Cessation of activity in red nucleus neurons during stimulation of the medial medulla in decerebrate rats

Neuro-orchestration of sleep and wakefulness

Although considered an inactive state for centuries, sleep entails many active processes occurring at the cellular, circuit and organismal levels. Over the last decade, several key technological advances, including calcium imaging and optogenetic and chemogenetic manipulations, have facilitated a detailed understanding of the functions of different neuronal populations and circuits in sleep-wake regulation. Here, we present recent progress and summarize our current understanding of the circuitry underlying the initiation, maintenance and coordination of wakefulness, rapid eye movement sleep (REMS) and non-REMS (NREMS). We propose a de-arousal model for sleep initiation, in which the neuromodulatory milieu necessary for sleep initiation is achieved by engaging in repetitive pre-sleep behaviors that gradually reduce vigilance to the external environment and wake-promoting neuromodulatory tone. We also discuss how brain processes related to thermoregulation, hunger and fear intersect with sleep-wake circuits to control arousal. Lastly, we discuss controversies and lingering questions in the sleep field.

Sleep as a window on the sensorimotor foundations of the developing hippocampus

The hippocampal formation plays established roles in learning, memory, and related cognitive functions. Recent findings also suggest that the hippocampus integrates sensory feedback from self-generated movements to modulate ongoing motor responses in a changing environment. Such findings support the view of Bland and Oddie (Behavioural Brain Research, 2001, 127, 119-136) that the hippocampus is a site of sensorimotor integration. In further support of this view, we review neurophysiological evidence in developing rats that hippocampal function is built on a sensorimotor foundation and that this foundation is especially evident early in development. Moreover, at those ages when the hippocampus is first establishing functional connectivity with distant sensory and motor structures, that connectivity is preferentially expressed during periods of active (or REM) sleep. These findings reinforce the notion that sleep, as the predominant state of early infancy, provides a critical context for sensorimotor development, including development of the hippocampus and its associated network.

Cerebellar Modulation of Cortically Evoked Complex Movements in Rats

Intracortical microstimulation (ICMS) delivered to the motor cortex (M1) via long- or short-train duration (long- or short-duration ICMS) can evoke coordinated complex movements or muscle twitches, respectively. The role of subcortical cerebellar input in M1 output, in terms of long- and short-duration ICMS-evoked movement and motor skill performance, was evaluated in rats with bilateral lesion of the deep cerebellar nuclei. After the lesion, distal forelimb movements were seldom observed, and almost 30% of proximal forelimb movements failed to match criteria defining the movement class observed under control conditions. The classifiable movements could be evoked in different cortical regions with respect to control and many kinematic variables were strongly affected. Furthermore, movement endpoints within the rat's workspace shrunk closer to the body, while performance in the reaching/grasping task worsened. Surprisingly, neither the threshold current values for evoking movements nor the overall size of forelimb movement representation changed with respect to controls in either long- or short-duration ICMS. We therefore conclude that cerebellar input via the motor thalamus is crucial for expressing the basic functional features of the motor cortex.

A new view of "dream enactment" in REM sleep behavior disorder

Patients with REM sleep behavior disorder (RBD) exhibit increased muscle tone and exaggerated myoclonic twitching during REM sleep. In addition, violent movements of the limbs, and complex behaviors that can sometimes appear to involve the enactment of dreams, are associated with RBD. These behaviors are widely thought to result from a dysfunction involving atonia-producing neural circuitry in the brainstem, thereby unmasking cortically generated dreams. Here we scrutinize the assumptions that led to this interpretation of RBD. In particular, we challenge the assumption that motor cortex produces twitches during REM sleep, thus calling into question the related assumption that motor cortex is primarily responsible for all of the pathological movements of RBD. Moreover, motor cortex is not even necessary to produce complex behavior; for example, stimulation of some brainstem structures can produce defensive and aggressive behaviors in rats and monkeys that are strikingly similar to those reported in human patients with RBD. Accordingly, we suggest an interpretation of RBD that focuses increased attention on the brainstem as a source of the pathological movements and that considers sensory feedback from moving limbs as an important influence on the content of dream mentation.

Sensorimotor processing in the newborn rat red nucleus during active sleep

Sensory feedback from sleep-related myoclonic twitches is thought to drive activity-dependent development in spinal cord and brain. However, little is known about the neural pathways involved in the generation of twitches early in development. The red nucleus (RN), source of the rubrospinal tract, has been implicated in the production of phasic motor activity during active sleep in adults. Here we hypothesized that the RN is also a major source of motor output for twitching in early infancy, a period when twitching is an especially abundant motor behavior. We recorded extracellular neural activity in the RN during sleep and wakefulness in 1-week-old unanesthetized rats. Neurons in the RN fired phasically before twitching and wake movements of the contralateral forelimb. A subpopulation of neurons in the RN exhibited a significant peak of activity after forelimb movement onset, suggesting reafferent sensory processing. Consistent with this observation, manual stimulation of the forelimb evoked RN responses. Unilateral inactivation of the RN using a mixture comprising GABAA, GABAB, and glycine receptor agonists caused an immediate and temporary increase in motor activity followed by a marked and prolonged decrease in twitching and wake movements. Altogether, these data support a causal role for the RN in infant motor behavior. Furthermore, they indicate that twitching, which is characterized by discrete motor output and reafferent input, provides an opportunity for sensorimotor integration and activity-dependent development of topography within the newborn RN.

