Descending pathways mediating cardiovascular response from dorsomedial hypothalamic nucleus

Physiological and anatomic methods were used to determine whether neurons i n the rostral ventrolateral medulla (RVLM), nucleus tractus solitarius (NTS ), or hypothalamic paraventricular nucleus (PVN) mediate the cardiovascular response evoked from the dorsomedial hypothalamic nucleus (DMH), which is believed to play a key role in mediating responses to stress. In urethane-a nesthetized rats, activation of neurons in the DMH by microinjection of bic uculline resulted in a large increase in arterial pressure, heart rate, and renal sympathetic nerve activity. The pressor and sympathoexcitatory respo nses, but not the tachycardic response, were greatly reduced after bilatera l muscimol injections into the RVLM even when baseline arterial pressure wa s maintained at a constant level. These responses were not reduced by musci mol injections into the PVN or NTS. Retrograde tracing experiments identifi ed many neurons in the DMH that projected directly to the RVLM. The results indicate that the vasomotor and cardiac components of the response evoked from the DMH are mediated by pathways that are dependent and independent, r espectively, of neurons in the RVLM.