Etomidate reduces excitability of the neurons and suppresses the function of nAChR ventral horn in the spinal cord of neonatal rats

Abstract

To investigate the effects of etomidate on electrophysiological properties and nicotinic acetylcholine receptors (nAChRs) of ventral horn neurons in the spinal cord. The spinal cord containing lumbosacral enlargement was isolated from 19 neonatal SD rats aged 7-12 days. The spinal cord were sliced and digested with papain (0.18 g/30 mL artificial cerebrospinal fluid) and incubated for 40 min. At the ventral horn, acute mechanical separation of neurons was performed with fire-polished Pasteur pipettes, and perforated patch-clamp recordings combined with pharmacological methods were employed on the adherent healthy neurons. In current-clamp mode, the spontaneous action potential (AP) of the ventral horn neurons in the spinal cord was recorded. The effects of pretreatment with different concentrations of etomidate on AP recorded in the ventral horn neurons were examined. In the voltage-clamp mode, nicotine was applied to induce inward currents in the ventral horn neurons, and the effect of pretreatment with etomidate on the inward currents induced by nicotine were examined with different etomidate concentrations, different holding potentials and different use time. The isolated ventral horn neurons were in good condition with large diverse somata and intact processes. The isolated spinal ventral horn neurons (n=21) had spontaneous action potentials, and were continuously perfused for 2 min with 0.3, 3.0 and 30.0 μmol/L etomidate. Compared with those before administration, the AP amplitude, spike potential amplitude and overshoot were concentration-dependently suppressed (P < 0.01), and spontaneous discharge frequency was obviously reduced (P < 0.01, n=12). The APs of the other 9 neurons were completely abolished by etomidate at 3.0 or 30 μmol/L. At the same holding potential (VH=-70 mV), pretreatment with 0.3, 3.0 or 30.0 μmol/L etomidate for 2 min concentration-dependently suppressed the current amplitude induced by 0.4 mmol/L nicotine (P < 0.01, n=7). At the holding potentials of - 30, - 50, and - 70 mV, pretreatment with 30.0 μmol/L etomidate for 2 min voltage-dependently suppressed the current amplitude induced by 0.4 mmol/L nicotine (P < 0.01, n=6 for each holding potential). During the 6 min of 30.0 μmol/L etomidate pretreatment, the clamped cells were exposed to 0.4 mmol/L nicotine for 4 times at 0, 2, 4, and 6 min (each exposure time was 2 s), and the nicotinic current amplitude decreased gradually as the number of exposures increased. But at the same concentration, two nicotine exposures (one at the beginning and the other at the end of the 6 min pretreatment) resulted in a significantly lower inhibition rate compared with 4 nicotine exposures (P < 0.01, n=6). etomidate reduces the excitability of the spinal ventral neurons in a concentration-dependent manner and suppresses the function of nAChR in a concentration-, voltage-, and use-dependent manner.