Sympathetic cutaneous vasomotor alerting responses (SCVARs) are associated with hippocampal theta rhythm in non-moving conscious rats

Alerting stimuli that cause sympathetic cutaneous vasomotor alerting responses (“SCVARs”) in the thermoregulatory vascular bed of the rabbit ear pinna also induce theta (5-8 Hz) rhythm in the hippocampal EEG, a marker that the animal is engaged with the environment. The present study determines the relation between SCVARs in the thermoregulatory tail artery vascular bed and hippocampal EEG theta rhythm in Sprague-Dawley rats. A Doppler ultrasonic flow probe chronically implanted around the base of the tail artery was used to measure SCVARs. Unipolar electrodes were implanted in the hippocampus (CA1 region) to measure EEG. Six standard non-noxious brief alerting stimuli were administered during continuous recording of tail blood flow and EEG. The SCVAR index was calculated as the percentage fall from pre-alerting blood flow values. After stimuli the SCVAR index for the tail artery blood flow was 84 ± 2% of the pre-alerting control. In contrast, the same stimuli caused little fall in blood flow within the superior mesenteric bed (SCVAR index = 18 ± 2%). The proportion of theta power in the total frequency range (0-20 Hz) increased significantly after alerting stimuli (46 ± 2% vs. 29 ± 1% before stimuli, p < 0.05). Theta proportion began to increase approximately 0.5 s after the stimuli and preceded SCVARs by approximately 1 s. The SCVAR index was correlated with the magnitude of the increment in theta power. Our study demonstrates that alerting responses resulting in selective vasoconstriction of the tail vascular bed are associated with hippocampal theta rhythm in conscious rats.