Intrathecal injection of brilliant blue G, a P2X7 antagonist, attenuates the exercise pressor reflex in rats

Abstract

Purinergic 2X (P2X) receptors on the endings of group III and IV afferents play a role in evoking the exercise pressor reflex. Particular attention has been paid to P2X3 receptors because their blockade in the periphery attenuated this reflex. In contrast, nothing is known about the role played by P2X receptors in the spinal cord in evoking the exercise pressor reflex in rats. P2X7 receptors, in particular, may be especially important in this regard because they are found in abundance on spinal glial cells and may communicate with neurons to effect reflexes controlling cardiovascular function. Consequently, we investigated the role played by spinal P2X7 receptors in evoking the exercise pressor reflex in decerebrated rats. We found that intrathecal injection of the P2X7 antagonist brilliant blue G (BBG) attenuated the exercise pressor reflex (blood pressure index: 294 ± 112 mmHg·s before vs. 7 ± 32 mmHg·s after; P < 0.05). Likewise, intrathecal injection of minocycline, which inhibits microglial cell output, attenuated the reflex. In contrast, intrathecal injection of BBG did not attenuate the pressor response evoked by intracarotid injection of sodium cyanide, a maneuver that stimulated carotid chemoreceptors. Moreover, injections of BBG either into the arterial supply of the contracting hindlimb muscles or into the jugular vein did not attenuate the exercise pressor reflex. Our findings support the hypothesis that P2X7 receptors on microglial cells within the spinal cord play a role in evoking the exercise pressor reflex.

Keywords: autonomic function; glia; purinergic receptors; spinal cord.

Fig. 1.

The exercise pressor reflex is inhibited following intrathecal injection of brilliant blue G (BBG). A: intrathecal injection of artificial cerebrospinal fluid (aCSF) does not influence expression of the pressor response to contraction; however, intrathecal injection of BBG inhibits the pressor response to contraction. B: neither injection of saline nor BBG into the hindlimb arterial circulation influences the pressor response to contraction. AP, aortic pressure.

Fig. 2.

The exercise pressor reflex is attenuated by intrathecal injection of brilliant blue G (BBG). The reflex was measured by the peak [change in peak blood pressure (Δpeak BP)] and the pressor response integrated over the period of the contraction [blood pressure index (BPI)] before and after intrathecal BBG. The measurements were normalized for the peak strength [ΔBP/Δtension (T)] and magnitude [BPI/time-tension index (TTI)] of the contractions. Mean baseline blood pressures are given below the vertical columns in the top left graph (means ± standard deviation). Individual data points (dark circles) within an experiment are connected by dotted lines. Data were compared with a paired t test. Statistical differences are noted with horizontal bars. aCSF, artificial cerebrospinal fluid.

Fig. 3.

The exercise pressor reflex is attenuated by intrathecal injection of minocycline (MinCy). The reflex was measured by the peak [change in peak blood pressure (Δpeak BP)] and the pressor response integrated over the period of the contraction [blood pressure index (BPI)] before and after intrathecal MinCy. The measurements were normalized for the peak strength [ΔBP/Δtension (T)] and magnitude [BPI/time-tension index (TTI)] of the contractions. Mean baseline blood pressures are given below the vertical columns in the top left graph (means ± standard deviation). Individual data points (dark circles) within an experiment are connected by dotted lines. Data were compared with a paired t test. Statistical differences are noted with horizontal bars. aCSF, artificial cerebrospinal fluid; MAP, mean arterial pressure.

Fig. 4.

The cyanide-induced carotid chemoreflex was not influenced by intrathecal injection of brilliant blue G (BBG). The chemoreflex was measured by the peak pressor [change in peak blood pressure (Δpeak BP)] and heart rate (Δpeak HR) before and after intrathecal BBG. Mean baseline blood pressures are given below the vertical columns in the top graph (means ± standard deviation). Individual data points (dark circles) within an experiment are connected by dotted lines. MAP, mean arterial pressure.

Fig. 5.

The exercise pressor reflex was not influenced by brilliant blue G (BBG) injected into the arterial supply of the hindlimb muscles. The reflex was measured by the peak [change in peak blood pressure (Δpeak BP)] and the pressor responses integrated over the period of the contraction [blood pressure index (BPI)] before and after intra-arterial BBG. The measurements were normalized for the peak strength [ΔBP/Δtension (T)] and magnitude [BPI/time-tension index (TTI)] of the contractions. Mean baseline blood pressures are given below the vertical columns in the top left graph (means ± standard deviation). Individual data points (dark circles) within an experiment are connected by dotted lines. Data were compared with a paired t test. MAP, mean arterial pressure.

Fig. 6.

The exercise pressor reflex is not influenced by brilliant blue G (BBG) injected intravenously. The reflex was measured by the peak [change in peak blood pressure (Δpeak BP)] and the pressor response integrated over the period of the contraction [blood pressure index (BPI)] before and after intravenous BBG. The measurements were normalized for the peak strength [ΔBP/Δtension (T)] and magnitude [BPI/time-tension index (TTI)] of the contractions. Mean baseline blood pressures are given below the vertical columns in the top left graph (means ± standard deviation). Individual data points (dark circles) within an experiment are connected by dotted lines. Data were compared with a paired t test. MAP, mean arterial pressure.

Fig. 7.

The exercise pressor reflex was not influenced by intrathecal injection of phrixotoxin-3 (Phrx3). The reflex was measured by the peak [change in peak blood pressure (Δpeak BP)] and the blood pressure response integrated over the period of the contraction [blood pressure index (BPI)] before and after intrathecal Phrx3. The measurements were normalized for the peak strength [ΔBP/Δtension (T)] and magnitude [BPI/time-tension index (TTI)] of the contractions. Mean baseline blood pressures are given below the vertical columns in the top left graph (means ± standard deviation). Individual data points (dark circles) within an experiment are connected by dotted lines. Data were compared with a paired t test. aCSF, artificial cerebrospinal fluid; MAP, mean arterial pressure.