Effect of the Opioid on Excitatory and Inhibitory Synaptic Inputs to Periaqueductal Gray-Projecting Neurons in the Amygdala

Opioids are potent analgesics, but the sites of their action and cellular mechanisms are not fully understood. The central nucleus of the amygdala (CeA) is important for opioid analgesia through the projection to the periaquaductal gray (PAG). In this study, we examined the effects of opioid receptor stimulation on inhibitory and excitatory synaptic inputs to PAG-projecting CeA neurons retrogradely labeled with a fluorescent tracer injected into the ventrolateral PAG of rats. Whole-cell voltageclamp recordings were performed on labeled CeA neurons in brain slices. The specific opioid receptor agonist, [D-Ala,NMe-Phe,Gly-ol]-enkephalin (DAMGO, 1 M), significantly reduced the frequency of miniature inhibitory postsynaptic currents (mIPSCs) without altering the amplitude and decay constant of mIPSCs in 47.6% (10 of 21) of cells tested. DAMGO also significantly decreased the peak amplitude of evoked IPSCs in 69% (9 of 13) of cells examined. However, DAMGO did not significantly alter the frequency of miniature excitatory postsynaptic currents (EPSCs) and the amplitude of evoked EPSCs in 69% (9 of 13) and 83% (10 of 12) of labeled cells, respectively. The IPSCs were blocked by the GABAA receptor antagonist bicuculline, whereas the EPSCs were largely abolished by the non-N-methyl-D-aspartate antagonist 6-cyano-7nitroquinoxaline-2,3-dione. The immunoreactivity of opioid receptors was colocalized with synaptophysin, a presynaptic marker, in close appositions to labeled CeA neurons. These results suggest that activation of opioid receptors on presynaptic terminals primarily attenuates GABAergic synaptic inputs to PAG-projecting neurons in the CeA. The amygdala plays a central role in the emotional interpretation of sensory information, especially painand fearrelated behavior (Pitkanen et al., 1997; LeDoux, 2000). The endogenous opioids involved in the stress-induced analgesia are probably produced within the amygdala complex, especially the central nucleus (CeA) and stria terminalis (Roberts et al., 1982; da Costa Gomez and Behbehani, 1995; Wiedenmayer et al., 2002). The CeA is an important site for pain perception and analgesia produced by environmental stress and opioids. For example, the CeA receives afferent inputs from the spinal cord dorsal horn and parabrachial nucleus (Bernard et al., 1993; Burstein and Potrebic, 1993). The spinopontoamygdaloid pathway has been shown to specifically transmit nociceptive information (Bernard and Besson, 1990; Bester et al., 2000). Also, this amygdaloid nucleus contains all major opioid receptors, including the opioid receptor (Paden et al., 1987). Furthermore, lesioning the CeA largely eliminates the antinociceptive effect of systemic morphine in both tail-flick and formalin tests in rats (Manning and Mayer, 1995a,b; Manning, 1998). This effect may be produced through the projection from the CeA to periaqueductal gray (PAG) (Rizvi et al., 1991; da Costa Gomez and Behbehani, 1995). Thus, CeA neurons not only respond to noxious stimuli but also play an important role in opioid-