Rj. Callister et P. Sah, THE REMOVAL OF ACETYLCHOLINE BY DIFFUSION AT NICOTINIC SYNAPSES IN THE RAT OTIC GANGLION, Journal of physiology, 505(1), 1997, pp. 165-175
1. We have examined the clearance of synaptically released acetylcholi
ne in the otic ganglion when acetylcholinesterase was blocked with ese
rine. 2. Intracellular recordings were made from otic ganglion neurone
s, in vitro. The decay of the excitatory postsynaptic potential (EPSP)
, in response to stimulation of afferent fibres, was greatly prolonged
in the presence of eserine. Low frequency (0.05-4 Hz) repetitive syna
ptic stimulation led to a slow depolarization of the postsynaptic cell
that persisted throughout the period of stimulation. This slow depola
rization was blocked by the nicotinic antagonists mecamylamine (100 mu
M) or (+) tubocurarine (100 mu M), but was unaffected by atropine (1
mu M), indicating that the response was due to the activation of nicot
inic receptors. 3. Following 2 Hz synaptic stimulation (30 s), the rat
e of rise of the slow depolarization had a time constant of 3.1 +/- 0.
4 s and a peak amplitude of 12 +/- 1 mV. Upon cessation of stimulation
, the depolarization decayed to resting levels with a time constant of
18.3 +/- 1.5 s (n = 23). At increasing stimulation frequencies the ra
te of rise of the depolarization increased. Lowering the probability o
f release, by adding cadmium to the perfusing solution or by lowering
extracellular calcium, slowed the rise time of the response. 4. Both t
he onset and decay kinetics of the slow depolarization had a low tempe
rature sensitivity indicating that they reflect diffusional processes.
5. Repetitive stimulation (2 Hz) of the afferent nerve supplying the
ganglion, in the presence of eserine, also caused a slow depolarizatio
n in cells in which we could not demonstrate a synaptic input. This in
dicates that synaptically released acetylcholine can spill over onto n
earby neurones. 6. We conclude that at parasympathetic synapses, under
physiological conditions, transmitter action is terminated by the enz
ymatic degradation of acetylcholine. When acetylcholinesterase is bloc
ked, acetylcholine accumulates because its removal by diffusion is slo
w.