SUMMARY1. 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; ) 5-10 ltM) 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 inspiratory-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 0-07-037 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-HT211c receptor antagonist ketanserin and mimicked by the 5-HT2/1l agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCI (DOI). However, the 5-HIT 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.
We present three hypotheses related to the nervous system control of breathing in mammals: 1) that neural mechanisms controlling breathing change with state and that the relationship between mechanisms in different states can be described in terms of either modulation or a basic transformation of properties, or both; 2) that the mechanisms generating respiratory rhythm and pattern are separate; and 3) that conditional pacemaker cell activity is the basis for respiratory rhythm in highly reduced states.
The morphology of neonatal rat phrenic motoneurons was studied following retrograde labeling with horseradish peroxidase, which resulted in Golgi-like fills of phrenic motoneuron somata and dendrites. At birth, these neurons have well-developed dendritic trees with many characteristics described for phrenic motoneurons in the adult rat. The dendrites form tightly fasciculated bundles that emerge from the phrenic nucleus primarily along four axes: ventromedial, ventrolateral, dorsolateral, and rostral/caudal, with smaller and more variable projections directly lateral and ventral. Although sparse, some dendritic appendages were also present, and in a few animals, somata clustering was apparent. The most significant difference between adult and neonatal rat phrenic motoneurons is in the extent to which medially and laterally projecting dendrites extend beyond the borders of the ipsilateral gray matter. In the neonate, unlike the adult, these dendrites project extensively past the gray/white border to the edge of the hemicord. Ventromedial dendrites occasionally cross to the contralateral ventral horn in the ventral white commissure and laterally projecting dendrites could be seen reaching the edge of the cord, turning and traveling rostrally or caudally for up to 100 microns. Phrenic motoneurons are not unique in having long dendrites at birth. A brief comparative study showed that neonatal cervical, thoracic, and lumbar motoneurons also have long dendrites that project to the medial and lateral borders of the hemicord.
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