High susceptibility to hypoxia of afferent synaptic transmission in the goldfish sacculus.

1. The effect of hypoxia on synaptic transmission between hair cells and afferent fibers was examined in the sacculus of goldfish. For this, we recorded potentials, intracellularly, from large afferent fibers. Anoxia was introduced by perfusing the gill with water deprived of oxygen or by halting the water flow to the gill. 2. The ear of the goldfish is most sensitive to hypoxia. Sound-evoked afferent activities were profoundly depressed within several minutes after the introduction of hypoxia. 3. The depressed afferent activity was attributed to a reduction in the amplitude of sound-evoked excitatory postsynaptic potentials (EPSPs) generated at afferent fiber terminals, since no significant change was detected in the resting and action potentials of afferent fibers or in intensity of the threshold current required to set up an action potential. Also, there was no marked change in the electrical activity of hair cells, determined by the finding that the amplitude of intramacularly recorded microphonic potentials and that of the coupling potentials was not altered. 4. A statistical analysis of the amplitude of sound-evoked EPSPs revealed that the binomial parameter n decreased during hypoxia, in parallel with a reduction in the amplitude of EPSPs, while the binomial parameter p either remained unaltered or was augmented. No change was found in the quantal size, thereby indicating that the sensitivity of the postsynaptic membrane remained unchanged. These results indicate that presynaptic mechanisms within hair cells, especially those playing a role in transmitter release or in replenishment of the latter, are suppressed during hypoxia.