Carla Gambarana

A Chronic Stress that Impairs Reactivity in Rats Also Decreases Dopaminergic Transmission in the Nucleus Accumbens: A Microdialysis Study

Abstract: 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.

Region-specific expression of messenger RNAs encoding GABAa receptor subunits in the developing rat brain

The distribution and levels of messenger RNAs encoding the alpha 1, beta 1, beta 2, beta 3, and gamma 2 subunits of the GABAA receptor in the developing and adult rat brain were investigated using quantitative in situ hybridization histochemistry and subunit-specific probes. Regional localization of the subunit messenger RNAs was determined with film autoradiography and expression in identified neuronal cell populations was examined using higher resolution techniques. Each of the GABAA receptor subunit messenger RNAs exhibits a distinct pattern of localization in the developing and adult brain. Of the subunits examined, the alpha 1, beta 2, and gamma 2 are the most abundant and are found in many brain regions, including the olfactory bulb, cortex, hippocampus, thalamic nuclei, and inferior colliculus. In addition, these subunit messenger RNAs are prominent in the cerebellum where virtually all cells of the deep cerebellar nuclei and Purkinje cell layer are labeled. The levels of most of the subunit messenger RNAs, with the exception of that encoding the beta 1 subunit, increase during postnatal development. While the alpha 1, beta 2, and gamma 2 subunit messenger RNAs rise in parallel in many regions and identified cell populations, different subsets of receptor subunit messenger RNAs are co-ordinately expressed at other sites. The greatest increases in subunit messenger RNA levels occur in the cerebellar cortex during the second postnatal week, a period coincident with cerebellar maturation. The co-distribution of different GABAA receptor subunit messenger RNAs in various regions of the developing and adult nervous systems supports the hypothesis that multiple receptor compositions exist. Moreover, that different subunit messenger RNAs exhibit coordinate changes in expression in different regions and cell populations suggests that receptor gene expression is modulated by cell type-specific signals. The temporal changes in subunit messenger RNA levels in the cerebellum raise the possibility that synaptogenesis may play a role in receptor gene regulation in this brain region.

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.

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 animals 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.

Dizocilpine antagonizes the effect of chronic imipramine on learned helplessness in rats

Dizocilpine coadministered with imipramine (IMI) through an SC-implanted osmotic minipump completely prevents the occurrence of behavioral supersensitivity to quinpirole, as well as the decrease of dopamine D1 and beta-adrenergic receptor function. The present report shows that, in the same experimental conditions, dizocilpine completely antagonized the capacity of IMI to prevent the development of the learned helplessness behavior in rats. Thus suggesting that the blockade of NMDA receptors also antagonizes the antidepressant effect of IMI. Interestingly, rats acutely treated with dizocilpine 30 min before the inescapable shock session behaved similarly to naive animals during the escape test session.

A novel γ subunit of the GABAA receptor identified using the polymerase chain reaction

We have utilized a polymerase chain reaction (PCR) strategy to identify a novel subunit, γ3, of the GABAA receptor. The γ3 cDNA encodes a mature protein of 450 amino acids that contains structural features typically conserved among subunits of the GABAA receptor family. The γ3 subunit shares approximately 66% sequence identity with the γ2 subunit but only 38% and 29% with α1 and β1 subunits, respectively. Localization of the γ3 mRNA indicates that it is widely distributed throughout the mouse brain in a pattern similar to that observed for mRNAs encoding the γ2 subunits.

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 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.

Efficacy of an Hypericum Perforatum (St. John’s Wort) extract in preventing and reverting a condition of escape deficit in rats.

The treatment of unselected depressed patients with an hydro-alcoholic extract of Hypericum perforatum has been reported to have an efficacy similar to that of classical antidepressants. In the present report, the effects of H. perforatum were studied on three animal models of depression: (i) an acute form of escape deficit (ED) induced by an unavoidable stress; (ii) a chronic model of ED, which can be maintained by the administration of mild stressors on alternate days; (iii) a model of anhedonia based on the finding that repeated stressors prevent the development of an appetitive behavior induced by vanilla sugar in satiated rats fed ad libitum. H. perforatum acutely protected animals from the sequelae of unavoidable stress; such an effect was partially prevented by the administration of SCH 23390 or (-)-pindolol. Moreover, H. perforatum reverted the ED maintained by repeated stressors and preserved the animals capacity to learn to operate for earning a positive reinforcer. It was concluded that H. perforatum contains some active principle(s) endowed with antidepressant activity.

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.

Selective modifications in the nucleus accumbens of dopamine synaptic transmission in rats exposed to chronic stress

Stressful events are accompanied by modifications in dopaminergic transmission in distinct brain regions. As the activity of the neuronal dopamine (DA) transporter (DAT) is considered to be a critical mechanism for determining the extent of DA receptor activation, we investigated whether a 3‐week exposure to unavoidable stress, which produces a reduction in DA output in the nucleus accumbens shell (NAcS) and medial prefrontal cortex (mPFC), would affect DAT density and DA D1 receptor complex activity in the NAcS, mPFC and caudate–putamen (CPu). Rats exposed to unavoidable stress showed a decreased DA output in the NAcS accompanied by a decrease in the number of DAT binding sites, and an increase in the number of DA D1 binding sites and Vmax of SKF 38393‐stimulated adenylyl cyclase. In the mPFC, stress exposure produced a decrease in DA output with no modification in DAT binding or in DA D1 receptor complex activity. Moreover, in the CPu stress exposure induced no changes in DA output or in the other neurochemical variables examined. This study shows that exposure to a chronic unavoidable stress that produces a decrease in DA output in frontomesolimbic areas induced several adaptive neurochemical modifications selectively in the nucleus accumbens.