Cholinergic Activation of Startle Motoneurons by a Pair of Cerebral
Interneurons in the Pteropod Mollusc, Clione limacina.
Norekian, Tigran P. and Richard A. Satterlie.
Department of Zoology, Arizona State University, Tempe, AZ 85287-1501, USA
and Friday Harbor Laboratories, Friday Harbor, WA 98250, USA.
APStracts 3:0224N, 1996.
The holoplanktonic pteropod mollusc Clione limacina exhibits an active escape
behavior which is characterized by fast swimming away from the source of
potentially harmful stimuli. The initial phase of escape behavior is a startle
response which is controlled by pedal motoneurons whose activity is
independent of the normal swim pattern generator. In this study, a pair of
cerebral interneurons is described which produce strong activation of the d-
phase Startle motoneurons, which control dorsal flexion of the wings. These
interneurons were designated cerebral startle (Cr-St) interneurons. Each Cr-St
neuron has a small cell body on the dorsal surface of the cerebral ganglia and
one large axon which runs into the ipsilateral cerebral-pedal connective and
the neuropile of the ipsilateral pedal ganglion. Each spike in a Cr-St neuron
produces a fast, high amplitude (up to 50 mV) EPSP in the d-phase Startle
motoneurons. This 1 spike:1 EPSP ratio and the stable short synaptic latencies
(2 ms) persist in high Mg ++ /high Ca ++ sea water suggesting monosynaptic
connections. Synaptic transmission between Cr-St neurons and Startle
motoneurons exhibits a very slow synaptic depression as a number of spikes in
Cr-St neurons is required to achieve a noticeable decrease in EPSP amplitude.
Synaptic transmission between Cr-St interneurons and Startle motoneurons
appears to be cholinergic. 20 [mu]M atropine and 50 [mu]M d-tubocurarine
reversibly block EPSPs in Startle neurons produced by spike activity in Cr-St
interneurons. Hexamethonium only partially blocks EPSPs in Startle neurons,
and much higher concentrations are required. Exogenous acetylcholine (1 [mu]M)
produces a dramatic depolarization of Startle motoneurons in high Mg ++ sea
water, and this depolarization is reversibly blocked by atropine. Nicotine
also has a depolarizing effect on Startle motoneurons, although higher
concentrations are required. Cr-St interneurons and Startle motoneurons are
also electrically coupled, however the coupling is weak. Stimuli which are
known to initiate escape responses in intact animals, such as tactile
stimulation of the tail or wings, produce excitatory inputs to Cr-St
interneurons. In addition, tactile stimulation of the lips and buccal cones,
which is known to trigger prey capture reactions in Clione , also produces
excitatory inputs to Cr-St interneurons and Startle motoneurons, suggesting
involvement of the startle neuronal system in prey capture behavior of Clione.
Received 22 April 1996; accepted in final form 18 September 1996.
APS Manuscript Number J334-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 November 1996