Intrinsic, striatal tyrosine hydroxylase-immunoreactive (TH-i) cells have received little consideration. In this study we have characterized these neurons and their regulatory response to nigrostriatal dopaminergic deafferentation. TH-i cells were observed in the striatum of both control and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys; TH-i cell counts, however, were 3.5-fold higher in the striatum of MPTPlesioned monkeys. To establish the dopaminergic nature of the TH-i cells, sections were double-labeled with antibodies to dopamine transporter (DAT). Immunofluorescence studies demonstrated that nearly all TH-i cells were double-labeled with DAT, suggesting that they contain the machinery to be functional dopaminergic neurons. Two types of TH-i cells were identified in the striatum: small, aspiny, bipolar cells with varicose dendrites and larger spiny, multipolar cells. The aspiny cells, which were more prevalent, corresponded morphologically to the GABAergic interneurons of the striatum. Doublelabel immunofluorescence studies using antibodies to TH and glutamate decarboxylase (GAD 67 ), the synthetic enzyme for GABA, showed that 99% of the TH-i cells were GAD 67 -positive. Very few (Ͻ1%) of the TH-i cells, however, were immunoreactive for the calcium-binding proteins calbindin and parvalbumin. In summary, these results demonstrate that the dopaminergic cell population of the striatum responds to dopamine denervation by increasing in number, apparently to compensate for loss of extrinsic dopaminergic innervation. Moreover, this population of cells corresponds largely with the intrinsic GABAergic cells of the striatum. This study also suggests that the adult primate striatum does retain some intrinsic capacity to compensate for dopaminergic cell loss.

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Introduction Flibanserin is a novel pharmacologic agent in late-stage clinical testing for hypoactive sexual desire disorder (HSDD) in premenopausal women. Aim The aim of this article is to review the hypothetical mechanism of action of flibanserin in HSDD. Methods A literature review was conducted of all published works on flibanserin and on related studies of serotonin (5-HT)1A receptors and 5-HT2A receptors, including their actions on monoamines and on sexual function. Main Outcome Measures The main outcome measures are preclinical pharmacologic actions, especially changes in regional monoamines following treatment with flibanserin. Results At clinically relevant doses, flibanserin acts predominantly at 5-HT1A receptors as an agonist and secondarily at 5-HT2A receptors as an antagonist. Additional binding actions within an order of magnitude of its 5-HT1A affinity, which are not likely to be clinically relevant, include weaker antagonist actions at 5-HT2C and 5-HT2B receptors, and less defined activity at dopamine (DA) D4 receptors. The 5-HT1A actions of flibanserin are only seen postsynaptically, which is unlike other agents such as buspirone that act at presynaptic 5-HT1A receptors. Furthermore, the postsynaptic actions of chronic flibanserin administration appear to demonstrate a preference for some populations of postsynaptic 5-HT receptors, particularly those that are located on the prefrontal cortex (PFC) pyramidal neurons, which regulate monoamine release in certain selective brain regions. Conclusions The regional selectivity of flibanserin results in a unique pattern of monoamine modulation. Sustained increases in baseline of DA and norepinephrine (NE) are observed in the PFC, and flibanserin dosing increases DA and NE levels above the basal changes. Conversely, flibanserin induces transient decreases in 5-HT levels in some brain areas such as the PFC, nucleus accumbens, and hypothalamus, but not in other brain areas such as the hippocampus. Therefore, since DA and NE are excitatory and 5-HT is inhibitory to sexual desire and arousal, it is tempting to postulate that the actions of flibanserin on serotonin receptors at the PFC pyramidal neurons, resulting in increased DA and NE yet reduced 5-HT in the PFC, are the mechanistic underpinnings of enhancing sexual desire in HSDD.

These results support a partial agonist activity for aripiprazole at 5-HT(1A) receptors in vitro and in vivo, and suggest important interactions with other 5-HT-receptor subtypes. This receptor activity profile may contribute to the antipsychotic activity of aripiprazole in humans.