The recent identification of the trace amine-associated receptor (TAAR)1 provides an opportunity to dissociate the effects of trace amines on the dopamine transporter from receptor-mediated effects. To separate both effects on a physiological level, a Taar1 knockout mouse line was generated. Taar1 knockout mice display increased sensitivity to amphetamine as revealed by enhanced amphetamine-triggered increases in locomotor activity and augmented striatal release of dopamine compared with wild-type animals. Under baseline conditions, locomotion and extracellular striatal dopamine levels were similar between Taar1 knockout and wild-type mice. Electrophysiological recordings revealed an elevated spontaneous firing rate of dopaminergic neurons in the ventral tegmental area of Taar1 knockout mice. The endogenous TAAR1 agonist p-tyramine specifically decreased the spike frequency of these neurons in wild-type but not in Taar1 knockout mice, consistent with the prominent expression of Taar1 in the ventral tegmental area. Taken together, the data reveal TAAR1 as regulator of dopaminergic neurotransmission.

The trace amine-associated receptor 1 (TAAR1), activated by endogenous metabolites of amino acids like the trace amines p-tyramine and β-phenylethylamine, has proven to be an important modulator of the dopaminergic system and is considered a promising target for the treatment of neuropsychiatric disorders. To decipher the brain functions of TAAR1, a selective TAAR1 agonist, RO5166017, was engineered. RO5166017 showed high affinity and potent functional activity at mouse, rat, cynomolgus monkey, and human TAAR1 stably expressed in HEK293 cells as well as high selectivity vs. other targets. In mouse brain slices, RO5166017 inhibited the firing frequency of dopaminergic and serotonergic neurons in regions where Taar1 is expressed (i.e., the ventral tegmental area and dorsal raphe nucleus, respectively). In contrast, RO5166017 did not change the firing frequency of noradrenergic neurons in the locus coeruleus, an area devoid of Taar1 expression. Furthermore, modulation of TAAR1 activity altered the desensitization rate and agonist potency at 5-HT 1A receptors in the dorsal raphe, suggesting that TAAR1 modulates not only dopaminergic but also serotonergic neurotransmission. In WT but not Taar1 −/− mice, RO5166017 prevented stress-induced hyperthermia and blocked dopamine-dependent hyperlocomotion in cocaine-treated and dopamine transporter knockout mice as well as hyperactivity induced by an NMDA antagonist. These results tie TAAR1 to the control of monoamine-driven behaviors and suggest anxiolyticand antipsychotic-like properties for agonists such as RO5166017, opening treatment opportunities for psychiatric disorders.drug discovery | serotonin | depression | schizophrenia | anxiety

The biochemical and behavioral effects of a nonpeptidic, selective, and brain-penetrant agonist at the ORL1 receptor are reported herein. This low molecular weight compound {(1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triazaspiro[4.5]decan-4-one} has high affinity for recombinant human ORL1 receptors and has 100-fold selectivity for ORL1 over other members of the opioid receptor family. It is a full agonist at these receptors and elicits dose-dependent anxiolytic-like effects in a set of validated models of distinct types of anxiety states in the rat (i.e., elevated plus-maze, fear-potentiated startle, and operant conflict). When given systemically, the compound has an efficacy and potency comparable to those of a benzodiazepine anxiolytic such as alprazolam or diazepam. However, this compound is differentiated from a classical benzodiazepine anxiolytic by a lack of efficient anti-panic-like activity, absence of anticonvulsant properties, and lack of effects on motor performance and cognitive function at anxiolytic doses (0.3 to 3 mg͞kg i.p.). No significant change in intracranial self-stimulation performance and pain reactivity was observed in this dose range. Higher doses of this compound (>10 mg͞kg) induced disruption in rat behavior. These data confirm the notable anxiolytic-like effects observed at low doses with the orphanin FQ͞nociceptin neuropeptide given locally into the brain and support a role for orphanin FQ͞nociceptin in adaptive behavioral fear responses to stress.T he ORL1 orphan receptor was identified from a human cDNA library on the basis of close homology (Ϸ65% in the transmembrane domains) with the -, ␦-, and -opioid receptors (1, 2). Classical opioid ligands do not bind to ORL1, but orphanin FQ͞nociceptin (OFQ͞N), a 17-amino acid neuropeptide purified from brain extracts, was found to be the natural ligand of the G protein-coupled receptor ORL1 (3, 4). OFQ͞N, its precursor peptide, and its receptor ORL1 are located in corticolimbic regions involved in the integration of the emotional components of fear and stress as well as in the spinal cord, with a pattern distinct from that of opioid peptides and receptors in rodents (5-9). The expression of OFQ͞N or its receptor in the amygdaloid complex, septohippocampal region, periaqueductal gray matter, locus coeruleus, and dorsal raphe nucleus suggests that major brain neuronal systems may be sensitive to the action of OFQ͞N. Such sensitivity has widespread implications for many aspects of behavior including arousal, attention, neuroendocrine control, fear, and anxiety (10). In brain slices, OFQ͞N has potent inhibitory actions on neurons in the dorsal raphe nucleus, the locus coeruleus, the periaqueductal gray matter, and the amygdala (11)(12)(13)(14). In general, OFQ͞N plays an inhibitory role on synaptic transmission in the central nervous system and thereby may contribute to a reduction in responsiveness to stress. When given intracerebroventricularly to rodents, OFQ͞N reduces elementary stress-induced physiological respon...

