Neurons In Reticular Formation Research Articles

Mapping the vocal circuitry of Alston's singing mouse with pseudorabies virus.

Vocalizations are often elaborate, rhythmically structured behaviors. Vocal motor patterns require close coordination of neural circuits governing the muscles of the larynx, jaw, and respiratory system. In the elaborate vocalization of Alston's singing mouse (Scotinomys teguina) each note of its rapid, frequency-modulated trill is accompanied by equally rapid modulation of breath and gape. To elucidate the neural circuitry underlying this behavior, we introduced the polysynaptic retrograde neuronal tracer pseudorabies virus (PRV) into the cricothyroid and digastricus muscles, which control frequency modulation and jaw opening, respectively.Each virus singly labels ipsilateral motoneurons (nucleus ambiguus for cricothyroid, and motor trigeminal nucleus for digastricus). We find that the two isogenic viruses heavily and bilaterally colabel neurons in the gigantocellular reticular formation, a putative central pattern generator. The viruses also show strong colabeling in compartments of the midbrain including the ventrolateral periaqueductal gray and the parabrachial nucleus, two structures strongly implicated in vocalizations. In the forebrain, regions important to social cognition and energy balance both exhibit extensive colabeling. This includes the paraventricular and arcuate nuclei of the hypothalamus, the lateral hypothalamus, preoptic area, extended amygdala, central amygdala, and the bed nucleus of the stria terminalis. Finally, we find doubly labeled neurons in M1 motor cortex previously described as laryngeal, as well as in the prelimbic cortex, which indicate these cortical regions play a role in vocal production. The progress of both viruses is broadly consistent with vertebrate-general patterns of vocal circuitry, as well as with circuit models derived from primate literature.

Intrinsic properties and synaptic connectivity of Phox2b-expressing neurons in rat rostral parvocellular reticular formation

The paired-like homeobox 2b gene (Phox2b) is critical for the development of the autonomic nervous system. We have previously demonstrated the distinct characteristics of Phox2b-expressing (Phox2b+) neurons in the reticular formation dorsal to the trigeminal motor nucleus (RdV), which are likely related to jaw movement regulation. In this study, we focused on Phox2b+ neurons in the rostral parvocellular reticular formation (rPCRt), a critical region for controlling orofacial functions, using 2–11-day-old Phox2b-EYFP rats. Most Phox2b+ rPCRt neurons were glutamatergic, but not GABAergic or glycinergic. Approximately 65 % of Phox2b+ rPCRt neurons fired at a low frequency, and approximately 24 % of Phox2b+ rPCRt neurons fired spontaneously, as opposed to Phox2b+ RdV neurons. Stimulation of the RdV evoked inward postsynaptic currents in more than 50 % of Phox2b+ rPCRt neurons, while only one Phox2b+ rPCRt neuron responded to stimulation of the nucleus of the solitary tract. Five of the 10 Phox2b+ neurons sent their axons that ramified within the trigeminal motor nucleus (MoV). Of these, the axons of the two neurons terminated within both the MoV and rPCRt. Our findings suggest that Phox2b+ rPCRt neurons have distinct electrophysiological and synaptic properties that may be involved in the motor control of feeding behavior.

Intrinsic brainstem circuits comprised of Chx10-expressing neurons contribute to reticulospinal output in mice.

Glutamatergic reticulospinal neurons in the gigantocellular reticular nucleus (GRN) of the medullary reticular formation can function as command neurons, transmitting motor commands to spinal cord circuits to instruct movement. Recent advances in our understanding of this neuron-dense region have been facilitated by the discovery of expression of the transcriptional regulator, Chx10, in excitatory reticulospinal neurons. Here, we address the capacity of local circuitry in the GRN to contribute to reticulospinal output. We define two subpopulations of Chx10-expressing neurons in this region, based on distinct electrophysiological properties and soma size (small and large), and show that these populations correspond to local interneurons and reticulospinal neurons, respectively. Using focal release of caged glutamate combined with patch clamp recordings, we demonstrated that Chx10 neurons form microcircuits in which the Chx10 local interneurons project to and facilitate the firing of Chx10 reticulospinal neurons. We discuss the implications of these microcircuits in terms of movement selection.NEW & NOTEWORTHY Reticulospinal neurons in the medullary reticular formation integrate inputs from higher regions to effectively instruct spinal motor circuits. Using photoactivation of neurons in brainstem slices, we studied connectivity of reticular formation neurons that express the transcriptional regulator, Chx10. We show that a subpopulation of these neurons functions as local interneurons that affect descending commands. The results shed light on the internal organization and microcircuit formation of reticular formation neurons.

