Regional cerebral blood flow responses to neurochemical stimulation of the substantia innominata in the anaesthetized rat.

Since electrical stimulation of neurones may activate not only cell bodies but also neuronal fibres, this study aimed to test a selectively cholinergic neurochemical stimulation of the rat substantia innominata (SI) by the local microinjection of carbachol; the effects of this acetylcholine agonist were compared with glutamate. Cortical and subcortical cerebral blood flow (CBF) were measured in anaesthetized rats with the [14C]iodoantipyrine method by the tissue sampling technique immediately following the intracerebral (SI) microinjection of saline, 50 nmol of carbachol or glutamate. Carbachol microinjection into the SI induced a transient but significant vasodilation in frontoparietal motor (+28%) and temporal (+41%) cortices, that lasted for less than 10 min. Glutamate did not elicit any significant CBF modifications when compared to control rats although a significant interhemispheric asymmetry after microinjection was observed in the frontoparietal motor cortex. This latter observation would suggest that the glutamate-induced cortical response is less pronounced than that elicited by carbachol. Overall, these results demonstrate that a selective cholinergic stimulation of the SI can induce a transient cortical vasodilation and further confirms the hypothesis of a muscarinic modulation of CBF via this basal structure.