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Articles by J. C. Longhurst
Total Records ( 2 ) for J. C. Longhurst
  S. C Tjen A Looi , P Li and J. C. Longhurst

A long-loop pathway, involving the hypothalamic arcuate nucleus (ARC), ventrolateral periaqueductal gray (vlPAG), and the rostral ventrolateral medulla (rVLM), is essential in electroacupuncture (EA) attenuation of sympathoexcitatory cardiovascular reflex responses. The ARC provides excitatory input to the vlPAG, which, in turn, inhibits neuronal activity in the rVLM. Although previous studies have shown that endocannabinoid CB1 receptor activation modulates -aminobutyric acid (GABA)-ergic and glutamatergic neurotransmission in the dorsolateral PAG in stress-induced analgesia, an important role for endocannabinoids in the vlPAG has not yet been observed. We recently have shown (Fu LW, Longhurst JC. J Appl Physiol; doi:10.1152/japplphysiol.91648.2008) that EA reduces the local vlPAG concentration of GABA, but not glutamate, as measured with high-performance liquid chromatography from extracellular samples collected by microdialysis. We, therefore, hypothesized that, during EA, endocannabinoids, acting through CB1 receptors, presynaptically inhibit GABA release to disinhibit the vlPAG and ultimately modulate excitatory reflex blood pressure responses. Rats were anesthetized, ventilated, and instrumented to measure heart rate and blood pressure. Gastric distention-induced blood pressure responses of 18 ± 5 mmHg were reduced to 6 ± 1 mmHg by 30 min of low-current, low-frequency EA applied bilaterally at pericardial P 5–6 acupoints overlying the median nerves. Like EA, microinjection of the fatty acid amide hydrolase inhibitor URB597 (0.1 nmol, 50 nl) into the vlPAG to increase endocannabinoids locally reduced the gastric distention cardiovascular reflex response from 21 ± 5 to 3 ± 4 mmHg. This inhibition was reversed by pretreatment with the GABAA antagonist gabazine (27 mM, 50 nl), suggesting that endocannabinoids exert their action through a GABAergic receptor mechanism in the vlPAG. The EA-related inhibition from 18 ± 3 to 8 ± 2 mmHg was reversed to 14 ± 2 mmHg by microinjection of the CB1 receptor antagonist AM251 (2 nmol, 50 nl) into the vlPAG. Pretreatment with gabazine eliminated reversal following CB1-receptor blockade. Thus EA releases endocannabinoids and activates presynaptic CB1 receptors to inhibit GABA release in the vlPAG. Reduction of GABA release disinhibits vlPAG cells, which, in turn, modulate the activity of rVLM neurons to attenuate the sympathoexcitatory reflex responses.

  L. W Fu and J. C. Longhurst

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.

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