Enkephalinergic mechanisms in midbrain (dPAG) in modulation of hypothalamically-induced predatory attack behaviour

Present study was carried out in nine cats which did not attack the rats spontaneously. Predatory attack on an anaesthetized rat was elicited by electrical stimulation of lateral hypothalamus at a mean current strength of 690 microA. The attack was accompanied by minimal affective display and culminated in neck biting. Microinjections of delta-alanine methionine enkephaline (DAME) in 250 ng dose in dorsal periaqueductal gray completely suppressed the predatory attack. There was a significant increase in the threshold current strength for affective display components while the somatic components were completely inhibited even when the current strength was increased to 1000 microA. Microinjections of naloxone, an opioid antagonist in 1 microgram dose reversed the DAME blocking effect and the thresholds returned to control levels within 10 min of microinjections. Microinjections of naloxone alone in similar dose facilitated the response as indicated by a decrease in threshold current strengths for both affective display and somatomotor components. Control injections of saline in similar volumes (0.5 microliter) failed to produce any change. These findings indicate that hypothalamically induced predatory attack is inhibited by enkephalinergic mechanisms operating at the dPAG level in the midbrain.