An afterhyperpolarization recorded in striatal cells 'in vitro': effect of dopamine administration

Parahorizontal slices of the neostriatum from young adult male rats were maintained at the fluid interface of a perfusion chamber. Intracellular recordings were made with potassium acetate filled micropipettes. The mean resting membrane potential of the neurones was -84 mV and all produced action potentials which overshot 0 mV. All the cells were silent at resting membrane potentials and only fired in response to depolarizing pulses delivered through the intracellular electrode. When long pulses were applied, the firing rate within the train of action potentials evoked was slower at the end of the pulse. In a very few cells at resting membrane potential, but in all cells depolarised from a holding potential of -65 mV, the resulting train of action potentials was followed by a slow hyperpolarizing potential. The potential observed was smaller in amplitude than the afterhyperpolarization seen in hippocampal pyramidal cells in vitro, but its voltage sensitivity was similar. Iontophoresis of dopamine (DA) close to the recording pipette resulted in two distinct actions. Firstly, DA reduced the number of action potentials following a short pulse, by increasing the threshold for spike initiation. Secondly, it also inhibited the hyperpolarizing potential which followed trains of action potentials even when allowance was made for the increased firing threshold. The inhibition of this hyperpolarization was accompanied by an increase in the number of spikes per pulse. Whether these two actions of DA are mediated by a single receptor/membrane action is unclear. Nevertheless, they may help to reconcile the contradictory literature regarding the action of DA upon extra-cellular responses in striatal cells.(ABSTRACT TRUNCATED AT 250 WORDS)