Impramine, fluoxetine and clozapine differently affected reactivity to positive and negative stimuli in a model of motivational anhedonia in rats.

Anhedonia is a relevant symptom in depression and schizophrenia. Chronic stress exposure induces in rats escape deficit, disrupts the dopaminergic response to palatable food and the competence to acquire sucrose self-administration (SA), thus configuring a possible model of motivational anhedonia. Repeated lithium administration reverts stress effects and brings back to control values the breaking point (BP) score, a measure of reward motivation. In this study, we tested on this model two antidepressants, imipramine and fluoxetine, and two antipsychotics, haloperidol and clozapine. The dopaminergic response to sucrose consumption was studied in non food-deprived rats in terms of dopamine D1 receptor signaling in the nucleus accumbens shell (NAcS). More specifically, we studied the modifications in dopamine and cAMP-regulated phosphoprotein of Mr 32,000 (DARPP-32) phosphorylation pattern following sucrose consumption. Fluoxetine reverted the escape deficit and showed no effects on dopaminergic response and sucrose SA. Imipramine reverted sucrose SA and dopamine response deficit in half of the rats and the escape deficit in all animals. Haloperidol did not affect stress-induced deficits. Clozapine-treated rats recovered the dopaminergic response to sucrose consumption and the competence to acquire sucrose SA, although they still showed the escape deficit, thus confirming that motivation toward reward may be dissociated from that to punishment escape. These results indicate that imipramine or fluoxetine are not endowed with a rapid onset antianhedonic effect. On the other hand, clozapine treatment showed a motivational antianhedonic activity similar to that observed after lithium treatment.

Influence of palatability on motivation to operate for caloric and non-caloric food in non food-deprived and food-deprived rats.

Palatability is the hedonic food component that is considered to override the homeostatic mechanisms that control food intake, and we compared how much effort non food-deprived and food-deprived rats were willing to spend in order to earn a palatable caloric (sucrose) or non-caloric (saccharin) snack. We first studied the dopaminergic response, in terms of dopamine levels and dopamine and cAMP-regulated phosphoprotein Mr 32,000 (DARPP-32) phosphorylation pattern, to two consecutive palatable caloric or non-caloric snacks in the nucleus accumbens shell (NAcS) of non food-deprived and fasted rats. We report that non food-deprived rats developed rapid habituation in the NAcS dopaminergic response to the second consumption of both caloric and non-caloric palatable food, while food-deprived rats developed rapid habituation only to saccharin. Next, we show that in self-administration experiments, non food-deprived rats spent a similar effort when operating for sucrose or saccharin. However, the same rats showed an increased response specifically for sucrose after 18-h fasting. After pre-feeding devaluation, rats reduced their response to sucrose but not for saccharin. These results strengthen the hypothesis that food intake is mainly controlled by palatability in non food-deprived rats and by caloric content in food-deprived rats. Moreover, they show that rapid habituation development was associated with a similar, basal working activity aimed at ingesting both caloric and non-caloric food, as observed in non food-deprived rats consuming sucrose or saccharin and in fasted rats consuming saccharin. Conversely, lack of habituation, as present in fasted rats consuming a caloric food, was associated with extra energy expenditure.

Gastric cancer surveillance in a high-risk population in tuscany (Central Italy): preliminary results.

BACKGROUND/AIMS: The surveillance of subjects at high risk for developing gastric cancer (GC) may represent an effective strategy for reducing specific morbidity and mortality. The aim of this study was to identify GC at its initial phase and to identify precancerous lesions in a group of GC high-risk subjects. METHODS: We enrolled first-degree relatives of patients affected by GC who resided in a GC high-risk area (Tuscany, Central Italy). The study's protocol included the collection of several individual measurements, including a blood sample for the determination of specific biomarkers, an upper digestive tract endoscopy with detailed gastric biopsies and Helicobacter pylori (Hp) treatment followed by a specific check. RESULTS: We enrolled 167 subjects who were members of 128 different familial groups with GC history. We identified 1 case of initial-phase GC, 1 gastric dysplasia type II, 32 intestinal metaplasia, 10 gastric atrophy, and 21 atrophic chronic gastritis. 81 subjects were Hp-positive and underwent eradication therapy. CONCLUSION: This study of a GC high-risk Italian population reveals positive results in terms of population compliance, the identification of specific gastric lesions requiring close follow-up and successful therapy for Hp infection. To define future surveillance strategies, a longer follow-up of these patients is necessary.

