Cholinergic systems in the rat brain: IV. Descending projections of the pontomesencephalic tegmentum

Identification of the mechanism mediating genioglossus muscle suppression in REM sleep

RATIONALE

Inhibition of pharyngeal motoneurons accompanies REM sleep and is a cause of hypoventilation and obstructive sleep apnea in humans. One explanation posits that the neurotransmitters glycine and γ-aminobutyric acid are responsible for REM sleep motor inhibition. However, blockade of that mechanism at cranial motor nuclei increases motor activity in all sleep-wake states, and least of all in REM sleep, arguing against it as a major mechanism of REM sleep pharyngeal motor inhibition.

OBJECTIVES

To identify the mechanism of REM sleep inhibition at the hypoglossal motor pool.

METHODS

Genioglossus and diaphragm activities were recorded in 34 rats across sleep-wake states. Microdialysis probes were implanted into the hypoglossal motor pool.

MEASUREMENTS AND MAIN RESULTS

Here we show that muscarinic receptor antagonism at the hypoglossal motor pool prevents the inhibition of genioglossus activity throughout REM sleep; likewise, with G-protein-coupled inwardly rectifying potassium (GIRK) channel blockade. Importantly, the genioglossus activating effects of these interventions were largest in REM sleep and minimal or often absent in other sleep-wake states. Finally, we showed that muscarinic inhibition of the genioglossus is functionally linked to GIRK channel activation.

CONCLUSIONS

We identify a powerful cholinergic-GIRK channel mechanism operating at the hypoglossal motor pool that has its largest inhibitory influence in REM sleep and minimal or no effects in other sleep-wake states. This mechanism is the major cause of REM sleep inhibition at a pharyngeal motor pool critical for effective breathing.

Muscarinic acetylcholine receptors enhance neonatal mouse hypoglossal motoneuron excitability in vitro

In brain stem slices from neonatal ( postnatal days 0–4) CD-1 mice, muscarinic ACh receptors (MAChRs) increased rhythmic inspiratory-related and tonic hypoglossal nerve discharge and depolarized single hypoglossal motoneurons (HMs) via an inward current without changing input resistance. These responses were blocked by the MAChR antagonist 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide (4-DAMP; 100 nM). MAChRs shifted voltage-dependent activation of the hyperpolarization-activated cation current to more positive levels. MAChRs increased the HM repetitive firing rate and decreased rheobase, with both effects being blocked by 4-DAMP. Muscarinic agonists reduced the afterhyperpolarization of single action potentials (APs), suggesting that small-conductance Ca2+-dependent K+ current inhibition increased the HM firing rate. Muscarinic agonists also reduced the AP amplitude and slowed its time course, suggesting that MAChRs inhibited voltage-gated Na+ channels. To compare muscarinic excitation of single HMs to muscarinic excitatory effects on motor output in thicker brain stem slices requiring higher extracellular K+ for rhythmic activity, we tested the effects of muscarinic agonists on single HM excitability in high-K+ artificial cerebrospinal fluid (aCSF). In high-K+ aCSF, muscarinic agonists still depolarized HMs and altered AP size and shape, as in standard aCSF, but did not increase the steady-state firing rate, decrease afterhyperpolarization, or alter threshold potential. These results indicate that the basic cellular response of HMs to muscarinic receptors is excitatory, via a number of distinct mechanisms, and that this excitatory response will be largely preserved in rhythmically active brain stem slices.

Vestibular responses in the macaque pedunculopontine nucleus and central mesencephalic reticular formation

The pedunculopontine nucleus (PPN) and central mesencephalic reticular formation (cMRF) both send projections and receive input from areas with known vestibular responses. Noting their connections with the basal ganglia, the locomotor disturbances that occur following lesions of the PPN or cMRF, and the encouraging results of PPN deep brain stimulation in Parkinson's disease patients, both the PPN and cMRF have been linked to motor control. In order to determine the existence of and characterize vestibular responses in the PPN and cMRF, we recorded single neurons from both structures during vertical and horizontal rotation, translation, and visual pursuit stimuli. The majority of PPN cells (72.5%) were vestibular-only (VO) cells that responded exclusively to rotation and translation stimuli but not visual pursuit. Visual pursuit responses were much more prevalent in the cMRF (57.1%) though close to half of cMRF cells were VO cells (41.1%). Directional preferences also differed between the PPN, which was preferentially modulated during nose-down pitch, and cMRF, which was preferentially modulated during ipsilateral yaw rotation. Finally, amplitude responses were similar between the PPN and cMRF during rotation and pursuit stimuli, but PPN responses to translation were of higher amplitude than cMRF responses. Taken together with their connections to the vestibular circuit, these results implicate the PPN and cMRF in the processing of vestibular stimuli and suggest important roles for both in responding to motion perturbations like falls and turns.

5-HT1A receptor-responsive pedunculopontine tegmental neurons suppress REM sleep and respiratory motor activity

Serotonin type 1A (5-HT(1A)) receptor-responsive neurons in the pedunculopontine tegmental nucleus (PPTn) become maximally active immediately before and during rapid eye movement (REM) sleep. A prevailing model of REM sleep generation indicates that activation of such neurons contributes significantly to the generation of REM sleep, and if correct then inactivation of such neurons ought to suppress REM sleep. We test this hypothesis using bilateral microperfusion of the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 10 μm) into the PPTn; this tool has been shown to selectively silence REM sleep-active PPTn neurons while the activity of wake/REM sleep-active PPTn neurons is unaffected. Contrary to the prevailing model, bilateral microperfusion of 8-OH-DPAT into the PPTn (n = 23 rats) significantly increased REM sleep both as a percentage of the total recording time and sleep time, compared with both within-animal vehicle controls and between-animal time-controls. This increased REM sleep resulted from an increased frequency of REM sleep bouts but not their duration, indicating an effect on mechanisms of REM sleep initiation but not maintenance. Furthermore, an increased proportion of the REM sleep bouts stemmed from periods of low REM sleep drive quantified electrographically. Targeted suppression of 5-HT(1A) receptor-responsive PPTn neurons also increased respiratory rate and respiratory-related genioglossus activity, and increased the frequency and amplitude of the sporadic genioglossus activations occurring during REM sleep. These data indicate that 5-HT(1A) receptor-responsive PPTn neurons normally function to restrain REM sleep by elevating the drive threshold for REM sleep induction, and restrain the expression of respiratory rate and motor activities.

Morphological and functional characterization of cholinergic interneurons in the dorsal horn of the mouse spinal cord

Endogenous acetylcholine is an important modulator of sensory processing, especially at the spinal level, where nociceptive (pain-related) stimuli enter the central nervous system and are integrated before being relayed to the brain. To decipher the organization of the local cholinergic circuitry in the spinal dorsal horn, we used transgenic mice expressing enchanced green fluorescent protein specifically in cholinergic neurons (ChAT::EGFP) and characterized the morphology, neurochemistry, and firing properties of the sparse population of cholinergic interneurons in this area. Three-dimensional reconstruction of lamina III ChAT::EGFP neurons based either on their intrinsic fluorescence or on intracellular labeling in live tissue demonstrated that these neurons have long and thin processes that grow preferentially in the dorsal direction. Their dendrites and axon are highly elongated in the rostrocaudal direction, beyond the limits of a single spinal segment. These unique morphological features suggest that dorsal horn cholinergic interneurons are the main contributors to the plexus of cholinergic processes located in lamina IIi, just dorsal to their cell bodies. In addition, immunostainings demonstrated that dorsal horn cholinergic interneurons in the mouse are γ-aminobutyric acidergic and express nitric oxide synthase, as in rats. Finally, electrophysiological recordings from these neurons in spinal cord slices demonstrate that two-thirds of them have a repetitive spiking pattern with frequent rebound spikes following hyperpolarization. Altogether our results indicate that, although they are rare, the morphological and functional features of cholinergic neurons enable them to collect segmental information in superficial layers of the dorsal horn and to modulate it over several segments.

