The Role of the I h Current in Rhythmic Stability of the PreBötzinger Complex.

The generation of rhythmicity is a fundamental property of the nervous system. Common amongst all rhythmogenic networks is the ability to reconfigure and quickly adapt to changes in metabolic, environmental, and behavioral demands. Yet, this flexibility, and plasticity must be counter-balanced by the need to maintain stability. A group of neurons collectively known as the preBötzinger complex (preBötC) within the ventral respiratory column of the medulla assemble in a network that is both necessary and sufficient to maintain inspiratory rhythm generation. Multiple ionic conductances contribute to rhythm generation. One known for its role in the generation of rhythmic activity across multiple neural networks is the hyperpolarization activated cation current (I ). The I current is a voltage-dependent mixed cationic current that is activated upon phasic hyperpolarization of neurons. The role of I in rhythmogenesis within the preBötC is inconclusive. Some reports find that blocking I has minimal effect, while others conclude that this increases the rate of rhythmic bursting throughout the network. To date, I was specifically studied in rhythmically active neurons with unknown transmitter phenotype. Herein we sought to determine the distribution of I amongst the population of tonically and rhythmically active excitatory and inhibitory neurons throughout the preBötC. Additionally, we tested whether removing I in-vitro and in-vivo renders the network more susceptible to perturbations. Using in-vitro patch clamping we found that I showed greatest expression amongst excitatory (87% of DBX1 ), and inhibitory (89% of VGAT ) tonically active cell populations. Removing I using ZD7288 silenced nearly all tonically spiking neurons. This led to a minimal effect on the rhythmic activity of fully synchronized population bursts, but significantly increased the incidence of less synchronized burstlet activity. Moreover, removing I and silencing tonic activity within the network rendered the preBötC more susceptible to suppression following opioid (DAMGO) and CNQX administration. This effect was recapitulated in-vivo with microdialysis of ZD7288 into the preBötC, which caused normally well-tolerated doses of morphine to become lethal via terminal apnea. In summary, I plays a complex role within the rhythm generating network of the preBötC. Under optimal conditions, I is not necessary for maintaining fully synchronized bursting within the network. Yet, I becomes increasingly important during perturbations that challenge network function, suggesting this ionic conductance contributes to maintaining the robustness of breathing during everchanging physiological and environmental demands.