In vivo control of 5-hydroxytryptamine release by terminal autoreceptors in rat brain areas differentially innervated by the dorsal and median raphe nuclei
Selective serotonin reuptake inhibitors (SSRIs) reduce the 5-HT release in vivo. This effect is due to the activation of somatodendritic 5-HT1A receptors and it displays a regional pattern comparable to that of selective 5-HT1A agonists, i.e., preferentially in forebrain areas innervated by the dorsal raphe nucleus (DRN). However, despite a comparatively lower 5-HT1A-mediated inhibition of 5-HT release and a greater density of serotonergic uptake sites in hippocampus, the net elevation produced by the systemic administration of SSRIs is similar in various forebrain areas, regardless of the origin of serotonergic fibres. As terminal autoreceptors may also limit the SSRI-induced elevations of 5-HT in the extracellular brain space, we reasoned that a differential control of 5-HT release by terminal autoreceptors in DRN- and median raphe-innervated areas might be accountable. To examine this possibility, we have conducted a regional microdialysis study in the DRN, MRN and four forebrain regions preferentially innervated either by the DRN (frontal cortex, striatum) or the median raphe nucleus (MRN; dorsal and ventral hippocampus) using freely moving rats. Dialysis probes were perfused with 1 µM of the SSRI citalopram to augment the endogenous tone on terminal 5-HT autoreceptors. The non-selective 5-HT1 antagonist methiothepin (10 and 100 µM, dissolved in the dialysis fluid) increased extracellular 5-HT in frontal cortex and dorsal hippocampus in a concentration-dependent manner. The 5-HT1B/1D antagonist GR 127935 was ineffective at 10 µM and tended to reduce 5-HT in dorsal hippocampus at 100 µM. The local infusion of 100 µM methiothepin significantly elevated the extracellular 5-HT concentration to 142–173% of baseline (mean values of 260 min post-administration) in the DRN, MRN, frontal cortex, striatum and hippocampus (dorsal and ventral). Comparable elevations were noted in the four forebrain regions examined. As observed in frontal cortex and dorsal hippocampus, the perfusion of 10 µM GR 127935 did not elevate 5-HT in DRN, MRN, striatum or ventral hippocampus. Because the stimulated 5-HT release in the DRN has been suggested to be under control of 5-HT1B/1D receptors, we examined the possible contribution of these receptor subtypes to the effects of methiothepin in the DRN. The perfusion of sumatriptan (0.01–10 µM) or GR 127935 (0.01–10 µM) did not significantly modify the 5-HT concentration in dialysates from the DRN. Thus, the present data suggest that the comparable effects of SSRIs in DRN- and MRN-innervated forebrain regions are not explained by a preferential attenuation of 5-HT release by terminal 5-HT1B autoreceptors in hippocampus, an area with a low inhibitory influence of somatodendritic 5-HT1A receptors. Methiothepin-sensitive autoreceptors (possibly 5-HT1B) appear to play an important role not only in the projection areas but also with respect to the control of 5-HT release in the DRN and MRN. In addition, our findings indicate that GR 127935 is not an effective antagonist of the actions of 5-HT at rat terminal autoreceptors.
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