A wide array of interdisciplinary experiments have served to strengthen the general premise that some central synaptic effects of neurotransmitters may be mediated by cyclic nucleotides. Specific instances of such second messenger mediation are most strongly supported for certain noradrenergic connections of the locus coeruleus (LC) and for dopaminergic connections within the caudate nucleus. In these sites catecholamines selectively activate intraneuronal cyclic AMP synthesis, and exogenously applied cyclic AMP closely mimics the biophysical actions of the catecholamine on target neurons. In cerebellar cortex, iontophoresis of norepinephrine, stimulation of LC, or iontophoresis of cyclic AMP lead to hyperpolarization and increased membrane resistance. Although overtly inhibitory when examined in isolation, the effects of the LC system coupled to adenylate cyclase can initiate a holistic set of target cell responses which can enhance or "enable" the actions of other synaptic inputs to the target cells. Electrophysiologic and immunocytochemical evidence suggests that this heterosynaptic interaction may arise from phosphorylation of the synaptic membrane substrate of cyclic AMP-dependent protein kinase, or Protein 1.