Cyclic AMP endogenously enhances synaptic strength of developing glutamatergic synapses in serum-free microcultures of rat hippocampal neurons.

The time course of development of autaptic and synaptic connections and the contribution of endogenously activated cAMP signaling to the regulation of AMPA/kainate receptor-mediated synaptic transmission were studied in microcultures of isolated single hippocampal neurons or of pairs of neurons grown on astrocytic islands in serum-free culture medium. Standard whole cell patch clamp techniques were employed to monitor evoked and spontaneous autaptic and synaptic currents. Glutamatergic synaptic transmission became detectable after 4 days in vitro (DIV). After 9-10 DIV more than 80% of the neurons had developed glutamatergic autaptic and synaptic connections. Elevation of intracellular cAMP levels by application of forskolin (20 microM) or IBMX (200 microM) to autaptic neurons resulted in enhanced autaptic current amplitudes (forskolin: 146 +/- 9%, IBMX: 177 +/- 21% of control) and impaired paired pulse facilitation (PPF). Likewise, intracellular application of cAMP via the patch pipette into autaptic neurons or into the presynaptic neuron of a synaptically connected pair also resulted in enhanced autaptic/synaptic current amplitudes (170 +/- 16% of control). In contrast, injection of cAMP into the postsynaptic neuron of a synaptic pair failed to significantly enhance the synaptic responses. The magnitude of the cAMP-mediated enhancement depended on the initial autaptic/synaptic strength observed in an individual cell, with small autapses/synapses being enhanced more effectively. Application of an inhibitor of cAMP-mediated processes (Rp-cAMPS) reversibly reduced autaptic/synaptic current amplitudes (to 75 +/- 5% of control). Taken together, these results suggest that cAMP-mediated processes endogenously enhance the efficacy of developing glutamatergic autaptic and synaptic connections in serum-free microcultures of isolated hippocampal neurons.