Glycinergic Inhibition Contributes to the Generation of Rostral Scratch Motor Patterns in the Turtle Spinal Cord

Abstract

Cutaneous stimulation within the rostral scratch receptive field in a low spinal-immobilized turtle elicits a fictive rostral scratch reflex characterized by robust rhythmic motor output from ipsilateral hindlimb muscle nerves and weaker, alternating motor discharge in contralateral nerves. Simultaneous bilateral stimulation elicits bilateral rostral scratch motor patterns in which activity on the right and left sides alternates.We investigated the role of glycinergic inhibition in the generation and coordination of fictive rostral scratch motor patterns. Glycine (2 or 5 mM) and strychnine (5-50 M), a glycine antagonist, were superfused over the anterior spinal hindlimb enlargement while fictive rostral scratch motor output was recorded bilaterally from hindlimb muscle nerves in the form of electroneurograms (ENGs). Although glycine reduced rostral scratch burst frequencies, strychnine tended to increase burst frequency. Strychnine also changed the shape of hip flexor ENG bursts, resulting in more abrupt burst onsets, indicating an earlier recruitment of motor neurons with large ENG spikes. During bilateral stimulation, strychnine increased the variability of interlimb phase values (left vs right hip flexor bursts) but did not abolish right-left alternation.These results indicate that glycinergic neurons in or near the anterior hindlimb enlargement contribute to the overall timing of the rostral scratch rhythm and to the recruitment timing of individual hip flexor motor neurons within each scratch burst. Our data also indicate that glycinergic mechanisms contribute to, but are not critically important for, maintaining an alternating interlimb coordination during bilateral scratch motor patterns.

Reciprocal Interactions in the Turtle Hindlimb Enlargement Contribute to Scratch Rhythmogenesis

“…According to the ''bilateral shared core'' model (Stein et al 1995), HF and HE modules, comprising the core of the scratch rhythm generator, interact via crossed and uncrossed synaptic pathways: HF modules make reciprocal inhibitory connections with contralateral HF and ipsilateral HE modules and mutual excitatory connections with contralateral HE modules. These connections are supported by more recent studies (Currie 1997;Currie and Lee 1997;Stein et al 1998). We disconnected as much HE circuitry as possible from the right and left rostral scratch networks by completely transecting the spinal cord at the caudal end of segment D 9 or D 8 in the anterior enlargement; this created D 3 -D 9 and D 3 -D 8 preparations, respectively.…”

“…In D 3 -end preparations (low spinal animals with a complete cord transection between segments D 2 and D 3 ), unilaterally evoked fictive rostral scratching is characterized by rhythmic alternation between ipsilateral hip flexor (HF) and hip extensor (HE) bursts and monoarticular knee extensor (KE) discharge during the late HF phase (Robertson et al 1985). There is also weaker rhythmic motor output from contralateral hindlimb (Currie and Lee 1997;Stein et al 1995Stein et al , 1998 and preenlargement (Currie and Gonsalves 1997) muscle nerves that alternates with ipsilateral activity. Simultaneous bilateral stimulation in the right and left rostral receptive fields elicits bilateral rostral scratch motor patterns in which homologous nerve activity alternates on the right and left sides (Currie and Gonsalves 1997;Currie and Lee 1997;Stein et al 1995Stein et al , 1998.…”

“…There is also weaker rhythmic motor output from contralateral hindlimb (Currie and Lee 1997;Stein et al 1995Stein et al , 1998 and preenlargement (Currie and Gonsalves 1997) muscle nerves that alternates with ipsilateral activity. Simultaneous bilateral stimulation in the right and left rostral receptive fields elicits bilateral rostral scratch motor patterns in which homologous nerve activity alternates on the right and left sides (Currie and Gonsalves 1997;Currie and Lee 1997;Stein et al 1995Stein et al , 1998. These results establish that there is an alternating phase relationship, probably involving reciprocal inhibition, between mirror-image hip circuitry (e.g., HF) on the right and left sides and between hip antagonist circuitry (HF and HE) on the same side during fictive rostral scratching.…”

Location of Spinal Cord Pathways That Control Hindlimb Movement Amplitude and Interlimb Coordination During Voluntary Swimming in Turtles

“…Extensive evidence indicates that crossed commissural fibers in the turtle hindlimb enlargement can also contribute to right-left coordination during scratch reflex Gonsalves 1997, 1999;Currie and Lee 1997;Currie and Stein 1989;Stein et al 1995Stein et al , 1998, flexion reflex (Currie and Lee 1996), and chemically activated hindlimb motor patterns in isolated turtle spinal cords (Currie 1999). New experiments are needed to gauge the relative contributions of longitudinal (interenlargement) propriospinal and crossed commissural coordinating mechanisms during turtle locomotion and to identify and characterize the spinal interneurons that underlie these functions.…”

Key central pattern generators of the spinal cord

“…NMDA and AMPA locally applied (hindlimb enlargement level in the spinal cord) were reported to activate the CPG for scratching in both in vivo and in vitro turtle preparations (Currie, 1999). It has also been shown that glycinergic mechanisms contribute to, but are not critically important for, maintaining an alternating interlimb coordination during bilateral scratch motor patterns (Currie and Lee, 1997). Endogenous glutamate released in the spinal cord during scratching was reported to contribute to hindlimb motoneuron depolarization via ionotropic and mGlu1 receptor activation and Ca v 1.3 voltage-gated calcium channelmediated conductances (Alaburda and Hounsgaard, 2003), and other not yet fully characterized high conductances (Alaburda et al, 2005).…”