Pharmacokinetic and Pharmacodynamic Characterization of Tetrahydrocannabinol-Induced Cannabinoid Dependence After Chronic Passive Cannabis Smoke Exposure in Rats

Introduction: Cannabis is the most widely used illicit drug in the US, and cannabis use among young adults continues to rise. Previous studies have shown that chronic administration of delta 9-tetrahydrocannabinol (THC), the main psychoactive component of cannabis, induces dependence in animal models. Because smoking is the most frequent route of THC self-administration, it is critical to investigate the effects of cannabis smoke inhalation. The goal of the current study was to develop a rat model to characterize the pharmacokinetics (PKs) of THC after cannabis smoke inhalation, and to determine if chronic cannabis smoke inhalation leads to the development of cannabis dependence. Materials and Methods: For the PK study, male Wistar rats were administered THC intravenously (1 mg/kg) or exposed to smoke from 5 or 10 sequentially smoked cannabis cigarettes (5.3% THC) in an automated smoking machine. Plasma samples were collected from 10 min to 10 hours post smoke exposure (or intravenous administration) and analyzed using liquid chromatography-mass spectrometry to characterize the PK of THC. A three-compartment PK model was used to characterize the PKs. In a separate study, three groups of male Wistar rats were trained in an intracranial self-stimulation (ICSS) procedure, and exposed to smoke from burning 5 or 10 cannabis cigarettes (or clean air control conditions), 5 days/week for 4 weeks. Discussion and Conclusions: Across exposure days, the change from baseline in ICSS thresholds for cannabis smoke-exposed groups was significantly lower and response latencies were significantly faster in the cannabis smoke-exposed groups compared to controls, suggesting that chronic cannabis smoke exposure has rewarding properties. Acute administration of the CB1 receptor antagonist rimonabant (0.3, 1.0, 3.0 mg/kg) induced a dose-dependent increase in ICSS thresholds in the smoke-exposed rats, suggestive of dependence and withdrawal. Finally, an effect compartment PK-pharmacodynamic model was used to describe the relationship between THC concentrations and changes in ICSS thresholds after cannabis smoke exposure.

Effects in rats of adolescent exposure to cannabis smoke or THC on emotional behavior and cognitive function in adulthood

RATIONALE

Cannabis use is common among adolescents and some research suggests that adolescent cannabis use increases the risk for depression, anxiety, and cognitive impairments in adulthood. In human studies, however, confounds may affect the association between cannabis use and the development of brain disorders.

OBJECTIVES

These experiments investigated the effects of adolescent exposure to either cannabis smoke or THC on anxiety- and depressive-like behavior and cognitive performance in adulthood in Long-Evans rats.

METHODS

Adolescent rats of both sexes were exposed to either cannabis smoke from postnatal days (P) 29-49 or ascending doses of THC from P35-45. When the rats reached adulthood (P70), anxiety-like behavior was investigated in the large open field and elevated plus maze, depressive-like behavior in the sucrose preference and forced swim tests, and cognitive function in the novel object recognition test.

RESULTS

Despite sex differences on some measures in the open field, elevated plus maze, forced swim, and novel object recognition tests, there were no effects of either adolescent cannabis smoke or THC exposure, and only relatively subtle interactions between exposure conditions and sex, such that sex differences on some performance measures were slightly attenuated.

CONCLUSION

Neither cannabis smoke nor THC exposure during adolescence produced robust alterations in adult behavior after a period of abstinence, suggesting that adverse effects associated with adolescent cannabis use might be due to non-cannabinoid concomitants of cannabis use.

