Excitability properties of medial forebrain bundle axons of A9 and A10 dopamine cells.

A9 and A10 units identified as dopaminergic were recorded with extracellular micropipettes. The units were antidromically activated by electrical stimulation at the level of the preoptic area. The absolute refractory periods ranged from 1.2 to 2.5 ms. During the 2-8 ms of the relative refractory period, conduction was slower than normal by up to 1.5 ms. The time constant, C, of the strength-duration curve ranged from 0.4 to 0.6 ms. The current (I)-distance (D) relationship, tested by moving the stimulating electrode past the axon, was approximately parabolic (I = K D exp 2), with the constant of the equation, K, ranging from 900 to 2000 microA/mm exp 2, for 0.5 ms pulses. This relationship allows calculation of the radius of the field of dopamine axon excitation at any current. These high K values show that axons of dopamine cells cannot be activated unless high current densities are delivered, even when electrodes are placed near the axons. These data allow determination of the extent to which dopamine axons can be the directly activated substrates for behaviors, such as self-stimulation and circling, which are evoked by electrical stimulation of the medial forebrain bundle or internal capsule.