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Electroacupuncture modulates vlPAG release of GABA through presynaptic cannabinoid CB₁ receptors

¹Department of Medicine, ²Department of Physiology and Biophysics, and ³Susan Samueli Center for Integrative Medicine, School of Medicine, University of California, Irvine, California

Submitted 29 December 2008 ; accepted in final form 7 April 2009

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 (vlPAG), and rostral ventrolateral medulla (rVLM). Neurons in the ARC provide excitatory input to the vlPAG, whereas the vlPAG inhibits neuronal activity in the rVLM. -Aminobutyric acid (GABA) and glutamate (Glu) have been identified in the vlPAG. 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 vlPAG. Since it is uncertain whether ECs influence GABA and Glu in the vlPAG, the present study tested the hypothesis that EA modulates the release of these neurotransmitters in the vlPAG 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 vlPAG 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-EA levels. In contrast, sham EA did not change the vlPAG GABA concentration. Blockade of CB₁ receptors with AM251, a selective CB₁ receptor antagonist, reversed the EA-modulated changes in GABA concentration, whereas microinjection of vehicle into the vlPAG did not alter EA-modulated GABA changes. In addition, we observed no changes in the vlPAG 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 vlPAG, most likely through a presynaptic CB₁ receptor mechanism.

somatic afferents; sympathoexcitatory reflex; cannabinoid type 1 receptor; microdialysis

This article has been cited by other articles:

				S. C. Tjen-A-Looi, P. Li, and J. C. Longhurst Processing cardiovascular information in the vlPAG during electroacupuncture in rats: roles of endocannabinoids and GABA J Appl Physiol, June 1, 2009; 106(6): 1793 - 1799. [Abstract] [Full Text] [PDF]