Franco Borsini

Role of the serotonergic system in the forced swimming test

The effect of manipulations aimed at modifying the function of the 5-HT system has been reviewed. 5-HT uptake inhibitors are devoid of any activity in rats and induce an anti-immobility effect in mice. The so-called 5-HT1A agonists reduce the immobility time with some differences in mice and rats, mice being less sensitive. None of the procedure aimed at reducing 5-HT function reduced immobility time. Therefore, the 5-HT system does not play a tonic role in animals performing the forced swimming test. The involvement of possible brain regions mediating the anti-immobility effects of 5-HT mimetic drugs has been discussed.

The inhibitory effect of 8-OH-DPAT on the firing activity of dorsal raphe serotoninergic neurons in rats is attenuated by lesion of the frontal cortex

The dose-response inhibitory effect of 8-OH-DPAT on the firing rate of dorsal raphe serotoninergic neurons was shifted 10-fold to the right after acute fronto-cortical deafferentation. This finding suggests that the inhibitory effect of 8-OH-DPAT on the dorsal raphe firing rate might be mediated indirectly by the frontal cortex.

Effect of chronic intermittent restraint stress on hippocampal expression of marker proteins for synaptic plasticity and progenitor cell proliferation in rats.

Chronic restraint stress may change hippocampal mRNA levels of markers for synaptic plasticity such as synaptophysin, growth-associated protein 43 (GAP-43), and brain-derived neurotrophic factor (BDNF). In order to examine the relation between that stressor and those biochemical markers on protein level as well as the Ki-67 protein, a marker of progenitor cell proliferation, we subjected rats to chronic intermittent restraint stress for 6 h per day for 14 days excluding the weekends. This stress intensity caused a significant increase in adrenal gland weight and decrease in body weight gain. However, we did not find significant alteration of protein expression levels for synaptophysin, GAP-43, and BDNF by using Western blot analysis. Unlike these findings, the hippocampal protein expression of Ki-67 was significantly reduced by using both Western blot and immunohistochemical analyses. This reduction of Ki-67 expression in chronically stressed rats was correlated with increased adrenal gland weight and decreased body weight gain. All marker proteins used did not show any changes of hippocampal expression level after a single restraint stress session of 3 h. In conclusion, chronic intermittent restraint stress caused changes in the physiological stress response in rats, and a decrease of hippocampal progenitor cells using the Ki-67 protein as marker which indicates a suppression of adult neurogenesis. The results might contribute to understand the relationship between stress and cellular neurobiology of depression, since chronic antidepressant treatment have been shown to increase adult neurogenesis in the rat hippocampus.

BIMG 80, a novel potential antipsychotic drug: evidence for multireceptor actions and preferential release of dopamine in prefrontal cortex.

Abstract: In radioligand binding studies, BIMG 80, a new putative antipsychotic, displayed good affinity at certain serotonin (5‐HT1A, 5‐HT2A, 5‐HT6), dopamine (D1, D2L, D4), and noradrenergic (α1) receptors. The effect of acute subcutaneous BIMG 80, clozapine, haloperidol, risperidone, amperozide, olanzapine, and Seroquel was then investigated on dopamine release in medial prefrontal cortex, nucleus accumbens, and striatum in freely moving rats using the microdialysis technique. Four different neurochemical profiles resulted from the studies: (a) Systemic administration of BIMG 80, clozapine, and amperozide produced greater percent increases in dopamine efflux in medial prefrontal cortex than in the striatum or the nucleus accumbens. (b) Haloperidol induced a similar increase in dopamine concentrations in the striatum and nucleus accumbens with no effect in the medial prefrontal cortex. (c) Risperidone and olanzapine stimulated dopamine release to a similar extent in all brain regions investigated. (d) Seroquel failed to change significantly dopamine output both in the medial prefrontal cortex and in the striatum. Because an increase in dopamine release in the medial prefrontal cortex may be predictive of effectiveness in treating negative symptoms and in the striatum may be predictive of induction of extrapyramidal side effects, BIMG 80 appears to be a potential antipsychotic compound active on negative symptoms of schizophrenia with a low incidence of extrapyramidal side effects.

