An investigation of two animal models of anxiety : central administration of corticotropin-releasing factor on the behaviour and neurochemistry of rats, and the effect of pharmacotherapy on spontaneous stereotypical behaviour and NMDA receptor function in mice

[摘要] ENGLISH ABSTRACT:Corticotropin-releasing factor (CRF) studyBackground: Hypothalamic pituitary adrenal (HPA)-axis dysfunction is acommon symptom of patients with anxiety disorders like posttraumatic stressdisorder (PTSO), panic disorder and obsessive-compulsive disorder (OCD).Depressive patients also have HPA-axis dysfunction similar to patients withanxiety disorders. PTSD patients usually show decreased basal plasma cortisollevels whereas OCD and depressive patients show an increase in plasmacortisol. A blunted adrenocorticotropin hormone (ACTH) response to CRF isseen in these patients and mostly an increased level of CRF in theircerebrospinal fluid (CSF). It has been proposed that the CSF level of CRF is areflection of activity of both hypothalamic and extra-hypothalamic CRF systems.Since the amygdala has been shown to be involved in the endocrine andbehavioural response to stress, and CRF is involved in the mediation of thisresponse, we investigated whether chronic elevation of CRF in the amygdala isinvolved in development of the symptoms of psychiatric disorders. Wechronically injected rats with CRF in this area to see what the effect is on theirbehaviour and HPA-axis response. It has been shown that the hippocampalserotonergic (5-HT) system is involved in both anxiety and depression and thatthe 5-HT system is regulated by CRF. We therefore measured hippocampal 5-HT1A receptor density and affinity in CRF-injected and control rats. Materials and methods: Male Sprague-Dawley and Wistar rats werestereotaxically implanted with unilateral and bilateral chronic cannulae in thebasolateral amygdala (BLA). After recovery unilaterally implanted rats wereinjected with 10ng (n=7) or 1DOng(n=6) of CRF or saline (n=6) daily for 5 days.Behaviour was tested on day 5 in the elevated plus-maze and open field. Ratswith bilateral implants were injected with 100ng CRF (n=19) or saline (n=17) oneither side for 5 days. On day 5, behaviour was tested on the elevated plusmazeand open field. Group 1 was tested at baseline levels (n=6 for saline andn=7 for CRF) and group 2 after 5 min. of restraint (n=11 for saline and n=12 forCRF). The stress response of all rats was tested 2 days later. The rats weredivided into 3 groups: The first group was decapitated at baseline level (n=6 foreach group), the second and third groups restrained for 10 min. and decapitated15 min. (n=6 for saline and n=7 for CRF) and 60 min. (n=5 for saline and n=6 forCRF) after restraint stress. Blood was collected for plasma ACTH andcorticosterone determinations. A group of naïve rats was also included in thisexperiment to control for the possible effect of the operation (n=6 for each timepoint). Hippocampi were dissected out and used for 5-HT1A radioligand bindingstudies.Results: Rats that were unilaterally injected with 1DOngCRF, showed significantincrease in the amount of entries into the open arms as well as the amount oftime spent in the open arms of the elevated plus-maze compared to controls and rats injected with 10ng CRF. There were no differences between the groups inother parameters of the elevated plus-maze or open field behaviour. There wereno significant differences in behaviour of bilateral injected rats compared tocontrols, but an increase of grooming in the open field was observed in CRFinjectedrats that were stressed before behavioural tests. The ACTH andcorticosterone response of rats were normal as seen by a significant increase inboth concentrations 15 min. after stress and a return to near basal values 60 min.post stress. There were no differences in plasma ACTH concentrations betweenthe groups at any time point. The basal corticosterone level of CRF-injected ratswas however significantly lower than controls, but no difference were found 15 or60 min. post stress. There were no significant differences in hippocampal 5-HT1Areceptor densities or affinities of CRF-injected and control rats.Conclusions: We did not observe increased anxiety levels or decreased activityin CRF injected rats. Instead, we observed increased activity in unilateralinjected rats. We suspect that the lower dosage of CRF may have this effect onthe behaviour of rats, since other authors have also found this result. The lack ofdifferences in the 5-HT1A receptor populations of CRF-injected rats and controlrats also confirms this result, since there was no increase in anxiety levels ofCRF-injected rats. The chronic elevation of CRF in the SLA caused decreasedbasal levels of corticosterone in the rats and we speculate that CRF causedadrenal insufficiency although the mechanism is unknown.Stereotypical behaviour studyBackground: OCD affects 1-2% of the adult human population and is amongstthe most common psychiatric disorders. This disorder is characterized byobsessions and compulsions. These compulsions or repetitive behaviours aresuggested to be the cause of a hyperactive cortical-striatal-thalamic-cortical(CSTC) circuit in the brain, because of imbalance of the direct and indirectpathways. CSTC pathways are modulated by both 5-HT and dopaminergic (DA)neurons and it has been suggested that a hyperglutamatergic state exists in thefrontal cortex of OCD patients. Two types of animal models of stereotypicalbehaviour are used to investigate neurotransmitter abnormalities related to OCD.These are drug induced stereotypies and environmentally induced (spontaneous)stereotypies. It has been shown that in deermice (Peromyscus manicula tis),drug induced stereotypies are topographically different from spontaneousstereotypies, and our aims were to characterize a deermice model ofspontaneous stereotypy for OCD in terms of face, predictive and constructvalidity. We injected adult deermice, showing spontaneous stereotypies, for atime period of 8 weeks with risperidone (D2/5-HT2 antagonist), citalopram(selective serotonin reuptake inhibitor) and inositol (a metabolic precursor to thephosphatidylinositol second messenger cycle). All these drugs have been shownto improve symptoms of OCD in humans, and we investigated whether the drugscan reduce stereotypies in deermice. Materials and methods: 40 Adult deermice were raised and housed in standardlaboratory cages and randomly divided into four groups (6 females and 4 malesper group). After baseline recordings of behaviour (3 times per week for a totalof 15 min.) the mice were injected daily for 8 weeks with risperidone, citalopram,inositol or saline. Video recordings were made for the 8 week trial and ratedafterwards by raters that were blind to the medication status of the animals. A 5sec. interval scoring system was used for ratings in which the absence orpresence of a stereotypy (backward somersault) was noted. After 8 weeks oftreatment, the mice were decapitated, brains were dissected and the frontalcortices were stored in liquid nitrogen until radioligand binding studies wereperformed on NMDA receptors.Results: There were no significant differences in the amount of somersaultsbetween saline injected and drug treated groups when the data was analysedusing an ANOVA with repeated measures. There was a significant differencebetween the control group and drug treated groups at week 8 in male mice whenthe data was analysed using a Mann-Whitney test. The amount of somersaultsshown by saline injected mice increased over the 8 week trial while it stayedmore constant in all three drug treated groups. There were no significantdifferences between the control group and treatment groups in Bmax and Kdvalues of NMDA receptors in the frontal cortex. There was a trend towardsincreased receptor densities in all treatment groups compared to the controlgroup and a decrease in affinity in the risperidone group.Conclusions: We found limited evidence for the involvement of both 5-HT andDA systems in the development of spontaneous stereotypical behaviour ofdeermice. Risperidone, citalopram and inositol were useful in suppressing theincrease in somersaults observed in the control group towards week 8 of the trial.This increase was presumably due to stress from handling and injections. Thefact that there was a trend towards increased receptor densities in all treatmentgroups and decreased affinity in the risperidone group also point to theinvolvement of 5-HT and DA in spontaneous stereotypies. The limitation of thisstudy was small group numbers and excessive stress experienced by theanimals.

[发布机构] Stellenbosch University