Axonal projections from the pontine pneumotaxic region to the nucleus raphe magnus in cats

Lesion of Serotonergic Afferents to the Retrotrapezoid Nucleus Impairs the Tachypneic Response to Hypercapnia in Unanesthetized Animals

Hypercapnia promotes an increase in pulmonary ventilation due to the stimulation of brainstem chemosensory cells that are connected to the respiratory network. Among these cells are the raphe serotonergic neurons which widely send projections to distinct central respiratory compartments. Nevertheless, the physiological role of specific raphe serotonergic projections to other chemosensitive sites on the emergence of hypercapnia ventilatory response in vivo still remains to be elucidated. Here we investigated whether the ventilatory response to hypercapnia requires serotonergic inputs to the chemosensitive cells of the retrotrapezoid nucleus (RTN) in the ventrolateral medulla. To test this, pulmonary ventilation was evaluated under baseline conditions and during hypercapnia (7% CO₂) in unanesthetized juvenile Holtzman rats (60-90 g) that received bilateral microinjections of either vehicle (control) or anti-SERT-SAP (0.1 mM, 10 pmol/100 nl) toxin in the RTN to retrogradely destroy serotonergic afferents to this region. Fifteen days after microinjections, baseline ventilation was not different between anti-SERT-SAP (n = 8) and control animals (n = 9). In contrast, the ablation of RTN-projecting serotonergic neurons markedly attenuated the hypercapnia-induced increase in respiratory frequency which was correlated with reduced numbers of serotonergic neurons in the raphe obscurus and magnus, but not in the raphe pallidus. The increase in tidal volume during hypercapnia was not significantly affected by anti-SERT-SAP microinjections in the RTN. Our data indicate that serotoninergic neurons that send projections to the RTN region are required for the processing of ventilatory reflex response during exposure to high CO₂ in unanesthetized conditions.

Neurochemistry of the Kölliker-Fuse nucleus from a respiratory perspective

The Kölliker-Fuse nucleus (KF) is a functionally distinct component of the parabrachial complex, located in the dorsolateral pons of mammals. The KF has a major role in respiration and upper airway control. A comprehensive understanding of the KF and its contributions to respiratory function and dysfunction requires an appreciation for its neurochemical characteristics. The goal of this review is to summarize the diverse neurochemical composition of the KF, focusing on the neurotransmitters, neuromodulators, and neuropeptides present. We also include a description of the receptors expressed on KF neurons and transporters involved in each system, as well as their putative roles in respiratory physiology. Finally, we provide a short section reviewing the literature regarding neurochemical changes in the KF in the context of respiratory dysfunction observed in SIDS and Rett syndrome. By over-viewing the current literature on the neurochemical composition of the KF, this review will serve to aid a wide range of topics in the future research into the neural control of respiration in health and disease.

A Subregion of the Parabrachial Nucleus Partially Mediates Respiratory Rate Depression from Intravenous Remifentanil in Young and Adult Rabbits

BACKGROUND

The efficacy of opioid administration to reduce postoperative pain is limited by respiratory depression. We investigated whether clinically relevant opioid concentrations altered the respiratory pattern in the parabrachial nucleus, a pontine region contributing to respiratory pattern generation, and compared these effects with a medullary respiratory site, the pre-Bötzinger complex.

METHODS

Studies were performed in 40 young and 55 adult artificially ventilated, decerebrate rabbits. We identified an area in the parabrachial nucleus where α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid microinjections elicited tachypnea. Two protocols were performed in separate sets of animals. First, bilateral microinjections of the μ-opioid receptor agonist [D-Ala, N-MePhe, Gly-ol]-enkephalin (100 μM) into the "tachypneic area" determined the effect of maximal μ-opioid receptor activation. Second, respiratory rate was decreased with continuous IV infusions of remifentanil. The opioid antagonist naloxone (1 mM) was then microinjected bilaterally into the "tachypneic area" of the parabrachial nucleus to determine whether the respiratory rate depression could be locally reversed.

