Functional characterization of 5‐HT1D autoreceptors on the modulation of 5‐HT release in guinea‐pig mesencephalic raphe, hippocampus and frontal cortex
Authors: Mansari Mostafa El , Blier Pierre 
Published: July 2012

The aims of the present study were (i) to characterize further the pharmacology of 5‐HT1D autoreceptors modulating 5‐HT release in guinea‐pig mesencephalic raphe, hippocampus and frontal cortex; (ii) to determine whether 5‐HT1D receptors in the mesencephalic raphe are located on 5‐HT neurones; (iii) to determine whether 5‐HT1D autoreceptors are coupled to G proteins; and (iv) to assess their sensitivity following long‐term 5‐HT reuptake blockade and inhibition of type‐A monoamine oxidase.

In mesencephalic raphe, hippocampus and frontal cortex slices, the 5‐HT1D/1B receptor agonist, sumatriptan and the 5‐HT1 receptor agonist, 5‐methoxytryptamine (5‐MeOT) but not the 5‐HT1B receptor agonist, CP93129, inhibited electrically the evoked release of [3H]‐5‐HT in a concentration‐dependent manner. This effect was antagonized by the 5‐HT1D/1B receptor antagonist GR127935 in the three structures, but not by the 5‐HT1A receptor antagonist, (+)‐WAY100635 in mesencephalic raphe slices. These results confirm the presence of functional 5‐HT1D autoreceptors controlling 5‐HT release within the mesencephalic raphe as well as in terminal regions.

The inhibitory effect of sumatriptan on K+‐evoked release of [3H]‐5‐HT was not reduced by the addition of the Na+ channel blocker, tetrodotoxin to the superfusion medium, suggesting that these 5‐HT1D receptors in the mesencephalic raphe are located on 5‐HT neurones and may be considered autoreceptors.

The in vitro treatment with the alkylating agent N‐ethylmaleimide (NEM) was used to determine whether these 5‐HT1D autoreceptors are coupled to G proteins. The inhibitory effect of sumatriptan on electrically evoked release of [3H]‐5‐HT was attenuated in NEM‐pretreated slices from mesencephalic 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 sumatriptan, another subtype of 5‐HT autoreceptor is activated by 5‐MeOT in the hippocampus.

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 [3H]‐5‐HT was enhanced in mesencephalic raphe, hippocampus and frontal cortex slices. There was an attenuation of the capacity of sumatriptan to inhibit the evoked release of [3H]‐5‐HT from mesencephalic raphe slices but not from frontal cortex and hippocampus slices. Only in the latter structure was the suppressant effect of 5‐MeOT attenuated. After a 3‐week treatment with the reversible type‐A monoamine oxidase inhibitor, befloxatone (0.75 mg kg−1 day−1) and 48 h washout period, the effectiveness of sumatriptan and 5‐MeOT on the evoked release of [3H]‐5‐HT was unaltered in the same brain structures.

The enhancement of [3H]‐5‐HT release by long‐term paroxetine treatment is possibly due to a desensitization of 5‐HT1D autoreceptors activated by sumatriptan in mesencephalic raphe and by terminal 5‐HT autoreceptors activated by 5‐MeOT in hippocampus. 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 desensitized either after paroxetine or befloxatone treatment, as previously reported.