An enkephalinergic mechanism involved in amygdaloid suppression of affective defence behavior elicited from the midbrain periaqueductal gray in the cat

A series of recent studies in our laboratory have provided evidence that opioid peptides powerfully suppress feline affective defense behavior at the level of the midbrain periaqueductal gray (PAG). In the present study, we tested the hypothesis that the central (CE) nucleus of the amygdala constitutes a significant inhibitory input to the PAG which utilizes enkephalins as its neurotransmitter or neuromodulator. Cannula-electrodes were implanted into the PAG for the elicitation of affective defense behavior as well as for infusion of opioid antagonists. Monopolar stimulating electrodes were also implanted into the central, lateral and medial amygdaloid nuclei from which suppression or facilitation of affective defense behavior could be obtained. Initially, 4 trials of concurrent, subseizure stimulation of the CE or lateral amygdala at very low (100 μA, 60 Hz) currents and PAG resulted in an immediate suppression of this response which displayed a time dependent decline after 30 min. In the next stage of the experiment, naloxone (2.7, 18.9 and 27.5 nM) was microinjected through the cannula-electrode into the PAG affective defense site and the experimental procedures noted above were repeated. Naloxone treatment (at 27.5 and 18.9 nM) blocked the suppressive effects of CE and lateral amygdaloid stimulation in a dose and time dependent manner. Further analysis revealed that this effect is likely mediated via the μ receptor since the suppressive effects of amygdaloid stimulation were blocked by the selective μ antagonist, β-Funaltrexamine (0.05 and 0.2 nM) but not by the selective σ-antagonist, ICI 174,864 (0.7 nM). Moreover, facilitation of affective defense behavior following dual stimulation of the medial amygdala and PAG was not affected by naloxone administration (27.5 nM). The final stage of the study was designed to further test the hypothesis that neurons in the CE contain cell bodies that are both enkephalinergic and project to the dorsal aspect of the PAG associated with the expression of affective defense behavior. This aspect involved the combination of retrograde tracing of amygdaloid neurons into the PAG following microinjections of the retrograde tracer, Fluoro-Gold, and the immunocytochemical analysis of amygdaloid cells that immunoreact positively for Met-enkephalin. The results indicated the presence of many retrogradely labelled and immunocytochemically positive cells within the CE and adjoining regions of the lateral nucleus as well as other