Unit activity in medial thalamus: comparative effects of caffeine an amphetamine

The effects of caffeine and d-amphetamine sulfate were evaluated upon single unit activity of medial thalamus in chloral hydrate anesthetized rats. Single unit activity recorded extracellularly with platinum-iridium microelectrodes. It was found that caffeine, 0.1 to 0.5 mg/kg i.v., markedly suppressed and that amphetamine, 0.1 to 1 mg/kg i.v., markedly augmented the spontaneous firing rates of medial thalamic neurons. It was also demonstrated that amphetamine was capable of activating the medial thalamic neuronal activity while it was suppressed by caffeine. On the other hand, while the firing rate was increased by a previous amphetamine injection, administration of caffeine suppressed the amphetamine-induced augmentation of neuronal activity. The data imply that these two central nervous system (CNS) stimulants, caffeine and amphetamine, have different mechanisms of action at least at the medial thalamic site. Moreover, since medial thalamus participates in the recruitment phenomenon and exerts a stabilizing action on the cerebral cortex, our findings that caffeine suppressed this brain area suggest that the medial thalamus may be an important site of action for the arousal induced by caffeine. In addition, the demonstration of inhibitory effects of caffeine on the medial thalamus in this study and excitatory effects on brain stem reticular formation from our previous studies indicate that caffeine has differential actions on different brain areas. This observation broadens the generally held belief that caffeine excites at all levels of the CNS to specifically include inhibition of inhibitory influences in at least one functional system of the brain.