Role of the Raphe Pallidus in Mediation of the Cardiovascular Response to Stress

Brainstem nuclei, like the raphe pallidus (RaPa), are rich in serotonin (5HT) and are thought to mediate protective responses to stress including increases in BP and HR. Dysfunction in 5HT secretion may disrupt these responses and therefore increases an infant's risk for Sudden Infant Death Syndrome (SIDS). It was hypothesized that alterations in neurotransmission in the RaPa will block normal cardiovascular responses to various stressors. Male Sprague‐Dawley rats were instrumented with biotelemetric probes implanted into the descending aorta to measure BP and HR. A microinjection guide cannula was also sterotaxically implanted into the brainstem, targeting the RaPa. Following a 7 day recovery rats were microinjected with either muscimol (GABA‐A agonist) to block neurotransmission, 8‐OH‐DPAT to activate the inhibitory 5HT 1A receptor, or artificial cerebral spinal fluid (ACSF). Then they were immediately exposed to one of three stressors: handling, restraint, or air jet. ACSF elicited the expected increase in HR and BP following stress where as the stress response was completely attenuated following 8‐OH‐DPAT or Muscimol microinjection. These data suggest, cardiovascular responses to stress are mediated by the RaPa area of the brainstem, and likely involve the 5HT 1A receptor. Determining how the nervous system, specifically 5HT secretion, coordinates the body's cardiovascular response to stress will help explain how dysfunction in 5HT neurotransmission could lead to an increased risk for SIDS.