The cellular mechanism of thalamic ponto-geniculo-occipital waves

Metabotropic glutamate receptor function and regulation of sleep-wake cycles

Metabotropic glutamate (mGlu) receptors are the most abundant family of G-protein coupled receptors and are widely expressed throughout the central nervous system (CNS). Alterations in glutamate homeostasis, including dysregulations in mGlu receptor function, have been indicated as key contributors to multiple CNS disorders. Fluctuations in mGlu receptor expression and function also occur across diurnal sleep-wake cycles. Sleep disturbances including insomnia are frequently comorbid with neuropsychiatric, neurodevelopmental, and neurodegenerative conditions. These often precede behavioral symptoms and/or correlate with symptom severity and relapse. Chronic sleep disturbances may also be a consequence of primary symptom progression and can exacerbate neurodegeneration in disorders including Alzheimer's disease (AD). Thus, there is a bidirectional relationship between sleep disturbances and CNS disorders; disrupted sleep may serve as both a cause and a consequence of the disorder. Importantly, comorbid sleep disturbances are rarely a direct target of primary pharmacological treatments for neuropsychiatric disorders even though improving sleep can positively impact other symptom clusters. This chapter details known roles of mGlu receptor subtypes in both sleep-wake regulation and CNS disorders focusing on schizophrenia, major depressive disorder, post-traumatic stress disorder, AD, and substance use disorder (cocaine and opioid). In this chapter, preclinical electrophysiological, genetic, and pharmacological studies are described, and, when possible, human genetic, imaging, and post-mortem studies are also discussed. In addition to reviewing the important relationships between sleep, mGlu receptors, and CNS disorders, this chapter highlights the development of selective mGlu receptor ligands that hold promise for improving both primary symptoms and sleep disturbances.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Sleep neurobiology from a clinical perspective

Many neurochemical systems interact to generate wakefulness and sleep. Wakefulness is promoted by neurons in the pons, midbrain, and posterior hypothalamus that produce acetylcholine, norepinephrine, dopamine, serotonin, histamine, and orexin/hypocretin. Most of these ascending arousal systems diffusely activate the cortex and other forebrain targets. NREM sleep is mainly driven by neurons in the preoptic area that inhibit the ascending arousal systems, while REM sleep is regulated primarily by neurons in the pons, with additional influence arising in the hypothalamus. Mutual inhibition between these wake- and sleep-regulating regions likely helps generate full wakefulness and sleep with rapid transitions between states. This up-to-date review of these systems should allow clinicians and researchers to better understand the effects of drugs, lesions, and neurologic disease on sleep and wakefulness.

Brain activity and temporal coupling related to eye movements during REM sleep: EEG and MEG results

EEG and MEG REM sleep gamma activity was studied immediately before rapid eye movement onset (PRE-EM), during REM sleep with eye movements away from eye movement onset -phasic-REM (Ph-REM)--and during REM sleep without eye movements, or tonic REM (T-REM). For this purpose, activity was segmented into three different time windows: of 62.5, 250 and 500 ms. Two strategies were used: one a statistical comparison of changes between T-REM, Ph-REM and PRE-EM; the other a descriptive approach using principal component analysis. Significant findings showed that both EEG and MEG gamma activity are higher directly before eye movement onset in PRE-EM periods and during Ph-REM than during T-REM; temporal coupling of electrical activity between the frontal and parietal regions is decreased, while temporal coupling between the right frontal and midline is increased. Just before eye movement onset, larger recording sites become related. For the first time, results showed a close temporal link between power and temporal coupling of fast oscillations andrapid eye movements in REM sleep, indicating increased activation, uncoupling between the left frontal executive areas and posterior sensory association regions and increased coupling between the right frontal attentional and midline alerting systems. Brain activity is reorganized by phasic events.

Diurnal gene expression patterns of T-type calcium channels and their modulation by ethanol

The transient (T-type) calcium channel participates in the generation of normal brain rhythms as well as abnormal rhythms associated with a range of neurological disorders. There are three different isoforms of T-type channels and all are particularly enriched in the thalamus, which is involved in generating many of these rhythms. We report a novel means of T-type channel regulation in the thalamus that involves diurnal regulation of gene expression. Using real time polymerase chain reaction we detected a diurnal pattern of gene expression for all T-type channel transcripts. The peak of gene expression for the CaV3.1 transcript occurred close to the transition from active to inactive (sleep) states, while expression for both CaV3.2 and CaV3.3 peaked near the transition of inactive to active phase. We assessed the effect of chronic consumption of ethanol on these gene expression patterns by examining thalamic tissues of ethanol-consuming cohorts that were housed with the controls, but which received ethanol in the form of a liquid diet. Ethanol consumption resulted in a significant shift of peak gene expression of approximately 5 h for CaV3.2 toward the normally active phase of the mice, as well as increasing the overall gene expression levels by approximately 1.7-fold. Peak gene expression was significantly increased for both CaV3.2 and CaV3.3. Measurements of CaV3.3 protein expression reflected increases in gene expression due to ethanol. Our results illustrate a novel regulatory mechanism for T-type calcium channels that is consistent with their important role in generating thalamocortical sleep rhythms, and suggests that alterations in the pattern of gene expression of these channels could contribute to the disruption of normal sleep by ethanol.

Bursting of thalamic neurons and states of vigilance

This article addresses the functional significance of the electrophysiological properties of thalamic neurons. We propose that thalamocortical activity, is the product of the intrinsic electrical properties of the thalamocortical (TC) neurons and the connectivity their axons weave. We begin with an overview of the electrophysiological properties of single neurons in different functional states, followed by a review of the phylogeny of the electrical properties of thalamic neurons, in several vertebrate species. The similarity in electrophysiological properties unambiguously indicates that the thalamocortical system must be as ancient as the vertebrate branch itself. We address the view that rather than simply relays, thalamic neurons have sui generis intrinsic electrical properties that govern their specific functional dynamics and regulate natural functional states such as sleep and vigilance. In addition, thalamocortical activity has been shown to be involved in the genesis of several neuropsychiatric conditions collectively described as thalamocortical dysrhythmia syndrome.

Functional organization of lemniscal and nonlemniscal auditory thalamus

Thalamic nuclei of the mammalian auditory system exhibit remarkable parallelism in their anatomical pathways and the patterns of synaptic signalling. This has led to the theory that lemniscal, or core thalamocortical projection, carries tonotopically organized and auditory specific information whereas the nonlemniscal thalamocortical pathway forms part of an integrative system that plays an important role in polysensory integration, temporal pattern recognition, and certain forms of learning. Recent experimental evidence derived from molecular, cellular and behavioural studies indeed supports the conjecture that lemniscal and nonlemniscal pathways are involved in distinctive auditory functions.

