The Transformation Of Excitability In Mesencephalic V Neurons

Mes V neurons which are the primary sensory neurons of oral cavity and jaw closer muscles are located within the brainstem. In our previous study, it has been demonstrated that there are three types of excitability in Mes V neurons; type 1: low injected current produce spike discharges with a latency and the firing frequency increased continuously with the injected current intensity increase; type 2: relatively high injected current produce high frequency spike train without latency. The firing frequency was less sensitive to the injected current intensity; type 3: only fired a single or a little AP at the relatively high injected current. Pharmacological experiment indicated that the types of excitability can transformed by regulated INaP and/or I4-AP. But it belongs to the category of pharmacological experiment for completing it by using ion channel regulators (riluzole & 4-AP). In order to imply physiological meaning of excitatory transformation we need physiological experiment evidence that is the excitatory transformation is implemented by neurotransmitters exist in the brain and consistent with physiological phenomenon.5-HT is one of the important neurotransmitter that can regulate much physiological activity in brain. It has demonstrated there is high density of 5-HT receptors in Mes V neurons (5-HT2A and 5-HT3). The 5-HT2A is coupled to phospholipase C, via G protein; the 5-HT3 receptors directly activate a 5-HT-gated cation channel. Other literature indicated that 5-HT modulated persistent sodium currents and membrane excitability via cyclic AMP-protein kinase A cascade in Mes V neurons.In our study, the mesencephalic V (Mes V) neurons slices from neonatal rats were adopted as specimen. Whole-cell recording by infrared visual patch clamp was used to investigate the effect of 5-HT in transformation of excitability.1. Rita presumed hysteresis phenomenon in neuron discharges from limit cycle when studied squid giant axon with HH equation and confirmed this phenomenon in experiment in 1970s'.We first observed hysteresis phenomenon in type 2 excitability neurons in Mes V. When applied triangular stimulated currents which are increased and then decreased slowly, neurons discharged with increasing currents and ceased with decreasing, but the currents in cessation are smaller (hystersis) than the currents when discharge. This phenomenon cannot be explained by orthodox threshold theory. We also found that the Hysteresis phenomenon is abolished when type 2 excitability neurons transformed to type2 by applied 4-AP that is the currents are almost the same level at discharge and cessation. Nonlinear dynamic analysis was used to analyze this phenomenon and demonstrated that it is the character of subcritical Andronov-Hopf bifurcation that makes the discharges of type 2 Mes V neurons appear hysteresis.First: The effect of 5-HT on excitatory transformation in Mes V neurons.Main results:1. Three types of excitability have been observed in Mes V neurons.2. Serotonin transformed membrane excitability types of Mes V neurons from type 2 to type 3.3. The persistent sodium currents has been decreased under the effect of 5-HT.Main conclusion:1. In physiological environment, transition of neuron membrane excitability types is existence.2. Serotonin transformed membrane excitability types by modulate persistent sodium currents in Mes V neurons.Second: Discharges Hysteresis phenomenon in Mes V Neurons.Main results:1. Observed discharges hystersis phenomenon produced by triangle train current stimulus in type 2 excitation Mes V neurons.2. Type 2 excitation neurons transformed to type 1 and the hystersis disappeared when application 4-AP.3. According to non-linear dynamics bifurcation theory, type 1 and type 2 are related to a saddle-node bifurcation and an Andronov–Hopf bifurcation respectively.Main conclusion: 1. It is the character of subcritical Andronov-Hopf bifurcation that makes the discharges of type 2 Mes V neurons appear hysteresis.2. The types of bifurcation have changed, so discharges hystersis disappeared when type 2 transformed to type 1.