Slowly developing potentiation in goldfish retino-tectal synapses is masked by prior stimulation; an in vitro study

Field potential responses to local electrical stimulation were analyzed in slices of goldfish optic tectum (OT) in vitro. The post-synaptic component of retino- tectal origin was suppressed by DNQX, suggesting glutamatergic f ield excitatory post-synaptic potential (fEPSP). Repetitive stimulation at 0.1 Hz for 30 min resulted in a slowly developing potentiation in the fEPSP in a population of slices examined. When stimulation at a lower frequency (0.017 Hz for 30 min) preceded the conditioning, the 0.1 Hz stimulation fai led to induce the potentiation even though the 0.017 Hz stimulation by itse lf caused no lasting changes. Induction of the potentiation was partially b locked by AP5 (100 mu M), suggesting an involvement of NMDA receptors, howe ver the effect was not significant. We also examined a possible involvement of adenosine, which is supposed to be co-released with glutamate at synapt ic terminals. Exogenously applied adenosine (200 mu M) slowly depressed fEP SP, and the depression was blocked bq DPCPX (Al receptor antagonist: 400 nM ) but not by DMPX (A2 antagonist: 4 mu M). However, DPCPX failed to unmask the potentiation after low-frequency stimulation (0.017 Hz for 30 min). Mor eover, the potentiability was not correlated with sensitivity to exogenous adenosine. The Al depression may not underlie the masking effect. (C) 1999 Elsevier Science Inc. All rights reserved.