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Vol. 282, Issue 3, 1581-1590, 1997
Department of Pharmacology, Stritch School of Medicine, Loyola
University Chicago, Maywood Illinois
The aim of the present study was to determine whether the
previously observed desensitization of hypothalamic
5-hydroxytryptamine1A (5-HT1A) receptors,
during daily injections of fluoxetine, is mediated by sustained
blockade of 5-HT reuptake. In the present study, we examined the time
course effects of another 5-HT uptake inhibitor, paroxetine. Paroxetine
reduced the oxytocin, adrenal corticotropic hormone and corticosterone
responses to a challenge with the 5-HT1A agonist
8-hydroxy-2-(dipropylamino)tetralin. These reductions in hormone
responses were significant after 3 daily injections and reached a
maximum after 7 daily paroxetine injections. These hormone responses
remained maximally suppressed after 14 daily injections of paroxetine.
A single day of paroxetine treatment did not alter the hormone
responses to 8-hydroxy-2-(dipropylamino)tetralin. Repeated injections
of paroxetine did not reduce the density of 5-HT1A
receptors in any brain region but did produce a gradual reduction in
the levels of Gi and Go proteins in a
region-specific manner. The time course of the paroxetine-induced
reduction in the level of Gi1 and Gi3 proteins
in the hypothalamus was similar to the effect previously observed with
fluoxetine and was also similar to the time course of
paroxetine-induced reductions in oxytocin and adrenal corticotropic
hormone responses to 8-hydroxy-2-(dipropylamino)tetralin. In
conclusion, these results suggest that blockade of 5-HT uptake sites
produces a delayed and gradual desensitization of 5-HT1A receptors in the hypothalamus. This desensitization is not due to
changes in the density of hypothalamic 5-HT1A receptors.
Reduction in the hypothalamic level of Gi3 proteins may
play a role in the desensitization of 5-HT1A receptor
systems. However, reductions in Gi1 or Go
proteins cannot be excluded as potential mediators of the
desensitization of 5-HT1A receptor systems.
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