RESULTS

Average respiratory rate was 27 ± 10 breaths/min. First, [D-Ala, N-MePhe, Gly-ol]-enkephalin injections decreased respiratory rate by 62 ± 20% in young and 45 ± 26% in adult rabbits (both P < 0.001). Second, during IV remifentanil infusion, bilateral naloxone injections into the "tachypneic area" of the parabrachial nucleus reversed respiratory rate depression from 55 ± 9% to 20 ± 14% in young and from 46 ± 20% to 18 ± 27% in adult rabbits (both P < 0.001). The effects of bilateral [D-Ala, N-MePhe, Gly-ol]-enkephalin injection and IV remifentanil on respiratory phase duration in the "tachypneic area" of the parabrachial nucleus was significantly different from the pre-Bötzinger complex.

CONCLUSIONS

The "tachypneic area" of the parabrachial nucleus is highly sensitive to μ-opioid receptor activation and mediates part of the respiratory rate depression by clinically relevant administration of opioids.

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

Kölliker–Fuse neurons send collateral projections to multiple hypoxia-activated and nonactivated structures in rat brainstem and spinal cord

The Kölliker–Fuse nucleus (KFN) in dorsolateral pons has been implicated in many physiological functions via its extensive efferent connections. Here, we combine iontophoretic anterograde tracing with posthypoxia c-Fos immunohistology to map KFN axonal terminations among hypoxia-activated/nonactivated brain stem and spinal structures in rats. Using a set of stringent inclusion/exclusion criteria to align visualized axons across multiple coronal brain sections, we were able to unequivocally trace axonal trajectories over a long rostrocaudal distance perpendicular to the coronal plane. Structures that were both richly innervated by KFN axonal projections and immunopositive to c-Fos included KFN (contralateral side), ventrolateral pontine area, areas ventral to rostral compact/subcompact ambiguus nucleus, caudal (lateral) ambiguus nucleus, nucleus retroambiguus, and commissural–medial subdivisions of solitary tract nucleus. The intertrigeminal nucleus, facial and hypoglossal nuclei, retrotrapezoid nucleus, parafacial region and spinal cord segment 5 were also richly innervated by KFN axonal projections but were only weakly (or not) immunopositive to c-Fos. The most striking finding was that some descending axons from KFN sent out branches to innervate multiple (up to seven) pontomedullary target structures including facial nucleus, trigeminal sensory nucleus, and various parts of ambiguus nucleus and its surrounding areas. The extensive axonal fan-out from single KFN neurons to multiple brainstem and spinal cord structures("one-to-many relationship"’) provides anatomical evidence that KFN may coordinate diverse physiological functions including hypoxic and hypercapnic respiratory responses, respiratory pattern generation and motor output,diving reflex, modulation of upper airways patency,coughing and vomiting abdominal expiratory reflex, as well as cardiovascular regulation and cardiorespiratory coupling.

Hypoxia-excited neurons in NTS send axonal projections to Kölliker-Fuse/parabrachial complex in dorsolateral pons

Hypoxic respiratory and cardiovascular responses in mammals are mediated by peripheral chemoreceptor afferents which are relayed centrally via the solitary tract nucleus (NTS) in dorsomedial medulla to other cardiorespiratory-related brainstem regions such as ventrolateral medulla (VLM). Here, we test the hypothesis that peripheral chemoafferents could also be relayed directly to the Kölliker-Fuse/parabrachial complex in dorsolateral pons, an area traditionally thought to subserve pneumotaxic and cardiovascular regulation. Experiments were performed on adult Sprague-Dawley rats. Brainstem neurons with axons projecting to the dorsolateral pons were retrogradely labeled by microinjection with choleras toxin subunit B (CTB). Neurons involved in peripheral chemoreflex were identified by hypoxia-induced c-Fos expression. We found that double-labeled neurons (i.e. immunopositive to both CTB and c-Fos) were localized mostly in the commissural and medial subnuclei of NTS and to a lesser extent in the ventrolateral NTS subnucleus, VLM and ventrolateral pontine A5 region. Extracellular recordings from the commissural and medial NTS subnuclei revealed that some hypoxia-excited NTS neurons could be antidromically activated by electrical stimulations at the dorsolateral pons. These findings demonstrate that hypoxia-activated afferent inputs are relayed to the Kölliker-Fuse/parabrachial complex directly via the commissural and medial NTS and indirectly via the ventrolateral NTS subnucleus, VLM and A5 region. These pontine-projecting peripheral chemoafferent inputs may play an important role in the modulation of cardiorespiratory regulation by dorsolateral pons.

