The mammalian circadian timing system contains both a light- and food-entrainable oscillator, located in the suprachiasmatic (SCN) and dorsomedial (DMH) hypothalamic nucleus, respectively. Additional studies have suggested the presence of a Methamphetamine-Sensitive Circadian Oscillator (MASCO) that is also independent of the SCN but whose location in the brain remains unresolved. To locate MASCO, we assessed circadian clock gene expression in the brain as well as recorded body temperature (Tb) and locomotor activity (LMA) in mice provided daily daytime methamphetamine (MA) injections (ip) for at least 8 days. Following repeated MA but not following saline (SA) administration, anticipatory elevations were evident preceding subsequent injections. Also, rhythmic oscillations of clock genes (Per1, Bmal1) were clearly evident in the DMH and the SCN. However, rhythmic oscillations were evident in the SCN regardless of treatment, while they were evident in the DMH only after MA exposure. To assess further the role of the DMH in the control of methamphetamine-entrainable circadian rhythms, I will first determine if cell-specific lesions in DMH will disrupt expression of these MA-entrainable Tb and LMA. Telemetry transmitters will be implanted for collection of Tb and LMA data in mice receiving injections of the neurotoxin ibotenic acid into the DMH. After recovery from surgery, Tb and LMA will be assessed following daily daytime MA injections (ip, 5 mg/kg, 8 days). I predict that anticipatory increases in Tb and LMA associated with MA entrainment should decrease in correspondence with the loss of cells in the DMH. In the second experiment, I will assess if clock gene expression in the DMH is necessary and sufficient for MA-entrainable rhythms. To test this question, Bmal1 mutant -/- mice (which lack circadian rhythms) will receive injections of an adeno-associated virus (AAV) injections containing the Bmal1 gene into the DMH, while a subset of these animals will receive control injections (AAV-GFP and/or SA) into this nucleus. Following recovery from surgery, I will collect Tb and LMA data from animals from both groups that receive daily MA injections (ip, ZT 5;8 days;5 mg/kg). MA-entrainable rhythms (Tb and LMA) should emerge following repeated daily MA injections in those animals in which the Bmal1 gene was restored in the DMH, but not in other hypothalamic nuclei. Collectively these experiments will provide important insight into a novel mechanism whereby drugs of abuse control daily physiological and behavioral patterns via their action on a specific hypothalamic region. Ultimately, these findings may shed new light on our understanding of the etiological roots of drug abuse and addiction.
Drug addiction is generally believed to result in part from repeated drug exposure which alters brain structure and function thereby enhancing drug-seeking behavior. The proposed experiments are designed to provide insight to a novel mechanism whereby drugs of abuse control daily physiological and behavioral patterns via their action on a specific brain area. Ultimately this work should add to our understanding of the neurobiology underlying drug abuse and addiction.
