Thyrotropin-releasing hormone (TRH) depolarizes a subset of inspiratory neurons in the newborn mouse brain stem in vitro

Breathing matters

Breathing is a well-described, vital and surprisingly complex behaviour, with behavioural and physiological outputs that are easy to directly measure. Key neural elements for generating breathing pattern are distinct, compact and form a network amenable to detailed interrogation, promising the imminent discovery of molecular, cellular, synaptic and network mechanisms that give rise to the behaviour. Coupled oscillatory microcircuits make up the rhythmic core of the breathing network. Primary among these is the preBötzinger Complex (preBötC), which is composed of excitatory rhythmogenic interneurons and excitatory and inhibitory pattern-forming interneurons that together produce the essential periodic drive for inspiration. The preBötC coordinates all phases of the breathing cycle, coordinates breathing with orofacial behaviours and strongly influences, and is influenced by, emotion and cognition. Here, we review progress towards cracking the inner workings of this vital core.

Inspiratory-Activated Airway Vagal Preganglionic Neurones Excited by Thyrotropin-Releasing Hormone via Multiple Mechanisms in Neonatal Rats

The airway vagal preganglionic neurons (AVPNs) providing projections to intrinsic tracheobronchial ganglia are considered to be crucial to modulation of airway resistance in physiological and pathological states. AVPNs classified into inspiratory-activated AVPNs (IA-AVPNs) and inspiratory-inhibited AVPNs (II-AVPNs) are regulated by thyrotropin-releasing hormone (TRH)-containing terminals. TRH causes a direct excitatory current and attenuates the phasic inspiratory glycinergic inputs in II-AVPNs, however, whether and how TRH influences IA-AVPNs remains unknown. In current study, TRH regulation of IA-AVPNs and its mechanisms involved were investigated. Using retrogradely fluorescent labeling method and electrophysiology techniques to identify IA-AVPNs in brainstem slices with rhythmic inspiratory hypoglossal bursts recorded by a suction electrode, the modulation of TRH was observed with patch-clamp technique. The findings demonstrate that under voltage clamp configuration, TRH (100 nM) caused a slow excitatory inward current, augmented the excitatory synaptic inputs, progressively suppressed the inhibitory synaptic inputs and elicited a distinctive electrical oscillatory pattern (OP). Such a current and an OP was independent of presynaptic inputs. Carbenoxolone (100 μM), a widely used gap junction inhibitor, fully suppressed the OP with persistence of TRH-induced excitatory slow inward current and augment of the excitatory synaptic inputs. Both tetrodotoxin (1 μM) and riluzole (20 μM) functioned to block the majority of the slow excitatory inward current and prevent the OP, respectively. Under current clamp recording, TRH caused a slowly developing depolarization and continuously progressive oscillatory firing pattern sensitive to TTX. TRH increased the firing frequency in response to injection of a square-wave current. The results suggest that TRH excited IA-AVPNs via the following multiple mechanisms: (1) TRH enhances the excitatory and depresses the inhibitory inputs; (2) TRH induces an excitatory postsynaptic slow inward current; (3) TRH evokes a distinctive OP mediated by gap junction.

Physiological and morphological properties of Dbx1-derived respiratory neurons in the pre-Botzinger complex of neonatal mice

Breathing in mammals depends on an inspiratory-related rhythm that is generated by glutamatergic neurons in the pre-Bötzinger complex (preBötC) of the lower brainstem. A substantial subset of putative rhythm-generating preBötC neurons derive from a single genetic line that expresses the transcription factor Dbx1, but the cellular mechanisms of rhythmogenesis remain incompletely understood. To elucidate these mechanisms, we carried out a comparative analysis of Dbx1-expressing neurons (Dbx1(+)) and non-Dbx1-derived (Dbx1(-)) neurons in the preBötC. Whole-cell recordings in rhythmically active newborn mouse slice preparations showed that Dbx1(+) neurons activate earlier in the respiratory cycle and discharge greater magnitude inspiratory bursts compared with Dbx1(-) neurons. Furthermore, Dbx1(+) neurons required less input current to discharge spikes (rheobase) in the context of network activity. The expression of intrinsic membrane properties indicative of A-current (IA) and hyperpolarization-activated current (Ih) tended to be mutually exclusive in Dbx1(+) neurons. In contrast, there was no such relationship in the expression of currents IA and Ih in Dbx1(-) neurons. Confocal imaging and digital morphological reconstruction of recorded neurons revealed dendritic spines on Dbx1(-) neurons, but Dbx1(+) neurons were spineless. The morphology of Dbx1(+) neurons was largely confined to the transverse plane, whereas Dbx1(-) neurons projected dendrites to a greater extent in the parasagittal plane. The putative rhythmogenic nature of Dbx1(+) neurons may be attributable, in part, to a higher level of intrinsic excitability in the context of network synaptic activity. Furthermore, Dbx1(+) neuronal morphology may facilitate temporal summation and integration of local synaptic inputs from other Dbx1(+) neurons, taking place largely in the dendrites, which could be important for initiating and maintaining bursts and synchronizing activity during the inspiratory phase.

Understanding the rhythm of breathing: so near, yet so far

Breathing is an essential behavior that presents a unique opportunity to understand how the nervous system functions normally, how it balances inherent robustness with a highly regulated lability, how it adapts to both rapidly and slowly changing conditions, and how particular dysfunctions result in disease. We focus on recent advancements related to two essential sites for respiratory rhythmogenesis: (a) the preBötzinger Complex (preBötC) as the site for the generation of inspiratory rhythm and (b) the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) as the site for the generation of active expiration.