Anti-anhedonic actions of the novel serotonergic agent flibanserin, a potential rapidly-acting antidepressant

Chronic exposure to mild unpredictable stress has previously been found to depress the consumption of palatable sweet solutions and to block the formation of conditioned place preferences; these effects are reversed by chronic treatment with tricyclic or atypical antidepressant drugs. The present study was designed to evaluate the antidepressant-like activity in this model of flibaserin (BIMT-17), a novel serotonergic agent with 5-HT1A receptor agonist and 5-HT2 receptor antagonist properties. Two experiments were conducted, using rats (experiment 1) and mice (experiment 2). In experiment 1, decreases in sucrose intake were seen in rats exposed to chronic mild stress, but the effect was unreliable in this study, and sucrose testing was terminated after 7 weeks of stress. Beginning after 5 weeks of stress, groups of control and stressed animals were treated daily with vehicle, fluoxetine (5 mg/kg) or flibanserin (5, 10 or 20 mg/kg). After 6 weeks of treatment, all animals were tested for acquisition of food-reinforced place preference conditioning. Conditioning was seen in all groups other than the vehicle-treated stressed animals. We also tested the locomotor stimulant effect of a single injection of the dopamine D2/D3 receptor agonist quinpirole (0.2 mg/kg). The effect of quinpirole was potentiated by fluoxetine in control animals, and by both fluoxetine and flibanserin (all doses) in stressed animals. In experiment 2, long-lasting decreases in sucrose intake were seen in mice exposed to chronic mild stress. The effects were reversed by chronic (4 weeks) treatment with fluoxetine (5 mg/kg) or flibanserin (2.5 or 5 mg/kg); the full effect of flibanserin was seen after the first injection. All animals received a single injection of raclopride (0.1 mg/kg) immediately prior to a sucrose intake test on day 27 of drug treatment. Raclopride decreased sucrose intake only in the three drug-treated stressed groups. The results support a rapid antidepressant-like action of flibanserin, and suggest that this effect involves sensitization of dopamine D2/D3 receptor-mediated transmission.

Region-dependent effects of flibanserin and buspirone on adenylyl cyclase activity in the human brain

The mode of action of antidepressant drugs may be related to mechanisms of receptor adaptation, involving overall the serotonin 1A (5-HT1A) receptor subtype. However, so far, the clinical effectiveness of selective compounds acting at this level has proved disappointing. This could be explained by the heterogeneity of 5-HT1A receptors within the central nervous system. In animals, two 5-HT1A agonists, flibanserin and buspirone, have shown different pharmacological properties, depending on the brain region. Since no evidence supports this observation in humans, this study sought to investigate whether these two drugs exert different effects on 5-HT1A receptor activation in three different human brain areas: the prefrontal cortex, hippocampus and raphe nuclei. 5-HT1A-mediated inhibition of forskolin-stimulated adenylyl cyclase (AC) was taken as an index of 5-HT1A receptor activation. Flibanserin significantly reduced the activity of AC post-synaptically, i.e. in the prefrontal cortex [EC50 (mean +/- S.E.M.), 28 +/- 10.2 nM; Emax, 18 +/- 2.3%] and in the hippocampus (EC50, 3.5 +/- 3.1 nM; Emax, 20 +/- 4.0%), but had no effect in the raphe nuclei, i.e. at pre-synaptic level. Vice versa, buspirone was only slightly but significantly effective in the raphe (EC50, 3.0 +/- 2.8 nM; Emax, 12 +/- 1.9%). Agonist effects were sensitive to the 5-HT1A antagonists WAY-100135 and pindobind 5-HT1A in the cortex and raphe nuclei, whereas buspirone antagonized flibanserin in the hippocampus. These findings suggest a region-related action of flibanserin and buspirone on forskolin-stimulated AC activity in human brain.

BIMT 17, A 5-HT1A RECEPTOR AGONIST/5-HT2A RECEPTOR ANTAGONIST, DIRECTLY ACTIVATES POSTSYNAPTIC 5-HT INHIBITORY RESPONSES IN THE RAT CEREBRAL CORTEX

BIMT 17 (1-[2-[4-(3-trifluoromethyl phenyl) piperazin-1-yl] ethyl] benzimidazol- [1H]-2-one), a 5-HT1A receptor agonist/5-HT2A receptor antagonist (see Borsini et al., accompanying paper), in a dose range of 1–10 mg/kg i.v., dose-dependently inhibited the electrical activity of rat medial prefronto-cortical neurons, whereas buspirone, in a dose range of 0.1–1000 μg/kg, increased it. 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and 1-[2-(2-thenoylamino)ethyl]-4[1-(7-methoxynaphthyl)] piperazine (S 14671) presented biphasic patterns of response; they increased electrical activity at doses in the range of 0.1–10 μg/kg and 0.1–3 μg/kg i.v. respectively, and reduced it at higher doses, 30–300 μg/kg and 10–30 μg/kg i.v., respectively.The inhibitory effect of BIMT 17 on the firing rate of neurons in the frontal cortex was antagonized by the 5-HT1A antagonists tertatolol and WAY 100135, and was still present after destruction of serotonin (5-HT) containing neuronal endings by the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT; 150 μg/rat, given intraventricularly), which reduced the cortical 5-HT content by 85%. This destruction of 5-HT neurons, while suppressing the ability of 8-OH-DPAT to inhibit the firing rate at high doses, did not change the excitatory action of this compound at low doses. The addition of ritanserin, a 5-HT2A receptor antagonist, potentiated both the excitatory and inhibitory effects of 8-OHDPAT on neuronal electrical activity. Direct microiontophoretic application (100 nA/20 s) of 5-HT and BIMT 17, but not that of 8-OH-DPAT, onto medial prefronto-cortical neurons, decreased the firing rate of these neurons.These findings suggest that BIMT 17 directly inhibits the electrical activity of medial prefronto-cortical neurons through its dual mode of receptor interaction.

