Respiratory drive in hindlimb motoneurones of the anaesthetized female cat

Abstract

Anatomical studies have shown a monosynaptic projection from nucleus retroambiguus (NRA) to semimembranosus (Sm) motor nucleus in female cats, which is stronger in oestrus. Expiratory bulbospinal neurones are the best documented functional cell type in the NRA. If these cells participate in this projection, an expiratory drive would be expected in Sm motoneurones and this drive would be expected to be stronger in oestrus. In anaesthetized, paralyzed, ovariohysterectomized female cats, artificially ventilated to produce a strong respiratory drive (as monitored by phrenic nerve discharges), intracellular recordings were made from Sm motoneurones and from motoneurones in the surrounding hindlimb motor nuclei that are outside the focus of the NRA projection. The animals comprised two groups: either treated for 7 days with oestradiol benzoate (oestrous) or untreated (non-oestrous). Central respiratory drive potentials (CRDPs) were observed in most motoneurones of both groups, with amplitudes larger for the oestrous than for the non-oestrous group (1.58 ± 1.34 mV versus 0.89 ± 0.79 mV, mean ± S.D.). However, the CRDPs most often consisted of a maximum depolarization in early expiration, which declined in late expiration and into inspiration. This pattern is different from the incrementing firing pattern of most expiratory bulbospinal neurones. The CRDPs in Sm and semitendinosus motoneurones (located in the same motor column) were of similar size and frequency to CRDPs in motoneurones outside that column. The hypothesis that expiratory bulbospinal neurones are significantly involved in the projection was rejected. Alternative sources and possible functional roles for the CRDPs are discussed.