Evidence for the neurotransmitter role of norepinephrine in the ventral thalamic nucleus of the guinea pig: Localization, uptake and release

Abstract

The localization, uptake and release of norepinephrine (NE) by the isolated guinea pig ventral thalamic nucleus (VT) were studied to determine the possible neurotransmitter role of this catecholamine. The NE content in the VT was356±24ng/g wet weight and intraneuronal localization of NE was demonstrated in the VT by histofluorescence. [ 3H]NE accumulation into the VT slices was about 3 times that of [ 3H]NE (10 −7 M) in the medium 10 min after incubation. Kinetic analyses indicated 2 components of [ 3H]NE accumulation, one representing a high ( K m1 4 × 10 −7M and V max11.5pmol/mg/10min) and a low (K m2 1.8 × 10 10−6M and V max25pmol/mg/10min) affinity uptake system. Desmethylimipramine (10 −5 M) reduced the high affinity uptake of [ 3H]NE by 30% of the control value. Electrical stimulation of the slices increased the efflux of [ 3H]NE from tissues preloaded with [ 3H]NE, in a current- and frequency-dependent fashion. The release of [ 3H]NE induced by stimulation of up to 1 mA was inhibited by tetrodotoxin (10 −6 M) or Ca-free medium containing EGTA (10 −4 M). These findings provides strong evidence for the neurotransmitter role of NE in the VT.