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Major depression and anxiety disorders are a social and economic burden worldwide. Serotonergic signaling has been implicated in the pathophysiology of these disorders and thus has been a crucial target for pharmacotherapy. However, the precise mechanisms underlying these disorders are still unclear. Here, we used species-optimized lentiviral vectors that were capable of efficient and specific transduction of serotonergic neurons in mice and rats for elucidation of serotonergic roles in anxiety-like behaviors and active coping behavior in both species. Immunohistochemical analyses revealed that lentiviral vectors with an upstream sequence of tryptophan hydroxylase 2 gene efficiently transduced serotonergic neurons with a specificity of approximately 95% in both mice and rats. Electrophysiological recordings showed that these lentiviral vectors induced sufficient expression of optogenetic tools for precise control of serotonergic neurons. Using these vectors, we demonstrate that acute activation of serotonergic neurons in the dorsal raphe nucleus increases active coping with inescapable stress in rats and mice in a time-locked manner, and that acute inhibition of these neurons increases anxiety-like behaviors specifically in rats. These findings further our understanding of the pathophysiological role of dorsal raphe serotonergic neurons in different species and the role of these neurons as therapeutic targets in major depression and anxiety disorders.

Background Ketamine rapidly elicits antidepressive effects in humans and mice in which serotonergic activity is involved. Although α4β2 nicotinic acetylcholine receptor (α4β2 nAChR) in the dorsal raphe nucleus plays a key role in the ketamine-induced prefrontal serotonin release, the source of cholinergic afferents, and its role is unclear. Methods Prefrontal serotonin levels after ketamine injection were measured by microdialysis in rats. Electrolytic lesion of pedunculopontine tegmental nucleus and laterodorsal tegmental nucleus was made with constant direct current. Results Bilateral lesion of the pedunculopontine tegmental nucleus, but not laterodorsal tegmental nucleus, attenuated prefrontal serotonin release induced by systemic ketamine. Intra-pedunculopontine tegmental nucleus, but not intra-laterodorsal tegmental nucleus ketamine perfusion, increased prefrontal serotonin release. This increase was attenuated by intra-dorsal raphe nucleus injection of dihydro-β-erythroidine, an α4β2 nAChR antagonist, or NBQX, an AMPA receptor antagonist. Conclusions These results suggest the ketamine-induced serotonin release in medial prefrontal cortex is mediated by cholinergic neurons projecting from pedunculopontine tegmental nucleus to dorsal raphe nucleus via α4β2 nAChRs.

Spontaneous activity of serotonergic neurons of the dorsal raphe nucleus (DRN) regulates mood and motivational state. Potentiation of serotonergic function is one of the therapeutic strategies for treatment of various psychiatric disorders, such as major depression, panic disorder and obsessive-compulsive disorder. However, the control mechanisms of the serotonergic firing activity are still unknown. In this study, we examined the control mechanisms for serotonergic spontaneous activity and effects of chronic antidepressant administration on these mechanisms by using modified ex vivo electrophysiological recording methods. Serotonergic neurons remained firing even in the absence of glutamatergic and GABAergic ionotropic inputs, while blockade of L-type voltage dependent Ca2+ channels (VDCCs) in serotonergic neurons decreased spontaneous firing activity. L-type VDCCs in serotonergic neurons received gamma-aminobutyric acid B (GABAB) receptor-mediated inhibition, which maintained serotonergic slow spontaneous firing activity. Chronic administration of an antidepressant, citalopram, disinhibited the serotonergic spontaneous firing activity by weakening the GABAB receptor-mediated inhibition of L-type VDCCs in serotonergic neurons. Our results provide a new mechanism underlying the spontaneous serotonergic activity and new insights into the mechanism of action of antidepressants.

Obsessive-compulsive disorder (OCD) is a neuropsychiatric disorder characterized by the repeated rise of concerns (obsessions) and repetitive unwanted behavior (compulsions). Although selective serotonin reuptake inhibitors (SSRIs) is the first-choice drug, response rates to SSRI treatment vary between symptom dimensions. In this study, to find a therapeutic target for SSRI-resilient OCD symptoms, we evaluated treatment responses of quinpirole (QNP) sensitization-induced OCD-related behaviors in mice. SSRI administration rescued the cognitive inflexibility, as well as hyperactivity in the lateral orbitofrontal cortex (lOFC), while no improvement was observed for the repetitive behavior. D2receptor signaling in the central striatum (CS) was involved in SSRI-resistant repetitive behavior. An adenosine A2Aantagonist, istradefylline, which rescued abnormal excitatory synaptic function in the CS indirect pathway medium spiny neurons (MSNs) of sensitized mice, alleviated both of the QNP-induced abnormal behaviors with only short-term administration. These results provide a new insight into therapeutic strategies for SSRI-resistant OCD symptoms and indicate the potential of A2Aantagonists as a rapid-acting anti-OCD drug.

