Effect of raised Mg2+ on the antidromic activation of immature rat spinal motoneurones in vitro.

Currents elicited by electrical stimulation of the ventral root were recorded from motoneurones of the immature rat spinal cord in vitro using the patch pipette technique. Control medium contained Ca2+ (1.5 mM) and Mg2+ (0.75 mM). In nine preparations the mean amplitude of antidromic current responses was 5.12 +/- 0.41 nA. Raising Mg2+ (EC50 9.6 +/- 1.1 mM) to levels up to 50 mM produced an all-or-none maximal depression of the current responses by 69 +/- 1%. These levels of Mg2+ depressed currents elicited by depolarizing command steps by only 30% and compound action potentials recorded in ventral roots by only 17%. It is concluded that raised Mg2+ caused conduction block from the initial segment to the soma dendritic region of motoneurones. This non-synaptic depressant action of raised Mg2+ should be considered when raised Mg2+ is used in order to specifically block synaptic activity in vitro.