Angelo Ceci

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.

Activation of the A10 mesolimbic system by the σ-receptor agonist (+)SKF 10,047 can be blocked by rimcazole, a novel putative antipsychotic

This study evaluated with electrophysiological and behavioral techniques the ability of rimcazole, a novel putative antipsychotic and selective sigma-receptor ligand, to antagonize the stimulation of the mesocorticolimbic dopamine system by the sigma-agonist, (+)SKF 10,047. Rimcazole effectively blocked the (+)SKF 10,047-induced excitation of ventral tegmental dopamine neurons while having no effect on either spontaneous activity or apomorphine-elicited slowing of A10 firing. Rimcazole also antagonized the behavioral hyperactivity produced by (+)SKF 10,047, but not by d-amphetamine which is also mediated through the same mesolimbic dopamine pathway. These data provide further evidence that rimcazoles novel pharmacologic profile may involve a blockade of sigma-receptors on mesocorticolimbic dopamine neurons.

Anxiety-like behaviour in rats with mononeuropathy is reduced by the analgesic drugs morphine and gabapentin.

Abstract Anxiety has been described as an important comorbidity in patients suffering from chronic pain. However, in animals the connection between persistent pain and anxiety has hardly been investigated. Therefore, in the current study it was assessed whether chronic pain also causes anxiety‐like behaviour in animals and if it can be reversed by analgesic or anxiolytic drugs. Neuropathic pain was induced in rats by partial sciatic nerve ligation (PNL) and chronic constriction injury (CCI). Mechanical hypersensitivity was assessed by the “electronic algometer”, while anxiety‐like behaviour was measured by using the elevated plus maze. In both neuropathic pain models, rats exhibited mechanical hypersensitivity, whereas a significant increase in anxiety‐like behaviour was observed only in CCI rats (time spent in open arms decreased significantly from 99 ± 15.8 s in sham animals to 33.4 ± 7.5 s in CCI animals). Furthermore, midazolam (0.5 mg/kg; i.p.) significantly reduced anxiety‐like behaviour in both sham‐ and CCI‐operated animals without influencing mechanical hypersensitivity. Morphine (3 mg/kg; i.p.) and gabapentin (30 mg/kg; i.p.) significantly attenuated anxiety‐like behaviour in the CCI lesioned rats: morphine increased entries into open arms from 3.0 ± 0.4 to 7.7 ± 1.4 (P = 0.01), gabapentin elevated this value from 4.7 ± 1 to 7.5 ± 0.9 (P = 0.02). These data suggest that rats subjected to neuropathic pain models develop anxiety‐like behaviour which can be reversed by appropriate analgesic treatment. Morphine and gabapentin had no anxiolytic‐like effect in sham treated animals, thus their effect on anxiety‐like behaviour in the neuropathic pain model is likely indirect via their anti‐nociceptive properties.

Depression-like behaviour in rats with mononeuropathy is reduced by the CB2-selective agonist GW405833

ABSTRACT The current study assessed whether the chronic constriction injury (CCI) model of neuropathic pain causes depression‐like behaviour in animals, and if this depression‐like behaviour can be reversed by anti‐nociceptive and/or antidepressant drugs. CCI of the sciatic nerve in rats was selected as a neuropathic pain model, mechanical hypersensitivity was assessed by punctuate mechanical stimuli, and depression‐like behaviour was evaluated in the forced swimming test (FST) measuring the time of immobility, climbing and swimming. The CCI rats displayed a significant mechanical hypersensitivity (sham 27 ± 2 g, CCI 12 ± 2 g; P < 0.001) and a significant increase in time of immobility (sham 133 ± 14 s, CCI 201 ± 9 s; P < 0.001). As time of swimming was unchanged, immobility was increased at the expense of climbing behaviour (sham 105 ± 17 s, CCI 63 ± 9 s; P < 0.05). There was no difference in ambulation between sham and CCI animals. In sham and CCI animals, desipramine (20 mg/kg) significantly reduced immobility (sham + vehicle 134 ± 19 s, sham + desipramine 79 ± 13 s; P < 0.01, CCI + vehicle 195 ± 8 s, CCI + desipramine 140 ± 11 s; P < 0.05) and increased climbing behaviour (sham + vehicle 118 ± 21 s, sham + desipramine 182 ± 16 s; P < 0.05, CCI + vehicle 59 ± 8 s, CCI + desipramine 112 ± 14 s; P < 0.05) with little effect on mechanical hypersensitivity. In contrast in CCI animals the cannabinoid CB2‐selective agonist GW405833 (2,3‐dichloro‐phenyl)‐[5‐methoxy‐2‐methyl‐3‐(2‐morpholin‐4‐yl‐ethyl)‐indol‐1‐yl]‐methanone) (30 mg/kg) significantly attenuated immobility (CCI + vehicle 191 ± 7 s, GW405833 145 ± 14 s; P < 0.01) and mechanical hypersensitivity (CCI + vehicle 15 ± 1 g, CCI + GW405833 24 ± 1 g; P < 0.001). Moreover, differently from desipramine, GW405833 did not change the climbing behaviour. These data suggest that rats subjected to the CCI model of neuropathic pain develop depression‐like behaviour, which can be reversed by appropriate anti‐nociceptive treatment.