Extensive Cortical Convergence to Primate Reticulospinal Pathways.

Early evolution of the motor cortex included development of connections to brainstem reticulospinal neurons; these projections persist in primates. In this study, we examined the organization of corticoreticular connections in five macaque monkeys (one male) using both intracellular and extracellular recordings from reticular formation neurons, including identified reticulospinal cells. Synaptic responses to stimulation of different parts of primary motor cortex (M1) and supplementary motor area (SMA) bilaterally were assessed. Widespread short latency excitation, compatible with monosynaptic transmission over fast-conducting pathways, was observed, as well as longer latency responses likely reflecting a mixture of slower monosynaptic and oligosynaptic pathways. There was a high degree of convergence: 56% of reticulospinal cells with input from M1 received projections from M1 in both hemispheres; for SMA, the equivalent figure was even higher (70%). Of reticulospinal neurons with input from the cortex, 78% received projections from both M1 and SMA (regardless of hemisphere); 83% of reticulospinal cells with input from M1 received projections from more than one of the tested M1 sites. This convergence at the single cell level allows reticulospinal neurons to integrate information from across the motor areas of the cortex, taking account of the bilateral motor context. Reticulospinal connections are known to strengthen following damage to the corticospinal tract, such as after stroke, partially contributing to functional recovery. Extensive corticoreticular convergence provides redundancy of control, which may allow the cortex to continue to exploit this descending pathway even after damage to one area.SIGNIFICANCE STATEMENT The reticulospinal tract (RST) provides a parallel pathway for motor control in primates, alongside the more sophisticated corticospinal system. We found extensive convergent inputs to primate reticulospinal cells from primary and supplementary motor cortex bilaterally. These redundant connections could maintain transmission of voluntary commands to the spinal cord after damage (e.g., after stroke or spinal cord injury), possibly assisting recovery of function.

Эффекты стимуляции инфралимбической коры на реакции нейронов каудальной вентролатеральной ретикулярной формации, вызванные ноцицептивным раздражением толстой кишки крысы

The infralimbic cortex of the cerebral hemispheres is characterized by an extensive system of afferent and efferent connections with cortical, subcortical, bainstem structures and the spinal cord, which ensure its participation in the higher mechanisms of regulation of the functions of the visceral sphere. At the same time, the involvement of the infralimbic cortex in the mechanisms of cortical modulation of visceral nociception, both acute and chronic, seems to be poorly understood. In this work, using an experimental model of abdominal pain on two groups of urethane anesthetized rats (normal rats and rats with experimental colitis), we studied the effects of electrical stimulation of the infralimbic cortex on the neuronal (responses of neurons of the caudal ventrolateral reticular formation) and systemic (changes in arterial blood pressure) reactions induced by nociceptive distension of the colorectal region of colon. It has been found that in rats of both groups nociceptive colorectal distension was accompanied by both excitatory and inhibitory reactions of neurons of the ventrolateral reticular formation, as well as depressor changes in blood pressure. Electrical stimulation of the infralimbic cortex increased excitatory and decreased inhibitory neuronal responses to visceral painful stimulation and also enhanced nociceptive depressor responses in both normal rats and rats with colitis. This effect was more pronounced in the group of healthy animals. In general, the influence of the infralimbic cortex on nociceptive neurons of the ventrolateral reticular formation and vascular responses to visceral pain can be considered as facilitating visceral nociception. Quantitative differences in the effects of cortical stimulation in normal rats and rats with colitis are possibly related to differences in the brain processing of acute visceral pain and pain complicated by chronic inflammation. One of the possible mechanisms of cortical influence on visceral nociception may be modulation of the current activity of nociceptive neurons of the caudal ventrolateral reticular formation, directly or indirectly involved in the descending control of visceral pain.

Direct Injection of a Lentiviral Vector Highlights Multiple Motor Pathways in the Rat Spinal Cord.