Developmental origin of preBötzinger complex respiratory neurons

A subset of preBötzinger Complex (preBötC) neurokinin 1 receptor (NK1R) and somatostatin peptide (SST)-expressing neurons are necessary for breathing in adult rats, in vivo. Their developmental origins and relationship to other preBötC glutamatergic neurons are unknown. Here we show, in mice, that the "core" of preBötC SST(+)/NK1R(+)/SST 2a receptor(+) (SST2aR) neurons, are derived from Dbx1-expressing progenitors. We also show that Dbx1-derived neurons heterogeneously coexpress NK1R and SST2aR within and beyond the borders of preBötC. More striking, we find that nearly all non-catecholaminergic glutamatergic neurons of the ventrolateral medulla (VLM) are also Dbx1 derived. PreBötC SST(+) neurons are born between E9.5 and E11.5 in the same proportion as non-SST-expressing neurons. Additionally, preBötC Dbx1 neurons are respiratory modulated and show an early inspiratory phase of firing in rhythmically active slice preparations. Loss of Dbx1 eliminates all glutamatergic neurons from the respiratory VLM including preBötC NK1R(+)/SST(+) neurons. Dbx1 mutant mice do not express any spontaneous respiratory behaviors in vivo. Moreover, they do not generate rhythmic inspiratory activity in isolated en bloc preparations even after acidic or serotonergic stimulation. These data indicate that preBötC core neurons represent a subset of a larger, more heterogeneous population of VLM Dbx1-derived neurons. These data indicate that Dbx1-derived neurons are essential for the expression and, we hypothesize, are responsible for the generation of respiratory behavior both in vitro and in vivo.

Synaptically activated burst-generating conductances may underlie a group-pacemaker mechanism for respiratory rhythm generation in mammals

Breathing, chewing, and walking are critical life-sustaining behaviors in mammals that consist essentially of simple rhythmic movements. Breathing movements in particular involve the diaphragm, thorax, and airways but emanate from a network in the lower brain stem. This network can be studied in reduced preparations in vitro and using simplified mathematical models that make testable predictions. An iterative approach that employs both in vitro and in silico models argues against canonical mechanisms for respiratory rhythm in neonatal rodents that involve reciprocal inhibition and pacemaker properties. We present an alternative model in which emergent network properties play a rhythmogenic role. Specifically, we show evidence that synaptically activated burst-generating conductances-which are only available in the context of network activity-engender robust periodic bursts in respiratory neurons. Because the cellular burst-generating mechanism is linked to network synaptic drive we dub this type of system a group pacemaker.

Structure-function analysis of rhythmogenic inspiratory pre-Bötzinger complex networks in "calibrated" newborn rat brainstem slices

Inspiratory pre-Bötzinger complex (preBötC) networks remain active in perinatal rodent brainstem slices. Our analysis of (crescendo-like) inspiratory-related population and cellular bursting in novel histologically identified transversal preBötC slices in physiological (3 mM) superfusate [K(+)] revealed: (i) the preBötC extent sufficient for rhythm in thin slices is at most 175 microm. (ii) In 700 microm thick slices with unilaterally exposed preBötC, a <100 microm kernel generates a eupnea-like inspiratory pattern under predominant influence of caudally adjacent structures or thyrotropin-releasing hormone-like transmitters, but a mixed eupnea-sigh-like pattern when influence of rostral structures or substance-P-like transmitters dominates. (iii) Primarily presynaptic processes may underlie inhibition of rhythm by opioids or raising superfusate [Ca(2+)] from lower to upper physiological limits (1-1.5 mM). (iv) High K(+) reverses depression of rhythm by raised Ca(2+), opioids and anoxia. In summary, distinct activity patterns of spatiochemically organized isolated inspiratory networks depend on both an extracellular "Ca(2+)-K(+) antagonism" and slice dimensions. This explains some discrepant findings between studies and suggests use of "calibrated" slices and more uniform experimental conditions.

The chemical neuroanatomy of breathing

The chemical neuroanatomy of breathing must ultimately encompass all the various neuronal elements physiologically identified in brainstem respiratory circuits and their apparent aggregation into "compartments" within the medulla and pons. These functionally defined respiratory compartments in the brainstem provide the major source of input to cranial motoneurons controlling the airways, and to spinal motoneurons activating inspiratory and expiratory pump muscles. This review provides an overview of the neuroanatomy of the major compartments comprising brainstem respiratory circuits, and a synopsis of the transmitters used by their constituent respiratory neurons.

AMPA and metabotropic glutamate receptors cooperatively generate inspiratory-like depolarization in mouse respiratory neurons in vitro

Excitatory transmission mediated by AMPA receptors is critical for respiratory rhythm generation. However, the role of AMPA receptors has not been fully explored. Here we tested the functional role of AMPA receptors in inspiratory neurons of the neonatal mouse preBötzinger complex (preBötC) using an in vitro slice model that retains active respiratory function. Immediately before and during inspiration, preBötC neurons displayed envelopes of depolarization, dubbed inspiratory drive potentials, that required AMPA receptors but largely depended on the Ca(2+)-activated non-specific cation current (I(CAN)). We showed that AMPA receptor-mediated depolarization opened voltage-gated Ca(2+) channels to directly evoke I(CAN). Inositol 1,4,5-trisphosphate receptor-mediated intracellular Ca(2+) release also evoked I(CAN). Inositol 1,4,5-trisphosphate receptors acted downstream of group I metabotropic glutamate receptor activity but, here too, AMPA receptor-mediated Ca(2+) influx was essential to trigger the metabotropic glutamate receptor contribution to inspiratory drive potential generation. This study helps to elucidate the role of excitatory transmission in respiratory rhythm generation in vitro. AMPA receptors in preBötC neurons initiate convergent signaling pathways that evoke post-synaptic I(CAN), which underlies inspiratory drive potentials. The coupling of AMPA receptors with I(CAN) suggests that latent burst-generating intrinsic conductances are recruited by excitatory synaptic interactions among preBötC neurons in the context of respiratory network activity in vitro, exemplifying a rhythmogenic mechanism based on emergent properties of the network.

