Involvement of 5‐HT1A‐ and α2‐receptors in the decreased 5‐hydroxytryptamine release and metabolism in rat suprachiasmatic nucleus after intravenous 8‐hydroxy‐2‐ (n‐dipropylamino) tetralin

Abstract

1 The 5-hydroxytryptamine_1A-receptor agonist 8-hydroxy-2-(n-dipropylamino) tetralin (8-OH-DPAT, 0.1 mg kg⁻¹ i.v.) decreased the height of the extracellular 5-hydroxyindoleacetic acid (5-HIAA) oxidation peak recorded in the suprachiasmatic nucleus (SCN) of the anaesthetized rat by use of differential pulse voltammetry. 2 The decrease in extracellular 5-HIAA produced by 8-OH-DPAT could be partially attenuated by prior administration of the non-selective 5-HT receptor antagonist methiothepin (1 mg kg⁻¹ i.v.). The 5-HT₂-receptor antagonist ritanserin (0.2 mg kg⁻¹ i.v.) did not appear to block the effects of 8-OH-DPAT. 3 The selective ligand for 5-HT_1A recognition sites TVX Q 7821 (isapirone, 1 mg kg⁻¹ i.v.) decreased the extracellular level of 5-HIAA in the SCN but to a lesser extent than 8-OH-DPAT. The response to 8-OH-DPAT was attenuated by prior administration of TVX Q 7821 to a level suggesting that TVX Q 7821 had blocked the effect of intravenous 8-OH-DPAT. 4 Idazoxan (0.2 mg kg⁻¹ i.v.) an α₂-adrenoceptor antagonist, completely blocked the effect of 8-OH-DPAT on the 5-HIAA oxidation peak recorded in the SCN, whilst having no effect on the 5-HIAA oxidation peak when given alone. 5 At a dose of 0.5 mg kg⁻¹ i.v. idazoxan induced a 120% increase in the height of the indole oxidation peak, suggesting that 5-HT release and metabolism in the rat SCN may be influenced by tonic adrenergic inputs. 6 The data in this paper suggest that 5-HT_1A- and α₂-receptors are involved in the effects of i.v. administered 8-OH-DPAT on 5-HT release and metabolism in the SCN in vivo.