Effects of ST1936, a selective serotonin-6 agonist, on electrical activity of putative mesencephalic dopaminergic neurons in the rat brain.

The serotonin-6 (5-HT6) receptor is the most recently discovered serotonin receptor, and it represents an increasingly promising target for improving cognition in both normal and disease states. Recently, a new selective 5-HT6 receptor agonist, 2-(5 chloro-2-methyl-1H-indol-3-yl)-N,N-dimethylethanamine (ST1936), with nanomolar affinity for 5-HT6 receptors was described. We performed in-vivo electrophysiological studies to investigate the physiological role of 5-HT6 receptors in the control of the function of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA). Extracellular single-unit recordings were performed from putative dopamine-containing neurons in the SNc and VTA of anesthetised rats. In the SNc, acute systemic administration of ST1936 had no effects on basal firing activity of these dopamine neurons; however, in the VTA, ST1936 induced either dose-related increases (45% of cells) or decreases in basal activity of these dopaminergic neurons. Local application of ST1936 into the VTA caused excitation in all of the dopamine neurons, but had no effects on non-dopamine VTA neurons. Both effects of systemic and microiontophoretic ST1936 were completely reversed by the potent and selective 5-HT6 receptor antagonist 5-chloro-N-(4-methoxy-3-piperazin-1-ylphenyl)-3-methyl-2- benzothiophene-sulfonamide (SB271046). Systemic application of another 5-HT6 agonist, 2-(1-{6-chloroimidazo[2,1-b] [1,3]thiazole-5-sulfonyl}-1H-indol-3-yl)ethan-1-amine (WAY-181187), induced dose-dependent inhibition of these VTA dopaminergic neurons. ST1936 and WAY-181187 appear to have different effects on these VTA dopaminergic neurons, potentially due to different mechanisms of action or to the complexity of 5-HT6 receptor functions. Our data demonstrate the need for further investigations into the use of 5-HT6 receptor agonists to control cognitive disfunction, such as in schizophrenia and depression.

Serotonin receptor of type 6 (5-HT6) in human prefrontal cortex and hippocampus post-mortem: an immunohistochemical and immunofluorescence study.

Given the paucity of data on the distribution of serotonin (5-HT) receptors of type 6 (5-HT(6)) in the human brain, the aim of this study was to investigate their distribution in postmortem human prefrontal cortex, striatum and hippocampus by either immunohistochemical or immunofluorescence techniques. The brain samples were obtained from 6 subjects who had died for causes not involving primarily or secondarily the CNS. The 5-HT(6) receptor distribution was explored by the [(125)I]SB-258585 binding to brain membranes followed by immunohistochemical and immunofluorescence evaluations. A specific [(125)I]SB-258585 binding was detected in all the regions under investigation, whilst the content in the hippocampus and cortex being about 10-30 times lower than in the striatum. Immunohistochemistry and double-label immunofluorescence microscopy experiments, carried out in the prefrontal cortex and hippocampus only, since data in the striatum were already published, showed the presence of 5-HT(6) receptors in both pyramidal and glial cells of prefrontal cortex, while positive cells were mainly pyramidal neurons in the hippocampus. The heterogeneous distribution of 5-HT(6) receptors provides a preliminary explanation of how they might regulate different functions in different brain areas, such as, perhaps, brain trophism in the cortex and neuronal firing in the hippocampus. This study, taking into account all the limitations due to the postmortem model used, represents the starting point to explore the 5-HT(6) receptor functionality and its sub-cellular distribution.

Distribution of serotonin receptor of type 6 (5-HT6) in human brain post-mortem. A pharmacology, autoradiography and immunohistochemistry study.

The aim of this study was to investigate the distribution of serotonin (5-HT) receptors of type 6 (5-HT(6)) in postmortem human prefrontal cortex, striatum and hippocampus. The brain samples were obtained from 6 subjects who had died for causes not involving primarily or secondarily the CNS. The 5-HT(6) receptor distribution was explored by the [(125)I]SB-258585 binding to brain membranes followed by the pharmacological characterization, where possible, and by autoradiographic, immunohistochemical and immunofluorescence evaluations. A specific and saturable [(125)I]SB-258585 binding was detected in striatum only, with a pharmacological characterization consistent with that of a 5-HT(6) receptor. The autoradiography showed the presence of a specific [(125)I]SB-258585 binding distributed homogeneously in caudate, putamen and accumbens. The immunohistochemistry, carried out in the striatum only, coupled with the immunofluorescence with glial fibrillary acidic protein (GFAP) and parvalbumin (PV) showed the co-localization of 5-HT(6) receptor with PV, while indicating that this receptor subtype was expressed in neurons and not in astrocytes. Taken together, the present findings showed the presence of a higher density of 5-HT(6) receptors, as labeled by [(125)I]SB-258585, in striatum than in hippocampus and prefrontal cortex, and specifically within the neuronal body. In addition, they would suggest that striatum is one of the major potential CNS targets linked to 5-HT(6) receptor modulation.

Effects of the 5-HT(6) receptor agonist ST 1936 on depression- and anhedonia-like experimental models.