Analysis of perisaccadic field potentials in the occipitotemporal pathway during active vision

Eye movement potentials (EMPs) associated with saccades appear in both subcortical and cortical structures of the primate visual system. In this study, EMPs are recorded across sites in the occipitotemporal (OT) pathway of monkeys performing a pattern-recognition task. We characterize pair recordings of saccade-triggered local field potentials (LFPs) in early extrastriate and inferotemporal regions of the ventral visual pathway using time-frequency spectrograms. Parameters of the spectrograms, including the centroids of identified regions of interest in the time-frequency plane, are extracted and analyzed. Comparisons among the distributions of the extracted parameters reveal that the occipital lobe EMPs are largely postsaccadic events centered at 100 ms after saccade onset that are typically not influenced in timing by the direction of the saccade or the appearance of a stimulus transient appearing either before or after the saccade. The occipital lobe EMPs also demonstrate a significant shift in frequency content during their transient time course that is influenced, in a few cases, by saccade direction. Temporal lobe EMPs, on the other hand, may be centered in either the presaccadic or postsaccadic intervals; the time of their appearance is significantly influenced by the direction of the saccade. Temporal lobe EMPs demonstrate less frequency modulation than those recorded in the occipital lobe. The prevalence of EMPs in the OT pathway suggests that many cortical regions important for pattern recognition can be modulated by saccades. The timing and frequency characteristics of these signals suggest that the nature of this perisaccadic modulation varies across the cortex.

Slow oscillation in non-lemniscal auditory thalamus

In the present study, we investigated the oscillatory behavior of the auditory thalamic neurons through in vivo intracellular and extracellular recordings in anesthetized guinea pigs. Repeated acoustic stimulus and cortical electrical stimulation were applied to examine their modulatory effects on the thalamic oscillation. The time course of the spike frequency over each trial was obtained by summing all spikes in the onset period and/or the last time period of 100 or 200 msec in the raster display. Spectral analysis was made on the time course of the spike frequency. A slow-frequency oscillation ranging from 0.03 to 0.25 Hz (mean +/- SD, 0.11 +/- 0.05 Hz) was found in the medial geniculate body (MGB) together with a second rhythm of 5-10 Hz. The oscillation neurons had a mean auditory response latency of 17.3 +/- 0.3 msec, which was significantly longer than that of the non-oscillation neurons in lemniscal MGB (9.0 +/- 1.5 msec, p < 0.001, ANOVA) and similar to the non-oscillation neurons in the non-lemniscal MGB (17.6 +/- 5.4 msec, p = 0.811). They were located in the non-lemniscal nuclei of the auditory thalamus. Cortical stimulation altered the thalamic oscillation, leading to termination of the oscillation or to acceleration of the rhythm of the oscillation (the average rhythm changed from 0.07 +/- 0.03 to 0.11 +/- 0.04 Hz, n = 8, p = 0.066, t test). Acoustic stimulation triggered a more regular rhythm in the oscillation neurons. The present results suggest that only the non-lemniscal auditory thalamus is involved in the slow thalamocortical oscillation. The auditory cortex may control the oscillation of the auditory thalamic neurons.

Hallucinations: synchronisation of thalamocortical gamma oscillations underconstrained by sensory input

What we perceive is the product of an intrinsic process and not part of external physical reality. This notion is consistent with the philosophical position of transcendental idealism but also agrees with physiological findings on the thalamocortical system. gamma-Frequency rhythms of discharge activity from thalamic and cortical neurons are facilitated by cholinergic arousal and resonate in thalamocortical networks, thereby transiently forming assemblies of coherent gamma oscillations under constraints of sensory input and prefrontal attentional mechanisms. Perception and conscious experience may be based on such assemblies and sensory input to thalamic nuclei plays merely a constraining role in their formation. In schizophrenia, the ability of sensory input to modulate self-organisation of thalamocortical gamma activity may be generally reduced. If during arousal thalamocortical self-organisation is underconstrained by sensory input, then attentional mechanisms alone may determine the content of perception and hallucinations may arise.

Phase-locking of spontaneous and elicited ponto-geniculo-occipital waves is associated with acceleration of hippocampal theta waves during rapid eye movement sleep in cats

We investigated the temporal relationship between hippocampal theta waves and ponto-geniculo-occipital waves (PGO) during rapid eye movement sleep (REM sleep) in cats. In addition, we analyzed the relationship between hippocampal theta waves and PGO as elicited by tone stimulus (PGO(E)) in order to quantitively characterize the PGO wave generator mechanism. The results showed that a spontaneous PGO tended to be phase-locked to the theta wave, which was more clearly observed in the single PGO than in the cluster. However, cluster PGO(E) tended to be phase-locked as well as single PGO(E). It was therefore suggested that the generator of PGO is activated in relation to the hippocampal theta wave. An acceleration of the theta wave associated with PGO occurrence was found, and was more markedly observed than with the cluster PGO. Although the magnitude of it was less than in the spontaneous case, an acceleration around the PGO(E) was also observed. These results suggest that the generators of theta and PGO receive some common activations, especially when a cluster PGO is generated. The interaction between PGO and hippocampal theta waves is expected to be involved in the possible functions of REM sleep.

Rapid eye movement sleep deprivation modifies expression of long-term potentiation in visual cortex of immature rats

During rapid eye movement (REM) sleep, activity of non-retinal origin is propagated into central visual-system pathways in a manner similar, in pattern and intensity, to central visual-system activity that is exogenously generated in waking. It has been hypothesized that REM sleep, which is more abundantly represented early in life than later, functions to provide adjunct 'afferent' input for shaping synaptic connectivity during brain maturation. Here we present data that support this proposal. Recent studies have described a developmentally regulated form of in vitro long-term potentiation (LTP) in the visual cortex that is experience- and age-dependent. In immature rats, suppression of retinal activation of the visual system by removal of visual experience (dark rearing) extends the age when the developmentally regulated form of LTP can be produced. This study tests whether suppression of REM-state activation of the visual system also lengthens the developmental period in which this specific form of LTP can be elicited. Young rats were deprived of REM sleep by the multiple-small-platforms-over-water method during the typically latest week for induction of such LTP in slices of visual cortex. After this week, we could still induce LTP in slices from nearly all the REM-sleep-deprived rats (8/9) but not from age-matched rats that had not lost REM sleep (0/5). The control rats had been housed on large platforms that allow the animals to obtain REM sleep. Only body weights and the concentration of thyrotrophin-releasing hormone in the hypothalamus distinguished home-caged, normal-sleeping controls from both groups of platform animals. On all measures, stress levels were not dissimilar in the two platforms groups. After 7 days of behavioral suppression of REM sleep in immature rats, and consequent reduction of the intense, extra-retinal activity endogenously generated during this sleep state, we found that the period was extended in which developmentally regulated synaptic plasticity (LTP) could be elicited in slices of visual neocortex. These studies support the role of REM sleep and its associated neuronal activity in brain maturation.

