Spontaneous miniature hyperpolarizations of presynaptic nerve terminals in the chick ciliary ganglion

Intracellular recordings from presynaptic nerve terminals in the chick ciliary ganlion revealed the presence of spontaneous miniature hyperpolarizations in virtually all (86%) nerve terminals examined. These spontaneous events appeared as small, brief hyperpolarizations at resting potential and were observed to increase or decrease as the membrane potential was depolarized or hyperpolarized from rest, respectively. The hyperpolarizing potentials were sensitive to blockade by tetraethylammonium and Ba²⁺, while caffeine increased then abolished these events. The voltage fluctuations were unaffected by tetrodotoxin, low Ca²⁺ external solution or the synaptic blockers, picrotoxin and strychnine. These spontaneous, transient, miniature hyperpolarizations may be due to the brief and co-ordinated activation of between 15–60 Ca²⁺-dependent K⁺ channels following the release of Ca²⁺ from internal stores.