The activation of 5-HT1A receptors modulates autonomic and central functions such as mood and anxiety. A better understanding of serotonergic neurons is critical to the development of improved pharmacological intervention in mental illness. 5-HT1A receptors are found on the Bell bodies of serotonergic dorsa l raphe (DR) neurons where their function is not understood. Biochemical studies have shown that the coupling of this receptor can be altered by stimulation of the cellular enzyme protein kinase C (PKC), but with a few exceptions studies have not investigated the effect of PKC activation on 5-HT1A coupling to ion channels. In DR neurons there are two consequences of receptor activation (l) opening of an inwardly rectifying K+ channel and (2) inhibition of voltage dependent Ca2+ influx. In DR neurons we have found that a certain level of stimulation of PKC uncouples the effect of 5-HT from Ca2+ current inhibition but unexpectedly it had no effect on the coupling of 5-HT to the activation of K+ channels. 5-HT-induced inhibition of Ca2+ influx in nerve cell bodies and terminals during an action potential may decrease the size of Ca2+-activated K+ conductances. The 5-HT-induced opening of inwardly rectifying K+ channels would elicit a neuronal hyperpolarization. If our hypothesis is correct only the actions of 5-HT on Ca2+-dependent potentials would be decreased by activation of PKC. Our preliminary data suggests that PKC activation has a negative feedback effect on one branch of the 5-HT1A receptor transduction fork namely inhibition of Ca2+ influx. Assuming that 5-HT does not directly activate PKC in DR neurons, prolonged depolarization or activation of metabotropic receptors which both increase intracellular Ca2+ should physiologically activate PKC. This suggests the possibility that a signalling pathway may be altered in an activity dependent manner. The functional weight of one pathway could be increased, possibly at the expense of another, and this may represent a short term form of synaptic plasticity. The overall aims of this proposal are threefold: (1) to determine whether the activation of protein kinase in isolated DR neurons alters the modulation of both arms of the 5-HT1A receptor transduction pathway equally; or whether there is a selective action on the coupling of this receptor only to Ca2+ channels. (2) To explore the mechanism of this effect using the patch clamp technique. (3) To investigate whether these observations can be observed in a more physiological setting, by examining the effect of 5-HT and phorbol esters on the Ca2+ activated K+ conductance of DR neurons in the slice preparation.
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