Neuromodulation via Conditional Release of Endocannabinoids in the Spinal Locomotor Network

AbstractEndocannabinoids act as retrograde signals to modulate synaptic transmission. Little is known, however, about their significance in integrated network activity underlying motor behavior. We have examined the physiological effects of endocannabinoids in a neuronal network underlying locomotor behavior using the isolated lamprey spinal cord. Our results show that endocannabinoids are released during locomotor activity and participate in setting the baseline burst rate. They are released in response to mGluR1 activation and act as retrograde messengers. This conditional release of endocannabinoids can transform motoneurons and crossing interneurons into modulatory neurons by enabling them to regulate their inhibitory synaptic inputs and thus contribute to the modulation of the locomotor burst frequency. These results provide evidence that endocannabinoid retrograde signaling occurs within the locomotor network and contributes to motor pattern generation and regulation in the spinal cord

Short-Term Memory of Motor Network Performance via Activity-Dependent Potentiation of Na<sup style="border: 0px; font-size: 0.75em; font-weight: 100; margin: 0px; padding: 0px; vertical-align: super; line-height: 0;">+/K<sup style="border: 0px; font-size: 0.75em; font-weight: 100; margin: 0px; padding: 0px; vertical-align: super; line-height: 0;">+</sup> Pump Function</sup>

SummaryBrain networks memorize previous performance to adjust their output in light of past experience. These activity-dependent modifications generally result from changes in synaptic strengths or ionic conductances, and ion pumps have only rarely been demonstrated to play a dynamic role [1–4]. Locomotor behavior is produced by central pattern generator (CPG) networks and modified by sensory and descending signals to allow for changes in movement frequency, intensity, and duration [5–7], but whether or how the CPG networks recall recent activity is largely unknown. In Xenopus frog tadpoles, swim bout duration correlates linearly with interswim interval, suggesting that the locomotor network retains a short-term memory of previous output. We discovered an ultraslow, minute-long afterhyperpolarization (usAHP) in network neurons following locomotor episodes. The usAHP is mediated by an activity- and sodium spike-dependent enhancement of electrogenic Na+/K+ pump function. By integrating spike frequency over time and linking the membrane potential of spinal neurons to network performance, the usAHP plays a dynamic role in short-term motor memory. Because Na+/K+ pumps are ubiquitously expressed in neurons of all animals and because sodium spikes inevitably accompany network activity, the usAHP may represent a phylogenetically conserved but largely overlooked mechanism for short-term memory of neural network function

Neurons Controlling Aplysia Feeding Inhibit Themselves by Continuous NO Production

Neural activity can be affected by nitric oxide (NO) produced by spiking neurons. Can neural activity also be affected by NO produced in neurons in the absence of spiking?Applying an NO scavenger to quiescent Aplysia buccal ganglia initiated fictive feeding, indicating that NO production at rest inhibits feeding. The inhibition is in part via effects on neurons B31/B32, neurons initiating food consumption. Applying NO scavengers or nitric oxide synthase (NOS) blockers to B31/B32 neurons cultured in isolation caused inactive neurons to depolarize and fire, indicating that B31/B32 produce NO tonically without action potentials, and tonic NO production contributes to the B31/B32 resting potentials. Guanylyl cyclase blockers also caused depolarization and firing, indicating that the cGMP second messenger cascade, presumably activated by the tonic presence of NO, contributes to the B31/B32 resting potential. Blocking NO while voltage-clamping revealed an inward leak current, indicating that NO prevents this current from depolarizing the neuron. Blocking nitrergic transmission had no effect on a number of other cultured, isolated neurons. However, treatment with NO blockers did excite cerebral ganglion neuron C-PR, a command-like neuron initiating food-finding behavior, both in situ, and when the neuron was cultured in isolation, indicating that this neuron also inhibits itself by producing NO at rest.Self-inhibitory, tonic NO production is a novel mechanism for the modulation of neural activity. Localization of this mechanism to critical neurons in different ganglia controlling different aspects of a behavior provides a mechanism by which a humeral signal affecting background NO production, such as the NO precursor L-arginine, could control multiple aspects of the behavior

Neuromodulation via endocannabinoids and nitric oxide in the lamprey spinal cord

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

Parallel pathways from whisker and visual sensory cortices to distinct frontal regions of mouse neocortex

