Introduction: Spinal cord motoneurons express many serotonin (5-hydroxytryptamine; 5-HT) receptors that are activated when 5-HT is released from the descending tracts of the raphe nuclei. 5HT2C is one of 14 known 5-HT receptors and is shown here to interact with N-methyl- D- aspartate (NMDA) to enhance motoneuronal activity. Spinal cord injury and degenerative neurological disease processes that target the motoneuron, such as Amyotrophic Lateral Sclerosis and Huntington Disease, are major medical treatment problems that involve intrinsic NMDA and 5-HT interactions. Here we provide evidence that SRC tyrosine kinase mediates 5HT2C receptor enhancement of NMDA depolarization of spinal cord motoneurons using in situ electrophysiological recording of frog motoneuron membrane potentials.
Methods: A rana pipien frog (30-50g) was anesthesized by cooling. After the frog was decapitated, a spinal cord laminectomy and spinal cord hemisection was performed, preserving the IXnth dorsal and ventral root. The spinal cord is then transferred to a sucrose gap chamber where it is perfused with Ringer’s solution and pharmacological agents. Motoneuronal depolarization was recorded from the ventral root after stimulating the dorsal root ganglion.
Results: Our work shows that unlike other 5-HT2 receptors (5HT2A and 5HT2B), the 5HT2C interaction with NMDA is independent of G-proteins and Ca2+. Instead, electrophysiological experiments demonstrated that the 5HT2C agonist MK 212 enhances NMDA motoneuronal depolarization produced by activation of 5HT2C receptors. Furthermore, we were able to show the MK212 enhancement is mediated by a tyrosine kinase pathway, particularly the SRC kinase. This was substantiated with spinal neuronal cultures and co-immunoprecipitation experiments which revealed a protein association between the NMDA subunit GluN2A, 5HT2C receptors, and SRC in both mammalian and amphibian spinal neurons and mammalian synaptosomes.
Conclusions: There is a distinct multiprotein complex in the spinal cord by which 5-HT and NMDA coordinate intracellularly, via a SRC tyrosine kinase pathway, to mediate functional activity.
Patient Care: Better understanding of 5-HT and NMDA receptor signaling may provide further insight into the development of target therapeutic intervention in the pathologies affecting spinal cord motor pathways such as Amyotrophic Lateral Sclerosis and Huntington Disease.
Learning Objectives: By the conclusion of this session, participants will be able to: 1) Describe how descending input from raphe neurons in the brain modulates excitation of motoneurons in the spinal cord., 2) Understand the role of glutaminergic NMDA receptors in motoneuronal excitation., 3) Explain how the activation of serotonin 5-HT2C receptors modulates NMDA-mediated depolarization of motoneurons., 4) Incorporate the concept of how multi-protein complexes of 5-HT2C receptors, NMDA subunits and SRC modulate spinal cord activity.