Importance of neurokinin-1 receptors in the nucleus tractus solitarii of mice for the integration of cardiac vagal inputs

Unmyelinated vagal afferents from the heart terminate within the nucleus tractus solitarii (NTS) located in the dorsomedial medulla. The neurotransmitter and postsynaptic receptors mediating information from cardiac vagal receptors to the NTS are unknown. This study determined the effects of neurokinin-1 (NK1) receptor blockade on: (i) the reflex response evoked following aortic root injection of either veratridine (1-3 mu g/kg) or bradykinin (80-300 ng/kg) to stimulate cardiac receptors in in vivo anaesthetized mice; and (ii) the evoked synaptic response of cardioreceptive NTS neurons following both intraleft-ventricular injection of veratridine or bradykinin, and electrical stimulation of the ipsilateral vagus nerve in an arterially perfused working heart-brainstem preparation of mouse. Administration of CP-99,994 (0.75-1.5 mg/kg i.v.), a specific NK1 antagonist, attenuated significantly the evoked reflex bradycardia and depressor response following cardiac receptor (n = 6), but not pulmonary chemoreflex stimulation in vivo. From extracellular recordings of cardioreceptive NTS neurons, CP-99,994 reduced reversibly the total number of evoked spikes, peak firing frequency and response duration evoked by intraventricular injections of veratridine (n = 5) or bradykinin (n = 5). The number of evoked action potentials following electrical stimulation of the vagus nerve was also reduced. In five whole cell recordings of NTS neurons, both the evoked depolarization following cardiac receptor stimulation, and the peak amplitude and duration of vagus nerve-evoked EPSPs were reduced by CP-99 994; synaptic inputs from both peripheral chemoreceptors or pulmonary C-fibres were unaffected. These data support a selective involvement of NK1 receptors in the transmission of cardiac vagal afferent inputs to NTS neurons integrating cardiorespiratory information.