Extrasynaptic α₅GABA_A receptors on proprioceptive afferents produce a tonic depolarization that modulates sodium channel function in the rat spinal cord

Abstract

Activation of GABAA receptors on sensory axons produces a primary afferent depolarization (PAD) that modulates sensory transmission in the spinal cord. While axoaxonic synaptic contacts of GABAergic interneurons onto afferent terminals have been extensively studied, less is known about the function of extrasynaptic GABA receptors on afferents. Thus, we examined extrasynaptic α5GABAA receptors on low-threshold proprioceptive (group Ia) and cutaneous afferents. Afferents were impaled with intracellular electrodes and filled with neurobiotin in the sacrocaudal spinal cord of rats. Confocal microscopy was used to reconstruct the afferents and locate immunolabelled α5GABAA receptors. In all afferents α5GABAA receptors were found throughout the extensive central axon arbors. They were most densely located at branch points near sodium channel nodes, including in the dorsal horn. Unexpectedly, proprioceptive afferent terminals on motoneurons had a relative lack of α5GABAA receptors. When recording intracellularly from these afferents, blocking α5GABAA receptors (with L655708, gabazine, or bicuculline) hyperpolarized the afferents, as did blocking neuronal activity with tetrodotoxin, indicating a tonic GABA tone and tonic PAD. This tonic PAD was increased by repeatedly stimulating the dorsal root at low rates and remained elevated for many seconds after the stimulation. It is puzzling that tonic PAD arises from α5GABAA receptors located far from the afferent terminal where they can have relatively little effect on terminal presynaptic inhibition. However, consistent with the nodal location of α5GABAA receptors, we find tonic PAD helps produce sodium spikes that propagate antidromically out the dorsal roots, and we suggest that it may well be involved in assisting spike transmission in general. NEW & NOTEWORTHY GABAergic neurons are well known to form synaptic contacts on proprioceptive afferent terminals innervating motoneurons and to cause presynaptic inhibition. However, the particular GABA receptors involved are unknown. Here, we examined the distribution of extrasynaptic α5GABAA receptors on proprioceptive Ia afferents. Unexpectedly, these receptors were found preferentially near nodal sodium channels throughout the afferent and were largely absent from afferent terminals. These receptors produced a tonic afferent depolarization that modulated sodium spikes, consistent with their location.

Direct evidence for decreased presynaptic inhibition evoked by PBSt group I muscle afferents after chronic SCI and recovery with step‐training in rats

“…In rodents, most studies investigating presynaptic inhibition derive from reduced preparations of adult spinal cords ex vivo (Lucas-Osma et al 2018), neonates (Hochman et al 2010;Garcia-Ramirez et al 2014) in which the state of the spinal cord is different from adulthood, or adult anaesthetized preparations in which the excitatory/inhibitory state of the spinal cord is altered (Wall & Lidierth, 1997). Here, we show a decrease in the activation of lumbar PAD pathways associated with a reduction in presynaptic inhibition of the H-reflex Figure 5.…”

Direct evidence for decreased presynaptic inhibition evoked by PBSt group I muscle afferents after chronic SCI and recovery with step-training in the decerebrated rat

“…In rodents, most studies investigating presynaptic inhibition derive from reduced preparations of adult spinal cords ex vivo (Lucas-Osma et al, 2018), neonates in which the development of the nervous system is not yet fully complete (Hochman et al, 2010;Garcia-Ramirez et al, 2014), or adult anaesthetized preparation (Wall & Lidierth, 1997). Here, we show a decrease in the activation of lumbar PAD pathways associated with a reduction in presynaptic inhibition of the H-reflex in vivo in the adult decerebrated rat after a chronic low thoracic transection.…”

“…More recently, last order GABAergic interneurons specifically involved in presynaptic inhibition have been genetically identified in rodent models (Fink et al, 2014;Koch et al, 2017) and presynaptic inhibitory mechanisms were shown to rely not only on GABAergic signaling, but also on other neurotransmitters, neuromodulators and receptors that also could contribute to our results (Kremer & Lev-Tov, 1998;Hochman et al, 2010;Lucas-Osma et al, 2018). Finally, mechanisms other than PAD are also likely to contribute to presynaptic inhibition of Ia afferents and possibly contribute to the PAD-independent presynaptic inhibition of the H-reflex in our experiments (Price et al, 1984;Stuart & Redman, 1992;Hultborn et al, 1996;Curtis et al, 1997;Garcia-Ramirez et al, 2014).…”

“…This inhibition is described as the phasic PAD, in opposition to the tonic PAD, which regulates afferents transmission through extrasynaptic GABAA receptors (Lucas-Osma et al, 2018). While a change in tonic PAD has the potential to be of particular importance for the propagation of action potentials at branching points, our recording configuration for DRPs (6mm interelectrode distance and 0.1Hz high pass) prevented its estimation in this experiment.…”

“…The DRP represents the compound potential from activated afferents in the recorded rootlet at a given stimulation intensity. As the amplitude of the DRP reflects the amount of PAD evoked by the stimulated primary afferents (Lucas-Osma et al, 2018), we therefore sought to investigate whether chronic SCI affects the recruitment gain of DRPs evoked by tibial and PBSt afferents.…”