Excitatory amino acid receptors in rat locus coeruleus: an extracellular in vitro study

The goal of this study was to investigate whether locus coeruleus neurons of the rat are sensitive to agonists of the different excitatory amino acid receptors. All experiments were performed on a midpontine rat slice preparation. Bath-applied L-glutamate, kainate, N-methyl-D-aspartate (NMDA) and quisqualate induced concentration-dependent activations of all neurons which were reflected in an increase of the neurons' mean discharge rate. The rank order of cell activation was kainate approximately quisqualate greater than NMDA greater than L-glutamate. None of the agonists induced a bursting-type of discharge. The NMDA-receptor blocker DL-2-amino-5-phosphonovaleric acid (APV, 30 microM) selectively antagonized the NMDA-induced increase in cell firing. Kynurenic acid (100 microM) non-selectively attenuated the response to NMDA, kainate and quisqualate. Neither APV nor kynurenic acid per se had any effect on the spontaneous firing rate. If the Mg2+ concentration in the superfusion medium was lowered from 2 mM to nominally zero the response to NMDA was selectively increased. In conclusion, locus coeruleus neurons share with other neurons their sensitivity to agonists of all three types of excitatory amino acid receptors. However, in contrast to other neurons, they do not respond with a bursting type of discharge.