| Summary: | 碩士 === 國立臺灣大學 === 動物學研究所 === 96 === Tonic inhibition has emerged as a key regulator of neuronal excitability in the central nervous system. Under block of glutamatergic transmission, there was many spontaneous IPSC (sIPSC) activity could be recorded by whole cell patch-clamp recording in cardiac vagal neurons (CVNs) in nucleus ambiguous (NA). The sIPSC could only be completely blocked with GABAA receptor blocker picrotoxin (PTX) and glycine receptor antagonist strychnine (Stry), showing it was mediated by GABAA and glycine receptors. Interestingly, application of PTX was associated with a reduction in holding current (Ihold), which was not observed with Stry application. These results suggest that there is a tonic inhibitory current (Itonic) mediated by extrasynaptic GABAA receptors in CVNs. Application of GABAA receptor antagonist gabazine (GBZ) and Stry also completely blocked sIPSC but did not have effect on Ihold. The Itonic was insensitive to d subunit-containing GABAA receptor agonists ethanol and THODC, showing no role for d subunit in Itonic. Application γ subunit-containing GABAA receptor agonist zolpidem reduced the Ihold, implyingγ subunit-containing GABAA receptors were expressed in CVNs. Pentobarbital (Pento), a widely used anesthetic in experimental animals, has profound effect on cardiovascular functions. The best known effect of Pento on neurons is that it augments GABAA receptor function. To test whether this role also involved in cardiovascular effect of Pento, we examined the effect of Pento on CVNs in NA. Bath application of Pento resulted in significant increase in noise level and decay of sIPSC, and a large increase in Ihold (IPento) of ~120 pA, all of which were completely blocked by Ptx or GABAA receptor agonist bicuculline. Application of presynaptic action potential antagonist tetrodotoxin (TTX) reduced the frequency of sIPSC, showing a very low incident of miniature IPSC activity in CVN. The IPento was not affected by TTX, suggesting that it can not be ascribed to the increased chance of summation of phasic sIPSC. This argument is supported by the fact that IPento was insensitive to GBZ. Raising recording temperature which decrease tonic current also did not reduce IPento. In summary, it was γ subunit, not δ subunit, contributing to tonic current of GABAA receptor. The present results suggest that Pento can directly activate GABAA receptors located at synaptic and extrasynaptic sites in CVN. This increased phasic or tonic GABAA receptor conductance silences CVN and inhibits cardiovascular function.
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