Midbrain periaqueductal gray projections to the dorsomedial medulla in the rabbit

Role of ventrolateral periaqueductal gray neurons in the behavioral and cardiovascular responses to contextual conditioned fear and poststress recovery

We have previously shown that conditioned fear to context increases Fos expression in the caudal ventrolateral region of the periaqueductal gray in the rat. To understand the reason for this activation and its role in the expression of the contextual fear response, the ventrolateral periaqueductal gray was temporarily blocked with bilateral microinjections (0.4 microl) of the GABA agonist muscimol (0.2 mM) or the glutamate antagonist kynurenic acid (0.1 M). Cardiovascular changes and activity were recorded by radio-telemetry and the microinjections were made immediately before testing the conditioned response in the aversive context. Muscimol and kynurenic acid had the same effects: when compared to saline controls, freezing immobility and ultrasonic vocalizations were reduced and replaced by marked locomotor activity, and the increase in heart rate was enhanced; however, the increase in arterial blood pressure remained the same. Interesting changes were also observed when animals were returned to the safe context of their home box after fear (recovery). Basically, the recovery response was either prevented or delayed: instead of returning to resting immobility, the rats remained agitated in their home box with a moderately elevated activity, heart rate and blood pressure. However, the effect of ventrolateral periaqueductal gray blockade on heart rate, arterial pressure and activity did not appear to be specific to the fear response or its recovery because they were also observed in animals returned to the safe context of their home box immediately after injection. The later response was also a recovery response from the milder stress of handling and the injection procedure.We discuss the results by arguing that the ventrolateral periaqueductal gray is involved in the immobility component of both the fear response and poststress recovery responses. To explain our interpretation we consider the findings in relation to the classic descending defence-arousal system and the hyporeactive-hypotensive immobility pattern that has been attributed to the ventrolateral periaqueductal gray. We propose that there is a dual activation of the defence-arousal system and of the ventrolateral periaqueductal gray during fear, with the ventrolateral periaqueductal gray acting as a brake on the defence-arousal system. The role of this brake is to impose immobility and hold off active defence responses such as fight and flight. The result of this combination of arousal and immobility is a hyperreactive freezing immobility associated with ultrasonic vocalizations, and a pressor response accompanied with a slow rise in heart rate. Basically, the animal is tense and ready for action but temporarily immobilised. The ventrolateral periaqueductal gray also acts to impose immobility during recovery; however, this is without coactivation of the defence-arousal system. The result is a return to resting immobility, associated with a return to baseline blood pressure and heart rate. This is an active process that insures a faster and complete return to rest. We conclude that the ventrolateral periaqueductal gray is an immobility center involved not only in the fear response but also in poststress recovery responses.

Attenuation of cardiovascular reactions of vocalized and non-vocalized defence areas of periaqueductal grey following lesions in dorsomedial or ventrolateral medulla of cats

In the pretentorial periaqueductal grey (PAG), the region producing pressor responses, vocalization and other somatic and visceral signs (VPR) of the defence reaction and another region producing pressor responses (PR) were localized by electrical stimulation in adult cats, anesthetized with intraperitoneal chloralose (40 mg/kg) and urethane (400 mg/kg). The pressor responses included increases of systemic arterial pressure (SAP) and heart rate, increases of blood flow in the common carotid and femoral arteries and a decrease of blood flow in the superior mesenteric artery. The VPR was found in a relatively restricted region of the dorsolateral PAG, while PR was found scattered within the dorsal and ventral portions of the lateral PAG. The increase of SAP and the changes of blood flow in the sampled arteries were slightly greater during VPR than PR stimulation. Mild vocalization with a slight increase of SAP but marked increases of carotid and femoral blood flow (vasodilation) could be induced by microinjection of N-methyl D-aspartate (0.2 M, 200 nl) into the VPR and the blood flow increase, particularly that of the femoral artery, was greatly attenuated by atropine (1 mg/kg, i.v.). In order to ascertain the contribution of the medullary pressor areas to the VPR- and PR-induced responses, extensive lesions were made in the dorsomedial (DM) or vetrolateral medulla (VLM) by microinjections of kainic acid (KA, 0.024 M) in 27 of the 42 cats. The resting SAP and blood flow of the three arteries were reduced by lesioning of the VLM more than that of the DM. Responses of SAP and blood flow from activation of the PR and VPR, particularly the latter, were affected more after DM compared to VLM lesioning. These data suggest that, while the pretentorial PAG constitutes the 'defence area,' vocalization is confined exclusively to its dorsolateral region and that both the VLM and DM contribute to the cardiovascular components of defence reactions. The DM appears to have a greater contribution compared to the VLM.

Effect of lesions of the ventrolateral periaqueductal gray on the Pavlovian conditioned heart rate response in the rabbit

The present experiment sought to evaluate the contribution of the ventrolateral periaqueductal gray to the expression of the conditioned bradycardic heart rate response in the rabbit. This response occurs in the presence of a fear-arousing conditioned stimulus. Lesions of the ventrolateral periaqueductal gray made after training failed to affect retention of the conditioned response. The results suggest that this region of the midbrain plays a nonessential role in the expression of the conditioned bradycardic response.