Dopamine D1 receptor-dependent modifications in the dopamine and cAMP-regulated phosphoprotein of Mr 32 kDa phosphorylation pattern in striatal areas of morphine-sensitized rats.

Morphine sensitization is a model of latent, functionally inducible increase in dopamine D(1) receptor-mediated transmission, which may be unmasked by an external stimulus. Morphine-sensitized rats present dopamine D(1) receptor-dependent stereotypies upon morphine challenge and resilience to unavoidable stress-induced behavioral deficits. This tonic increase in dopamine D(1) dopaminergic transmission is counter-adaptive to an enhanced mu-opioid receptor-dependent signaling in striatal areas. Control and sensitized rats show a similar dopamine and cAMP-regulated phosphoprotein of M(r) 32 kDa (DARPP-32) phosphorylation pattern in striatal areas. Acute morphine administration induced an early increase and delayed decrease in phospho-threonine (Thr)34 DARPP-32 levels accompanied by a delayed increase in phospho-Thr75 DARPP-32 levels in the nucleus accumbens and caudate-putamen of sensitized rats, while it had no effects in control animals. The administration of a selective dopamine D(1) receptor antagonist (SCH 23390) before morphine challenge prevented the behavioral and neurochemical modifications in sensitized rats. 6-Methyl-2-(phenylethynyl)-pyridine, a selective metabotropic glutamate receptor 5 (mGluR(5)) antagonist, administered 1 h after morphine challenge, prevented the delayed phosphorylation changes, but it had no effect when administered before challenge. Moreover, the DARPP-32 phosphorylation pattern in the caudate-putamen of sensitized rats after unavoidable stress exposure was studied. The stress-induced neurochemical modifications and their sensitivity to receptor antagonists were similar to those observed after acute morphine administration. In conclusion, these results suggest that in the experimental conditions used an increase in dopamine output in striatal areas is followed by a complex neurochemical pattern, in which the initial stimulation of dopamine D(1) receptors triggers a sequence of signaling events that lead to an mGluR(5)-mediated increase in phospho-Thr75 DARPP-32 levels. Since DARPP-32 phosphorylated in Thr75 inhibits cAMP-dependent protein kinase (PKA) activity, the final result is a decrease in the dopamine D(1) receptor-dependent phosphorylation events.

Hypothalamus-pituitary-adrenal modifications consequent to chronic stress exposure in an experimental model of depression in rats.

UNLABELLED: The modifications in the hypothalamus-pituitary-adrenal (HPA) axis function induced by repeated unavoidable stress exposure, according to a standardized procedure used for inducing an experimental model of depression, were studied. Rats exposed to this procedure show hyporeactivity to both pleasurable and aversive stimuli and this condition is antagonized by the repeated administration of classical antidepressant drugs. We also studied whether imipramine administration during stress exposure would interfere with the possible modifications in the HPA axis. Rats were exposed to a 4-week stress procedure with and without imipramine treatment and then tested for escape, as compared with non-stressed control animals. Twenty-four hours later all rats were bled through a tail nick for plasma corticosterone measurement before and after dexamethasone (10 microg/kg) or corticotropin-releasing hormone (CRH, 1 microg/kg) administration. Rats were then killed, adrenals and thymus weighed, brain areas dissected out and frozen for glucocorticoid receptors (GRs) and corticotropin-releasing hormone receptor 1 (CRHR1) immunoblotting and for the assessment of hypothalamic corticotropin-releasing hormone levels. RESULTS: Rats exposed to a 4-week unavoidable stress showed escape deficit and their basal plasma corticosterone levels were higher than those of control animals. Moreover, they had decreased response to dexamethasone administration, adrenal hypertrophy, and decreased GR expression in the hippocampus, hypothalamus, medial prefrontal cortex and pituitary. No significant modifications in CRHR1 expression were observed in the pituitary nor in different discrete brain areas. CRH levels in the hypothalamus and the plasma corticosterone response to CRH administration were found to be higher in stressed rats than in controls. Imipramine treatment offset all the behavioral and neurochemical stress-induced modifications. In conclusion, the present results strengthen the assumption that the escape/avoidance behavioral deficit induced by inescapable stress exposure is accompanied by steadily increased HPA activity, and that imipramine effect is strongly related to a normalization of HPA axis activity.

