Cardiovascular and behavioral responses to conditioned fear after medullary raphe neuronal blockade.

Conditioned fear to context in the rat leads to a host of sympathetically mediated physiological changes, including a marked rise in mean arterial pressure, a delayed rise in heart rate and a marked cutaneous vasoconstriction, along with the behavioral responses of freezing and ultrasonic vocalization. In this study we examine the role of the rostral ventromedial medulla (RVM), which includes raphe nuclei pallidus and magnus, in the expression of these changes. RVM is a major premotor sympathetic and somatic center and an important integrating center in the descending emotional motor system. To evaluate its role, conditioned fear was tested after temporary blockade with microinjections (0.4 microl) of the GABA-A receptor agonist muscimol (0.2 mM) or the glutamate receptor antagonist kynurenic acid (0.1 M). Changes in mean arterial pressure, heart rate and activity were recorded by radio-telemetry. Cutaneous vasoconstriction in the tail was recorded indirectly by infrared thermography. Muscimol and kynurenic acid had different, almost complementary effects. Muscimol abolished the skin vasoconstrictor response and significantly reduced the tachycardic response, but did not reduce the pressor response significantly and had little effect on the somatic motor components, freezing and ultrasonic vocalization. In contrast, kynurenic acid abolished ultrasonic vocalization and significantly reduced freezing but had no effect on the cardiovascular components. The results show that neurons in the rostral ventromedial medulla are implicated in the expression of some of the cardiac, vascular and somatic motor components of conditioned fear. Most importantly, these cardiovascular components are not under local glutamatergic control whereas the somatic motor components are.