1. Whole-cell patch-clamp recordings were made from rostral ventrolateral medulla (RVLM) neurones of brainstem slices from 8- to 12-day-old rats. In the presence of tetrodotoxin (0.5 mu M), 5-HT (50 mu M) elicited an outward current (I-5.HT,I-outward) (10/44 neurones) associated with an increase in membrane conductance, and an inward current (I-5-HT,I-inward) (29/44 neurones) accompanied by a decrease or no significant change in membrane conductance. 2. The steady-state I-V relationship of I-5-HT,I-outward showed an inward rectification; the 5-HT-induced current, which reversed at -87.9 +/- 3.0 mV, was suppressed by 0.1 mM Ba2+. 3. Two types of steady-state I-V relationship for I-5-HT,I-inward were noted: type I I-5-HT,I-inward was characterized by a significant decrease in membrane conductance and reversed at a potential close to or negative to the theoretical K+ equilibrium potential (E-K), -94 mV, in 8/17 neurones; type II I-5-HT,I-inward was not associated with a significant change in membrane conductance and was relatively independent of membrane potential. 4. Both type I and type II I-5-HT,I-inward were significantly reduced in a low [Na+](o) solution. In this solution, I-5-HT,I-inward decreased with hyperpolarization and had a linear steady-state I-V relationship with a reversal potential of approximately -110 mV. The reversal potential of type I I-5-HT,I-inward shifted to about -80 mV as the [K+](o) was increased from 3.1 to 7.0 mM in low [Na+](o) solution. The type II I-5-HT,I-inward did not reverse at the estimate EK in the same solution. 5. While not affected by externally applied Cs+ (1 mM), I-5-HT,I-inward was significantly smaller in RVLM neurones patched with Cs+-containing electrodes; the current reversed at -11.9 +/- 6.4 mV in 8/15 responsive neurones. 6. It may be concluded that in rat RVLM neurones 5-HT increases an inwardly rectifying K+ conductance which may underlie the I-5-HT,I-outward and that a combination of varying degrees of K+ conductance decrease and a Cs+-insensitive, non-selective cation conductance increase may account for the two types of conductance change associated with I-5-HT,I-inward.
