The prefrontal cortex (PFC) is involved in higher brain functions altered in schizophrenia. Classical antipsychotics modulate cortico-limbic circuits mainly through subcortical D2 receptor blockade, whereas second generation (atypical) antipsychotics preferentially target cortical 5-HT receptors. Anatomical and functional evidence supports a PFC-based control of the brainstem monoaminergic nuclei. Using a combination of retrograde tracing experiments and in situ hybridization we report that a substantial proportion of PFC pyramidal neurons projecting to the dorsal raphe (DR) and/or ventral tegmental area (VTA) express 5-HT2A receptors. Cholera-toxin B application into the DR and the VTA retrogradely labeled projection neurons in the medial PFC (mPFC) and in orbitofrontal cortex (OFC). In situ hybridization of 5-HT2A receptor mRNA in the same tissue sections labeled a large neuronal population in mPFC and OFC. The percentage of DR-projecting neurons expressing 5-HT2A receptor mRNA was ∼60% in mPFC and ∼75% in OFC (n = 3). Equivalent values for VTA-projecting neurons were ∼55% in both mPFC and ventral OFC. Thus, 5-HT2A receptor activation/blockade in PFC may have downstream effects on dopaminergic and serotonergic systems via direct descending pathways. Atypical antipsychotics may distally modulate monoaminergic cells through PFC 5-HT2A receptor blockade, presumably decreasing the activity of neurons receiving direct cortical inputs.

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Derangements of the prefrontal cortex (PFC) and of brainstem monoaminergic systems occur in depression and schizophrenia. Anatomical and functional evidence supports a PFC control of the brainstem monoaminergic systems. Similarly, the PFC contains a high density of monoamine receptors for which antipsychotic drugs exhibit high affinity. This raises the possibility that pathological or drug-induced changes in PFC may subsequently alter monoaminergic activity. Recent data indicate that a substantial proportion of PFC pyramidal neurons projecting to the ventral tegmental area (VTA) or the dorsal raphe nucleus (DR) express the 5-HT 2A receptor mRNA, which suggests that atypical antipsychotic drugs affect serotonergic and dopaminergic function by targeting PFC 5-HT 2A receptors. Using electrophysiological and tract-tracing techniques we examined whether PFC pyramidal neurons projecting to DR are segregated from those projecting to the VTA. Sequential electrical stimulation of these nuclei in anaesthetized rats evoked antidromic potentials from both areas in the same pyramidal neurons of the medial PFC (60 %, n=30). A similar percentage of dual DR+VTA projection neurons (50 %) was obtained using the reciprocal collision test (n=85). Similarly, tracer application (Fluoro-Gold in VTA and cholera toxin B in DR, or vice versa) retrogradely labelled pyramidal neurons in PFC projecting to VTA (81¡18), to DR (52¡9) and to both nuclei (31¡4, n=5 rats). Overall, these results indicate that the PFC may simultaneously coordinate the activity of dopaminergic and serotonergic systems within a short temporal domain, supporting a concerted modulation of the ascending serotonergic and dopaminergic activity during antipsychotic drug treatment.