Modulation of the cardiovascular defence response by low frequency stimulation of a deep somatic nerve in rats

In rats anaesthetised with alphaxalone/alphadolone, electrical stimulation in the dorsal part of the periaqueductal grey matter (PAG) produced a pressor response with tachycardia and vasodilatation in the hind limb, a pattern known as the 'cardiovascular defence reaction' owing to its resemblance to fear-induced hemodynamic changes. Following a 20-min period of stimulation of the peroneal nerve at 10 Hz with current intensities sufficient to recruit group II and III fibres the pressor component of the response was significantly reduced compared to control rats. The maximum decrease of the PAG-evoked pressor response was about 50% (from 32.0 +/- 0.7 to 16.6 +/- 5.9 mmHg). The effect lasted for between 60 and 290 min and was not correlated to baseline blood pressure changes observed after the stimulation of the nerve. In contrast, the tachycardia and hind limb vasodilator components of the defence response as well as their baseline values remained unchanged. Resting blood pressure did not change significantly in control rats but showed a small progressive increase in stimulated rats which reached significance 90-100 min after the stimulation. These results suggest that the afferent input from high threshold fibres in a muscle nerve can produce a selective and long-lasting depression of the vasoconstrictor components of the midbrain-evoked cardiovascular defence response. This effect is discussed in relation to the long-lasting sympathoinhibitory effects of acupuncture-like stimulation or sustained physical exercise.