H-reflex down-conditioning greatly increases the number of identifiable GABAergic interneurons in rat ventral horn

Abstract

H-reflex down-conditioning increases GABAergic terminals on spinal cord motoneurons. To explore the origins of these terminals, we studied the numbers and distributions of spinal cord GABAergic interneurons. The number of identifiable GABAergic interneurons in the ventral horn was 78% greater in rats in which down-conditioning was successful than in naive rats or rats in which down-conditioning failed. No increase occurred in other spinal lamina or on the contralateral side. This finding supports the hypothesis that the corticospinal tract influence that induces the motoneuron plasticity underlying down-conditioning reaches the motoneuron through GABAergic interneurons in the ventral horn.

Cortical stimulation causes long-term changes in H-reflexes and spinal motoneuron GABA receptors

“…All procedures complied with the Guide for the Care and Use of Laboratory Animals (National Academies Press, Washington, DC, 2011) and had been approved by the Institutional Animal Care and Use Committee of the Wadsworth Center. The procedures for electrode implantation, H-reflex monitoring, SMC stimulation, motoneuron labeling, and immunohistochemical processing have been described previously (Chen et al 2007a;Pillai et al 2008;Wang et al 2006aWang et al , 2009 and are summarized here. GABA receptor labeling methods are described in detail.…”

“…The standard avidin-biotin complex (ABC)-peroxidase system (ABC Elite; Vector Laboratories, Burlingame, CA) was used to assess glutamic acid decarboxylase 67 immunoreactivity (GAD 67 -IR) in spinal interneurons and in synaptic terminals on soleus motoneurons (Pillai et al 2008;Wang et al 2006aWang et al , 2009 and GABA A (i.e., GABA A ␤ 2 ␤ 3 -subunit) and GABA B (i.e., GABA B1 subunit) receptor immunoreactivities (GABA A -IR and GABA B -IR, respectively) in soleus motoneurons. Of every four serial sections from each of the Experimental or Control group rats, one was processed for GAD 67 -IR, one for GABA A -IR, one for GABA B -IR, and one for other studies [e.g., KCC2-IR (see below)].…”

“…The protocol for quantification of GAD 67 -labeled interneurons is detailed by Wang et al (2009). Briefly, in the ventral horns of GAD 67 -labeled sections, GAD 67 -positive neurons that had a clearly defined somatic border, a nucleus and/or at least one dendritic process, an average soma diameter (average of the long and short somatic axes) under 25 m, and an average somatic area under 400 m 2 were identified as GABAergic interneurons and assigned a laminar location (Barber et al 1982;Molander et al 1984).…”

“…While much remains to be learned about this hierarchy and the sites and mechanisms of plasticity, it is clear that SMC activity modifies the H-reflex by producing changes in the motoneuron (e.g., in firing threshold), as well as elsewhere in the spinal cord. Recent studies showing that operantly conditioned H-reflex decreases are accompanied by marked changes in GABAergic interneurons in the ventral horn and in GABAergic terminals on spinal motoneurons suggest that changes in GABAergic function may play a key role in producing the motoneuron plasticity directly underlying H-reflex conditioning (Wang et al 2006a(Wang et al , 2009.Another recent study revealed that weak electrical stimulation of SMC, like the operant conditioning protocol, can change (i.e., increase) H-reflex size (Chen et al 2007a). Motivated by this finding and by the data suggesting that change in GABAergic function contributes to H-reflex conditioning, the present study asked three questions.…”

“…While much remains to be learned about this hierarchy and the sites and mechanisms of plasticity, it is clear that SMC activity modifies the H-reflex by producing changes in the motoneuron (e.g., in firing threshold), as well as elsewhere in the spinal cord. Recent studies showing that operantly conditioned H-reflex decreases are accompanied by marked changes in GABAergic interneurons in the ventral horn and in GABAergic terminals on spinal motoneurons suggest that changes in GABAergic function may play a key role in producing the motoneuron plasticity directly underlying H-reflex conditioning (Wang et al 2006a(Wang et al , 2009.…”