No abstract

In order to localize the cells expressing SHT,, receptors in the rat brain, we used in situ hybridization histochemistry to visualize the distribution of the mRNA coding for SHT,, receptors. Oligonucleotides derived from different parts of the coding region of the rat 5-HT,, receptor gene were used as hybridization probes. 5-HT,, binding sites were visualized on consecutive sections by receptor autoradiography using 3H-8-hydroxy-2-(di-rpropylamino)tetralin as ligand. The highest levels of hybridization were observed in the dorsal raphe nucleus, septum, hippocampus, entorhinal cortex, and interpeduncular nucleus. Positive hybridization signals were also present in other areas, such as the olfactory bulb; cerebral cortex; some thalamic and hypothalamic nuclei; several nuclei of the brainstem, including all the remaining raphe nuclei, nucleus of the solitary tract, and nucleus of the spinal tract of the trigeminus; and the dorsal horn of the spinal cord.The distribution and abundance of SHT,, receptor mRNA in different rat brain areas generally correlate with those of the binding sites, suggesting that 5-HT,, receptors are predominantly somatodendritic receptors.Several different types of S-HT receptors have been pharmacologically defined (Peroutka, 1990). The recent molecular cloning ofthe genes or the cDNAs for four ofthese receptors, 5-HT,,, 5-HT,,, 5-HT,,, and 5-HT, (see Hartig, 1989, for a review; Branchek et al., 199 l), has suggested that they all belong to the monomeric G-protein-coupled receptor family (Dohlman et al., 1987). The distribution of 5-HT,, receptors has been examined in the rat brain by means ofreceptor autoradiographic technique (Marcinkiewicz et al., 1984;Pazos and Palacios, 1985) using a selective tritium-labeled ligand, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (Arvidsson et al., 198 1;Gozlan et al., 1983). Recently, 5-HT,, receptors have been also visualized by immunoautoradiography (El Mestikawy et al., 1990), using an antibody against a synthetic peptide derived from the predicted sequence of the rat 5-HT,, receptor. The light microscopic autoradiographic techniques used until now did not allow the unequivocal establishment of the cellular localization of receptors, which could only be inferred indirectly (Palacios and Dietl, 1988). A presynaptic localization of 5-HT,, receptors on the cell body and dendrites of serotoninergic cells, which would be responsible for the autoinhibition of 5-HT cells in the dorsal raphe (Wang and Aghajanian, 1977) is supported by biochemical (Hjorth et al., 1982), lesion (Weissman-Nanopoulos et al., 1985;Verge et al., 1986) and electrophysiological (Aghajanian et al., 1988) experiments. The cellular localization of 5-HT,, receptors in the projection areas of the raphe nuclei is less clear (Palacios and Dietl, 1988). However, in the hippocampus, lesion experiments suggested a localization of the receptors to neurons intrinsic to this region (Hall et al., 1985) and electrophysiological experiments pointed in particular to the hippocampal pyramidal c...

The prefrontal cortex plays a key role in the control of higher brain functions and is involved in the pathophysiology and treatment of schizophrenia. Here we report that approximately 60% of the neurons in rat and mouse prefrontal cortex express 5-HT(1A) and/or 5-HT2A receptor mRNAs, which are highly co-localized (approximately 80%). The electrical stimulation of the dorsal and median raphe nuclei elicited 5-HT1A-mediated inhibitions and 5-HT2A-mediated excitations in identified pyramidal neurons recorded extracellularly in rat medial prefrontal cortex (mPFC). Opposite responses in the same pyramidal neuron could be evoked by stimulating the raphe nuclei at different coordinates, suggesting a precise connectivity between 5-HT neuronal subgroups and 5-HT1A and 5-HT2A receptors in pyramidal neurons. Microdialysis experiments showed that the increase in local 5-HT release evoked by the activation of 5-HT2A receptors in mPFC by DOI (5-HT2A/2C receptor agonist) was reversed by co-perfusion of 5-HT1A agonists. This inhibitory effect was antagonized by WAY-100635 and the prior inactivation of 5-HT1A receptors in rats and was absent in mice lacking 5-HT1A receptors. These observations help to clarify the interactions between the mPFC and the raphe nuclei, two key areas in psychiatric illnesses and improve our understanding of the action of atypical antipsychotics, acting through these 5-HT receptors.