Introducing proteins of interest into cells in the nervous system is challenging due to innate biological barriers that limit access to most molecules. Injection directly into spinal cord tissue bypasses these barriers, providing access to cell bodies or synapses where molecules can be incorporated. Combining viral vector technology with this method allows for introduction of target genes into nervous tissue for the purpose of gene therapy or tract tracing. Here a virus engineered for highly efficient retrograde transport (HiRet) is introduced at the synapses of propriospinal interneurons (PNs) to encourage specific transport to neurons in the spinal cord and brainstem nuclei. Targeting PNs takes advantage of the numerous connections they receive from motor pathways such as the rubrospinal and reticulospinal tracts, as well as their interconnection with each other throughout spinal cord segments. Representative tracing using the HiRet vector with constitutively active green fluorescent protein (GFP) shows high fidelity details of cell bodies, axons and dendritic arbors in thoracic PNs and in reticulospinal neurons in the pontine reticular formation. HiRet incorporates well into brainstem pathways and PNs but shows age dependent integration into corticospinal tract neurons. In summary, spinal cord injection using viral vectors is a suitable method for introduction of proteins of interest into neurons of targeted tracts.

Calcium-induced calcium release activates spontaneous miniature outward currents in newt medullary reticular formation neurons.

Neurons in the medullary reticular formation are involved in the control of postural and locomotor behaviors in all vertebrates. Reticulospinal neurons in this brain region provide one of the major descending projections to the spinal cord. Although neurons in the newt medullary reticular formation have been extensively studied using in vivo extracellular recordings, little is known of their intrinsic biophysical properties or of the underlying circuitry of this region. Using whole cell patch-clamp recordings in brain slices containing the rostromedial reticular formation from adult male newts, we observed spontaneous miniature outward currents (SMOCs) in ~2/3 of neurons. Although SMOCs superficially resembled inhibitory postsynaptic currents (IPSCs), they had slower risetimes and decay times than spontaneous IPSCs. SMOCs required intracellular Ca2+ release from ryanodine receptors and were also dependent on the influx of extracellular Ca2+. SMOCs were unaffected by apamin but were partially blocked by iberiotoxin and charybdotoxin, indicating that SMOCs were mediated by big-conductance Ca2+-activated K+ channels. Application of the sarco/endoplasmic Ca2+ ATPase inhibitor cyclopiazonic acid blocked the generation of SMOCs and also increased neural excitability. Neurons with SMOCs had significantly broader action potentials, slower membrane time constants, and higher input resistance than neurons without SMOCs. Thus, SMOCs may serve as a mechanism to regulate action potential threshold in a majority of neurons within the newt medullary reticular formation. NEW & NOTEWORTHY The medullary reticular formation exerts a powerful influence on sensorimotor integration and subsequent motor behavior, yet little is known about the neurons involved. In this study, we identify a transient potassium current that regulates action potential threshold in a majority of medullary reticular neurons.

Behavioural state-specific neurons in the mouse medulla involved in sleep-wake switching.

The medullary reticular formation (RF) is involved in the maintenance of several vital physiological functions and level of vigilance. In this study, in nonanesthetised, head-fixed mice, I examined the role of medullary RF neurons in the control of sleep-wake states, that is, wakefulness (W), slow-wave sleep (SWS) and paradoxical (or rapid eye movement) sleep (PS). I showed, for the first time, that the mouse medullary RF contains presumed SWS-promoting, SWS-on neurons that remain silent during W, display a sharp increase in discharge rate at sleep onset, and discharge tonically and selectively during SWS. In addition, I showed the presence in the medullary RF of both PS-on and PS-off neurons, which, respectively, commence discharging or cease firing selectively just prior to, and during, PS. PS-off neurons were located in the raphe nuclei and ventral medulla, while PS-on neurons were found in both the lateral part of the ventral gigantocellular reticular nucleus and the raphe nuclei, as were SWS-on neurons. PS-off and SWS-on neurons appear to play an important role in both the W-SWS and SWS-PS switches, while PS-on and PS-off neurons play an important role in the PS-W switch. The present findings on the trends in spike activity at the transitions from SWS to PS and from PS to W are in line with the reciprocal interaction hypothesis according to which PS occurs as a result of the cessation of discharge of PS-off neurons, while PS ends as a result of the start of discharge of PS-off neurons.

