A possible mechanism for the effect of neuromodulators and modifiable inhibition on long-term potentiation and depression of the excitatory inputs to hippocampal principal cells.

A postsynaptic mechanism for the influences of various neuromodulators and modifiable disynaptic inhibition on long-term potentiation and depression of the excitatory inputs to granule and pyramidal neurons in the hippocampus is described. According to this mechanism, facilitation of the induction of long-term depression/potentiation at the excitatory input to the inhibitory interneuron induced by the action of a neuromodulator on a receptor bound to a G(i/0)/(Gs or G(q/11)) protein can lead to decreases/increases in GABA release, weakening/strengthening of the inhibitory action on the target cell, and improvement in the conditions for induction of long-term potentiation/depression of the excitatory input to this cell. In the absence of inhibition, the same neuromodulator, activating the same type of receptors on the target cell, would facilitate induction of long-term depression/potentiation in that cell. The resultant effect of the action of the neuromodulator on the target cell depends on the ratio of the "strengths" of the excitatory and inhibitory inputs to the cell, on the presence on the interneuron and the target cell of the same or different types of receptors sensitive to this neurumodulator, and on the concentration of the neurumodulator, because of its different affinities for the receptors through which its differently directed effects on postsynaptic processes are mediated. Predictions based on this mechanism are in agreement with known experimental data.