Muscimol dialysis in the retrotrapezoid nucleus region inhibits breathing in the awake rat

Under anesthesia, inactivation of the retrotrapezoid nucleus (RTN) region m arkedly inhibits breathing and chemoreception. In conscious rats, we dialyz ed muscimol for 30 min to inhibit neurons of the RTN region reversibly. Dia lysis of artificial cerebrospinal fluid had no effect. Muscimol (1 or 10 mM ) significantly decreased tidal volume (VT) (by 16-17%) within 15 min. VT r emained decreased for 50 min or more, with recovery by 90 min. Ventilation ((V) over dot(E)) decreased significantly (by 15-20%) within 15 min and the n returned to baseline within 40 min as a result of an increase in frequenc y. This, we suggest, is a compensatory physiological response to the reduce d VT. Oxygen consumption was unchanged. In response to 7% CO2 in the 1 mM g roup, absolute (V) over dot(E) and change in (V) over dot(E) were significa ntly reduced (by 19-22%). In the 10 nM group, the response to dialysis incl uded a time-related increase in frequency and decrease in body temperature, wh:ich may reflect greater spread of muscimol. In the awake rat, the RTN r egion provides a portion of the tonic drive to breathe, as well as a portio n of the response to hypercapnia.