1. Intracellular recordings were made from neurones in the substantia nigra zona compacta in slices of rat mesencephalon in vitro. The majority of neurones fired action potentials spontaneously at 0.2-5.6 Hz. Dopamine, applied either by superfusion or from the tip of a pressurized pipette, prevented spontaneous action potential firing and hyperpolarized the membrane. 2. When the membrane potential was held negative to the threshold for action potential firing, the hyperpolarization evoked by dopamine was accompanied by a fall in input resistance. Under voltage clamp, dopamine produced an outward membrane current associated with an increase in membrane conductance. The effects of superfused dopamine on firing rate, membrane potential and membrane current were concentration dependent in the range 1-100 microM. 3. The reversal potential for the hyperpolarizations and the outward currents produced by dopamine was -109.7 +/- 1.7 mV (n = 12) when the potassium concentration was 2.5 mM and -74.0 +/- 5.0 mV (n = 4) when the potassium concentration was 10.5 mM. The change in reversal potentials in these and intermediate potassium concentrations was described by the Nernst equation. 4. The outward current induced by dopamine was reversibly reduced by barium (100-300 microM) and by high concentrations of tetraethylammonium (greater than or equal to 10 mM). Calcium-free solutions with cobalt (0.5-2 mM) did not reduce the current in response to dopamine during the first 5 min of their application. Currents and hyperpolarizations caused by dopamine were unaffected by tetrodotoxin (1 microM). 5. The hyperpolarization produced by dopamine was mimicked by the D2 receptor agonist quinpirole (LY 171555, 0.1-3 microM) and was blocked by the D2 receptor agonists domperidone and (-)-sulpiride. Agonists and antagonists at D1 receptors had no effect. 6. (-)-Sulpiride (30 nM-30 microM) produced a progressive shift to the right in the concentration-response curve to either dopamine or quinpirole. Schild analysis of the antagonism between (-)-sulpiride and quinpirole suggested competitive antagonism with a dissociation equilibrium constant for (-)-sulpiride of about 13 nM. 7. It is concluded that dopamine acts on D2 receptors on neurones of the rat substantia nigra pars compacta to increase the membrane potassium conductance.

Intracellular recordings were made from 475 rat substantia nigra zona compacta neurons in vitro. The region from which recordings were made was rich in catecholamine fluorescence. Two groups of neuron, termed principal neurons (95% of the total) and secondary neurons (5% of the total) were clearly distinguishable according to one or more of the following 4 electrophysiological properties. Secondary neurons (23 cells) (1) fired spontaneous action potentials at frequencies greater than 10 Hz, or were quiescent (30%); (2) had action potentials less than 1 msec in duration; (3) did not show time-dependent inward rectification with step hyperpolarization; and (4) had slope conductances of about 4 nS (between -75 and -90 mV). In contrast, principal neurons (1) fired spontaneous action potentials in the range 1-8 Hz, or were quiescent (33%); (2) had action potentials greater than 1 msec in duration; (3) showed pronounced time-dependent inward rectification; and (4) had steady-state membrane slope conductances of around 22 nS (between -75 and -90 mV). Secondary cells were not affected by dopamine but were hyperpolarized by baclofen, GABA, and the mu opioid receptor agonist Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGO). On the other hand, dopamine and baclofen inhibited firing and/or hyperpolarized all principal cells tested, but mu or delta opioid receptor agonists had no effect. The properties of these 2 cell types broadly correspond with those described by electrophysiological studies in vivo, in which case the majority, or principal, cells are believed to be dopaminergic.(ABSTRACT TRUNCATED AT 250 WORDS)

SUMMARY1. Intracellular recordings were made from 193 substantia nigra zona compacta neurones in slices of rat mesencephalon. All cells were hyperpolarized by baclofen; this was accompanied by a fall in input resistance. Cells voltage clamped at -60 mV showed an outward current associated with a conductance increase in response to baclofen. The baclofen effects were concentration dependent (effective range 0 3-30 lM); the concentration producing half the maximal effect was 1P5 ,UM. (-)-Baclofen was 300-700 times more potent than (+ )-baclofen.2. The potential change or membrane current caused by baclofen reversed polarity at -108-8 +1I1 mV (n = 10) when the potassium ion concentration was 2-5 mM, -96-0 + 2-8 mV (n = 3) in 4-5 mM-potassium and -766 + 1-7 mV (n = 5) in 10-5 mM-potassium. The relationship between reversal potential and potassium concentration conformed to the Nernst equation.3. Dopamine was also applied to 119 of these neurones; all exhibited either a hyperpolarization or an outward current.4. Baclofen and dopamine outward currents were reduced reversibly by barium (100-300 ,sM) and tetraethylammonium (10 mM). Superfusion for 5-10 min with solutions presumed to block calcium currents reduced, but did not abolish, responses to baclofen. The effect of baclofen persisted in tetrodotoxin (1 ,lM). 5. Superfusion of y-aminobutyric acid (GABA, 0 3-3 mM) caused either membrane depolarization or hyperpolarization, accompanied by a fall in input resistance. The depolarization was mimicked by muscimol (10 /,M) and blocked by bicuculline methiodide (10-100 ,tM); the hyperpolarization was resistant to bicuculline.Nipecotic acid (500 j,M) enhanced the effect of GABA, but was without effect upon the actions of muscimol and baclofen.6. The effect of dopamine was enhanced by cocaine (10 jiM) and antagonized by (-)-sulpiride (0-1-1 /uM), whereas the actions of baclofen were unaffected by cocaine or (-)-sulpiride. The maximum outward current produced by dopamine was approximately half that produced by baclofen.

1Intracellular recordings were made from presumed dopamine-containing neurones in the ventral tegmental area (VTA) in rat brain slices. 2 Nicotine (10-100 uM) and acetylcholine (ACh) depolarized the neurones. The depolarization caused by ACh was typically biphasic; both components were increased by neostigmine (0.1-10pM), but only the slower component was blocked by scopolamine (1-10 pM). 3 The nicotinic action of ACh, studied in the presence of neostigmine and scopolamine, persisted in the presence of tetrodotoxin (1 pM) and cobalt (2-5 mM). 4 ACh or carbachol (30pM) caused inward currents in neurones voltage-clamped near the resting potential. These currents reversed polarity at around -4mV, were blocked by hexamethonium (1-100pM) in a voltage-dependent manner, and showed desensitization with prolonged or repeated agonist applications. 5 Depolarizations caused by ACh and carbachol were reduced in slices pretreated with Kbungarotoxin, but were not changed by a-bungarotoxin. 6 These responses to ACh and nicotine resemble those previously described on autonomic ganglion cells. The direct action on VTA neurones may contribute to the positive reinforcement associated with nicotine consumption.