Acetylcholine-mediated neurotransmission within the nucleus raphe magnus exerts a key role in the organization of both interictal and postictal antinociception

The role of the acetylcholine-mediated system in the organization of postictal antinociception was investigated. For this purpose, nicotinic and muscarinic cholinergic receptor antagonists were microinjected into the nucleus raphe magnus (NRM), a key structure of the endogenous pain inhibitory system. After the tail-flick test baseline recording, male Wistar rats (N=8 per group) were submitted to stereotaxic surgery for the introduction of a guide cannula aiming at the NRM. Five days after surgery, atropine or mecamylamine (1 µg/0.2 µL, 3 µg/0.2 µL, or 5 µg/0.2 µL) was microinjected into the NRM. The tail-flick withdrawal latency was recorded immediately after peripheral treatment with pentylenetetrazole (PTZ) (64 mg/kg), in two different interictal time windows, and for 130 minutes after the last seizure evoked by intraperitoneal injection of PTZ. The blockade of GABA-mediated Cl(-) influx caused tonic-clonic convulsions in all animals followed by sustained postictal antinociception lasting 110 minutes after seizures; the nociceptive threshold was also found to be high in interictal periods. Pretreatment of the NRM with either atropine or mecamylamine antagonized both interictal and postictal antinociception, suggesting the involvement of cholinergic mechanisms recruiting muscarinic and nicotinic cholinergic receptors of the NRM in the organization of tonic-clonic seizure-induced antinociception.

Cholinergic inputs to laryngeal motoneurons functionally identified in vivo in rat: a combined electrophysiological and microscopic study

The intrinsic laryngeal muscles are differentially modulated during respiration as well as other states and behaviors such as hypocapnia and sleep. Previous anatomical and pharmacological studies indicate a role for acetylcholine at the level of the nucleus ambiguus in the modulation of laryngeal motoneuron (LMN) activity. The present study investigated the anatomical nature of cholinergic input to inspiratory- (ILM) and expiratory-modulated (ELM) laryngeal motoneurons in the loose formation of the nucleus ambiguus. Using combined in vivo intracellular recording, dye filling, and immunohistochemistry, we demonstrate that LMNs identified in Sprague-Dawley rat receive several close appositions from vesicular acetylcholine transporter-immunoreactive (VAChT-ir) boutons. ELMs receive a significantly greater number of close appositions (mean ± standard deviation [SD]: 47 ± 11; n = 5) than ILMs (32 ± 9; n = 8; t-test P < 0.05). For both LMN types, more close appositions were observed on the cell soma and proximal dendrites compared to distal dendrites (two-way analysis of variance [ANOVA], P < 0.0001). Using fluorescence confocal microscopy, almost 90% of VAChT-ir close appositions (n = 45 boutons on n = 4 ELMs) were colocalized with the synaptic marker synaptophysin. These results support a strong influence of cholinergic input on LMNs and may have implications in the differential modulation of laryngeal muscle activity.

The schizophrenia risk allele C of the TCF4 rs9960767 polymorphism disrupts sensorimotor gating in schizophrenia spectrum and healthy volunteers

In a large-scale meta-analysis, it has been recently shown that the transcription factor 4 (TCF4) gene is among the most prominent susceptibility genes for schizophrenia. Moreover, transgenic mice overexpressing TCF4 in the brain display a reduction of sensorimotor gating measured by prepulse inhibition (PPI) of the acoustic startle response (ASR). PPI is heritable and has been established as an important translational endophenotype of schizophrenia. We therefore investigated the impact of the schizophrenia susceptibility gene TCF4 (rs9960767) on sensorimotor gating of the ASR in healthy humans and in patients with a schizophrenia spectrum disorder. We assessed PPI, startle reactivity, and habituation of the ASR in two independent samples. The first sample consisted of 107 healthy volunteers from London, UK. The second sample was a schizophrenia spectrum group (n = 113) of 73 schizophrenia patients and 40 individuals at high risk for schizophrenia from Bonn, Germany (total sample n = 220). In both samples, PPI was strongly decreased in carriers of the schizophrenia risk allele C of the TCF4 gene (meta-analysis across both samples: p = 0.00002), whereas startle reactivity and habituation were unaffected by TCF4 genotype. Sensorimotor gating is modulated by TCF4 genotype, indicating an influential role of TCF4 gene variations in the development of early information-processing deficits in schizophrenia.

Choline supplementation mitigates trace, but not delay, eyeblink conditioning deficits in rats exposed to alcohol during development

Children exposed to alcohol prenatally suffer from a range of physical, neuropathological, and behavioral alterations, referred to as fetal alcohol spectrum disorders (FASD). Both the cerebellum and hippocampus are affected by alcohol exposure during development, which may contribute to behavioral and cognitive deficits observed in children with FASD. Despite the known neuropathology associated with prenatal alcohol exposure, many pregnant women continue to drink (heavy drinkers, in particular), creating a need to identify effective treatments for their children who are adversely affected by alcohol. We previously reported that choline supplementation can mitigate alcohol's effects on cognitive development, specifically on tasks which depend on the functional integrity of the hippocampus. The present study examined whether choline supplementation could differentially mitigate alcohol's effects on trace eyeblink classical conditioning (ECC, a hippocampal-dependent task) and delay ECC (a cerebellar-dependent task). Long-Evans rats were exposed to 5.25 g/kg/day alcohol via gastric intubation from postnatal days (PD) 4-9, a period of brain development equivalent to late gestation in humans. A sham-intubated control group was included. From PD 10-30, subjects received subcutaneous injections of 100 mg/kg choline chloride or vehicle. Beginning on PD 32-34, subjects were trained on either delay or trace eyeblink conditioning. Performance of subjects exposed to alcohol was significantly impaired on both tasks, as indicated by significant reductions in percentage and amplitude of conditioned eyeblink responses, an effect that was attenuated by choline supplementation on the trace, but not delay conditioning task. Indeed, alcohol-exposed subjects treated with choline performed at control levels on the trace eyeblink conditioning task. There were no significant main or interactive effects of sex. These data indicate that choline supplementation can significantly reduce the severity of trace eyeblink conditioning deficits associated with early alcohol exposure, even when administered after the alcohol insult is complete. These findings have important implications for the treatment of fetal alcohol spectrum disorders.

Cholinergic modulation of mesolimbic dopamine function and reward

The substantial health risk posed by obesity and compulsive drug use has compelled a serious research effort to identify the neurobiological substrates that underlie the development these pathological conditions. Despite substantial progress, an understanding of the neurochemical systems that mediate the motivational aspects of drug-seeking and craving remains incomplete. Important work from the laboratory of Bart Hoebel has provided key information on neurochemical systems that interact with dopamine (DA) as potentially important components in both the development of addiction and the expression of compulsive behaviors such as binge eating. One such modulatory system appears to be cholinergic pathways that interact with DA systems at all levels of the reward circuit. Cholinergic cells in the pons project to DA-rich cell body regions in the ventral tegmental area (VTA) and substantial nigra (SN) where they modulate the activity of dopaminergic neurons and reward processing. The DA terminal region of the nucleus accumbens (NAc) contains a small but particularly important group of cholinergic interneurons, which have extensive dendritic arbors that make synapses with a vast majority of NAc neurons and afferents. Together with acetylcholine (ACh) input onto DA cell bodies, cholinergic systems could serve a vital role in gating information flow concerning the motivational value of stimuli through the mesolimbic system. In this report we highlight evidence that CNS cholinergic systems play a pivotal role in behaviors that are motivated by both natural and drug rewards. We argue that the search for underlying neurochemical substrates of compulsive behaviors, as well as attempts to identify potential pharmacotherapeutic targets to combat them, must include a consideration of central cholinergic systems.

