early phases of extinction only F344 rats showed alterations in D2-like receptor binding, however in the latter phases a specific modulation occurred in both strains. These differences in basal D1-like and D2-like receptor binding, and their differential modulation after self-administration and during extinction, may be reflected in the greater vulnerability to opiate addiction shown by LEW strain.