4-Aminopyridine-sensitive outward currents in preBötzinger complex neurons influence respiratory rhythm generation in neonatal mice

We measured a low-threshold, inactivating K+ current, i.e. A-current (I(A)), in respiratory neurons of the preBötzinger complex (preBötC) in rhythmically active slice preparations from neonatal C57BL/6 mice. The majority of inspiratory neurons (21/34 = 61.8%), but not expiratory neurons (1/8 = 12.5%), expressed I(A). In whole-cell and somatic outside-out patches I(A) activated at -60 mV (half-activation voltage measured -16.3 mV) and only fully inactivated above -40 mV (half-inactivation voltage measured -85.6 mV), indicating that I(A) can influence membrane trajectory at baseline voltages during respiratory rhythm generation in vitro. 4-Aminopyridine (4-AP, 2 mm) attenuated I(A) in both whole-cell and somatic outside-out patches. In the context of rhythmic network activity, 4-AP caused irregular respiratory-related motor output on XII nerves and disrupted rhythmogenesis as detected with whole-cell and field recordings in the preBötC. Whole-cell current-clamp recordings showed that 4-AP changed the envelope of depolarization underlying inspiratory bursts (i.e. inspiratory drive potentials) from an incrementing pattern to a decrementing pattern during rhythm generation and abolished current pulse-induced delayed excitation. These data suggest that I(A) opposes excitatory synaptic depolarizations at baseline voltages of approximately -60 mV and influences the inspiratory burst pattern. We propose that I(A) promotes orderly recruitment of constituent rhythmogenic neurons by minimizing the activity of these neurons until they receive massive coincident synaptic input, which reduces the periodic fluctuations of inspiratory activity.

ATP sensitivity of preBötzinger complex neurones in neonatal rat in vitro: mechanism underlying a P2 receptor-mediated increase in inspiratory frequency

P2 receptor (R) signalling plays an important role in the central ventilatory response to hypoxia. The frequency increase that results from activation of P2Y(1)Rs in the preBötzinger complex (preBötC; putative site of inspiratory rhythm generation) may contribute, but neither the cellular nor ionic mechanism(s) underlying these effects are known. We applied whole-cell recording to rhythmically-active medullary slices from neonatal rat to define, in preBötC neurones, the candidate cellular and ionic mechanisms through which ATP influences rhythm, and tested the hypothesis that putative rhythmogenic preBötC neurones are uniquely sensitive to ATP. ATP (1 mm) evoked inward currents in all non-respiratory neurones and the majority of respiratory neurons, which included inspiratory, expiratory and putative rhythmogenic inspiratory neurones identified by sensitivity to substance P (1 microM) and DAMGO (50 microM) or by voltage-dependent pacemaker-like activity. ATP current densities were similar in all classes of preBötC respiratory neurone. Reversal potentials and input resistance changes for ATP currents in respiratory neurones suggested they resulted from either inhibition of a K(+) channel or activation of a mixed cationic conductance. The P2YR agonist 2MeSADP (1 mm) evoked only the latter type of current in inspiratory and pacemaker-like neurones. In summary, putative rhythmogenic preBötC neurones were sensitive to ATP. However, this sensitivity was not unique; ATP evoked similar currents in all types of preBötC respiratory neurone. The P2Y(1)R-mediated frequency increase is therefore more likely to reflect activation of a mixed cationic conductance in multiple types of preBötC neurone than excitation of one, highly sensitive group.

Anatomical and functional development of the pre-Bötzinger complex in prenatal rodents

Developmental anomalies of central respiratory neural control contribute to newborn mortality and morbidity. Elucidation of the cellular, molecular, trophic, and genetic mechanisms involved in the formation and function of respiratory nuclei during prenatal development will provide a foundation for understanding pathologies. The pre-Bötzinger Complex (pre-BötC) is a specific group of neurons located in the ventrolateral medulla that is critical for respiratory rhythmogenesis. Thus it has become a major focus of research. Here, we provide an overview of current knowledge regarding the anatomical and functional emergence of the rodent pre-BötC during the prenatal period.

