Effect Of Neurotransmitters Released From Primary Afferents On Dorsal Root Ganglion Neurons Membrane Potentials And Action Potentials

Dorsal root ganglion neurons are primary sensory neurons which is the pathway of sensory information transmition and also can analyze and integrate sensory information. Chemical substances such as ATP,GABA and SP in local surrounding can affect the function of DRG neurons terminal endings, such membrane potential, excitability. Research indicates that those chemical substance can be synthesized and released from the DRG neurons and also their receptors all exsit on the DRG neurons membrane. How do those neurotansmitters affect the function of DRG neurons and how do those changes which occurred on the membrane affect sensory information transmition? So this study try to observe the effect of SP, GABA, ATP on DRG neurons'membrane potentials and action potentials and the effect of low frenquent extra-stimulate on GABA-induced depolariztion using intracellular recording technique. The results are :1. SP evoked membrane depolarization in rat DRG neuronsAbout 42.9%(18/42)SP (10-8 ~3×10-6 mol/L) evoked a depolarizing response. The membrane depolarization induced by SP was concentration-dependent. The threshold was about 10-8 mol/L and the maximal response was achieved by 3×10-6 mol/L SP, The EC50 value was about 3×10-7 mol/L deduced from the concentration-response curve. SP induced DRG neurons depolariztion with a increase in conductance, the membrane conductance became greater with increased concentration of SP.2. ATP-induced depolarizationThe majority of cells (84.5%,224/265) examined in the present experiment were sensitive to bath application of ATP with a depolarizing response. In the present study, ATP-activated depolariztion from rat dorsal root ganglion neurons using intracellular recording technique are classified into three types ( F, I and S ) based on the characteristics of their activation and desensitization. There was statistical difference between the time of rising phase (R10-90) of types F, I and S and the time of decaying phase (D10-90) of types F, I and S. The incidence of types F, I and S was 1.8% (4/224), 4.6% (10/224) and 93.6% (210/224) respectively. The ATP (5×10-5 ~ 10-2 mol/L)-evoked response was in a concentration-dependent manner (type S). The threshold was about 10-4 mol/L and the maximal response was achieved by 10-2 mol/L GABA. The EC50 value was about 2×10-3 mol/L deduced from the concentration-response curve. The selective P2X2/3/P2X3 -R antagonist TNP-ATP(10-5 mol/L) inhibited ATP-evoked responses, the inhibiting ritio of TNP-ATP induced inhibition on ATP-activated membrane depolariztion were 76.9% (2×10-3 mol/L, n=3), 51.0% (5×10-3 mol/L, n=4). ATP induced DRG neurons depolariztion with a increase in conductance, the membrane conductance became greater with increased concentration of ATP.3.Effect of ATP on single DRG neuron action potentialResults were presented for 63 DRG neurons. The conduction velocities from the sciatic nerve to the DRG ranged from 2.52 m/s to 20.00 m/s, and the average conduction velocity was 9.68±0.40 m/s. They all belonged to type A fibres. According to the style of action potentials, the DRG neurons were classfied into type F and type H and the incidence of these types were 74.60% and 25.40% respectively.The average membrane potential of type F was depolarizted from -56.00±3.10 mV to -43.10±3.74 mV, the average spike amplitude of action potential of type F were decreased from 65.30±2.73 mV to 20.10±2.51 mV, the average maximum velocity of rising phase of type F were decreased from 117.35±12.60 m/s to 49.12±13.40 m/s, the overshooting was diminished after bath application of 5×10-3 mol/L ATP for five minutes (n=4, P<0.05). there were statistical difference when bath application balance salt solution insteading of 5×10-3 mol/L ATP (n=9) comparing with bath application of 5×10-3 mol/L ATP. There was no statistical difference between the duration of action potential and conduction velocity.4.Effect of low frenquent extra-stimulate on GABA-induced depolariztionGABA-induced depolariztion was inhibited by low frenquent extra-stimulate.The inhibitory effect of low frenquent extra-stimulate increased with the widen duration of stimulate and the time of inhibitory effect was prolonged with the increased stimulate frenquence. The ritio of inhibition was 38.7% and the inhibition time was more than ninety minutes when stimulating sciatic nerve (n=5, P<0.05).These results suggest that SP receptor, P2X recepter exsit on rat dorsal root ganglion; those chemical substances can affect the function of DRG neurons. SP can affect the function of DRG neurons by activing the non-selective cation channels combining with its selective NK1 receptor. ATP depolarize DRG neurons and make some sodium channels closed so that the amplitude of action potentials and Vmax decreased which result in decreaced releasing of neurotrasmitters from DRG neurons, the way is same like GABA-induced presynaptic inhibition by activing the non-selective cation channels possibly combining with P2X2/3-receptor. Extra-stimulation can make nociceptive information transmit easily through inhibiting the'presynaptic inhibition'evoked by GABA, thus play a role in the modulation of primary sensory information (especially pain).