Turtle Flexion Reflex Motor Patterns Show Windup, Mediated Partly by L-type Calcium Channels

Abstract

Windup is a form of multisecond temporal summation in which identical stimuli, delivered seconds apart, trigger increasingly strong neuronal responses. L-type Ca2+ channels have been shown to play an important role in the production of windup of spinal cord neuronal responses, initially in studies of turtle spinal cord and later in studies of mammalian spinal cord. L-type Ca2+ channels have also been shown to contribute to windup of limb withdrawal reflex (flexion reflex) in rats, but flexion reflex windup has not previously been described in turtles and its cellular mechanisms have not been studied. We studied windup of flexion reflex motor patterns, evoked with weak mechanical and electrical stimulation of the dorsal hindlimb foot skin and assessed via a hip flexor (HF) nerve recording, in spinal cord-transected and immobilized turtles in vivo. We found that an L-type Ca2+ channel antagonist, nifedipine, applied at concentrations of 50 μM or 100 μM to the hindlimb enlargement spinal cord, significantly reduced windup of flexion reflex motor patterns, while lower concentrations of nifedipine had no such effect. Nifedipine similarly reduced the amplitude of an individual flexion reflex motor pattern evoked by a stronger mechanical stimulus, in a dose-dependent manner, suggesting that L-type Ca2+ channels contribute to each flexion reflex as well as to multisecond summation of flexion reflex responses in turtles. We also found that we could elicit flexion reflex windup consistently using a 4-g von Frey filament, which is not usually considered a nociceptive stimulus. Thus, it may be that windup can be evoked by a wide range of tactile stimuli and that L-type calcium channels contribute to multisecond temporal summation of diverse tactile stimuli across vertebrates.

Windup of Nociceptive Flexion Reflex Depends on Synaptic and Intrinsic Properties of Dorsal Horn Neurons in Adult Rats

“…Two neuronal levels can elicit windup of their discharge within the flexion reflex circuit: motoneurons and DHNs [8,9,12,15,23]. We previously showed that, in control conditions, the windup of the flexion reflex was strictly correlated with DHNs windup [11].…”

“…It has been shown that windup of the flexion reflex in juvenile rats depends on a synaptic balance between excitation and inhibition that allows expression of intrinsic properties [11,13,14]. However, the flexion reflex is the output of a neural circuit that comprises both DHNs and motor neurons, and the latter ones also exhibit a windup of their discharge that presents the same molecular sensitivity as windup of DHNs [8,9,15]. Because of the different model used (juvenile versus adult, turtles), the site of recordings (DHNs, motor neurons, and flexion reflex), our understanding of windup remains partial.…”

Windup of Nociceptive Flexion Reflex Depends on Synaptic and Intrinsic Properties of Dorsal Horn Neurons in Adult Rat

“…Two neuronal levels can elicit windup of their discharge within the flexion reflex circuit: motoneurone and DHNs neurons [8,9,12,15,23]. We previously showed that in control conditions, the windup of the flexion reflex was strictly correlated with DHNs windup [11].…”

“…It has been shown that windup of the flexion reflex in juvenile rats depends on a synaptic balance between excitation and inhibition that allow expression of intrinsic properties [11,13,14]. However, the flexion reflex is the output of a neural circuit that comprise both DHNs and motor neurons and the latter ones also exhibit a windup of their discharge that present the same molecular sensitivity as windup of DHNs [8,9,15]. Because of the different model used (juvenile versus adult, turtles), the site of recordings (DHNs, motor neurons, flexion reflex), our understanding of windup remains partial.…”

Neurotransmitters and Motoneuron Contacts of Multifunctional and Behaviorally Specialized Turtle Spinal Cord Interneurons

“…Such information can provide a foundation for constructing hypotheses and for comparison to species in which multifunctional and behaviorally specialized interneurons have been elucidated in more detail, especially hatchling Xenopus tadpoles and larval zebrafish (Berkowitz et al, 2010). The turtle spinal cord has been a focus of research on spinal control of multiple kinds of natural limb movements for Ͼ4 decades (Lennard and Stein, 1977;Valk-Fai and Crowe, 1978;Stein, 2018) and has also been a focus for investigation of diverse cellular-and circuit-level questions of general interest, including the ion channel mechanisms and modulation of plateau potentials, windup, and long-lasting excitability (Hounsgaard and Kiehn, 1989;Currie and Stein, 1990;Hounsgaard and Kjaerulff, 1992;Russo and Hounsgaard, 1994;Delgado-Lezama et al, 1997, 1999Russo et al, 1997;Guertin and Hounsgaard, 1998;Svirskis and Hounsgaard, 1998;Alaburda and Hounsgaard, 2003;Alaburda et al, 2005;Guzulaitis et al, 2013;Reali and Russo, 2013;Johnson et al, 2017), the contribution of rhythmic inhibition to spinal central patterngenerating networks (CPGs; Robertson and Stein, 1988;Berkowitz and Stein, 1994b;Berg et al, 2007;Berkowitz, 2008;Stein, 2010;Petersen et al, 2014;Guzulaitis and Hounsgaard, 2017), the distribution, sparseness, and modularity of spinal cord CPGs (Mortin and Stein, 1989;Stein et al, 1995Stein et al, , 1998Currie and Gonsalves, 1997;Stein, 2008;Guzulaitis et al, 2014;Hao et al, 2014;Radosevic et al, 2019)…”