Cardiovascular-related discharge of periaqueductal gray neurons during sleep-waking states

The periaqueductal gray (PAG) contains topographically organized areas that, upon stimulation or lesion, lead to modifications in blood pressure and heart rate and redistribution of blood flow. We examined patterns of discharge of single neurons in the PAG in 5 undrugged, freely moving cats to determine if cardiovascular patterning changes observed during different sleep states were related to activity in this midbrain region. Cross-correlation histograms and linear regression techniques were used to calculate dependencies between neuronal discharge and cardiac activity. Fifty of 68 cells recorded (74%) showed a discharge timing relationship (cardiac cycle-by-cycle) and/or a tonic discharge correlation with the cardiac cycle. Nearly all (48 or 96%) of these dependencies were state related. The large proportion of neurons showing a state-related cardiac dependency suggests that the PAG may contribute to mediating different cardiac patterns observed in each state.