The amygdala is a medial forebrain structure with an established role in nociceptive modulation, including the expression of stress-induced hypoalgesia (SIH). Projections from the locus coeruleus increase levels of noradrenaline in the amygdala during acute stress. alpha(2)-Noradrenergic receptor agonists have significant clinical utility as analgesic agents. We therefore hypothesized that alpha(2)-noradrenergic activation of the amygdala may result in behaviorally measurable hypoalgesia. Lightly anesthetized rats underwent microinjection of the alpha(2)-noradrenergic agonist clonidine into the amygdala and intermittent measurement of thermal nociception using the tail-flick latency (TFL). Bilateral microinjection of clonidine into the central nucleus of the amygdala (CeA) resulted in a significant, dose-dependent increase in TFL. This effect was blocked by systemic pre-treatment with the alpha(2)-antagonist yohimbine or by local pre-injection of the alpha(2)-antagonist idazoxan but not by local pre-injection of the alpha(1)-antagonist WB-4101. When injected alone, no antagonist resulted in a significant change in TFL compared with baseline. Clonidine injection into the amygdala but outside the CeA, including the basolateral nucleus of the amygdala, did not significantly alter TFL. These results demonstrate that anatomically and pharmacologically specific activation of alpha(2)-receptors in the CeA in lightly anesthetized rats results in behaviorally measurable antinociception.

Stress-induced hypoalgesia (SIH) is an adaptive behavioral phenomenon mediated in part by the amygdala. Acute stress increases amygdalar noradrenaline levels and focal application of α 2 -adrenoceptor agonists in the central nucleus of the amygdala (CeA) is antinociceptive. We hypothesized that α 2 -adrenoceptor antagonist administration into the CeA may block SIH.Bilateral microinjections of drug or saline via chronically implanted CeA cannulae were followed by either a period of restraint stress or rest. The nocifensive paw-withdrawal latency (PWL) to a focused beam of light was measured. PWLs were longer in restrained rats, constituting SIH. Microinjection of the α 2 -adrenoceptor antagonist idazoxan into the CeA prior to restraint blocked SIH. Idazoxan administration in unrestrained rats had no effect. Microinjection of the α 2 -adrenoceptor agonist clonidine in unrestrained rats caused dose dependent hypoalgesia, mimicking the effects of environmental stress. α 2 -Adrenoceptor function in the CeA is necessary for restraintinduced SIH. Keywords microinjection; α 2 -adrenoceptor; idazoxan; clonidine; restraint; nociception; stress-induced hypoalgesia Hypoalgesia resulting from exposure to an acute stressor, or "stress-induced hypoalgesia" (SIH), is a well-described phenomenon in both humans and experimental animals (Hayes et al., 1978, Amit and Galina, 1986, Bodnar, 1986, van der Kolk et al., 1989, Rushen and Ladewig, 1991. SIH can be evoked by a variety of acute stressors including footshock and restraint, as well as more biologically relevant threat stimuli such as exposure to a predator or to odors from stressed conspecifics (Watkins et al., 1982b, Fanselow, 1985, Lester and Fanselow, 1985, Kavaliers, 1988, Rushen and Ladewig, 1991.SIH may represent one component of a broader shift in attention towards salient environmental cues under stressful contingencies (Selden et al., 1990b, Berridge and Waterhouse, 2003, Morilak et al., 2005. This allows the organism to ignore distracting nociceptive stimuli, and Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. (Amit and Galina, 1988). NIH Public AccessOne neurotransmitter implicated in SIH is noradrenaline, and systemic treatment with the α 2 -adrenoceptor antagonists yohimbine or idazoxan blocks the expression of SIH in rodents (Coderre and Rollman, 1984, Oluyomi and Hart, 1990, Tokuyama et al., 1991. However, the site or sites of action of noradrenaline in evoking SIH are unknown.SIH can be blocked by lesions of the central nucleus of the amygdala (CeA) (Helmstetter and Bellgowan, 1993). The CeA is densely populate...

Cerebellar Purkinje cells (PCs) are particularly sensitive to cerebral ischemia, and decreased GABAA receptor function following injury is thought to contribute to PC sensitivity to ischemia-induced excitotoxicity. Here we examined the functional properties of the GABAA receptors that are spared following ischemia in cultured Purkinje cells from rat and in vivo ischemia in mouse. Using subunit-specific positive modulators of GABAA receptors, we observed that oxygen and glucose deprivation (OGD) and cardiac arrest-induced cerebral ischemia cause a decrease in sensitivity to the β2/3-subunit-preferring compound, etomidate. However, sensitivity to propofol, a β-subunit-acting compound that modulates β1–3-subunits, was not affected by OGD. The α/γ-subunit-act-ing compounds, diazepam and zolpidem, were also unaffected by OGD. We performed single-cell reverse transcription–polymerase chain reaction on isolated PCs from acutely dissociated cerebellar tissue and observed that PCs expressed the β1-subunit, contrary to previous reports examining GABAA receptor subunit expression in PCs. GABAA receptor β1-subunit protein was also detected in cultured PCs by western blot and by immunohistochemistry in the adult mouse cerebellum and levels remained unaffected by ischemia. High concentrations of loreclezole (30 µm) inhibited PC GABA-mediated currents, as previously demonstrated with β1-subunit-containing GABAA receptors expressed in heterologous systems. From our data we conclude that PCs express the β1-subunit and that there is a greater contribution of β1-subunit-containing GABAA receptors following OGD.