Biogenic amine flux mediated by cloned transporters stably expressed in cultured cell lines: amphetamine specificity for inhibition and efflux

LLC-PK1 cells have been stably transfected with cDNAs encoding the human norepinephrine transporter (NET), rat dopamine transporter (DAT), and rat serotonin transporter. Using these cell lines, the specificity of each transporter toward agents that inhibit substrate influx and stimulate substrate efflux across the plasma membrane was examined. With 1-methyl-4-phenylpyridinium as a substrate for DAT and NET and serotonin as a substrate for the serotonin transporter, each transporter demonstrated a distinct pattern of inhibition by a panel of amphetamine derivatives and analogs, including amphetamine, methamphetamine (also known as "ecstasy"), p-chloroamphetamine, 3,4-methylenedioxymethamphetamine, methylphenidate (ritalin), and 5-methoxy-6-methyl-2-aminoindan. For each cell line expressing a single biogenic amine transporter, efflux of the accumulated substrate was stimulated by amphetamine derivatives, and this efflux was blocked by mazindol, an inhibitor of all three transporters. Of the amphetamine derivatives tested, some caused efflux at concentrations similar to those that inhibited transport. Other derivatives were much less effective at stimulating efflux than at inhibiting uptake. Methylphenidate caused little or no efflux, although it blocked uptake mediated by both NET and DAT. Other inhibitors of transport, such as cocaine, mazindol, citalopram, and nisoxetine, failed to stimulate efflux from these cells at concentrations that inhibited influx. The results suggest that potency toward individual plasma membrane biogenic amine transporters and the ability to release accumulated amine substrates are independent properties of each amphetamine derivative.

Ligand binding to the serotonin transporter: equilibria, kinetics, and ion dependence

The effects of Na+ and Cl- on the binding of [3H]imipramine and the cocaine analog [125I]-beta-carbomethoxy-3 beta-(4-iodophenyl)tropane([125I]-beta-CIT) to the human platelet serotonin transporter have been measured. The ion dependence of beta-CIT binding is consistent with binding beta-CIT together with one Na+ ion, but not in an ordered sequence. Imipramine affinity, like beta-CIT affinity, is increased by Na+, but imipramine binding involves at least two Na+ ions. This conclusion is based on the observation that both imipramine association rate constants and equilibrium affinity constants show a sigmoidal Na+ dependence. As with beta-CIT, the imipramine and Na+ binding sequence is not strictly ordered. Cl- increases imipramine affinity, apparently by slowing dissociation. beta-CIT binding occurs even in the absence of Na+ and Cl-. This provided a means to measure substrate and inhibitor affinity in both the presence and absence of cotransported ions. Nontransported inhibitors, such as imipramine and citalopram, as well as the transport substrates serotonin and 3,4-(methylenedioxy)methamphetamine all displaced beta-CIT binding in the absence of NaCl. In the absence of Cl-, Na+ increased the affinity of nontransported inhibitors but not of substrates. The results suggest that Na+ and Cl- induce independent changes in the transporter binding site and that binding of substrates and inhibitors is affected differently by these changes.

Stable expression of biogenic amine transporters reveals differences in inhibitor sensitivity, kinetics, and ion dependence

We have constructed stable cell lines expressing transporters for dopamine (DA), norepinephrine (NE), and serotonin (5-HT) by transfection with cloned cDNAs. The parental LLC-PK1 cell does not express any of these neurotransmitter transporters. Therefore, monoamine transport activities in each of these cell lines are due to the transfected DNA only, allowing comparison in the same background. Drug inhibition profiles for each cell line are distinct and as expected for each transporter. LLC-NET and LLC-DAT cells transported both NE and DA and both cell types exhibited a lower KM for DA transport than for NE transport. Analysis of Vmax data for LLC-NET cells suggests that substrate is bound to the NE transporter during the rate-limiting step(s) in transport. The cocaine analog 2-beta-carbomethoxy-3 beta-(4-[125I]iodophenyl)tropane binds to each cell type, and is displaced by transport substrate in each case. Binding and transport measurements on parallel cell cultures allowed estimation of turnover numbers for norepinephrine, dopamine, and serotonin transporters. All three transporters require external Na+ and Cl-. The Na+ concentration dependence suggests that a single Na+ ion is involved in transport catalyzed by norepinephrine and serotonin transporters while more than one Na+ ion participate in transport mediated by the dopamine transporter.

