Joaquín Martí

Effects of chronic stress on food intake in rats : influence of stressor intensity and duration of daily exposure

The effect of exposure to chronic intermittent stressors of differing intensities [handling, restraint, and immobilization (IMO)] and daily duration (15, 60, and 240 min of IMO) on changes in food intake was studied in adult male rats. Whereas handling did not caused anorexia, restraint slightly reduced food intake and IMO drastically reduced it. The effects were very similar after the 7th and 27th day of exposure to the stressors. Fourteen days of chronic IMO (2 h daily) resulted in decreased food intake as measured on days 1, 10, and 14 of treatment, the inhibition being slightly greater after the first stress session. The circadian rhythm of food intake, expressed as a percent of the total food eaten in a 24 h period, was found to be almost unaffected by chronic IMO, although IMO rats appeared to satiate sooner than control rats. Exposure of rats to chronic IMO (7 days) for 15, 60, and 240 min daily reduced food intake to the same extent in all IMO groups. Taken together, these results suggest that: a) the magnitude of the changes in food intake after chronic exposure to stressors is closely related to their intensity, and b) a severe stressor such as IMO reduces food intake to a certain level that was independent on its daily duration. After repeated exposure to the same stressor, only a slight tendency to recover normal food intake was observed.

Comparison of the behavioural and endocrine response to forced swimming stress in five inbred strains of rats

Some inbred strains of rats showed behavioural differences in the forced swimming test, which is considered a putative animal model of depression. In the present work, the behavioural and physiological responses to forced swimming were studied in male and female rats of five inbred strains of rats: Brown-Norway (BN), Fischer 344 (FIS), Lewis (LEW), Spontaneously Hypertensive (SHR) and Wistar-Kyoto (WKY). Physiological measures were aimed at characterizing emotional reactivity, a very important issue which has usually been approached by studying a single endocrine system, and its relationship to the forced swimming behaviour. The four indices of reactivity to stress used were serum glucose, ACTH, corticosterone and prolactin. No behavioural differences between sexes were observed in the forced swimming test. In addition, BN and WKY rats showed passive behaviour compared with the other three strains, the FIS strain being the most active. Whereas only minor differences were found in the resting levels of the variables studied with regard to either sex or strain, pituitary-adrenal (PA) and glucose responses to 15 min forced swimming differed among sexes and strains. Stress-induced hyperglycaemia was lowest in WKY and highest in SHR, being lower in females than in males. The lowest ACTH and corticosterone responses to forced swimming were observed in LEW and the highest in FIS. Female rats showed a clearly higher PA response to stress in all strains. Prolactin response to stress was very similar between sexes and strains. It might thus be concluded that: (i) there are important inter-strain differences in the forced swimming behaviour, with no differences between sexes; (ii) the various physiological indices of emotional reactivity follow a different trend and no warranted conclusion on differences in emotional reactivity should be based upon a single endocrine system or even only upon physiological measures; (iii) we cannot be sure, therefore, whether or not there are differences in emotionality between the strains studied in spite of well-established inter-strains differences in the forced swimming behaviour.

Influence of various acute stressors on the activity of adult male rats in a holeboard and in the forced swim test

The effects of various acute stressors on the activity of adult male rats in a holeboard and in the forced swim test were studied. When tested immediately or 24 h after 1 h exposure to noise, restraint in tubes or tail shock, no changes in either defecation rate or activity in the holeboard were observed. In contrast, immediately after 1 h immobilization in wood-boards, a reduction of the number of areas crossed and the number of head-dips was found. The inhibitory effect of immobilization on head-dips persisted 24 h later. The behavior of the rats in the forced swim test was classified into three categories: struggling, mild swim and immobility. The changes in behavior were critically dependent on the type of stressor, and more specifically on its intensity, that was evaluated with three different physiological parameters (serum prolactin, corticosterone and glucose levels). Thus, if tested immediately after stress, noise did not alter the response of the rats, restraint in tubes and tail shock-reduced immobility, and the latter stressor increased mild swim. In the second experiment, immobilization in wood-boards reduced struggling. Twenty-four hours after stress, noise, restraint in tubes or tail shock were without effect, but immobilized rats showed increased immobility and reduced mild swim activity. The present data clearly indicate that behavior of rats in a holeboard and in a forced swim situation are not related, and that acute stress could have a differential effect on the various categories of behavior in a forced swim situation.

The serum glucose response to acute stress is sensitive to the intensity of the stressor and to habituation

The reliability of serum glucose concentrations as an index of habituation to chronic stress was evaluated in adult male rats. The glucose response to immobilization was attenuated by six days of previous chronic exposure to the same stressor, the degree of reduction being related to the duration (15 min, 1 hr or 4 hr) of the daily exposure to immobilization. In another experiment, three groups of rats were exposed to one of three stressors (handling plus change of room, restraint in tubes, or immobilization by wood boards), 1 hr daily for 27 days. On day 28, when faced with the same acute stressor to which they were chronically exposed, the rats showed a consistent reduction in glucose response, regardless of the type of stressor used. In addition, in stress-naive rats serum glucose levels were related to the intensity of the stressor as assessed by three independent measures (food intake, body weight changes, and adrenal weight after chronic exposure to the stressor). These data indicate that, under appropriate conditions, glucose levels can be a good index of both the intensity of acute stress experienced by the rats and their habituation to repeated stress.

