Serotonin (5-HT), the endogenous nonselective 5-HT receptor agonist, activates the inositol-1,4,5-triphosphate/calcium $(InsP3/Ca^{2+})$ signaling pathway and exerts both stimulatory and inhibitory actions on cAMP production and neuromodulin secretion in rat hypothalamic neurons. Specific mRNA transcripts for 5-HT1A, 5-HT2C and 5-HT4 were identified in rat hypothalamic neurons. These experiments were supported by combined techniques such as cAMP and a $Ca^{2+}$ assays in order to elucidate the associated receptors and signaling pathways. The cAMP production and neuromodulin release were profoundly inhibited during the activation of the Gi-coupled 5-HT1A receptor. Treatment with a selective agonist to activate the Gq-coupled 5-HT2C receptor stimulated InsP3 production and caused $Ca^{2+}$ release from the sarcoplasmic reticulum. Selective activation of the Gs-coupled 5-HT4 receptor also stimulated cAMP production, and caused an increase in neuromodulin secretion. These findings demonstrate the ability of 5-HT receptor subtypes expressed in neurons to induce neuromodulin production. This leads to the activation of single or multiple G-proteins which regulate the $InsP3/Ca^{2+}/PLC-{gamma}$ and adenyl cyclase / cAMP signaling pathways.