Pontine-ventral respiratory column interactions through raphe circuits detected using multi-array spike train recordings

Recently, Segers et al. identified functional connectivity between the ventrolateral respiratory column (VRC) and the pontine respiratory group (PRG). The apparent sparseness of detected paucisynaptic interactions motivated consideration of other potential functional pathways between these two regions. We report here evidence for "indirect" serial functional linkages between the PRG and VRC via intermediary brain stem midline raphé neurons. Arrays of microelectrodes were used to record sets of spike trains from a total of 145 PRG, 282 VRC, and 340 midline neurons in 11 decerebrate, vagotomized, neuromuscularly blocked, ventilated cats. Spike trains of 13,843 pairs of neurons that included at least one raphé cell were screened for respiratory modulation and short-time scale correlations. Significant correlogram features were detected in 7.2% of raphé-raphé (291/4,021), 4.3% of VRC-raphé (292/6,755), and 4.0% of the PRG-raphé (124/3,067) neuron pairs. Central peaks indicative of shared influences were the most common feature in correlations between pairs of raphé neurons, whereas correlated raphé-PRG and raphé-VRC neuron pairs displayed predominantly offset peaks and troughs, features suggesting a paucisynaptic influence of one neuron on the other. Overall, offset correlogram features provided evidence for 33 VRC-to-raphé-to-PRG and 45 PRG-to-raphé-to-VRC correlational linkage chains with one or two intermediate raphé neurons. The results support a respiratory network architecture with parallel VRC-to-PRG and PRG-to-VRC links operating through intervening midline circuits, and suggest that raphé neurons contribute to the respiratory modulation of PRG neurons and shape the respiratory motor pattern through coordinated divergent actions on both the PRG and VRC.

5-HT1A, but not 5-HT2 and 5-HT7, receptors in the nucleus raphe magnus modulate hypoxia-induced hyperpnoea

AIM

In the present study, we assessed the role of 5-hydroxytryptamine (5-HT) receptors (5-HT(1A), 5-HT(2) and 5-HT(7)) in the nucleus raphe magnus (NRM) on the ventilatory and thermoregulatory responses to hypoxia.

METHODS

To this end, pulmonary ventilation (V(E)) and body temperature (T(b)) of male Wistar rats were measured in conscious rats, before and after a 0.1 microL microinjection of WAY-100635 (5-HT(1A) receptor antagonist, 3 microg 0.1 microL(-1), 56 mm), ketanserin (5-HT(2) receptor antagonist, 2 microg 0.1 microL(-1), 36 mm) and SB269970 (5-HT(7) receptor antagonist, 4 microg 0.1 microL(-1), 103 mm) into the NRM, followed by 60 min of severe hypoxia exposure (7% O(2)).

RESULTS

Intra-NMR microinjection of vehicle (control rats) or 5-HT antagonists did not affect V(E) or T(b) during normoxic conditions. Exposure of rats to 7% O(2) evoked a typical hypoxia-induced anapyrexia after vehicle microinjections, which was not affected by microinjection of WAY-100635, SB269970 or ketanserin. The hypoxia-induced hyperpnoea was not affected by SB269970 and ketanserin intra-NMR. However, the treatment with WAY-100635 intra-NRM attenuated the hypoxia-induced hyperpnoea.

CONCLUSION

These data suggest that 5-HT acting on 5-HT(1A) receptors in the NRM increases the hypoxic ventilatory response.

