Stimulation of chemosensory pathways and intracellular alkalinization mimic cAMP activation of endogenous bursting in feeding command neurones.

Cyclic nucleotides modulate the activity of diverse types of excitable cells (Twarog & Muneoka, 1973; Berridge, 1975; Triestman & Levitan, 1976; Greengard, 1980; Bernier, Castellucci, Kandel & Schwartz, 1982; Gillette, Gillette & Davis, 1982£; Evans, 1984). In the carnivorous gastropod Pleurobranchaea, it would appear that cAMP modulation of a single pair of neurones effects the expression of a complex coordinated rhythmic behaviour. These neurones, the ventral white cells (VWCs), command vigorous motor output from the neuronal feeding oscillator through an intrinsic capacity to generate minutes-long high intensity bursts of action potentials (Gillette, Gillette & Davis, 1980). This high frequency firing causes the action potentials to increase in duration (broaden). The increase in action potential duration is a specific antecedent to enhanced motor output through a Ca-dependent mechanism (Gillette, Gillette & Davis, 1982a). Elevated intraneuronal cAMP is a specific activator of prolonged VWC bursting in both the isolated nervous system and the isolated VWC soma (Gillette et al. 19826). The relationship between this endogenous effector (cAMP) and extrinsic activators of the motor programme has not been established. I report here that the electrical changes in VWCs activated by food stimulation of the chemosensory sites involved in food detection and ingestion, as well as the changes activated by alkalinizing intracellular pH-shifts, both resemble in detail the changes induced by augmentation of intracellular cAMP. All three treatments stimulate recurrent burst episodes and cause attenuation of the after-hyperpolarizations of single stimulated action potentials. Decay of the activation in the continued presence of food shows kinetics strikingly similar to the decay of cAMP or NH4 stimulation. This is consistent with a model wherein ingestion is modulated at this strategic neuronal site by a pH-sensitive mechanism which controls intracellular levels of cAMP. Pleurobranchaea californica were pre-tested for their response to food stimuli (squid homogenate, SH) and egestive stimuli (100% EtOH or 1 % Haemosol in sea water). Immediately after behavioural testing, the animal was dissected to leave the anterior half (hemi-animal) intact. The buccal ganglion was immobilized on a