FUNCTIONAL-CHARACTERIZATION OF 5-HT1D AUTORECEPTORS ON THE MODULATIONOF 5-HT RELEASE IN GUINEA-PIG MESENCEPHALIC RAPHE, HIPPOCAMPUS AND FRONTAL-CORTEX

1 The aims of the present study were (i) to characterize further the p harmacology of 5-HT1D autoreceptors modulating 5-HT release in guinea- pig mesencephalic raphe, hippocampus and frontal cortex; (ii) to deter mine whether 5-HT1D receptors in the mesencephalic raphe are located o n 5-HT neurones; (iii) to determine whether 5-HT1D autoreceptors are c oupled to G proteins; and (iv) to assess their sensitivity following l ong-term 5-HT reuptake blockade and inhibition of type-A monoamine oxi dase. 2 In mesencephalic raphe, hippocampus and frontal cortex slices, the 5-HT1D/1B receptor agonist, sumatriptan and the 5-HT1 receptor ag onist, 5-methoxytryptamine (5-MeOT) but not the 5-HT1B receptor agonis t, CP93129, inhibited electrically the evoked release of [H-3]-5-HT in a concentration-dependent manner. This effect was antagonized by the 5-HT1D/1B receptor antagonist GR127935 in the three structures, but no t by the 5-HT1A receptor antagonist, (+)-WAY100635 in mesencephalic ra phe slices. These results confirm the presence of functional 5-HT1D au toreceptors controlling 5-HT release within the mesencephalic raphe as well as in terminal regions. 3 The inhibitory effect of sumatriptan o n K+-evoked release of [H-3]-5-HT was not reduced by the addition of t he Na+ channel blocker, tetrodotoxin to the superfusion medium, sugges ting that these 5-HT1D receptors in the mesencephalic raphe are locate d on 5-HT neurones and may be considered autoreceptors. 4 The in vitro treatment with the alkylating agent N-ethylmaleimide (NEM) was used t o determine whether these 5-HT1D autoreceptors are coupled to G protei ns. The inhibitory effect of sumatriptan on electrically evoked releas e of [H-3]-5-HT was attenuated in NEM-pretreated slices from mesenceph alic raphe, hippocampus and frontal cortex, indicating that the 5-HT1D , autoreceptors activated by sumatriptan are coupled to G proteins in these three structures. Taken together with our previous results, this suggests that, in addition to the 5-HT1D autoreceptor activated by su matriptan, another subtype of 5-HT autoreceptor is activated by 5-MeOT in the hippocampus. 5 Following a 3-week treatment with the selective 5-HT reuptake inhibitor, paroxetine (10 mg kg(-1) day(-1)) and a 48 h washout period, the electrically evoked release of [H-3]-5-HT was enh anced in mesencephalic raphe, hippocampus and frontal cortex slices. T here was an attenuation of the capacity of sumatriptan to inhibit the evoked release of [H-3]-5-HT from mesencephalic raphe slices but not f rom frontal cortex and hippocampus slices. Only in the latter structur e was the suppressant effect of 5-MeOT attenuated. After a 3-week trea tment with the reversible type-A monoamine oxidase inhibitor, befloxat one (0.75 mg kg(-1) day(-1)) and 48 h washout period, the effectivenes s of sumatriptan and 5-MeOT on the evoked release of [H-3]-5-HT was un altered in the same brain structures. 6 The enhancement of [H-3]-5-HT release by long-term paroxetine treatment is possibly due to a desensi tization of 5-HT1D autoreceptors activated by sumatriptan in mesenceph alic raphe and by terminal 5-HT autoreceptors activated by 5-MeOT in h ippocampus. In the case of the frontal cortex, it appears that 5-MeOT and sumatriptan may act on the same 5-HT1D autoreceptor which is not d esensitized either after paroxetine or befloxatone treatment, as previ ously reported.