Multiple origins of cholinergic innervation of the cochlear nucleus

Acetylcholine (Ach) affects a variety of cell types in the cochlear nucleus (CN) and is likely to play a role in numerous functions. Previous work in rats suggested that the acetylcholine arises from cells in the superior olivary complex, including cells that have axonal branches that innervate both the CN and the cochlea (i.e. olivocochlear cells) as well as cells that innervate only the CN. We combined retrograde tracing with immunohistochemistry for choline acetyltransferase to identify the source of ACh in the CN of guinea pigs. The results confirm a projection from cholinergic cells in the superior olivary complex to the CN. In addition, we identified a substantial number of cholinergic cells in the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT) that project to the CN. On average, the PPT and LDT together contained about 26% of the cholinergic cells that project to CN, whereas the superior olivary complex contained about 74%. A small number of additional cholinergic cells were located in other areas, including the parabrachial nuclei.The results highlight a substantial cholinergic projection from the pontomesencephalic tegmentum (PPT and LDT) in addition to a larger projection from the superior olivary complex. These different sources of cholinergic projections to the CN are likely to serve different functions. Projections from the superior olivary complex are likely to serve a feedback role, and may be closely tied to olivocochlear functions. Projections from the pontomesencephalic tegmentum may play a role in such things as arousal and sensory gating. Projections from each of these areas, and perhaps even the smaller sources of cholinergic inputs, may be important in conditions such as tinnitus as well as in normal acoustic processing.

Cholinergic cells in the tegmentum send branching projections to the inferior colliculus and the medial geniculate body

The pontomesencephalic tegmentum (PMT) provides cholinergic input to the inferior colliculus (IC) and the medial geniculate body (MG). PMT cells are often characterized as projecting to more than one target. The purpose of this study was to determine whether individual PMT cholinergic cells, (1) innervate the auditory pathways bilaterally via collateral projections to left and right auditory thalamus; or, (2) innervate multiple levels of the auditory pathways via collateral projections to the auditory thalamus and inferior colliculus. We used multiple retrograde tracers to identify individual PMT cells that project to more than one target. We combined the retrograde tracer studies with immunohistochemistry for choline acetyltransferase to determine whether the projecting cells were cholinergic. We found that individual PMT cells send branching axonal projections to two or more auditory targets in the midbrain and thalamus. The collateral projection pattern that we observed most frequently was to the ipsilateral IC and ipsilateral MG. Cells projecting to both MGs were somewhat less common, followed by cells projecting to the contralateral IC and ipsilateral MG. Both cholinergic and non-cholinergic cells contribute to each of these projection patterns. Less often, we found cells that project to one IC and both MGs; there was no evidence for non-cholinergic cells in this projection pattern. It is likely that collateral projections from PMT cells could have coordinated effects bilaterally and at multiple levels of the ascending auditory pathways.

Rebound discharge in deep cerebellar nuclear neurons in vitro

Neurons of the deep cerebellar nuclei (DCN) play a critical role in defining the output of cerebellum in the course of encoding Purkinje cell inhibitory inputs. The earliest work performed with in vitro preparations established that DCN cells have the capacity to translate membrane hyperpolarizations into a rebound increase in firing frequency. The primary means of distinguishing between DCN neurons has been according to cell size and transmitter phenotype, but in some cases, differences in the firing properties of DCN cells maintained in vitro have been reported. In particular, it was shown that large diameter cells in the rat DCN exhibit two phenotypes of rebound discharge in vitro that may eventually help define their functional roles in cerebellar output. A transient burst and weak burst phenotype can be distinguished based on the frequency and pattern of rebound discharge immediately following a hyperpolarizing stimulus. Work to date indicates that the difference in excitability arises from at least the degree of activation of T-type Ca(2+) current during the immediate phase of rebound firing and Ca(2+)-dependent K(+) channels that underlie afterhyperpolarizations. Both phenotypes can be detected following stimulation of Purkinje cell inhibitory inputs under conditions that preserve resting membrane potential and natural ionic gradients. In this paper, we review the evidence supporting the existence of different rebound phenotypes in DCN cells and the ion channel expression patterns that underlie their generation.

Cholinergic cells of the pontomesencephalic tegmentum: connections with auditory structures from cochlear nucleus to cortex

Acetylcholine (ACh) is a neuromodulator that is likely to play a role in plasticity as well as other phenomena at many sites in the auditory system. The auditory cortex receives cholinergic innervation from the basal forebrain, whereas the cochlea receives cholinergic innervation from the superior olivary complex. Much of the remainder of the auditory pathways receives innervation from the pedunculopontine and laterodorsal tegmental nuclei, two nuclei referred to collectively as the pontomesencephalic tegmentum (PMT). The PMT provides the major source of ACh to the auditory thalamus and the midbrain, and is a substantial source (in addition to the superior olivary complex) of ACh in the cochlear nucleus. Individual cholinergic cells in the PMT often have axon branches that innervate multiple auditory nuclei, including nuclei on both sides of the brain as well as nuclei at multiple levels of the auditory system. The auditory cortex has direct axonal projections to the PMT cells, including cholinergic cells that project to the inferior colliculus or cochlear nucleus. The divergent projections of PMT cholinergic cells suggest widespread effects on the auditory pathways. These effects are likely to include plasticity as well as novelty detection, sensory gating, reward behavior, arousal and attention. Descending projections from the forebrain, including the auditory cortex, are likely to provide a high level of cognitive input to these cholinergic effects. Dysfunction associated with the cholinergic system may play a role in disorders such as tinnitus and schizophrenia.

The pedunculopontine tegmental nucleus: implications for a role in modulating spinal cord motoneuron excitability

There is evidence that deep brain stimulation (DBS) of the pedunculopontine tegmental nucleus (PPTg) improves parkinsonian motor signs. The mechanisms that mediate these effects and the modifications that occur in the PPTg in Parkinson's disease (PD) are not fully known and are the object of current debate. The aim of this paper was to critically review available data with respect to (1) the presence of PPTg neurons linked to reticulospinal projections, (2) the involvement of these neurons in modulating spinal reflexes, and (3) the participation of fibers close to or within the PPTg region in such modulation. The PPTg neurons are distributed in a large pontotegmental region, stimulation of which can evoke activity in hindlimb, shoulder and neck muscles, and potentiate motor responses evoked by stimulation of dorsal roots. This influence seems to be carried out by fast-conducting descending fibers, which likely run in the medial reticulospinal pathway. It is yet unclear which neurotransmitters are involved and on which elements of the gray matter of the spinal cord PPTg fibers synapse. The modulation of spinal cord activity which can be achieved by stimulating the PPTg region seems to be mediated not only by PPTg neurons, but also by tecto-reticular fibers which run in the pontotegmental area, and which likely are activated during PPTg-DBS. The importance of these fibers is discussed taking into account the degeneration of PPTg neurons in PD and the benefits in gait and postural control that PPTg-DBS exerts in PD. The potential usefulness of PPTg-DBS in other neurodegenerative disorders characterized by neuronal loss in the brainstem is also considered.

Effects of an oral dose of l-glutamic acid on circulating neurotransmitters: Possible roles of the C1(Ad) and the A5(NA) pontomedullary nuclei

Objective

Investigation of the effects of an oral administration of a small dose of l-glutamic acid on the two peripheral sympathetic branches (neural and adrenal) of the autonomic nervous system.

Research design and methods

Circulating neurotransmitters and cardiovascular parameters were assessed in 28 healthy volunteers before and after the administration of 500 mg of l-glutamic acid or placebo.

Results

The drug triggered a significant and sustained enhancement of the noradrenaline and dopamine circulating levels which were paralleled and positively correlated with the diastolic blood pressure increases. Conversely, both platelet and plasma serotonin showed significant falls throughout the test. Significant positive correlations were registered between noradrenaline, dopamine, and noradrenaline/dopamine ratio versus diastolic blood pressure but not versus systolic blood pressure or heart rate.

Conclusion

The above results allowed us to postulate that the drug provoked a significant enhancement of peripheral neural sympathetic activity and the reduction of adrenal sympathetic and parasympathetic drives. Both sympathetic branches are positively correlated with the A₅ noradrenergic and the C₁ adrenergic pontomedullary nuclei, which interchange inhibitory axons that act at post-synaptic α₂ inhibitory autoreceptors. In addition, we discussed the mechanisms able to explain why the drug acted preferentially at the A₅ noradrenergic rather than the C₁ adrenergic nuclei.