N-[[1-(5-fluoropentyl)-1H-indazol-3-yl]carbonyl]-3-methyl-D-valine methyl ester (5F-ADB) is one of the most potent synthetic cannabinoids and elicits severe psychotic symptoms in humans, sometimes causing death. To investigate the neuronal mechanisms underlying its toxicity, we examined the effects of 5F-ADB on midbrain dopaminergic and serotonergic systems, which modulate various basic brain functions such as those in reward-related behavior. 5F-ADB-induced changes in spontaneous firing activity of dopaminergic and serotonergic neurons were recorded by ex vivo electrophysiological techniques. In dopaminergic neurons, 5F-ADB (1 μM) significantly increased the spontaneous firing rate, while 5F-ADB failed to activate dopaminergic neurons in the presence of the CB1 antagonist AM251 (1 μM). However, the same concentration of 5F-ADB did not affect serotonergic-neuron activity. These results suggest that 5F-ADB activates local CB1 receptors and potentiates midbrain dopaminergic systems with no direct effects on midbrain serotonergic systems.

Spontaneous activity of serotonergic neurons of the dorsal raphe nucleus (DRN) regulates mood and motivational state. Potentiation of serotonergic function is one of the therapeutic strategies for treatment of various psychiatric disorders, such as major depression, panic disorder and obsessive-compulsive disorder. However, the control mechanisms of the serotonergic firing activity are still unknown. In this study, we examined the control mechanisms for serotonergic spontaneous activity and effects of chronic antidepressant administration on these mechanisms by using modified ex vivo electrophysiological recording methods. Serotonergic neurons remained firing even in the absence of glutamatergic and GABAergic ionotropic inputs, while blockade of L-type voltage dependent Ca2+ channels (VDCCs) in serotonergic neurons decreased spontaneous firing activity. L-type VDCCs in serotonergic neurons received gamma-aminobutyric acid B (GABAB) receptor-mediated inhibition, which maintained serotonergic slow spontaneous firing activity. Chronic administration of an antidepressant, citalopram, disinhibited the serotonergic spontaneous firing activity by weakening the GABAB receptor-mediated inhibition of L-type VDCCs in serotonergic neurons. Our results provide a new mechanism underlying the spontaneous serotonergic activity and new insights into the mechanism of action of antidepressants.

The spontaneously hypertensive rat (SHR)/NDmcr-cp (SHR-cp), which is a metabolic syndrome model rat, was reported to show hypercholesteremia, as compared with lean littermates. The serum total cholesterol level in SHR-cp at 18 weeks of age is higher than that of normotensive Wistar Kyoto rat (WKY), but that in SHR-cp at 10 weeks of age is the same. The objective of this study is to clarify whether there are differences in the system regulating serum cholesterol levels between SHR-cp and WKY at 10 weeks of age. Total serum cholesterol levels, and cholesterol levels of high density lipoprotein (HDL), low density lipoprotein (LDL), and very low density lipoprotein (VLDL) were similar in the two strains. However, the cholesterol levels in the liver of SHR-cp were lower than those of WKY. Next, mRNA levels of receptors (scavenger receptor class B type 1 [SRB1], LDL receptor [LDLR]) involved in uptake from serum to liver or enzymes of cholesterol catabolism (CYP7A1 and CYP8B1) and biosynthesis (mevalonate pyrophosphate decarboxylases [MPD]) in liver were compared between SHR-cp and WKY. High levels of MPD and LDLR and low levels of SRB1 were shown in SHR-cp, as compared with WKY. CYP7A1 and CYP8B1 levels were similar between SHR-cp and WKY. These results suggest that the serum cholesterol level in SHR-cp by the balance or regulation between the rise in cholesterol uptake and reduction in cholesterol biosynthesis in the liver is the same as that in WKY.

Effect of YM175, a new bisphosphonate, on vitamin D metabolism was studied in rats. When animals were treated with the compound, serum 1,25-dihydroxyvitamin D increased in a dose dependent manner. The effect was also detected in thyroparathyroidectomized animals. The effect appears to be due to the stimulation of renal production of the hormone, since renal 1-hydroxylase was also elevated in these animals. However, when kidneys were incubated with YM175 and then renal 1-hydroxylase activity was examined, the enzyme activity was not different from that of non-treated control kidney. We conclude therefore that YM175 indirectly stimulates renal 25-hydroxyvitamin D-1-hydroxylase by increasing circulating parathyroid hormone via an unknown mechanism independent of parathyroid hormone. This is the first direct demonstration of increase in the renal production of 1,25-dihydroxyvitamin D resulting from bisphosphonate treatment.