My research program uses vertebrate (i.e., rats, mice) and invertebrate (i.e., planarians, land snails) models to investigate the mechanisms by which drugs of abuse (e.g. cannabinoids, opioids, cocaine, amphetamines, and ecstasy) alter the functions of glutamate, dopamine, nitric oxide and GABA systems at the behavioral, physiological, and neurochemical levels. These studies extend to defining the natural functions of the endogenous ligands (e.g. endogenous opioids, endocannabinoids) and molecular pathways that are hijacked by drugs of abuse--for example, in ameliorating pain, modulating body temperature, inducing euphoria, modulating inflammation, or altering behaviors. Drug effects are evaluated in rat and mouse models using five different behavioral endpoints- body temperature, analgesia (e.g., tail-flick and hot plate assays), hyperactivity (e.g., wet dog shakes, locomotor activity, stereotypy), physical dependence (e.g. withdrawal syndrome), and psychological dependence (conditioned place preference). The neurochemical effects of drugs are determined using the technique of in vivo microdialysis coupled to high-performance liquid chromatography (HPLC). The microdialysis technique is useful for determining drug-induced, brain-region specific changes in neurotransmitter levels in conscious rats and mice. Glutamate, aspartate, and GABA are quantified using fluorescence detection whereas dopamine is quantified using electrochemical detection.

At the present time, my research is focused specifically on four important areas. One is the exploration of the connections between cannabinoid and glutamate systems as related to the role that cannabinoid-glutamate cross-talk plays in the behavioral and neurochemical effects of abused drugs. This research is supported by my recently funded NIH grant and includes extensive investigation into:
(1) The interactions between cannabinoids and glutamate systems in the limbic system and basal ganglia of rats and mice,
(2) The roles of endogenous cannabinoids, cannabinoid receptors, and the molecular pathways of endocannabinoid inactivation on glutamate signaling and release,
(3) The effects of novel cannabinoid compounds (antagonists, uptake blockers, and inhibitors of endocannabinoid metabolism) on glutamate systems, and
(4) The role of the cannabinoid system on glutamate-mediated effects such as the physical dependence, psychological dependence, toxicity, and hyperthermia caused by psychostimulants (e.g. cocaine, amphetamine, methamphetamine, ecstasy).
From my research, I hope to delineate the role of cannabinoid-glutamate interactions in drug dependence and to determine if the pharmacological manipulation of the endogenous cannabinoid system is a potential strategy in the clinical management of addiction. In a context broader than just addiction, I hope my results will be a first step in determining whether novel drugs that block endocannabinoid uptake and prevent endocannabinoid inactivation offer any therapeutic advantage over marijuana itself.

The second, and perhaps most exciting, area is the exploration of the role of beta-lactam antibiotics (e.g., ceftriaxone, penicillin) on glutamate systems and glutamate-mediated pathologies. Evidence indicates that these antibiotics do more than just kill bacteria. In a process separate from their antibacterial mechanism, the beta-lactam class of antibiotics was identified as the only practical pharmaceuticals capable of increasing the clearance of extracellular glutamate in the brain. The mechanism was an increase in the expression and functional activity of the GLT-1 transporter protein, the protein responsible for 90% of glutamate reuptake in the mammalian central nervous system. This finding is important because abnormally high levels of extracellular glutamate mediate a number of pathological conditions (e.g., physical dependence, withdrawal, psychological dependence, neurotoxicity, stroke, Parkinson's disease). Because extracellular glutamate is cleared primarily by the GLT-1 transporter, my first goal is to determine if beta-lactam antibiotics actually decrease extracellular glutamate levels in the rat brain (e.g., limbic system, basal ganglia) and, if so, whether the effect is mediated by an increase in the expression and activity of the GLT-1 transporter. Our preliminary evidence does suggest that the repeated administration of ceftriaxone decreases extracellular glutamate in the striatum and nucleus accumbens, two regions that mediate the rewarding effects of abused drugs (see CV under submitted papers). A long-range goal of mine is to test the hypothesis that beta-lactam antibiotics, through the activation of GLT-1 and reduction of extracellular glutamate, decrease the development, maintenance, and expression of the physical and psychological dependence caused by a broad range of addictive drugs. From such research, I hope to delineate a role for the GLT-1 transporter protein and antibiotics in addiction.

The third area is drug interactions. People rarely abuse only a single drug. Therefore, in a variety of physiological systems and in measurements of behavior (e.g., analgesia, hyperthermia), I examine the effects of more than one drug on the interaction of a drug with its receptor system or with other endogenous receptor systems--for example, cannabinoid and opioid drugs and their endogenous pathways.

The fourth area is the use of an invertebrate (planarian) model of physical dependence to determine if abused drugs (e.g., opioids, cannabinoids, amphetamines, cocaine, ecstasy, nicotine, caffeine, and benzodiazepines) produce abstinence-induced withdrawal by altering systems including, but not limited to, glutamate, nitric oxide, cyclic AMP, agmatine, and GABA. Traditionally, physical dependence data has been gathered primarily from rodents and primates, but both of these mammalian models have inherent limitations related to an inability to know drug concentration - rather than dose - due to pharmacokinetic factors (i.e., the two drugs can affect each other's absorption, distribution, metabolism, or excretion). Furthermore, the difficulty in quantifying withdrawal behaviors in mammals is one of the main reasons the exact mechanism of physical dependence, as well as effective treatments for ameliorating the dependence, remains elusive. This means that alternative models of physical dependence are needed. One attractive alternative to mammals is planarians. Planarians have a rich history of productive use for modeling mammalian behaviors because of their mammalian-like central nervous system (a primitive brain and spinal cord that is capable of learning and memory) and mammalian neurotransmitter systems (e.g., dopaminergic, glutamatergic, opioid, serotonin, nitric oxide). Our laboratory uses a convenient and sensitive endpoint- planarian locomotor velocity (pLMV) - to quantify abstinence-induced drug withdrawal in planarians. Significant advantages over mammalian models include: a sensitive measure of withdrawal; an ability to measure drug concentration; ability to obtain precise ED50 values; relative absence of pharmacokinetic interference between drugs. The planarian model is particularly useful for involving undergraduate, professional, and high school students in active research. The model is simple and inexpensive, but it provides students the opportunity to apply the concepts they learn in lecture (e.g., dose-response relationships, dependence, tolerance, withdrawal, concentration calculations) to actual experiments in the laboratory. The method in the planaria dependence experiments is simple. A piece of graphing paper is placed under a petri dish containing a solution of drug. A single planarian is then placed into the petri dish for 60 minutes. Planarians are removed from the drug solution and placed in another petri dish containing water or another drug for 5 minutes. During the 5-minute exposure, we count the number of times, per minute, that planarians cross a grid on the graphing paper. This procedure is repeated for all of our drug combinations. As in our rat model, HPLC is used to quantify the effects of withdrawal on neurotransmitter levels in planarians. Withdrawal of planarians from long-term exposure to an abused drug decreases planarian locomotor velocity (pLMV).