Topological Analysis of the Brainstem of the Australian Lungfish Neoceratodus forsteri.

This paper presents a survey of the cell masses in the brainstem of the Australian lungfish Neoceratodus forsteri, based ontransversely cut Bodian-stained serial sections, supplemented by immunohistochemical data from the recent literature. This study is intended to serve a double purpose. First it concludes and completes a series of publications on the structure of the brainstem in representative species of all groups of anamniote vertebrates. Within the framework of this comparative program the cell masses in the brainstem and their positional relations are analyzed in the light of the Herrick-Johnston concept, according to which the brainstem nuclei are arranged in four longitudinal, functional zones or columns, the boundaries of which are marked by ventricular sulci. The procedure employed in this analysis essentially involves two steps: first, the cell masses and large individual cells are projected upon the ventricular surface, and next, the ventricular surface is flattened out, that is, subjected to a one-to-one continuous topological transformation [J Comp Neurol. 1974;156:255-267]. The second purpose of the present paper is to complement our mapping of the longitudinal zonal arrangement of the cell masses in the brainstem of Neoceratoduswith a subdivision in transversely oriented neural segments. Five longitudinal rhombencephalic sulci - the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis and the sulcus medianus superior - and four longitudinal mesencephalic sulci - the sulcus tegmentalis medialis, the sulcus tegmentalis lateralis, the sulcus subtectalis and the sulcus lateralis mesencephali - could be distinguished. Two obliquely oriented grooves, present in the isthmic region - the sulcus isthmi dorsalis and ventralis - deviate from the overall longitudinal pattern of the other sulci. Although in Neoceratodus most neuronal perikarya are situated within a diffuse periventricular gray, 45 cell masses could be delineated. Ten of these are primary efferent or motor nuclei, eight are primary afferent or sensory centers, six are considered to be components of the reticular formation and the remaining 21 may be interpreted as "relay" nuclei. The topological analysis showed that in most of the rhombencephalon the gray matter is arranged in four longitudinal zones or areas, termed area ventralis, area intermedioventralis, area intermediodorsalis and area dorsalis. The sulcus intermedius ventralis, the sulcus limitans, and the sulcus intermedius dorsalis mark the boundaries between these morphological entities. These longitudinal zones coincide largely, but not entirely, with the functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the viscerosensory nucleus of the solitary tract, several general somatosensory and special somatosensory centers. The isthmus region does not exhibit a clear morphological zonal pattern. The mesencephalon is divisible into a ventral, primarily motor zone and a dorsal somatosensory zone. The boundary between these zones is marked by the sulcus tegmentalis lateralis, which may be considered as an isolated rostral extremity of the sulcus limitans. The results of this study are summarized in a "classical" topological map, as well as in a "modernized" version of this map, in which neuromere borders are indicated.

Brainstem nucleus MdV mediates skilled forelimb motor tasks.

Translating the behavioural output of the nervous system into movement involves interaction between brain and spinal cord. The brainstem provides an essential bridge between the two structures, but circuit-level organization and function of this intermediary system remain poorly understood. Here we use intersectional virus tracing and genetic strategies in mice to reveal a selective synaptic connectivity matrix between brainstem substructures and functionally distinct spinal motor neurons that regulate limb movement. The brainstem nucleus medullary reticular formation ventral part (MdV) stands out as specifically targeting subpopulations of forelimb-innervating motor neurons. Its glutamatergic premotor neurons receive synaptic input from key upper motor centres and are recruited during motor tasks. Selective neuronal ablation or silencing experiments reveal that MdV is critically important specifically for skilled motor behaviour, including accelerating rotarod and single-food-pellet reaching tasks. Our results indicate that distinct premotor brainstem nuclei access spinal subcircuits to mediate task-specific aspects of motor programs.

Parallel pathways from motor and somatosensory cortex for controlling whisker movements in mice.