High sensitivity to neuromodulator-activated signaling pathways at physiological [K+] of confocally imaged respiratory center neurons in on-line-calibrated newborn rat brainstem slices

The pre-Bötzinger complex (PBC) inspiratory center remains active in a transverse brainstem slice. Such slices are studied at high (8-10 mM) superfusate [K+], which could attenuate the sensitivity of the PBC to neuromodulators such as opiates. Findings may also be confounded because slice boundaries, drug injection sites, or location of rhythmogenic interneurons are rarely verified histologically. Thus, we first generated PBC slices with defined boundaries using novel "on-line histology" based on our finding that rostrocaudal extensions of brainstem respiratory marker nuclei are constant in newborn rats between postnatal days 0-4. At physiological superfusate [K+] (3 mM), 500- and 600-microm-thick slices with the PBC in the center and the caudal boundary 0.70 and 0.76 mm caudal to the facial motonucleus generated rhythm for >2 and approximately 4 h, respectively. Rhythm was abolished by low nanomolar concentrations of the mu-opiate receptor agonist DAMGO ([D-Ala2, N-Me-Phe4, Gly5-ol]enkephalin). After spontaneous arrest of bursting, rhythm was reactivated at clinically relevant or physiological concentrations by 3,5-dihydroxyphenylglycine, thyrotropin-releasing hormone, or rolipram, each affecting distinct second-messenger pathways. Two-photon/confocal Ca2+ imaging revealed that these agents reactivated the same PBC neurons initially active in 3 mM [K+]. The data show that "calibrated" PBC slices at physiological [K+] generate rhythm with a high sensitivity to neuromodulators for extended time periods, whereas spontaneous "in vitro apnea" is an important tool to study the interaction of signaling pathways that modulate rhythm. Our approaches and findings provide the basis for a pharmacological and structure-function analysis of the isolated respiratory center in a histologically well defined substrate at physiological [K+].

Neural tube patterning by Krox20 and emergence of a respiratory control

Recent data begin to bridge the gap between developmental events controlling hindbrain neural tube regional patterning and the emergence of breathing behaviour in the fetus and its vital adaptive function after birth. In vertebrates, Hox paralogs and Hox-regulating genes orchestrate, in a conserved manner, the transient formation of developmental compartments in the hindbrain, the rhombomeres, in which rhythmic neuronal networks of the brainstem develop. Genetic inactivation of some of these genes in mice leads to pathological breathing at birth pointing to the vital importance of rhombomere 3 and 4 derived territories for maintenance of the breathing frequency. In chick embryo at E7, we investigated neuronal activities generated in neural tube islands deriving from combinations of rhombomeres isolated at embryonic day E1.5. Using a gain of function approach, we reveal a role of the transcription factor Krox20, specifying rhombomeres 3 and 5, in inducing a rhythm generator at the parafacial level of the hindbrain. The developmental genes selecting and regionally coordinating the fate of CNS progenitors may hold further clues to conserved aspects of neuronal network formation and function. However, the most immediate concern is to take advantage of early generated rhythmic activities in the hindbrain to pursue their downstream cellular and molecular targets, for it seems likely that it will be here that rhythmogenic properties will eventually take on a vital role at birth.

Thyroliberin blocks the potassium A-current in neurons in the respiratory center of adult rats in vitro

Thyroliberin is a neuropeptide with marked respiratory activity. The neuronal mechanisms underlying this activity were addressed in experiments on transverse slices of brainstem from adult rats in conditions of membrane potential clamping to study effect effects of thyroliberin (10 nM) on the potassium A-current in neurons of two areas of the respiratory center--the ventrolateral areas of the solitary tract nucleus and the pre-Betzinger complex. The A-current, seen in all study neurons in the respiratory center, was partially and reversibly blocked by thyroliberin. A significant reduction in the amplitude of the current was accompanied by an increase in the inactivation constant. The effect of thyroliberin on the amplitude of the A-current was analogous to that of 5 mM 4-aminopyridine. These results show that the stimulatory effects of thyroliberin at the level of respiratory center neurons is at least partly explained by its ability to block the potassium A-current.

Sodium and calcium current-mediated pacemaker neurons and respiratory rhythm generation

The breathing motor pattern in mammals originates in brainstem networks. Whether pacemaker neurons play an obligatory role remains a key unanswered question. We performed whole-cell recordings in the preBotzinger Complex in slice preparations from neonatal rodents and tested for pacemaker activity. We observed persistent Na+ current (I(NaP))-mediated bursting in approximately 5% of inspiratory neurons in postnatal day 0 (P0)-P5 and in P8-P10 slices. I(NaP)-mediated bursting was voltage dependent and blocked by 20 mum riluzole (RIL). We found Ca2+ current (I(Ca))-dependent bursting in 7.5% of inspiratory neurons in P8-P10 slices, but in P0-P5 slices these cells were exceedingly rare (0.6%). This bursting was voltage independent and blocked by 100 microm Cd2+ or flufenamic acid (FFA) (10-200 microm), which suggests that a Ca2+-activated inward cationic current (I(CAN)) underlies burst generation. These data substantiate our observation that P0-P5 slices exposed to RIL contain few (if any) pacemaker neurons, yet maintain respiratory rhythm. We also show that 20 nm TTX or coapplication of 20 microm RIL + FFA (100-200 microm) stops the respiratory rhythm, but that adding 2 mum substance P restarts it. We conclude that I(NaP) and I(CAN) enhance neuronal excitability and promote rhythmogenesis, even if their magnitude is insufficient to support bursting-pacemaker activity in individual neurons. When I(NaP) and I(CAN) are removed pharmacologically, the rhythm can be maintained by boosting neural excitability, which is inconsistent with a pacemaker-essential mechanism of respiratory rhythmogenesis by the preBotzinger complex.

