1. We have investigated the electrical properties of neurons acutely dissociated from the substantia nigra zona compacta (SNZC) of the postnatal rat with whole cell patch-clamp recordings. Retrogradely labeled nigrostriatal neurons were identified with the use of rhodamine-labeled fluorescent latex microspheres. Over 90% of the rhodamine-labeled neurons in the SNZC demonstrated formaldehyde/glutaraldehyde-induced catecholamine fluorescence, indicating that they were dopaminergic (DA) neurons. 2. DA neurons had 15-20 μm ovoid or fusiform-shaped cell bodies with 2-3 thick proximal processes. Labeled neurons generated spontaneous action-potential activity in both regular and irregular patterns. These cells exhibited input resistances of 300-600 MΩ and action-potential amplitudes of 60-80 mV. Locally applied dopamine inhibited the spontaneous activity of these neurons by hyperpolaizing the cells. 3. Outward currents were examined with voltage-clamp recordings using a tetrodotoxin (TTX)-containing medium. In all DA cells, depolarizing voltage commands activated several components of outward current depending on the holding potential of the cell. When cells were held at -40 mV (or more positive), voltage steps activated a sustained outward current. If the membrane potential was held more negative than -50 mV, a rapidly activating and inactivating component of outward current response could also be detected. 4. From a hyperpolarized holding potential (-90 mV) the transient outward current activated with depolarizing commands to -55 mV, peaking within 5 ms. The current inactivated with a monoexponential time constant of 53 ± 4 (SE) ms. At more positive holding potentials (-40 mV) the steady-state inactivation of the current could be removed by applying a conditioning hyperpolarizing prepulse. In response to a fixed depolarizing voltage step, half-maximal inactivation occurred at about -65 mV. The transient current was blocked by 4-aminopyridine (4-AP). 5. The sustained outward currents were isolated by holding the cells at -40 mV. Two components of sustained outward current were distinguished by their sensitivity to the calcium channel blockers Co2+ (5 mM) and/or Cd2+ (200 μM). The current remaining in the presence of Co2+/Cd2+ was activated by depolarizing voltage commands more positive than -40 mV. It activated rapidly and inactivated negligibly over a 400-ms voltage step. This current was blocked by tetraethyl ammonium (TEA; 10 mM). The TEA-sensitive current accounted for 72% of the total sustained outward current. 6. The Cd2+/Co2+-sensitive sustained outward component of current response could be further dissected pharmacologically. A part of the Co2+/Cd2+-sensitive current was also inhibited by apamin (200 nM). The apamin-sensitive component activated with depolarizing commands to -10 mV or more depolarized and was resistant to TEA. The apamin-insensitive current activated with voltage commands more positive than -25 mV and was sensitive to TEA. The apamin-sensitive current and the apamin-insensitive current contributed 33 and 20% to the sustained outward current, respectively. 7. Anomalous rectification of voltage responses to hyperpolarizing current injections were also recorded in all DA neurons. Under voltage clamp, a slowly developing inwardly directed current was activated with hyperpolarizing commands more negative than -70 mV. It took 700-800 ms to peak and was attenuated by Cs2+ (1 mM). 8. We conclude that acutely dissociated postnatal nigrostriatal DA neurons identified utilizing retrogradely transported rhodamine-labeled microspheres exhibit electrophysiological characteristics that are similar to those recorded in other preparations of SNZC DA cells. All DA neurons exhibit five types of potassium currents that can be distinguished by their voltage sensitivities, kinetics, and pharmacologic properties.