Mice can gather tactile sensory information by actively moving their whiskers to palpate objects in their immediate surroundings. Whisker sensory perception therefore requires integration of sensory and motor information, which occurs prominently in the neocortex. The signalling pathways from the neocortex for controlling whisker movements are currently poorly understood in mice. Here, we delineate two pathways, one originating from primary whisker somatosensory cortex (wS1) and the other from whisker motor cortex (wM1), that control qualitatively distinct movements of contralateral whiskers. Optogenetic stimulation of wS1 drove retraction of contralateral whiskers while stimulation of wM1 drove rhythmic whisker protraction. To map brainstem pathways connecting these cortical areas to whisker motor neurons, we used a combination of anterograde tracing using adenoassociated virus injected into neocortex and retrograde tracing using monosynaptic rabies virus injected into whisker muscles. Our data are consistent with wS1 driving whisker retraction by exciting glutamatergic premotor neurons in the rostral spinal trigeminal interpolaris nucleus, which in turn activate the motor neurons innervating the extrinsic retractor muscle nasolabialis. The rhythmic whisker protraction evoked by wM1 stimulation might be driven by excitation of excitatory and inhibitory premotor neurons in the brainstem reticular formation innervating both intrinsic and extrinsic muscles. Our data therefore begin to unravel the neuronal circuits linking the neocortex to whisker motor neurons.

[Septal Activation and Control of Limbic Structures].

Coherent activation of limbic system structures as the main function of theta-rhythm is widely discussed in the literature. However until now does not exist the common view on its generation in these brain structures. The model of septal theta-rhythmic activation and control of limbic structures is suggested basing on the literature and own experimental data.

Anatomical evidence that the superior colliculus controls saccades through central mesencephalic reticular formation gating of omnipause neuron activity.

Omnipause neurons (OPNs) within the nucleus raphe interpositus (RIP) help gate the transition between fixation and saccadic eye movements by monosynaptically suppressing activity in premotor burst neurons during fixation, and releasing them during saccades. Premotor neuron activity is initiated by excitatory input from the superior colliculus (SC), but how the tectum's saccade-related activity turns off OPNs is not known. Since the central mesencephalic reticular formation (cMRF) is a major SC target, we explored whether this nucleus has the appropriate connections to support tectal gating of OPN activity. In dual-tracer experiments undertaken in macaque monkeys (Macaca fascicularis), cMRF neurons labeled retrogradely from injections into RIP had numerous anterogradely labeled terminals closely associated with them following SC injections. This suggested the presence of an SC-cMRF-RIP pathway. Furthermore, anterograde tracers injected into the cMRF of other macaques labeled axonal terminals in RIP, confirming this cMRF projection. To determine whether the cMRF projections gate OPN activity, postembedding electron microscopic immunochemistry was performed on anterogradely labeled cMRF terminals with antibody to GABA or glycine. Of the terminals analyzed, 51.4% were GABA positive, 35.5% were GABA negative, and most contacted glycinergic cells. In summary, a trans-cMRF pathway connecting the SC to the RIP is present. This pathway contains inhibitory elements that could help gate omnipause activity and allow other tectal drives to induce the bursts of firing in premotor neurons that are necessary for saccades. The non-GABAergic cMRF terminals may derive from fixation units in the cMRF.

The distribution of the cortical origin of the corticoreticular pathway in the human brain: a diffusion tensor imaging study.

We investigated the distribution of the cortical origin of the corticoreticular pathway (CRP) in the human brain. Forty normal subjects were recruited and CRPs from four cortical areas were reconstructed. The first cortical origin area of the CRP was the premotor cortex and the next was the primary motor cortex. Although the CRP fibers also originated from the primary somatosensory cortex and prefrontal cortex, they occupied the smallest portion among four regions of interest.

A novel neural substrate for the transformation of olfactory inputs into motor output.

It is widely recognized that animals respond to odors by generating or modulating specific motor behaviors. These reactions are important for daily activities, reproduction, and survival. In the sea lamprey, mating occurs after ovulated females are attracted to spawning sites by male sex pheromones. The ubiquity and reliability of olfactory-motor behavioral responses in vertebrates suggest tight coupling between the olfactory system and brain areas controlling movements. However, the circuitry and the underlying cellular neural mechanisms remain largely unknown. Using lamprey brain preparations, and electrophysiology, calcium imaging, and tract tracing experiments, we describe the neural substrate responsible for transforming an olfactory input into a locomotor output. We found that olfactory stimulation with naturally occurring odors and pheromones induced large excitatory responses in reticulospinal cells, the command neurons for locomotion. We have also identified the anatomy and physiology of this circuit. The olfactory input was relayed in the medial part of the olfactory bulb, in the posterior tuberculum, in the mesencephalic locomotor region, to finally reach reticulospinal cells in the hindbrain. Activation of this olfactory-motor pathway generated rhythmic ventral root discharges and swimming movements. Our study bridges the gap between behavior and cellular neural mechanisms in vertebrates, identifying a specific subsystem within the CNS, dedicated to producing motor responses to olfactory inputs.

