GABA B receptors mediate presynaptic inhibition of proprioceptive afferents and have reduced action after spinal cord injury

Abstract Despite a long history of studying presynaptic inhibition of the Ia afferent synapse that produces the monosynaptic EPSP on motoneurons, recent evidence has upset the conventional idea that GABA A receptors mediate this inhibition and instead suggests that there are mainly GABA B receptors at this synapse. However, without targeted access to the GABAergic neurons that activate these receptors, quantifying their functional contribution to presynaptic inhibition has proven difficult. We demonstrate here that focal optogenetic activation of terminals of a subpopulation of GAD2+ GABAergic neurons that exclusively project ventrally to Ia afferent synapses produce long-lasting presynaptic inhibition that is entirely mediated by GABA B receptors and simultaneously produces a characteristic brief GABA A receptor-mediated IPSP on the motoneurons. These ventral GAD2 neurons are recurrently activated by Ia afferents, contributing to post-activation depression with repeated afferent reflex testing, with a similar long time-course to post-activation depression of the H-reflex induced in humans from either repetitive activation of the same Ia afferents or from antagonist nerve conditioning. In contrast, focal activation of dorsally projecting GAD2 neurons does not directly cause presynaptic inhibition or postsynaptic IPSPs but does produce primary afferent depolarization. Following chronic spinal cord injury (SCI), the expression of GABA B receptors on the Ia terminal is halved, and in mice and humans, is associated with a similar decrease of GABA B receptor-mediated post-activation depression of Ia-EPSPs transmission, which is reversed by the GABA B receptor agonist baclofen. In summary, GABA B receptors mediate presynaptic inhibition, but are down regulated with SCI, contributing to reflex hyperexcitability associated with spasticity. Key Points Summary Presynaptic inhibition of Ia afferents is mediated by the recurrent activation of terminal GABA B receptors by a subpopulation of ventrally projecting GAD2+ interneurons. In contrast, dorsally projecting GAD2+ interneurons activate GABA A receptors on Ia afferent nodes to facilitate action potential conduction through branchpoints. Repetitive activation of Ia afferents at rates of every 10 s or faster produces post-activation depression via neurotransmitter depletion and from activation of terminal GABA B receptors. These ventrally projecting GAD2+ interneurons can also be activated by other afferents that then produce PAD-evoked spikes to produce post-activation depression from conditioning nerve stimulation. The reduction of GABA B receptors on the Ia terminal in spinal cord injury results in reduced presynaptic inhibition and post-activation depression, contributing to reflex hyperexcitability. Abstract legend: Schematic of GABAergic circuit producing presynaptic inhibition and primary afferent depolarization (PAD) in proprioceptive Ia afferents. We propose two populations of GAD2+ GABAergic interneurons, one with dorsal projections (purple) that activate GABA A receptors on the nodes of Ia afferents to produce PAD and subsequent facilitation of Ia afferent conduction, and another ventrally projecting population (pink) that activates GABA B receptors on the Ia afferent terminal to produce presynaptic inhibition via inhibition of VCa 2+ channels and reduction of neurotransmitter release and replenishment. Both are activated by first order interneurons (grey). Repetitive activation of Ia afferents (green extensor) recurrently activates twhe ventrally projecting GAD2+ neurons to activate terminal GABA B receptors and long-lasting post-activation depression of Ia EPSPs and reflexes as measured from ventral root recordings. Strong conditioning stimulation of other afferents (blue flexor) activates dorsal GAD2+ neurons that can produce PAD-evoked spikes in extensor afferents that orthodromically activate motoneurons to set up post-activation depression of subsequent extensor reflexes. Here, PAD is also evoked in other afferents (flexor) by dorsally projecting GAD2+ neurons (light pink branch) but without activation of the ventrally projecting GAD2+ neurons or presynaptic inhibition.