Synaptic Specializations of Melanopsin-Retinal Ganglion Cells in Multiple Brain Regions Revealed by Genetic Label for Light and Electron Microscopy.

The form and synaptic fine structure of melanopsin-expressing retinal ganglion cells, also called intrinsically photosensitive retinal ganglion cells (ipRGCs), were determined using a new membrane-targeted version of a genetic probe for correlated light and electron microscopy (CLEM). ipRGCs project to multiple brain regions, and because the method labels the entire neuron, it was possible to analyze nerve terminals in multiple retinorecipient brain regions, including the suprachiasmatic nucleus (SCN), olivary pretectal nucleus (OPN), and subregions of the lateral geniculate. Although ipRGCs provide the only direct retinal input to the OPN and SCN, ipRGC terminal arbors and boutons were found to be remarkably different in each target region. A network of dendro-dendritic chemical synapses (DDCSs) was also revealed in the SCN, with ipRGC axon terminals preferentially synapsing on the DDCS-linked cells. The methods developed to enable this analysis should propel other CLEM studies of long-distance brain circuits at high resolution.

Genetic inactivation of glutamate neurons in the rat sublaterodorsal tegmental nucleus recapitulates REM sleep behaviour disorder.

SEE SCHENCK AND MAHOWALD DOI101093/AWW329 FOR A SCIENTIFIC COMMENTARY ON THIS ARTICLE: Idiopathic REM sleep behaviour disorder is characterized by the enactment of violent dreams during paradoxical (REM) sleep in the absence of normal muscle atonia. Accumulating clinical and experimental data suggest that REM sleep behaviour disorder might be due to the neurodegeneration of glutamate neurons involved in paradoxical sleep and located within the pontine sublaterodorsal tegmental nucleus. The purpose of the present work was thus to functionally determine first, the role of glutamate sublaterodorsal tegmental nucleus neurons in paradoxical sleep and second, whether their genetic inactivation is sufficient for recapitulating REM sleep behaviour disorder in rats. For this goal, we first injected two retrograde tracers in the intralaminar thalamus and ventral medulla to disentangle neuronal circuits in which sublaterodorsal tegmental nucleus is involved; second we infused bilaterally in sublaterodorsal tegmental nucleus adeno-associated viruses carrying short hairpin RNAs targeting Slc17a6 mRNA [which encodes vesicular glutamate transporter 2 (vGluT2)] to chronically impair glutamate synaptic transmission in sublaterodorsal tegmental nucleus neurons. At the neuroanatomical level, sublaterodorsal tegmental nucleus neurons specifically activated during paradoxical sleep hypersomnia send descending efferents to glycine/GABA neurons within the ventral medulla, but not ascending projections to the intralaminar thalamus. These data suggest a crucial role of sublaterodorsal tegmental nucleus neurons rather in muscle atonia than in paradoxical sleep generation. In line with this hypothesis, 30 days after adeno-associated virus injections into sublaterodorsal tegmental nucleus rats display a decrease of 30% of paradoxical sleep daily quantities, and a significant increase of muscle tone during paradoxical sleep concomitant to a tremendous increase of abnormal motor dream-enacting behaviours. These animals display symptoms and behaviours during paradoxical sleep that closely mimic human REM sleep behaviour disorder. Altogether, our data demonstrate that glutamate sublaterodorsal tegmental nucleus neurons generate muscle atonia during paradoxical sleep likely through descending projections to glycine/GABA premotor neurons in the ventral medulla. Although playing a role in paradoxical sleep regulation, they are, however, not necessary for inducing the state itself. The present work further validates a potent new preclinical REM sleep behaviour disorder model that opens avenues for studying and treating this disabling sleep disorder, and advances potential regions implicated in prodromal stages of synucleinopathies such as Parkinson's disease.

The ventral portion of the anterior pretectal nucleus controls descending mechanisms that initiate neuropathic pain in rats.

Stimulating the dorsal anterior pretectal nucleus (dAPtN) in rats is more effective than stimulating the ventral APtN (vAPtN) at reducing tail-flick latency, whereas stimulation of the vAPtN is more effective at reducing postoperative pain behaviour. This study examines whether a cell lesion caused by injecting N-methyl-D-aspartate into the dAPtN or vAPtN changes the withdrawal threshold of a rat hind paw during different phases of the tactile hypersensitivity induced by a chronic constriction injury (CCI) of the contralateral sciatic nerve. The number of Fos immunoreactive cells in the APtN was also evaluated. The rats whose vAPtN was lesioned 2 days before CCI had more intense tactile hypersensitivity 2 days after CCI than that of the control group, but the groups were not different 7 days after the CCI. The rats whose vAPtN was lesioned 5 days after CCI had withdrawal thresholds that did not differ significantly 7 days after the CCI. The tactile hypersensitivity of the rats whose dAPtN was lesioned 2 days before or 5 days after CCI was not different from that of the control on the second and seventh days after the CCI. The number of Fos immunoreactive cells in the vAPtN and dAPtN increased 2 days after CCI, but did not differ from that in the control 7 days after CCI. We conclude that vAPtN and dAPtN cells are activated by nerve injury; the vAPtN exerts inhibitory control of the initial phase of neuropathic pain whereas the dAPtN does not appear to exert an inhibitory effect in neuropathic processing.

Descending mechanisms activated by the anterior pretectal nucleus initiate but do not maintain neuropathic pain in rats.

BACKGROUND: The anterior pretectal nucleus (APtN) activates descending mechanisms of pain control. This study evaluated whether the APtN also controls neuropathic pain in rats. METHODS: The hypersensitivity to mechanical stimulation with an electronic von Frey apparatus and the number of Fos-immunoreactive (Fos-ir) neurons in the APtN were evaluated in rats before and after chronic constriction injury of the sciatic nerve. RESULTS: The tactile hypersensitivity was characterized by an initial phase (the 2 days following the injury) and a maintenance phase (the subsequent 7 days). The injection of 2% lidocaine (0.25 µL) or N-methyl-D-aspartate (2.5 µg/0.25 µL) into the APtN intensified the tactile hypersensitivity observed 2 days after injury but did not alter the tactile hypersensitivity observed 7 and 14 days after injury. The injection of naloxone (10 ng/0.25 µL) or methysergide (40 pg/0.25 µL) but not atropine (100 ng/0.25 µL) into the APtN also intensified the tactile hypersensitivity observed 2 days after the injury. A significant increase in the number of Fos-ir cells was found in the contralateral APtN 2 days but not 7 or 14 days after the injury. Electrical stimulation of the APtN reduced the tactile hypersensitivity at 2, 7 and 14 days after the nerve ligation. CONCLUSION: APtN exerts a tonic inhibitory influence on persistent pain. The results point out to an important role of opioid and serotonergic mediation into the APtN to inhibit hyperalgesia during the initial phase of neuropathic pain.