Neural correlates of an injury-free model of central sensitization induced by opioid withdrawal in humans.

Preclinical evidence suggests that opioid withdrawal induces central sensitization (CS) that is maintained by supraspinal contributions from the descending pain modulatory system (DPMS). Here, in healthy human subjects we use functional magnetic resonance imaging to study the supraspinal activity during the withdrawal period of the opioid remifentanil. We used a crossover design and thermal stimuli on uninjured skin to demonstrate opioid withdrawal-induced hyperalgesia (OIH) without a CS-inducing peripheral stimulus. Saline was used in the control arm to account for effects of time. OIH in this injury-free model was observed in a subset of the healthy subjects (responders). Only in these subjects did opioid infusion and withdrawal induce a rise in activity in the mesencephalic-pontine reticular formation (MPRF), an area of the DPMS that has been previously shown to be involved in states of CS in humans, which became significant during the withdrawal phase compared with nonresponders. Paradoxically, this opioid withdrawal-induced rise in MPRF activity shows a significant negative correlation with the behavioral OIH score indicating a predominant inhibitory role of the MPRF in the responders. These data illustrate that in susceptible individuals central mechanisms appear to regulate the expression of OIH in humans in the absence of tissue injury, which might have relevance for functional pain syndromes where a peripheral origin for the pain is difficult to identify.

Control of visually guided behavior by distinct populations of spinal projection neurons.

A basic question in the field of motor control is how different actions are represented by activity in spinal projection neurons. We used a new behavioral assay to identify visual stimuli that specifically drive basic motor patterns in zebrafish. These stimuli evoked consistent patterns of neural activity in the neurons projecting to the spinal cord, which we could map throughout the entire population using in vivo two-photon calcium imaging. We found that stimuli that drive distinct behaviors activated distinct subsets of projection neurons, consisting, in some cases, of just a few cells. This stands in contrast to the distributed activation seen for more complex behaviors. Furthermore, targeted cell by cell ablations of the neurons associated with evoked turns abolished the corresponding behavioral response. This description of the functional organization of the zebrafish motor system provides a framework for identifying the complete circuit underlying a vertebrate behavior.

Internal pallidum and substantia nigra control different parts of the mesopontine reticular formation in primate.

The locomotor area has recently emerged as a target for deep brain stimulation to lessen gait disturbances in advanced parkinsonian patients. An important step in choosing this target is to define anatomical limits of its 2 components, the pedunculopontine nucleus and the cuneiform nucleus, their connections with the basal ganglia, and their output descending pathway. Based on the hypothesis that pedunculopontine nucleus controls locomotion whereas cuneiform nucleus controls axial posture, we analyzed whether both nuclei receive inputs from the internal pallidum and substantia nigra using anterograde and retrograde tract tracing in monkeys. We also examined whether these nuclei convey descending projections to the reticulospinal pathway. Pallidal terminals were densely distributed and restricted to the pedunculopontine nucleus, whereas nigral terminals were diffusely observed in the whole extent of both the pedunculopontine nucleus and the cuneiform nucleus. Moreover, nigral terminals formed symmetric synapses with pedunculopontine nucleus and cuneiform nucleus dendrites. Retrograde tracing experiments confirmed these results because labeled cell bodies were observed in both the internal pallidum and substantia nigra after pedunculopontine nucleus injection, but only in the substantia nigra after cuneiform nucleus injection. Furthermore, anterograde tracing experiments revealed that the pedunculopontine nucleus and cuneiform nucleus project to large portions of the pontomedullary reticular formation. This is the first anatomical evidence that the internal pallidum and the substantia nigra control different parts of the brain stem and can modulate the descending reticulospinal pathway in primates. These findings support the functional hypothesis that the nigro-cuneiform nucleus pathway could control axial posture whereas the pallido-pedunculopontine nucleus pathway could modulate locomotion.

Cerebellar Golgi cells in the rat receive convergent peripheral inputs via a lateral reticular nucleus relay.

