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Previously-reported evidence showed that freezing to a context previously associated with footshock is impaired by lesion of the ventral periaqueductal gray (vPAG). It has also been shown that stepwise increase in the intensity of the electrical stimulation of the dorsal periaqueductal gray (dPAG) produces alertness, then freezing, and finally escape. These aversive responses are mimicked by microinjections of GABA receptor antagonists, such as bicuculline, or blockers of the glutamic acid decarboxylase (GAD), such as semicarbazide, into the dPAG. In this work, we examined whether the expression of these defensive responses could be the result of activation of ventral portion of the periaqueductal gray. Sham- or vPAG electrolytic-lesioned rats were implanted with an electrode in the dPAG for the determination of the thresholds of freezing and escape responses. The vPAG electrolytic lesions were behaviorally verified through a context-conditioned fear paradigm. Results indicated that lesion of the vPAG disrupted conditioned freezing response to contextual cues associated with footshocks but did not change the dPAG electrical stimulation for freezing and escape responses. In a second experiment, lesion of the vPAG also did not change the amount of freezing and escape behavior produced by microinjections of semicarbazide into the dPAG. These findings indicate that freezing and escape defensive responses induced by dPAG stimulation do not depend on the integrity of the vPAG. A discussion on different neural circuitries that might underlie different inhibitory and active defensive behavioral patterns that animals display during threatening situations is presented.

The dose-effect relationships of intraventricularly injected bradykinin, Gly-Arg-Met-Lys-bradykinin (GAML-bradykinin), synthetic substance P and angiotensin II on lever-lifting behavior of rabbits in a variable-interval (VI) 72-second schedule of sweetened water presentation were determined. All peptides used caused dose-dependent decreases in overall rates of VI responding during the experimental session in the following order of potency: angiotensin II greater than bradykinin = substance P greater than GAML-bradykinin. The angiotensin II dose-effect curve was less steep than those of the other peptides. The administration of nearly equimolar doses of the bradykinin potentiating peptides, BPP5a and BPP9a, slightly decreased overall VI response rates and caused a 10- to 20-fold potentiation of the rate-decreasing effect of bradykinin on VI responding. Both angiotensin II and bradykinin caused pauses in responding of dose-dependent duration at the beginning of the experimental session that were followed by normal VI responding. The effect of GAML-bradykinin on VI performance was similar to that of bradykinin and angiotensin II but had a delay of onset of 3 to 6 minutes. In contrast, substance P caused actual decreases in response output and pauses of variable duration interspersed between periods of regular VI responding. At the doses used, both bradykinin-potentiating peptides caused uniform decreases in VI responding throughout the experimental session. Gross behavioral changes caused by the peptides were also observed. After the intraventricular injection of bradykinin or GAML-bradykinin, rabbits showed decreased motility, ptosis, miosis and lowered ears; after angiotensin II, animals remained motionless but with wide open eyes, fully raised ears and no miosis. In turn, substance P caused restlessness and increased locomotion. These results together with reported evidence on other powerful central actions of bradykinin, angiotensin and substance P and on the existence of components of their releasing and destroying enzymatic systems in the brain suggest that linear peptides may play a role in the functioning of the central nervous system.

The intraventricular injection of 1 mug of bradykinin (BK) in rats anesthetized with urethane (1.5 g/kg i.p.) caused an increase in mean arterial blood pressure with little or no change in pulse pressure or heart rate. A similar hypertensive response followed the local administration of 0.5 mug of BK at the pars ventralis of the lateral septal area, whereas local application at other subcortical regions, known to be involved in cardiovascular regulation, caused no effect. Injections of 0.5 or 1 mug of synthetic substance P or 1 mug of 9-desarginine-bradykinin at the pars ventralis of the lateral septal area caused no change in blood pressure. In addition, bilateral electrolytical lesions placed in the lateral septal area either markedly reduced or completely blocked the pressor response to intraventricular BK. These results suggest that the pars ventralis of the lateral septal area is involved in the pressor action of BK in the central nervous system. They also indicate that this brain region responds fairly specifically to BK and that local vascular changes are unlikely to be involved in the mediation of the central action of BK.