Regulation Of Action Potential Encoding On The Secretion Of Nerve Cells

Action potentials (APs) are the principal physiological stimuli for neurotransmitter secretion in neurons. One of its important functions is to trigger the secretion of the synapse. The AP firing is very complex. Most studies on stimulus-secretion coupling have been performed under voltage clamp using artificial electrical stimuli. And it is found that the change of presynaptic stimulus can result in difference of postsynaptic action. To investigate the modulatory effects of AP codes on neural secretion, we introduce a capacitance method to study AP-induced secretion in single cells. Real action potential was used as stimulus template. The action potential pattern was defined by a 4-parameter "code function"—F(n, m, f, d), where the four coding parameters (n, m, f, d) are defined as follows. A series of consecutive APs is defined as a "burst": "n" is the number of APs in a burst; "m" is the number of bursts in the whole AP pattern; "f" is the frequency of APs in a burst; and "d" is the interval between two adjacent bursts (when m = 1, "d" has no meaning). With this method, cell secretion evoked by stimulation with an AP code was quantified in real time by membrane capacitance (Cm) in rat adrenal chromaffin cells (RACCs) and rat dorsal root ganglion (DRG) neurons. We found: 1. Our data confirmed that secretion increased when AP-frequency "f" was increased. However, we found no further increase of secretion at higher frequencies (RACC, "f" ( 7 Hz; DRG neuron, "f" ( 10 Hz). 2. The relationship between secretion and AP numbers ("n") was not linear. 3. When "d" was longer than 1 second, the change of "d" had no effect on total secretion. 4. When the total number of APs and AP frequency were fixed, "m" can modulate secretion. The 4-burst AP-code evoked secretion 2.2 ( 0.1 times greater than that caused by a single burst on RACCs, when total number of APs was 40. However on DRG neurons, a single burst evoked secretion 1.5 ( 0.2 times greater than that caused by 4-burst AP-code. Our data indicated, besides "f" and "n", the physiological "m" effect may play a key role in AP-mediated neural information transfer within a single neuron and among the elements of a neural network. And this modulation has variety between different cells and tissues. This could be attributed to the multiformity of secretion kinetics.