Raphe magnus nucleus is involved in ventilatory but not hypothermic response to CO2

There is evidence that serotonin [5-hydroxytryptamine (5-HT)] is involved in the physiological responses to hypercapnia. Serotonergic neurons represent the major cell type (comprising 15-20% of the neurons) in raphe magnus nucleus (RMg), which is a medullary raphe nucleus. In the present study, we tested the hypothesis 1) that RMg plays a role in the ventilatory and thermal responses to hypercapnia, and 2) that RMg serotonergic neurons are involved in these responses. To this end, we microinjected 1) ibotenic acid to promote nonspecific lesioning of neurons in the RMg, or 2) anti-SERT-SAP (an immunotoxin that utilizes a monoclonal antibody to the third extracellular domain of the serotonin reuptake transporter) to specifically kill the serotonergic neurons in the RMg. Hypercapnia caused hyperventilation and hypothermia in all groups. RMg nonspecific lesions elicited a significant reduction of the ventilatory response to hypercapnia due to lower tidal volume (Vt) and respiratory frequency. Rats submitted to specific killing of RMg serotonergic neurons showed no consistent difference in ventilation during air breathing but had a decreased ventilatory response to CO(2) due to lower Vt. The hypercapnia-induced hypothermia was not affected by specific or nonspecific lesions of RMg serotonergic neurons. These data suggest that RMg serotonergic neurons do not participate in the tonic maintenance of ventilation during air breathing but contribute to the ventilatory response to CO(2). Ultimately, this nucleus may not be involved in the thermal responses to CO(2).

Purinergic modulation of respiration via medullary raphe nuclei in rats

The involvement of P2X receptors in raphe nuclei in respiratory control was investigated. Experiments were done on urethane anesthetized, spontaneously breathing or paralyzed and artificially ventilated adult rats. We found that microinjection of ATP (0.1-0.2 M, 10-70 nl) into raphe magnus (RM) caused dose-dependent decreases in integrated phrenic amplitude and respiratory frequency, whereas injection of ATP into raphe pallidus (RP) caused dose-dependent increases in phrenic amplitude and respiratory frequency. Microinjection of pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (0.02 M, 50 nl), a broad-spectrum P2X receptor antagonist, into the RM or RP did not cause any significant change in respiration, but partially blocked the respiratory effects of ATP that was subsequently injected into the same sites within the RM or RP. These findings indicate that the ATP-P2X mediated neurotransmission could contribute to the respiratory control by affecting the activities of raphe nuclei.

Cytoarchitecture of pneumotaxic integration of respiratory and nonrespiratory information in the rat

The "pneumotaxic center" in the Kölliker-Fuse and medial parabrachial nuclei of dorsolateral pons (dl-pons) plays an important role in respiratory phase switching, modulation of respiratory reflex, and rhythmogenesis. Recent electrophysiological and neural tracing data implicate additional pneumotaxic nuclei in (and a broader role for) the dl-pons in integrating respiratory and nonrespiratory information. Here, we examined the cytoarchitecture of the greater pneumotaxic center and its integrating function by using combined extracellular recording and juxtacellular labeling of unit respiratory rhythmic neurons in dl-pons in urethane-anesthetized, vagotomized, paralyzed, and servo-ventilated adult Sprague Dawley rats. Perievent histogram analysis identified four major types of neuronal discharge patterns: inspiratory, expiratory (with three subdivisions), inspiratory-expiratory, and expiratory-inspiratory phase spanning, sometimes with mild tonic background activity. Most recorded neurons were localized in the Kölliker-Fuse and medial parabrachial nuclei, but some were also found in lateral parabrachial nucleus, intertrigeminal nucleus, principal trigeminal sensory nucleus, and supratrigeminal nucleus. The majority of labeled neurons had large and spatially extended dendritic trees that spanned several of these dl-pons subnuclei, often with terminal dendrites ending in the ventral spinocerebellar tract. The distal sections of the primary and higher-order dendrites exhibited rich varicosities, sometimes with dendritic spines. Axons of some labeled neurons were traced all the way to the ventrolateral pons (vl-pons). These findings extend and generalize the classical definition of the pneumotaxic center to include extensive somatic-axonal-dendritic integration of complex descending and ascending respiratory information as well as nociceptive and possibly musculoskeletal and trigeminal information in multiple dl-pons and vl-pons structures in the rat.

