Protein and surface expression of HCN2 and HCN4 subunits in mesocorticolimbic areas after cocaine sensitization.

The Ih is a mixed depolarizing current present in neurons which, upon activation by hyperpolarization, modulates neuronal excitability in the mesocorticolimbic (MCL) system, an area which regulates emotions such as pleasure, reward, and motivation. Its biophysical properties are determined by HCN protein expression profiles, specifically HCN subunits 1-4. Previously, we reported that cocaine-induced behavioral sensitization increases HCN2 protein expression in all MCL areas with the Ventral Tegmental Area (VTA) showing the most significant increase. Recent evidence suggests that HCN4 also has an important expression in the MCL system. Although there is a significant expression of HCN channels in the MCL system their role in addictive processes is largely unknown. Thus, in this study we aim to compare HCN2 and HCN4 expression profiles and their cellular compartmental distribution in the MCL system, before and after cocaine sensitization. Surface/intracellular (S/I) ratio analysis indicates that VTA HCN2 subunits are mostly expressed in the cell surface in contrast to other areas tested. Our findings demonstrate that after cocaine sensitization, the HCN2 S/I ratio in the VTA was decreased whereas in the Prefrontal Cortex it was increased. In addition, HCN4 total expression in the VTA was decreased after cocaine sensitization, although the S/I ratio was not altered. Together, these results demonstrate differential cocaine effects on HCN2 and HCN4 protein expression profiles and therefore suggest a diverse Ih modulation of cellular activity during cocaine addictive processes.

aPKC-Mediated Persistent Increase in AMPA/NMDA Ratio in the VTA Participates in the Neuroadaptive Signal Necessary to Induce NAc Synaptic Plasticity After Cocaine Administration.

Chronic cocaine exposure produces enduring neuroadaptations in the brain's reward system. Persistence of early cocaine-evoked neuroadaptations in the ventral tegmental area (VTA) is necessary for later synaptic alterations in the nucleus accumbens (NAc), suggesting a temporal sequence of neuroplastic changes between these two areas. However, the molecular nature of the signal that mediates this sequential event is unknown. Here we used the behavioral sensitization model and the aPKC inhibitor of late-phase LTP maintenance, ZIP, to investigate if a persistent increase in AMPA/NMDA ratio plays a role in the molecular mechanism that allows VTA neuroadaptations to induce changes in the NAc. Results showed that intra-VTA ZIP microinfusion successfully blocked cocaine-evoked synaptic enhancement in the VTA and the expected AMPA/NMDA ratio decrease in the NAc following cocaine sensitization. ZIP microinfusions also blocked the expected AMPA/NMDA ratio increase in the NAc following cocaine withdrawal. These results suggest that a persistent increase in AMPA/NMDA ratio, mediated by aPKCs, could be the molecular signal that enables the VTA to elicit synaptic alterations in the NAc following cocaine administration.