Serotonin 5-HT(6) receptor agonists and antagonists have been proposed as possible useful compounds in the treatment of psychiatric disorders such as depression. This study was aimed at characterizing ST 1936, a new 5-HT(6) receptor agonist, as a possible antidepressant/anti-anhedonic drug by studying its effects on three experimental models of depression. These models are based on the behavioral sequelae induced in rats by unavoidable stressors that result in decreased reactivity to avoidable stressors (escape deficit, ED) and an anhedonia-like condition based on the disruptive effect of stress on the competence to acquire an instrumental vanilla sugar-sustained appetitive behavior (VAB). The repeated administration of ST 1936 prevented the development of ED, but did not revert a condition of chronic ED. The protective effect of ST 1936 was antagonized by co-administration of SB 271046, a 5-HT(6) receptor antagonist, indicating that the 5-HT(6) receptor stimulation is crucial for triggering a plasticity process that resulted in the prevention of ED development. ST 1936 administration in rats undergoing VAB training did not interfere with its acquisition, whereas SB 271046 administered in similar conditions prevented VAB acquisition. Moreover, ST 1936 administration in rats trained in the Y-maze while exposed to a chronic stress protocol consistently antagonized the stress-disrupting effect, and also this effect was antagonized by SB 271046 coadministration. It was concluded that a tonic 5-HT(6) receptor activity was crucial for VAB acquisition, and that pharmacological stimulation of 5-HT(6) receptors reinstated a stress-reduced hedonic competence with an efficacy similar to that of classical antidepressant drugs.

ST1936 stimulates cAMP, Ca2+, ERK1/2 and Fyn kinase through a full activation of cloned human 5-HT6 receptors.

5-HT(6) receptor is one of the most recently cloned serotonin receptors, and it might play important roles in Alzheimer's disease, depression, and learning and memory disorders. Availability of only very few 5-HT(6) receptor agonists, however, does not allow examining their contribution in psychopharmacological processes. Therefore, a new 5-HT(6) receptor agonist, ST1936, was synthesized. ST1936 binds to human 5-HT(6) receptors with good affinity (K(i)=28.8 nM). ST1936 also exhibited some moderate binding affinity for 5HT(2B), 5HT(1A), 5HT(7) receptors and adrenergic α receptors. ST1936 behaved as a full 5-HT(6) agonist on cloned cells and was able to increase Ca(2+) concentration, phosphorylation of Fyn kinase, and regulate the activation of ERK1/2 that is a downstream target of Fyn kinase. These effects were completely antagonized by two 5-HT(6) receptor antagonists, SB271046 and SB258585. The other 5-HT(6) receptor agonist, WAY181187 also increased Fyn kinase activity. These results suggest that both ST1936 and WAY181187 mediate 5-HT(6) receptor-dependent signal pathways, such as cAMP, Fyn and ERK1/2 kinase, as specific agonists.

5-HT6 pharmacology inconsistencies.

5-HT(6) receptors are relatively recently-discovered receptors. After an uncertain beginning, where results were ambiguous, findings are now apparently more congruent. Nevertheless, discrepancies still exist. The aim of the present manuscript is to point out some of these discrepancies, in order to reflect on the current status of the field of the 5-HT(6) receptor neuropharmacology, and where the field should move next. Examples of 5-HT(6) receptor ligand-induced changes in behavior, neurochemistry and binding highlight areas where discrepancies remain and further experimental attention is needed. Possible methodological as well as conceptual issues underlying the inconsistencies are considered in an effort to increase awareness of the need for more thorough consideration of these aspects in future research.

2,4-Dicarboxy-pyrroles as selective non-competitive mGluR1 antagonists: further characterization of 3,5-dimethyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester and structure-activity relationships.

Following the disclosure of 3,5-dimethyl pyrrole-2,4-dicarboxylic acid 2-propyl ester 4-(1,2,2-trimethyl-propyl) ester [3,5-dimethyl PPP] as a potent and selective mGluR1 non-competitive antagonist, we report here further in vivo characterization of this important tool and disclose the investigation of the C-5 position, which led to very potent compounds.

Single-unit activity in the lateral nucleus of the amygdala and overlying areas of the striatum in freely behaving rats: rates, discharge patterns, and responses to acoustic stimuli.

Acoustic responses of single units were examined in awake, freely behaving rats in the lateral nucleus of the amygdala (AL). Recordings were made from a movable bundle of 9 microwires. Most cells had very low rates of spontaneous activity (about 3 spikes/s average). Firing rates increased during sleep states. Short-latency auditory responses (12-25 ms) were found in the dorsal subnucleus (ALd) of the AL. Cells in the ALd most typically responded in a sustained fashion. Some of the cells in the ALd showed preferences for high frequencies, tone bursts, or frequency-modulated stimuli with center frequencies above 12 kHz. Response latencies were considerably longer in other areas of the amygdala. Our results corroborate the main findings of a previous study (F. Bordi & J. LeDoux, 1992) that examined the acoustic response properties of single cells in the AL in anesthetized rats. Together the findings from awake and anesthetized rats provide the most precise information about sensory processing in amygdala neurons available to date.