Firing properties of respiratory rhythm generating neurons in the absence of synaptic transmission in rat medulla in vitro

SummaryIt has previously been demonstrated that Pre-I neurons, localized in the rostral ventrolateral medulla, are important in the generation of the primary respiratory rhythm in brainstemspinal cord preparations from newborn rats. To investigate whether or not Pre-I neurons have endogenous pacemaker properties, we examined Pre-I neuron activity before and after chemical synaptic transmission was blocked by incubation in a low Ca2+ (0.2 mM), high Mg2+ (5 mM) solution (referred to here as low Ca). After incubation for about 30 min in low Ca, 28 (52%, type-1) out of 54 neurons tested in 27 preparations retained apparent rhythmic (phasic) activity after complete disappearance of C4 inspiratory activity. Sixteen neurons (30%, type-2) fired tonically and 10 (18%, type-3) were silent. We examined the effects of synaptic blockade on 14 inspiratory neurons in the RVL. The firing of all 14 neurons in 9 preparations disappeared concomitantly with the disappearance of C4 activity in low Ca. When the pH of the low Ca solution was lowered with a decrease in NaHCO3 concentration from 7.4 to 7.1, the firing rate of the Pre-I neurons (type-1) increased from 12 to 18/min. In conclusion, the generator of respiratory rhythm in the newborn rat is probably a neuronal network with chemical synapses that functions mainly through the endogenous Pre-I pacemaker cells. Intrinsic chemoreception in the rhythm generator is probably important in frequency control of respiratory rhythm.

[1]  T. Murakoshi,et al.  A pharmacological study on respiratory rhythm in the isolated brainstem‐spinal cord preparation of the newborn rat , 1985, British journal of pharmacology.

[2]  K. Inenaga,et al.  Excitation of neurones in the rat paraventricular nucleus in vitro by vasopressin and oxytocin. , 1986, The Journal of physiology.

[3]  H. Haas,et al.  Synchronized bursting of CA1 hippocampal pyramidal cells in the absence of synaptic transmission , 1982, Nature.

[4]  M. Otsuka,et al.  The effects of substance P and baclofen on motoneurones of isolated spinal cord of the newborn rat. , 1980, The Journal of experimental biology.

[5]  P. L. Marino,,et al.  Effects of CO-2 and extracellular H+ iontophoresis on single cell activity in the cat brainstem. , 1975, Journal of applied physiology.

[6]  E. Bruce,et al.  Central chemoreceptors. , 1987, Journal of applied physiology.

[7]  M. Kuno,et al.  Differential effects of carbon dioxide and pH on central chemoreceptors in the rat in vitro. , 1985, The Journal of physiology.

[8]  J. Jefferys,et al.  Low‐calcium field burst discharges of CA1 pyramidal neurones in rat hippocampal slices. , 1984, The Journal of physiology.

[9]  Ikuo Homma,et al.  Respiratory rhythm generator neurons in medulla of brainstem-spinal cold preparation from newborn rat , 1987, Brain Research.

[10]  Ikuo Homma,et al.  Primary respiratory rhythm generator in the medulla of brainstem-spinal cord preparation from newborn rat , 1988, Brain Research.

[11]  N. Kogo,et al.  Locations of medullary neurons with non-phasic discharges excited by stimulation of central and/or peripheral chemoreceptors and by activation of nociceptors in cat , 1988, Brain Research.

[12]  Ikuo Homma,et al.  Inhibitory synaptic inputs to the respiratory rhythm generator in the medulla isolated from newborn rats , 1990, Pflügers Archiv.

[13]  T. Suzue,et al.  Respiratory rhythm generation in the in vitro brain stem‐spinal cord preparation of the neonatal rat. , 1984, The Journal of physiology.

[14]  T. Murakoshi,et al.  Electrophysiology of reflexes in an isolated brainstem-spinal cord preparation of the newborn rat , 1983 .

[15]  S. Kelso,et al.  Effect of synaptic blockade on thermosensitive neurons in hypothalamic tissue slices. , 1982, The American journal of physiology.

[16]  M. I. Cohen,et al.  Neurogenesis of respiratory rhythm in the mammal. , 1979, Physiological reviews.

[17]  Jack L. Feldman,et al.  In vitro brainstem-spinal cord preparations for study of motor systems for mammalian respiration and locomotion , 1987, Journal of Neuroscience Methods.

[18]  Y. Fukuda,et al.  Effect of H+ on spontaneous neuronal activity in the surface layer of the rat medulla oblongata in vitro , 1977, Pflügers Archiv.

[19]  I. Homma,et al.  Localization of respiratory rhythm-generating neurons in the medulla of brainstem-spinal cord preparations from newborn rats , 1987, Neuroscience Letters.