The Respiratory Network: Plasticity, Network Dynamics, Intrinsic and Synaptic Properties, and Neuromodulation

 

The Ramirez laboratory aims at unraveling the neuronal mechanisms that underlies breathing. These mechanisms include respiratory rhythm generation, neuromodulation, short- and long-term plasticity and the developmental changes of cellular and network properties induced by hypoxia. These processes alter the network configuration and involve cascades of molecular and cellular events that are of basic scientific and clinical interest.

Respiratory Rhythm Generation.  The Ramirez laboratory studies the cellular mechanisms that underlie respiratory rhythm generation in the pre-Bötzinger complex. This area in the ventrolateral medulla has been called the “noeud vital” and is essential for breathing (Ramirez, 2011). Lesioning the pre-Bötzinger complex in vivo abolishes breathing (Ramirez et al. 1998), while isolating the pre-Bötzinger complex in a slice preparation preserves respiratory rhythmic activity (Lieske et al. 2000) (click here for a movie showing a rhythmically active slice). This preparation generates not only normal respiratory activity (“eupneic activity”), but also sighs and gasps (Figure below shows mapping of sighs, eupneic activity and gasp).It must be emphasized that other brain areas also contribute to the generation of breathing. Indeed breathing is probably one of the most integrated behaviors as it is also affected and controlled by cognition, emotions such as fear, vocalization and changes in blood gases.

The Sigh:  Neurobiology and Significance.   Most of the time when you wake up at night – you wake up with a sigh. When your emotions change – in pain or in love – you sigh. When you get hypoxic during sleep, sighs are initiated and are the first activities before you arouse. Sighs are large amplitude breaths that are also critical to prevent your alveoli from collapsing. If you don’t sigh for a long time, you develop atelectasis. Hence it is important that we all sigh spontaneously. Children sigh more frequent than adults (see figure below). Sighs are typically triggered by a normal breath and followed by a brief period of post-sigh apnea. The Ramirez laboratory studies the cellular mechanisms that are responsible for the generation of the sigh. While sensory inputs from the lung can trigger sighs, the sighs are generated within the central nervous system. Sighs are centrally generated within the ventrolateral medulla in the area called the pre-Bötzinger complex. Isolated in a slice preparation the pre-Bötzinger complex continues to generate sighs (see Figures below). The Ramirez lab published several articles on the neurobiology of the sigh. Important for the generation of sighs are the so called P/Q type calcium currents which together with the N-type calcium current. These currents provide the excitatory synaptic drive and together with metabotropic GluR III (mGluR8) receptors and the persistent sodium these membrane properties generate the sigh burst (see Lieske et al. 2000; Lieske and Ramirez, 2006a,b; Tryba et al. 2008).

 

 

 

Neuromodulation.  The Ramirez laboratory studies the role of neuromodulation in regulating cellular and network functions involved in respiratory rhythm generation.  Through the use of a broad range of in vitro and in vivo electrophysiological approaches, we are currently investigating how neuromodulators define different network states, how they regulate different forms or respiratory activities, and how changes in oxygen affect peptidergic and aminergic modulation of the preBötzinger complex. Neuromodulators differentially regulate membrane properties. The example shows in the upper trace a spontaneously active spiking neurons, which becomes an intrinsically bursting neuron in the presence of alpha-1 noradrenergic agonist. In a series of publications we demonstrated how norepinephrine, serotonin and substance P can differentially induce bursting properties that depend either on CAN current (like the example shown on the figure) or on the persistent sodium current. For further details see publications: Pena and Ramirez, 2002, 2004; Viemari and Ramirez, 2006; Tryba et al. 2006; Tryba et al. 2008; Doi and Ramirez, 2010; Viemari et al. 2011).

 

 

 

Network Reconfiguration.  The respiratory network can assume different configurations that underlie normal breathing, sighs and gasps. Modulators play critical roles in mediating this reconfiguration, but changes in the network reconfiguration also alter the responses to neuromodulators. We employ computational modeling and in vitro, in vivo and imaging techniques to address how different network assemblies affect rhythmogenesis. These studies are also critical for understanding state-dependent changes in breathing during the sleep and wake cycle. The figure shows various aspects of the reconfiguration of the respiratory network as it transitions from normal respiratory activity (“eupneic activity”) into gasping. (For more details see e.g. Pena et al. 2004, Thoby-Brisson and Ramirez, 2001.)

 


                                          

 

Designing Novel Optogenetic Tools.  We are currently developing novel genetically-encoded probes designed to optically control mitochondrial function and to sense intracellular Ca2+. These probes will be used to measure and perturb neuronal activity non-invasively in targeted neuronal populations, in in vitro brain slice and in vivo transgenic mouse preparations.