Distribution and medullary projection of respiratory neurons in the dorsolateral pons of the rat

The dorsolateral pons around the parabrachial nucleus including the Kölliker-Fuse nucleus is closely linked with the medullary respiratory center and plays an important role in respiratory control. We aimed to elucidate the firing properties, detailed distributions, and medullary projections of pontine respiratory neurons in pentobarbitone-anesthetized, paralyzed, and artificially ventilated rats with intact vagi. A total of 235 respiratory neurons were recorded from the dorsolateral pons in and around the Kölliker-Fuse nucleus. Six types of firing patterns were identified: inspiratory, expiratory-inspiratory phase spanning, inspiratory-expiratory phase spanning, decrementing expiratory, augmenting expiratory, and whole-phase expiratory patterns. Of these, the inspiratory neurons and the expiratory-inspiratory phase spanning neurons, which constituted the largest population (61%), were characterized most carefully by changing lung inflation levels, since under some conditions both showed similar firing patterns. Many (58%) of the 133 respiratory neurons examined were antidromically activated by electrical stimulation of the medulla. They were activated from the ventrolateral medulla around the ventral respiratory group and the Bötzinger complex and from the dorsomedial medulla around the nucleus tractus solitarii and the hypoglossal nucleus. The projections to the dorsomedial medulla were bilateral in many cases, and those to the ventrolateral medulla were unilateral. Of these medullary projections, two specific projections could be characterized in detail. First, many expiratory-inspiratory phase spanning neurons projected to the hypoglossal nucleus, suggesting that these pontine neurons are important premotor neurons of the hypoglossal motoneurons. This projection explains well the hypoglossal inspiratory activity, which is often dissociated from the phrenic inspiratory activity. Second, most whole-phase expiratory neurons that were distributed medially to the KF nucleus sent their axons toward the spinal cord via the midline medulla. These findings provide a new insight into the pontine control of medullary and spinal respiratory function.

Nitric oxide pathway in the nucleus raphe magnus modulates hypoxic ventilatory response but not anapyrexia in rats

Nucleus raphe magnus (NRM) is one of the cellular groups of the brainstem that is involved in the physiologic responses to hypoxia and contains nitric oxide (NO) synthase. In the present study, we assessed the role of NO pathway in the NRM on the hypoxic ventilatory response (HVR) and anapyrexia (a regulated decrease in body temperature). To this end, pulmonary ventilation (VE) and body temperature (Tb) of male Wistar rats were measured before and after microinjection of N-monomethyl-L-arginine (L-NMMA, a nonselective nitric oxide synthase inhibitor, 12.5 microg/0.1 microl) into the NRM, followed by hypoxia. Control rats received microinjection of saline. Under resting conditions, L-NMMA treatment did not affect pulmonary VE or Tb. Typical hypoxia-induced hyperventilation and anapyrexia were observed after saline treatment. L-NMMA into the NRM reduced the HVR but did not affect hypoxia-induced anapyrexia. In conclusion, the present study indicates that NO in the NRM is involved in HVR, exerts an inhibitory modulation on the NRM neurons but does not mediate hypoxia-induced anapyrexia.

Medullary raphe neuron activity is altered during fictive cough in the decerebrate cat

Chemical lesions in the medullary raphe nuclei region influence cough. This study examined whether firing patterns of caudal medullary midline neurons were altered during cough. Extracellular neuron activity was recorded with microelectrode arrays in decerebrated, neuromuscular-blocked, ventilated cats. Cough-like motor patterns (fictive cough) in phrenic and lumbar nerves were elicited by mechanical stimulation of the intrathoracic trachea. Discharge patterns of respiratory and nonrespiratory-modulated neurons were altered during cough cycles (58/133); 45 increased and 13 decreased activity. Fourteen cells changed firing rate during the inspiratory and/or expiratory phases of cough. Altered patterns in 43 cells were associated with the duration of, or extended beyond, the cough episodes. The different response categories suggest that multiple factors influence the discharge patterns during coughing: e.g., respiratory-modulated and tonic inputs and intrinsic connections. These results suggest involvement of midline neurons (i.e., raphe nuclei) in the cough reflex.