Duration of inhibition of ventral tegmental area dopamine neurons encodes a level of conditioned fear

It is widely accepted that midbrain dopamine (DA) neurons encode actual and expected reward values by phasic alterations in firing rate. However, how DA neurons encode negative events in the environment is still unclear because some DA neurons appear to be depressed and others excited by aversive stimuli. Here, we show that exposing fear-conditioned rats to stimuli predicting electrical shock elicited three types of biphasic responses, each of which contained an inhibitory pause, in neurochemically identified ventral tegmental area (VTA) DA neurons. The duration of the inhibitory pause in these responses of VTA DA neurons was in direct proportion to the increase in respiratory rate reflecting the level of conditioned fear. Our results suggest that the duration of inhibition of VTA DA neurons encodes negative emotional values of signals predicting aversive events in the environment.

Induction of long-lasting depolarization in medioventral medulla neurons by cholinergic input from the pedunculopontine nucleus

Stimulation of the pedunculopontine nucleus (PPN) is known to induce changes in arousal and postural/locomotor states by activation of such descending targets as the caudal pons and the medioventral medulla (MED). Previously, PPN stimulation was reported to induce prolonged responses (PRs) in intracellularly recorded caudal pontine neurons in vitro. The present study used intracellular recordings in semihorizontal slices from rat brain stem (postnatal days 12-21) to determine responses in MED neurons following PPN stimulation. One-half (40/81) of MED neurons showed PRs after PPN stimulation. MED neurons with PRs had shorter duration action potential, longer duration afterhyperpolarization, and higher amplitude afterhyperpolarization than non-PR MED neurons. PR MED neurons were significantly larger (568 +/- 44 microm2) than non-PR MED neurons (387 +/- 32 microm2). The longest mean duration PRs and maximal firing rates during PRs were induced by PPN stimulation at 60 Hz compared with 10, 30, or 90 Hz. The muscarinic cholinergic agonist carbachol induced depolarization in all PR neurons tested, and the muscarinic cholinergic antagonist scopolamine reduced or blocked carbachol- and PPN stimulation-induced PRs in all MED neurons tested. These findings suggest that PPN stimulation-induced PRs may be due to activation of muscarinic receptor-sensitive channels, allowing MED neurons to respond to a transient, frequency-dependent depolarization with long-lasting stable states. PPN stimulation appears to induce PRs in large MED neurons using parameters known best to induce locomotion.

The neural substrates of infant sleep in rats

Sleep is a poorly understood behavior that predominates during infancy but is studied almost exclusively in adults. One perceived impediment to investigations of sleep early in ontogeny is the absence of state-dependent neocortical activity. Nonetheless, in infant rats, sleep is reliably characterized by the presence of tonic (i.e., muscle atonia) and phasic (i.e., myoclonic twitching) components; the neural circuitry underlying these components, however, is unknown. Recently, we described a medullary inhibitory area (MIA) in week-old rats that is necessary but not sufficient for the normal expression of atonia. Here we report that the infant MIA receives projections from areas containing neurons that exhibit state-dependent activity. Specifically, neurons within these areas, including the subcoeruleus (SubLC), pontis oralis (PO), and dorsolateral pontine tegmentum (DLPT), exhibit discharge profiles that suggest causal roles in the modulation of muscle tone and the production of myoclonic twitches. Indeed, lesions in the SubLC and PO decreased the expression of muscle atonia without affecting twitching (resulting in “REM sleep without atonia”), whereas lesions of the DLPT increased the expression of atonia while decreasing the amount of twitching. Thus, the neural substrates of infant sleep are strikingly similar to those of adults, a surprising finding in light of theories that discount the contribution of supraspinal neural elements to sleep before the onset of state-dependent neocortical activity.

Unexpectedly, the anatomy and neurophysiology of brainstem areas associated with sleep in the neonatal rat are strikingly similar to the adult

Active medullary control of atonia in week-old rats

Muscle atonia is a central feature of adult REM sleep which has recently been demonstrated to be a component of sleep in rats as young as 2 days of age (P2). The neural generation of atonia, which depends on mesopontine and medullary structures, is not fully understood in adults and has never been described in infants. In the present experiments we used electrical stimulation in decerebrated pups to identify an inhibitory area within the medial medulla of P7-10 rats. Muscle tone inhibition was consistently found on or near the midline within the ventromedial medulla, dorsal to the inferior olive, in an area that includes the nucleus gigantocellularis, nucleus paramedianus, and raphe obscurus. Chemical infusions in the same region revealed inhibitory responses to quisqualic acid but not to carbachol or corticotropin-releasing factor. Next, extracellular recordings within the medullary inhibitory area revealed neurons with atonia-on profiles; tone-on neurons were also found, typically at more lateral sites. Finally, in non-decerebrated pups, chemical lesions within the inhibitory area resulted in significant reductions in atonia durations, as well as decoupling of atonia from a second component of infant sleep, myoclonic twitching; specifically, twitches occasionally occurred during periods of high muscle tone, a condition reminiscent of "REM without atonia" as described in adults. In summary, we document the existence of an area within the ventromedial medulla of infant rats that (i) causes atonia when stimulated; (ii) contains units that exhibit atonia-related discharge profiles during sleep-wake cycling; and (iii) when lesioned, results in the partial loss of atonia and decoupling of the components of sleep. All together, these findings demonstrate that muscle atonia is actively regulated very early in ontogeny.

Pharmacotherapy for cataplexy

A variety of medications representing several major drug classes improve cataplexy in patients with narcolepsy. These include aminergic reuptake inhibitors such as venlafaxine and clomipramine as well as sodium oxybate. This review is intended to familiarize readers with the safety and efficacy of these medications, thus enabling clinicians to optimize their management of cataplexy.