Induction of a calcium-dependent long-term enhancement of excitability in the rat olfactory bulb

We have investigated whether a transient increase in extracellular calcium concentration is able to induce long-term modification of neuronal excitability in the olfactory bulb. High-calcium artificial cerebrospinal fluid containing picrotoxin (Ca-PTX solution) was applied locally near the mitral cell layer through a push-pull device for 10 min in anaesthetized rats. Changes in the neuronal excitability were monitored through electrically-evoked field potentials. Application of the Ca-PTX solution induced a rapid increase in the granule cell response amplitude, whereas mitral/tufted cells response amplitude increased more progressively and reached its maximum within a few hours. The increase in mitral/tufted cells response and granule cells response reached between 30 and 100% in experiments which lasted for 4-8 h. Pre-application of amino-phosphonovalerate (NMDA receptor blocker) potently reduced both short- and long-term enhancement produced by the Ca-PTX solution. Neither application of the high calcium solution alone nor the picrotoxin solution alone induced long-term changes. The results point out the possible importance of Ca2+ and NMDA receptors in persistent forms of olfactory bulb plasticity. Relevance of this phenomenon in normal olfactory bulb physiology remains to be examined.