The effects of serotonin on phrenic motoneurones were studied in an in vitro preparation of the isolated brainstem and spinal cord from neonatal rats. 2. Serotonin (5-HT; > or = 5-10 microM) increased inspiratory-modulated phrenic nerve activity and produced a small amount of tonic activity during expiration. Inspiratory-modulated activity of the fourth cervical ventral root also increased, but was accompanied by robust tonic activity, which often obscured the rhythmic activity. 3. Serotonin, in both normal and tetrodotoxin-containing medium, depolarized phrenic motoneurones and increased cell input resistance. Serotonin also increased inspriatory-modulated firing as well as the response of phrenic motoneurones to injected current. The y-intercept of the relationship between firing frequency and injected current (f-I) was increased, but the slope was not affected. There was no bistable firing behaviour. 4. Under voltage clamp conditions, 5-HT produced a tonic inward current of 0.07-0.37 nA. This current increased with less negative holding potentials and decreased with more negative holding potentials (-75 to -90 mV) but did not reverse. 5. In addition, 5-HT decreased inspiratory-modulated synaptic current by 23 +/- 6%. The degree of attenuation was not affected by holding potential. The time course of the decrease in inspiratory-modulated synaptic current was similar to the changes seen in tonic inward current and input resistance. 6. Depolarization, tonic inward current, and shift in the f-I relationship produced by 5-HT were antagonized by the 5-HT2/1C receptor antagonist ketanserin and mimicked by the 5-HT2/1C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI). However, the 5-HT induced decrease in inspiratory-modulated synaptic current was not reduced by ketanserin nor mimicked by DOI. 7. We conclude that exogenously applied 5-HT simultaneously increases cell excitability and decreases inspiratory-modulated synaptic current in phrenic motoneurones via different types of receptors. When these responses occurred simultaneously, the increase in excitability predominated and the net effect was an augmentation of inspiratory-modulated phrenic motoneurone activity.
