Excitatory amino acid-receptor-mediated EPSPs in rat dorsolateral septal nucleus neurones in vitro.

1. Intracellular recordings were made from rat dorsolateral septal nucleus (DLSN) neurones in vitro. We investigated depolarizations resulting from pressure application of excitatory amino acids and compared these to synaptically evoked excitatory postsynaptic potentials (EPSPs). 2. EPSPs evoked by focal fimbrial afferent stimulation in saline with 30-50 microM-bicuculline and 1.2 mM-Mg2+ yielded a linear amplitude-voltage relationship: their reversal potential was -3 mV. These EPSPs exhibited little sensitivity to 2-amino-5-phosphonopentanoate (APV), an N-methyl-D-aspartate(NMDA)-receptor-specific antagonist, but were markedly depressed by kynurenic acid, a broad-spectrum excitatory amino acid antagonist. 3. In Mg2(+)-free solution, the amplitude and the duration of EPSPs were increased markedly masking the following inhibitory postsynaptic potential (IPSP) and the late hyperpolarizing potential (LHP). These facilitated and broadened EPSPs were sensitive to APV or Mg2+. The APV or Mg2(+)-sensitive component of the EPSP obtained by digital subtraction suggests a slower time course for the NMDA-receptor-mediated EPSP compared to the non-NMDA-receptor-mediated EPSP. On the other hand, in normal Mg2+ solution an EPSP evoked by either a single strong stimulus or by repetitive stimuli had APV-sensitive components. 4. The depolarizing potentials induced by pressure application of glutamate, kainate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), quisqualate or NMDA were compared. The amplitude-voltage relationship of depolarizations induced by NMDA obtained in a normal Mg2+ solution was non-linear, but approached linearity when the same responses were recorded in a Mg2(+)-free solution. Depolarizations induced by kainate, AMPA and quisqualate were linear in their amplitude-voltage relationship in the presence or absence of Mg2+. APV blocked NMDA-induced depolarizations specifically, while kynurenic acid blocked all the depolarizations induced by NMDA, quisqualate, or kainate. 5. Our data demonstrate the existence of NMDA-receptor-mediated synaptic potentials in the rat DLSN, the characteristics of which are similar to those in other central nervous system regions.