Plasticity in respiratory motor control: intermittent hypoxia and hypercapnia activate opposing serotonergic and noradrenergic modulatory systems

Experimental results consistently show that the respiratory control system is plastic, such that environmental factors and experience can modify its performance. Such plasticity may represent basic neurobiological principles of learning and memory, whereby intermittent sensory stimulation produces long-term alterations (i.e. facilitation or depression) in synaptic transmission depending on the timing and intensity of the stimulation. In this review, we propose that intermittent chemosensory stimulation produces long-term changes in respiratory motor output via specific neuromodulatory systems. This concept is based on recent data suggesting that intermittent hypoxia produces a net long-term facilitation of respiratory output via the serotonergic system, whereas intermittent hypercapnia produces a net long-term depression by a mechanism associated with the noradrenergic system. There is suggestive evidence that, although both respiratory stimuli activate both modulatory systems, the balance is different. Thus, these opposing modulatory influences on respiratory motor control may provide a 'push-pull' system, preventing unchecked and inappropriate fluctuations in ventilatory drive.

Parabrachial area and nucleus raphe magnus inhibition of corneal units in rostral and caudal portions of trigeminal subnucleus caudalis in the rat

The cornea has been used extensively as a means to selectively stimulate trigeminal nociceptive neurons. The aim of this study was to determine the effects of descending modulatory control pathways on corneal unit activity by comparing the effects of conditioning stimulation of the pontine parabrachial area (PBA CS) and nucleus raphe magnus (NRM CS). Electrical stimulation of the cornea at A- and C-fiber intensities was used to activate neurons in two regions of the trigeminal spinal nucleus, the subnucleus interpolaris/caudalis transition (Vi/Vc, 'rostral units') and laminae I-II at the subnucleus caudalis/cervical cord transition (Vc/C1, 'caudal units'), in chloralose-anesthetized rats. Corneal units were further classified according to convergent cutaneous receptive field properties and PBA projection status. None of 48 rostral and 23/28 caudal units projected to the ipsilateral or contralateral PBA. PBA CS inhibited the cornea-evoked responses (<75% change from control) of approximately 65% of rostral and caudal units regardless of neuronal class. For rostral corneal units, PBA CS inhibited A- and C-fiber input equally (15+/-3 and 18+/-14% of control, respectively), whereas among caudal units, A-fiber input was inhibited more than C-fiber input (26+/-5 and 64+/-12% of control, respectively, P<0.01). The magnitude of NRM CS inhibition on cornea-evoked activity of both rostral and caudal units was not different from that seen after PBA CS. Glutamate microinjections into PBA also inhibited rostral and caudal corneal units (6/9 tested). These results indicate that corneal input to rostral and caudal units is modified by activation of descending controls from the PBA and NRM. The significance for processing corneal sensory information is discussed in terms of functional differences between rostral and caudal neurons.

Cardiorespiratory responses to glutamate microinjected into the medullary raphé

The involvement of the medullary raphé in modulating cardiorespiratory activity was examined by microinjecting L-glutamate (L-Glu) into the raphé of rats. Animals were vagotomized, paralyzed, artificially ventilated, maintained at 37 degrees C, and instrumented to record arterial blood pressure (BP) and phrenic nerve activity (PNA). Mock cerebrospinal fluid (mCSF, 10 nl, pH 7.4; control) and L-Glu dissolved in mCSF (10, 100, 1000 mM; 10 nl; pH 7.4; experimental) were microinjected into the raphé. L-Glu affected both BP and PNA in a dose dependent manner. Blood pressure was reduced by 6.30 +/- 0.97 and 12.98 +/- 1.29% by 100 and 1000 mM L-Glu, respectively, without affecting heart rate. PNA increased by 23 and 38% with 100 and 1000 mM L-Glu, respectively. Mock CSF and 10 mM L-Glu had no effect. It is concluded that there are sites in the medullary raphé that affect blood pressure only and other sites which can affect both blood pressure and respiration.

