Hormonal status and age differentially affect tolerance to the disruptive effects of delta-9-tetrahydrocannabinol (Δ(9)-THC) on learning in female rats.

The effects of hormone status and age on the development of tolerance to Δ(9)-THC were assessed in sham-operated (intact) or ovariectomized (OVX) female rats that received either intraperitoneal saline or 5.6 mg/kg of Δ(9)-THC daily from postnatal day (PD) 75-180 (early adulthood onward) or PD 35-140 (adolescence onward). During this time, the four groups for each age (i.e., intact/saline, intact/THC, OVX/saline, and OVX/THC) were trained in a learning and performance procedure and dose-effect curves were established for Δ(9)-THC (0.56-56 mg/kg) and the cannabinoid type-1 receptor (CB1R) antagonist rimonabant (0.32-10 mg/kg). Despite the persistence of small rate-decreasing and error-increasing effects in intact and OVX females from both ages during chronic Δ(9)-THC, all of the Δ(9)-THC groups developed tolerance. However, the magnitude of tolerance, as well as the effect of hormone status, varied with the age at which chronic Δ(9)-THC was initiated. There was no evidence of dependence in any of the groups. Hippocampal protein expression of CB1R, AHA1 (a co-chaperone of CB1R) and HSP90ß (a molecular chaperone modulated by AHA-1) was affected more by OVX than chronic Δ(9)-THC; striatal protein expression was not consistently affected by either manipulation. Hippocampal brain-derived neurotrophic factor expression varied with age, hormone status, and chronic treatment. Thus, hormonal status differentially affects the development of tolerance to the disruptive effects of delta-9-tetrahydrocannabinol (Δ(9)-THC) on learning and performance behavior in adolescent, but not adult, female rats. These factors and their interactions also differentially affect cannabinoid signaling proteins in the hippocampus and striatum, and ultimately, neural plasticity.

Chronic administration during early adulthood does not alter the hormonally-dependent disruptive effects of delta-9-tetrahydrocannabinol (Δ9-THC) on complex behavior in female rats.

This study examined whether chronic Δ(9)-THC during early adulthood would produce the same hormonally-dependent deficits in learning that are produced by chronic Δ(9)-THC during adolescence. To do this, either sham-operated (intact) or ovariectomized (OVX) female rats received daily saline or 5.6 mg/kg of Δ(9)-THC i.p. for 40 days during early adulthood. Following chronic administration, and a drug-free period to train both a learning and performance task, acute dose-effect curves for Δ(9)-THC (0.56-10 mg/kg) were established in each of the four groups (intact/saline, intact/THC, OVX/saline and OVX/THC). The dependent measures of responding under the learning and performance tasks were the overall response rate and the percentage of errors. Although the history of OVX and chronic Δ(9)-THC in early adulthood did not significantly affect non-drug or baseline behavior under the tasks, acute administration of Δ(9)-THC produced both rate-decreasing and error-increasing effects on learning and performance behavior, and these effects were dependent on their hormone condition. More specifically, both intact groups were more sensitive to the rate-decreasing and error-increasing effects of Δ(9)-THC than the OVX groups irrespective of chronic Δ(9)-THC administration, as there was no significant main effect of chronic treatment and no significant interaction between chronic treatment (saline or Δ(9)-THC) and the dose of Δ(9)-THC administered as an adult. Post mortem examination of 10 brain regions also indicated there were significant differences in agonist-stimulated GTPγS binding across brain regions, but no significant effects of chronic treatment and no significant interaction between the chronic treatment and cannabinoid signaling. Thus, acute Δ(9)-THC produced hormonally-dependent effects on learning and performance behavior, but a period of chronic administration during early adulthood did not alter these effects significantly, which is contrary to what we and others have shown for chronic administration during adolescence.

Ovarian hormones and chronic administration during adolescence modify the discriminative stimulus effects of delta-9-tetrahydrocannabinol (Δ9-THC) in adult female rats.