Projections from auditory cortex to midbrain cholinergic neurons that project to the inferior colliculus

We have shown that auditory cortex projects to cholinergic cells in the pedunculopontine tegmental nucleus (PPT) and laterodorsal tegmental nucleus (LDT). PPT and LDT are the sources of cholinergic projections to the inferior colliculus, but it is not known if the cortical inputs contact the cholinergic cells that project to the inferior colliculus. We injected FluoroRuby into auditory cortex in pigmented guinea pigs to label cortical projections to PPT and LDT. In the same animals, we injected Fast Blue into the left or right inferior colliculus to label PPT and LDT cells that project to the inferior colliculus. We processed the brain to identify cholinergic cells with an antibody to choline acetyltransferase, which was visualized with a green fluorescent marker distinguishable from both FluoroRuby and Fast Blue. We then examined the PPT and LDT to determine whether boutons of FluoroRuby-labeled cortical axons were in close contact with cells that were double-labeled with the retrograde tracer and the immunolabel. Apparent contacts were observed ipsilateral and, less often, contralateral to the injected cortex. On both sides, the contacts were more numerous in PPT than in LDT. The results indicate that auditory cortex projects directly to brainstem cholinergic cells that innervate the ipsilateral or contralateral inferior colliculus. This suggests that cortical projections could elicit cholinergic effects on both sides of the auditory midbrain.

Brainstem pathology and non-motor symptoms in PD

Parkinson's disease (PD) is considered a multisystem disorder involving dopaminergic, noradrenergic, serotoninergic, and cholinergic systems, characterized by motor and non-motor symptoms. The causes of the non-motor symptoms in PD are multifactorial and unlikely to be explained by single lesions. However, several evidence link them to damage of specific brainstem nuclei. Numerous brainstem nuclei are engaged in fundamental homeostatic mechanisms, including gastrointestinal regulation, pain perception, mood control, and sleep-wake cycles. In addition, these nuclei are locally interconnected in a complex manner and are subject to supraspinal control. The objective of this review is to provide a better overview of the current knowledge about the consequences of the involvement of specific brainstem nuclei to the most prevalent non-motor symptoms occurring in PD. The multidisciplinary efforts of research directed to these non-nigral brainstem nuclei, in addition to the topographical and chronological spread of the disease - especially in the prodromal stages of PD, are discussed.

Differential effects of M1 and 5-hydroxytryptamine1A receptors on atypical antipsychotic drug-induced dopamine efflux in the medial prefrontal cortex

Systemic administration of the M(1) receptor agonists N-desmethylclozapine (NDMC) and 4-[3-(4-butylpiperidin-1-yl)-propyl]-7-fluoro-4H-benzo[1,4]oxazin-3-one (AC260584) increase dopamine (DA) efflux in rat medial prefrontal cortex (mPFC). This increase is blocked by systemic administration of both telenzepine, a preferential M(1) receptor antagonist, and N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide (WAY-100635), a 5-hydroxytryptamine(1A) receptor antagonist. The present study sought to determine whether DA efflux in the mPFC induced by the atypical antipsychotic drugs clozapine, risperidone, and olanzapine is also mediated by M(1) receptor stimulation and, specifically, to determine whether these effects are mediated M(1) receptors in the mPFC through use of in vivo microdialysis in awake, freely moving Sprague-Dawley rats. Telenzepine (3 mg/kg) significantly attenuated clozapine- (20 mg/kg), olanzapine- (10 mg/kg), and risperidone- (1.0 mg/kg) induced increases in mPFC DA efflux. Local mPFC perfusion of NDMC, AC260584, clozapine, risperidone, or olanzapine (10-500 microM), significantly increased DA efflux in the mPFC. Local mPFC perfusion of telenzepine (0.1 microM) prevented increases in mPFC DA efflux induced by systemic administration of AC260584 (10 mg/kg), NDMC (20 mg/kg), and clozapine (10 mg/kg), but not by risperidone (1.0 mg/kg) or olanzapine (10 mg/kg). However, local mPFC perfusion of WAY-100635 (0.1 microM) prevented mPFC DA efflux induced by clozapine, risperidone, and olanzapine, but not by AC260584 or NDMC. These results suggest that the AC260584-, NDMC-, and clozapine-induced DA efflux in the mPFC is mediated directly by mPFC M(1) receptors.

Rostral ventral medulla cholinergic mechanism in pain-induced analgesia

The ascending nociceptive control (ANC), a novel spinostriatal pain modulation pathway, mediates a form of pain-induced analgesia referred to as noxious stimulus-induced antinociception (NSIA). ANC includes specific spinal cord mechanisms as well as circuitry in nucleus accumbens, a major component of the ventral striatum. Here, using the trigeminal jaw-opening reflex (JOR) in the rat as a nociceptive assay, we show that microinjection of the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine into the rostral ventral medulla (RVM) blocks NSIA, implicating RVM as a potentially important link between ANC and the PAG-RVM-spinal descending pain modulation system. A circuit connecting nucleus accumbens to the RVM is proposed as a novel striato-RVM pathway.

Inputs to serotonergic neurons revealed by conditional viral transneuronal tracing

Descending projections arising from brainstem serotonergic (5HT) neurons contribute to both facilitatory and inhibitory controls of spinal cord "pain" transmission neurons. Unclear, however, are the brainstem networks that influence the output of these 5HT neurons. To address this question, here we used a novel neuroanatomical tracing method in a transgenic line of mice in which Cre recombinase is selectively expressed in 5HT neurons (ePet-Cre mice). Specifically, we injected the conditional pseudorabies virus recombinant (BA2001) that can replicate only in Cre-expressing neurons. Because BA2001 transports exclusively in a retrograde manner, we were able to reveal a subset of the neurons and circuits that are located upstream of the Cre-expressing 5HT neurons. We show that diverse brainstem regions differentially target the 5HT neurons of the dorsal raphe (DR) and the nucleus raphe magnus of the rostroventral medulla (RVM). Among these are several catecholaminergic and cholinergic cell groups, the periaqueductal gray, several brainstem reticular nuclei, and the nucleus of the solitary tract. We conclude that a brainstem 5HT network integrates somatic and visceral inputs arising from various areas of the body. We also identified a circuit that arises from projection neurons of deep spinal cord laminae V-VIII and targets the 5HT neurons of the NRM, but not of the DR. This spinoreticular pathway constitutes an anatomical substrate through which a noxious stimulus can activate 5HT neurons of the NRM and in turn could trigger descending serotonergic antinociceptive controls.

Central cholinergic regulation of respiration: nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are expressed in brainstem and spinal cord regions involved in the control of breathing. These receptors mediate central cholinergic regulation of respiration and effects of the exogenous ligand nicotine on respiratory pattern. Activation of alpha4* nAChRs in the preBötzinger Complex (preBötC), an essential site for normal respiratory rhythm generation in mammals, modulates excitatory glutamatergic neurotransmission and depolarizes preBötC inspiratory neurons, leading to increases in respiratory frequency. nAChRs are also present in motor nuclei innervating respiratory muscles. Activation of post- and/or extra-synaptic alpha4* nAChRs on hypoglossal (XII) motoneurons depolarizes these neurons, potentiating tonic and respiratory-related rhythmic activity. As perinatal nicotine exposure may contribute to the pathogenesis of sudden infant death syndrome (SIDS), we discuss the effects of perinatal nicotine exposure on development of the cholinergic and other neurotransmitter systems involved in control of breathing. Advances in understanding of the mechanisms underlying central cholinergic/nicotinic modulation of respiration provide a pharmacological basis for exploiting nAChRs as therapeutic targets for neurological disorders related to neural control of breathing such as sleep apnea and SIDS.