Ocular dipping movements in an electrocution cardiac arrest survivor

Background : Abnormal eye movements can occur in patients with neurological diseases. These have been described in unconscious patients due to anoxic coma and often disappear as the patient regains consciousness. Ocular dipping movements are rarely seen in practice in pediatric patients. Methods : A one year old male patient was brought unconscious with electrocution injury. With resuscitation, he became hemodynamically stable. He could not recognize his mother or respond to visual/auditory stimuli; and had generalized hypertonia. Cranial nerves were normal. He showed abnormal involuntary conjugate vertical saccadic eye movements, consisting of slow downward movement (for 1-2 seconds) followed by a relatively faster return to the primary mid-position. These persisted continuously for 48 hours at irregular intervals of 1-2 seconds. The patient was awake but non-responsive and not interested in surroundings. Results : These eye movements are called ocular dipping (OD) . In a normal person, saccadic movements are meant for stable fixation and are maintained by pause-cell neurons positioned in the pontine raphe. These prevent the occurrence of unwanted saccadic pulses by inhibiting the saccadic premotor burst neurons in the paramedian pontine reticular formation and midbrain. Injury to pause cells leads to extraneous saccades without fixation. It appears that cortical and basal ganglia malfunction causes ocular dipping. OD is mainly seen in anoxic coma and creuztfelt Jacob disease. A 53-year-old man developed ocular dipping following prolonged hypotension.Our patient was in vegetative state with normal respiratory and cardiac function suggesting that major midbrain and medullary centers were intact. Conclusion : The self limiting nature of OD suggests that these are caused by diffuse dysfunction rather than a single structural locus.

Homeostatic Changes in GABA and Acetylcholine Muscarinic Receptors on GABAergic Neurons in the Mesencephalic Reticular Formation following Sleep Deprivation.

We have examined whether GABAergic neurons in the mesencephalic reticular formation (RFMes), which are believed to inhibit the neurons in the pons that generate paradoxical sleep (PS or REMS), are submitted to homeostatic regulation under conditions of sleep deprivation (SD) by enforced waking during the day in mice. Using immunofluorescence, we investigated first, by staining for c-Fos, whether GABAergic RFMes neurons are active during SD and then, by staining for receptors, whether their activity is associated with homeostatic changes in GABAA or acetylcholine muscarinic type 2 (AChM2) receptors (Rs), which evoke inhibition. We found that a significantly greater proportion of the GABAergic neurons were positively stained for c-Fos after SD (∼27%) as compared to sleep control (SC; ∼1%) and sleep recovery (SR; ∼6%), suggesting that they were more active during waking with SD and less active or inactive during sleep with SC and SR. The density of GABAARs and AChM2Rs on the plasma membrane of the GABAergic neurons was significantly increased after SD and restored to control levels after SR. We conclude that the density of these receptors is increased on RFMes GABAergic neurons during presumed enhanced activity with SD and is restored to control levels during presumed lesser or inactivity with SR. Such increases in GABAAR and AChM2R with sleep deficits would be associated with increased susceptibility of the wake-active GABAergic neurons to inhibition from GABAergic and cholinergic sleep-active neurons and to thus permitting the onset of sleep and PS with muscle atonia.

Morphology of Dbx1 respiratory neurons in the preB\xf6tzinger complex and reticular formation of neonatal mice

The relationship between neuron morphology and function is a perennial issue in neuroscience. Information about synaptic integration, network connectivity, and the specific roles of neuronal subpopulations can be obtained through morphological analysis of key neurons within a microcircuit. Here we present morphologies of two classes of brainstem respiratory neurons. First, interneurons derived from Dbx1-expressing precursors (Dbx1 neurons) in the preBötzinger complex (preBötC) of the ventral medulla that generate the rhythm for inspiratory breathing movements. Second, Dbx1 neurons of the intermediate reticular formation that influence the motor pattern of pharyngeal and lingual movements during the inspiratory phase of the breathing cycle. We describe the image acquisition and subsequent digitization of morphologies of respiratory Dbx1 neurons from the preBötC and the intermediate reticular formation that were first recorded in vitro. These data can be analyzed comparatively to examine how morphology influences the roles of Dbx1 preBötC and Dbx1 reticular interneurons in respiration and can also be utilized to create morphologically accurate compartmental models for simulation and modeling of respiratory circuits.