Dopamine and cyclic AMP-regulated phosphoprotein-32 phosphorylation pattern in cocaine and morphine-sensitized rats.

This study reports some of the modifications in dopaminergic signalling that accompany cocaine and morphine behavioural sensitization. Cocaine-sensitized rats showed increased phosphorylation of dopamine- and cyclic AMP-regulated phosphoprotein Mr 32 kDa (DARPP-32) at threonine-75 (Thr75) and decreased DARPP-32 phosphorylation at Thr34, in the caudate-putamen (CPu) and nucleus accumbens (NAc) 7 days after sensitization assessment. Conversely, in morphine-sensitized rats, no apparent modifications in DARPP-32 phosphorylation pattern were observed. Morphine-sensitized rats have increased binding and coupling of micro -opioid receptors and increased dopaminergic transmission in striatal areas and, upon morphine challenge, exhibit dopamine D1 receptor-dependent stereotypies. Thus, the DARPP-32 phosphorylation pattern was studied in morphine-sensitized rats at different times after morphine challenge. Morphine challenge increased levels of phospho-Thr75 DARPP-32 and decreased levels of phospho-Thr34 DARPP-32 in a time-dependent manner in the CPu and NAc. In order to assess whether these modifications were related to modified cyclic AMP-dependent protein kinase (PKA) activity, the phosphorylation levels of two other PKA substrates were examined, the GluR1 and NR1 subunits of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and NMDA receptors respectively. The phosphorylation levels of GluR1 and NR1 subunits decreased in parallel with those of phospho-Thr-34 DARPP-32, supporting the hypothesis that morphine challenge elicited a decrease in PKA activity in morphine-sensitized rats.

Acquisition of a palatable-food-sustained appetitive behavior in satiated rats is dependent on the dopaminergic response to this food in limbic areas.

Rats exposed to repeated unavoidable stress show decreased dopamine output in the nucleus accumbens shell (NAcS) and do not acquire vanilla sugar (VS)-sustained appetitive behavior (VAB). Rats treated with lithium for 3 weeks also show decreased NAcS dopamine output, yet they acquire VAB. Feeding a novel palatable food increases extraneuronal dopamine levels in the NAcS and medial prefrontal cortex (mPFC) in rats. In order to investigate the role of food-induced dopamine release in VAB acquisition, we studied by microdialysis the dopaminergic response in the NAcS and mPFC to the presentation and consumption of VS in satiated control rats, and in satiated rats exposed to repeated stress or lithium treatment. The dopaminergic response to VS was also studied in rats familiar with VS, or that had acquired VAB. In control rats, VS feeding was accompanied by increased dopamine output in the NAcS and mPFC, and one-trial habituation to this effect developed in the NAcS. Rats exposed to a 7-day stress showed reduced interest in VS pellets, and when fed VS they did not show a dopaminergic response in the NAcS and mPFC. Lithium-treated rats rapidly ate VS pellets and showed increased dopamine output in the NAcS and mPFC, with no habituation in the NAcS response. Rats familiar with VS and rats that had already learned VAB ate VS pellets. The first group showed a lower dopaminergic response to VS consumption than the control group, but the latter showed no dopaminergic response in the NAcS and mPFC. We propose that the limbic dopaminergic response to a novel palatable food plays a role in associative learning and that it is predictive of the competence to learn an appetitive behavior. Moreover, in our experimental conditions a phasic increase in mesolimbic dopamine no longer signals the VS stimulus once it has become a reinforcer in an appetitive task.

Dizocilpine infusion has a different effect in the development of morphine and cocaine sensitization: behavioral and neurochemical aspects.