Comparison of cholinergic and histaminergic axons in the lateral geniculate complex of the macaque monkey

The cholinergic and histaminergic projections have important neuromodulatory functions in the ascending visual pathways, so we compared the pattern and mode of innervation of the two projections in the lateral geniculate complex (dorsal lateral geniculate nucleus and pregeniculate nucleus) of the macaque monkey. Brain tissue from macaques was immunoreacted by means of antibodies to choline acetyltransferase (ChAT) or to histamine and processed for light and electron microscopy. A dense plexus of thin, highly branched ChAT-immunoreactive axons laden with varicosities was found in all layers of the dLGN including the koniocellular laminae and in the pregeniculate nucleus. ChAT label was more dense in magnocellular layers 1 and 2 than in parvocellular layers 3-6 and relatively sparse in the interlaminar zones. Varicosities associated with the cholinergic axons had an average of three conventional asymmetric synapses per varicosity, and these appeared to contact dendrites of both thalamocortical cells and interneurons. Histamine-immunoreactive axons were distributed homogeneously throughout all laminar and interlaminar zones of the dLGN, but were denser in the pregeniculate nucleus than in the dLGN. Histaminergic axons branched infrequently and were typically larger in caliber than cholinergic axons. The overwhelming majority of varicosities were found en passant and rarely displayed conventional synapses, despite the abundance of synaptic vesicles, and were not associated preferentially with specific cellular structures. The innervation of the macaque dLGN complex by cholinergic and histaminergic systems is consistent with their proposed role in state dependent modulation of thalamic activity. The dense and highly synaptic innervation by cholinergic axons supports the proposal of additional involvement of these axons in functions related to eye movements.

Ponto-geniculo-occipital-wave suppression amplifies lateral geniculate nucleus cell-size changes in monocularly deprived kittens

We have previously shown that during the post-natal critical period of development of the cat visual system, 1 week of instrumental rapid eye movement (REM) sleep deprivation (IRSD) during 2 weeks of monocular deprivation (MD) results in significant amplification of the effects of solely the 2-week MD on cell-size in the binocular segment of the lateral geniculate nucleus (LGN) [36,40]. In this study, we examined whether elimination of ponto-geniculo-occipital (PGO)-wave phasic activity in the LGN during REM sleep (REMS), rather than suppression of all REMS state-related activity, would similarly yield enhanced plasticity effects on cell-size in LGN. PGO-activity was eliminated in LGN by bilateral pontomesencephalic lesions [8,32]. This method of removing phasic activation at the level of the LGN preserved sleep and wake proportions as well as the tonic activities (low voltage, fast frequency ECoG and low amplitude EMG) that characterize REM sleep. The lesions were performed in kittens on post-natal day 42, at the end of the first week of the 2-week period of MD, the same age when IRSD was started in the earlier study. LGN interlaminar cell-size disparity increased in the PGO-wave-suppressed animals as it had in behaviorally REM sleep-deprived animals. Smaller A1/A-interlaminar ratios reflect the increased disparity effect in both the REM sleep- and PGO-suppressed groups compared to animals subjected to MD-alone. With IRSD, the effect was achieved because the occluded eye-related, LGN A1-lamina cells tended to be smaller relative to their size after MD-alone, whereas after PGO-suppressing lesions, the A1-lamina cells retained their size and the non-occluded eye-related, A-lamina cells tended to be larger than after MD-alone. Despite this difference, for which several possible explanations are offered, these A1/A-interlaminar ratio data indicate that in conjunction either with suppression of the whole of the REMS state or selective removal of REM sleep phasic activity at the LGN, altered visual input evokes more LGN cell plasticity during the developmental period than it would otherwise. These data further support involvement of the REM sleep state in reducing susceptibility to plasticity changes and undesirable variability in the course of normative CNS growth and maturation.

Neuronal activity in the lateral geniculate nucleus associated with ponto-geniculo-occipital waves lacks lamina specificity

Ponto-geniculo-occipital (PGO) waves are spontaneously occurring field potentials recorded in the dorsal lateral geniculate nucleus (LGN) just prior to and during rapid eye movement (REM) sleep. Facilitated discharge rates of LGN neurons are associated with PGO waves. In kittens during the critical period of visual system development, both visual experience and PGO waves appear capable of influencing the course of development through activity-dependent mechanisms. Retinal innervation of LGN segregates into eye-specific laminae and is critical to supporting the role of binocular visual experience in development. We sought to determine whether neuronal activity associated with PGO waves also exhibits lamina specificity. PGO wave-related discharges were examined in LGN neurons identified as to lamina location in adult cats administered urethane anesthesia and the reserpine-like compound, RO4-1284. Spontaneous activity of LGN neurons was related to the occurrence of PGO-like waves in all cells studied. No factors could be found that differentiated lamina location and PGO wave-related discharges. We conclude that the PGO wave influence on neuronal activity in the visual system is fundamentally different from that derived from visual experience. The implications of this difference for the role of the two sources of activation in the control of neural activity in development are discussed.

The fasciculus retroflexus controls the integrity of REM sleep by supporting the generation of hippocampal theta rhythm and rapid eye movements in rats

The fasciculus retroflexus (FR) fiber bundle comprises the intense cholinergic projection from the medial division of the habenula nucleus (Hbn) of the epithalamus to the interpeduncular nucleus (IPN) of the limbic midbrain. Due to the widespread connections of the Hbn and IPN, it could be surmised that the FR is integrated in the processings of various subsystems that are known to be involved in the sleep-wake mechanisms; relevant sites include the limbic forebrain and midbrain areas and more caudal pontine structures. Consequently, the present study addressed the significance of the FR in the spontaneous sleep-wake stage-associated variations of the different activity patterns of frontal cortex and hippocampal electroencephalograms (EEGs), the electrooculogram, and body movements, in freely behaving rats that had been subjected to either bilateral electrolytic lesioning of the FR or control operations. The evolution of different state combinations was assessed by the combinatory analysis of different activity stages appearing on the 6-h records. As compared to the control-operated group, the FR lesioning substantially reduced the time spent in rapid eye movement (REM) sleep by 79%, moderately decreased the duration of the intermediate state of sleep by 29%, and quiet waking state by 44%, but had virtually no effects on the durations of different types of non-REM sleep (i.e., drowsiness that which involved quiet sleep or slow-wave sleep containing delta and spindle state components) or on the times of active waking behavior that corresponded to the body movements. Quantitative decomposition analyses revealed marked variations in the frontal cortex and hippocampal activity as well as REM during the course of the extracted sleep-wake stages described and there were also some group differences. Of those individual features that were used to determine different sleep-wake stages, the overall hippocampal theta time (41% decrease) and single REM frequency (71% reduction during the REM sleep) were most affected. In contrast, the various properties of desynchronization/synchronization patterns of frontal cortex EEGs were consistently hardly influenced by the FR lesioning. Therefore, the present data suggest the involvement of the FR in the REM sleep processes by establishing prominent associations with the limbic and REM control mechanisms that involve the hippocampus and plausibly pontine ocular activity networks.