Golgi cells are important players in the function of the cerebellar cortex, controlling the flow of incoming information from mossy fibres to the granule cells, which excite other cortical neurons. We recently showed that in anaesthetized rats most Golgi cells respond to stimulation of afferents from a very wide peripheral receptive field with a long-lasting depression of firing. These responses are mediated via a crossed ascending afferent pathway but the supraspinal part of this pathway is unknown. Here we have examined the hypothesis that the lateral reticular nucleus, a brainstem nucleus with known broad afferent convergence that projects mossy fibres to much of the cerebellum, is involved. First, we showed that single-pulse electrical microstimulation within the lateral reticular nucleus can elicit long-lasting depressions in Golgi cells, which are qualitatively similar to those evoked by peripheral afferent stimulation. Second, we showed that the amplitude of the depressions of Golgi cell firing evoked by peripheral stimulation can be reduced by pharmacological manipulation of the lateral reticular nucleus, either ipsilateral or contralateral to the stimulus site, with local injections of either the GABA(A) receptor agonist muscimol or the AMPA receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione. This evidence suggests that the lateral reticular nucleus is a relay nucleus in the brainstem for peripheral afferent information in a pathway that generates Golgi cell long-lasting depression responses.

Mapping of neural pathways that influence diaphragm activity and project to the lumbar spinal cord in cats.

During breathing, the diaphragm and abdominal muscles contract out of phase. However, during other behaviors (including vomiting, postural adjustments, and locomotion) simultaneous contractions are required of the diaphragm and other muscle groups including abdominal muscles. Recent studies in cats using transneuronal tracing techniques showed that in addition to neurons in the respiratory groups, cells in the inferior and lateral vestibular nuclei (VN) and medial pontomedullary reticular formation (MRF) influence diaphragm activity. The goal of the present study was to determine whether neurons in these regions have collateralized projections to both diaphragm motoneurons and the lumbar spinal cord. For this purpose, the transneuronal tracer rabies virus was injected into the diaphragm, and the monosynaptic retrograde tracer Fluoro-Gold (FG) was injected into the Th13-L1 spinal segments. A large fraction of MRF and VN neurons (median of 72 and 91%, respectively) that were infected by rabies virus were dual-labeled by FG. These data show that many MRF and VN neurons that influence diaphragm activity also have a projection to the lumbar spinal cord and thus likely are involved in coordinating behaviors that require synchronized contractions of the diaphragm and other muscle groups.

The macaque midbrain reticular formation sends side-specific feedback to the superior colliculus.

The central mesencephalic reticular formation (cMRF) likely plays a role in gaze control, as cMRF neurons receive tectal input and provide a bilateral projection back to the superior colliculus (SC). We examined the important question of whether this feedback is excitatory or inhibitory. Biotinylated dextran amine (BDA) was injected into the cMRF of M. fascicularis monkeys to anterogradely label reticulotectal terminals and retrogradely label tectoreticular neurons. BDA labeled profiles in the ipsi- and contralateral intermediate gray layer (SGI) were examined electron microscopically. Postembedding GABA immunochemistry was used to identify putative inhibitory profiles. Nearly all (94.7%) of the ipsilateral BDA labeled terminals were GABA positive, but profiles postsynaptic to these labeled terminals were exclusively GABA negative. In addition, BDA labeled terminals were observed to contact BDA labeled dendrites, indicating the presence of a monosynaptic feedback loop connecting the cMRF and ipsilateral SC. In contrast, within the contralateral SGI, half of the BDA labeled terminals were GABA positive, while more than a third were GABA negative. All the postsynaptic profiles were GABA negative. These results indicate the cMRF provides inhibitory feedback to the ipsilateral side of the SC, but it has more complex effects on the contralateral side. The ipsilateral projection may help tune the "winner-take-all" mechanism that produces a unified saccade signal, while the contralateral projections may contribute to the coordination of activity between the two colliculi.

Anatomy of the brainstem: a gaze into the stem of life.