The stimulation of glutamate receptors plays a relevant role in the development of behavioral sensitization to psychostimulants, while less clear results have been obtained on their role in morphine sensitization. We addressed this issue by comparing the development of cocaine and morphine sensitization under a continuous s.c. infusion of the N-methyl-D-aspartate (NMDA) antagonist dizocilpine (0.1 mg/kg/24 h). Moreover, we studied the expression of NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunits in discrete limbic areas of rats sensitized to morphine or cocaine with or without the concomitant dizocilpine infusion. It was observed that dizocilpine infusion did not prevent the development of morphine sensitization, while it prevented the development of tolerance to morphine-induced analgesia. Finally, morphine-sensitized animals did not present any modification in the subunit expression of glutamate receptors in the brain areas examined. In agreement with previous results, we found that dizocilpine infusion prevented the development of cocaine sensitization. Moreover, we observed that rats sensitized to cocaine presented a significant increase in the levels of GLUR1, NR1 and NR2B, in the nucleus accumbens, and of NR2B in the hippocampus compared to control animals. Such modifications were absent in rats administered cocaine under dizocilpine infusion. We conclude that: (i) morphine sensitization is a neuroadaptive phenomenon which does not appear to require NMDA receptor activity in order to develop; (ii) cocaine sensitization is clearly dependent on NMDA receptor activity, as dizocilpine infusion prevented the occurrence of glutamate receptors modifications as well as the development of sensitization.

Long-term behavioral and neurochemical effects of chronic stress exposure in rats.

Rats exposed to acute unavoidable stress develop a deficit in escaping avoidable aversive stimuli that lasts as long as unavoidable stress exposure is repeated. A 3-week exposure to unavoidable stress also reduces dopamine (DA) output in the nucleus accumbens shell (NAcS). This study showed that a 7-day exposure to unavoidable stress induced in rats an escape deficit and a decrease in extraneuronal DA basal concentration in the NAcS. Moreover, animals had reduced DA and serotonin (5-HT) accumulation after cocaine administration in the medial pre-frontal cortex (mPFC) and NAcS, compared with control animals. After a 3-week exposure to unavoidable stress, escape deficit and reduced DA output in the NAcS were still significant at day 14 after the last stress administration. In the mPFC we observed: (i) a short-term reduction in DA basal levels that was back to control values at day 14; (ii) a decrease in DA accumulation at day 3 followed by a significant increase beyond control values at day 14; (iii) a significant reduction in 5-HT extraneuronal basal levels at day 3, but not at day 14. Finally, a significant decrease in 5-HT accumulation following cocaine administration was present in the NAcS and mPFC at day 3, but not at day 14. In conclusion, a long-term stress exposure induced long-lasting behavioral sequelae associated with reproducible neurochemical modifications.

Dopamine output in the nucleus accumbens shell is related to the acquisition and the retention of a motivated appetitive behavior in rats.

Chronic stress exposure consistently impairs the reactivity to aversive and pleasurable stimuli in rats; these behavioral modifications are associated with a decrease in dopamine output in the nucleus accumbens shell (NAcS). However, when rats that have already acquired an appetitive behavior are exposed to chronic stress, they develop an impaired reactivity to avoidable aversive stimuli while retaining the appetitive behavior. The dissociation between these two behavioral traits was used to study whether the decreased dopaminergic activity in the NAcS was connected to either of the two deficits. Dopamine output was studied through microdialysis as dopamine accumulation following re-uptake inhibition by cocaine. When rats that had previously acquired the appetitive behavior were exposed to chronic stress, they showed a dopaminergic transmission in the NAcS similar to that of controls and significantly higher than that of chronically stressed animals. Thus, dopamine output in the NAcS was consistently associated to the acquisition and maintenance of appetitive behavior, while the expression of a deficit in avoidance appeared to be independent of it.

Animal models for the study of antidepressant activity.

Three behavioral paradigms are presented for the study of the mechanism of action of antidepressant treatments and for the screening of new antidepressant drugs. The first model (acute escape deficit) exploits the decreased ability of a rat exposed to an unavoidable stress to avoid a noxious stimulus, and it allows us to evaluate the preventive activity of a treatment on the development of escape deficit. The second paradigm (chronic escape deficit) begins as acute escape deficit, that is then indefinitely sustained by the repeated administration of mild stressors; this model allows us to evaluate the efficacy of a treatment to revert the escape deficit. The third is a model of anhedonia based on the finding that exposure to repeated unavoidable stress prevents the acquisition of an appetitive behavior induced and maintained by a highly palatable food (vanilla sugar) in rats fed ad libitum; this paradigm assesses the efficacy of a treatment to restore an animal's motivation. A long-term (2 to 3 week) treatment with classical antidepressants, such as imipramine or fluoxetine, resulted in a clear-cut preventive and/or revertant activity in the three models.