The mechanism that controls the selective vulnerability of striatal neurons in Huntington's disease is unclear. Brain-derived neurotrophic factor (BDNF) protects striatal neurons and is regulated by Huntingtin through the interaction with the neuron-restrictive silencer factor. Here, we demonstrate that the downregulation of BDNF by mutant Huntingtin depends on the length and levels of expression of the CAG repeats in cell cultures. To analyze the functional effects of these changes in BDNF in Huntington's disease, we disrupted the expression of bdnf in a transgenic mouse model by cross-mating bdnf ϩ/Ϫ mice with R6/1 mice. Thus, we compared transgenic mice for mutant Huntingtin with different levels of BDNF. Using this double mutant mouse line, we show that the deficit of endogenous BDNF modulates the pathology of Huntington's disease. The decreased levels of this neurotrophin advance the onset of motor dysfunctions and produce more severe uncoordinated movements. This behavioral pathology correlates with the loss of striatal dopamine and cAMPregulated phosphoprotein-32-positive projection neurons. In particular, the insufficient levels of BDNF cause specific degeneration of the enkephalinergic striatal projection neurons, which are the most affected cells in Huntington's disease. This neuronal dysfunction can specifically be restored by administration of exogenous BDNF.Therefore, the decrease in BDNF levels plays a key role in the specific pathology observed in Huntington's disease by inducing dysfunction of striatal enkephalinergic neurons that produce severe motor dysfunctions. Hence, administration of exogenous BDNF may delay or stop illness progression.

We examined the in vivo effects of the hallucinogen 4-iodo-2,5-dimethoxyamphetamine (DOI). DOI suppressed the firing rate of 7 of 12 dorsal raphe (DR) serotonergic (5-HT) neurons and partially inhibited the rest (ED(50) = 20 microg/kg, i.v.), an effect reversed by M100907 (5-HT(2A) antagonist) and picrotoxinin (GABA(A) antagonist). DOI (1 mg/kg, s.c.) reduced the 5-HT release in medial prefrontal cortex (mPFC) to 33 +/- 8% of baseline, an effect also antagonized by M100907. However, the local application of DOI in the mPFC increased 5-HT release (164 +/- 6% at 100 microm), an effect antagonized by tetrodotoxin, M100907, and BAY x 3702 (5-HT(1A) agonist) but not by SB 242084 (5-HT(2C) antagonist). The 5-HT increase was also reversed by NBQX (AMPA-KA antagonist) and 1S,3S-ACPD (mGluR 2/3 agonist) but not by MK-801 (NMDA antagonist). AMPA mimicked the 5-HT elevation produced by DOI. Likewise, the electrical-chemical stimulation of thalamocortical afferents and the local inhibition of glutamate uptake increased the 5-HT release through AMPA receptors. DOI application in mPFC increased the firing rate of a subgroup of 5-HT neurons (5 of 10), indicating an enhanced output of pyramidal neurons. Dual-label fluorescence confocal microscopic studies demonstrated colocalization of 5-HT(1A) and 5-HT(2A) receptors on individual cortical pyramidal neurons. Thus, DOI reduces the activity of ascending 5-HT neurons through a DR-based action and enhances serotonergic and glutamatergic transmission in mPFC through 5-HT(2A) and AMPA receptors. Because pyramidal neurons coexpress 5-HT(1A) and 5-HT(2A) receptors, DOI disrupts the balance between excitatory and inhibitory inputs and leads to an increased activity that may mediate its hallucinogenic action.