Effects of i.c.v. lithium chloride administration on monoamine concentration in rat mediobasal hypothalamus

Efficacy of serotonin inhibition in mouse models of bone loss

In a proof-of-concept study it was shown that decreasing synthesis of gut serotonin through a small molecule inhibitor of Tph1 could prevent and treat ovariectomy-induced osteoporosis in young mice and rats. In this study, we define the minimal efficacy of this Tph1 inhibitor, demonstrate that its activity is improved with the duration of treatment, and show that its anabolic effect persists on interruption. Importantly, given the prevalence of osteoporosis in the aging population, we then show that Tph1 inhibition rescues ovariectomy-induced bone loss in aged mice. It also cures the low bone mass of Lrp5-deficient mice through a sole anabolic effect. Lastly, we provide evidence that inhibition of gut serotonin synthesis can work in concert with an antiresorptive agent to increase bone mass in ovariectomized mice. This study provides a more comprehensive view of the anabolic efficacy of Tph1 inhibitors and further establishes the spectrum of their therapeutic potential in the treatment of bone-loss disorders.

Serotonin and dopamine in the parabrachial nucleus of rats during conditioned taste aversion learning

A microdialysis technique was used to monitor changes in serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA) and dopamine (DA) in the extracellular space of the parabrachial nucleus (PBN) of rats to estimate the contribution of these neurotransmitter systems to the acquisition of conditioned taste aversion (CTA). A significant (280%) enhancement of 5-HT was found immediately after saccharin drinking (CS). I.p. injection of unconditioned stimulus LiCl alone (after water drinking) also increased level of 5-HT (200%). However, when saccharin intake was followed by injection of LiCl (CS-US pairing), no change in 5-HT was observed. 5-HIAA and DA were unaffected by any of the above treatments. Thus in spite of elevation of 5-HT in PBN following saccharin consumption alone (CS) or LiCl administration alone (US) no changes in 5-HT occurred after pairing of both stimuli (CS-US). Our work demonstrates that participation of 5-HT in acquisition of CTA appears to be unlikely, and also DA appears not to be engaged in this acquisition at all. At the level of the PBN 5-HT participates mainly in CS and/or US stimuli processing, where this phenomenon has close relationship to other important physiological mechanisms, involved in behavioral control. Such as anxiety, alimentation intake.

Lithium-induced gene expression of inducible cyclic adenosine monophosphate early repressor in the rat adrenal gland

Lithium has acute and chronic effects on the hypothalamic-pituitary-adrenal gland (HPA) axis that are important for both therapeutic (e.g., treatment of mood disorders) and experimental (e.g., as the toxin in conditioned taste aversion studies) applications. We visualized lithium-induced activation of the HPA axis in rats by the adrenal expression of inducible cAMP early repressor (ICER), which is activated by elevated intracellular cAMP. We have shown that 1) intraperitoneal lithium chloride (LiCl) induces transient expression of ICER and c-fos mRNAs in the rat adrenal cortex and increases plasma level of corticosterone; 2) the cortical expression of ICER mRNA by LiCl occurs in a dose-dependent manner; 3) adrenal induction of ICER expression is delayed compared with c-fos expression; 4) dexamethasone pretreatment (4 mg/kg) blocks corticosterone release and adrenocortical ICER induction either by systemic LiCl (76 mg/kg) or by restraint stress; and 5) intracerebroventricular LiCl (127 microg/5 microl) is sufficient for adrenocortical, but not medullary, ICER induction. These results suggest that adrenocortical ICER expression could serve as a reliable marker for lithium-induced activation of the HPA axis. Understanding the activation of immediate-early genes such as c-fos or ICER in response to a single LiCl injection is an important first step in understanding the long-term changes in gene expression elicited by lithium that are involved in its therapeutic and toxic effect. The pattern and mechanism by which lithium stimulates ICER transcription in the adrenal gland would serve as a useful model system in future studies of lithium.

Nitric oxide is involved in lithium-induced immediate early gene expressions in the adrenal medulla

This study was conducted to determine if nitric oxide (NO) is involved in lithium-induced expression of c-Fos and inducible cAMP early repressor (ICER) in the adrenal gland. Rats received an intraperitoneal injection of isotonic lithium (76 mg/kg) with either an intracerebroventricle (i.c.v., 250 microg) or intraperitoneal (i.p., 30 mg/kg) N(omega)-nitro-L-arginine methyl ester (L-NAME) pretreatment. The adrenal expression of c-Fos and ICER was examined by in situ hybridization 1 h after the lithium injection. The cortical c-Fos/ICER expression induced by lithium was not modulated by L-NAME pretreatment. However, lithium-induced medullary expression of c-Fos was attenuated by central L-NAME, and ICER by systemic L-NAME. These results suggest that nitric oxide is, at least partly, involved in lithium-induced c-Fos/ICER expression in the adrenal medulla, and that central nitric oxide may play a different role from peripheral nitric oxide in lithium-induced activation of adrenal medulla.

Roles of chemical mediators in the taste system

Recent advances in neural mechanisms of taste are reviewed with special reference to neuroactive substances. In the first section, taste transduction mechanisms of basic tastes are explained in two groups, whether taste stimuli directly activate ion channels in the taste cell membrane or they bind to cell surface receptors coupled to intracellular signaling pathways. In the second section, putative transmitters and modulators from taste cells to afferent nerves are summarized. The candidates include acetylcholine, catecholamines, serotonin, amino acids and peptides. Studies favor serotonin as a possible neuromodulator in the taste bud. In the third section, the role of neuroactive substances in the central gustatory pathways is introduced. Excitatory and inhibitory amino acids (e.g., glutamate and GABA) and peptides (substance P and calcitonin gene-related peptide) are proved to play roles in transmission of taste information in both the brainstem relay and cortical gustatory area. In the fourth section, conditioned taste aversion is introduced as a model to study gustatory learning and memory. Pharmacobehavioral studies to examine the effects of glutamate receptor antagonists and protein kinase C inhibitors on the formation of conditioned taste aversion show that both glutamate and protein kinase C in the amygdala and cortical gustatory area play essential roles in taste aversion learning. Recent molecular and genetic approaches to disclose biological mechanisms of gustatory learning are also introduced. In the last section, behavioral and pharmacological approaches to elucidate palatability, taste pleasure, are described. Dopamine, benzodiazepine derivatives and opioid substances may play some roles in evaluation of palatability and motivation to ingest palatable edibles.