Dopamine autoreceptor mediation of the effects of apomorphine on serotonin neurons

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Abstract

The effects of direct apomorphine (APO) infusion to the dorsal raphe and the substantia nigra on serotonergic neurons were examined in male rats. The results showed that APO infusion to the dorsal raphe failed to produce a significant effect on serotonin (5-HT) neurons in the dorsal raphe or 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the corresponding projection site, the striatum. Conversely, direct APO infusion to the substantia nigra mimicked the effects of systemic APO, namely, elevated 5-HT fluorescence in the dorsal raphe and increased 5-HT and 5-HIAA concentrations in the striatum. Serotonin neurons in the median raphe and its projection site, the hippocampus, were unaffected. Furthermore, horseradish peroxidase injection to the dorsal raphe resulted in specific cell labelling in the substantia nigra and fiber labelling in the ventral tegmental area. Together with previous findings that the serotonergic actions of systemic APO were antagonized by haloperidol or intraventricular 6-hydroxydopamine pretreatment; and the selective dopamine (DA) autoreceptor agonist 3-3-hydroxyphenyl-N-n-propyl-piperidine mimicked the effects of APO on 5-HT neurons, these results suggest that the observed effects of APO on the mesostriatal serotonergic system are probably mediated through DA autoreceptors in the substantia nigra and possibly by a direct nigroraphe pathway.

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  • Serotonin/dopamine interaction in learning

    2008, Progress in Brain Research
    Citation Excerpt :

    From this, an interaction can be inferred on analyzing data from similar tests (similar in regard to cognitive process), but few studies have evaluated this interaction until now. Less is known about the possible modulation of 5-HTergic function by the DA system, despite the evidence that DArgic terminals on RN proceed into the SN (Afifi and Kaelber, 1965; Pasquier et al., 1977; Sakai et al., 1977; Lee and Geyer, 1984; Kalén et al., 1988), and the high density of D2 receptors on raphe neurons (Bouthenet et al., 1987). Future work must address a more integrative approach to the organization of cognition by multiple neurotransmitter systems, because this is the better focus by which significant progress may be made in the development of therapeutic strategies.

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