Differentiating hemodynamic responses in rat primary somatosensory cortex during non-noxious and noxious electrical stimulation by optical imaging

Nociception in the primary somatosensory (S1) cortex remains in need of further elucidation. The spatiotemporal comparison on changes of the cerebral blood volume evoked by graded peripheral electrical stimulation was performed in rat contralateral somatosensory cortex with optical intrinsic signal imaging (OISI, optical reflectance at 550 nm). Non-noxious electrical stimulus was applied with 5 Hz pulses (0.5 ms peak duration) for 2 s at the threshold current for muscle twitch, while noxious stimulus was delivered at currents of 10× and 20× amplitude of the predetermined threshold. Although the dimensions of peak response defined in the spatial domain (cerebral blood volume increase) in the S1 cortex presented no significant difference under non-/noxious stimuli, its early response component (about 1 s after stimulation onset) revealed by OISI technique was suggested to differentiate the loci of activated cortical region due to different stimulation in this study. The magnitude and duration of the optical intrinsic signal (OIS) response was found increasing with the varying stimulus intensity. Regions activated by the delivery of a noxious stimulus were surrounded by a ring of inverted optical intrinsic signal, the amplitude of that was inversely proportional to the strength of the optical signal attributable to activation. Intense stimuli significantly augmented the inverted optical signal in magnitude and spatial extent. These results indicated that noxious stimulation evoked different response patterns in the contralateral S1 cortex. The magnitude-dependent inverted optical signal might contribute to the differentiation of nociceptive input in the S1 cortex.