Sources of cholinergic input to the inferior colliculus

We combined retrograde tracing with immunohistochemistry for choline acetyltransferase to identify the source of cholinergic input to the inferior colliculus (IC) in guinea pigs. Injection of a retrograde tracer into one IC labeled cells in many brainstem nuclei. Retrogradely-labeled cells that were also immunoreactive for choline acetyltransferase were identified in two nuclei in the midbrain tegmentum: the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT). More PPT and LDT cells project ipsilaterally than contralaterally to the IC and, on both sides, there are more projecting cells in the PPT than in the LDT. Double-labeled cells were not found in any other brainstem nucleus. A common feature of cholinergic cells in PPT and LDT is collateral projections to multiple targets. We placed different retrograde tracers into each IC to identify cells in PPT and LDT that project to both ICs. In both PPT and LDT, a substantial proportion (up to 57%) of the immunoreactive cells that contained tracer from the contralateral IC also contained tracer from the ipsilateral IC. We conclude that acetylcholine in the IC originates from the midbrain tegmental cholinergic nuclei: PPT and LDT. These nuclei are known to participate in arousal, the sleep/wake cycle and prepulse inhibition of acoustic startle. It is likely that the cholinergic input to the IC is directly associated with these functions.

The development of nicotinic receptors in the human medulla oblongata: inter-relationship with the serotonergic system

Maternal cigarette smoking during pregnancy adversely affects fetal development and increases the risk for the sudden infant death syndrome (SIDS). In SIDS we have reported abnormalities in the medullary serotonergic (5-HT) system, which is vital for homeostatic control. In this study we analyzed the inter-relationship between nicotinic receptors (nAChRs), to which nicotine in cigarette smoke bind, and the medullary 5-HT system in the human fetus and infant as a step towards determining the mechanisms whereby smoking increases SIDS risk in infants with 5-HT defects. Immunohistochemistry for the alpha4 nAChR subunit and 5-HT neurons was applied in fetal and infant medullae (15-92 postconceptional weeks, n=9). The distribution of different nAChRs was determined from 39-82 postconceptional weeks (n=5) using tissue autoradiography for 3H-nicotine, 3H-epibatidine, 3H-cytisine, and 125I-bungarotoxin; the findings were compared to laboratory 5-HT1A and 5-HT transporter binding data, and 5-HT neuronal density. Alpha4 immunoreactivity was ubiquitously expressed in medullary nuclei related to homeostatic functions from 15 weeks on, including rhombic lip germinal cells. At all ages, alpha4 co-localized with 5-HT neurons, indicating a potential site of interaction whereby exogenous nicotine may adversely affect 5-HT neuronal development and function. Binding for heteromeric nAChRs was highest in the inferior olive, and for homomeric nAChRs, in the vagal complex. In the paragigantocellularis lateralis, 5-HT1A receptor binding simultaneously increased as alpha7 binding decreased across infancy. This study indicates parallel dynamic and complex changes in the medullary nicotinic and 5-HT systems throughout early life, i.e., the period of risk for SIDS.

Serotoninergic dorsal raphe neurons possess functional postsynaptic nicotinic acetylcholine receptors

Very few neurons in the telencephalon have been shown to express functional postsynaptic nicotinic acetylcholine receptors (nAChRs), among them, the noradrenergic and dopaminergic neurons. However, there is no evidence for postsynaptic nAChRs on serotonergic neurons. In this study, we asked if functional nAChRs are present in serotonergic (5-HT) and nonserotonergic (non-5-HT) neurons of the dorsal raphe nucleus (DRN). In rat midbrain slices, field stimulation at the tegmental pedunculopontine (PPT) nucleus evoked postsynaptic currents (eEPSCs) with different components in DRN neurons. After blocking the glutamatergic and GABAergic components, the remaining eEPSCs were blocked by mecamylamine and reduced by either the selective alpha7 nAChR antagonist methyllycaconitine (MLA) or the selective alpha4beta2 nAChR antagonist dihydro-beta-eritroidine (DHbetaE). Simultaneous addition of MLA and DHbetaE blocked all eEPSCs. Integrity of the PPT-DRN pathway was assessed by both anterograde biocytin tracing and antidromic stimulation from the DRN. Inward currents evoked by the direct application of acetylcholine (ACh), in the presence of atropine and tetrodotoxin, consisted of two kinetically different currents: one was blocked by MLA and the other by DHbetaE; in both 5-HT and non-5-HT DR neurons. Analysis of spontaneous (sEPSCs) and evoked (eEPSCs) synaptic events led to the conclusion that nAChRs were located at the postsynaptic membrane. The possible implications of these newly described nAChRs in various physiological processes and behavioral events, such as the wake-sleep cycle, are discussed.

Molecular targets for treating cognitive dysfunction in schizophrenia

Cognitive impairment is a core feature of schizophrenia as deficits are present in the majority of patients, frequently precede the onset of other positive symptoms, persist even with successful treatment of positive symptoms, and account for a significant portion of functional impairment in schizophrenia. While the atypical antipsychotics have produced incremental improvements in the cognitive function of patients with schizophrenia, overall treatment remains inadequate. In recent years, there has been an increased interest in developing novel strategies for treating the cognitive deficits in schizophrenia, focusing on ameliorating impairments in working memory, attention, and social cognition. Here we review various molecular targets that are actively being explored for potential drug discovery efforts in schizophrenia and cognition. These molecular targets include dopamine receptors in the prefrontal cortex, nicotinic and muscarinic acetylcholine receptors, the glutamatergic excitatory synapse, various serotonin receptors, and the gamma-aminobutyric acid (GABA) system.

Modulation of respiratory pattern and upper airway muscle activity by the pedunculopontine tegmentum: role of NMDA receptors

The pedunculopontine tegmental nucleus (PPT) is postulated to have important functions relevant to the regulation of rapid eye movement (REM) sleep and arousal, and various motor control systems including respiration. We have recently shown that pharmacologic activation of a neuronal subpopulation within the PPT, induced by micropipette injection of glutamate in nanoliter volumes, can produce respiratory rhythm disturbances and changes in genioglossus muscle activity in anesthetized rats. The aim of this study was to determine whether the respiratory pattern disturbance and increased genioglossus muscle tone induced by glutamate injection within the PPT are mediated by activation of N-methyl-D-aspartate (NMDA) receptors within the PPT. Experiments were performed in eight adult male spontaneously breathing Sprague-Dawley rats anesthetized using nembutal. Respiratory movements were monitored by piezoelectric strain gauge. Three-barrel glass pipettes were used to pressure inject glutamate (as a probe for respiratory modulating sites), ketamine (an NMDA channel blocker), and oil-red dye (to aid in histological verification of the injection sites) within the PPT. Electroencephalograms were recorded from the sensorimotor cortex, the hippocampus, and the pons, contralateral to the injection site. Electromyograms (EMGs) were recorded from the genioglossus muscle. The typical response to glutamate injection within the PPT respiratory-modulating region was immediate apnea followed by tachypnea and increased genioglossal tonic activity. The noncompetitive NMDA receptor channel-antagonist ketamine, injected at the same site and in the same volume as glutamate (5 nl), blocked respiratory dysrhythmia and genioglossal EMG responses to subsequent glutamate injections. For the first time, the present results suggest that respiratory rhythm and upper airway muscle tone are controlled by the activation of pedunculopontine tegmental nucleus NMDA receptors.

