Microinjection of Naltrexone into the Central, but not the Basolateral, Amygdala Blocks the Anxiolytic Effects of Diazepam in the Plus Maze

Abstract

The amygdala is involved in behavioral and physiological responses to fear, and the anxiolytic properties of several drugs are localized to this region. Activation of endogenous opioid systems is known to occur in response to stress and a growing body of literature suggests that opioid systems regulate the properties of anxiolytic drugs. These experiments sought to elucidate the role of opioid receptors in the central (CeA) and basolateral (BLA) nuclei of the amygdala in regulating the anxiolytic properties of ethanol and diazepam. Male rats fitted with cannula received bilateral microinjections of the nonselective opioid receptor antagonist naltrexone (NAL) immediately followed by systemic delivery of either ethanol (1 g/kg) or diazepam (2 mg/kg) in the elevated plus maze. Both diazepam and ethanol decreased anxiety-like behavior. Delivery of NAL into the CeA blocked the anxiolytic properties of diazepam. Delivery of NAL into the BLA slightly increased open arm avoidance, but had no effect on the anxiolytic properties of diazepam. Microinjection of NAL into either nucleus failed to block the effects of ethanol. These results were specific to the anxiolytic properties of diazepam, since baseline behaviors were unaffected by microinjection of NAL. Microinjection of lidocaine produced results distinct from NAL and failed to block the anxiolytic actions of diazepam. These studies indicate distinct roles for opioid receptor systems in the CeA and BLA in regulating the anxiolytic properties of diazepam in the elevated plus maze. Further, opioid receptor systems in the CeA and BLA do not regulate the anxiolytic properties of ethanol in this test.

The role of delta opioid receptors in the anxiolytic actions of benzodiazepines

“…The systemic administration of MOR and DOR receptor antagonists failed to modify the behavioral effects of DZ in this test. These results differ from previous findings demonstrating that the anxiolytic effects of DZ in the elevated plus maze test were attenuated by injections of the nonselective opioid receptor antagonist naltrexone into the central, but not the basolateral, amygdala (Kang et al 2000;Burghardt and Wilson 2006). These differences could be due to influences of systemically administered opioid receptor antagonists on DOR and MOR in several different brain areas that might be offsetting the amygdalar effects of these drugs, and/ or the use of receptor selective antagonists.…”

“…Non-selective antagonists (naloxone), however, can block the decreases in locomotor activity in mice after high but not low doses of chlordiazepoxide (Rodgers et al 1985). Further, we consistently observe that various treatments (benzodiazepine administration, opioid antagonists, chronic running) have distinct effects on the anxiety and activity measures in the elevated plus maze, suggesting that separate neural substrates underlie changes in these behavioral endpoints (Kang et al 2000;Burghardt et al 2004;Burghardt and Wilson 2006). The present data suggest that DOR is an integral component of the relationship between amygdalar ENK and diazepam on anxiety-related behaviors, but not activity-related behaviors.…”

“…These effects are reversed by systemic administration of the non-selective opioid receptor antagonist naloxone. Localized injections of the non-selective antagonist naltrexone in the central nucleus of the amygdala, but not the basolateral amygdala, attenuate the anxiolytic influences of diazepam in the elevated plus maze (Kang et al 2000;Burghardt and Wilson 2006). Amygdalar lesions, administration of peptides, and anxiolytic drugs can show divergent results in differing animal tests of anxiety or fear, and it has been suggested that these models assess differing aspects of fear or anxiety responses (Green 1991).…”

The Role of Amygdalar Mu-Opioid Receptors in Anxiety-Related Responses in Two Rat Models

“…The effects of enkephalin overexpression are reversed by systemic administration of the non-selective opioid receptor antagonist naloxone (Kang et al, 2000) and the DOR antagonist naltrindole (Primeaux et al, 2006). We have also shown that localized injections of the non-selective antagonist naltrexone in the CEA, but not the basolateral nucleus, attenuate the anxiolytic influences of diazepam in the elevated plus maze (Burghardt and Wilson, 2006).…”

“…The amygdala plays a key role in mood behaviors and emotional memory processes (Charney et al, 1998;Davis et al, 1994;Davis, 1992;Panksepp, 1990) in addition to being a key structure in mediating the anxiolytic effects of drugs such as benzodiazepines (BZs) Treit, 1995, 1994;Burghardt and Wilson, 2006;Petersen et al, 1985;Scheel-Kruger and Petersen, 1982;Senders and Shekhar, 1995;Menard and Treit, 1999;Kang et al, 2000). The endogenous opioid system also helps to mediate many responses associated with stress or anxiety (Panksepp, 1990;Drolet et al, 2001), and opioid processes in amygdala appear to play some role in the control of anxiety and the anxiolytic effects of BZs, although the opioid peptides and opioid receptor subtypes mediating such effects are unclear.…”