Serotonin suppresses the slow afterhyperpolarization in rat intralaminar and midline thalamic neurones by activating 5-HT7 receptors

While the highest expression level of 5-HT7 receptors in the brain is observed in intralaminar and midline thalamic neurotics, the physiological role of these receptors in this structure is unknown. In vivo recordings have shown that stimulation of the serotonergic raphe nuclei can alter the response of these neurotics to a nociceptive stimulus, suggesting that serotonin modulates their firing properties. Using the patch-clamp technique in rat thalamic brain slices, we demonstrate that activation of 5-HT7 receptors can strongly modulate the excitability of intralaminar and midline thalamic neurotics by inhibiting the calcium-activated potassium conductance that is responsible for the slow afterhyperpolarization (sAHP) following a spike discharge. This sAHP was inhibited after activation of the cAMP pathway, either by bath application of forskolin or intracellular perfusion with 8-bromo-cAMP. The inhibitory effect of 5-HT7 receptors on sAHPs was blocked by the protein kinase A antagonist R-p-cAMPS. Calcium-imaging experiments showed no change in intracellular calcium levels during the 5-HT7 response, indicating that in these neurotics, a global calcium signal was not necessary to activate the cAMP cascade. Finally, bath application of serotonin produced a strong increase in cytosolic cAMP concentration, as measured using the fluorescent probe FICRhR, and an inhibition of the sAHP. Taken together, these results suggest that 5-HT7 receptors are implicated in the effect of 5-HT on sAHP in intralaminar and midline thalamic neurotics, an effect that is mediated by the cAMP second-messenger cascade.