Serotonergic dorsal raphe nucleus projections to the cholinergic and noncholinergic neurons of the pedunculopontine tegmental region: a light and electron microscopic anterograde tracing and immunohistochemical study

The serotonergic dorsal raphe nucleus is considered an important modulator of state-dependent neural activity via projections to cholinergic neurons of the pedunculopontine tegmental nucleus (PPT). Light and electron microscopic analysis of anterogradely transported biotinylated dextran, combined with choline acetyltransferase (ChAT) immunohistochemistry, were employed to describe the synaptic organization of mesopontine projections from the dorsal raphe to the PPT. In a separate set of experiments, we utilized immunohistochemistry for the serotonin transporter (SERT), combined with ChAT immunohistochemistry at the light and electron microscopic levels, to determine whether PPT neurons receive serotonergic innervation. The results of these studies indicate that: (1) anterogradely labeled and SERT-immunoreactive axons and presumptive boutons invest the PPT at the light microscopic level; (2) at the ultrastructural level, dorsal raphe terminals in the PPT pars compacta synapse mainly with dendrites and axosomatic contacts were not observed; (3) approximately 12% of dorsal raphe terminals synapse with ChAT-immunoreactive dendrites; and (4) at least 2-4% of the total synaptic input to ChAT-immunoreactive dendrites is of dorsal raphe and/or serotonergic origin. This serotonergic dorsal raphe innervation may modulate cholinergic PPT neurons during alterations in behavioral state. The role of these projections in the initiation of rapid eye movement (REM) sleep and the ponto-geniculo-occipital waves that precede and accompany REM sleep is discussed.

Rapid eye movement sleep deprivation in kittens amplifies LGN cell-size disparity induced by monocular deprivation

The abundance of rapid eye movement (REM) sleep in the neonatal mammal and its subsequent decline in the course of development, as well as the dramatic and widespread enhancement of CNS activity during REM sleep, led us to propose that this state plays a functional role in the normative physiological and structural maturation of the brain [54]. When, after 1 week of monocular deprivation (MD), a second week of MD was coupled with behavioral deprivation of REM sleep, the structural alteration in the visual system provoked by MD alone (interlaminar relay cell-size disparity in the lateral geniculate nucleus (LGN) was amplified. With the addition of REM deprivation during MD, the LGN cells connected to the surgically patched eye, which are smaller than normal after MD, became even smaller, whereas the LGN cells receiving input from the seeing eye, which display compensatory hypertrophy after MD, grew even larger. We believe that the interlaminar disparity effect widened because during REM deprivation, the already vision-compromised LGN cells associated with the patched eye also lose the ascending brainstem activation reaching them during the REM state. Loss of the two main sources of 'afference' by these LGN cells permits their seeing-eye LGN counterparts to gain even greater advantage in the competition for synaptic connections in cortex, which is reflected in the relative soma sizes of the LGN relay cells. It is likely that the relatively abundant REM state in early maturation provides symmetric stimulation to all LGN relay cells, irrespective of eye of innervation. The symmetric activation propagated from brainstem to LGN acts to 'buffer' abnormal, asymmetric visual input and, thereby diminishes the extreme, asymmetric structural alteration that results from MD in the absence of REM sleep. We conclude that REM sleep-generated CNS discharge in development has the effect of 'protecting' the CNS against excessive plasticity changes. This is consistent with the possibility that REM sleep plays a role in the genetically programmed processes that direct normative brain development.

Midlatency auditory evoked potentials and the startle response in the rat

The P13 midlatency auditory evoked potential in the rat is (i) sleep state dependent, (ii) undergoes rapid habituation and (iii) is blocked by the cholinergic antagonist scopolamine. As such, the P13 potential in the rat shows the same characteristics as the P1 (or P50) potential in the human. These potentials are thought to be mediated, at least in part, by the cholinergic arm of the reticular activating system. Previous studies have linked the reticular activating system with the startle response. The present study was undertaken to explore this relationship by simultaneously recording the P13 potential and the electromyographically recorded startle response using stimuli designed to elicit each response. Simultaneous recordings from the vertex and neck musculature following auditory click stimuli showed that: (i) the mean threshold of the P13 potential was 69.3 +/- 1.9 dB, while that for the startle response was 87.9 +/- 6.4 dB; (ii) the P13 potential was present during waking and paradoxical sleep, but absent during slow-wave sleep, while the startle response was present reliably only during waking; (iii) both responses habituated in response to paired stimuli, but the startle response was more habituated than the P13 potential; and (iv) both responses were facilitated by trains of stimuli in a similar manner. Recordings carried out from the auditory cortex verified that the primary cortical response had properties different from the P13 potential; i.e. it was present during all sleep-wake states, had a lower threshold and did not habituate rapidly. Finally, different patterns of startle responses were detected in the neck muscles. In every case, the P13 potential occurred during the middle, inhibitory phase of the startle response. These results suggest that the P13 potential and the startle response share response features, but the P13 potential appears to be more sensitive to auditory stimulation and to sleep-wake states. The startle response may be modulating descending systems by priming the spinal cord to respond in a "fight vs flight" fashion. On the other hand, the P13 response may be modulating ascending systems by triggering thalamocortical activity and resetting descending systems to allow novel motor strategies.

Progressive cortical synchronization of ponto-geniculo-occipital potentials during rapid eye movement sleep

Phasic events, termed ponto-geniculo-occipital potentials, appear in the brainstem, thalamus and cerebral cortex during rapid eye movement sleep. In the cat, the species of choice for ponto-geniculo-occipital studies, these field potentials are usually recorded from the lateral geniculate thalamic nucleus and visual cortex. However, the fact that brainstem cholinergic neurons play a crucial role in the transfer of ponto-geniculo-occipital potentials to the thalamus, coupled with the evidence that mesopontine tegmental neurons project to virtually all thalamic nuclei, together explain why ponto-geniculo-occipital potentials are recorded over widespread territories, beyond the visual thalamocortical system. Here we demonstrate, by means of multi-site unit and field potential recordings from sensory, motor and association cortical areas in behaving cats, that: (i) ponto-geniculo-occipital potentials appear synchronously over the neocortex; and (ii) that their cortical synchronization develops progressively from the period preceding rapid eye movement sleep by 30-90 s (pre-rapid eye movement), to reach the highest degree of intracortical coherence during later epochs of rapid eye movement sleep. We propose that the widespread coherence of cortical ponto-geniculo-occipital potentials underlies the synchronization of fast oscillations (30-40 Hz) during rapid eye movement sleep over many, functionally distinct cortical territories implicated in dreaming, as brainstem-induced ponto-geniculo-occipital-like potentials are consistently followed by such fast oscillations.

