Dopaminergic antagonists prevent long-term maintenance of posttetanic LTP in the CA1 region of rat hippocampal slices.

The involvement of dopaminergic mechanisms in the induction and maintenance of posttetanic long-term potentiation (LTP) was investigated on CA1 cells of rat hippocampal slices. The presence of the dopamine receptor blocker domperidone in a concentration of 1 microM during tetanization with 3 trains of 100 impulses (100 Hz) and a train interval of 10 min influences neither the synaptic transmission nor the induction of LTP. However, the potentiation of both the population spike and the population EPSP gradually decreases, thus significantly differing from control LTP about 4 h after initiation and reaching the level of non-tetanized controls about 7-8 h after tetanization. The simultaneous presence of 1 microM apomorphine during tetanization abolishes this effect of domperidone indicating the specific dopaminolytic nature of its action. Also the presence of the dopamine antagonists sulpiride and flupenthixol, respectively, in a concentration of 1 microM during tetanization likewise prevents the occurrence of the late LTP maintenance. The determination of [14C]dopamine in 2 min fractions from the superfused slices after preloading during a preincubation period revealed that a low frequency stimulation of the Schaffer collaterals with 0.33 Hz does not influence the spontaneous efflux of dopamine, whereas the tetanization with an impulse train of 100 Hz produces a significantly enhanced release. The observations suggest that dopaminergic influences during and immediately after tetanization at least additionally contribute to the induction of postsynaptic mechanisms subserving a late, long-lasting maintenance of potentiation. The results also support the assumed existence of different subsequent stages of LTP.