Emerging evidence suggests alterations in cardiovascular function could be in part mediated by neuroplasticity of central autonomic networks [1-2]. In particular the periaqueductal gray (PAG) is a powerful modulator of cardiovascular parameters [3-4]. The PAG expresses hyperpolarization-activated cyclic nucleotide gated (HCN) channels throughout its columns. HCN channels play essential roles in the physiology of both the cardiovascular system and neural circuits. Therefore, we examined the functional involvement of HCN channels in the PAG in modulating cardiovascular activity in normotensive and hypertensive rats. Methods: HCN channel modulating compounds were injected into the PAG and changes in cardiorespiratory parameters were assessed. Finally we compared hypertensive and normotensive animals in their response to PAG HCN channel modulation. Heart rate and breathing rate of Sprague Dawley (SD), Wistar Kyoto (WKY) and Spontaneous Hypertensive Rats (SHR) (270-390g) was recorded in response to in vivo stereotactic injection via pulse oximetry of the right foot under general anesthesia (1-3% Isoflurane). For injection into the dorsal PAG (dPAG) the following coordinates were used [5]: -7.3mm caudal to Bregma, 0.6mm lateral from the medial suture, 4.0mm deep from the surface of the brain. All data are presented as mean±SEM with ANOVA for multiple comparisons.Results: Injection of the non-specific HCN channel blocker ZD7288 (500nl, 20µg/0.5µl; n=5) into the dPAG of SD rats, significantly decreased heart rate from 367±7 to 308±6bpm (p<0.001). Similarly breathing rate was also significantly decreased (from 55±2 brpm to 44±1 brpm, n=5, p<0.01). Injection of the synthetic cGMP analog 8-Bromo-cGMP (500nl, 10mM; n=6) into the dPAG of SD rats, significantly increased heart rate from 331±5 to 350±3bpm (p<0.005). Injecting ZD7288, 8-Bromo-cGMP or saline outside the PAG did not change heart rate or breathing rate. Finally, SHR rats showed a significantly stronger reduction in heart rate in response to ZD7288 injection (20µg/0.5µl) into the PAG then WKY rats (SHR: from 360±6 to 285±5bpm; WKY: from 300±6 to 275±5bpm, n=9, p<0.001). Conclusions: These data demonstrate that HCN channels in the dPAG are functionally involved in the regulation of cardiorespiratory activity. Blocking PAG HCN channels reduces heart rate and breathing rate, while facilitating HCN channel activity increases heart rate. Furthermore our data shows that hypertensive rats are more sensitive to HCN antagonism than their normotensive counterparts. This suggests that PAG HCN activity is more pronounced in hypertensive phenotype. PAG HCN channels are thus a potential substrate of central autonomic neuroplasticity that could contribute to alterations in cardiovascular functioning in disease states when sympathetic activity is abnormally elevated.