The normal breathing rhythm in mammals is hypothesized to be generated by neurokinin-1 receptor (NK1R)-expressing neurons in the preBötzinger complex (preBötC), a medullary region proposed to contain the kernel of the circuits generating respiration. If this hypothesis is correct, then complete destruction of preBötC NK1R neurons should severely perturb and perhaps even fatally arrest breathing. Here we show that specific and near complete bilateral (but not unilateral) destruction of preBötC NK1R neurons results in both an ataxic breathing pattern with markedly altered blood gases and pH, and pathological responses to challenges such as hyperoxia, hypoxia and anesthesia. Thus, these ~600 neurons seem necessary for the generation of normal breathing in rats.Breathing is an exceptionally reliable and continuous mammalian behavior that regulates blood gases and pH at rest and in response to diverse challenges such as exercise, sleep or altitude. The rhythm underlying breathing is postulated to depend critically on neurons in the preBötC1 , 2. Two recent developments allowed us to determine in awake adult rats whether this critical regulatory behavior would be affected by lesions in the preBötC. First, the extent of the preBötC can be anatomically defined by the sub-population of propriobulbar respiratory neurons within the ventrolateral respiratory column expressing NK1R3 -5 . Second, NK1R neurons can be specifically lesioned by substance P conjugated to saporin (SP-SAP) 6 , an effect that takes several days, allowing for complete recovery from surgery6. ResultsSP-SAP was effective in eliminating preBötC NK1R neurons in adult rats. Injections of 0.1-0.2 pmol of SP-SAP or 0.3 pmol each of unconjugated saporin and SP were made in the preBötC. Unilateral SP-SAP injection transiently produced sighs (large inspiratory efforts followed by prolonged expiration), consistent with the effects of injection of SP alone into the pre-BötC in vitro 7 and in vivo (P.A.G., D.R.M. and J.L.F., unpublished observations). All rats returned to normal behavior upon recovery from surgery. Two to eighteen days after injection, rats were perfused and their medullas were stained for NK1R immunoreactivity (n = 20 NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript lesion extent was estimated by counting NK1R immunopositive neuronal soma in the rostral medulla inside a circle of 600-μm diameter approximating the preBötC (ventrolateral to the nucleus ambiguus) and in a rectangle (1600 × 1070 μm) outside this circle (Fig. 1, inset). Counts were estimated from four transverse sections beginning within 100 μm of the rostral border of the lateral reticular nucleus and spanning the preBötC 3,4 containing approximately 12-15% of the total preBötC volume. Uninjected controls (n = 4) had 35 ± 5.1 (mean ± s.e.m., per side) NK1R neurons within the preBötC (~600 preBötC NK1R neurons total, which we estimate represent less than 10% of all preBötC neurons) and 82 ± 9 NK1R neurons outside the preBötC. Most of the latter were i...

Inspiration and active expiration are commonly viewed as antagonistic phases of a unitary oscillator that generates respiratory rhythm. This view conflicts with observations we report here in juvenile rats, where by administration of fentanyl, a selective µ-opiate agonist, and induction of lung reflexes, we separately manipulated the frequency of inspirations and expirations. Moreover, completely transecting the brainstem at the caudal end of the facial nucleus abolished active expirations, while rhythmic inspirations continued. We hypothesize that inspiration and expiration are generated by coupled, anatomically separate rhythm generators, one generating active expiration located close to the facial nucleus in the region of the retrotrapezoid nucleus/parafacial respiratory group, the other generating inspiration located more caudally in the preBötzinger Complex.

Data from perinatal and juvenile rodents support our hypothesis that the preBötzinger complex generates inspiratory rhythm and the retrotrapezoid nucleus–parafacial respiratory group (RTN/pFRG) generates active expiration (AE). Although the role of the RTN/pFRG in adulthood is disputed, we hypothesized that its rhythmogenicity persists but is typically silenced by synaptic inhibition. We show in adult anesthetized rats that local pharmacological disinhibition or optogenetic excitation of the RTN/pFRG can generate AE and transforms previously silent RTN/pFRG neurons into rhythmically active cells whose firing is correlated with late-phase active expiration. Brief excitatory stimuli also reset the respiratory rhythm, indicating strong coupling of AE to inspiration. The AE network location in adult rats overlaps with the perinatal pFRG and appears lateral to the chemosensitive region of adult RTN. We suggest that (1) the RTN/pFRG contains a conditional oscillator that generates AE, and (2) at rest and in anesthesia, synaptic inhibition of RTN/pFRG suppresses AE.