Acquisition of an appetitive behavior reverses the effects of long-term treatment with lithium in rats.

Rats exposed to a long-term treatment with lithium chloride develop a deficit of avoidance accompanied by a reduction in the basal levels of extraneuronal dopamine and in dopamine accumulation in the nucleus accumbens shell after acute uptake inhibition. Such a condition is similar to that of an experimental model of depression induced by exposing rats to a chronic stress procedure. Rats exposed to chronic stress are also unable to acquire an appetitive behavior sustained by a highly palatable food. Thus, it was studied whether rats fed a diet containing lithium would develop an appetitive behavior induced by a pure hedonic stimulus. Rats on the lithium diet developed a clear-cut escape deficit condition accompanied by a decreased dopamine output in the nucleus accumbens shell; nevertheless, they learned the appetitive behavior within a similar period to controls. The development of the appetitive behavior coincided with the recovery of the capacity to avoid a noxious stimulus and with the return of the dopaminergic transmission in the nucleus accumbens shell to values similar to those of control rats. It may be concluded that the mechanism of action underlying the behavioral and neurochemical sequelae of a chronic stress is distinct from that of the analogous effects produced by lithium.

Long-term lithium administration abolishes the resistance to stress in rats sensitized to morphine.

Morphine sensitized rats appear protected from the sequelae of an unavoidable stress: when exposed to stress (after a 7-day morphine wash-out) and then tested for escape, they perform like naive animals. This protection appears similar to that induced by chronic imipramine treatment, as it is antagonized by the inhibition of D(1)-dopamine receptors before exposure to unavoidable stress. Repeated unavoidable stress induces in rats a condition characterized by hyporeactivity to noxious stimuli and reverted by long-term antidepressant treatments, and this state is regarded as an experimental model of depression. The resistance to stress in morphine sensitized rats could be considered as the behavioral counterpart of the sensitivity to stress in control rats, i.e. as a model of mania. The aim of the present study was to validate such a putative model by studying whether the resistance to stress induced by morphine sensitization would respond to a long-term administration of lithium, the reference antimanic drug. Long-term lithium treatment induces in rats a condition of hyporeactivity to noxious stimuli, accompanied by decreased levels of dopamine in the nucleus accumbens shell. In morphine sensitized rats chronic lithium abolished the resistance to stress, but it did not modify the D(1)-dopamine receptor mediated response to morphine, nor did it modify the levels of extraneuronal dopamine in the nucleus accumbens shell. Thus, lithium treatment abolished the resistance to stress in morphine sensitized rats, conferring predictive validity to the paradigm. Moreover, it did so through a mechanism which appeared to be independent of D(1)-dopamine receptor activity.

Rats sensitized to morphine are resistant to the behavioral effects of an unavoidable stress.

In agreement with the results of other authors, rats sensitized to morphine and challenged with 5 mg/kg of morphine after 7 days of wash-out showed intense stereotyped movements, the expression of which was selectively antagonized by SCH 23390. Sensitized rats were exposed to an unavoidable stress (which consistently produces an escape deficit in control animals) after 3, 7 and 21 days of morphine wash-out. Twenty-four hours after the unavoidable stress, animals were tested for their capacity to escape and their performance was compared to that of control-stressed and naive rats. Morphine sensitization completely prevented the development of escape deficit. This protective effect was similar to that induced by a chronic imipramine treatment and, like the effect of imipramine, it was antagonized by the administration of SCH 23390 before the unavoidable stress. However, it was not affected by the administration of naloxone. Moreover, when rats presenting a clear-cut escape deficit, induced by a 10-day treatment with SKF 38393, were exposed to the morphine sensitization protocol, a complete recovery of their capacity to avoid a noxious stimulus was observed. Finally, the down-regulation of both the number of D(1)-dopamine receptors and of the coupled adenylyl cyclase activity in the pre-frontal cortex (PFC) produced by long-term SKF 38393 administration was reverted by the superimposed morphine sensitization. Thus, the condition of morphine sensitization appears to share several common effects with chronic imipramine treatment.