Projection of respiratory neurons in rat medullary raphé nuclei to the phrenic nucleus

The present study was undertaken to investigate firing patterns, locations, and projections to the phrenic motor nucleus of respiratory neurons in medullary raphe nuclei of rat. Experiments were performed on spontaneously breathing rats anesthetized with sodium pentobarbital. Extracellular spikes of single respiratory neurons were explored in midline medullary tegmentum. A total of 107 respiratory neurons was recorded in the raphe magnus, obscurus and pallidus. They were classified into the following eight types based on the relation of their firing patterns to the phase of respiration: (1) Inspiratory (I) throughout (n = 42); (2) I-late (n = 9); (3) I-decrementing (n = 1); (4) Pre-I (n = 2); (5) I-frequency modulated (n = 13); (6) Post-I (n = 12); (7) Expiratory (E) (n = 23) and (8) E-frequency modulated neurons (n = 5). Twenty of the 45 respiratory neurons examined were antidromically activated from the phrenic motor nucleus at the C4 spinal level with thresholds of 2-58 microA and latencies of 0.4-2.4 ms. Among the 20 neurons, 11 neurons were I-throughout, five were I-frequency modulated and four were E neurons. These results suggest that there is a population of neurons in the medullary raphe nuclei that projects to the phrenic motor nucleus at the C4 spinal level. It is possible that this projection may, in part, mediate the control of the diaphragmatic muscle motor neurons located in the C4 segments.

Vestibular nucleus projections to the parabrachial nucleus in rabbits: implications for vestibular influences on the autonomic nervous system

Acute vestibular dysfunction and motion sickness are characterized by autonomic effects such as pallor, nausea, and vomiting. Previous anatomic and physiologic studies suggest that one potential mediator of these effects may be light, direct vestibular nuclear projections to the nucleus tractus solitarius and the dorsal motor nucleus of the vagus nerve. This study presents evidence for relatively dense, direct projections from the vestibular nuclei to the parabrachial nucleus. Male albino rabbits received injections of Phaseolus vulgaris leucoagglutinin into the vestibular nuclei. The tracer was visualized immunocytochemically with standard techniques. Anterogradely labeled axons were traced bilaterally from the vestibular nuclei to the parabrachial nuclear complex, where they terminated around somata in the Kölliker-Fuse nucleus, external medial parabrachial nucleus, medial parabrachial nucleus, and lateral parabrachial nucleus. Less dense terminations were observed in the ventrolateral aspect of the medullary reticular formation, the subtrigeminal nucleus, lateral tegmental field, and nucleus ambiguus. These findings have several important implications. First, they suggest that vestibular input converges directly at brain stem levels with visceral sensory input in both nucleus of the solitary tract and the parabrachial complex. Second, they suggest that vestibular input influences brain stem autonomic outflow via two parallel pathways: (1) direct, light projections to the nucleus of the solitary tract, dorsal motor nucleus of the vagus nerve, and ventrolateral medullary reticular formation; and (2) denser projection to regions of the parabrachial nucleus that project to these brain stem regions. Finally, since the parabrachial nucleus regions that receive vestibular input also project to the hypothalamus and the insular and infralimbic prefrontal cortex, the parabrachial nucleus may serve as an important relay and integrative structure for the cognitive impairment and vegetative symptoms associated with motion sickness, vestibular dysfunction, and responses to altered gravitational environments.