Respiratory motor activity: influence of neuromodulators and implications for sleep disordered breathing

Sleep, especially rapid-eye-movement sleep, causes fundamental modifications of respiratory muscle activity and control mechanisms, modifications that can predispose individuals to sleep-related breathing disorders. One of the most common of these disorders is obstructive sleep apnea (OSA) that affects approximately 4% of adults. OSA is caused by repeated episodes of pharyngeal airway obstruction that can occur hundreds of times per night, leading to recurrent asphyxia, arousals from sleep, daytime sleepiness, and adverse cardiovascular and cerebrovascular consequences. OSA is caused by the effects of sleep on pharyngeal muscle tone in individuals with already narrow upper airways. Moreover, since OSA occurs only in sleep, this disorder by definition is a state-dependent process ultimately caused by the influence of sleep neural mechanisms on the activity of pharyngeal motoneurons. This review synthesizes recent findings relating to control of pharyngeal muscle activity across sleep-wake states, with special emphasis on the influence of neuromodulators acting at the hypoglossal motor nucleus that inervates the genioglossus muscle of the tongue. The results of such basic physiological studies may be relevant to identifying and developing new pharmacological strategies to augment pharyngeal muscle activity in sleep, especially rapid-eye-movement sleep, as potential treatments for OSA.

Vesicular acetylcholine transporter–immunoreactive axon terminals enriched in the pontine nuclei of the mouse

Information to the cerebellum enters via many afferent sources collectively known as precerebellar nuclei. We investigated the distribution of cholinergic terminal-like structures in the mouse precerebellar nuclei by immunohistochemistry for vesicular acetylcholine transporter (VAChT). VAChT is involved in acetylcholine transport into synaptic vesicles and is regarded as a reliable marker for cholinergic terminals and preterminal axons. In adult male mice, brains were perfusion-fixed. Polyclonal antibodies for VAChT, immunoglobulin G-peroxidase and diaminobenzidine were used for immunostaining. In the mouse brain, immunoreactivity was seen in almost all major cholinergic cell groups including brainstem motoneurons. In precerebellar nuclei, the signal could be detected as diffusely beaded terminal-like structures. It was seen heaviest in the pontine nuclei and moderate in the pontine reticulotegmental nucleus; however, it was seen less in the medial solitary nucleus, red nucleus, lateral reticular nucleus, inferior olivary nucleus, external cuneate nucleus and vestibular nuclear complex. In particular, VAChT-immunoreactive varicose fibers were so dense in the pontine nuclei that detailed distribution was studied using three-dimensional reconstruction of the pontine nuclei. VAChT-like immunoreactivity clustered predominantly in the medial and ventral regions suggesting a unique regional difference of the cholinergic input. Electron microscopic observation in the pontine nuclei disclosed ultrastructural features of VAChT-immunoreactive varicosities. The labeled bouton makes a symmetrical synapse with unlabeled dendrites and contains pleomorphic synaptic vesicles. To clarify the neurons of origin of VAChT-immunoreactive terminals, VAChT immunostaining combined with wheat germ agglutinin-conjugated horseradish peroxidase retrograde labeling was conducted by injecting a retrograde tracer into the right pontine nuclei. Double-labeled neurons were seen bilaterally in the laterodorsal tegmental nucleus and pedunculopontine tegmental nucleus. It is assumed that mesopontine cholinergic neurons negatively regulate neocortico-ponto-cerebellar projections at the level of pontine nuclei.

Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

Mesopontine cholinergic projections to the hypoglossal motor nucleus

Mesopontine cholinergic (ACh) neurons have increased discharge during wakefulness, rapid eye movement (REM) sleep, or both. Hypoglossal (12) motoneurons, which play an important role in the control of upper airway patency, are postsynaptically excited by stimulation of nicotinic receptors, whereas muscarinic receptors presynaptically inhibit inputs to 12 motoneurons. These data suggest that ACh contributes to sleep/wake-related changes in the activity of 12 motoneurons by acting within the hypoglossal motor nucleus (Mo12), but the origins of ACh projections to Mo12 are not well established. We used retrograde tracers to assess the projections of ACh neurons of the mesopontine pedinculopontine tegmental (PPT) and laterodorsal tegmental (LDT) nuclei to the Mo12. In six Sprague-Dawley rats, Fluorogold or B subunit of cholera toxin, were pressure injected (5-20nl) into the Mo12. Retrogradely labeled neurons, identified as ACh using nitric oxide synthase (NOS) immunohistochemistry, were found bilaterally in discrete subregions of both PPT and LDT nuclei. Most retrogradely labeled PPT cells (96%) were located in the PPT pars compacta region adjacent to the ventrolateral tip of the superior cerebellar peduncle. In the LDT, retrogradely labeled neurons were located exclusively in its pars alpha region. Over twice as many ACh neurons projecting to the Mo12 were located in the PPT than LDT. The results demonstrate direct mesopontine ACh projections to the Mo12. These projections may contribute to the characteristic of wakefulness and REM sleep increases, as well as REM sleep-related decrements, of 12 motoneuronal activity.

Muscarinic receptors and alpha2-adrenoceptors interact to modulate the respiratory rhythm in mouse neonates

The respiratory rhythm generator (RRG) is modulated by several endogenous substances, including acetylcholine (ACh) and noradrenaline (NA) that interact in several modulatory processes. To know whether ACh and NA interacted to modulate the RRG activity, we used medullary "en bloc" and slice preparations from neonatal mice where the RRG has been shown to receive a facilitatory modulation from A1/C1 neurons, via a continuous release of endogenous NA and activation of alpha2 adrenoceptors. Applying ACh at 25 microM activated the RRG but ACh had no effects at 50 microM. Applying the ACh receptor agonists nicotine and muscarine facilitated and depressed the RRG, respectively. After yohimbine pre-treatment that blocked the alpha2 facilitation, the nicotinic facilitation was not altered, the muscarinic depression was reversed and ACh 50 microM significantly facilitated the RRG. After L-tyrosine pre-treatment that potentiated the alpha2 facilitation, the muscarinic depression was enhanced. Thus, ACh regulates the RRG activity via nicotinic and muscarinic receptors, the muscarinic receptors interacting with alpha2 adrenoceptors.

The effects of ageing on the distribution of vesicular acetylcholine transporter immunoreactive inputs to pelvic motoneurons of male Wistar rats

Age-related changes in the number and size of large cholinergic terminals immunoreactive for vesicular acetylcholine transporter (VAChT), were documented for the dorsolateral nucleus (DLN), retrodorsolateral nucleus (RDLN) and spinal nucleus of the bulbospongiosus (SNB) of the lumbosacral spinal cord of male rats. The most significant changes were a large increase in the number and size of cholinergic terminals within the DLN of aged animals, together with a small decrease in terminal number within the RDLN. No significant age-associated differences in VAChT labeling were seen within the SNB. In both age groups, SNB motoneurons projecting to the levator ani muscle received about 9 to 10 contacts from large cholinergic terminals. Ultrastructural examination of the terminals revealed structures likely to be postsynaptic subsurface cisterns that are characteristic of type C terminal boutons. Since both the DLN and SNB contain motoneurons innervating pelvic muscles and sphincters, these findings provide further evidence for a central cholinergic influence on micturition and sexual reflexes and suggest that this may remain robust in the face of ageing.

Estradiol increases alpha7 nicotinic receptor in serotonergic dorsal raphe and noradrenergic locus coeruleus neurons of macaques

Acetylcholine, acting on presynaptic nicotinic receptors (nAChRs), modulates the release of neurotransmitters in the brain. The rat dorsal raphe nucleus (DR) and the locus coeruleus (LC) receive cholinergic input and express the alpha7nAChR. In previous reports, we demonstrated that estradiol (E) administration stimulates DR serotonergic and LC noradrenergic function in the macaque. In addition, it has been reported that E induces the expression of the alpha7nAChR in rats. We questioned whether E increased the expression of the alpha7nAChR in the macaque DR and LC. We utilized double immunostaining to study the effect of a simulated preovulatory surge of E on the expression of the alpha7nAChR in the DR and the LC and to determine whether alpha7nAChR colocalizes with serotonin and tyrosine hydroxylase (TH) in macaques. There was no difference in the number of alpha7nAChR-positive neurons between ovariectomized (OVX) controls and OVX animals treated with a silastic capsule containing E (Ecap). However, supplemental infusion of E for 5-30 hours to Ecap animals (Ecap + inf) significantly increased the number of alpha7nAChR-positive neurons in DR and LC. In addition, supplemental E infusion significantly increased the number of neurons in which alpha7nAChR colocalized with serotonin and TH. These results constitute an important antecedent for study of the effects of nicotine and ovarian steroid hormones in the physiological functions regulated by the DR and the LC in women.

