The actions of pentobarbitone, procaine and tetrodotoxin on synaptic transmission in the olfactory cortex of the guinea-pig

Abstract

1 It has been suggested that the depression of excitatory synaptic potentials produced by general anaesthetics can be attributed to a partial blockade of impulse conduction in the terminal branches of axons. This hypothesis has been tested by comparing the actions of pentobarbitone, procaine and tetrodotoxin (TTX) on synaptic transmission in the guinea-pig olfactory cortex. 2 Pentobarbitone (0.1-0.3mM) depressed the evoked synaptic potentials without any significant depression of impulse conduction in the afferent fibres of the lateral olfactory tract (1.o.t). It had no effect on the electrical excitability of either the l.o.t axons or the postsynaptic neurones. 3 Tetrodotoxin (TTX; 1-5x10(-8 M) slowed conduction of impulses in the l.o.t. and decreased the amplitude of the l.o.t compound action potential in proportion to the concentration applied. All concentrations of TTX elevated the electrical threshold of the l.o.t. axons and there was evidence to suggest that the threshold of the postsynaptic neurones was also elevated. The synaptic potentials were depressed in direct proportion to the depression of the l.o.t. compound action potential. 4 Procaine (0.1-0.5 mM) exhibited a pattern of activity intermediate between pentobarbitone and TTX. The most marked effect, seen at all concentrations tested, was a slowing of impulse conduction and a decrease in the electrical excitability of the l.o.t. axons. 5 It is concluded that general anaesthetics (exemplified by pentobarbitone) depress synaptic transmission by interfering with the processes involved in chemical transmission and not by blocking impulse conduction in the terminal branches of afferent nerves.