Dual response modes in lateral geniculate neurons: mechanisms and functions

Relay cells of the lateral geniculate nucleus, like those of other thalamic nuclei, manifest two distinct response modes, and these represent two very different forms of relay of information to cortex. When relatively hyperpolarized, these relay cells respond with a low threshold Ca2+ spike that triggers a brief burst of conventional action potentials. These cells switch to tonic mode when depolarized, since the low threshold Ca2+ spike, being voltage dependent, is inactivated at depolarized levels. In this mode they relay information with much more fidelity. This switch can occur under the influence of afferents from the visual cortex or parabrachial region of the brain stem. It has been previously suggested that the tonic mode is characteristic of the waking state while the burst mode signals an interruption of the geniculate relay during sleep. This review surveys the key properties of these two response modes and discusses the implications of new evidence that the burst mode may also occur in the waking animal.

A functional role for REM sleep in brain maturation

The biological function of REM sleep is defined in terms of the functions of neural processes that selectively operate during the REM sleep state. The high amounts of REM sleep expressed by the young during a period of central nervous system plasticity suggest that one function of REM sleep is in development. The phenomenon of activity-dependent development has been clearly shown to be one mechanism by which early sensory experience can affect the course of neural development. Activity-dependent development may be a ubiquitous process in brain maturation by which activity in one brain region can influence the developmental course of other regions. We hypothesize an ontogenetic function of REM sleep; namely, the widespread control of neuronal activity exerted by specific REM sleep processes help to direct brain maturation through activity-dependent developmental mechanisms. Preliminary tests of the hypothesis have been conducted in the developing feline visual system, which has long been known to incorporate information derived from visual experience in establishing neuronal connectivity. We find that suppression of REM sleep processes by an instrumental REM deprivation procedure results in a significant enhancement of the effects of altered visual experience by monocular occlusion. Bilateral brainstem lesions that selectively block the occurrence of ponto-geniculo-occipital (PGO) waves are sufficient to produce similar results. These data indicate that the propagation of phasic influences during REM sleep interacts with other processes subserving neural development. This source of influence appears not to derive from the environment but rather stems from an intrinsic source of genetic origin. Examination of the neural activity associated with PGO waves in the lateral geniculate nucleus reveals a distribution of facilitatory influence markedly different from that induced by visual experience. We conclude that REM sleep directs the course of brain maturation in early life through the control of neural activity.

Electrophysiology of the dorsolateral mesopontine reticular formation during Pavlovian conditioning in the rabbit

Extracellular single-unit recording methods were used to study the activity of neurons within a restricted portion of the dorsolateral mesopontine reticular formation, an area which includes the parabrachial, pedunculopontine tegmental and cuneiform nuclei. Recordings were obtained during presentations of unfamiliar and familiar sensory stimuli, during Pavlovian differential conditioning procedures that elicited conditioned bradycardia, and while stimulating the amygdaloid central nucleus to identify neurons that projected to, or received projections from, the amygdaloid central nucleus. Activity in most dorsolateral mesopontine reticular neurons was altered during sensory stimulation, and the convergence of auditory and somatic inputs onto single neurons was common. Moreover, neural responses were often of a different magnitude and/or direction to auditory stimuli that were unfamiliar vs familiar vs reinforced (paired with pinna stimulation), and many of these differentially responsive neurons were activated orthodromically by stimulation of the amygdaloid central nucleus. In contrast, neurons activated antidromically by stimulation of the amygdaloid central nucleus were relatively quiescent during all phases of the experiment. Results are discussed in relation to current hypotheses concerning the functional significance of various neuronal subpopulations within the dorsolateral mesopontine reticular formation during Pavlovian conditioning.

The time course of sigma activity and slow-wave activity during NREMS in cortical and thalamic EEG of the cat during baseline and after 12 hours of wakefulness

The extrapolation from recent neurophysiological findings concerning the dependency of spindle and slow-wave oscillations of thalamocortical neurons on membrane potential to macroscopic EEG events, predicts a reciprocal relation between spindle activity and slow-wave activity (SWA) in thalamic and cortical EEG during non-rapid-eye-movement sleep (NREMS). To test this hypothesis, the EEG recorded in 8 cats, from the nucleus centralis lateralis of the thalamus and from the skull during a 12-h baseline dark period and during a 12-h recovery dark period, following a 12-h sleep deprivation, were analyzed. Per 12-s epoch, sleep-wake behaviour was determined and spectral power density was computed in the slow-wave frequency range (0.5-4.0 Hz) and in the spindle frequency region (sigma activity: 11.0-14.5 Hz). To analyze the development of EEG power densities in the course of NREMS and during the transition from NREMS to REMS, the last epoch of wakefulness and the first 15 epochs of NREMS, as well as the last epochs of NREMS and the first epoch of REMS were selected from the NREM-REM cycles. For each animal the values were averaged over 4-h intervals. In the cortical EEG, SWA was minimal at NREMS onset and increased progressively in the course of NREMS. SWA declined sharply prior to REMS. sigma Activity increased gradually towards a uniform level after NREMS onset. During the transition to REMS, sigma activity initially increased and then decreased rapidly. In the thalamic EEG, the time course of SWA paralleled that of the cortex. However, the development of sigma activity during the first part of NREMS differed: in the thalamic EEG, sigma activity was maximal during the beginning of NREMS and slightly decreased thereafter. After sleep deprivation, SWA within NREMS was markedly enhanced in both the cortical and the thalamic EEG. Sigma activity was attenuated in the thalamic EEG, whereas in the cortical EEG it was temporarily elevated. The present data show that, in the thalamic EEG, an inverse relation exists between spindle and slow-wave activity during baseline NREMS. This relation is preserved after sleep deprivation. In the cortical EEG, a reciprocal relation between spindling and SWA is less evident.

