In the central nervous system (CNS), neurons and glia communicate via extracellular signalling molecules. It has been proposed that the activity-dependent release of potassium (K⁺) or glutamate (Glu) is a way in which axons communicate with glia in CNS white matter (Ransom & Orkand, 1996). Previously, we presented evidence that Glu induced raised cytosolic [Ca²⁺]_i in glial cells of the neonatal rat optic nerve (Butt et al. 2001). Here, the interactions between K⁺ and Glu were studied in the developing rat optic nerve (RON), using K⁺-sensitive microelectrodes. Rats, aged postnatal day (P) 4-25, were humanely killed by anaesthetic overdose (I.P. admistration of sodium pentobarbitol), and the optic nerves were immediately removed to a brain slice chamber and perfused with oxygenated artificial cerebrospinal fluid (ACSF) containing (mM): NaCl, 133; KCl, 3; CaCl₂, 1.5; NaH₂PO₄, 1.2; MgCl₂, 1.0; D-glucose, 10; Hepes, 10; pH 7.4. Glass micropipettes were pulled to a tip size of approximately 10 µm, and either back-filled with 0.1 M KCl, for the reference electrode, or silanised and the tip filled with K⁺-sensitive resin (Fluka, potassium ionophore cocktail B) and then backfilled with 0.1 M KCl. K⁺-sensitive microelectrodes were calibrated and tested for a Nernstian response at the beginning and end of each experiment, using a series of NaCl:KCl solutions. Optic nerves were impaled with both reference and K⁺-sensitive microelectrodes and the change in [K⁺]_o was measured by differential recording (WPI Duo 773). Solutions were perfused over the nerve surface at approximately 5 ml min^-1 via a six-way tap that allowed the rapid change-over of solutions. Test solutions were applied for 1-2 min and a recovery period of 8-10 min was allowed between tests. In neonatal nerves, P4-7, application of Glu induced increased [K⁺]_o in a concentration-dependent manner (0.1-10 mM), with a maximal response at 5 mM and a half-maximal response at approximately 1 mM, which was used in subsequent experiments. The mean (± S.E.M.) response to 1 mM Glu at P4-7 was an increase in [K⁺]_o of 3.7 ± 0.02 mM (n = 28). Changes in [K⁺]_o were measured during application of Glu (1 mM), kainic acid (KA, 0.1 mM) or quisqualate (Quis, 0.1 mM), which act on ionotropic (i) GluR and metabotropic (m) GluR. In paired experiments (n = 5-12), all ligands evoked an increase in [K⁺]_o, KA (7.0 ± 0.7 mM) > Quis (4.1 ± 0.2 mM) > Glu (3.7 ± 0.04), which were significantly decreased (P < 0.05, paired t tests) by the non-NMDA receptor antagonist CNQX (10 µM). NMDA failed to evoke any response (n = 5). The mGluR agonist (trans-ACPD, 0.1 mM) and antagonist (EC4PG, 0.1 mM) had no effect. The Glu-evoked increase in [K⁺]_o was not affected by either DHK (0.05 mM) or PDC (0.05 or 0.5 mM), respectively blockers of the glial (GLT-1) and neuronal (EAAC1) Glu transporters. The Glu-evoked increase in [K⁺]_o attenuated with age, and responses to Glu, KA and Quis were significantly reduced in P25 nerves. The results demonstrate functional iGluR in the RON and support a role for Glu and K⁺ in non-synaptic signalling in the development of this white matter tract.

Butt, A.M., Pugh, M.E. & James, G. (2001). J. Physiol. 531.P, 106P.

Ransom, B.R. & Orkand, R.K. (1996). Trends Neurosci. 19, 352-357.