Excitatory amino acid-mediated transmission of inspiratory drive to phrenic motoneurons

Abstract

1. The role of excitatory amino acids (EAAs) in the bulbospinal transmission of inspiratory drive was studied by intracellular and single-electrode voltage-clamp recordings from phrenic motoneurons in the in vitro neonatal rat brain stem spinal cord. 2. In all brain stem-spinal cord preparations there were spontaneously generated rhythmic membrane depolarizations and associated spiking of phrenic motoneurons during the inspiratory phase of the respiratory cycle. The envelope of the motoneuron drive potential had a rapid onset to peak (50 ms) followed by a plateau/declining phase that lasted 400-700 ms. The peak potential was approximately 10-20 mV above base-line potential. The drive current under voltage clamp had a similar shape and duration to the drive potential with a peak current greater than 1.5 nA. 3. The involvement of EAAs in the bulbospinal transmission of inspiratory drive was demonstrated by checking the effects of various EAA receptor antagonists on the phrenic motoneuron inspiratory drive. When kynurenic acid (KYN), an antagonist acting on all three subtypes of EAA receptors, was applied to the solution bathing the spinal cord, the motoneuron action potentials were abolished, and the amplitude of inspiratory drive potential was significantly reduced. To further classify the role of the different EAA receptor subtypes in the synaptic transmission of inspiratory drive, the effects on the drive potential of either 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a specific non-N-methyl-D-aspartic acid (non-NMDA) receptor antagonist, or DL-2-amino-5-phosphonovaleric acid (AP5), DL-2-amino-7-phosphonoheptanoic acid (AP7), and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imin emaleate (MK-801), NMDA receptor antagonists, were investigated. Bath or local application of CNQX induced a dose-dependent decrease of the inspiratory drive potential without changing intrinsic motoneuron membrane properties. On the other hand, application of AP7 or MK 801 had a small effect on the inspiratory drive potential or the inspiratory drive current when the motoneuron membrane potential was clamped near end-expiratory potentials (-60 to -75 mV). 4. To establish the presence of EAA receptors on the phrenic motoneuronal membrane and to provide information on the available receptor subtypes for action of the endogenously released transmitter, we tested the effects of agonists for the major EAA receptor subtypes after blocking synaptic transmission (produced by axonal action potentials) by bath application of tetrodotoxin (TTX).(ABSTRACT TRUNCATED AT 400 WORDS)