Plasticity of first-order sensory synapses: interactions between homosynaptic long-term potentiation and heterosynaptically evoked dopaminergic potentiation.

Persistent potentiations of the chemical and electrotonic components of the eighth nerve (NVIII) EPSP recorded in vivo in the goldfish reticulospinal neuron, the Mauthner cell, can be evoked by afferent tetanization or local dendritic application of an endogenous transmitter, dopamine (3-hydroxytyramine). These modifications are attributable to the activation of distinct intracellular kinase cascades. Although dopamine-evoked potentiation (DEP) is mediated by the cAMP-dependent protein kinase (PKA), tetanization most likely activates a Ca2+-dependent protein kinase via an increased intracellular Ca2+ concentration. We present evidence that the eighth nerve tetanus that induces LTP does not act by triggering dopamine release, because it is evoked in the presence of a broad spectrum of dopamine antagonists. To test for interactions between these pathways, we applied the potentiating paradigms sequentially. When dopamine was applied first, tetanization produced additional potentiation of the mixed synaptic response, but when the sequence was reversed, DEP was occluded, indicating that the synapses potentiated by the two procedures belong to the same or overlapping populations. Experiments were conducted to determine interactions between the underlying regulatory mechanisms and the level of their convergence. Inhibiting PKA does not impede tetanus-induced LTP, and chelating postsynaptic Ca2+ with BAPTA does not block DEP, indicating that the initial steps of the induction processes are independent. Pharmacological and voltage-clamp analyses indicate that the two pathways converge on functional AMPA/kainate receptors for the chemically mediated EPSP and gap junctions for the electrotonic component or at intermediaries common to both pathways. A cellular model incorporating these interactions is proposed on the basis of differential modulation of synaptic responses via receptor-protein phosphorylation.