The spatial organization of mouse frontal cortex is poorly understood. Here, we used voltage-sensitive dye to image electrical activity in the dorsal cortex of awake head-restrained mice. Whisker-deflection evoked the earliest sensory response in a localized region of primary somatosensory cortex and visual stimulation evoked the earliest responses in a localized region of primary visual cortex. Over the next milliseconds, the initial sensory response spread within the respective primary sensory cortex and into the surrounding higher order sensory cortices. In addition, secondary hotspots in the frontal cortex were evoked by whisker and visual stimulation, with the frontal hotspot for whisker deflection being more anterior and lateral compared to the frontal hotspot evoked by visual stimulation. Investigating axonal projections, we found that the somatosensory whisker cortex and the visual cortex directly innervated frontal cortex, with visual cortex axons innervating a region medial and posterior to the innervation from somatosensory cortex, consistent with the location of sensory responses in frontal cortex. In turn, the axonal outputs of these two frontal cortical areas innervate distinct regions of striatum, superior colliculus, and brainstem. Sensory input, therefore, appears to map onto modality-specific regions of frontal cortex, perhaps participating in distinct sensorimotor transformations, and directing distinct motor outputs. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License

Membrane potential correlates of sensory perception in mouse barrel cortex

Neocortical activity can evoke sensory percepts, but the cellular mechanisms remain poorly understood. We trained mice to detect single brief whisker stimuli and report perceived stimuli by licking to obtain a reward. Pharmacological inactivation and optogenetic stimulation demonstrated a causal role for the primary somatosensory barrel cortex. Whole-cell recordings from barrel cortex neurons revealed membrane potential correlates of sensory perception. Sensory responses depended strongly on prestimulus cortical state, but both slow-wave and desynchronized cortical states were compatible with task performance. Whisker deflection evoked an early (<50 ms) reliable sensory response that was encoded through cell-specific reversal potentials. A secondary late (50-400 ms) depolarization was enhanced on hit trials compared to misses. Optogenetic inactivation revealed a causal role for late excitation. Our data reveal dynamic processing in the sensory cortex during task performance, with an early sensory response reliably encoding the stimulus and later secondary activity contributing to driving the subjective percept

Voltage-sensitive dye imaging of mouse neocortex during a whisker detection task

Sensorimotor processing occurs in a highly distributed manner in the mammalian neocortex. The spatiotemporal dynamics of electrical activity in the dorsal mouse neocortex can be imaged using voltage-sensitive dyes (VSDs) with near-millisecond temporal resolution and similar to 100-mu m spatial resolution. Here, we trained mice to lick a water reward spout after a 1-ms deflection of the C2 whisker, and we imaged cortical dynamics during task execution with VSD RH1691. Responses to whisker deflection were highly dynamic and spatially highly distributed, exhibiting high variability from trial to trial in amplitude and spatiotemporal dynamics. We differentiated trials based on licking and whisking behavior. Hit trials, in which the mouse licked after the whisker stimulus, were accompanied by overall greater depolarization compared to miss trials, with the strongest hit versus miss differences being found in frontal cortex. Prestimulus whisking decreased behavioral performance by increasing the fraction of miss trials, and these miss trials had attenuated cortical sensorimotor responses. Our data suggest that the spatiotemporal dynamics of depolarization in mouse sensorimotor cortex evoked by a single brief whisker deflection are subject to important behavioral modulation during the execution of a simple, learned, goal-directed sensorimotor transformation. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License

Serotonergic modulation of locomotion in zebrafish: endogenous release and synaptic mechanisms

Serotonin (5-HT) plays an important role in shaping the activity of the spinal networks underlying locomotion in many vertebrate preparations. At larval stages in zebrafish, 5-HT does not change the frequency of spontaneous swimming; and it only decreases the quiescent period between consecutive swimming episodes. However, it is not known whether 5-HT exerts similar actions on the locomotor network at later developmental stages. For this, the effect of 5-HT on the fictive locomotor pattern of juvenile and adult zebrafish was analyzed. Bath-application of 5-HT (1-20 mum) reduced the frequency of the NMDA-induced locomotor rhythm. Blocking removal from the synaptic cleft with the reuptake inhibitor citalopram had similar effects, suggesting that endogenous serotonin is modulating the locomotor pattern. One target for this modulation was the mid-cycle inhibition during locomotion because the IPSPs recorded in spinal neurons during the hyperpolarized phase were increased both in amplitude and occurrence by 5-HT. Similar results were obtained for IPSCs recorded in spinal neurons clamped at the reversal potential of excitatory currents (0 mV). 5-HT also slows down the rising phase of the excitatory drive recorded in spinal cord neurons when glycinergic inhibition is blocked. These results suggest that the decrease in the locomotor burst frequency induced by 5-HT is mediated by a potentiation of mid-cycle inhibition combined with a delayed onset of the subsequent depolarization