Cyclic adenosine 3'5'-monophosphate potentiates excitatory amino acid and synaptic responses of rat spinal dorsal horn neurons.

Intracellular recordings were made from rat dorsal horn neurons in the in vitro slice preparation to study the actions of cyclic adenosine 3',5'-monophosphate (cyclic AMP). In the presence of TTX, bath application of the membrane permeable analogue of cyclic AMP, 8-Br cyclic AMP (25-100 microM) caused a small depolarization of the resting membrane potential accompanied by a variable change in membrane input resistance. In addition, 8-Br cyclic AMP caused a long-lasting increase in the spontaneous synaptic activity and the amplitude of presumed monosynaptic excitatory postsynaptic potentials evoked in the substantia gelatinosa neurons by orthodromic stimulation of a lumbar dorsal root. When the fast voltage-sensitive Na conductance was blocked by TTX, 8-Br cyclic AMP enhanced in a reversible manner, the depolarizing responses of a proportion of dorsal horn neurons to N-methyl-D-aspartic acid (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), quisqualic acid (QA) and kainic acid (KA). The effects of 8-Br cyclic AMP on the resting membrane potential and the NMDA response of dorsal horn neurons were mimicked by reducing phosphodiesterase activity with bath application of 3-isobutyl-1-methylxanthine, but not by cyclic AMP applied extracellularly. Moreover, we have found that intracellular application of a protein inhibitor of cyclic AMP-dependent protein kinase (PKI) into dorsal horn neurons prevents the 8-Br cyclic AMP-induced potentiation of the NMDA response of these cells. These results suggest that in the rat spinal dorsal horn the activation of the adenylate cyclase-cyclic AMP-dependent protein kinase system may be involved in the enhancement of the sensitivity of postsynaptic excitatory amino acid (NMDA, AMPA, KA) receptors and modulation of primary afferent neurotransmission, including nociception.