Effect of chronic clonidine treatment on transmitter release from sympathetic varicosities of the guinea-pig vas deferens

摘要

Previous studies have demonstrated that chronic pre-synaptic inhibition of transmitter release by morphine evokes a counter-adaptive response in the sympathetic nerve terminals that manifests itself as an increase in transmitter release during acute withdrawal. In the present study we examined the possibility that other pre-synaptically acting drug such as clonidine also evoke a counter-adaptive response in the sympathetic nerve terminals. In chronically saline treated(CST)preparations, clonidine(0.5μM)completely abolished evoked Transmitter release from sympathetic varicosities bathed in an extracellular calcium concentration ([Ca~2+]_o)of 2 mM.