Amphetamine derivatives interact with both plasma membrane and secretory vesicle biogenic amine transporters

The interaction of fenfluramine, 3,4-methylenedioxymethamphetamine (MDMA), and p-chloroamphetamine (PCA) with the platelet plasma membrane serotonin transporter and the vesicular amine transporter were studied using both transport and binding measurements. Fenfluramine is apparently a substrate for the plasma membrane transporter, and consequently inhibits both serotonin transport and imipramine binding. Moreover, fenfluramine exchanges with internal [3H]serotonin in a plasma membrane transporter-mediated reaction that requires NaCl and is blocked by imipramine. These properties are similar to those of MDMA and PCA as previously described. In adrenal chromaffin granule membrane vesicles containing the vesicular amine transporter, fenfluramine inhibited serotonin transport and dissipated the transmembrane pH difference (delta pH) that drives amine uptake. The use of [3H]reserpine-binding measurements to determine drug interaction with the vesicular amine transporter allowed assessment of the relative ability of MDMA, PCA, and fenfluramine to bind to the substrate site of the vesicular transporter. These measurements permit a distinction between inhibition of vesicular serotonin transport by directly blocking vesicular amine transport and by dissipating delta pH. The results indicate that MDMA and fenfluramine inhibit by both mechanisms but PCA dissipates delta pH without blocking vesicular amine transport directly.

Binding of the cocaine analog 2 beta-carbomethoxy-3 beta-(4-[125I]iodophenyl)tropane to serotonin and dopamine transporters: different ionic requirements for substrate and 2 beta-carbomethoxy-3 beta-(4-[125I]iodophenyl)tropane binding

The iodinated cocaine analog 2 beta-carbomethoxy-3 beta-(4- [125I]iodophenyl)tropane (beta-[125I]CIT) binds with high affinity to the platelet plasma membrane serotonin transporter, as previously reported for dopamine transporters from rat brain [Eur. J. Pharmacol. 194:133-134 (1991)]. Unlabeled beta-CIT also inhibits serotonin transport by platelet membrane vesicles. In both rat striatal membranes and platelet plasma membranes, beta-[125I]CIT binding was found to be pH dependent, with a pKa of 6.4-6.9, and did not require the presence of Cl-. Na+ dramatically stimulated beta-[125I]CIT binding to both serotonin and dopamine transporters, although a small fraction of beta-[125I]CIT binding to the serotonin transporter was observed in the absence of Na+. The substrates serotonin and dopamine competed with beta-[125I]CIT for binding to their respective transporters. However, substrate affinity was enhanced by Cl-, whereas beta-[125I]CIT binding affinity was not. [3H]Imipramine binding to the platelet serotonin transporter and [3H]GBR-12935 binding to the dopamine transporter were not inhibited by decreasing the pH from 8 to 6.5. Likewise, the ability of serotonin to compete with [3H]imipramine binding and that of dopamine to inhibit [3H]GBR-12935 binding were equal at pH 6.5 or 8. Thus, beta-[125I]CIT binding to biogenic amine transporters is distinct from serotonin or dopamine binding by virtue of its inhibition by H+ and its insensitivity to Cl-.

Non-neurotoxic amphetamine derivatives release serotonin through serotonin transporters

3,4-Methylenedioxymethamphetamine (MDMA) and several other amphetamine derivatives cause degeneration of serotonergic nerve terminals. These drugs also release serotonin from nerve terminals both in vivo and in vitro. Two non-neurotoxic derivatives of MDMA were tested in membrane vesicle model systems to determine whether they also lacked the ability to release serotonin. 3-Methoxy-4-methylamphetamine (MMA) and 5-methoxy-6-methyl-2-aminoindan (MMAI) both inhibited imipramine binding to serotonin transporters in platelet plasma membrane vesicles and both inhibited Na+ gradient-driven serotonin transport into those vesicles. Significantly, both MMA and MMAI released [3H]serotonin from plasma membrane vesicles, apparently by a process of exchange. The half-maximal concentrations for this effect were comparable to that reported for MDMA. In addition to their effects on plasma membrane transporters, MMA and MMAI both inhibited serotonin transport into chromaffin granule membrane vesicles catalyzed by the vesicular biogenic amine transporter. At higher concentrations, these compounds also caused release of [3H]serotonin from chromaffin granule membrane vesicles and dissipated the transmembrane pH difference (delta pH). Although MMAI effects on the serotonin transporter were similar to those of MDMA, the two compounds had different effects on dopamine transporters. MDMA and methamphetamine inhibited binding of a cocaine analog to the dopamine transporter and released dopamine accumulated by cells expressing dopamine transporters, but similar concentrations of MMAI were inactive.

