Aim: 

A5 pontine region and hypothalamic defence area (HDA) stimulation in rats produces similar cardiovascular changes: tachycardia and hypertension. We have demonstrated functional interactions between these two regions inhibiting A5 neurones with muscimol (Dawid-Milner et al. 2003). We aim to characterise the role of angiotensin II in the cardiorespiratory response to HDA stimulation.

Methods: 

Experiments were carried out in spontaneously breathing anaesthetised rats (sodium pentobarbitone, n=8). The response evoked by electrical stimulation of the HDA (1ms pulses, 20-50mA, at 100Hz, 5s) was analysed before and after the microinjection of angiotensin II into the A5 region (50pmol, 50nl, 5s). At the end of the experiments animals were humanely killed.

Results: 

Angiotensin II into the A5 region produced (10 minutes after the microinjection) an increase of mean arterial pressure (from 922.5 to 1093.7 mmHg, p<0.001) and a decrease of respiratory rate (from 791.9 to 616.2 cpm, p<0.01). No changes were observed in heart rate (from 34210.4 to 34010.6 bpm). The compared cardiorespiratory response evoked by HDA stimulation before and after angiotensin II showed a decrease in the amplitude of the pressor response (from 55.86.3 to 31.86.5 mmHg p<0.05) and of the tachypnoea (from 36.510.6 to 7.81.7 cpm, p<0.05) with a disappearance of the tachycardia (from 33.83.1 to 0.18 bpm, p<0.05).

Conclusion: 

These results suggest a role for A5 region Angiotensin II receptors in the modulation of the cardiorespiratory response evoked from HDA stimulation. The abolishment of the heart rate component of the defence reaction suggests also its importance modulating the cardiac component the baroreceptor reflex.