Evidence for the functional compartmentalization of spike generating regions of rat midbrain dopamine neurons recorded in vitro.

The regulation of spike generation in rat midbrain dopamine (DA) neurons was investigated using in vitro intracellular recordings. DA neurons fired long (greater than 1.8 ms) action potentials that exhibited comparatively depolarized spike thresholds (approx. -35 to -45 mV). Depolarization of the DA neuron increased the duration and the threshold of subsequent action potentials. The action potential was composed of two distinct components, a fast (0.8-1.5 ms duration) initial segment (IS) spike which triggered a slow (1.5-3 ms duration) somatodendritic (SD) component. Cobalt application (2 mM) blocked the SD spike component and revealed fast TTX-sensitive spikes. These fast spikes were also observed in untreated neurons following large hyperpolarizing pulses, and showed consistent changes in threshold and amplitude during membrane depolarization. Administration of 4-aminopyridine decreased the threshold of this TTX-sensitive spike, whereas tetraethylammonium (TEA) had no effect. When the fast spike was blocked by TTX, depolarization was ineffective in triggering further spike activity. However, after the administration of TEA (but not 4-AP), high threshold cobalt-sensitive spike activity could be triggered by relatively small depolarizations. TEA increased the duration of the SD portion of the action potential without altering the action potential threshold. The effect of 4-AP on spike threshold and the increase in SD spike duration caused by TEA were similar in nature to the changes in action-potential waveforms produced by polarizing the DA neuron membrane. Drawing from evidence gathered here and in previous in vivo studies, the properties of the TTX-sensitive fast spike are consistent with those of the IS spike component of the action potential, whereas the SD component is similar in nature to the high threshold calcium spike. One hypothesis that can be drawn from these studies is that dendritic and axonal spiking regions may exist in different functional subcompartments of the DA neuron, and may be independently modulated by pharmacologically distinct conductances. Under these conditions, synaptic influences could exist to modulate dendritic excitability and thus regulate putative dendritic spike-dependent functions, such as neuronal activity state, electrical coupling, and dendritic DA synthesis and release.