Chronic intermittent hypoxia effects on NTS glutamatergic neurotransmission and intrinsic properties of reticulospinal respiratoy and presympathetic neurons of ventral medulla of rats

Author(s)

Carlos Eduardo Lopes Almado

• Document type: Doctoral Thesis
• Press: Ribeirão Preto.
• Institution: Universidade de São Paulo (USP). Faculdade de Medicina de Ribeirão Preto (PCARP/BC)
• Defense date: 2012-05-04

Examining board members

Ricardo Mauricio Xavier Leão; Leonardo Resstel Barbosa Moraes; Thiago dos Santos Moreira; Lígia Araujo Naves; Wamberto Antonio Varanda

• Advisor: Ricardo Mauricio Xavier Leão; Benedito Honório Machado

Abstract

ln this study we assessed the effects of chronic intermittent hypoxia (CIH) on the glutamatergic neurotransmission in the nucleus of the solitary tract (NTS) and on the intrinsic properties of respiratory and pre-sympathetic reticulospinal neurons of ventrolateral medulla. For this purpose, we submitted juvenile male Wistar rats (P19-P21) to 10 days of CIH (6% O2 for 30-40 s, every 9 minutes, 8 hours/day) or to 10 days of normoxia (20.8% O2) for the same period to be used as controls. We used the whole cell patch-clamp technique to study the neurotransmission and the electrophysiological properties of neurons. We observed that CIH reduced the amplitude of TS-eEPSCs selectively on NTS second order neurons, classified in accordance to their standard deviation (SD) of their EPSC latencies. The study of the underlying mechanisms of this reduction in eEPSCs revealed that CIH does not change vesicular release probability (PR) of TS synapses on these neurons, since short-term depression of their TS-EPSCs was not changed. Additionally, we documented that CIH produced no changes in postsynaptic non-NMDA receptors because we observed no differences in the amplitude of quantal eEPSCs (quantal size, q) obtained by means of replacement of extracellular calcium by strontium (producing asynchronous eEPSCs, aeEPSCs), or by means of minimal stimulation of TS. We concluded that CIH reduces the TS-eEPSC by reducing the quantal content (m, which is defined as the product of PR and functional release sites/synapses, n) by reducing the parameter n. We also investigated the effects of CIH on NMDA eEPSCs and we verified that these currents were similarly depressed after CIH. This last finding suggests that the reduction in n did not occur by selective internalization of post-synaptic non-NMDA receptors, but likely by complete silencing of specific TS synapses onto second-order neurons. We observed that the CIH-induced depression was reverted 3 weeks after the end of CIH also by contribution of compensatory mechanisms distinct from the one that produced the original depression. We also evaluated the effects of CIH on intrinsic electrophysiological properties of respiratory and pre-sympathetic reticulo-spinal neurons of ventrolateral medulla responsible for respiratory rhythm generation and sympathetic efferent activity, respectively. CIH produced no changes in passive membrane properties or intrinsic action potential firing rate of these neurons suggesting that CIH is not producing its physiological effects by affecting the intrinsic firing of these neurons. We conclude that CIH produces neurotransmission depression in the NTS by silencing of specific TS synapses onto second-order NTS neurons, an effect that is compensated after 3 weeks by increases in PR. Furthermore, CIH seems to produce no changes in intrinsic properties of reticulospinal neurons of ventrolateral medulla. (AU)