Effect of antipsychotic drugs and selective dopaminergic antagonists on dopamine-induced facilitatory activity in prelimbic cortical pyramidal neurons. An in vitro study

Intracellular recordings were obtained from 119 pyramidal neurons localized in prelimbic cortex, five in the dorsal cingulate cortex, one in the infralimbic cortex, one in the border of prelimbic and cingulate cortex and two in the border of prelimbic and infralimbic cortex. The passive membrane properties of these pyramidal neurons (i.e. resting membrane potential, input membrane resistance, shape of the tetrodotoxin-sensitive action potentials, spike frequency adaptation with a prominent postspike afterhyperpolarization, tetrodotoxin-sensitive inward rectification in the depolarizing direction and the absence of bursting) suggested that they resembled regular spiking or intrinsically bursting pyramidal neurons. Bath application of dopamine (EC50 of 1.8 microM) produced a reversible facilitatory effect on all 119 pyramidal neurons localized in the middle layer of the prelimbic cortex. No consistent change in membrane potential was detected during the application of dopamine. No effect of dopamine was noted on the nine pyramidal neurons that were not localized in the prelimbic cortex. The facilitatory effect of dopamine in prelimbic cortex was concentration dependently antagonized by haloperidol, risperidone, quetiapine, clozapine and by the selective D4 dopaminergic receptor antagonist L-745,870, but not by the selective D2/D3 dopaminergic receptor antagonist (-)-sulpiride. (+)-SCH 23390, which is a selective D1/D5 dopamine receptor antagonist, produced, similarly to dopamine, a facilitatory effect per se, and an additive effect when co-administered with dopamine. These results provide evidence that dopamine has a facilitatory effect specifically on pyramidal neurons localized in the middle layer of prelimbic cortex. Antipsychotic drugs and L-745,870 block this effect of dopamine.

BIMT 17, a 5-HT2A receptor antagonist and 5-HT1A receptor full agonist in rat cerebral cortex

In the search for antidepressant agents with a rapid onset of action, we have found that compound BIMT 17 (1-[2-[4-(3-trifluoromethylphenyl)piperazin1-yl]ethyl]benzimidazol-[1H]-2-one) shows a good affinity for cerebral cortical 5-HT1A (pKi = 7.72) and 5-HT2A (pKi = 6.90) receptors, with no appreciable affinity for the other 5-HT receptor subtypes, including 5-HT2C. BIMT 17 reduced forskolin-stimulated cAMP accumulation in the cerebral cortex (pEC50 = 6.09) and in the hippocampus (pEC50 = 6.50), and antagonized 5-HT-induced phosphatidylinositol turnover (pKi = 6.96) in the cerebral cortex. The effect on cAMP accumulation was blocked by the 5-HT1A receptor antagonist tertatolol. Buspirone, 8-OH-DPAT and S 14671 {1-[2-(2-thenoylamino)ethyl]-4[1-(7-methoxynaphtyl)]piperazine, claimed to be 5-HT1A receptor agonists, did not reduce forskolin-stimulated cAMP formation in the cerebral cortex.On the basis of these data, it was concluded that BIMT 17 was the only compound that behaved as a full agonist with respect to the CAMP response in the cortex, while exerting concurrent agonism at 5-HT1A receptors and antagonism at 5-HT2A receptors. These characteristics might explain the peculiar behaviour of BIMT 17 in mimicking the inhibitory action of 5-HT on the basal firing rate of the cortical neurons (see accompanying paper).

Effect of DAU 6215, a novel 5-HT3 receptor antagonist, on scopolamine-induced amnesia in the rat in a spatial learning task

The effects of different doses (1, 10, 30, and 100 micrograms/kg, IP) of a new 5-hydroxytryptamine3 (5-HT3) receptor antagonist, 3-alpha-tropanyl)1H-benzimidazolone-3-carboxamide chloride (DAU 6215), on memory and performance deficits induced by SC 0.2 mg/kg scopolamine were assessed in the Morris water maze task. No effect was observed on the performance of rats treated with DAU 6215 alone. The doses of 10 and 30 micrograms/kg DAU 6215 attenuated these scopolamine-induced behavioral deficits.