Pharmacological characterization of ionotropic excitatory amino acid receptors in young and aged rat basal forebrain

Abstract

Ionotropic glutamate receptors were characterized in acutely dissociated medial septum/nucleus of diagonal band neurons from one- to four-month- and 24–26-month-old male Fischer 344 rats. Whole-cell patch-clamp recordings were used to study glutamate, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, kainate and N-methyl-d-aspartate-induced currents. Pharmacological properties of these ionotropic receptors were studied across different age groups by comparing concentration response curves and EC50 for agonist-induced currents, as well as dissociation constants (Kb) for competitive receptor antagonists. Our results suggest that non-N-methyl-d-aspartate receptors on medial septum/nucleus of diagonal band neurons were predominantly of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate type and display biophysical and pharmacological properties similar to other central neurons. However, peak α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate induced-currents were enhanced in aged (35.0±4.4pA/pF) compared to young cells (16.2±1.7pA/pF, P<0.005), and the EC50 shifted to the right (4.4±0.6 in young compared to 8.8±1.3μM in aged, P<0.05). The Kb for 6,7-dinitroquinoxaline-2,3-dione inhibition of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-induced currents likewise shifted to the right (0.16±0.02 in young and 0.29±0.04μM in aged, P<0.05) suggesting an age-related decrease in affinity for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors. N-methyl-d-aspartate-induced currents were generated in standard physiological solutions with the addition of 1mM glycine and the removal of Mg2+. The N-methyl-d-aspartate responses were predictably modulated by magnesium and glycine, and were antagonized by the competitive antagonist 2-amino-5-phosphonovaleric acid. No age-related change in N maximum, EC50, magnesium sensitivity, glycine sensitivity or Kb for 2-amino-5-phosphonovaleric acid was observed.Overall, our results suggest that ionotropic glutamate receptors in the medial septum/nucleus of diagonal band have a similar pharmacological profile compared to glutamate receptors in other brain regions. More importantly, these data suggest that while medial septum/nucleus of diagonal band cells maintain N-methyl-d-aspartate receptors during ageing, a significant increase in current density and decrease in receptor affinity for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors, during this same time period, may provide a mechanism for age-related changes in neuronal plasticity and excitotoxicity in the basal forebrain.