Tyramine, octopamine, and tryptamine belong to a family of endogenous amines called trace amines (TAs) that have structural and metabolic similarities to the 'classical' neurotransmitters serotonin, dopamine, and nor-epinephrine. While receptors for TAs were recently identified, their physiological relevance remains elusive. This project seeks to reveal that these substances control leg movements by acting on the spinal cord circuits responsible for locomotion. TAs are applied directly to the spinal cord to characterize their modulation of neural circuit function. In addition, electrical recordings from limb muscles are coupled with video-based analyses of limb movements to allow quantitative study of locomotor cycle-based activity patterns, detailing the effects of TAs on motor performance. The advances made will spur investigations that impact our understanding of how the spinal cord is 'engineered' to control locomotion. Overall, this project establishes ground-breaking science and methodology that incorporates cross-disciplinary mentoring of underrepresented engineering students from the Georgia Institute of Technology with training in the biological sciences at Emory, and a broadening of their activities through participation in conferences.
Trace amines (TAs), tryptamine, tyramine, octopamine, and β-phenylethylamine, named for their low endogenous concentrations in mammals, are related to the classical monoamine transmitters, but have been understudied and thought of as false transmitters. They share structural, physiological, pharmacological, and metabolic similarities with the monoamines, including synthesis by the aromatic-L-amino acid decarboxylase (AADC) enzyme. In 2001, a new class of receptors preferentially activated by the TAs, termed trace amine-associated receptors (TAARs), was discovered establishing a mechanism for TA actions independent of classic monoaminergic mechanisms. While the TAs and some of their receptors are present in the mammalian central nervous system (CNS), their physiologic role remains uncertain. We hypothesized that the TAs are found intrinsically in the spinal cord, and that they are able to modulate spinal neural networks. Using immunohistochemistry, numerous spinal neurons were identified that express AADC, the TAs (octopamine, tryptamine, and tyramine), and TAARs (TAAR1 and TAAR4). Similar results were seen for AADC and TAAR1 with in situ hybridization. The most consistent observation was for labeling D cells associated with the central canal and in motoneurons. Overall, these results provided evidence for the presence of an anatomical substrate onto which the TAs could have intrinsic biological actions in the spinal cord. Using exogenous application of the TAs in the isolated spinal cord in vitro, and in vivo in the mid-thoracic chronically spinalized, we showed that the TAs could induce rhythmic locomotor-like activity. TA injection-induced hindlimb motor rhythms observed in chronic spinalized animals, supports TA spinal actions independent of the descending monoaminergic systems. TA applications recruited a variety of rhythmic motor patterns in the isolated spinal cord. This ranged from locomotor activity to complex patterns including, an episodic form of locomotion where there were locomotor bouts with intervening quiescent periods. Recordings were then made electromyographically (EMG) from various hindlimb muscles: (i) to compare the relationship between the TAs and serotonin (5-HT) evoked motor coordination and, (ii) to examine the ability of the TAs to modulate ongoing locomotion. The TAs produced both the continuous and episodic patterns on muscles and also generally increased both frequency and amplitude of ongoing 5-HT locomotor frequency. Overall, the research provided an important step towards establishing the TAs as bona fide endogenous neuromodulators with their own actions. These studies improve our understanding of the TAs by establishing a spinal cord substrate for TA to have intrinsic biological actions on the spinal motor circuitry. We also demonstrated that the TAs appear to be the first irrefutable amine neuromodulatory system intrinsic to the mammalian spinal cord. Since the TAs can modulate sensory and motor systems, control of their release and/or receptor activation may provide new therapeutic strategies for the management of spinal cord dysfunction, including after loss of descending monoaminergic systems as occur after spinal cord injury.
