WHOLE-CELL RECORDINGS OF LUMBAR MOTONEURONS DURING LOCOMOTOR LIKE ACTIVITY IN THE IN-VITRO NEONATAL RAT SPINAL-CORD

Whole cell current-and voltage-clamp recordings were obtained from lum bar motoneurons in the isolated neonatal rat spinal cord to characteri ze the behavior of motoneurons during neurochemically induced locomoto r-like activity. Bath application of serotonin (10-100 mu M) in combin ation with N-methyl-D-aspartate (1-12 mu M) initially produced tonic m embrane depolarization (mean = 26 mV), increased input resistance, dec reased rheobase, and increased spike inactivation in response to depol arizing current pulse injections. After the initial tonic depolarizati on, rhythmic fluctuations of the motoneuron membrane potential (locomo tor drive potentials; LDPs) developed that were modulated phasically i n association with ventral root discharge. The peak and trough voltage levels of the I,DP fluctuated above and below the membrane potential recorded immediately before the onset of rhythmic activity. Similarly, firing frequency was modulated above and below prelocomotion firing r ates (in those motoneurons that displayed neurochemically induced toni c firing immediately before the onset of rhythmic activity). These obs ervations are consistent with an alternation between phasic excitatory and inhibitory synaptic drives. The amplitude of LDPs and rhythmic ex citatory drive current increased with membrane depolarization from -80 to -40 mV and then decreased with further depolarization, thus displa ying nonlinear voltage-dependence. Faster frequency, small amplitude v oltage fluctuations were observed superimposed on the depolarized phas e of LDPs. In some motoneurons, the trajectory of these superimposed f luctuations was consistent with a synaptic origin, whereas in other ce lls, the regular sinusoidal appearance of the fluctuations and the occ urrence of superimposed plateau potentials were more compatible with t he activation of an intrinsic membrane property. One motoneuron displa yed exclusively excitatory phasic drive, and another motoneuron was ch aracterized by inhibitory phasic drive alone, during rhythmic activity . These findings are compatible with the concept of a central pattern generator that is capable of delivering both excitatory and inhibitory drive to motoneurons during locomotion. The data also suggest that th e rhythmic excitatory and inhibitory outputs of the hypothetical half- center model can be dissociated and operate in isolation.