Patch clamp studies of motor neurons in spinal cord slices: a tool for high-resolution analysis of drug actions.

AIM

To develop a tool for detailed analysis of spinally acting anesthetic and analgesic agents.

METHODS

Studies were done on visually identified motor neurons in 400 microns thick spinal cord slices from 14-23 d old rats using patch clamp techniques. Ethanol was used as a prototype general anesthetic agent.

RESULTS

Cell bodies in the ventrolateral horn identified as motor neurons by retrograde fluorescent labeling had a mean dimension of 32 +/- 5 microns (x +/- s, n = 25). Mean resting potential was -62.8 +/- 2.4 mV; input resistance was 44 +/- 24 M omega (n = 19). Threshold was -44 +/- 7 mV, and action potential amplitude 101 +/- 9 mV from baseline. Ethanol concentrations at and below 50-200 mmol/L decreased motor neuron excitability to the injected current; there was no effect on resting potential, but a variable reversible increase in input resistance. Ethanol reversibly depressed the excitatory postsynaptic potential, with a dose-response relationship similar to that previously observed for the population excitatory postsynaptic potential in intact spinal cord in vitro. Ethanol also reversibly depressed currents evoked by glutamate, reducing total charge transfer to 40% +/- 26% of control (x +/- s; n = 4).

CONCLUSION

Reduction of connectivity in this relatively thick slice preparation does not significantly modify drug actions. The actions of ethanol on excitatory synaptic transmission observed in intact spinal cord are in part due to postsynaptic effects on motor neurons.