Ryanodine receptor/Ca(2+) release mechanisms in rhythmically active respiratory neurons of cats in vivo

Inhibition of endoplasmic reticulum Ca²⁺ ATPase in preBötzinger complex of neonatal rat does not affect respiratory rhythm generation

PreBötzinger complex (preBötC) neurons in the brainstem underlie respiratory rhythm generation in vitro. As a result of network interactions, preBötC neurons burst synchronously to produce rhythmic premotor inspiratory activity. Each inspiratory neuron has a characteristic 10-20 mV, 0.3-0.8 s synchronous depolarization known as the inspiratory drive potential or inspiratory envelope, topped by action potentials (APs). Mechanisms involving Ca(2+) fluxes have been proposed to underlie the initiation of the inspiratory drive potential. An important source of intracellular Ca(2+) is the endoplasmic reticulum (ER) in which active Ca(2+) sequestration is mediated by a class of transporters termed sarco/endoplasmic reticulum Ca(2+) ATPases (SERCAs). We aim to test the hypothesis that disruption of Ca(2+) sequestration into the ER affects respiratory rhythm generation. We examined the effect of inhibiting SERCA on respiratory rhythm generation in an in vitro slice preparation. Bath application of the potent SERCA inhibitors thapsigargin or cyclopiazonic acid (CPA) for up to 90 min did not significantly affect the period or amplitude of respiratory-related motor output or integral and duration of inspiratory drive in preBötC neurons. We promoted the depletion of intracellular Ca(2+) stores by a transient bath application of 30 mM K(+) (high K(+)) in the continuous presence of thapsigargin or CPA. After washing out the high K(+), respiratory rhythm period and amplitude returned to baseline values. These results show that after inhibition of SERCA and depletion of intracellular Ca(2+) stores, respiratory rhythm remains substantially the same, suggesting that this source of Ca(2+) does not significantly contribute to rhythm generation in the preBötC in vitro.

Somatic Ca2+ transients do not contribute to inspiratory drive in preBötzinger Complex neurons

PreBötzinger Complex (preBötC) neurons are postulated to underlie respiratory rhythm generation. The inspiratory phase of the respiratory cycle in vitro results from preBötC neurons firing synchronous bursts of action potentials (APs) on top of 10-20 mV, 0.3-0.8 s inspiratory drive potentials. Is the inspiratory drive in individual neurons simply the result of the passive integration of inspiratory-modulated synaptic currents or do active processes modulate these currents? As somatic Ca(2+) is known to increase during inspiration, we hypothesized that it affects inspiratory drive. We combined whole cell recording in an in vitro slice preparation with Ca(2+) microfluorometry to detect single inspiratory neuron somatic Ca(2+) transients with high temporal resolution ( approximately mus). In neurons loaded with either Fluo-4 or Oregon Green BAPTA 5 N, we observed Ca(2+) transients associated with each AP. During inspiration, significant somatic Ca(2+) influx was a direct consequence of activation of voltage-gated Ca(2+) channels by APs. However, when we isolated the inspiratory drive potential in active preBötC neurons (by blocking APs with intracellular QX-314 or by hyperpolarization), we did not detect somatic Ca(2+) transients; yet, the parameters of inspiratory drive were the same with or without APs. We conclude that, in the absence of APs, somatic Ca(2+) transients do not shape the somatic inspiratory drive potential. This suggests that in preBötC neurons, substantial and widespread somatic Ca(2+) influx is a consequence of APs during the inspiratory phase and does not contribute substantively to the inspiratory drive potential. Given evidence that the Ca(2+) buffer BAPTA can significantly reduce inspiratory drive, we hypothesize that dendritic Ca(2+) transients amplify inspiratory-modulated synaptic currents.

[Role of Ca(2+) in the pacemaker-like property of spinal motoneurons]

It is generally accepted that locomotion in vertebrate species is produced by signals coded and integrated by neurons of the spinal cord. In fact, the basic features of locomotion, including patterns and rhythms, are generated by a network of neurons called the CPG (central pattern generator) essentially localized in the lumbar segments of the spinal cord. However, the detailed mechanisms underlying the rhythmic aspect of CPG-generated locomotion are not fully understood. Here, we report data of studies that focus on the role of Ca(2+)-related mechanisms involved in the expression of the pacemaker property of lumbar motoneurons that innervate the hindlimbs. In fact, it has become increasingly clear that Ca(2+) plays a determinant function in the expression of this active and conditional rhythmic property. In addition to NMDA-mediated currents (NMDA is an agonist of the calcium permeable ionotropic glutamatergic receptor) and to a Ca(2+)-dependent K(+) current that were found twenty years ago to contribute to intrinsic voltage oscillations in motoneurons, a pivotal role for voltage-gated channels (e.g., CaV1.3) and intracellular Ca(2+) concentrations ([Ca(2+)]i) have recently been shown. Increasing evidence of a role for metabotropic receptor subtypes, calmodulin (a calcium binding protein), ryanodine and IP3-sensitive intracellular stores of Ca(2+) suggests that additional mechanisms are yet to be identified. A detailed understanding of the complex role of Ca(2+) in mediating the auto-rhythmic property of spinal neurons may contribute to the development of novel therapeutic approaches to induce locomotion after spinal cord injury.