In vitro recordings from area postrema neurons demonstrate responsiveness to adrenomedullin

Adrenomedullin (ADM) is a recently discovered 52-amino acid peptide that exerts potent vasodilatory effects in the periphery and influences the control of body fluid balance when injected centrally. In this study extracellular single-unit recordings were obtained from 94 AP neurons in rat brain slices. Bath application of ADM (10(-7) M) excited 47% (32 of 68) of cells tested, and these effects were found to be dose dependent from 10(-7) to 10(-9) M. Excitation was maintained during synaptic blockade in a low-Ca2+ artificial cerebrospinal fluid solution, demonstrating direct actions of ADM on these neurons. The remaining cells were either unaffected (n = 25) or inhibited (n = 11) by ADM. ADM (10(-7) M) also influenced the spontaneous activity of 9 (7 inhibited, 2 excited) of 16 neurons located in the nucleus tractus solitarii (NTS). However, these effects could be eliminated during synaptic blockade, suggesting indirect actions of the peptide on NTS neurons. These data demonstrate that a specific population of CNS neurons within the AP are directly influenced by ADM and suggest that ADM may exert its effects on the central control of fluid balance through direct actions at this circumventricular organ.