Mechanisms of membrane potential oscillation in bursting neurons on the snail, Helix pomatia

Abstract

1. 1. The mechanism of generation of membrane potential (MP) oscillations was studied in identified bursting neurons from the snail Helix pomatia. 2. 2. Long-lasting stimulation of an identified peptidergic interneuron produced a persistent bursting activity in a non-active burster. 3. 3. External application of calcium channel blockers (1 mM Cd 2+ or 5 mM La 2+) resulted in a transient increase in the slow-wave amplitude and subsequent prevention of pacemaker activity generation in bursting neurons. Application of these blockers together with endogenous neuropeptide initiating bursting activity generation, increased MP wave amplitude without prevention of bursting activity generation. 4. 4. Replacement of all NaCl in normal Ringer's solution with isoosmotic CaCl 2, glucose or Tris-HCl produced a reversible block of bursting activity generation. Stationary current-voltage relation (CVR) of bursting neuron membrane has a region of negative resistance (NRR) and does not intersect the potential axis in threshold region for action potential (AP) generation in normal Ringer's solution. In Na-free solution stationary CVR is linear and intersects the potential axis near — 52 mV. 5. 5. Novel potential- and time-dependent outward ( E rev = − 58 mV) current, I B , activated by hyperpolarization was found in the bursting neuron membrane. Having achieved a maximal value, this current decayed with a time constant of about 1 sec. Hyperpolarization inactivated maximal conductance, g B , responsible for I B , and depolarization abolished inactivation of g B . 6. 6. Short-lasting (0.01 sec) hyperpolarization of the bursting neuron membrane by inward current pulse induced the development of prolonged hyperpolarization wave lasting up to 10 sec. 7. 7. These results suggest that: (a) persistent bursting activity of RPal neuron in the snail Helix pomatia is not endogenous but is due to a constant activation of peptidergic synaptic inputs of these neurons; (b) Ca 2+ ions do not play a pivotal role in the ionic mechanism of MP oscillations but play a determining role in the process of secretion of a peptide initiating bursting activity by the interneuron presynaptic terminal; (c) depolarizing phase of the MP wave is due to specific properties of stationary CVR and hyperpolarization phase is due to regenerative properties of hyperpolarization-activated outward current I B . The minimal mathematical version of MP oscillations based on the experimental data is presented.