Psychostimulants, including methamphetamine (MA), activate the mesolimbic circuits in the brain. The rewarding properties and addiction potential of these psychoactive drugs are correlated with their affinity at the dopamine transporter (DAT). MA increases extracellular levels of dopamine (DA) and glutamate in several areas of the mesolimbic system, and there are many reports that blockade of glutamate receptors interferes with neuroadaptive behavioral effects to psychostimulants. This proposal examines the hypothesis that MA potentiates NMDA synaptic currents in midbrain DA neurons in a DAT- and trace amine-associated receptor 1 (TAAR1)-dependent manner.
Dr. Richards created a selected mouse line model of high and low genetic risk for voluntary MA drinking (MADR). Multiple replicate sets of the MA high drinking (MAHDR) and MA low drinking (MALDR) lines demonstrate reliable outcomes for the selection response, including lack of differences in consumption of nonpsychoactive substances with bitter, sweet and salty tastes, and differential sensitivity to rewarding (MAHDR > MALDR) and aversive (MAHDR < MALDR) effects of MA. Quantitative trait locus mapping and functional and molecular studies resulted in the remarkable finding that Taar1 gene variants account for ~60% of the heritable component of voluntary MA intake. MAHDR mice possess the Taar1m1J allele that expresses a non-functional form of the TAAR1 receptor; MALDR mice possess the reference Taar1+ allele that expresses functional TAAR1. A CRISPR-Cas9 strategy was used to create MAHDR-Taar1+/+ knock-in mice in which Taar1m1J was replaced with Taar1+; high MA intake was converted to MALDR-like low MA intake. These results support a causal role of functional TAAR1 in reducing MA intake.
Dr. Ingram’s electrophysiological studies find that MA increases NMDA-GluN2B receptor signaling in midbrain DA neurons via intracellular TAAR1. MA-induced potentiation of NMDA synaptic currents in VTA DA neurons is greater in MALDR than MAHDR mice. Further, RNA-Seq results comparing the MADR lines implicate genes involved in glutamate-mediated synaptic plasticity in the outcome of selective breeding for differential MA intake. We hypothesize that TAAR1 function is critical in regulating glutamatergic signaling onto DA neurons in the VTA which is necessary for initiating addiction-related behaviors. This Project focuses on genetic risk and on the effects of acute MA exposure on glutamatergic synaptic currents onto midbrain DA neurons and will use 2 genetic mouse models with different Taar1 genotypes and MA intake profiles. Use of the MAHDR-Taar1+/+ knock-in mice will provide data about the specific role of Taar1.
- Aim 1: Changes in glutamate transmission associated with risk for MA use. Synaptic release of glutamate activates ligand-gated AMPA, NMDA and metabotropic mGluR5 receptors on DA neurons. We will examine whether properties of glutamatergic signaling onto midbrain DA neurons segregate with risk for high and low MA intake in 2 genetic mouse models, and will quantify glutamate receptor subtype mRNA and proteins, as well as synaptic function, in the VTA. We hypothesize that glutamate receptor signaling is disrupted in mice with high genetic risk for MA consumption.
- Aim 2: Changes in glutamate transmission associated with acute in vivo MA administration. Our preliminary data show that AMPH pre-exposure enhances AMPH-induced potentiation of NMDA receptor currents, indicating a “priming” effect. We will determine if sensitivity to this priming effect is related to genetic risk for MA intake, dependent on TAAR1 function, and dependent on changes in presynaptic glutamate release or postsynaptic glutamate receptors. We hypothesize that absence of TAAR1 has a pre-priming effect that shifts DA neurons toward MA-induced plasticity, and thus, the priming effect will be greater in MAHDR mice.
- Aim 3: Determine if TAAR1 limits glutamate signaling onto specific DA neuron populations. VTA DA neurons are heterogeneous with respect to their projections and specific projections may be related to risk for MA intake. We will use retrograde labeling of VTA DA neurons to determine if there is differential projection to the nucleus accumbens (NAc) or medial prefrontal cortex (mPFC) that corresponds with differential genetic risk. Loss of TAAR1 function increases MA drinking and reward sensitivity and decreases sensitivity to aversive effects of MA; thus, we hypothesize that MA potentiation of NMDA synaptic currents and “priming” will be greater in the VTA DA neurons projecting to the NAc.