Acute Ca2+-dependent desensitization of 5-HT1A receptors is mediated by activation of protein kinase A (PKA) in rat serotonergic neurons

This report investigates acute changes in the sensitivity of 5-HT1A receptors in dorsal raphe (dr) neurons in response to elevated serotonin. DR neurons were isolated from adult rats and measurements of inhibition of Ca2+ current by 5-HT were obtained using the whole cell patch clamp technique. During a 10-min application of 5-HT (with normal [Ca2+]i∼100 nM) a desensitization occurred. The response to 20 nM 5-HT decreased by 66% relative to control and remained depressed for about 30 min. When the internal [Ca2+] was buffered to <1 nM only a weak transient desensitization occurred that was surmountable with higher [5-HT]. Adenylyl cyclase activation with forskolin mimicked the desensitization and selective inhibition of protein kinase A (PKA), but not protein kinase C (PKC), partially antagonized the desensitization induced by 5-HT. To measure the activity of PKA and phosphatase enzymes, dr slices were incubated with the selective agonist dipropyl-5-carboxamidotryptamine (DP-5-CT, 1 μM) for 10 min and the phosphorylation of the PKA substrate Kemptide was followed using ATP-γ32P. DP-5-CT inhibited the cAMP stimulated maximal activity of PKA but raised basal PKA activity, thus increasing the percentage of PKA in the active state (activity ratio), an effect that was prevented by the selective 5-HT1A antagonist WAY100635. DP-5-CT also caused a significant inhibition of phosphatase activity. These data support a model in the dr where 5-HT1A-receptor stimulation of PKA promotes phosphorylation of a target and phosphatase inhibition leading to heterologous desensitization. The effect would be expected to have physiological consequences for 5-HT-mediated inhibitory post synaptic potentials and the Ca2+ component of the action potentials of dr neurons.