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.

ORIGINAL RESEARCH—BASIC SCIENCE: Acute and Repeated Flibanserin Administration in Female Rats Modulates Monoamines Differentially Across Brain Areas: A Microdialysis Study

INTRODUCTION Hypoactive sexual desire disorder (HSDD) is defined as persistent lack of sexual fantasies or desire marked by distress. With a prevalence of 10% it is the most common form of female sexual dysfunction. Recently, the serotonin-1A (5-HT(1A)) receptor agonist and the serotonin-2A (5-HT(2A)) receptor antagonist flibanserin were shown to be safe and efficacious in premenopausal women suffering from HSDD in phase III clinical trials. AIM The current study aims to assess the effect of flibanserin on neurotransmitters serotonin (5-HT), norepinephrine (NE), dopamine (DA), glutamate, and gamma-aminobutyric acid (GABA) in brain areas associated with sexual behavior. METHODS Flibanserin was administered to female Wistar rats (280-350 g). Microdialysis probes were stereotactically inserted into the mPFC, NAC, or MPOA, under isoflurane anesthesia. The extracellular levels of neurotransmitters were assessed in freely moving animals, 24 hours after the surgery. MAIN OUTCOME MEASURES Dialysate levels of DA, NE, and serotonin from medial prefrontal cortex (mPFC), nucleus accumbens (NAC), and hypothalamic medial preoptic area (MPOA) from female rats. RESULTS Acute flibanserin administration decreased 5-HT and increased NE levels in all tested areas. DA was increased in mPFC and MPOA, but not in the NAC. Basal levels of NE in mPFC and NAC and of DA in mPFC were increased upon repeated flibanserin administration, when compared to vehicle-treated animals. The basal levels of 5-HT were not altered by repeated flibanserin administration, but basal DA and NE levels were increased in the mPFC. Glutamate and GABA levels remained unchanged following either repeated or acute flibanserin treatment. CONCLUSIONS Systemic administration of flibanserin to female rats differentially affects the monoamine systems of the brain. This may be the mechanistic underpinning of flibanserins therapeutic efficacy in HSDD, as sexual behavior is controlled by an intricate interplay between stimulatory (catecholaminergic) and inhibitory (serotonergic) systems.

Mouse chronic social stress increases blood and brain kynurenine pathway activity and fear behaviour: Both effects are reversed by inhibition of indoleamine 2,3-dioxygenase

Psychosocial stress is a major risk factor for mood and anxiety disorders, in which excessive reactivity to aversive events/stimuli is a major psychopathology. In terms of pathophysiology, immune-inflammation is an important candidate, including high blood and brain levels of metabolites belonging to the kynurenine pathway. Animal models are needed to study causality between psychosocial stress, immune-inflammation and hyper-reactivity to aversive stimuli. The present mouse study investigated effects of psychosocial stress as chronic social defeat (CSD) versus control-handling (CON) on: Pavlovian tone-shock fear conditioning, activation of the kynurenine pathway, and efficacy of a specific inhibitor (IDOInh) of the tryptophan-kynurenine catabolising enzyme indoleamine 2,3-dioxygenase (IDO1), in reversing CSD effects on the kynurenine pathway and fear. CSD led to excessive fear learning and memory, whilst repeated oral escitalopram (antidepressant and anxiolytic) reversed excessive fear memory, indicating predictive validity of the model. CSD led to higher blood levels of TNF-α, IFN-γ, kynurenine (KYN), 3-hydroxykynurenine (3-HK) and kynurenic acid, and higher KYN and 3-HK in amygdala and hippocampus. CSD was without effect on IDO1 gene or protein expression in spleen, ileum and liver, whilst increasing liver TDO2 gene expression. Nonetheless, oral IDOInh reduced blood and brain levels of KYN and 3-HK in CSD mice to CON levels, and we therefore infer that CSD increases IDO1 activity by increasing its post-translational activation. Furthermore, repeated oral IDOInh reversed excessive fear memory in CSD mice to CON levels. IDOInh reversal of CSD-induced hyper-activity in the kynurenine pathway and fear system contributes significantly to the evidence for a causal pathway between psychosocial stress, immune-inflammation and the excessive fearfulness that is a major psychopathology in stress-related neuropsychiatric disorders.

Tramadol reduces anxiety-related and depression-associated behaviors presumably induced by pain in the chronic constriction injury model of neuropathic pain in rats.

