Neuromodulation via endocannabinoids and nitric oxide in the lamprey spinal cord

Abstract

The overall objective of this thesis is to increase our understanding of neural networks generating locomotion. These networks called Central Pattern Generators (CPG), are localized in the spinal cord and can generate the basic locomotor pattern in the absence of sensory or supraspinal inputs. However, locomotor behaviour needs to be adapted to the changing environmental conditions, and this is proposed to be a result of neuromodulatory systems which can change the efficacy of synaptic transmission, and the intrinsic properties of CPG neurons. The work presented here focuses on the role of metabotropic glutamate receptors (mGluRs) in the spinal locomotor network. We show that a brief activation of postsynaptic mGluR1 results in a long-term potentiation of the locomotor frequency associated with a long-term depression of the mid-cycle inhibition and potentiation of the on-cycle excitation. These effects are blocked by a cannabinoid receptor 1 (CB1) antagonists and nitric oxide synthase (NOS) inhibitors, suggesting that endocannabinoids and nitric oxide (NO) are involved. Overall, endocannabinoids and NO can shift the levels of excitation and inhibition, in favor of excitation to induce the long-term potentiation of the locomotor frequency. Endocannabinoids are released on demand following activation of mGluR1 at the postsynaptic site and inhibit presynaptic glycinergic transmission. This de novo retrograde signaling via endocannabinoids enables network neurons to control their synaptic input and thus the level of their activation. We show that 2-Arachydonylglycerol (2-AG) is the primary endocannabinoid released by activation of mGluR1 and mediates the potentiation of the locomotor frequency and the associated depression of mid-cycle inhibition. In the lamprey spinal cord NOS is found in grey matter neurons and provides an intrinsic NO tone which enhances the locomotor frequency. NO increases the locomotor frequency by reducing mid-cycle inhibition via presynaptic mechanisms, and by increasing the excitatory drive via both pre-and postsynaptic mechanisms. Finally, endogenous activation of mGluR1, cannabinoid and NO signaling facilitates the excitatory drive underlying locomotion and thus contribute to the pattern generation within the spinal cord. The endogenous NO signaling is acting downstream CB1, while approximately 30% of the endocannabinoid tone is dependent on mGluR1 activation. In summary we propose novel modulatory signaling pathways within the spinal CPG and suggest that neuromodulation is a core process embedded within the CPG function that shapes the generation of locomotion