Reticular Formation and Pain: The Past and the Future.

The involvement of the reticular formation (RF) in the transmission and modulation of nociceptive information has been extensively studied. The brainstem RF contains several areas which are targeted by spinal cord afferents conveying nociceptive input. The arrival of nociceptive input to the RF may trigger alert reactions which generate a protective/defense reaction to pain. RF neurons located at the medulla oblongata and targeted by ascending nociceptive information are also involved in the control of vital functions that can be affected by pain, namely cardiovascular control. The RF contains centers that belong to the pain modulatory system, namely areas involved in bidirectional balance (decrease or enhancement) of pain responses. It is currently accepted that the imbalance of pain modulation towards pain facilitation accounts for chronic pain. The medullary RF has the peculiarity of harboring areas involved in bidirectional pain control namely by the existence of specific neuronal populations involved in antinociceptive or pronociceptive behavioral responses, namely at the rostroventromedial medulla (RVM) and the caudal ventrolateral medulla (VLM). Furthermore the dorsal reticular nucleus (also known as subnucleus reticularis dorsalis; DRt) may enhance nociceptive responses, through a reverberative circuit established with spinal lamina I neurons and inhibit wide-dynamic range (WDR) neurons of the deep dorsal horn. The components of the triad RVM-VLM-DRt are reciprocally connected and represent a key gateway for top-down pain modulation. The RVM-VLM-DRt triad also represents the neurobiological substrate for the emotional and cognitive modulation of pain, through pathways that involve the periaqueductal gray (PAG)-RVM connection. Collectively, we propose that the RVM-VLM-DRt triad represents a key component of the “dynamic pain connectome” with special features to provide integrated and rapid responses in situations which are life-threatening and involve pain. The new available techniques in neurobiological studies both in animal and human studies are producing new and fascinating data which allow to understand the complex role of the RF in pain modulation and its integration with several body functions and also how the RF accounts for chronic pain.

Neurons in the pontomedullary reticular formation receive converging inputs from the hindlimb and labyrinth.

The integration of inputs from vestibular and proprioceptive sensors within the central nervous system is critical to postural regulation. We recently demonstrated in both decerebrate and conscious cats that labyrinthine and hindlimb inputs converge onto vestibular nucleus neurons. The pontomedullary reticular formation (pmRF) also plays a key role in postural control, and additionally participates in regulating locomotion. Thus, we hypothesized that like vestibular nucleus neurons, pmRF neurons integrate inputs from the limb and labyrinth. To test this hypothesis, we recorded the responses of pmRF neurons to passive ramp-and-hold movements of the hindlimb and to whole-body tilts, in both decerebrate and conscious felines. We found that pmRF neuronal activity was modulated by hindlimb movement in the rostral-caudal plane. Most neurons in both decerebrate (83% of units) and conscious (61% of units) animals encoded both flexion and extension movements of the hindlimb. In addition, hindlimb somatosensory inputs converged with vestibular inputs onto pmRF neurons in both preparations. Pontomedullary reticular formation neurons receiving convergent vestibular and limb inputs likely participate in balance control by governing reticulospinal outflow.

Functional properties of nociceptive neurons in the nucleus centralis lateralis of the cat thalamus.

We studied functional properties of nociceptive neurons in the thalamic intralaminar nuclei, especially the nucleus centralis lateralis (CL) in urethane-chloralose anesthetized cats. Nociceptive neurons were found in the intralaminar nuclei, i.e. CL, nucleus centralis medialis (CeM) and nucleus parafascicularis (Pf). One third of these nociceptive neurons had their receptive fields throughout the body, including the orofacial area. Namely, they received convergent nociceptive input from the trigeminal and spinal nerve territories. These neurons with widely complex receptive fields were similar to those we found in the middle third of the caudal bulbar reticular formation (RF). Electrical stimulation of the CL antidromically activated about half of the caudal bulbar RF neurons tested. Furthermore, electrical stimulation of the cingulate gyrus antidromically activated some of CL neurons. These findings suggest that most of the CL neurons receiving nociceptive input from throughout the body receive information directly or indirectly via the caudal bulbar RF, and that certain of this information is then conveyed to the cingulate gyrus.

