Ionic basis of the membrane potential responses of rat dorsal vagal motoneurones to HEPES buffer

The effects of 10 mM HEPES (N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid) buffered artificial cerebrospinal fluid (aCSF) on membrane potential and the action potential were studied in 93 dorsal vagal motoneurones (DVMs) using an in vitro slice preparation of the rat medulla. Changing from bicarbonate/CO2 aCSF to HEPES aCSF resulted in a depolarisation of 6.0 +/- 0.6 mV and an increase in input resistance (R(In); n = 61). In the presence of 5 mM 4-AP, HEPES either had little effect (n = 9) or hyperpolarised the membrane (n = 10). Mn2+ (3 mM) or Ni2+ (200 mu M) abolished the hyperpolarisation and its associated increase in R(In). In voltage-clamp studies 5 mM 4-AP eliminated a transient outward current and Ni2+ blocked an inactivating inward current. It is concluded that HEPES buffer reduces the contribution of the A current to resting membrane potential and also reduces a Ni2+-sensitive transient I-Ca.