Varying expressions of alerting mechanisms in wakefulness and across sleep states

Alerting stimuli, such as intense tones, presented to cats in wakefulness (W) elicit the orienting response (OR) and/or the acoustic startle reflex (ASR) in conjunction with elicited ponto-geniculo-occipital waves (PGOE) from the lateral geniculate body (LGB) and elicited waves from the thalamic central lateral nucleus (CLE). Alerting stimuli presented during rapid eye movement sleep (REM) and non-rapid eye movement sleep (NREM) also elicit PGOE. We presented tones in W, REM and NREM to determine whether CLE could be obtained in sleep and to examine the patterns of responsiveness of PGOE and CLE across behavioral states. Also, we recorded ASR and OR and compared the response patterns of behavioral and central correlates of alerting. The subjects were 7 cats; all exhibited spontaneously occurring waves in LGB and CL. All cats exhibited PGOE and 5 cats exhibited CLE in W, REM and NREM. PGOE and CLE showed less evidence of habituation than did ASR and OR. The pattern of responsiveness of CLE across behavioral states was different from that found for PGOE, and spontaneous CL waves were much rarer than the LGB waves. ASR was elicited in 5 cats during W trials, and in 3 cats during REM trials. OR habituated rapidly in W and did not occur in REM and NREM. The data indicate that central mechanisms of alerting function in sleep states as well as in W and suggest that CLE and PGOE reflect activity in mechanisms underlying cortical desynchronization and visual processes which may act in concert during alerting.

Enhancement of tonic and phasic events of rapid eye movement sleep following bilateral ibotenic acid injections into centralis lateralis thalamic nucleus of cats

The excitotoxin ibotenic acid (1.2-2.6 microliters of 50 micrograms/microliters) was injected bilaterally into the thalamic centralis lateralis nucleus of chronically implanted cats in order to study the effects of tonic excitation followed by destruction of perikarya on the sleep-waking cycle and its electrographic correlates. Ibotenate injections were performed under mild ketamine anaesthesia. Immediately afterwards, the animals showed behavioural arousal accompanied first by ocular nystagmiform movements and then by pontogeniculooccipital waves. By 6-10 h post-injection, the numbers of rapid eye movement sleep episodes, but not their duration, increased compared to the preinjection control period. The injection sites were histologically confirmed using conventional Thionin stains. Additional control was provided by retrograde transport of wheat-germ agglutinin conjugated with horseradish peroxidase. The present results suggest that a population of neurons important for ocular saccades, pontogeniculooccipital waves, and the state of desynchronized sleep is present in the internal medullary lamina, in particular in the centralis lateralis nuclei.

Serotonin hyperpolarizes cholinergic low-threshold burst neurons in the rat laterodorsal tegmental nucleus in vitro

Serotonergic suppression of cholinergic neuronal activity implicated in the regulation of rapid eye movement sleep and its associated phenomenon, pontogeniculooccipital waves, has long been postulated, but no direct proof has been available. In this study, intracellular and whole-cell patch-clamp recording techniques were combined with enzyme histochemistry to examine the intrinsic electrophysiological properties and response to serotonin (5-HT) of identified cholinergic rat laterodorsal tegmental nucleus neurons in vitro. Sixty-five percent of the recorded neurons demonstrated a prominent low-threshold burst, and of these, 83% were cholinergic. In current-clamp recordings 64% of the bursting cholinergic neurons tested responded to the application of 5-HT with a membrane hyperpolarization and decrease in input resistance. This effect was mimicked by application of the selective 5-HT type 1 receptor agonist carboxamidotryptamine maleate. Whole-cell patch-clamp recordings revealed that the hyperpolarizing response was mediated by an inwardly rectifying K+ current. Application of 5-HT decreased excitability and markedly modulated the discharge pattern of cholinergic bursting neurons: during a 5-HT-induced hyperpolarization these neurons exhibited no rebound burst after hyperpolarizing current input and a burst in response to depolarizing current input. In the absence of 5-HT, the relatively depolarized cholinergic bursting neurons responded to an identical hyperpolarizing current input with a burst and did not produce a burst after depolarizing current input. These data provide a cellular and molecular basis for the hypothesis that 5-HT modulates rapid eye movement sleep phenomenology by altering the firing pattern of bursting cholinergic neurons.

The effects of changing state on elicited ponto-geniculo-occipital (PGO) waves

Waves similar to ponto-geniculo-occipital (PGO) waves occurring spontaneously in the lateral geniculate body (LGB), pons, and occipital cortex during rapid eye movement (REM) sleep can be elicited in the LGB and the cortex by tones in waking (W), non-rapid eye movement sleep (NREM), and REM. In W, the elicited waves (PGOE) sometimes accompany orienting responses (OR). We have hypothesized that REM is a state resembling exaggerated "orienting" in part because spontaneous PGO waves similar to PGOE accompanying OR are constantly observed in REM. The present experiment tested whether: (1) PGOE and OR were strongly correlated in W across a large number of tone presentations as might be predicted if PGOE were central wave form markers for a state of orienting; and (2) recovery of responsiveness of PGOE to tones would then be greater in REM than NREM, as might be expected if REM but not NREM were a state in which central mechanisms of orienting were highly active. Tones were presented in W and then in REM and NREM to six cats in order to measure the degree of habituation of OR and PGOE simultaneously. PGOE and OR exhibited a degree of independence: the former were readily produced in W despite the rapid decline in OR across trials. Recovery in the amplitude of PGOE occurred in both NREM and REM. The recovery tended to be greater in REM than NREM, although this was not statistically significant. Refinements of the theory that REM represents a state of exaggerated internal orienting are discussed.

Low-frequency oscillatory activities intrinsic to rat and cat thalamocortical cells