The brainstem has an ectodermal origin and is composed of 4 parts: the diencephalon, mesencephalon, pons, and medulla oblongata. It serves as the connection between the cerebral hemispheres with the medulla and the cerebellum and is responsible for basic vital functions, such as breathing, heartbeat blood pressure, control of consciousness, and sleep. The brainstem contains both white and gray matter. The gray matter of the brainstem (neuronal cell bodies) is found in clumps and clusters throughout the brainstem to form the cranial nerve nuclei, the reticular formation, and pontine nuclei. The white matter consists of fiber tracts (axons of neuronal cells) passing down from the cerebral cortex--important for voluntary motor function--and up from peripheral nerves and the spinal cord--where somatosensory pathways travel--to the highest parts of the brain. The internal structure of brainstem, although complex, presents a systematical arrangement and is organized in 3 laminae (tectum, tegmentum, and basis), which extend its entire length. The motor pathway runs down through the basis, which is located at the most anterior part. The cranial nerve nuclei are settled into the middle layer (the tegmentum), just in front of the 4th ventricle and are placed, from medial to lateral, on the basis of their function: somatic motor, visceral motor, visceral sensory, and somatic sensory. All the somatosensory tracts run upward to the thalamus crossing the tegmentum in front of the cranial nerve nuclei. The tectum, formed by the quadrigeminal plate and the medullary velum, contains no cranial nuclei, no tracts and no reticular formation. The knowledge of precise anatomical localization of a lesion affecting the brainstem is crucial in neurological diagnosis and, on this basis, is essential to be familiar with the location of the mayor tracts and nuclei appropriately. Nowadays, current magnetic resonance imaging techniques, although still macroscopic, allow the fine internal structure of the brainstem to be viewed directly and make it possible to locate the main intrinsic structures that justify the symptoms of the patient. In this article we discuss the anatomy of the brainstem and highlight the features and landmarks that are important in interpreting magnetic resonance imaging.

[Neuroanatomic bases of hypocretin actions on the ascending reticular activating system: a contribution to narcolepsy physiopathology]. / Bases neuroanatómicas de la acción de las hipocretinas en el sistema reticular ascendente de activación: su aportación a la fisiopatología de la narcolepsia.

The present study describes at the ultrastructural level the hypocretinergic innervation of brainstem reticular formation neurons that project to the medial frontal cortex in the rat. In addition, we assess, by using dual immunofluorescence techniques, the proportion of those reticular neurons containing specific neurotransmitters. Our results indicate that hypocretinergic axons make asymmetric synapses with neurons within the locus coeruleus, ventral tegmental area, dorsal raphe nucleus and laterodorsal tegmental nucleus that target the medial frontal cortex, Hypocretins could facilitate wakefulness and cortical activation, therefore, by activation of those neurons with cortical projections in these four reticular nuclei.

Neonatal maternal separation increases brain-derived neurotrophic factor and tyrosine kinase receptor B expression in the descending pain modulatory system.

Neonatal maternal separation (NMS) could trigger long-term changes in the central neuronal responses to nociceptive stimuli in rats. Stress-induced visceral hyperalgesia is closely associated with the dysfunction of descending pain modulatory systems. Brain-derived neurotrophic factor (BDNF) not only has an important role in long-term synaptic plasticity but also in facilitating descending pain. The present study aimed to investigate changes in the expression of BDNF and its receptor tyrosine kinase receptor B (TrkB) in the amygdala and the rostral ventromedial medulla (RVM) after NMS and colorectal distention (CRD) stimulation in rats. Male Wistar rat pups were subjected to 180 min of daily NMS or not handled for 13 consecutive days. Expression of BDNF and TrkB following NMS and CRD stimulation was determined using immunohistochemistry. The results revealed an increase in the expression of BDNF and TrkB in the amygdala after NMS. An interactive effect of NMS and CRD on the expression of TrkB, but not BDNF, was found in the RVM. Furthermore, a significant interactive effect of NMS and CRD on the colocalization coefficient of TrkB and phospho-extracellular signal-regulated kinase expression in both the amygdala and RVM were found. These data demonstrate that NMS increases BDNF and TrkB expression in the descending pain systems, which may contribute to the development of NMS-induced visceral hyperalgesia.

Neural pathways associated with hypothalamically elicited attach behavior in cats.

Small electrolytic lesions were made in cats through electrodes, which, when stimulated, elicited either quiet biting attack or affective paw strike attack upon rats. The Nauta method for impregnating degenerating axoplasm was used to reveal that degeneration resulting from lesions at quiet attack sites followed largely along the course of the medial forebrain bundle, while the degeneration after lesions of affective attack sites was concentrated more heavily in the periventricular system.

Forebrain projections to brainstem nuclei involved in the control of mandibular movements in rats.

