Pharmacological characterization of the rhythmic synaptic drive onto lumbosacral motoneurons in the chick embryo spinal cord

Abstract

The isolated spinal cord of the chick embryo generates episodes of rhythmic bursting in which sartorius (hip flexor) and femorotibialis (knee extensor) motoneurons exhibit characteristic patterns of activity. At the beginning of each cycle both sets of motoneurons discharge synchronously. Following this brief synchronous activation sartorius motoneurons stop firing at the time of peak femorotibialis activity, producing a period of alternation between the two sets of motoneurons. Intracellular recording from motoneurons has suggested that the pause is mediated by a synaptically induced shunt conductance. However, the pharmacological basis for this shunt and the nature of the excitatory drive to motoneurons is unknown. To address these questions we have investigated the pharmacology of the rhythmic, synaptic drive to lumbosacral motoneurons using local and bath application of several excitatory and inhibitory antagonists, and documenting their effects on motor output in E10-E12 chick embryos. Local application of bicuculline or picrotoxin over sartorius motoneurons abolished the pause in firing recorded from the sartorius muscle nerve. As a consequence, the pattern of sartorius and femorotibialis activity was similar and the motoneurons were coactive. The pause in sartorius firing was shortened following local application of the glycine antagonist strychnine the nicotinic, cholinergic antagonists mecamylamine, and dihydro-beta-erythroidine and several excitatory amino acid antagonists. Application of the GABA uptake inhibitor nipecotic acid depressed the slow potentials and discharge recorded from the sartorius muscle nerve. These findings suggest that the pause is determined primarily by synaptic inputs acting at motoneuron GABAA receptors with contributions from glycinergic, cholinergic, and glutamatergic inputs. The actions of locally applied GABA onto spinal neurons are consistent with these findings because the neurotransmitter depolarizes spinal neurons and reduces their input resistance. Local application of bicuculline, but not strychnine, onto segments containing femorotibialis motoneurons altered the amplitude and duration of femorotibialis discharge and changed the profile of the slow potentials recorded from the muscle nerve. This finding implicates GABAergic inputs in the regulation of femorotibialis discharge. The pause in sartorius firing was still present and a pause in firing appeared in each cycle of femorotibialis discharge following bath application of bicuculline or strychnine. The pause in both sets of motoneurons could be abolished by local application of the NMDA receptor antagonist AP-5 onto the motoneurons, but not by local application of bicuculline. This action of AP-5 was in contrast to its activity in normal Tyrode's solution where it shortened the pause slightly.(ABSTRACT TRUNCATED AT 400 WORDS)

Limb Movements During Embryonic Development in the Chick: Evidence for a Continuum in Limb Motor Control Antecedent to Locomotion

“…Analysis for the E18 embryo shown produced a power coefficient (solid line, R 2 ϭ 0.28) that was similar to the exponential coefficient (dashed line,dination (Bradley 1999). However, leg movements exhibiting a disassociation of knee and ankle excursions at E13 raise the possibility that intralimb coordination breaks down during subsequent development (Sharp et al 1999). Our findings extend these observations to E18 and reveal five critical points for advancing our understanding of embryonic leg movements.…”

Modeling Spontaneous Activity in the Developing Spinal Cord Using Activity-Dependent Variations of Intracellular Chloride

“…Closely related to this point is the fact that we assumed that the effects of the chloride conductance are exclusively excitatory, but the balance of excitatory and inhibitory effects of GABA depends on the synaptic location (Sernagor et al, 1995;Gulledge and Stuart, 2003). In fact, it has been shown previously that the conductance can sometimes be functionally inhibitory, presumably because of shunting (O'Donovan, 1989;Sernagor et al, 1995;Marchetti et al, 2002).…”

Selective Fasciculation and Divergent Pathfinding Decisions of Embryonic Chick Motor Axons Projecting to Fast and Slow Muscle Regions

“…As described previously (Landmesser and O'Donovan, 1984a;O'Donovan, 1989;Ho and O'Donovan, 1993;Sernagor et al, 1995;Sholomenko and O'Donovan, 1995), stimulation of descending input with a single electrical shock to the thoracic cord activates a central pattern generator in the lumbosacral cord, setting off a series of highly stereotyped motoneuron bursts (or cycles) characteristic for each muscle. Figure 9A shows the EMG to one such muscle, the AITIB, after a single stimulus to the cord.…”

“…The different activity patterns recorded from the fast and slow regions of the AITIB and IFIB indicate that these fast and slow motoneurons are differentially activated by the central pattern generator (CPG) (O'Donovan, 1989), presumably via differential innervation by local interneurons (O'Donovan, 1989;Ho and O'Donovan, 1993;Sernagor et al, 1995). Because fast-and slow-projecting motoneurons were mixed within the motor pool, selective connectivity suggests the recognition of specific molecular cues on the motoneuron cell bodies/dendrites by local interneurons.…”

“…Additionally, differences in intrinsic membrane properties (transmitter receptors, channels, etc.) between fast and slow motoneurons may have contributed to these differences in activation (Sernagor et al, 1995).…”