Endocannabinoid-dependent disinhibition of orexinergic neurons: electrophysiological evidence in leptin-knockout obese mice

Objectives In the ob/ob mouse model of obesity, chronic absence of leptin causes a significant increase of orexin (OX) production by hypothalamic neurons and excessive food intake. The altered OX level is linked to a dramatic increase of the inhibitory innervation of OX producing neurons (OX neurons) and the over expression of the endocannabinoid 2-arachidonoylglycerol (2-AG) by OX neurons of ob/ob mice. Little is known about the function of the excitatory synapses of OX neurons in ob/ob mice, and their modulation by 2-AG. In the present study, we fill this gap and provide the first evidence of the overall level of activation of OX neurons in the ob/ob mice. Methods We performed in vitro whole-cell patch-clamp recordings on OX neurons located in the perifornical area of the lateral hypothalamus in acute brain slices of wt and ob/ob mice. We identified OX neurons on the basis of their electrophysiological membrane properties, with 96% of concordance with immunohistochemisty. Results We found that OX neurons of ob/ob mice are innervated by less efficient and fewer excitatory synapses than wt mice. Consequently, ob/ob OX neurons show more negative resting membrane potential and lower action potential firing frequency than wt. The bath application of the cannabinoid type 1 receptor agonist WIN55,212-2, depresses both the excitatory and the inhibitory synapses in ob/ob animals, but only the excitatory synapses in wt animals. Finally, the physiologic release of 2-AG induces a prevalent depression of inhibition (disinhibition) of OX neurons in ob/ob animals but not in wt. Conclusions In ob/ob mice, chronic absence of leptin induces a 2-AG mediated functional disinhibition of OX neurons. This helps explain the increase of OX production and, consequently, the excessive food intake of ob/ob mice

The timing of activity is a regulatory signal during development of neural connections.

In PNS and CNS remarkable rearrangements occur soon after the connections are laid down in the course of embryonic life. These processes clearly follow the period of developmental cell death and mostly take place during the very beginning of postnatal life. They consist in changes of the peripheral fields of neurons, marked by elimination of many inputs, while others undergo further maturation and strengthening. Along the efforts to uncover the signals that regulate development, it turned out that while the initial construction of the circuits is heavily based on chemical cues, the subsequent rearrangement is markedly influence by activity. Here we describe experiments testing the influence on developmental plasticity of a particular aspect of activity, the timing of nerve impulses in the competing inputs. Two recent investigations are reviewed, indicating strikingly similar developmental features in quite different systems, neuromuscular and visual. A sharp contrast between the effects of synchrony and asynchrony emerges, indicating that Hebb-related activity rules are important not only for learning but also for development

Whisker-related afferents in superior colliculus

Whisker-related afferents in superior colliculus. J Neurophysiol 115: 2265\u20132279, 2016. First published February 10, 2016; doi:10.1152/jn.00028.2016.\u2014Rodents use their whiskers to explore the environment, and the superior colliculus is part of the neural circuits that process this sensorimotor information. Cells in the intermediate layers of the superior colliculus integrate trigeminotectal afferents from trigeminal complex and corticotectal afferents from barrel cortex. Using histological methods in mice, we found that trigeminotectal and corticotectal synapses overlap somewhat as they innervate the lower and upper portions of the intermediate granular layer, respectively. Using electrophysiological recordings and optogenetics in anesthetized mice in vivo, we showed that, similar to rats, whisker deflections produce two successive responses that are driven by trigeminotectal and corticotectal afferents. We then employed in vivo and slice experiments to characterize the response properties of these afferents. In vivo, corticotectal responses triggered by electrical stimulation of the barrel cortex evoke activity in the superior colliculus that increases with stimulus intensity and depresses with increasing frequency. In slices from adult mice, optogenetic activation of channelrhodopsin-expressing trigeminotectal and corticotectal fibers revealed that cells in the intermediate layers receive more efficacious trigeminotectal, than corticotectal, synaptic inputs. Moreover, the efficacy of trigeminotectal inputs depresses more strongly with increasing frequency than that of corticotectal inputs. The intermediate layers of superior colliculus appear to be tuned to process strong but infrequent trigeminal inputs and weak but more persistent cortical inputs, which explains features of sensory responsiveness, such as the robust rapid sensory adaptation of whisker responses in the superior colliculus

Lesson from the neuromuscular junction: role of pattern and timing of nerve activity in synaptic development

[No abstract available

Spike timing plays a key role in synapse elimination at the neuromuscular junction.

Nerve impulse activity produces both developmental and adult plastic changes in neural networks. For development, however, its precise role and the mechanisms involved remain elusive. Using the classic model of synapse competition and elimination at newly formed neuromuscular junctions, we asked whether spike timing is the instructive signal at inputs competing for synaptic space. Using a rat strain whose soleus muscle is innervated by two nerves, we chronically evoked different temporal spike patterns in the two nerves during synapse formation in the adult. We found that asynchronous activity imposed upon the two nerves promotes synapse elimination, provided that their relative spikes are separated by 25 ms or more; remarkably, this elimination occurs even though an equal number of spikes were evoked in the competing axons. On the other hand, when spikes are separated by 20 ms or less, activity is perceived as synchronous, and elimination is prevented. Thus, in development, as in adult plasticity, precise spike timing plays an instructive role in synaptic modification

Adult rat motor neurons do not re-establish electrical coupling during axonal regeneration and muscle reinnervation.