Long-term depression at the mossy fiber-deep cerebellar nucleus synapse

Several lines of evidence have indicated that the deep cerebellar nuclei (DCN) are a site of memory storage for certain forms of motor learning, most notably associative eyelid conditioning. In particular, these experiments, together with network models, have implicated the excitatory glutamatergic synapse between mossy fibers and DCN neurons in this memory trace. However, to date, evidence for persistent use-dependent change in the strength of this synapse has been almost entirely absent. Here, we report that high-frequency burst stimulation of mossy fibers, either alone or paired with postsynaptic depolarization, gives rise to long-term depression (LTD) of the mossy fiber-DCN synapse. This form of LTD is not associated with changes in the paired-pulse ratio and is blocked by loading with a postsynaptic Ca2+ chelator but not by bath application of an NMDA receptor antagonist. Mossy fiber-DCN LTD requires activation of a group I metabotropic glutamate receptor (mGluR) and protein translation. Unlike mGluR/translation-dependent LTD in other brain regions, this form of LTD requires mGluR1 and is mGluR5 independent.

Cyclic nucleotides modulate genioglossus and hypoglossal responses to excitatory inputs in rats

RATIONALE

Previous studies modulating pharyngeal muscle activity with pharmacologic approaches have targeted membrane receptors on pharyngeal motoneurons. Whether modulation of intracellular pathways can increase pharyngeal muscle activity, however, has not been investigated but is relevant to pharmacologic treatments of obstructive sleep apnea.

OBJECTIVES

To determine if modulating the second messenger cyclic adenosine-3'-5'-monophosphate (cAMP) at the hypoglossal motor nucleus (HMN) will increase genioglossus activity across sleep- wake states.

METHODS

Forty-eight rats were implanted with electroencephalogram and neck electrodes to record sleep-wake states and genioglossus and diaphragm electrodes for respiratory muscle recordings. Microdialysis probes were inserted into the HMN to perfuse artificial cerebrospinal fluid and (1) forskolin (500 microM, adenylyl cyclase activator to increase cAMP), (2) a cAMP analog (500 microM), (3) iso-butyl-methylxanthine (IBMX; 300 microM, phosphodiesterase inhibitor), or (4) a cyclic guanosine-3'-5'-monophosphate (cGMP) analog (500 microM, 8-Br-cGMP).

MEASUREMENTS AND MAIN RESULTS

Forskolin and the cAMP analog at the HMN increased respiratory-related and tonic genioglossus activities in wakefulness and non-REM sleep but not REM sleep. IBMX did not affect genioglossus activity in awake or sleeping rats. However, IBMX abolished the robust excitatory responses to serotonin and phenylephrine at the HMN, but responses to non-N-methyl-D-aspartate receptor activation remained. These effects of IBMX were mimicked by 8-Br-cGMP.

CONCLUSIONS

Genioglossus responses to manipulation of cAMP at the HMN are differentially modulated by sleep-wake state. Selective abolition of serotonin and phenylephrine responses after IBMX suggests that under conditions of nonspecific phosphodiesterase inhibition the HMN is unresponsive to certain, otherwise potent, excitatory inputs. Similar responses with 8-Br-cGMP suggest this effect is likely mediated by cGMP pathways.

Cholinergic modulation of vibrissal receptive fields in trigeminal nuclei

In sensory systems, it is usually considered that mesopontine cholinergic neurons exert their modulatory action in the thalamus by enhancing the relay of sensory messages during states of neural network desynchronization. Here, we report a projection heretofore unknown of these cholinergic cells to the interpolar division of the brainstem trigeminal complex in rats. After FluoroGold injection in the interpolar nucleus, a number of retrogradely labeled cells were found bilaterally in the pedunculopontine tegmental nucleus, and immunostaining revealed that the vast majority of these cells were also positive for choline acetyltransferase. Immunostaining for the acetylcholine vesicular transporter confirmed the presence of cholinergic terminals in the interpolar nucleus, where electron microscopy showed that they make symmetric and asymmetric synaptic contacts with dendrites and axon terminals. In agreement with these anatomical data, recordings in slices showed that the cholinergic agonist carbachol depolarizes large-sized interpolaris cells and increases their excitability. Local application of carbachol in vivo enhances responses to adjacent whiskers, whereas systemic administration of the cholinergic antagonist scopolamine produces an opposite effect. Together, these results show that mesopontine cholinergic neurons exert a direct, effective control over receptive field size at the very first relay stations of the vibrissal system in rodents. As far as receptive field synthesis in the lemniscal pathway relies on intersubnuclear projections from the spinal complex, it follows that cholinergic modulation of sensory transmission in the interpolar nucleus will have a direct bearing on the type of messages that is forwarded to the thalamus and cerebral cortex.

Nocifensive reflex-related on- and off-cells in the pedunculopontine tegmental nucleus, cuneiform nucleus, and lateral dorsal tegmental nucleus

Cholinergic projections from the pedunculopontine tegmental nucleus (PPTg) to the rostral ventromedial medulla (RVM) have been implicated in nociceptive modulation. The goal of this study was to identify neurons with nocifensive reflex-related activity in the mesopontine tegmentum including the PPTg. This study used the same behavioral neurophysiological classification system to identify neurons as has been extensively described in the RVM. Extracellular microelectrode recording was conducted in lightly anesthetized rats. Changes in firing associated with the noxious heat-evoked tail flick reflex were used to classify neurons as "on-cells" (displayed a burst in neuronal activity associated with the reflex), "off-cells" (displayed a pause in activity), and neutral cells (showed no response). Of 188 neurons studied in 23 rats, 77 were classified as on-cells, 14 as off-cells, the remainder as neutral cells. Recordings during periods without noxious stimulation found that some of the on- and off-cells displayed spontaneous transitions between active and silent periods termed cell cycling. The distribution of on- and off-cells in the mesopontine tegmentum overlapped and included the cholinergic PPTg and lateral dorsal tegmental nucleus identified by NADPH diaphorase staining, as well as the cuneiform nucleus and periaqueductal gray. The mesopontine tegmentum thus contains nocifensive reflex-related neurons with neurophysiological characteristics similar to those reported in the RVM. Neurons showing reflex-related activity were frequently encountered in the cholinergic PPTg and LDTg. Further studies will be required to determine whether these neurons modulate nociception through a link to the RVM.

Injection of glutamate into the pedunculopontine tegmental nuclei of anesthetized rat causes respiratory dysrhythmia and alters EEG and EMG power

The pedunculopontine tegmental nucleus (PPT) has been shown to have important functions relevant to the regulation of behavioral states and various motor control systems, including breathing control. Our previous work has shown that the activation of neurons within the PPT, a structure that is typically active during rapid eye movement (REM) sleep, can produce respiratory disturbances in freely moving and anesthetized rats. The aim of this study was to test the hypothesis that respiratory modulation by the PPT in anesthetized rats can be evoked in the absence of other signs of an REM-sleep-like state. We characterized electroencephalogram (EEG) and electromyogram (EMG) changes during respiratory disturbances induced by glutamatergic stimulation of the PPT in spontaneously breathing, adult male Sprague-Dawley rats anesthetized with a ketamine/xylazine combination or with nembutal. Respiratory movements were monitored by a piezoelectric strain gauge. Two-barrel glass pipettes were used to pressure inject glutamate, to probe for respiratory effective sites within the PPT, and to inject oil red dye at the end of the experiments for histological verification of the injection sites. The EEGs were recorded from the sensorimotor cortex, hippocampus, and from the pons contralateral from the injection site. The EMGs were recorded from the genioglossus muscle. The initial response to glutamate injection into the respiratory modulating region of the PPT was always a respiratory pattern disturbance. Subsequent activation of EMG and EEG often occurred in ketamine/xylazine-anesthetized rats, but REM-sleep-like patterns were not observed. Respiratory pattern and EMG power changes in nembutal-anesthetized rats were similar, but EEG activation was never observed. Thus, we conclude that respiratory suppression produced by the local activation of PPT neurons may not necessarily be accompanied by an REM-sleep-like cortical state in this anesthetized model.

