Acetyl- l-carnitine induces a sustained potentiation of the afterhyperpolarization

Abstract

Acetyl- l-carnitine is known to improve many aspects of the neural activity even if its exact role in neurotransmission is still unknown. This study investigates the effects of acetyl- l-carnitine in T segmental sensory neurons of the leech Hirudo medicinalis. These neurons are involved in some forms of neural plasticity associated with learning processes. Their physiological firing is accompanied by a large afterhyperpolarization that is mainly due to the Na +/K + ATPase activity and partially to a Ca 2+-dependent K + current. A clear-cut hyperpolarization and a significant increase of the afterhyperpolarization have been recorded in T neurons of leeches injected with 2 mM acetyl- l-carnitine some days before. Acute treatments of 50 μM acetyl- l-carnitine induced similar effects in T cells of naive animals. In the presence of apamin, a pharmacological blocker of Ca 2+-dependent K + channel, acetyl- l-carnitine still enhanced the residual afterhyperpolarization, suggesting an effect of the drug on the Na +/K +ATPase. Acetyl- l-carnitine also increased the hyperpolarization induced by intracellular injection of Na + ions. Therefore, acetyl- l-carnitine seems to be able to exert a positive sustained effect on the Na +/K + ATPase activity in leech T sensory neurons. Moreover, in these cells, widely arborized, the afterhyperpolarization seems to play an important role in determining the action potential transmission at neuritic bifurcations. A computational model of a T cell has been previously developed considering detailed data for geometry and the modulation of the pump current. Herein, we showed that to a larger afterhyperpolarization, due to the acetyl- l-carnitine-induced effects, corresponds a decrement in the number of action potentials reaching synaptic terminals.