Bidirectional plasticity in the primate inferior olive induced by chronic ethanol intoxication and sustained abstinence

The brain adapts to chronic ethanol intoxication by altering synaptic and ion-channel function to increase excitability, a homeostatic counterbalance to inhibition by alcohol. Delirium tremens occurs when those adaptations are unmasked during withdrawal, but little is known about whether the primate brain returns to normal with repeated bouts of ethanol abuse and abstinence. Here, we show a form of bidirectional plasticity of pacemaking currents induced by chronic heavy drinking within the inferior olive of cynomolgus monkeys. Intracellular recordings of inferior olive neurons demonstrated that ethanol inhibited the tail current triggered by release from hyperpolarization (I-tail). Both the slow deactivation of hyperpolarization-activated cyclic nucleotide-gated channels conducting the hyperpolarization-activated inward current and the activation of Ca(v)3.1 channels conducting the T-type calcium current (I-T) contributed to I-tail, but ethanol inhibited only the I-T component of I-tail. Recordings of inferior olive neurons obtained from chronically intoxicated monkeys revealed a significant up-regulation in I-tail that was induced by 1 y of daily ethanol self-administration. The up-regulation was caused by a specific increase in I-T which (i) greatly increased neurons' susceptibility for rebound excitation following hyperpolarization and (ii) may have accounted for intention tremors observed during ethanol withdrawal. In another set of monkeys, sustained abstinence produced the opposite effects: (i) a reduction in rebound excitability and (ii) a down-regulation of I-tail caused by the down-regulation of both the hyperpolarization-activated inward current and I-T. Bidirectional plasticity of two hyperpolarization-sensitive currents following chronic ethanol abuse and abstinence may underlie persistent brain dysfunction in primates and be a target for therapy.