Mechanisms for central cannabinoid receptor 1 evoked sympathoexcitation/pressor response in conscious rats

Abstract

The main goal of the current study was to elucidate the molecular mechanisms implicated in central cannabinoid receptor 1 (CB1R)-evoked sympathoexcitation/pressor response. In pursuit of this goal, the candidate first characterized the centrally elicited hemodynamic effects of CB1R activation (WIN55,212-2) in a conscious rat model. The results showed that the pressor response elicited by central CB1R stimulation was associated with enhanced neuronal activity of presympathetic neurons in the rostral ventrolateral medulla (RVLM). The findings of multiple in vivo and in vitro studies have suggested a functional crosstalk between central CB1R and orexins. Therefore, the candidate hypothesized that orexin-A/orexin receptor 1 (OX1R) signaling in the RVLM is essential for the central CB1R-mediated pressor response. In support of this hypothesis, central CB1R activation elevated orexin-A level in the RVLM and inhibition of orexin-A/OX1R signaling abrogated the CB1R-evoked pressor effect. On the other hand, central orexin-A/OX1R-evoked pressor response was not affected by prior blockade of central CB1R. Colocalization studies have unraveled close proximity of orexin-A/OX1R to CB1R immuno-reactive neurons and punctate structures in the RVLM, which support the pharmacological findings. Furthermore, the present study delineated a novel role for PI3K/Akt/ERK1/2 signaling in RVLM and nucleus tractus solitarii (NTS) in the sympathoexcitation elicited by central CB1R activation in conscious rats. The latter findings inferred a causal role for a down-regulated PI3K/Akt signaling in the RVLM and NTS in the central CB1R-evoked pressor response. This conclusion is supported by the exacerbation of WIN55,212-2 evoked pressor response following PI3K/Akt inhibition (wortmannin). By contrast, ERK1/2 phosphorylation was enhanced in the same neuronal pools and the pharmacological and molecular studies suggest that this effect, which is mediated, at least partly, via the reduction in PI3K/Akt signaling, contributes to the central CB1R-evoked pressor response. Finally, the findings demonstrated that direct CB1R activation in the RVLM enhanced the activity of two distinct neuronal pools (catecholaminergic and nitroxidergic), which are essential for the central regulation of cardiovascular function. These latter neuronal responses may be linked to the modulation of brainstem GABAergic neurotransmission and subsequently to the central CB1R-evoked sympathoexcitatory and pressor response. The findings yield new insight into a functional crosstalk between CB1R and OX1R signaling in the RVLM, a neuronal pool that is heavily implicated in blood pressure control and in hypertension.