Effects of thyroliberin on membrane potential and the pattern of spontaneous activity of neurons in the respiratory center in in vitro studies in rats

Patch-clamp experiments on transverse brainstem slices from rats were performed to study the effects of thyroliberin (10(-8) M) on the membrane potential and spontaneous activity of neurons in two areas of the respiratory center: the ventrolateral area of the solitary tract nucleus and the pre-Botzinger complex. Thyroliberin induced membrane depolarization of neurons in the respiratory center and increased their spike activity. The pattern of activity of neurons in the pre-Botzinger complex showed decreases in the time intervals between the beginnings of bursts in response to thyroliberin. In some cases, thyroliberin led to the appearance of spike activity in initially "silent" neurons; "silent" neurons in the solitary tract nucleus became tonically active, while those in the pre-Botzinger complex showed burst activity. These results provide evidence for the existence of an indirect regulatory influence for thyroliberin on respiratory center neurons, operating at the membrane level.

Cardiorespiratory neurons of the rat ventrolateral medulla contain TASK-1 and TASK-3 channel mRNA

The potassium channels TASK-1 and TASK-3 are neuronal 'leak' channels that are inhibited by extracellular acidification and numerous neurotransmitters. Here we tested whether in the rat TASK-1 and TASK-3 mRNAs are present in ventrolateral medulla neurons that regulate respiration or circulation (C1 adrenergic neurons). The mRNAs were identified by in situ hybridization. Respiratory neurons were identified by anatomical markers (neurokinin-1 receptors, NK1R, or somatostatin) or directly by juxtacellular labeling of bulbospinal neurons. C1 neurons were identified by the presence of tyrosine-hydroxylase. TASK-1 and TASK-3 transcripts were present in all hypoglossal and facial motor neurons, in most C1 cells (85-95%), in most small and large NK1R-ir neurons (>90%) of the ventral respiratory group (VRG) and in all the inspiratory-augmenting bulbospinal neurons of the rostral ventral respiratory group. In conclusion, TASK channels are expressed by respiratory cells with putative rhythmogenic function and by premotor and motor neurons. TASK channels presumably mediate excitatory effects of numerous transmitters on these neurons and may contribute to their pH-sensitivity.

Modulation of AMPA receptors by cAMP-dependent protein kinase in preBötzinger complex inspiratory neurons regulates respiratory rhythm in the rat

We hypothesize that phosphorylation of AMPA receptors or associated synaptic proteins modulates the excitability of respiratory neurons in the preBötzinger Complex (preBötC), affecting respiratory rhythm. Using neonatal rat medullary slices that spontaneously generate respiratory rhythm, we examined the role of the cAMP-PKA pathway (PKA: cAMP-dependent protein kinase) in modulating glutamatergic synaptic transmission, the excitability of inspiratory neurons in the preBötC and respiratory rhythm. Microinjection of forskolin, an activator of adenylate cyclase, into the preBötC with or without the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), decreased the period (increased the frequency) of respiratory-related rhythmic motor output in the hypoglossal nerve (XIIn) to 84 % (without IBMX) and to 72 % (with IBMX) of the pre-injection baseline. In the presence of MK-801, a non-competitive NMDA receptor antagonist, microinjection of forskolin plus IBMX decreased the period to 66 % of baseline levels. Microinjection of Rp-adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS), a PKA inhibitor, increased the period to 145 % of baseline levels. Concurrent microinjection of Rp-cAMPS and forskolin had no effect on the period. Bath application of 7beta-deacetyl-7beta-[gamma-(morpholino)butyryl]-forskolin hydrochloride (7Db-forskolin, a water-soluble derivative of forskolin): (1) decreased the period to 67 % of baseline levels without affecting the amplitude of integrated XIIn inspiratory discharge, (2) induced a tonic inward current of 29 pA and enhanced inspiratory drive current (the amplitude increased to 183 % and the integral increased to 184 % of baseline) in voltage-clamped (holding potential = -60 mV) preBötC inspiratory neurons and (3) increased the frequency to 195 % and amplitude to 118 % of spontaneous excitatory postsynaptic currents (sEPSCs) during expiratory periods. Dideoxy-forskolin did not have these effects. Intracellular perfusion with the catalytic subunit of PKA (cPKA) into preBötC inspiratory neurons progressively enhanced inspiratory drive currents and, in the presence of TTX, increased the inward currents induced by local ejection of AMPA; the latter currents were blocked by 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulphonamide (NBQX, an AMPA/kainate receptor antagonist). The effects of cPKA were blocked by co-application of PKA inhibitor (6-22) amide (PKI). These results suggest that phosphorylation of postsynaptic AMPA receptors through the cAMP-PKA pathway modulates both tonic and phasic excitatory amino acid synaptic transmission and excitability of inspiratory neurons in the preBötC and, therefore, regulates respiratory rhythm. Moreover, the basal level of endogenous PKA activity appears to be a determinant of resting respiratory frequency.

