Neurotransmitters and neuromodulators controlling the hypoxic respiratory response in anaesthetized cats

The classical respiratory response to acute hypoxia consists of an initial augmentation of central respiratory activity followed by a secondary depression which progresses to complete respiratory arrest in hypoxic apnoea (Richter et al. 1991). A variety of mechanisms which generate or contribute to this biphasic respiratory response has been implicated. Hypoxic activation of arterial chemoreceptors increases excitatory synaptic drive of respiratory neurones under in vivo conditions (Lawson et al. 1989). However, augmentation of central respiratory activity also occurs in in vitro preparations in which chemoreceptor afferents are deleted (Volker et al. 1995; Ramirez et al. 1998). Hence, activation of medullary chemosensory neurones (Kawai et al. 1996) or direct stimulation of respiratory neurones (Volker et al. 1995) contribute to the increased respiratory response to hypoxia. Another factor leading to augmentation results from reduction of Na+-K+ pump activity leading to an increase in extracellular potassium levels and direct depolarization of axon terminals and postsynaptic neurones (Acker & Richter, 1985; Haddad & Donnelly, 1990; Trippenbach et al. 1990; Richter, 1996). The processes which lead to persistent secondary depression of central respiratory activity and finally to hypoxic apnoea are, as yet, unclear. However, it appears that depression of neuronal excitability and synaptic interactions between bulbar respiratory neurones play an important role (Richter et al. 1991). Mechanisms underlying such disturbances may involve: (i) a generalized release and local accumulation of inhibitory neurotransmitters (Neubauer et al. 1990; Haddad & Jiang, 1992; Young et al. 1992; Katoh et al. 1997) and/or neuromodulators such as adenosine, catecholamines, serotonin and opioids (Runold et al. 1989; Neubauer et al. 1990; Moss et al. 1993; Bentue-Ferrer et al. 1994; Yan et al. 1995); (ii) activation of ATP-sensitive potassium (KATP) channels of respiratory neurones due to a decrease in intracellular ATP levels (Jiang & Haddad, 1991; Haddad & Jiang, 1993; Pierrefiche et al. 1996); and (iii) accumulation of metabolic byproducts such as adenosine (Schmidt et al. 1995), which augment KATP channel currents in postsynaptic neurones (Mironov et al. 1998) and depress the otherwise increased Ca2+ influx in cell bodies and axon terminals and consequently release of neurotransmitters. A previous intracellular investigation (Richter et al. 1991) showed that, during hypoxia, there is an orderly temporal sequence of membrane potential changes in medullary respiratory neurones. The findings pointed to an involvement of neuromodulatory mechanisms that disrupt synaptic interactions between respiratory neurones. Since then, however, the identity, time course and functional consequences of such neuromodulatory processes within the respiratory network have not been further analysed. Therefore, in the present investigation we made sequential, short time scale measurements of glutamate (Glu), γ-aminobutyric acid (GABA), serotonin (5-HT) and adenosine (Ado) content in the extracellular fluid of the ventral medullary respiratory group (VRG) before, during and after acute hypoxic periods and correlated them with the changes in respiratory neuronal activities. All these neurochemicals have been shown to exert prominent effects on the central respiratory rhythm during normoxia (Bonham, 1995). In addition, we made complementary electrophysiological measurements to determine whether serotonin 5-HT-1A receptors (Lalley, 1994; Lalley et al. 1994; Richter et al. 1997) and increased potassium currents (Richter et al. 1991) in VRG neurones are causal for the transition from hypoxic augmentation to depression of the respiratory network. Preliminary reports of some of these findings have been given in abstract form (Schmidt-Garcon et al. 1994).