Current consensus holds that a single medullary network generates respiratory rhythm in mammals. Pre-Bötzinger Complex inspiratory (I) neurons, isolated in transverse slices, and preinspiratory (pre-I) neurons, found only in more intact en bloc preparations and in vivo, are each proposed as necessary for rhythm generation. Opioids slow I, but not pre-I, neuronal burst periods. In slices, opioids gradually lengthened respiratory periods, whereas in more intact preparations, periods jumped nondeterministically to integer multiples of the control period (quantal slowing). These findings suggest that opioid-induced quantal slowing results from transmission failure of rhythmic drive from pre-I neurons to preBötC I networks, depressed below threshold for spontaneous rhythmic activity. Thus, both I (in the slice), and pre-I neurons are sufficient for respiratory rhythmogenesis.

Delineating neurons that underlie complex behaviors is of fundamental interest. Using adenoassociated virus 2, we expressed the Drosophila allatostatin receptor in somatostatin (Sst)-expressing neurons in the preBötzinger Complex (preBötC). Rapid silencing of these neurons in awake rats induced a persistent apnea without any respiratory movements to rescue their breathing. We hypothesize that breathing requires preBötC Sst neurons and that their sudden depression can lead to serious, even fatal, respiratory failure.The preBötC in the ventrolateral medulla is hypothesized to be a kernel for the generation of respiratory rhythm in vitro and in vivo 1-4 . Adult rats with slow (~days), toxin-induced neurodegeneration of > 80% of neurokinin 1 receptor (NK1R)-expressing preBötC neurons survive with an ataxic rhythm during wakefulness and apnea during sleep 1,5 . Whether this pathological breathing pattern is driven by neurons that normally control respiratory-related muscles, including those underlying volitional or emotional behaviors, or by a compensatory reorganization in response to the slow neurodegeneration is unknown. To eliminate adaptation resulting from slow lesions, we rapidly (~minutes) decreased excitability in a glutamatergic subpopulation of preBötC neurons that express Sst 6 . This population overlaps with neurons expressing NK1R; 28 ± 2% of preBötC Sst neurons expressed NK1R and 41 ± 1% of preBötC NK1R neurons expressed Sst (5-6-week-old rats, n = 3; Supplementary Fig. 1 online), consistent with their overlap in neonatal rat preBötC 7 . We hypothesized that preBötC Sst neurons are essential for normal breathing and predicted that silencing these neurons would cause acute apnea in awake adult rats; as hypoxia and hypercapnea worsened with apnea, we presumed that other mechanisms, particularly in relation to volitional or emotional drives, would restore breathing, as appears to be the case in central congenital hypoventilation syndrome 8 . Rapid, reversible silencing of genotypic neuronal subpopulations can illuminate their role in behavior. This can be done by targeted expression of allatostatin receptor (AlstR), a G protein-coupled receptor that is neither expressed nor activated by any endogenous ligand in mammals 9-11 . Mammalian cortical and spinal cord neurons that are made to express AlstR can be rapidly and reversibly inactivated in vitro 12,13 and in vivo under anesthesia 10 by administration of allatostatin, which opens K + channels 9,13 to hyperpolarize them. WeCorrespondence should be addressed to J.L.F. (feldman@ucla.edu). 3 These authors contributed equally to this work.Note: Supplementary information is available on the Nature Neuroscience website. expressed AlstR and enhanced green fluorescent protein (EGFP) in targeted preBötC neurons and studied the effect of allatostatin application on breathing in adult rats. NIH Public AccessTo obtain stable and reliable expression of exogenous genes in adult rat preBötC neurons, we used adeno-associated virus 2 (AAV2) to ensure high infecti...