Correlation of intrinsic optical signal, cerebral blood flow, and evoked potentials during activation of rat somatosensory cortex

Abstract

This study was undertaken to test the hypothesis that cerebral blood flow (CBF) and the intrinsic optical signal could be dissociated by altering adenosine receptor activity and to uncover the origin of the optic signal using a cranial window in the anesthetized rat. In anesthetized, ventilated, and temperature-controlled rats with closed cranial windows, the authors evaluated simultaneously the alterations in pial arteriolar diameter, intrinsic optical signals (690 nm), and somatosensory evoked potentials during cortical activation evoked by contralateral sciatic nerve stimulation (SNS). To dissociate the vascular and intrinsic signal, they topically applied the adenosine receptors antagonists theophylline (5 microM), which affects A1 and A2A receptors, and 8-cyclopentyl-1,3-dipropylxanthine (CPX, 1 microM), which blocks the A(1) receptor. The former interacts primarily with the vasculature whereas the latter influences the parenchyma exclusively. During 20 seconds of contralateral SNS, pial arterioles in the hindlimb somatosensory cortex dilated in a characteristic peak and shoulder pattern. As compared with mock cerebrospinal fluid alone, theophylline significantly (p<0.05) attenuated SNS-induced vasodilation (mean+/-standard deviation 8.1+/-2.5% vs 21.7+/-1.9%; 4 rats in each group). In contrast, CPX potentiated vasodilation significantly (p<0.05) during SNS (54.7+/-15.8% for the CPX group vs 20.1+/-1.9% for the controls; 5 rats in each group). The change in optical signal persisted after cessation of SNS in all the animals. Thus, the pattern of change of the optical signal was distinctly different from the pattern of changes in arteriolar diameter (which returned rapidly to baseline). Moreover, the optical signal during SNS was increased by 50% by theophylline and by almost 5-fold by CPX (p<0.05). The area of change of the intrinsic signal was also increased by the topical application of theophylline and CPX. The somatosensory evoked potential recordings revealed no significant changes after theophylline application, but CPX caused a small diminution of the N1 wave (p<0.01). The noncongruent temporal profiles of the changes in pial arteriolar diameter and optical signal, imaged at 690 nm, indicate that the optical signal at 690 nm is not related to CBF. Alteration of adenosine receptor activity independently changed cortical activity, as measured by the optical signal, and CBF, as determined by pial arteriolar diameter. Manipulation of the adenosine receptor activity during increased cortical activity confirmed the temporal dissociation of optical signal and CBF and provided further evidence for the role of adenosine in regulating CBF.