Cardiovascular Effect of Dorsal Periaqueductal Gray During Lipopolysaccharide-induced Hypotension

The central mechanism related to the cardiovascular response to lipopolysaccharide (LPS)-induced hypotension is not entirely known, but it is suggested that numerous brain areas such as dorsal periaqueductal gray (dPAG) are involved in this process. In the current work, the cardiovascular effect of the dPAG during LPS-induced hypotension is investigated.

The study animals (rats) were divided into four groups: control (saline microinjected into dPAG), lidocaine 2%, LPS (intravenously injected), and lidocaine + LPS. Catheterization of the femoral artery and vein was performed to record blood pressure and LPS injection, respectively. Saline and lidocaine were microinjected into the dPAG nucleus then the LPS injection was performed. The changes (Δ) in systolic blood pressure (SBP), mean arterial pressure (MAP), and heart rate (HR) were measured and compared with those of the control and LPS groups.

LPS significantly declined ΔMAP and ΔSBP (P<0.05) but did not change the ΔHR compared to the control. Lidocaine did not significantly affect basic ΔSBP, ΔMAP, and ΔHR compared to the control. Injection of lidocaine before LPS significantly attenuated the reduction of ΔSBP and ΔMAP evoked by LPS (P<0.05).

Our data showed that blockade of the dPAG by lidocaine significantly ameliorates the hypotension induced by LPS. this finding confirms the involvement of the dPAG in cardiovascular regulation during LPS-induced hypotension.

Inactivation of the dPAG by lidocaine significantly ameliorates cardiovascular responses in hypotensive rats.LPS significantly lowers blood pressure and does not affect the heart rate.

The mechanism of hypotension induced by endotoxin is not yet clear. However, it is often attributed to the direct effect of lipopolysaccharide (LPS) as a component of the outer wall of Gram-negative bacteria and other vascular mediators, including tumor necrosis factor (TNF) and nitric oxide (NO). One possibility is that the initial drop in LPS-induced arterial hypertension is mediated by a central mechanism. The ventral region of the transcranial gray matter is involved in lowering blood pressure, and the dorsal region is involved in increasing blood pressure. The dorsolateral region of the transcranial gray matter (dlPAG) also causes tachycardia, vasodilation in muscles, and tachypnea. dlPAG contains cells that produce NO and serotonin (5HT) and 5HT1 and 5HT2 receptors, which may play a role in hypotension due to stimulation of this region. LPS (1 mg/kg or higher IV) typically elicits a biphasic hypotensive response in rats. The first stage of this response begins immediately after LPS injection. The second phase begins about 1 hour after LPS injection. Thus, endotoxic hypertension begins through a central mechanism and further suggests that hypotension may play a critical role in developing fatal hypotension, representing the second stage of septic shock. Although dlPAG is an important site for regulating cardiovascular responses, its role in hypotension induced by LPS has not been investigated. We investigated the role of this nucleus in cardiovascular changes after LPS injection.