p-Chloroamphetamine induces serotonin release through serotonin transporters

p-Chloroamphetamine (PCA) interacts with serotonin transporters in two membrane vesicle model systems by competing with serotonin for transport and stimulating efflux of accumulated serotonin. In plasma membrane vesicles isolated from human platelets, PCA competes with [3H]imipramine for binding to the serotonin transporter with a KD of 310 nM and competitively inhibits serotonin transport with a KI of 4.8 nM. [3H]Serotonin efflux from plasma membrane vesicles is stimulated by PCA in a Na(+)-dependent and imipramine-sensitive manner characteristic of transporter-mediated exchange. In membrane vesicles isolated from bovine adrenal chromaffin granules, PCA competitively inhibits ATP-dependent [3H]serotonin accumulation with a KI of 1.7 microM and, at higher concentrations, stimulates efflux of accumulated [3H]serotonin. Stimulation of vesicular [3H]serotonin efflux is due in part to dissipation of the transmembrane pH difference (delta pH) generated by ATP hydrolysis. Part of PCA's ability to stimulate efflux may be due to its transport by the vesicular amine transporter. Flow dialysis experiments demonstrated uptake of [3H]PCA into chromaffin granule membrane vesicles in response to the delta pH generated in the presence of Mg2+ and ATP. In plasma membrane vesicles, no accumulation was observed using an NaCl gradient as the driving force. We conclude that rapid nonmediated efflux of transported PCA prevents accumulation unless PCA is trapped inside by a low internal pH.

The molecular mechanism of "ecstasy" [3,4-methylenedioxy-methamphetamine (MDMA)]: serotonin transporters are targets for MDMA-induced serotonin release

MDMA ("ecstasy") has been widely reported as a drug of abuse and as a neurotoxin. This report describes the mechanism of MDMA action at serotonin transporters from plasma membranes and secretory vesicles. MDMA stimulates serotonin efflux from both types of membrane vesicle. In plasma membrane vesicles isolated from human platelets, MDMA inhibits serotonin transport and [3H]imipramine binding by direct interaction with the Na(+)-dependent serotonin transporter. MDMA stimulates radiolabel efflux from plasma membrane vesicles preloaded with [3H]serotonin in a stereo-specific, Na(+)-dependent, and imipramine-sensitive manner characteristic of transporter-mediated exchange. In membrane vesicles isolated from bovine adrenal chromaffin granules, which contain the vesicular biogenic amine transporter, MDMA inhibits ATP-dependent [3H]serotonin accumulation and stimulates efflux of previously accumulated [3H]serotonin. Stimulation of vesicular [3H]serotonin efflux is due to dissipation of the transmembrane pH difference generated by ATP hydrolysis and to direct interaction with the vesicular amine transporter.

Binding of the cocaine analog 2 beta-[3H] carboxymethoxy-3 beta-(4-fluorophenyl)tropane to the serotonin transporter

The cocaine analog 2 beta-carboxymethoxy-3 beta-(4-fluorophenyl)-tropane (CFT) binds to platelet plasma membrane vesicles. [3H]CFT binding is blocked equally well by cocaine and imipramine. Specific (cocaine-sensitive) binding requires Na+ and is inhibited by H+ and Cl- ions. At 150 mM Na2SO4 and pH 9.5, the KD for [3H]CFT is 232 +/- 71 nM. The number of specific [3H]CFT binding sites on the membrane vesicles is equal to the number of serotonin transporters, as measured by [3H]imipramine binding. Binding of imipramine and CFT appeared to be mutually competitive. These results suggest that [3H]CFT and cocaine bind to the serotonin transporter at a site close to but distinct from the antidepressant binding site.