Gap junctions (GJs) between neurons are present in both the newborn and the adult nervous system, and although important roles have been suggested or demonstrated in a number of instances, in many other cases a full understanding of their physiological role is still missing. GJs are expressed in the rodent lumbar cord at birth and mediate both dye and electrical coupling between motor neurons. This expression has been proposed to mediate: (i) fast synchronization of motoneuronal spike activity, in turn linked to the process of refinement of neuromuscular connections, and (ii) slow synchronization of locomotor-like oscillatory activity. Soon after birth this coupling disappears. Since in the adult rat regeneration of motor fibers after peripheral nerve injury leads to a recapitulation of synaptic refinement at the target muscles, we tested whether GJs between motor neurons are transiently re-expressed. We found that in conditions of maximal responsiveness of lumbar motor neurons (such as no depression by anesthetics, decerebrate release of activity of subsets of motor neurons, use of temporal and spatial summation by antidromic and orthodromic stimulations, testing of large ensembles of motor neurons) no firing is observed in ventral root axons in response to antidromic spike invasion of nearby counterparts. We conclude that junctional coupling between motor neurons is not required for the refinement of neuromuscular innervation in the adult

Enhanced thalamocortical synaptic transmission and dysregulation of the excitatory-inhibitory balance at the thalamocortical feed-forward inhibitory microcircuit in a genetic mouse model of migraine

Migraine is a complex brain disorder, characterized by attacks of unilateral headache and global dysfunction in multisensory information processing, whose underlying cellular and circuit mechanisms remain unknown. The finding of enhanced excitatory, but unaltered inhibitory, neurotransmission at intracortical synapses in mouse models of familial hemiplegic migraine (FHM) suggested the hypothesis that dysregulation of the excitatory-inhibitory balance in specific circuits is a key pathogenic mechanism. Here, we investigated the thalamocortical (TC) feed-forward inhibitory microcircuit in FHM1 mice of both sexes carrying a gain-of-function mutation in CaV2.1. We show that TC synaptic transmission in somatosensory cortex is enhanced in FHM1 mice. Due to similar gain-of-function of TC excitation of layer 4 excitatory and fast-spiking inhibitory neurons elicited by single thalamic stimulations, neither the excitatory-inhibitory balance nor the integration time window set by the TC feed-forward inhibitory microcircuit were altered in FHM1 mice. However, during repetitive thalamic stimulation, the typical shift of the excitatory-inhibitory balance towards excitation and the widening of the integration time window were both smaller in FHM1 compared to wild-type mice, revealing a dysregulation of the excitatory-inhibitory balance, whereby the balance is relatively skewed towards inhibition. This is due to an unexpected differential effect of the FHM1 mutation on short-term synaptic plasticity at TC synapses on cortical excitatory and fast-spiking inhibitory neurons. Our findings point to enhanced transmission of sensory, including trigeminovascular nociceptive, signals from thalamic nuclei to cortex and TC excitatory-inhibitory imbalance as mechanisms that may contribute to headache, increased sensory gain, and sensory processing dysfunctions in migraine

NMDA receptors are the basis of persistent network activity in neocortex slices

During behavioral quiescence the neocortex generates spontaneous slow oscillations that consist of Up and Down states. Up states are short epochs of persistent activity but their underlying source is unclear. In neocortex slices of adult mice, we monitored several cellular and network variables during the transition between a traditional buffer, that does not cause Up states, and a lower divalent cation buffer, that leads to the generation of Up states. We found that the resting Vm and input resistance of cortical cells did not change with the development of Up states. The synaptic efficacy of excitatory postsynaptic potentials mediated by non-NMDA receptors was slightly reduced but this is unlikely to facilitate the generation of Up states. On the other hand, we identified two variables that are associated with the generation of Up states; an enhancement of the intrinsic firing excitability of cortical cells and an enhancement of NMDA mediated responses evoked by electrical or optogenetic stimulation. The fact that blocking NMDA receptors abolishes Up states indicates that the enhancement in intrinsic firing excitability alone is insufficient to generate Up states. NMDA receptors have a crucial role in the generation of Up states in neocortex slices

Schematic diagram illustrating the preparation used for the acute electrophysiological experiment.

<p>Dorsal roots (DRs), ventral roots (VRs), dorsal root ganglia (DRG) and the lumbar spinal cord are outlined. After intercollicular decerebration, the ether anesthesia is discontinued and a laminectomy exposes the lumbar cord, DRs L3 through S2 are cut, DRs L4 and L5 are mounted on stimulating electrodes, and finally VRs L4 and L5 are isolated and mounted on diphasic recording electrodes. A tiny fascicle of VR axons is initially cut and diverted on monophasic recording electrodes. After a series of antidromic (sciatic nerve) and orthodromic (DRs) stimulations are performed, while recording from VR axons both diphasically and monophasically, a further tiny bundle of axons is shifted to the monophasic recording electrodes and the stimulation series repeated. This procedure is reproduced about 10 times until most, although not all, VR axons are diverted to the monophasic recording electrodes. The cell bodies in the ventral horn and the axons of the motor neurons invaded antidromically (from spikes elicited in the sciatic nerve) are represented in black, while those <i>possibly</i> receiving an electrically-transmitted depolarization and their axons are represented in gray. Arrows and question marks depict the possible electrical coupling of motor neurons.</p