Models of respiratory rhythm generation in the pre-Bötzinger complex. III. Experimental tests of model predictions

We used the testable predictions of mathematical models proposed by Butera et al. to evaluate cellular, synaptic, and population-level components of the hypothesis that respiratory rhythm in mammals is generated in vitro in the pre-Bötzinger complex (pre-BötC) by a heterogeneous population of pacemaker neurons coupled by fast excitatory synapses. We prepared thin brain stem slices from neonatal rats that capture the pre-BötC and maintain inspiratory-related motor activity in vitro. We recorded pacemaker neurons extracellularly and found: intrinsic bursting behavior that did not depend on Ca(2+) currents and persisted after blocking synaptic transmission; multistate behavior with transitions from quiescence to bursting and tonic spiking states as cellular excitability was increased via extracellular K(+) concentration ([K(+)](o)); a monotonic increase in burst frequency and decrease in burst duration with increasing [K(+)](o); heterogeneity among different cells sampled; and an increase in inspiratory burst duration and decrease in burst frequency by excitatory synaptic coupling in the respiratory network. These data affirm the basis for the network model, which is composed of heterogeneous pacemaker cells having a voltage-dependent burst-generating mechanism dominated by persistent Na(+) current (I(NaP)) and excitatory synaptic coupling that synchronizes cell activity. We investigated population-level activity in the pre-BötC using local "macropatch" recordings and confirmed these model predictions: pre-BötC activity preceded respiratory-related motor output by 100-400 ms, consistent with a heterogeneous pacemaker-cell population generating inspiratory rhythm in the pre-BötC; pre-BötC population burst amplitude decreased monotonically with increasing [K(+)](o) (while frequency increased), which can be attributed to pacemaker cell properties; and burst amplitude fluctuated from cycle to cycle after decreasing bilateral synaptic coupling surgically as predicted from stability analyses of the model. We conclude that the pacemaker cell and network models explain features of inspiratory rhythm generation in vitro.

Electrical coupling and excitatory synaptic transmission between rhythmogenic respiratory neurons in the preBötzinger complex

Breathing pattern is postulated to be generated by brainstem neurons. However, determination of the underlying cellular mechanisms, and in particular the synaptic interactions between respiratory neurons, has been difficult. Here we used dual recordings from two distinct populations of brainstem respiratory neurons, hypoglossal (XII) motoneurons, and rhythmogenic (type-1) neurons in the preBötzinger complex (preBötC), the hypothesized site for respiratory rhythm generation, to determine whether electrical and chemical transmission is present. Using an in vitro brainstem slice preparation from newborn mice, we found that intracellularly recorded pairs of XII motoneurons and pairs of preBötC inspiratory type-1 neurons showed bidirectional electrical coupling. Coupling strength was low (<0.10), and the current that passed between two neurons was heavily filtered (corner frequency, <10 Hz). Dual recordings also demonstrated unidirectional excitatory chemical transmission (EPSPs of approximately 3 mV) between type-1 neurons. These data indicate that respiratory motor output from the brainstem involves gap junction-mediated current transfer between motoneurons. Furthermore, bidirectional electrical coupling and unidirectional excitatory chemical transmission are present between type-1 neurons in the preBötC and may be important for generation or modulation of breathing rhythm.

Effects of neuroactive substances on the morphine-induced respiratory depression; an in vitro study

Effects of different neuroactive substances on morphine-induced respiratory depression were studied in medullary respiration-related structures using in vitro brainstem-spinal cord preparation from 1 to 4-day-old rats. Application of morphine (10 microM) reduced respiratory rhythm (fR) as measured by C4 ventral root activity. The depressant effects of morphine were reversed by acetylcholine (10 microM), substance P (50 nM), thyrotropin releasing hormone (TRH) (100 nM) and forskolin (10 microM). The adenosine receptor antagonist, theophylline (100 microM), the dopamine receptors antagonist, haloperidol (10 microM), the cyclooxygenase inhibitor, indomethacin (10 microM) and the phospholipase A(2) inhibitor, quinacrine (10 microM) had no effect on morphine-induced respiratory depression.

Neuropharmacology of control of respiratory rhythm and pattern in mature mammals

This review summarizes the current understanding of the neurotransmitters and neuromodulators that are involved, firstly, in respiratory rhythm and pattern generation, where glutamate plays an essential role in the excitatory mechanisms and glycine and gamma-aminobutyric acid mediate inhibitory postsynaptic effects, and secondly, in the transmission of input signals from the central and peripheral chemoreceptors and of motor outputs to respiratory motor neurons. Finally, neuronal mechanisms underlying respiratory modulations caused by respiratory depressants and excitants, such as general anesthetics, benzodiazepines, opioids, and cholinergic agents, are described.

Respiratory network function in the isolated brainstem-spinal cord of newborn rats

The in vitro brainstem-spinal cord preparation of newborn rats is an established model for the analysis of respiratory network functions. Respiratory activity is generated by interneurons, bilaterally distributed in the ventrolateral medulla. In particular non-NMDA type glutamate receptors constitute excitatory synaptic connectivity between respiratory neurons. Respiratory activity is modulated by a diversity of neuroactive substances such as serotonin, adenosine or norepinephrine. Cl(-)-mediated IPSPs provide a characteristic pattern of membrane potential fluctuations and elevation of the interstitial concentration of (endogenous) GABA or glycine leads to hyperpolarisation-related suppression of respiratory activity. Respiratory rhythm is not blocked upon inhibition of IPSPs with bicuculline, strychnine and saclofen. This indicates that GABA- and glycine-mediated mutual synaptic inhibition is not crucial for in vitro respiratory activity. The primary oscillatory activity is generated by neurons of a respiratory rhythm generator. In these cells, a set of intrinsic conductances such as P-type Ca2+ channels, persistent Na+ channels and G(i/o) protein-coupled K+ conductances mediates conditional bursting. The respiratory rhythm generator shapes the activity of an inspiratory pattern generator that provides the motor output recorded from cranial and spinal nerve rootlets in the preparation. Burst activity appears to be maintained by an excitatory drive due to tonic synaptic activity in concert with chemostimulation by H+. Evoked anoxia leads to a sustained decrease of respiratory frequency, related to K+ channel-mediated hyperpolarisation, whereas opiates or prostaglandins cause longlasting apnea due to a fall of cellular cAMP. The latter observations show that this in vitro model is also suited for analysis of clinically relevant disturbances of respiratory network function.

