Cerebral circulatory response to hypercapnia: effects of lesions of central dopaminergic and serotoninergic neuron systems.

The present study explores the possibility that the central dopaminergic and serotoninergic neuron systems influence CBF under normocapnic and hypercapnic conditions. In the first part of the study the effect of unilateral 6-hydroxydopamine lesion of the nigrostriatal dopamine pathway on local cerebral blood flow (1-CBF) was measured autoradiographically with [14C]iodoantipyrine as the diffusible tracer. The lesion caused no major effect on CBF under normocapnic or hypercapnic conditions. However, the circulatory response to hypercapnia was slightly enhanced (about 10%) in the denervated caudate-putamen. It is suggested that under hypercapnic conditions the pronounced increase in blood flow in the caudate-putamen is normally modulated by a slight vasoconstriction caused by dopamine release from the nigrostriatal system. In the second part of the study the effect of intraventricular 5,7-dihydroxytryptamine on cerebral metabolic rate for oxygen (CMRO2) and CBF was evaluated using a 133xenon modification of the Kety-Schmidt inert gas technique. The lesion, which removed about 90% of cortical 5-hydroxytryptamine, had no effect on the circulatory response to hypercapnia, not did it alter CMRO2. Under normocapnic conditions, though, the lesion seemed to induced a minor increase in CMRO2, which indicates that the serotoninergic system exerts a depressant resting tone on metabolic rate in the brain.