1. Low-frequency membrane potential oscillations recorded intracellularly from thalamocortical (TC) cells of the rat and cat dorsal lateral geniculate nucleus (dLGN) and of the rat ventrobasal nucleus (VB) maintained in vitro were investigated. On the basis of their electrophysiological and pharmacological properties, four types of activity were distinguished and named: the pacemaker oscillations, the spindle-like oscillations, the 'very slow' oscillations and the 'N-methyl-D-aspartate' (NMDA) oscillations. 2. The pacemaker oscillations (95 out of 173 cells) consisted of rhythmic, large-amplitude (10-30 mV) depolarizations which occurred at a frequency of 1.8 +/- 0.3 Hz (range, 0.5-2.9 Hz) and could often give rise to single or a burst of action potentials. Pacemaker oscillations were observed when the membrane potential was moved negative to -55 and positive to -80 mV, but in a given cell the upper and lower limits of this voltage range were separated by only 13.1 +/- 0.5 mV. Above -45 mV tonic firing consisting of single action potentials was seen in the cells showing this or the other types of low-frequency oscillations. 3. The spindle-like oscillations were observed in thirty-nine (out of 173) TC cells and consisted of rhythmic (2.1 +/- 0.3 Hz), large-amplitude depolarizations (and often associated burst firing) similar to the pacemaker oscillations but occurring in discrete periods every 5-25 s and lasting for 1.5-28 s. The spindle-like oscillations were observed when the membrane potential was moved negative to -55 and positive to -80 mV and in two cells they were transformed into continuous pacemaker oscillations by depolarization of the membrane potential to -60 mV. 4. Pacemaker and spindle-like oscillations were unaffected by tetrodotoxin (TTX) or by selective blockade of NMDA, non-NMDA, GABAA, GABAB, nicotinic, muscarinic, alpha- and beta-noradrenergic receptors. 5. The 'very slow' oscillations consisted of a TTX-insensitive, slow hyperpolarization-depolarization sequence (5-15 mV in amplitude) which lasted up to 90 s and was observed in nine dLGN cells and in two VB cells. The pacemaker and the spindle-like oscillations were recorded in one cell each which also showed the 'very slow' oscillations. 6. The 'NMDA' oscillations were observed only in a 'Mg(2+)-free' medium (0 mM-Mg2+, 2-4 mM-Ca2+; 64 out of 72 cells) and consisted of large-amplitude (10-25 mV) depolarizations that did not occur at regular intervals and were intermixed with smaller depolarizations present on the baseline and on the falling phase of the larger ones.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of stimulus intensity on elicited ponto-geniculo-occipital waves

Waves similar to spontaneous ponto-geniculo-occipital (PGO) waves of paradoxical sleep (PS) in cats are elicited by tones and can be considered a form of evoked potential termed PGOE. The aims of the present experiment were to determine: (1) the effects of tone intensity on the probability of producing PGOE; (2) the effects of intensity on the amplitude and latency of PGOE across slow-wave sleep (SWS) and PS; (3) whether the effects of intensity on PGOE are similar to those on a particular form of auditory evoked potential known as the mid-latency response (MLR). Increasing the intensity of the stimulus from 60 to 100 in 10 dB increments resulted in increased probability, increased amplitude, and decreased latency of PGOE in both SWS and PS. This pattern was similar to published findings with MLR, and latencies of PGOE (roughly 60-100 msec) fell within the range of 'C wave' type of MLR recorded in intralaminar nuclei of the thalamus of cats. The possibility that PGOE and MLR share underlying mechanisms and represent the same phenomenon is discussed with particular attention to the function of the mechanisms during alerting and orienting.

Paradoxical sleep and its chemical/structural substrates in the brain

As originally named for the ostensibly contradictory appearance of rapid eye movements and low voltage fast cortical activity during behavioral sleep, paradoxical sleep or rapid eye movement sleep, represents a distinct third state, in addition to waking and slow wave sleep, in mammals and birds. It is an internally generated state of intense tonic and phasic central activation that is contemporaneous with the inhibition of sensory input and motor output. In early studies, it was established that the state of paradoxical sleep was generated within the brainstem, and particularly within the pons. Pharmacological studies indicated an important role for acetylcholine as a neurotransmitter in the generation of this state. Local injections of cholinergic agonists into the pontine tegmentum triggered a state of paradoxical sleep marked by phasic ponto-geniculo-occipital spikes in association with cortical activation and neck muscle atonia. Following the immunohistochemical identification of choline acetyl transferase-containing neurons and their localization to the dorsolateral ponto-mesencephalic tegmentum, neurotoxic lesions of this major cholinergic cell group could be performed to assess its importance in paradoxical sleep. Destruction of the majority of the cholinergic cells, which are concentrated within the laterodorsal tegmental and pedunculopontine tegmental nuclei but extend also into the locus coeruleus and parabrachial nuclei in the cat, resulted in a loss or diminishment of the state of paradoxical sleep, ponto-geniculo-occipital spiking and neck muscle atonia. These deficits were correlated with the loss of choline acetyltransferase-immunoreactive neurons in the region, so as to corroborate results of pharmacological studies and single unit recording studies indicating an active role of these cholinergic cells in the generation of paradoxical sleep and its components. These cells provide a cholinergic innervation to the entire brainstem reticular formation that may be critical in the generation of the state which involves recruitment of massive populations of reticular neurons. Major ascending projections into the thalamus, including the lateral geniculate, may provide the means by which phasic (including ponto-geniculo-occipital spikes) and tonic activation is communicated in part to the cerebral cortex. Descending projections through the caudal dorsolateral pontine tegmentum and into the medial medullary reticular formation may be involved in the initiation of sensorimotor inhibition. Although it appears that the pontomesencephalic cholinergic neurons play an important, active role in the generation of paradoxical sleep, this role may be conditional upon the simultaneous inactivity of noradrenaline and serotonin neurons, evidence for which derives from both pharmacological and recording studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition by carbachol microinjections of presumptive cholinergic PGO-on neurons in freely moving cats

The effects of microinjections of a cholinergic agonist, carbachol (0.2 micrograms/0.2 microliters), were examined on a population of presumptive cholinergic mesopontine PGO-on neurons that presents a tonic pattern of discharge during waking and exhibits short spike bursts preceding the onset of dorsal lateral geniculate PGO waves during paradoxical sleep and slow wave sleep just prior to it. PGO-on neurons were activated antidromically by the stimulation of the dorsal lateral geniculate, pulvinar and/or medial and intralaminar thalamic nuclei. They were all characterized by a long spike duration and a slow conduction velocity. Microinjections of carbachol near unit recording sites in freely moving cats induced a complete suppression of the spontaneous tonic activity during waking, but did not suppress the spontaneous phasic burst activity during sleep. Carbachol microinjections also resulted in a marked reduction in responsiveness of PGO-on neurons to orthodromic stimulation. These spike depressant effects lasted for approximately 90-120 min and were reversed completely by a local or systemic administration of atropine sulfate. These findings point to a direct inhibition of central cholinergic PGO-on neurons via a muscarinic autoreceptor and a difference in the mechanisms underlying the generation of tonic and phasic burst activity of PGO-on neurons occurring during waking and sleep.

