Chemical Excitation of the Proventriculus of the Polychaete Worm Syllis Spongiphila

Abstract

1. The myoepithelial cells of the proventriculus of Syllis spongiphila (which are composed of only one or two sarcomeres that may reach 40 μm in length) are innervated by excitatory and inhibitory nerve fibres. Experiments were performed to examine the chemical excitation of these cells. We found that acetylcholine (ACh) mimics the excitatory transmitter and that the postsynaptic receptors appear to be nicotinic. 2. Bath application of ACh (10−5 to 10−4M), carbamylcholine (Carb, 10−4 to 10−3M) or nicotine (10−4 to 10−3M), but not muscarine (10−4 to 10−3 M), elicited overshooting depolarizations followed by prolonged depolarized plateaus. Iontophoresis of ACh or Carb (0.1-1.0 M) elicited depolarizing responses which, when elicited in trains, exhibited increased rise times to peak and decreased amplitudes to a plateau level; these results suggest the occurrence of desensitization of the postsynaptic receptors. Depolarizations elicited by indirect stimulation or by iontophoresis of ACh were reduced in amplitude by D-tubocurarine (10−5 to 5 × 10−4M) in a dose-related and reversible manner. Atropine (10−6 to 10−4M) had no effect on responses elicted by indirect stimulation. 3. Small potential changes superimposed on responses to cholinergic agents applied in the bath or iontophoretically were abolished in Ca2+-free ASW, Ca2+-free+1 mM EGTA ASW or in ASW containing 4-6 mM Mn2+. These small potential changes are likely to be from activity in neurones that receive cholinergic synapses. 4. Carb and nicotine applied in the bath consistently induced more overshooting peaks than did ACh, and the plateaus of the responses to ACh were of lower amplitude than those of the responses to Carb. In addition, responses to iontophoretic pulses of ACh were prolonged in the presence of 10−4 and 10−3 neostigmine. These data suggest the presence of an esterase in the nerve-muscle junctions of the proventriculus.