Endogenous opioids seem to play a critical role in the regulation of mood states. For example, there is accumulating evidence that stimulation of -opioid receptors, upon which the endogenous opioid dynorphin acts, can produce depressivelike behaviors in laboratory animals. Here we examined whether systemic administration of salvinorin A (SalvA), a potent and highly selective -opioid agonist, would produce depressivelike effects in the forced swim test (FST) and intracranial selfstimulation (ICSS) test, which are behavioral models often used to study depression in rats. We extracted, isolated, and purified SalvA from Salvia divinorum plant leaves and examined its effects on behavior in the FST and ICSS test across a range of doses (0.125-2.0 mg/kg) after systemic (intraperitoneal) administration. SalvA dose dependently increased immobility in the FST, an effect opposite to that of standard antidepressant drugs. Doses of SalvA that produced these effects in the FST did not affect locomotor activity in an open field. Furthermore, SalvA dose dependently elevated ICSS thresholds, an effect similar to that produced by treatments that cause depressive symptoms in humans. At a dose that caused the depressivelike effects in both the FST and ICSS assays, SalvA decreased extracellular concentrations of dopamine (DA) within the nucleus accumbens (NAc), a critical component of brain reward circuitry, without affecting extracellular concentrations of serotonin (5-HT). These data provide additional support for the hypothesis that stimulation of brain -opioid receptors triggers depressive-like signs in rats and raise the possibility that decreases in extracellular concentrations of DA within the NAc contribute to these effects.Although much research on depression has focused on brain norepinephrine and serotonin (5-HT) systems, there is substantial evidence that other systems have important roles in the neurobiology of mood and affective disorders. For example, the mesolimbic dopamine (DA) systemwhich projects from the ventral tegmental area to the nucleus accumbens (NAc)-contributes importantly to the hedonic (rewarding) effects of food, sexual behavior, and addictive drugs (see Wise, 1998;Nestler and Carlezon, 2005). It has been proposed that disruption of DA function within the NAc causes anhedonia (reduced ability to experience reward) (Wise, 1982), a hallmark sign of clinical depression. The mesolimbic DA system is modulated by noradrenergic and serotonergic inputs (Pasquier et al., 1977), as well as endogenous opioid peptides (Devine et al., 1993;Shippenberg and Rea, 1997;Svingos et al., 1999). Agents that selectively affect the function of -opioid receptors cause profound alterations in mood in humans (Pfeiffer et al., 1986;Roth et al., 2002) and motivated behaviors in laboratory animals (Shippenberg and Herz, 1987;Todtenkopf et al., 2004), suggesting that manipulations targeting brain -opioid systems might be useful in the study and treatment of depressive disorders.

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Salvinorin (Sal) A is a naturally occurring, selective opioid receptor (KOPR) agonist with a short duration of action in vivo. Pharmacological properties of a C(2) derivative, 2-methoxymethyl (MOM)-Sal B, were characterized. MOM-Sal B bound to KOPR with high selectivity and displayed ϳ3-fold higher affinity than U50,488H [(trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide methanesulfonate] and Sal A. It acted as a full agonist at KOPR in guanosine 5Ј-O-(3-[ 35 S]thio)triphosphate binding and was ϳ5-and ϳ7-fold more potent than U50,488H and Sal A, respectively. In Chinese hamster ovary cells stably expressing KOPR, all three agonists internalized or down-regulated KOPR to similar extents, with MOM-Sal B being the most potent. In mice, MOM-Sal B (0.05-1 mg/kg s.c.) caused immediate and dose-dependent immobility lasting ϳ3 h, which was blocked by norbinaltorphimine. In contrast, ambulation in a Y-maze was increased when rats received MOM-Sal B (1-5 mg/kg s.c.). In addition, MOM-Sal B (0.5-5 mg/kg i.p.) produced antinociception (hot-plate test) and hypothermia in a dose-dependent manner in rats. MOM-Sal B was more potent than U50,488H in both tests and more efficacious than U50,488H in the hot-plate test. These latter two in vivo effects were blocked by norbinaltorphimine, indicating KOPR-mediated actions. Sal A at 10 mg/kg elicited neither antinociception nor hypothermia 30 min after administration to rats. In summary, MOM-Sal B is a potent and efficacious KOPR agonist with longer lasting in vivo effects than Sal A.Activation of the opioid receptor (KOPR), one of three major types of opioid receptors, produces many effects including analgesia, dysphoria, antipruritis, water diuresis, and hypothermia (Liu-Chen, 2004). Many selective nonpeptide KOPR agonists have been synthesized; most are arylacetamide compounds, including U50,488H, U69,593, ICI 204,448, and asimadoline. Nalfurafine is an exception, being an epoxymorphinan.Chewing or smoking the leaves of Salvia divinorum or drinking juice of crushed leaves causes hallucinations in humans (Siebert, 1994). Salvinorin (Sal) A (Fig. 1), a neoclerodane diterpene, was isolated and identified as the main active ingredient (Valdés, 1994). In a large scale screening of G protein-coupled receptors, transporters, and ion channels by radioligand binding assays, Roth et al. (2002) found that Sal A was a highly potent and selective agonist for the KOPR. Since then, several pharmacological studies on Sal A have been performed. Sal A binds to KOPR with a high affinity similar to U50,488H, a prototypic selective KOPR agonist. It does not show significant affinities to and ␦ opioid receptors or the nociceptin/orphanin FQ receptor . Sal A is reported to be more selective for KOPR than U50,488H or U69,593 (Beguin et al., 2008). Sal A acts as a very potent full agonist at KOPR in [35 S]GTP␥S binding, intracellular calcium mobilization, and potassium conductance assays (Chavkin et al., 2004;Wang et al., 2005). Sal A

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