Initial findings from our laboratory have indicated that muscarinic enhancement of K+-evoked release of dopamine from perifused striatal slices is reduced after exposure to${}^{56}{\rm Fe}\text{-particle}$ irradiation. This finding suggested that there is a radiation-induced deficit in muscarinic receptor sensitivity. Subsequent findings have indicated that at least part of the loss in sensitivity may occur as a result of alterations in the initial steps of the signal transduction process and involve muscarinic receptor-G protein coupling/uncoupling. The present study was carried out to localize this deficit further by determining carbachol-stimulated low- K m guanosine triphosphatase (GTPase) activity in striatal and hippocampal tissue obtained from rats exposed to 0, 0.1 or 1.0 Gy of${}^{56}{\rm Fe}\text{-particle}$ irradiation. In addition, to examine the specificity of the effect of${}^{56}{\rm Fe}\text{-particle}$ irradiation, α1-adrenergic-stimulated$\text{low-}{\rm K}_{{\rm m}}$ GTPase activity was also examined in these tissues. The results showed that there was a high degree of specificity in the effects of${}^{56}{\rm Fe}$ particles. Decrements were observed in muscarinic-stimulated$\text{low-}{\rm K}_{{\rm m}}$ GTPase in striatum but not in hippocampus, and${}^{56}{\rm Fe}\text{-particle}$ irradiation did not affect α1-adrenergic$\text{low-}{\rm K}_{{\rm m}}$ GTPase activity in either brain tissue.

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