SK Ca2+-activated K+ channel ligands alter the firing pattern of dopamine-containing neurons in vivo.

Apamin-sensitive, SK channels play an important role in generating the rhythmic firing patterns exhibited by midbrain dopamine neurons in vitro. However, their contribution to the firing properties of these cells in intact animals has yet to be determined. In the present series of experiments, extracellular single unit recording techniques were used to assess the central effects of prototypical SK channel ligands on the firing pattern of dopamine neurons in the substantia nigra of the chloral hydrate anesthetized rat. I.v. administration of the SK channel blocker apamin (0.4 mg/kg), increased bursting activity in approximately 50% of the dopamine neurons tested without altering average firing rate. The majority of these cells responded slowly to the effects of apamin, gradually transitioning from an irregular single spike to a phasic discharge composed of the same relative proportion of long (>or=three spike) and short (two spike) bursts as "natural" bursting activity recorded in drug naive animals. Local administration of apamin increased bursting activity in all cells tested. Systemic administration of the SK channel opener, 1-ethyl-2-benzimidazolinone (5-25 mg/kg) also had no effect on average firing rate but suppressed bursting activity and increased the precision of firing. The effects of 1-ethyl-2-benzimidazolinon on firing pattern were abolished when recording electrodes contained apamin (125 microM). These results suggest that SK channels actively contribute to the spontaneous firing patterns exhibited by dopamine neurons in vivo and provide additional support for the proposition that this channel could serve as a useful target for modifying their activity.