Bistability of alpha‐motoneurones in the decerebrate cat and in the acute spinal cat after intravenous 5‐hydroxytryptophan.

Abstract

SUMMARY1. In the preceding paper (Crone, Hultborn, Kiehn, Mazieres & Wigstr6m, 1988) it was shown that a short-lasting synaptic excitation ('on' stimulus) of extensor motoneurones (primarily triceps surae) in the decerebrate cat often resulted in a maintained excitability increase, which could be reset by a short-lasting inhibitory stimulus train ('off' stimulus). In the present experiments intracellular recording from triceps surae motoneurones and the electroneurogram (ENG activity) from triceps surae nerve branches were performed in parallel.2. Sustained firing of individual triceps surae motoneurones was most often recorded in parallel with the maintained ENG activity following a synaptic 'on' stimulus. When the motoneurone was silenced, by a hyperpolarizing current through the microelectrode, there was no sign of on-going synaptic excitation during the maintained ENG activity following an 'on' stimulus. It was therefore suggested that voltage-dependent intrinsic properties of the motoneurones themselves could be responsible for the maintained firing.3. In confirmation of this hypothesis it was found that short-lasting depolarizing current pulses through the recording microelectrode could trigger a self-sustained firing in the motoneurone provided that the bias current (i.e. the holding potential) was kept within certain limits. Hyperpolarizing current pulses terminated the firing. When the spike-generating mechanism was inactivated (by long-lasting excessive depolarization) similar depolarizing and hyperpolarizing current pulses could initiate and terminate plateau potentials in the motoneurones. By grading the depolarizing current pulses it was found that the plateau potentials were of all-or-none character, typically around 10 mV in amplitude. The two levels of excitability which can be triggered by short-lasting excitation and inhibition of the motoneurones is referred to as 'bistable' behaviour of the motoneurones. 4. After an acute spinal transection, in the unanaesthetized cat, the bistable behaviour of the motoneurones disappeared. However, it reappears following intravenous injection of the serotonin precursor 5-hydroxytryptophan (50-120 mg/kg).* To whom reprint requests should be sent.

Serotonin-Driven Long-Range Inhibitory Connections in the Cerebellar Cortex

“…6 D). This pattern resembles that observed in spinal motoneurons in the presence of serotonin, which has been linked to a nifedipine-sensitive calcium current (Hounsgaard et al, 1988;Hounsgaard and K iehn, 1989). The actual mechanism generating depolarizing plateau in L ugaro cells may thus involve a modulation of both potassium and calcium currents, or may even be more complicated, as for the calcium-dependent afterdepolarization mediated by I h in thalamic neurons (L üthi and McCormick, 1999).…”

Beginning at the end: Repetitive firing properties in the final common pathway

“…Recognizing that activity in descending serotonergic pathways was decreased in the commonly used anesthetised cat preparation (Engberg et al, 1968), Hultborn and colleagues attempted to evoke plateau potentials in this preparation, but were unable to do so. Following administration of 5-HTP to enhance serotonergic transmission, or clonidine to mimic noradrenergic transmission, however, plateau potentials could indeed be evoked in anesthetised animals (Conway et al, 1988;Crone et al, 1988;Hounsgaard et al, 1988). The conclusion drawn from these experiments was that monoamines are required for expression of plateau potentials.…”

“…The spinal cord could then be removed and motoneuron properties studied in vitro. In the acutely transected spinal cord, PICs are not present in either cat lumbar motoneurons (Hounsgaard et al, 1988) or rat sacral motoneurons (Bennett et al, 2001). Following chronic injury, however, Bennett et al (2001) showed that PICs are readily expressed.…”

“…Cognizant of the then-recent work of Hounsgaard et al (1988), we postulated that plateau potentials were recruited during fictive locomotion in an all-or-none fashion, and that hyperpolarizing current turned them off, thereby preventing repetitive firing. This prompted a series of experiments with Hultborn in which we demonstrated that the excitatory synaptic input to motoneurons during fictive locomotion activated voltagedependent channels.…”

Lamina VII neurons are rhythmically active during locomotor-like activity in the neonatal rat spinal cord

“…Plateau potentials are mediated by nifedipine-sensitive Ct* entry through L-type channels (Hounsgaard and Kiehn, 1989). In turtle and cat motoneurons bistability is not an endogenous property, but is dependent upon the presence of monoamines (Hounsgaard et al, 1988;Conway et al, 1988). Thus the modulatory action of 5-HT, and possibly the other monoamines, on NMDA currents and bistability probably plays a considerable role in the initiation and the regulation of locomotion.…”