Orphanin FQ (OFQ, Nociceptin) is a recently discovered 17-amino acid neuropeptide that is structurally related to the opioid peptides but does not bind opioid receptors. OFQ has been proposed to act as an anti-opioid peptide, but its widespread sites of action in the brain suggest that it may have more general functions. Here we show that OFQ plays an important role in higher brain functions because it can act as an anxiolytic to attenuate the behavioral inhibition of animals acutely exposed to stressful͞anxiogenic environmental conditions. OFQ anxiolytic-like effects were consistent across several behavioral paradigms generating different types of anxiety states in animals (light-dark preference, elevated plus-maze, exploratory behavior of an unfamiliar environment, pharmacological anxiogenesis, operant conf lict) and were observed at low nonsedating doses (0.1-3 nmol, intracerebroventricular). Like conventional anxiolytics, OFQ interfered with regular sensorimotor function at high doses (>3 nmol). Our results show that an important role of OFQ is to act as an endogenous regulator of acute anxiety responses. OFQ, probably in concert with other major neuropeptides, exerts a modulatory role on the central integration of stressful stimuli and, thereby, may modulate anxiety states generated by acute stress.Orphanin FQ (OFQ, Nociceptin) is a 17-amino acid neuropeptide that is structurally related to the opioid peptides but does not act on , ␦, or opioid receptor subtypes (1, 2). OFQ selectively binds its own receptor (OFQR), which is also sequentially related to the opioid receptors, yet does not bind opioid ligands (3-8). The OFQR couples to G proteins to modulate second messenger systems and cell excitability (9-11). When delivered intracerebroventricularly (i.c.v.) in a large dose range (0.1-10 nmol), OFQ was found to block stress and opioid-mediated antinociception (12, 13), to stimulate feeding in satiated rats (14), and to increase or decrease, depending on dosage, locomotion (1, 15) or nociception (16, 17) in rodents. OFQ, its precursor, and OFQR are present in several brain regions involved in integration of the emotional components of fear and stress such as the amygdaloid complex, thalamic and hypothalamic regions, or central gray regions (1-8, 18, 19). This has led us to investigate whether OFQ might also have a role in higher brain functions and would control behavioral responses to stress that relate to anxiety states. To investigate this hypothesis, a battery of behavioral models of anxiety and fear (light-dark aversion, elevated plus-maze, exploratory behavior of an unfamiliar environment, pharmacological anxiogenesis, operant conflict) were used on mice and rats. In these assays, fear-like responses of a composite nature are generated by exposure to various stressful environmental conditions (20-27). These paradigms have been established for their sensitivity to conventional anxiolytic tranquilizers and anxiogenic compounds of various structural classes and mechanisms of action. They were pha...