Cholinergic neurotransmission in the preBötzinger Complex modulates excitability of inspiratory neurons and regulates respiratory rhythm

We investigated whether there is endogenous acetylcholine (ACh) release in the preBötzinger Complex (preBötC), a medullary region hypothesized to contain neurons generating respiratory rhythm, and how endogenous ACh modulates preBötCneuronal function and regulates respiratory pattern. Using a medullary slice preparation from neonatal rat, we recorded spontaneous respiratory-related rhythm from the hypoglossal nerve roots (XIIn) and patch-clamped preBötC inspiratory neurons. Unilateral microinjection of physostigmine, an acetylcholinesterase inhibitor, into the preBötC increased the frequency of respiratory-related rhythmic activity from XIIn to 116+/-13% (mean+/-S.D.) of control. Ipsilateral physostigmine injection into the hypoglossal nucleus (XII nucleus) induced tonic activity, increased the amplitude and duration of the integrated inspiratory bursts of XIIn to 122+/-17% and 117+/-22% of control respectively; but did not alter frequency. In preBötC inspiratory neurons, bath application of physostigmine (10 microM) induced an inward current of 6.3+/-10.6 pA, increased the membrane noise, decreased the amplitude of phasic inspiratory drive current to 79+/-16% of control, increased the frequency of spontaneous excitatory postsynaptic currents to 163+/-103% and decreased the whole cell input resistance to 73+/-22% of control without affecting the threshold for generation of action potentials. Bath application of physostigmine concurrently induced tonic activity, increased the frequency, amplitude and duration of inspiratory bursts of XIIn motor output. Bath application of 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, 2 microM), a M3 muscarinic acetylcholine receptor (mAChR) selective antagonist, increased the input resistance of preBötC inspiratory neurons to 116+/-9% of control and blocked all of the effects of physostigmine except for the increase in respiratory frequency. Dihydro-beta-erythroidine (DH-beta-E; 0.2 microM), an alpha4beta2 nicotinic receptor (nAChR) selective antagonist, blocked all the effects of physostigmine except for the increase in inspiratory burst amplitude. In the presence of both 4-DAMP and DH-beta-E, physostigmine induced opposite effects, i.e. a decrease in frequency and amplitude of XIIn rhythmic activity. These results suggest that there is cholinergic neurotransmission in the preBötC which regulates respiratory frequency, and in XII nucleus which regulates tonic activity, and the amplitude and duration of inspiratory bursts of XIIn in neonatal rats. Physiologically relevant levels of ACh release, via mAChRs antagonized by 4-DAMP and nAChRs antagonized by DH-beta-E, modulate the excitability of inspiratory neurons and excitatory neurotransmission in the preBötC, consequently regulating respiratory rhythm.

Opposing muscarinic and nicotinic modulation of hypoglossal motor output to genioglossus muscle in rats in vivo

The genioglossus (GG) muscle of the tongue, innervated by the hypoglossal motor nucleus (HMN), helps maintain an open airway for effective breathing. In vitro studies in neonatal rodents have separately characterized muscarinic and nicotinic receptor influences at the HMN but the net effects of combined nicotinic and muscarinic receptor activation and increased endogenous acetylcholine have not been determined in adult animals in vivo. Urethane-anaesthetized, tracheotomized and vagotomised rats were studied. Microdialysis perfusion of acetylcholine into the HMN significantly decreased respiratory-related GG activity (28.5 +/- 11.0% at a threshold dose of 0.1 mm). Application of the cholinergic agonists carbachol and muscarine have similar suppression effects (GG activity was decreased 11.8 +/- 4.3 and 20.5 +/- 5.8%, respectively, at 0.01 microm). Eserine, an acetylcholinesterase inhibitor, also decreased the amplitude of respiratory-related GG activity (36.4 +/- 11.3% at 1.0 microm) indicating that endogenous acetylcholine modulates GG activity. Although these results showed that suppression of GG activity predominates during cholinergic stimulation at the HMN, application of the nicotinic receptor agonist dimethyl-4-phenylpiperazinium iodide significantly increased tonic and respiratory-related GG activity (156 +/- 33% for respiratory activity at 1.0 mm) showing that excitatory responses are also present. Consistent with this, 100 microm carbachol decreased GG activity by 44.2 +/- 7.5% of control, with atropine (10 microm) reducing this suppression to 13.8 +/- 4.0% (P < 0.001). However, the nicotinic receptor antagonist dihydro-beta-erythroidine (100 microm) increased the carbachol-mediated suppression to 69.5 +/- 5.9% (P = 0.011), consistent with a role for nicotinic receptors in limiting the overall suppression of GG activity during cholinergic stimulation. Application of eserine to increase endogenous acetylcholine also showed that inhibitory muscarinic and excitatory nicotinic receptors together determine the net level of GG activity during cholinergic stimulation at the HMN. The results suggest that acetylcholine has mixed effects at the HMN with muscarinic-mediated GG suppression masking nicotinic excitation.

Persistent changes in the intrinsic excitability of rat deep cerebellar nuclear neurones induced by EPSP or IPSP bursts

The deep cerebellar nuclei (DCN) are the major output of the cerebellum, and have been proposed as a site of memory storage for certain forms of motor learning. Microelectrode and whole-cell patch recordings were performed on DCN neurones in acute slices of juvenile rat cerebellum. DCN neurones display tonic and bursting basal firing patterns. In tonically firing neurones, a stimulus consisting of EPSP bursts produced a brief increase in dendritic Ca(2+) concentration and a persistent increase in the number of spikes elicited by a depolarizing test pulse, along with a decrease in spike threshold. In intrinsically bursting DCN neurones, EPSP bursts induced an increase in the number of depolarization-evoked spikes in some neurones, but in others produced a change to a more tonic firing pattern. Application of IPSP bursts evoked a large number of rebound spikes and an associated dendritic Ca(2+) transient, which also produced a persistent increase in the number of spikes elicited by a test pulse. Intracellular perfusion of the Ca(2+) chelator BAPTA prevented the increase in intrinsic excitability. Thus, rapid changes in intrinsic excitability in the DCN may be driven by bursts of both EPSPs and IPSPs, and may result in persistent changes to both firing frequency and pattern.

Expression of α-7 nAChRs on spinal cord–brainstem neurons controlling inspiratory drive to the diaphragm

In the present study, we determined whether alpha-7 subunit containing nicotinic acetylcholine receptors (nAChRs) are expressed by neurons within the pre-Botzinger complex (pre-BotC), bulbospinal, and phrenic motor nuclei in the rat. alpha-7 Immunohistochemistry combined with cholera toxin B (CTB), a retrograde tracer was used to detect expression of alpha-7 nAChRs by phrenic motor and bulbospinal neurons. Neurokinin-1 receptor immunoreactivity was used as a marker for pre-BotC neurons. Of the CTB-positive neurons in the phrenic nuclei, 60% exhibited immunoreactivity for alpha-7 nAChRs. Of the bulbospinal neurons in the paramedian reticular nuclei (PMn), gigantocellular nuclei (Gi), raphe nuclei, rostral ventrolateral medulla (RVLM) and nucleus tractus solitarius, 20-50% were found to express alpha-7 nAChR immunoreactivity. Of the peudorabies virus (PRV) labeled bulbospinal neurons in PMn, Gi, raphe and RVLM, 9-12% co-expressed alpha-7 nAChRs. Immunoreactivity for alpha-7 nAChRs was also detected in 57% of the neurokinin-1 receptor containing neurons presumed to reside in pre-BotC. These findings suggest that nicotinic cholinergic regulation of the chest wall pumping muscles may occur at multiple levels of the central nervous system.