Afferent projections to the Bötzinger complex from the upper cervical cord and other respiratory related structures in the brainstem in cats: retrograde WGA-HRP tracing

Following injection of WGA-HRP (30-60 nl, 5%) into the Bötzinger complex (Böt.c), a group of expiratory neurons in the vicinity of the retrofacial nucleus, a number of labeled neurons were observed, predominantly ipsilaterally, in the intermediate zone of the upper cervical cord at the C1 and C2 segments, the retrotrapezoid nucleus (RTN) in the ventrolateral medulla and the parabrachial-Kölliker-Fuse nuclear complex in the rostral pons. In addition, clusters of labeled cells were also observed in and around the solitary tract nucleus, nuclei ambiguus and retroambiguus, and nucleus raphe magnus. Control injections into the magnocellular tegmental field adjacent to the Böt.c resulted in a diffuse distribution of labeled neurons in the reticular formation. These results demonstrate that the Böt.c receives convergent monosynaptic axonal projections from the upper cervical spinal cord, the pontine pneumotaxic area, the RTN and several other respiratory related structures in the medulla.

Organization of the efferent projections from the spinal cervical enlargement to the parabrachial area and periaqueductal gray: a PHA-L study in the rat

The organization of efferent projections from the spinal cervical enlargement to the parabrachial (PB) area and the periaqueductal gray (PAG) was studied in the rat by using microinjections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into different laminae around the C7 level. The results demonstrated two areas of cervical enlargement which project in different ways to the PB area and PAG. First, the superficial laminae (I, II) showed a very dense projection, with a clear contralateral dominance at the coronal level where the inferior colliculus merges with the pons, to a restricted "superficial" portion of the PB area, namely the lateral crescent area, the dorsal lateral, the superior lateral (PBsl), and the outer portion of the external lateral PB subnuclei. Less dense projections were observed in the Kölliker-Fuse nucleus (KF) and in the ventrolateral/lateral quadrant of the caudal and mid PAG. By contrast, the labeling was weak or absent in the other PB subnuclei and the outer adjacent regions; in particular, no, or very little, labeling was found in the cuneiform nucleus. The PB area appeared to be the supraspinal target that received the densest projection from laminae I and II. Projections were less dense in the PAG and the thalamus and markedly less in other sites such as the ventrolateral medulla, the subnucleus reticularis dorsalis, and the nucleus of the solitary tract. Second, the reticular portion of lamina V, the medial portion of laminae IV-VI up to X and lamina VIII, showed bilateral projections with a weak ipsilateral dominance and a high to medium density on a very restricted portion of the PB area, namely the internal lateral PB subnucleus. A lesser projection was also observed in the adjacent portion of the PBsl, the KF, and the lateral quadrant of the PAG. These results suggest that signals carried by neurons from lamina I-II converge on a restricted superficial portion of the PB area and the ventral part of the lateral quadrant of the PAG. These results are discussed in the context of the role of the spino-PB and spino-PAG pathways in nociception.

Differential projections to the raphe nuclei from the medial parabrachial-Kölliker-Fuse (NPBM-KF) nuclear complex and the retrofacial nucleus in cats: retrograde WGA-HRP tracing

After injection of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP, 2 x 30 nl) [corrected] into the nucleus raphe magnus (NRM) of 6 cats, a number of retrogradely labelled neurons were observed in the rostral pons, mainly in the pontine pneumotaxic area, i.e., the medial parabrachial and Kölliker-Fuse (NPBM-KF) nuclear complex, and the tegmental field. In addition, a cluster of labelled cells was observed in the retrofacial nucleus (RFN) and its adjacent areas in the medulla. Control injections of the same volume of WGA-HRP into the medullary magnocellular tegmental field (2 mm lateral to the NRM) resulted in a much lower number of labelled neurons in the areas described above, and the labelled cells in the tegmental fields were predominant ipsilateral to the injected side. Injections into the nucleus raphe pallidus caudal to the NRM resulted in a diffuse distribution of labelled neurons, mainly in the tegmental fields of the pons and medulla. This study demonstrates that the NRM receives specific convergent projections from the NPBM-KF complex and the RFN in the medulla. It is suggested that these pathways are involved in the control of respiration.