Depression and anxiety are common comorbidities of neuropathic pain (NP). Pharmacological preclinical studies on NP have given abundant information on the effects of drugs on reflex measures of stimulus-evoked pain. However, few preclinical studies focus on relief of comorbidities evoked by NP. In this study, we investigated the effects of tramadol on nociceptive reflex, depression-associated and anxiety-related behaviors in a NP model in rats. We used chronic constriction injury (CCI) of the sciatic nerve as an animal model of neuropathic pain. We performed electronic von Frey tests (evF) to measure mechanical sensitivity, elevated plus maze tests (EPM) to record anxiety-related behaviors and forced swimming tests (FST) to evaluate depression-associated behaviors. In the evF, CCI rats showed a decrease of 82% of the paw withdrawal threshold (PWT) compared to sham (P<0.001). Tramadol increased the PWT by 336% in CCI rats (P<0.001) and by 16% in sham (P<0.05). On the EPM, CCI rats spent 45% less time than sham on the open arms of the maze (P<0.05). Tramadol increased the time spent on the open arms of CCI rats by 67% (P<0.05) and had no significant effect on sham. During the FST, CCI rats showed 28% longer immobility than sham (P<0.01). Tramadol reduced the immobility time in CCI rats by 22% (P<0.001), while having no effect on sham. Tramadol reversed the changes in mechanical sensitivity as well as anxiety-related and depression-associated behaviors that are caused by injury of the sciatic nerve with only minor effects in the absence of injury. These data suggest that tramadol relieves chronic pain and its indirect consequences and comorbidities, and that this study also is a model for pharmacological studies seeking to investigate the effect of drugs on the major disabling symptoms of NP.

Effect of high trait anxiety on mechanical hypersensitivity in male rats

Recently, we have shown that neuropathic pain is associated with anxiety-like behaviour in rats with sciatic nerve lesion. An enhanced pain perception has also been described in patients with anxiety disorders. However, there is only limited knowledge about the relationship between anxiety and pain in animals. To investigate whether trait anxiety influences nociception, we measured mechanical hypersensitivity following chronic constriction injury (CCI) in rats selectively bred for high (HAB) or low (LAB) anxiety-like behaviour. Pain sensitivity was assessed before surgery and at day 7, 14, 21, 36 and 57 after CCI by determination of withdrawal thresholds. Additionally, we examined pain-induced anxiety-like behaviour using the elevated plus-maze (EPM). HAB and LAB rats exhibited similar levels of mechanical hypersensitivity 7 days post-injury. However, at day 14 and 21 after surgery, mechanical-induced pain thresholds were significantly decreased in HAB rats (p<0.05) in comparison to LAB rats. From day 21 onward HAB rats displayed a faster return of paw withdrawal threshold to baseline level when compared to LAB rats (p<0.01). In the EPM anxiety-like behaviour was observed following CCI injury in HAB and LAB rats on top of low and high trait anxiety reflected in a reduced number of entries in open arms. These findings indicate that trait anxiety increases mechanical hypersensitivity in CCI rats during the chronic phase of pain, thereby suggesting that affective processes modulate even simple pain-related behaviour. Furthermore, we demonstrated that neuropathic pain in the CCI model increases anxiety-like behaviour even in LAB rats.

Chronic treatment with DAU 6215, a new 5-HT3 receptor antagonist, causes a selective decrease in the number of spontaneously active dopaminergic neurons in the rat ventral tegmental area

Electrophysiological techniques were used to study the effects of the new compound, DAU 6215 ((3-alpha-tropanyl) 1H-benzimidazolone-3-carboxamide chloride), a selective 5-HT3 receptor antagonist, on the activity of dopamine (DA)-containing neurons in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA). Acute i.v. injections of DAU 6215 did not cause any change in the basal firing rate of DA neurons in the SNc or in the VTA. Pretreatment with DAU 6215 did not modify the inhibitory effect of apomorphine on the firing rate of midbrain DA neurons. Acute s.c. administration of DAU 6215 caused a significant increase in the number of spontaneously active DA neurons in the VTA but not in the SNc. This effect was similar to that of acute clozapine, whereas acute haloperidol caused a significant increase of spontaneously active DA neurons in both the SNc and the VTA. Repeated consecutive s.c. administration of DAU 6215 and clozapine for 21 days produced a significant decrease in the number of spontaneously active DA neurons in the VTA but not in the SNc. Chronic haloperidol (21 days) decreased the number of DA cells both in the SNc and VTA. The effect of chronic DAU 6215 on the activity of VTA DA neurons was reversed by apomorphine, suggesting that these neurons were probably under a state of depolarization block. These findings indicate that DAU 6215 may have potential antipsychotic activity, probably associated with a low incidence of extrapyramidal side-effects.