Membrane potentials and microenvironment of rat dorsal vagal cells in vitro during energy depletion.

Abstract

1. Brainstem slices were taken from mature rats. In the dorsal vagal nucleus (DVNX), membrane potentials (Em) of neurons (DVNs) and glia, as well as extracellular oxygen, K+ and pH (Po2, aKO, pHO), were analysed during metabolic disturbances.2. Postsynaptic potentials of DVNs, elicited by repetitive electrical stimulation of the solitary tract (TS), led to a secondary glial depolarization of up to 25 mV, a fall in Po. of up to 150 mmHg, a rise in extracellular aKo of up to 9 mm, and a fall in pHo of about 0-2 pH units.3. Hypoxic superfusates produced tissue anoxia, leading to an aK. increase of less than 2 mM and a pHo fall of 024 + 0 04 pH units (mean + S.D.). Glucose-free solution evoked, after a delay of more than 8 min, a slow rise in aKo of 1-9 + 0-8 mm, accompanied by a mean increase in pHo of 0-24 + 0-13 pH units. After pre-incubation in glucose-free solution, anoxia elevated aKo by up to 15 mm, whereas the anoxia-induced pHo decrease was completely blocked. 4. In 45 of 118 DVNs, anoxia elicited a persistent hyperpolarization of 15'6 + 5 0 mV. In the remaining DVNs, anoxic exposure either did not produce a change in Em (37 %) or led to a depolarization of less than 10 mV (25 %). A stable depolarization of 9 + 3 8 mV was detected in glial cells during anoxia. Similar responses were revealed in oxygenated glucose-free solution after a delay of 12-60 min.5. The metabolism-related hyperpolarizations were blocked by 100-500 /uM tolbutamide or 20-100 AM glibenclamide, leading to recovery of spontaneous (0-5-6 Hz) spike discharge. In these cells, 400-500 uM diazoxide evoked hyperpolarizations and blockade of spontaneous activity. 6. In DVNs and glial cells, a progressive depolarization of up to 40 mV in amplitude developed during anoxic exposure after pre-incubation in glucose-free solution. 7. The results show that oxygen or glucose depletion does not impair the viability of DVNX cells. The contribution of neuronal ATP-sensitive K+ (KATP) channels to this tolerance is discussed.

Protective role of neuronal KATP channels in brain hypoxia

“…This is certainly not due to the fact that the cells were whole-cell recorded with patch-electrodes containing 1-2·mmol·l -1 ATP. A similar lack of occurrence of a terminal depolarisation or massive Cai increase is observed when the dorsal vagal neurons are recorded with sharp microelectrodes (Cowan and Martin, 1992;Ballanyi et al, 1996a) or optically in a non-invasive manner using a membrane-permeable form of fura-2 . According to these results, it is proposed that the tolerance to anoxia of these mammalian neurons is rather due to a low resting metabolic rate in conjunction with effective utilisation of anaerobic metabolism (Ballanyi et al, 1996a;Trapp et al, 1996;Ballanyi and Kulik, 1998).…”

“…Interestingly, these cells are particularly resistant to oxygen depletion, at least in rats. They respond to hypoxic solutions resulting in anoxia of the dorsal vagal nucleus (Ballanyi et al, 1996a) with a sustained K + channel-mediated hyperpolarisation ( Fig.·2A; Cowan and Martin, 1992;Trapp and Ballanyi, 1995). Neither hypoxic anoxia nor chemical block of aerobic metabolism with K. Ballanyi Fig.·1.…”

“…Subpopulations of mammalian neurons (Ballanyi et al, 1996a;Richter and Ballanyi, 1996) are highly tolerant to anoxia in a fashion similar to brain structures of some coldblooded vertebrates (Lutz and Nilsson, 1994;Hochachka and Lutz, 2001). By elucidating the mechanisms of the anoxia tolerance of these mammalian neurons, it may be possible to develop pharmacological strategies to protect the brain from the consequences of hypoxic-anoxic insults.…”

“…Brain section taken from Paxinos (1982 Trapp and Ballanyi, 1995;Ballanyi and Kulik, 1998). By contrast, a progressive depolarisation due to extracellular accumulation of glutamate is revealed in slices of dorsal vagal neurons preincubated in glucosefree superfusate and subsequently exposed to hypoxic or chemical anoxia in the absence of glucose, mimicking ischemia in vitro (Ballanyi et al, 1996a;Kulik et al, 2000). The anoxic hyperpolarisation and the underlying outward current and increase in membrane conductance of the dorsal vagal neurons are blocked by the sulfonylurea KATP channel blockers tolbutamide, glibenclamide and gliquidone (Figs·2A,·3A).…”

The Na+/H+ Exchanger-1 as a New Molecular Target in Stroke Interventions

“…In hippocampal slices, hypoxia induces a significant drop in both pH i and pH o , and a brief alkaline peak is also occasionally observed (Fujiwara et al, 1992;Melzian et al, 1996;Roberts & Chih, 1997). In slice preparations from various brain regions, hypoxia causes acidosis with an approximately 0.8-1.2 pH i unit drop (Ballanyi et al, 1996;Knopfel et al, 1998;Pirttila & Kauppinen, 1994). Cytosolic calcium changes are observed during ischemia in cortical brain slices that can be only partially inhibited by combined blockade of ion channels (Bickler and Hansen, 1994).…”