Mechanisms of Generation of Pacemaker Activity by Identified Helix Neurons

We studied the mechanisms of generation of pacemaker activity in identified neurons of Helix pomatia. For this purpose, we isolated the PPa2 and PPa7 neurons generating spontaneous rhythmic monomodal activity and PPa1 neuron with bursting activity. It was demonstrated that isolated PPa2 and PPa7 cells produce endogenous rhythmic activity that was not considerably modified by external application of 1 mM CdCl₂. Sometimes, only low-amplitude dendritic action potentials (AP) were observed instead of generation of full-amplitude somatic AP. In contrast, isolation of the PPa1 neuron eliminated its bursting activity, but subsequent application of oxytocin on this neuron recovered such activity. This finding shows that the bursting activity of the PPa1 neuron is of an exogenous nature. Application of 1 mM CdCl₂ suppressed this bursting activity, but when Cd²⁺ was applied against the background of superfusion of the neuron with Ringer solution containing a bursting activity-initiating neuropeptide obtained from the molluscan CNS, this blocker was incapable of suppressing the bursting activity. A blocker of the hyperpolarization-activated current (I _h , H current), Cs⁺ (10 mM) exerted no noticeable effect on the activity of the studied neurons. Our findings allow us to conclude that the pacemaker activity is initiated within the dendritic tree of a cell and is then electrotonically spread to the soma, where full-amplitude AP are generated. It seems probable that Ca²⁺ ions and H current are not directly involved in generation of the pacemaker activity in the studied snail neurons.