Gang Song, Ph.D., Research ScientistHarvard-MIT Division of Health Sciences and Technology

Massachusetts Institute of Technology

Neuroscience of Breathing –Don’t take my breath away: Pace-maker mechanisms of respiratory

rhythmogenesis

normal

vagotomy

Apneusis

50 sec

Gasping

Changes of breathing pattern after sequential brainstem sectioning. (Adapted from Thomas Lumsden, 1923, J. Physiol.)

1

2

3

Smith, J. C. et al. J Neurophysiol 98: 3370-3387 2007;doi:10.1152/jn.00985.2007

Example of a parasagittal section (neutral red stain) of the brain stem after rostral to caudal serial transections (250- to 350-{micro}m-thick slices indicated by arrows) made

with a piezo-driven microvibratome (custom built) from an arterially perfused brain stem-spinal cord preparation of a 4-wk-old rat

Brainstem transection

performed on neonatal

rat brainstem identified

pre-Bötzinger complex

as the respiratory

pace-maker (Smith et.

al. Science, 1991)

Del Negro et al., J Neurosci, 2005

Pre-Bot pace-making neurons express NK1 receptors.

Onimaru and Homma, Brain Res, 1987

Parafacial group of pace-making neuronswas reported 4 years before the pre-Bot group.

No anatomical marker was identified until now.

Onimaru and Homma, J Neurosci, 2003

Parafacial pace-making neurons fire first

Janczewski and Feldman, J Physiol, 2006

Airflow

Inspir.

Expir.

Airflow

Inspir.

Expir.

Adult rat, anesthesia, bilateral vagotomy, hypoxia …

A tale of two pacemakersA tale of two pacemakers

pre-Bötzinger complex (preBötC) pre-Bötzinger complex (preBötC) Smith et al. (1991)Smith et al. (1991) Monophasic: inspiratory burstsMonophasic: inspiratory bursts Two sub-types of pacemakers: Two sub-types of pacemakers:

NaP current (del Negro et al., 2002)NaP current (del Negro et al., 2002) CAN current (Thoby-Brisson & Ramirez, 2001)CAN current (Thoby-Brisson & Ramirez, 2001)

Parafacial respiratory group (pFRG)Parafacial respiratory group (pFRG) Onimaru & Homma (1987, 2003)Onimaru & Homma (1987, 2003) Rostral to preBötCRostral to preBötC;; overlaps retrotrapezoid nucleus overlaps retrotrapezoid nucleus Biphasic: Pre-I and post-I burstsBiphasic: Pre-I and post-I bursts Possibly NaP current (Onimaru et al., 1997)Possibly NaP current (Onimaru et al., 1997)

Embarrassment of RichesEmbarrassment of Riches

Two-rhythms hypothesis Two-rhythms hypothesis preBötC: inspiratory rhythm generator (IRG)preBötC: inspiratory rhythm generator (IRG) pFRG: expiratory rhythm generator (ERG)pFRG: expiratory rhythm generator (ERG) Q: How do the two Q: How do the two harmonizeharmonize with one another? with one another?

Master rhythm hypothesisMaster rhythm hypothesis pFRG paces preBötC pFRG paces preBötC pFRG: ERG-cum-IRGpFRG: ERG-cum-IRG

Q: pFRG lesions didn’t abolish respiratory Q: pFRG lesions didn’t abolish respiratory rhythm: why?rhythm: why?

Handshake • pFRG → preBötC excitation

• preBötC → pFRG reverse inhibition/hyperpolarization

• pFRG post-hyperpolarization rebound

• post-I inspiratory off-switch

Pacemakers Handshake HypothesisPacemakers Handshake Hypothesis

Model FormulationModel Formulation

NaP conditional pacemaker models base on NaP conditional pacemaker models base on Hodgkin-Huxley formalism (Butera et al., 1999)Hodgkin-Huxley formalism (Butera et al., 1999)

Each pacemaker represents the lumped effect of the Each pacemaker represents the lumped effect of the pFRG or preBötC rhythmogenic networkpFRG or preBötC rhythmogenic network

preBötC pacemaker less excitable than pFRG preBötC pacemaker less excitable than pFRG (hence lower burst frequency)(hence lower burst frequency)

Simulates Phase Resetting of pFRG burstsSimulates Phase Resetting of pFRG bursts

Model simulation of experimental data from Onimaru et al., Brain Res. (1988)

Simulate Phase Resetting of preBötC burstsSimulate Phase Resetting of preBötC bursts

Model simulation of experimental data from Onimaru et al., Brain Res. (1988)

preBötC veto of handshakepreBötC veto of handshake

No Yes

Stimulated cycle

Post-stim. cycle

Model simulation of experimental data from Onimaru et al., Brain Res. (1988)

Simulates fractional Quantal Breathing (preBötC under depression)Simulates fractional Quantal Breathing (preBötC under depression)

Skipped cycle shorter

than full cycles –

no rebound excitation

Model simulation of experimental data from Mellon et al., Neuron (2003)

ConclusionsConclusions Model supports two-rhythms hypothesis Model supports two-rhythms hypothesis andand master rhythm master rhythm

hypothesishypothesis

ERG-IRG neither totally autonomous nor totally master-slave, ERG-IRG neither totally autonomous nor totally master-slave, but collaborative in partnershipbut collaborative in partnership

Both can oscillate, but it takes two to tangoBoth can oscillate, but it takes two to tango

preBötC is prime mover but preBötC is prime mover but pFRG is “better half”: pFRG is “better half”:

it strengthens it strengthens preBötC by synchronizing itpreBötC by synchronizing it

It backs up preBötC when it fails or falls shortIt backs up preBötC when it fails or falls short

Fail-safe redundancy avoids “putting all eggs in one basket”Fail-safe redundancy avoids “putting all eggs in one basket”

Handshake mechanism may be evolutionarily conservedHandshake mechanism may be evolutionarily conserved

Thank you for your attention!