Modulation of respiratory frequency by peptidergic input to rhythmogenic neurons in the preBötzinger complex

Neurokinin-1 receptor (NK1R) and mu-opioid receptor (muOR) agonists affected respiratory rhythm when injected directly into the preBötzinger Complex (preBötC), the hypothesized site for respiratory rhythmogenesis in mammals. These effects were mediated by actions on preBötC rhythmogenic neurons. The distribution of NK1R+ neurons anatomically defined the preBötC. Type 1 neurons in the preBötC, which have rhythmogenic properties, expressed both NK1Rs and muORs, whereas type 2 neurons expressed only NK1Rs. These findings suggest that the preBötC is a definable anatomic structure with unique physiological function and that a subpopulation of neurons expressing both NK1Rs and muORs generate respiratory rhythm and modulate respiratory frequency.

Respiratory responses to thyrotropin-releasing hormone microinjected into the rabbit medulla oblongata

We investigated the respiratory role of thyrotropin-releasing hormone (TRH) input to medullary structures involved in the control of breathing in anesthetized, vagotomized, paralyzed, and artificially ventilated rabbits. Microinjections (10-20 nl) of 1 or 10 mM TRH were performed in different regions of the ventral respiratory group (VRG), namely the rostral expiratory portion or Bötzinger complex (Böt. c.), the inspiratory portion, the transition zone between these two neuronal pools, and the caudal expiratory component. TRH microinjections were also performed in the dorsal respiratory group (DRG) and the area postrema (AP). Injection sites were localized by using stereotaxic coordinates and extracellular recordings of neuronal activity; their locations were confirmed by subsequent histological control. TRH microinjections in the Böt. c. and the directly caudally located region where a mix of inspiratory and expiratory neurons were encountered elicited depressant respiratory responses. TRH microinjections were completely ineffective at sites within the inspiratory and the caudal expiratory components of the VRG. TRH microinjections in either the DRG or the AP induced excitatory effects on inspiratory activity. The results show for the first time that TRH may exert inhibitory influences on respiration at medullary levels by acting on rostral expiratory neurons and that not only the DRG, as previously suggested, but also the AP may mediate TRH-induced excitatory effects on respiration.

TRH microdialysis into the RTN of the conscious rat increases breathing, metabolism, and temperature

Thyrotropin-releasing hormone (TRH) injected into the retrotrapezoid nucleus (RTN) of anesthetized rats produces a large, prolonged stimulation of ventilatory output (C. L. Cream, A. Li, and E. E. Nattie. J. Appl. Physiol. 83: 792-799, 1997). Here we inject or dialyze TRH into the RTN of conscious rats. In 6 of 17 injections (200 nl, 3.1 +/- 1.7 mM), ventilation (VE) increased 31% by 10 min, with recovery by 60 min. With dialysis, each animal of one group (n = 5) received, in random order, 10 mM TRH, 10 mM TRHOH (a metabolite of TRH), and artificial cerebrospinal fluid (aCSF); each animal of a second group (n = 5) received aCSF and 1 mM TRH. TRHOH and aCSF had no sustained effects. TRH (1 mM) increased VE (32%, P < 0.02, by 10 min, with recovery by 60 min), O(2) consumption (VO(2); 19%, P < 0. 03), and body (rectal) temperature (T(re); 0.5 degrees C, P < 0.09). TRH (10 mM) increased VE (78%, P < 0.01, by 10 min, with no recovery at 60 min), VO(2) (48%, P < 0.01), and T(re) (1.0 degrees C, P < 0. 01). TRH also induced arousal. The tissue volume affected in dialysis, estimated by spread of dialyzed fluorescein (332.3 mol wt, mol wt of TRH = 362.4), was 1,580 +/- 256 nl for 10 mM (n = 5) and 590 +/- 128 nl for 1 mM (n = 5). We conclude that 1) the RTN is involved in the integration of VE, VO(2), T(re), and arousal and 2) TRH may establish the responsiveness of RTN neurons.

The pre-Bötzinger complex participates in generating the respiratory effects of thyroliberin

Experiments on anesthetized rats were performed to study the effects of microinjection of thyroliberin (10 fM-100 microM) into the area of the pre-Bötzinger complex on respiratory and circulatory parameters. Thyroliberin dose-dependently increased respiration frequency, with shortening of inspiration and expiration. Tidal volume and the amplitude of the integrated EMG recorded from the inspiratory muscles decreased after administration of concentrated solutions. Using this dosage method, thyroliberin had weak effects on systemic hemodynamics. The data suggest that structures located in the area of the pre-Bötzinger complex take part in generating the respiratory effects of thyroliberin.

