title: Role of adenosine in NMDA receptor modulation in the cerebral cortex of an anoxia-tolerant turtle (Chrysemys picta belli).
abstract: Accumulation of the neuromodulator adenosine in the anoxia-tolerant turtle brain may play a key role in a protective decrease in excitatory neurotransmission during anoxia. Since excitatory neurotransmission is mediated largely by Ca2+ entry through N-methyl-D-aspartate (NMDA) receptors, we measured the effect of adenosine on NMDA-mediated Ca2+ transients in normoxic and anoxic turtle cerebrocortical sheets. Intracellular [Ca2+] was measured fluorometrically with the Ca2+-sensitive dye Fura-2. Baseline intracellular [Ca2+] and [ATP] were also measured to assess cortical sheet viability and potential toxic effects of NMDA. Baseline [Ca2+] did not change significantly under any condition, ranging from 109 +/- 22 to 187 +/- 26 nmoll-1. Throughout normoxic and 2h anoxic protocols, and after single and multiple NMDA exposures, [ATP] did not change significantly, ranging from 16.0 +/- 1.9 to 25.3 +/- 4.9 nmol ATP mg-1 protein. Adenosine caused a reduction in the normoxic NMDA-mediated increase in [Ca2+] from a control level of 287 +/- 35 to 103 +/- 22 nmoll-1 (64%). This effect is mediated by the A1 receptor since 8-phenyltheophylline (a specific A1 antagonist) effectively blocked the adenosine effect and N6-cyclopentyladenosine (a specific A1 agonist) elicited a similar decrease in the NMDA-mediated response. Cortical sheets exposed to anoxia alone exhibited a 52% decrease in the NMDA-mediated [Ca2+] rise, from 232 +/- 30 to 111 +/- 9 nmoll-1. The addition of adenosine had no further effect and 8-phenyltheophylline did not antagonize the observed decrease. Therefore, the observed down-regulation of NMDA receptor activity during anoxia must involve additional, as yet unknown, mechanisms.
authors.affiliation: Department of Anesthesia, University of California, San Francisco 94143-0542, USA.
authors.forename: L T
authors.initials: LT
authors.lastname: Buck
authors.forename: P E
authors.initials: PE
authors.lastname: Bickler
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology* / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D005470:Fluorometry; D016257:Fura-2; D005971:Glutamates / Q000633:toxicity; D002534:Hypoxia, Brain / Q000378:metabolism / Q000662:veterinary*; D008297:Male; D016202:N-Methylaspartate / Q000633:toxicity; D058915:Purinergic P1 Receptor Antagonists; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism*; D018047:Receptors, Purinergic P1 / Q000378:metabolism; D013806:Theophylline / Q000031:analogs & derivatives / Q000494:pharmacology; D014426:Turtles / Q000378:metabolism*
chemical_list: D005971:Glutamates; D058915:Purinergic P1 Receptor Antagonists; D016194:Receptors, N-Methyl-D-Aspartate; D018047:Receptors, Purinergic P1; C048599:N(6)-cyclopentyladenosine; D016202:N-Methylaspartate; D013806:Theophylline; C028322:8-phenyltheophylline; D000241:Adenosine; D002118:Calcium; D016257:Fura-2