Ventromedial medulla inhibitory neuron inactivation induces REM sleep without atonia and REM sleep behavior disorder.

Despite decades of research, there is a persistent debate regarding the localization of GABA/glycine neurons responsible for hyperpolarizing somatic motoneurons during paradoxical (or REM) sleep (PS), resulting in the loss of muscle tone during this sleep state. Combining complementary neuroanatomical approaches in rats, we first show that these inhibitory neurons are localized within the ventromedial medulla (vmM) rather than within the spinal cord. We then demonstrate their functional role in PS expression through local injections of adeno-associated virus carrying specific short-hairpin RNA in order to chronically impair inhibitory neurotransmission from vmM. After such selective genetic inactivation, rats display PS without atonia associated with abnormal and violent motor activity, concomitant with a small reduction of daily PS quantity. These symptoms closely mimic human REM sleep behavior disorder (RBD), a prodromal parasomnia of synucleinopathies. Our findings demonstrate the crucial role of GABA/glycine inhibitory vmM neurons in muscle atonia during PS and highlight a candidate brain region that can be susceptible to α-synuclein-dependent degeneration in RBD patients.

Antenatal glucocorticoid treatment induces adaptations in adult midbrain dopamine neurons, which underpin sexually dimorphic behavioral resilience.

We demonstrated previously that antenatal glucocorticoid treatment (AGT, gestational days 16-19) altered the size and organization of the adult rat midbrain dopaminergic (DA) populations. Here we investigated the consequences of these AGT-induced cytoarchitectural disturbances on indices of DA function in adult rats. We show that in adulthood, enrichment of striatal DA fiber density paralleled AGT-induced increases in the numbers of midbrain DA neurons, which retained normal basal electrophysiological properties. This was co-incident with changes in (i) striatal D2-type receptor levels (increased, both sexes); (ii) D1-type receptor levels (males decreased; females increased); (iii) DA transporter levels (males increased; females decreased) in striatal regions; and (iv) amphetamine-induced mesolimbic DA release (males increased; females decreased). However, despite these profound, sexually dimorphic changes in markers of DA neurotransmission, in-utero glucocorticoid overexposure had a modest or no effect on a range of conditioned and unconditioned appetitive behaviors known to depend on mesolimbic DA activity. These findings provide empirical evidence for enduring AGT-induced adaptive mechanisms within the midbrain DA circuitry, which preserve some, but not all, functions, thereby casting further light on the vulnerability of these systems to environmental perturbations. Furthermore, they demonstrate these effects are achieved by different, often opponent, adaptive mechanisms in males and females, with translational implications for sex biases commonly found in midbrain DA-associated disorders.

New aspects in the pathophysiology of rapid eye movement sleep behavior disorder: the potential role of glutamate, gamma-aminobutyric acid, and glycine.

Rapid eye movement sleep behavior disorder (RBD) is a parasomnia characterized by the occurrence of intense movements during rapid eye movement (REM) sleep, also named paradoxical sleep. The neuronal dysfunctions at the origin of the loss of atonia in RBD patients are not known. One possibility is that RBD is due to the degeneration of neurons inducing the muscle atonia of REM sleep. Therefore, in our paper we review data on the populations of neurons responsible for the atonia of REM sleep before discussing their potential role in RBD. We first review evidence that motoneurons are tonically hyperpolarized by gamma-aminobutyric acid (GABA) and glycine and phasically excited by glutamate during REM sleep. Then, we review data indicating that the atonia of REM sleep is induced by glycinergic/GABAergic REM-on premotoneurons contained within the raphe magnus and the ventral and alpha gigantocellular reticular nuclei localized in the ventral medullary reticular formation. These neurons are excited during REM sleep by a direct projection from glutamatergic REM-on neurons localized in the pontine sublaterodorsal tegmental nucleus (SLD). From these results, we discuss the possibility that RBD is due to a specific degeneration of descending REM-on glutamatergic neurons localized in the caudal SLD or that of the REM-on GABA/glycinergic premotoneurons localized in the ventral medullary reticular formation. We then propose that movements of RBD are induced by descending projections of cortical motor neurons before discussing possible modes of action of clonazepam and melatonin.

The lateral hypothalamic area controls paradoxical (REM) sleep by means of descending projections to brainstem GABAergic neurons.

It has recently been shown that the ventrolateral part of the periaqueductal gray (VLPAG) and the adjacent dorsal deep mesencephalic nucleus (dDpMe) contain GABAergic neurons gating paradoxical sleep (PS) onset by means of their projection to the glutamatergic PS-on neurons of the sublaterodorsal tegmental nucleus (SLD). To determine the mechanisms responsible for the cessation of activity of these GABAergic PS-off neurons at the onset and during PS, we combined the immunostaining of c-FOS, a marker of neuronal activation, with cholera toxin b subunit (CTb) retrograde tracing from the VLPAG/dDpMe in three groups of rats (control, PS deprived, and PS hypersomniac). We found that the lateral hypothalamic area (LH) is the only brain structure containing a very large number of neurons activated during PS hypersomnia and projecting to the VLPAG/dDpMe. We further demonstrated that 44% of these neurons express the neuropeptide melanin concentrating hormone (MCH). We then showed that bilateral injections in the LH of two inhibitory compounds, clonidine (an α-2 adrenergic agonist) and muscimol (a GABAa agonist) induce an inhibition of PS. Furthermore, after muscimol injections in the LH, the VLPAG/dDpMe contained a large number of activated neurons, mostly GABAergic, and projecting to the SLD. Altogether, our results indicate for the first time that the activation of a population of LH neurons, in part MCH containing, is necessary for PS to occur. Furthermore, our results strongly suggest that these neurons trigger PS by means of their inhibitory projection to the PS-off GABAergic neurons located in the VLPAG/dDpMe.