The effects of long-term administration of rubidium or lithium on reactivity to stress and on dopamine output in the nucleus accumbens in rats.

Rubidium and lithium are alkali metals belonging to the same periodic series as sodium, potassium and cesium. In the present report the effects of lithium and rubidium on animal reactivity to stressful stimuli and on dopamine output in the nucleus accumbens were studied. A dose-response curve with rubidium, administered acutely before exposure to unavoidable stress, showed a maximal protective activity on escape deficit development at the dose of 0. 41 mEq/kg. Rubidium injected at doses of 0.008-0.08 mEq/kg 72 h before the unavoidable stress had the same efficacy as the acute 0. 41 mEq/kg dose. Tolerance to the effect of rubidium developed after 9 days of treatment and, on day 15, rats presented a spontaneous escape deficit. The acute effect of lithium, administered for 3.5 days at the dose of 0.8 mEq/kg, i.p. twice a day before the exposure to unavoidable stress, was analogous to that of rubidium, but after repeated treatment a spontaneous escape deficit developed. Rats showing an escape deficit secondary to chronic stress also presented decreased extraneuronal dopamine concentrations in the nucleus accumbens. Accordingly, microdialysis studies showed significantly lower extracellular dopamine levels in rats chronically treated with lithium or rubidium compared to control animals. Cocaine (5 mg/kg i. p.) administered acutely increased extracellular dopamine concentrations in control rats, as well as in rats chronically stressed or chronically treated with lithium or rubidium. However, the dopamine increase was significantly higher in controls compared to the other groups. In conclusion, long-term treatment with lithium or rubidium, or the exposure to chronic stress, produced a condition of behavioral hypo-reactivity accompanied by a decreased dopamine output in the nucleus accumbens.

A chronic stress that impairs reactivity in rats also decreases dopaminergic transmission in the nucleus accumbens: a microdialysis study.

Chronic stress induces in rats a decreased reactivity toward noxious stimuli (escape deficit), which can be reverted by antidepressant treatments. The present study reports that this condition of behavioral deficit is accompanied by a decreased level of extracellular dopamine in the nucleus accumbens shell. To assess whether this finding was the result of a decreased release or of an enhanced removal of dopamine, we acutely administered cocaine, and 2 h later d-amphetamine, to stressed and control rats. The increases in dopamine output observed in stressed animals after cocaine administration were significantly lower than those observed in control rats; whereas the total amount of dopamine released after d-amphetamine administration was similar in both groups of rats. These data suggest a reduced activity of dopaminergic neurons as the possible mechanism underlying dopamine basal level reduction in stressed animals. It is interesting that the stress group showed a locomotor response to cocaine not different from control rats, thus suggesting a condition of sensitization to dopamine receptor stimulation. Imipramine administered daily concomitantly with stress exposure completely reverted the escape deficit condition of chronically stressed rats. Moreover, stressed rats treated with imipramine showed basal and cocaine stimulated levels of extraneuronal dopamine similar to those observed in control animals.

Palatable food induces an appetitive behaviour in satiated rats which can be inhibited by chronic stress.

An experimental model for the study of antidepressant treatments was devised by exploiting the response maintained by vanilla pellets in rats freely fed on a standard diet. The apparatus used was a Y-maze and the rats were trained, during 10-12 consecutive sessions, to earn a vanilla pellet placed at the end of the one of the two divergent arms. Animals exposed to repeated unavoidable stressors during the training phase did not develop the appetitive behaviour. Rats previously trained on the Y-maze, however, did not modify their performance under the effect of repeated stressors. Long-term treatment with imipramine and fluoxetine, given for 2 weeks before training and during the whole of the training phase, was able to antagonize the disrupting effect of chronic stress on the acquisition of the Y-maze. Finally, vanilla pellet consumption in trained animals induced a consistent increase in extraneuronal dopamine in the nucleus accumbens, as measured by microdialysis.