Reversal of Dopamine Inhibition of Dopaminergic Neurons of the Ventral Tegmental Area is Mediated by Protein Kinase C

Abstract

Adaptation of putative dopaminergic (pDA) neurons in the ventral tegmental area (VTA) to drugs of abuse may alter information processing related to reward and reinforcement and is an important factor in the development of addiction. We have demonstrated that prolonged increases in the concentration of dopamine (DA) result in a time-dependent decrease in sensitivity of pDA neurons to DA, which we termed DA inhibition reversal (DIR). In this study, we used extracellular recordings to examine factors mediating DIR. A 40 min administration of DA (2.5-10 mM), but not the DA D2 receptor agonist quinpirole (50-200 nM), resulted in inhibition of neuronal firing followed by DIR. In the presence of 100 nM cocaine, inhibition followed by DIR was seen with much lower DA concentrations. Reversal of quinpirole inhibition could be induced by an activator of protein kinase C, but not of protein kinase A. Inhibitors of protein kinase C or phospholipase C blocked the development of DIR. Disruption of intracellular calcium release also prevented DIR. Reduction of extracellular calcium or inhibition of store-operated calcium entry blocked DIR, but the L-type calcium channel blocker nifedipine did not. DIR was age-dependent and not seen in pDA VTA neurons from rat pups younger than 15 days postnatally. Our data indicate that DIR is mediated by protein kinase C, and implicate a conventional protein kinase C. This characterization of DIR gives insight into the regulation of autoinhibition of pDA VTA neurons, and the resulting long-term alteration in information processing related to reward and reinforcement.

Reversal of Dopamine D2 Agonist-Induced Inhibition of Ventral Tegmental Area Neurons by Gq-Linked Neurotransmitters Is Dependent on Protein Kinase C, G Protein-Coupled Receptor Kinase, and Dynamin

“…We have also demonstrated that DIR is mediated by PLC and cPKC, and is dependent on both extracellular calcium influx and intracellular calcium release (Nimitvilai et al, 2012a). In addition, agonists of either 5-HT 2 or neurotensin receptors, both of which are linked to PLC activation, produced a decrease in sensitivity of D2 receptor to the D2 agonist quinpirole (Nimitvilai et al, 2012c), suggesting that this phenomenon is mediated through the Gq/PLC/PKC pathway.…”

“…In all of our previous studies in which agonists and antagonists were delivered via the recording pipette (Nimitvilai et al, 2012b), the effective concentration of drugs were 10-fold higher than the effective concentration used in the extracellular medium. The concentrations of drugs used in the present study were likewise 10-fold higher than the concentrations reported in the literature for selective action.…”

“…To allow time for the drug to diffuse from the pipette to the cell, the effects of bath-applied drugs were tested no less than 20 minutes after initiating the recording; this pipette-application method has produced comparable results to the administration of drugs through the extracellular medium in the cases in which both methods were tested (data not shown), with the advantage of more localized application and reduced expense. Such local delivery of drugs through recording pipettes has been used by our laboratory and others (Pesavento et al, 2000;Nimitvilai et al, 2012a). One disadvantage of this method is that the exact concentration of drug received by the neurons from which we recorded is unknown.…”

“…Time-dependent reversal of DA inhibition occurs with moderate concentrations of DA alone or the D2 agonist quinpirole in the presence of D1-like receptor agonist (Nimitvilai and Brodie, 2010;Nimitvilai et al, 2012a). This phenomenon is dependent on calcium and is mediated by activation of the PLC and cPKC pathway (Nimitvilai et al, 2012a).…”

“…Activation of the D1/D5 receptor linked to phosphatidylinositide (PI) accumulation, but not those that cause adenylyl cyclase (AC) activation, produced a decrease in sensitivity of the D2 receptor to its agonist (Nimitvilai et al, 2012c). DIR also requires the activation of phospholipase C (PLC) and conventional protein kinase C (cPKC), without involvement of AC, cAMP, or protein kinase A (Nimitvilai et al, 2012a). Recently, we have demonstrated that some (e.g., 5-HT 2 and neurotensin) but not all (e.g., a 1 -adrenergic and group I metabotropic glutamate) Gq-coupled receptors that stimulate the PLC and PKC pathway can mediate the reversal of D2 agonist inhibition (Nimitvilai et al, 2012c).…”