Mandibular movements occur through the triggering of trigeminal motoneurons. Aberrant movements by orofacial muscles are characteristic of orofacial motor disorders, such as nocturnal bruxism (clenching or grinding of the dentition during sleep). Previous studies have suggested that autonomic changes occur during bruxism episodes. Although it is known that emotional responses increase jaw movement, the brain pathways linking forebrain limbic nuclei and the trigeminal motor nucleus remain unclear. Here we show that neurons in the lateral hypothalamic area, in the central nucleus of the amygdala, and in the parasubthalamic nucleus, project to the trigeminal motor nucleus or to reticular regions around the motor nucleus (Regio h) and in the mesencephalic trigeminal nucleus. We observed orexin co-expression in neurons projecting from the lateral hypothalamic area to the trigeminal motor nucleus. In the central nucleus of the amygdala, neurons projecting to the trigeminal motor nucleus are innervated by corticotrophin-releasing factor immunoreactive fibers. We also observed that the mesencephalic trigeminal nucleus receives dense innervation from orexin and corticotrophin-releasing factor immunoreactive fibers. Therefore, forebrain nuclei related to autonomic control and stress responses might influence the activity of trigeminal motor neurons and consequently play a role in the physiopathology of nocturnal bruxism.

Opioids modulate pain facilitation from the dorsal reticular nucleus.

During chronic pain, the supraspinal pain modulatory system undergoes plastic changes with enhancement of facilitation transmission at the spinal cord. The changes induced by chronic pain at descending modulation often affect opioidergic modulation, and were never described for a key facilitatory component of the system, the dorsal reticular nucleus (DRt). Neurochemical characterization of the DRt-spinal pathway showed that delta-opioid receptors are positioned as to indirectly modulate the activity of non-projecting DRt neurons, whereas neurons expressing mu-opioid receptors project to the spinal dorsal horn or act as interneurons, the latter of which co-expressing GABA(B) receptors. In monoarthritic rats, the expression of mu-opioid receptors decreased in the DRt whereas the levels of endogenous enkephalin remained unaltered. To increase the opioidergic inhibition of the DRt, we locally injected selective agonists of delta- and mu-opioid receptors or a viral vector containing the human preproenkephalin transgene. Injection of the Herpes Simplex viral vector encoding preproenkephalin induced thermal hypoalgesia in non-inflamed animals and hyperalgesia in monoarthritic rats. The opioid agonists [D-Ala(2), Glu(4)]-deltorphin (DELT) and [D-Ala(2), NMePhe(4)Gly-ol(5)]-enkephalin (DAMGO) induced thermal hyperalgesia in both non-inflamed and monoarthritic rats, but with lower doses in the latter group. The present study shows that opioidergic neurons at the DRt are modulated by GABAergic cells herein controlling the descending facilitation of pain transmission. The DRt exhibits plastic changes during chronic inflammatory pain, with decrease opioid receptor expression which may account for increased descending facilitation during chronic pain.

Blockade of GABA, type A, receptors in the rat pontine reticular formation induces rapid eye movement sleep that is dependent upon the cholinergic system.

The brainstem reticular formation is an area important to the control of rapid eye movement (REM) sleep. The antagonist of GABA-type A (GABA(A)) receptors, bicuculline methiodide (BMI), injected into the rat nucleus pontis oralis (PnO) of the reticular formation resulted in a long-lasting increase in REM sleep. Thus, one factor controlling REM sleep appears to be the number of functional GABA(A) receptors in the PnO. The long-lasting effect produced by BMI may result from secondary influences on other neurotransmitter systems known to have long-lasting effects. To study this question, rats were surgically prepared for chronic sleep recording and additionally implanted with guide cannulas aimed at sites in the PnO. Multiple, 60 nl, unilateral injections were made either singly or in combination. GABA(A) receptor antagonists, BMI and gabazine (GBZ), produced dose-dependent increases in REM sleep with GBZ being approximately 35 times more potent than BMI. GBZ and the cholinergic agonist, carbachol, produced very similar results, both increasing REM sleep for about 8 h, mainly through increased period frequency, with little reduction in REM latency. Pre-injection of the muscarinic antagonist, atropine, completely blocked the REM sleep-increase by GBZ. GABAergic control of REM sleep in the PnO requires the cholinergic system and may be acting through presynaptic modulation of acetylcholine release.