Nicotine (NIC) abuse involves activation of midbrain dopamine (DA) neurons and NIC addiction involves neuroadaptive changes in the mesolimbic DA reward system. GABA neurons in the midbrain ventral tegmental area (VTA) express α4β2-containing nicotinic acetylcholine receptors (nAChRs), whose activation increases GABAergic input to DA neurons. However, this initial effect is decreased after chronic NIC treatment (as in the case of smokers) by inducing nAChR desensitization. Thus, GABA neuron inhibition results in increased DA release in limbic structures such as the nucleus accumbens. To support this hypothesis, we evaluated the effects of acute and chronic NIC on GAD-67 positive neurons in the VTA of GAD GFP mice using in vivo and in vitro electrophysiological methods. In in vivo studies in naïve mice, stimulation of the peduncopontine tegmental nucleus (PPT) activated VTA GABA neurons orthodromically and antidromically. Orthodromic activation of VTA GABA neuron spikes by PPT stimulation was blocked by the nAChR mecamylamine (1 mg/kg). Acute systemic NIC (0.15-0.5 mg/kg IV) had mixed overall effects on VTA GABA neuron firing rate, but in situ microelectrophoretic application of NIC produced a brisk and consistent enhancement (200-500 %) of VTA GABA neuron firing rate that showed no acute tolerance or sensitization with repeated, periodic current application. Local NIC activation was blocked by systemic administration of mecamylamine. Compared to 12 day chronic saline injections, chronic NIC injections (2 mg/kg IP/day) significantly increased VTA GABA neuron firing rate. In in vitro studies, compared to 12 day chronic saline injections, chronic NIC injections decreased DA neuron firing rate. In addition, chronic NIC increased DA neuron, but decreased GABA neuron GABA-mediated sIPSCs. These findings demonstrate that there is reciprocal innervation between the PPT and VTA and that cholinergic input from the PPT is excitatory on VTA GABA neurons. Moreover, VTA GABA neurons are excited by acute NIC and sensitize to chronic NIC, suggesting that α4β2 nAChR subunits on these neurons may play an important role in the adaptations to chronic NIC. Thus, quantitative molecular studies are ongoing to determine specific alterations in nAChRs on VTA GABA neurons to chronic NIC.
| Identifer | oai:union.ndltd.org:BGMYU2/oai:scholarsarchive.byu.edu:etd-3950 |
| Date | 13 March 2011 |
| Creators | Taylor, Devin Hardy |
| Publisher | BYU ScholarsArchive |
| Source Sets | Brigham Young University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | http://lib.byu.edu/about/copyright/ |
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