Calcium oscillations in rhythmically active respiratory neurones in the brainstem of the mouse

1. The rhythmically active respiratory network in the brainstem slice of the mouse was investigated under in vitro conditions using patch clamp and microfluorometric techniques. Rhythmic respiratory activity persisted over the whole course of an experiment. 2. Electrophysiologically recorded rhythmic activity in respiratory neurones was accompanied by oscillations in intracellular calcium, which displayed a maximal concentration of 300 nM and decayed to basal levels with a mean time constant of 1.6 +/- 0.9 s. 3. Elevations of calcium concentrations were highly correlated with the amplitude of rhythmic membrane depolarization of neurones, indicating that they were initiated by a calcium influx across the plasma membrane through voltage-gated calcium channels. 4. Voltage clamp protocols activating either high voltage-activated (HVA) or both HVA and low voltage-activated (LVA) calcium channels showed that intracellular calcium responses were mainly evoked by calcium currents through HVA channels. 5. Somatic calcium signals depended linearly on transmembrane calcium fluxes, suggesting that calcium-induced calcium release did not substantially contribute to the response. 6. For calcium elevations below 1 microM, decay time constants were essentially independent of the amplitude of calcium rises, indicating that calcium extrusion was adequately approximated by a linear extrusion mechanism. 7. Cytosolic calcium oscillations observed in neurones of the ventral respiratory group provide further evidence for rhythmic activation of calcium-dependent conductances or second messenger systems participating in the generation and modulation of rhythmic activity in the central nervous system.

Synaptic inhibition in the isolated respiratory network of neonatal rats

Gramicidin-perforated patch-clamp recording revealed phasic Cl(-)-mediated hyperpolarizations in respiratory neurons of the brainstem-spinal cord preparation from newborn rats. The in vitro respiratory rhythm persisted after block of gamma-aminobutyric acid (GABA), i.e. GABAA, receptor-mediated inhibitory postsynaptic potentials (IPSPs) with bicuculline and/or glycinergic IPSPs with strychnine. In one class of expiratory neurons, bicuculline unmasked inspiration-related excitatory postsynaptic potentials (EPSPs), leading to spike discharge. Bicuculline also blocked hyperpolarizations and respiratory arrest due to bath-applied muscimol, whereas strychnine antagonized similar responses to glycine. The reversal potential of respiration-related IPSPs and responses to GABA, muscimol or glycine was not affected by CO2/HCO3(-)-free solutions, but shifted from about -65 mV to values more positive than -20 mV upon dialysis of the cells with 144 instead of 4 mM Cl-. Impairment of GABA uptake with nipecotic acid or glycine uptake with sarcosine evoked a bicuculline- or strychnine-sensitive decrease of respiratory frequency which could lead to respiratory arrest. Also, the GABAB receptor agonist baclofen led to reversible suppression of respiratory rhythm. This in vitro apnoea was accompanied by a K+ channel-mediated hyperpolarization (reversal potential -88 mV) of tonic cells, whereas membrane potential of neighbouring respiratory neurons remained almost unaffected. Both baclofen-induced hyperpolarization and respiratory depression were antagonised by 2-OH-saclofen, which did not affect respiration-related IPSPs per se. The results show that synaptic inhibition is not essential for rhythmogenesis in the isolated neonatal respiratory network, although (endogenous) GABA and glycine have a strong modulatory action. Hyperpolarizing IPSPs mediated by GABAA and glycine receptors provide a characteristic pattern of membrane potential oscillations in respiratory neurons, whereas GABAB receptors rather appear to be a feature of non-respiratory neurons, possibly providing excitatory drive to the network.

PreBötzinger complex and pacemaker neurons: hypothesized site and kernel for respiratory rhythm generation

Identification of the sites and mechanisms underlying the generation of respiratory rhythm is of longstanding interest to physiologists and neurobiologists. Recently, with the development of novel experimental preparations, especially in vitro en bloc and slice preparations of rodent brainstem, progress has been made In particular, a site in the ventrolateral medulla, the preBötzinger Complex, is hypothesized to contain neuronal circuits generating respiratory rhythm. Lesions or disruption of synaptic transmission within the preBötzinger Complex, either in vivo or in vitro, can abolish respiratory activity. Furthermore, the persistence of respiratory rhythm following interference with postsynaptic inhibition and the subsequent discovery of neurons with endogenous bursting properties within the preBötzinger Complex have led to the hypothesis that rhythmogenesis results from synchronized activity of pacemaker or group-pacemaker neurons.

RTN TRH causes prolonged respiratory stimulation

We injected thyrotropin-releasing hormone (TRH; 10 nl; 0.25, 0.5, 1.0, or 10 mM), its inactive free acid form (TRHOH; 1 mM), or a metabolite with low TRH-receptor binding affinity, histidine-proline diketopiperazine (cHP; 1 mM), into the retrotrapezoid nucleus of anesthetized rats. Injection location was verified by anatomic analysis. Lower doses (0.25-0.5 mM) significantly increased both the product of integrated phrenic amplitude and frequency (Phr . f) and f for 20-30 min compared with artificial cerebrospinal fluid control injections. Higher doses (1. 0-10 mM) produced greater and long-lasting stimulation of Phr . f, Phr, and f and of blood pressure. This stimulation reached values 150% of baseline and durations of 270 min after a single injection. TRHOH (1 mM ) or cHP (1 mM) had no effect on Phr but increased f, as did 1 mM TRH. We conclude that TRH has a very powerful stimulatory effect in the retrotrapezoid nucleus region on Phr . f, with the Phr response seemingly specific for TRH receptors. Similar responses of f to TRHOH and cHP suggest it may be nonspecific.

The neuronal mechanisms of respiratory rhythm generation

New, improved in vivo and in vitro approaches have led to a better understanding of the mechanisms that generate respiratory rhythm, which depends on a complex interaction between network and intrinsic membrane properties. The pre-Bötzinger complex in the ventrolateral medulla is particularly important for respiratory rhythm generation. This complex can be studied in isolation, and it contains all the known types of respiratory neurons that are now amenable to detailed cellular and molecular analyses.