Functional properties of dopamine neurons and co-expression of vasoactive intestinal polypeptide in the dorsal raphe nucleus and ventro-lateral periaqueductal grey.

The dorsal raphe nucleus (DRN) and ventrolateral periaqueductal grey (vlPAG) regions contain populations of dopamine neurons, often considered to be a dorsal caudal extension of the A10 group [mostly found in the ventral tegmental area (VTA)]. Recent studies suggest they are involved in promoting wakefulness and mediate some of the antinociceptive and rewarding properties of opiates. However, little is known about their electrophysiological properties. To address this, we used Pitx3-GFP and tyrosine hydroxylase (TH)-GFP mice to carry out targeted whole-cell recordings from this population in acute brain slices. We found that DRN/vlPAG dopamine neurons have characteristics similar to most VTA dopamine neurons, but distinct from dorsal raphe serotonin neurons. They fire broad action potentials at a relatively slow, regular rate, exhibit a hyperpolarization-activated inward current and delayed repolarization, and show spike-frequency adaptation in response to prolonged depolarization. In addition, they receive fast excitatory and inhibitory synaptic inputs. Moreover, we found co-expression of vasoactive intestinal polypeptide in small, periaqueductal dopamine neurons, but generally not in larger, more ventral dopamine neurons.

Phasic excitation of dopamine neurons in ventral VTA by noxious stimuli.

Midbrain dopamine neurons play central roles in reward processing. It is widely assumed that all dopamine neurons encode the same information. Some evidence, however, suggests functional differences between subgroups of dopamine neurons, particularly with respect to processing nonrewarding, aversive stimuli. To directly test this possibility, we recorded from and juxtacellularly labeled individual ventral tegmental area (VTA) dopamine neurons in anesthetized rats so that we could link precise anatomical position and neurochemical identity with coding for noxious stimuli. Here, we show that dopamine neurons in the dorsal VTA are inhibited by noxious footshocks, consistent with their role in reward processing. In contrast, we find that dopamine neurons in the ventral VTA are phasically excited by footshocks. This observation can explain a number of previously confusing findings that suggested a role for dopamine in processing both rewarding and aversive events. Taken together, our results indicate that there are 2 functionally and anatomically distinct VTA dopamine systems.

Muscarinic-2 and orexin-2 receptors on GABAergic and other neurons in the rat mesopontine tegmentum and their potential role in sleep-wake state control.

Acetylcholine (ACh) plays an important role in the promotion of paradoxical sleep (PS) with muscle atonia through the muscarinic-2 receptor (M2R) in the mesopontine tegmentum. Conversely, orexin (Orx or hypocretin) appears to be critical for the maintenance of waking with muscle tone through the orexin-2 (or hypocretin-B) receptor (Orx2R), which is lacking in dogs having narcolepsy with cataplexy. In dual-immunostained material viewed under fluorescence microscopy, we examined the presence and distribution of M2R or Orx2R labeling on all neuronal nuclei (NeuN)-stained neurons or on glutamic acid decarboxylase (GAD)-stained neurons through the mesopontine tegmentum. Applying stereological analysis, we determined that many neurons bear M2Rs on their membrane ( approximately 6,300), including relatively large, non-GABAergic cells, which predominate (>75%) in the oral and caudal pontine (PnO and PnC) reticular fields, and small, GABAergic cells ( approximately 2,800), which predominate (>80%) in the mesencephalic (Mes) reticular formation. Many neurons bear Orx2Rs on their membrane ( approximately 6,800), including relatively large, non-GABAergic cells, which predominate (>70%) through all reticular fields, and comparatively few GABAergic cells ( approximately 700). In triple-immunostained material viewed by confocal microscopy, many large neurons in PnO and PnC appear to bear both M2Rs and Orx2Rs on their membrane, indicating that ACh and Orx could exert opposing influences of inhibition vs. excitation on putative reticulo-spinal neurons and thus attenuate vs. facilitate activity and muscle tone. A few GABAergic cells bear both receptors and could as PS inhibitor neurons serve under these different influences to control PS effector neurons and accordingly gate PS and muscle atonia appropriately across sleep-wake states.

Characterization of subpopulations of neurons producing melanin-concentrating hormone in the rat ventral diencephalon.

Neurons producing melanin-concentrating hormone (MCH) are involved in a large array of functions. Some of these functions may be mediated by specific subpopulations. One such subpopulation was characterized by the expression of the neurokinin 3 receptor and the 'cocaine- and amphetamine-regulated transcript' (CART) peptide, while another expresses neither one of these two molecules. MCH+/CART+ axons were traced throughout the brain and showed a strikingly different pattern of distribution than that of MCH+/CART- axons. Particularly, many MCH+/CART+ axons are observed in the telencephalon, while MCH+/CART- projections are mostly directed toward the brainstem. Calbindin, a protein involved in calcium homeostasis, has been largely used in many structures of the brain for the identification of neuronal phenotypes. However, few MCH neurons were labeled for this protein. On the other hand, neurons producing the peptides hypocretins (Hcrt), and codistributed with the MCH neurons, were all labeled for calbindin. Thus, at least two subpopulations of MCH neurons can be distinguished on the basis of neuronal phenotypes and connections. These neurons may be involved in distinct circuitry and in distinct functions.