Monoclonal antibody Py recognizes neurofilament heavy chain and is a selective marker for large diameter neurons in the brain.

Almost 30years ago, the monoclonal antibody Py was developed to detect pyramidal neurons in the CA3 region of the rat hippocampus. The utility of this antibody quickly expanded when several groups discovered that it could be used to identify very specific populations of neurons in the normal, developing, and diseased or injured central nervous system. Despite this body of literature, the identity of the antigen that the Py antibody recognizes remained elusive. Here, immunoprecipitation experiments from the adult rat cortex identified the Py antigen as neurofilament heavy chain (NF-H). Double immunolabeling of sections through the rat brain using Py and NF-H antibodies confirmed the identity of the Py antigen, and reveal that Py/NF-H+ neurons appear to share the feature of being particularly large in diameter. These include the neurons of the gigantocellular reticular formation, pyramidal neurons of layers II/III and V of the cortex, cerebellar Purkinje neurons as well as CA3 pyramidal neurons. Taken together, this finding gives clarity to past work using the monoclonal Py antibody, and immediately expands our understanding of the importance of NF-H in neural development, functioning, and disease.

Revisiting Antagonist Effects in Hypoglossal Nucleus: Brainstem Circuit for the State-Dependent Control of Hypoglossal Motoneurons: A Hypothesis.

We reassessed and provided new insights into the findings that were obtained in our previous experiments that employed the injections of combined adrenergic, serotonergic, GABAergic, and glycinergic antagonists into the hypoglossal nucleus in order to pharmacologically abolish the depression of hypoglossal nerve activity that occurred during carbachol-induced rapid-eye-movement (REM) sleep-like state in anesthetized rats. We concluded that noradrenergic disfacilitation is the major mechanism that is responsible for approximately 90% of the depression of hypoglossal motoneurons, whereas the remaining 10% can be explained by serotonergic mechanisms that have net inhibitory effect on hypoglossal nerve activity during REM sleep-like state. We hypothesized that both noradrenergic and serotonergic state-dependent mechanisms indirectly control hypoglossal motoneuron excitability during REM sleep; their activities are integrated and mediated to hypoglossal motoneurons by reticular formation neurons. In addition, we proposed a brainstem neural circuit that can explain the new findings.

Neurochemical changes in the rostral ventromedial nucleus of the medulla oblongata in rats with developing neuropathic pain

The dynamics of neurochemical rearrangements in neurons in the ventromedial reticular formation of the medulla oblongata in rats during the development of neuropathic pain were investigated. Distinct populations of monoaminergic and NOergic projecting neurons and cells that metabolize agmatine were found in the nucleus. The change in the activity of these neurons during the initiation (week 1) and formation (4 weeks) of neuropathic pain has several phases. Decreased synthesis of catecholamines, increased catabolism of agmatine, and activation of NO synthesis were found in neurons by the end of the first week. At 4 weeks after the surgery, the parameters of the catecholaminergic system returned to the control level, NOergic activity was significantly reduced and the catabolism of agmatine continued to increase. These findings suggest the involvement of bulbar NOergic, mono- and polyaminergic neurotransmitter mechanisms in the initiation and maintenance of neuropathic pain.

A Convergent and Essential Interneuron Pathway for Mauthner-Cell-Mediated Escapes

The Mauthner cell (M-cell) is a command-like neuron in teleost fish whose firing in response to aversive stimuli is correlated with short-latency escapes [1-3]. M-cells have been proposed as evolutionary ancestors of startle response neurons of the mammalian reticular formation [4], and studies of this circuit have uncovered important principles in neurobiology that generalize to more complex vertebrate models [3]. The main excitatory input was thought to originate from multisensory afferents synapsing directly onto the M-cell dendrites [3]. Here, we describe an additional, convergent pathway that is essential for the M-cell-mediated startle behavior in larval zebrafish. It is composed of excitatory interneurons called spiral fiber neurons, which project to the M-cell axon hillock. By invivo calcium imaging, we found that spiral fiber neurons are active in response to aversive stimuli capable of eliciting escapes. Like M-cell ablations, bilateral ablations of spiral fiber neurons largely eliminate short-latency escapes. Unilateral spiral fiber neuron ablations shift the directionality of escapes and indicate that spiral fiber neurons excite the M-cell in a lateralized manner. Their optogenetic activation increases the probability of short-latency escapes, supporting the notion that spiral fiber neurons help activate M-cell-mediated startle behavior. These results reveal that spiral fiber neurons are essential for the function of the M-cell in response to sensory cues and suggest that convergent excitatory inputs that differ in their input location and timing ensure reliable activation of the M-cell, a feedforward excitatory motif that may extend to other neural circuits.

