Electroacupuncture modulates v1PAG release of GABA through presynaptic cannabinoid CB1 receptors

Fu L-W, Longhurst JC. Electroacupuncture modulates v1PAG release of GABA through presynaptic cannabinoid CB1 receptors. J Appl Physiol 106: 1800-1809, 2009. First published April 9, 2009; doi:10.1152/japplphysiol.91648.2008.-Previous studies have demonstrated that electroacupuncture (EA) attenuates sympathoexcitatory reflex responses by activating a long-loop pathway involving the hypothalamic arcuate nucleus (ARC), midbrain ventrolateral periaqueductal gray (v1PAG), and rostral ventrolateral medulla (rVLM). Neurons in the ARC provide excitatory input to the v1PAG, whereas the v1PAG inhibits neuronal activity in the rVLM. gamma-Aminobutyric acid (GABA) and glutamate (Glu) have been identified in the v1PAG. Endocannabinoids (ECs), acting as atypical neurotransmitters, inhibit the release of both neurotransmitters in the hypothalamus and midbrain through a presynaptic cannabinoid type 1 (CB1) receptor mechanism. The EC system has been observed in the dorsal but not in the v1PAG. Since it is uncertain whether ECs influence GABA and Glu in the v1PAG, the present study tested the hypothesis that EA modulates the release of these neurotransmitters in the v1PAG through a presynaptic CB1 receptor mechanism. We measured the release of GABA and Glu simultaneously by using HPLC to assess samples collected with microdialysis probes inserted unilaterally into the v1PAG of intact anesthetized rats. Twenty-eight min of EA (2 Hz, 2-4 mA, 0.5 ms) at the P5-6 acupoints reduced the release of GABA by 39% during EA and by 44% 15 min after EA. Thirty-five minutes after EA, GABA concentrations returned to pre-EAlevels. In contrast, sham EA did not change the v1PAG GABA concentration. Blockade of CB1 receptors with AM251, a selective CB1 receptor antagonist, reversed the EA-modulated changes in GABA concentration, whereas micro-injection of vehicle into the v1PAG did not alter EA-modulated GABA changes. In addition, we observed no changes in the v1PAG Glu concentrations during EA, although the baseline concentration of Glu was much higher than that of GABA (3,541 +/- 373 vs. 33.8 +/- 8.7 nM, Glu vs. GABA). These results suggest that EA modulates the sympathoexcitatory reflex responses by decreasing the release of GABA, but not Glu, in the v1PAG, most likely through a presynaptic CB1 receptor mechanism.