Regions in the periaqueductal gray activated by static muscle contraction in cats

The periaqueductal gray of the midbrain (PAG) is involved in the automatic regulation of the cardiovascular system. The purpose of this study was to determine if static contraction of the triceps surae muscle, which increases arterial blood pressure and heart rate, activates neuronal cells in the PAG. Static muscle contraction was induced by electrical stimulation of the L7 and S1 ventral roots of the spinal cord in anesthesized cats. Immunocytochemical staining of the PAG for Fos-like immunoreactivity (FLI) was determined in barointact animals during muscle contraction (n=6), barodenervated animals during muscle contraction (n=4) and barodenervated control animals (n=3). In addition, intravenous infusion of phenylephrine (PE) was used to selectively activate arterial baroreceptors in a fourth group of animals (n=4). The increase in mean arterial pressure during contraction in barointact (60±13 mmHg) and barodenervated (49±6 mmHg) animals was not different from the PE-induced increase in pressure (59±11 mmHg). Extensive FLI was observed within the ventromedial regions (VM) of the rostral PAG, and the dorsolateral (DL), lateral (L) and ventrolateral (VL) regions of the middle and caudal PAG in animals treated with PE and barointact animals with muscle contraction. There was no difference in the number of FLI-positive in any regions of the PAG when blood pressure was increased by PE or by muscle contraction in barointact cats. However, muscle contraction caused higher number of FLI in the VM region of the rostal PAG, the VL region of the middle PAG, and the L and VL regions of the caudal PAG in the barointact animals than in barodenervated animals. Finally, the number of FLI in the DL and L regions of the middle PAG is greater in barodenervated animals with muscle contraction than in barodenervated control animals. Thus, these results indicate that stafic muscle contraction activates neuronal cells in regions of the PAG. Furthermore, afferent activity from skeletal muscle activates neurons in specific regions of the PAG independent of arterial baroreceptor activation. Therefore, neuronal cells in the PAG may play a role in determining the cardiovascular responses during the exercise pressor reflex.