Locomotor-Activated Neurons of the Cat. I. Serotonergic Innervation and Co-Localization of 5-HT₇, 5-HT_2A, and 5-HT_1A Receptors in the Thoraco-Lumbar Spinal Cord

Abstract

Monoamines are strong modulators and/or activators of spinal locomotor networks. Thus monoaminergic fibers likely contact neurons involved in generating locomotion. The aim of the present study was to investigate the serotonergic innervation of locomotor-activated neurons within the thoraco-lumbar spinal cord following induction of hindlimb locomotion. This was determined by immunohistochemical co-localization of serotonin (5-HT) fibers or 5-HT(7)/5-HT2A/5-HT1A receptors with cells expressing the activity-dependent marker c-fos. Experiments were performed on paralyzed, decerebrate cats in which locomotion was induced by electrical stimulation of the mesencephalic locomotor region. Abundant c-fos immunoreactive cells were observed in laminae VII and VIII throughout the thoraco-lumbar segments of locomotor animals. Control sections from the same segments showed significantly fewer labeled neurons, mostly within the dorsal horn. Multiple serotonergic boutons were found in close apposition to the majority (80-100%) of locomotor cells, which were most abundant in lumbar segments L3-7. 5-HT7 receptor immunoreactivity was observed on cells across the thoraco-lumbar segments (T7-L7), in a dorsoventral gradient. Most locomotor-activated cells co-localized with 5-HT7, 5-HT2A, and 5-HT1A receptors, with largest numbers in laminae VII and VIII. Co-localization of c-fos and 5-HT7 receptor was highest in the L5-L7 segments (>90%) and decreased rostrally (to approximately 50%) due to the absence of receptors on cells within the intermediolateral nucleus. In contrast, 60-80 and 35-80% of c-fos immunoreactive cells stained positive for 5-HT2A and 5-HT1A receptors, respectively, with no rostrocaudal gradient. These results indicate that serotonergic modulation of locomotion likely involves 5-HT(7)/5-HT2A/5-HT1A receptors located on the soma and proximal dendrites of serotonergic-innervated locomotor-activated neurons within laminae VII and VIII of thoraco-lumbar segments.

Controlling Specific Locomotor Behaviors through Multidimensional Monoaminergic Modulation of Spinal Circuitries

“…Taken together, these results are consistent with recent findings that showed the localization of 5HT1A, 5TH2A, and 5HT7 receptors onto specific and common locomotor-activated neurons with distinct densities along the rostrocaudal extent of the lumbosacral spinal cord (Noga et al, 2009). Thus, these findings reinforce the viewpoint (Hochman et al, 2001; Grillner, 2006; Jordan et al, 2008; Courtine et al, 2009; Hagglund et al, 2010) that descending monoaminergic systems orchestrate the parallel modulation of a widely distributed and heterogeneous, but highly integrated and synergistic, system of circuits that can generate a range of specific gait patterns when recruited in different combinations (Fig.…”

“…8A), but the extent of this modulation was lesser than expected (Liu et al, 2009). This general facilitation of rhythmicity, coordination and flexion is consistent with in vitro electrophysiological studies (Liu and Jordan, 2005) and immunohistochemistry analyses (Noga et al, 2009) that demonstrated the preferential localization of locomotor-related 5HT1A and 5HT7 receptors in upper lumbar segments where it has been suggested that the primary circuitry to drive locomotor outputs is located (Kiehn, 2006), and which essentially contain flexor motoneurons.…”

Peripheral nerve grafts after cervical spinal cord injury in adult cats

“…The immediate early gene cfos is rapidly expressed in neurons in response to stimulation (Morgan and Curran, 1991) and cFos-ir is a marker of transynaptic neuronal activation (Luckman et al, 1994). This approach has been used widely as an indicator of neuronal excitation in the cat and rat spinal cord (Ahn et al, 2006; Ichiyama et al, 2008; Noga et al, 2009). We found cFos-ir neurons in the intermediate gray adjacent to the PNG-spinal cord interface (Figure 7) indicating synaptic activation through connections made with regenerated axons.…”

Preclinical evidence supporting the clinical development of central pattern generator-modulating therapies for chronic spinal cord-injured patients

“…However, dual immunohistochemical experiments recently showed locomotor activity-labeled (c-fos) 5-HT1A-, 5-HT2A- or 5-HT7- positive neurons in the cat lumbar cord (specifically in laminae VII-VIII, Noga et al, 2009) suggesting that some of the locomotor activity-related receptors identified recently in in vivo models may indeed be located on CPG neurons (Hochman et al, 2012). …”