Marijuana abuse during adolescence may alter its abuse liability during adulthood by modifying the interoceptive (discriminative) stimuli produced, especially in females due to an interaction with ovarian hormones. To examine this possibility, either gonadally intact or ovariectomized (OVX) female rats received 40 intraperitoneal injections of saline or 5.6 mg/kg of Δ9-THC daily during adolescence, yielding 4 experimental groups (intact/saline, intact/Δ9-THC, OVX/saline, and OVX/Δ9-THC). These groups were then trained to discriminate Δ9-THC (0.32-3.2 mg/kg) from saline under a fixed-ratio (FR) 20 schedule of food presentation. After a training dose was established for the subjects in each group, varying doses of Δ9-THC were substituted for the training dose to obtain dose-effect (generalization) curves for drug-lever responding and response rate. The results showed that: 1) the OVX/saline group had a substantially higher mean response rate under control conditions than the other three groups, 2) both OVX groups had higher percentages of THC-lever responding than the intact groups at doses of Δ9-THC lower than the training dose, and 3) the OVX/Δ9-THC group was significantly less sensitive to the rate-decreasing effects of Δ9-THC compared to other groups. Furthermore, at sacrifice, western blot analyses indicated that chronic Δ9-THC in OVX and intact females decreased cannabinoid type-1 receptor (CB1R) levels in the striatum, and decreased phosphorylation of cyclic adenosine monophosphate response element binding protein (p-CREB) in the hippocampus. In contrast to the hippocampus, chronic Δ9-THC selectively increased p-CREB in the OVX/saline group in the striatum. Extracellular signal-regulated kinase (ERK) was not significantly affected by either hormone status or chronic Δ9-THC. In summary, these data in female rats suggest that cannabinoid abuse by adolescent human females could alter their subsequent responsiveness to cannabinoids as adults and have serious consequences for brain development.

Long-term behavioral and pharmacodynamic effects of delta-9-tetrahydrocannabinol in female rats depend on ovarian hormone status.

Abuse of Δ9-THC by females during adolescence may produce long-term deficits in complex behavioral processes such as learning, and these deficits may be affected by the presence of ovarian hormones. To assess this possibility, 40 injections of saline or 5.6 mg/kg of Δ9-THC were administered i.p. daily during adolescence to gonadally intact or ovariectomized (OVX) female rats, yielding four treatment groups (intact/saline, intact/THC, OVX/saline, and OVX/ THC). Δ9-THC (0.56-10 mg/kg) was then re-administered to each of the four groups during adulthood to examine their sensitivity to its disruptive effects. The behavioral task required adult subjects to both learn (acquisition component) different response sequences and repeat a known response sequence (performance component) daily. During baseline (no injection) and control (saline injection) sessions, OVX subjects had significantly higher response rates and lower percentages of error in both behavioral components than the intact groups irrespective of saline or Δ9-THC administration during adolescence; the intact group that received Δ9-THC had the lowest response rates in each component. Upon re-administration of Δ9-THC, the groups that received adolescent ovariectomy alone, adolescent Δ9-THC administration alone, or both treatments were found to be less sensitive to the rate-decreasing effects, and more sensitive to the error-increasing effects of Δ9-THC than the control group (i.e. intact subjects that received saline during adolescence). Neurochemical analyses of the brains from each adolescent-treated group indicated that there were also persistent effects on cannabinoid type-1 (CB-1) receptor levels in the hippocampus and striatum that depended on the brain region and the presence of ovarian hormones. In addition, autoradiographic analyses of the brains from adolescent-treated, but behaviorally naïve, subjects indicated that ovariectomy and Δ9-THC administration produced effects on receptor coupling in some of the same brain regions. In summary, chronic administration of Δ9-THC during adolescence in female rats produced long-term effects on operant learning and performance tasks and on the cannabinoid system that were mediated by the presence of ovarian hormones, and that altered their sensitivity to Δ9-THC as adults.