Brainstem genesis of reserpine-induced ponto-geniculo-occipital waves: an electrophysiological and morphological investigation

Several experimental results indicate that the peribrachial (PB) cholinergic area of the pedunculopontine nucleus is the final relay for the transfer of brainstem-generated pontogeniculo-occipital (PGO) waves to the thalamus. However, the mechanisms underlying the PGO-related activity of PB neurons remain unknown. In order to study these mechanisms, single unit recordings in the PB area were performed in reserpinized cats. Because PGO waves are closely related to rapid eye movements, our microelectrode explorations were also aimed to some structures of the preoculomotor network, namely, the superior colliculus (SC) and parts of the central tegmental field (FTC). We have found several classes of PGO-on cells in the PB area, most of them descharging 80 ms or less before the peak of PGO waves. These cell-classes comprised high-frequency bursting cells, slow-frequency bursting cells, and neurons discharging single spikes or doublets. Intracellular recordings showed that PGO-on single spikes arise from conventional excitatory postsynaptic potentials. Among PGO-related cells in structures outside the PB limits, it was found that most SC cells discharge during or after the PGO, whereas FTC cells increase their discharge rate several hundreds of ms before PGO waves, thus indicating that PGO waves are elaborated long before the activation of PB neurons. Massive retrograde labeling was found in FTC following horseradish peroxidase injections into the PB area. We suggest that long-lead FTC neurons provide an excitatory input to PGO-on PB neurons.

Membrane resistance increase induced in thalamic neurons by stimulation of brainstem cholinergic afferents

The synaptic response induced in thalamic relay cells by stimulation of peribrachial cholinergic afferents was studied in cats maintained under urethane anesthesia. Recordings were performed in the ventral lateral nucleus after destruction of the reticular thalamic complex and cortical lesion. Some animals were also pretreated with reserpine (1.2 mg/kg) 24 h before recording sessions. Repetitive stimulation (25-60 Hz for 200-600 ms) of the peribrachial area produced a slow depolarization and tonic firing in thalamic cells. The depolarization was associated with an increase in membrane resistance and its amplitude decreased during membrane hyperpolarization. The reactivity of thalamic relay cells to a corticothalamic volley was also increased during the slow depolarization. Similar results were obtained in reserpinized animals. On the basis of previous data obtained in vitro, it is concluded that the slow depolarization resulted from the activation of muscarinic receptors by brainstem cholinergic afferents.

Single cholinergic mesopontine tegmental neurons project to both the pontine reticular formation and the thalamus in the rat

Microinjections of the cholinergic agonist carbachol into a caudal part of the pontine reticular formation of the rat induce a rapid eye movement sleep-like state. This carbachol-sensitive region of the pontine reticular formation is innervated by cholinergic neurons in the pedunculopontine and laterodorsol tegmental nuclei. The same population of cholinergic neurons also project heavily to the thalamus, where there is good evidence that acetylcholine facilitates sensory transmission and blocks rhythmic thalamocortical activity. The present study was undertaken to examine the degree to which single cholinergic neurons in the mesopontine tegmentum project to both the carbachol-sensitive region of the pontine reticular formation and the thalamus, by combining double fluorescent retrograde tracing and immunofluorescence with a monoclonal antibody to choline acetyltransferase in the rat. The results indicated that a subpopulation (5-21% ipsilaterally) of cholinergic neurons in the mesopontine tegmentum projects to both the thalamus and the carbachol-sensitive site of the pontine reticular formation, and these neurons represented the majority (45-88%) of cholinergic neurons projecting to the pontine reticular formation site. The percentage of cholinergic neurons with dual projections was higher in the pedunculopontine tegmental nucleus (6-27%) than in the laterodorsal tegmental nucleus (4-11%). In addition, mixed with cholinergic neurons in the mesopontine tegmentum, there was a small population of dually projecting neurons that did not appear to be cholinergic. Mesopontine cholinergic neurons with dual projections may simultaneously modulate neuronal activity in the pontine reticular formation and the thalamus, and thereby have the potential of concurrently regulating different aspects of rapid eye movement sleep.

The effects of brainstem peribrachial stimulation on perigeniculate neurons: the blockage of spindle waves

The mode of action of afferents arising from the brainstem peribrachial region at the midbrain-pontine junction on neurons recorded from the reticular thalamic sector adjacent to the lateral geniculate nucleus (perigeniculate cells) was investigated at the intracellular level in the cat. Experiments were performed in cats under barbiturate or urethane anaesthesia and in non-anaesthetized deafferented animals. Most cats were pretreated with reserpine (1-2 mg/kg) and were also acutely deprived of their retinal and cortical visual inputs. It was found that peribrachial stimulation produced a short train of fast-rising depolarizations followed by a long-lasting period of hyperpolarization in all perigeniculate neurons. Although the latest part of the early depolarizations preceding the hyperpolarization resulted from a parallel activation of lateral geniculate relay neurons by peribrachial afferents, those occurring at shortest latencies appear to result from a direct excitation produced by peribrachial afferents. Furthermore, these early excitatory postsynaptic potentials persisted under deep barbiturate anaesthesia, a condition that prevents activation of thalamic relay neurons by peribrachial stimulation. The evoked hyperpolarization decreased with membrane hyperpolarization, was associated with a 40-50% increase in membrane conductance and was insensitive to Cl injections. It was no longer observed within one hour after i.v. injection of scopolamine. However, the depolarizing responses were not depressed by this muscarinic antagonist. Iontophoretic applications of scopolamine also removed peribrachial-evoked inhibition of synaptic responses triggered by optic chiasma stimulation. The peribrachial input exerted a powerful control on the oscillatory behavior of perigeniculate neurons. Spindle oscillations which are generated within the reticular thalamic complex were readily blocked by peribrachial stimulation. It is then concluded that the transition from an oscillatory to a relay mode of operation in the thalamus is controlled at least in part by a muscarinic inhibition of reticular thalamic neurons. The synaptic mechanism responsible for the early depolarization remains to be elucidated.

The effects of brainstem peribrachial stimulation on neurons of the lateral geniculate nucleus

The intracellular effects of brainstem peribrachial stimulation on lateral geniculate neurons were investigated in the cat. Experiments were performed in cats under barbiturate or urethane anaesthesia and in non-anaesthetized deafferented animals. Most animals were pretreated with reserpine and were acutely deprived of their retinal and visual cortical inputs. Short trains of stimuli triggered a transient depolarization in most relay neurons (latency: 20-30 ms; duration: 200-300 ms). This depolarization could be interrupted by a short-duration unitary inhibitory postsynaptic potential. The depolarization increased with membrane hyperpolarization and was associated with an increase in membrane conductance. The inhibitory postsynaptic potential had an intrathalamic origin and likely resulted from parallel activation of intrageniculate interneurons. The above responses were largely enhanced in reserpinized cats and were completely abolished by small doses of barbiturates. Iontophoretic applications of the nicotinic blocker, hexamethonium, eliminated peribrachial-evoked discharges in these cells, while similar applications of the muscarinic antagonist, scopolamine, were devoid of any effect. The conclusion is reached that the depolarization of lateral geniculate relay neurons by peribrachial afferents represents a direct postsynaptic effect and does not result from a global disinhibitory mechanism involving inhibition of perigeniculate cells and intrageniculate interneurons. This peribrachial-evoked transient excitation of relay neurons results from a nicotinic mechanism.