Interactions between hypocretinergic and GABAergic systems in the control of activity of neurons in the cat pontine reticular formation

Anatomical studies have demonstrated that hypocretinergic and GABAergic neurons innervate cells in the nucleus pontis oralis (NPO), a nucleus responsible for the generation of active (rapid eye movement (REM)) sleep (AS) and wakefulness (W). Behavioral and electrophysiological studies have shown that hypocretinergic and GABAergic processes in the NPO are involved in the generation of AS as well as W. An increase in hypocretin in the NPO is associated with both AS and W, whereas GABA levels in the NPO are elevated during W. We therefore examined the manner in which GABA modulates NPO neuronal responses to hypocretin. We hypothesized that interactions between the hypocretinergic and GABAergic systems in the NPO play an important role in determining the occurrence of AS or W. To determine the veracity of this hypothesis, we examined the effects of the juxtacellular application of hypocretin-1 and GABA on the activity of NPO neurons, which were recorded intracellularly, in chloralose-anesthetized cats. The juxtacellular application of hypocretin-1 significantly increased the mean amplitude of spontaneous EPSPs and the frequency of discharge of NPO neurons; in contrast, the juxtacellular microinjection of GABA produced the opposite effects, i.e., there was a significant reduction in the mean amplitude of spontaneous EPSPs and a decrease in the discharge of these cells. When hypocretin-1 and GABA were applied simultaneously, the inhibitory effect of GABA on the activity of NPO neurons was reduced or completely blocked. In addition, hypocretin-1 also blocked GABAergic inhibition of EPSPs evoked by stimulation of the laterodorsal tegmental nucleus. These data indicate that hypocretin and GABA function within the context of a neuronal gate that controls the activity of AS-on neurons. Therefore, we suggest that the occurrence of either AS or W depends upon interactions between hypocretinergic and GABAergic processes as well as inputs from other sites that project to AS-on neurons in the NPO.

A significant portion of Dbx1 neurons in the intermediate reticular formation are XII inspiratory premotoneurons

Reduced tonic and inspiratory drive to XII motoneurons (MNs) that innervate the genioglossus muscle (GG, a tongue protruder) is implicated in obstructive sleep apnea. Reduced excitability of glutamatergic inspiratory XII premotoneurons (preMNs) during sleep may contribute. Little is known about XII inspiratory preMNs due to their sparse distribution in the intermediate reticular formation (IRt). We hypothesized that ipsilaterally‐projecting XII inspiratory preMNs derive from Dbx1 progenitors, a transcription factor expressed in the IRt. Rhythmic, transverse medullary slices (550 μm) were produced from Dbx1ERCreT2; R26tdTomato neonatal mice (postnatal day 0‐5). Approximately 20% of the Dbx1 IRt neurons received inspiratory drive. In total we identified 29 inspiratory‐modulated Dbx1 IRt neurons via whole‐cell recording. 10 of 24 neurons were antidromically activated from the ipsilateral XII nucleus. Anatomical reconstruction of biocytin‐filled neurons revealed 10 commissural axons and 4 ipsilateral axons projecting to the XII nucleus. Using in situ hybridization, 57.3% (range 54.0‐58.9%) of Dbx1 IRt neurons were glutamatergic. Laser ablation of individual Dbx1 IRt neurons caused a significant (36 ± 3.6%) drop in ipsilateral, but not contralateral, XII inspiratory burst amplitude, and had no effect on inspiratory burst period. Thus, a subpopulation of Dbx1‐derived neurons in the IRt are XII inspiratory preMNs that contribute significantly to XII inspiratory output. This discovery will aid study of XII inspiratory preMNs. Funding: CIHR, NSERC, WCHRI, AIHS, CFI, NIH.