Examination of the periaqueductal gray as a site for controlling arterial pressure in the conscious spontaneously hypertensive rat.

Our understanding of central nervous system regulation of the set-point of arterial pressure remains incomplete, especially in conditions of hypertension. The ventrolateral periaqueductal gray (vlPAG) is of particular interest given that its acute activation induces hypotension and sympatho-inhibition in anaesthetised, normotensive animals, and recent preliminary studies have shown that vlPAG stimulation can reduce blood pressure in refractory hypertensive patients. To assist our mechanistic understanding, we investigated whether electrical stimulation of the vlPAG had depressor actions in a model of neurogenic hypertension, the spontaneously hypertensive (SH) rat. We found that electrical stimulation of the lateral and vlPAG (2-6 V, 20-40 Hz, 0.18-0.2 ms pulse width) decreased arterial pressure (-19 ± 4 mm Hg, n = 8) and heart rate (median - 18 bpm) in anaesthetised SH rats. In contrast, in conscious freely-moving SH rats fitted with blood pressure telemetry, stimulation of this same region produced failed to evoked a hypotensive response (n = 13; either no change, n = 9; or an increase in arterial pressure of 23 ± 4 mm Hg, n = 4). The hypotensive action of the vlPAG observed in anaesthetised animals has been attributed to inhibition of pre-sympathetic neurones originating in the rostral ventrolateral medulla. We therefore used an un-anaesthetised, decerebrate SH rat preparation to investigate whether activation of vlPAG neurons produced sympatho-inhibition that might be below the threshold at which a peripheral vascular response could be observed. Only sympatho-excitatory responses to electrical and excitatory amino acid microinjections were observed, and these were evoked from both the dorsal and ventral PAG; no responses were evoked from the vlPAG. We conclude that the vlPAG is not a reliable antihypertensive locus in the awake SH rat. We discuss the potential importance of the state-dependency of the hypotensive response that can be evoked from the vlPAG, which has important implications for translating to humans.