Forced swimming behavior is not related to the corticosterone levels ain the test: A study with four inbred rat strains

The behavior of four inbred strains of rats in the holeboard and the forced swimming tests, and its relationship with a physiological index of stress (serum corticosterone) were studied in adult male rats. The strains were: Fisher 344 (FIS), Lewis (LEW), Spontaneously hypertensive (SHR), and Wistar-Kyoto (WKY). In the holeboard, SHR rats were the most active and WKY the less active, the other strains showing intermediate levels of activity. During the first exposure to forced swimming WKY were far more passive than the other three strains and the same was observed during the second exposure. When corticosterone levels after this second exposure to water was determined, LEW rats showed lower values than the other three strains. Therefore, no apparent relationship between behavior and stress-induced corticosterone secretion exists. Although a single point measurement of only on physiological index of stress has important limitations, the present data do not give support to a strong relationship between the behavior of the animals in the forced swimming test and emotional reactivity to stress. It is therefore possible that forced swimming behavior might not be mainly a panic-like reaction, but the result of the tendency of the animals to adopt passive strategies in inescapable situations. Although more studies are needed to firmly establish this assumption, WKY rats might be, at least potentially, a useful model of depressive-like behavior.

Effects of diazepam and desipramine in the forced swimming test: influence of previous experience with the situation

The effects of desipramine and diazepam on the behavior of rats in the forced swimming test were studied. Desipramine significantly enhanced struggling behavior, the intensity of its effect being greater in rats previously exposed to the forced swimming test for 1 or 4 days before drug administration. The effect of desipramine on immobility was significantly only in rats previously exposed to the test. Acute diazepam administration did not modify the behavior of rats in the test. Chronic administration (6 days) of two different doses of diazepam did not alter the behavior of rats in the forced swimming test, but significantly decreased the defecation rate, suggesting that the drug was effective as an anxiolytic. The present results suggest that the behavior of rats in the forced swimming test might be related to mood states rather than to fear/anxiety and that efficacy of the test to detect antidepressant activity might be enhanced by repeated exposure of the rats to the situation.

Inhibition of catecholamine synthesis depresses behavior of rats in the holeboard and forced swim tests: Influence of previous chronic stress

Catecholaminergic pathways in the brain are activated during stress and are presumably involved in the control of physiological and behavioral changes triggered by stress. When repeatedly stressed, adaptive changes have been observed in catecholaminergic activity in the brain. In the present experiment, it was assessed whether or not chronic exposure to immobilization (IMO) altered the influence of catecholamines on behavior in the holeboard and forced swim test by administering alpha-methyl-p-tyrosine (an inhibitor of catecholamine synthesis). Adult Sprague-Dawley rats were used. Chronic stress amortiguated the inhibitory effect of acute IMO on some but not all behaviors in the two tests. Whereas previous chronic IMO exacerbated the effects of the drug on struggling and immobility in the forced swim test, no change in response to the drug as a consequence of chronic IMO was observed in the holeboard test. The present data suggest that chronic IMO-induced changes in the catecholaminergic control of some behaviors might be related to depression-like states in rats. The actual physiological meaning of these changes and the specific receptors involved remain to be elucidated.

Generation and survival of midbrain dopaminergic neurons in weaver mice

Generation and survival of midbrain dopaminergic (DA) neurons were investigated using tyrosine hydroxylase (TH) immunocytochemistry combined with tritiated thymidine autoradiography at appropriate anatomical levels throughout the anteroposterior (A/P) axes of the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA). The wild‐type (+/+) and homozygous weaver (wv/wv) mice used here were the offspring of pregnant dams injected with the radioactive precursor when the mesencephalic neurons were being produced (gestational days 11–15). Data reveal that, at postnatal day 90, depletion of TH‐stained cells in the wv/wv presented an A/P pattern of increasing severity and, therefore, the DA cells located in posterior parts of the SNc or the VTA appear to be more vulnerable than the settled anterior neurons. When the time of neuron origin is inferred for each level of these cell groups, it is found that the neurogenesis span is similar for both experimental groups, although significant deficits in the frequency of wv/wv late‐generated neurons were observed in any level considered. On the other hand, it has been found that TH‐positive neurons were settled along the extent of the SNc and the VTA following precise and differential neurogenetic gradients. Thus, the acute rostrocaudal increase in the proportion of late‐generated neurons detected in both +/+ DA‐cell groups is disturbed in the weaver homozygotes due to the indicated A/P depletion.

Chronic stress induced changes in LH secretion: The contribution of anorexia associated to stress

The effects of chronic intermittent immobilization (IMO) on serum LH levels of adult male rats were studied. Chronic IMO (2 h daily for 13 days) did not alter basal LH levels, but abolished the LH response to acute stressors (IMO and tailshock). The inhibition of LH caused by acute exposure to IMO for 4 or 18 h was similar in control and chronic IMO rats. Also the LH response to exogenous LHRH administration was normal in chronically stressed rats. When a group of rats eating the same amount of food as that eaten by immobilized rats was introduced (pair-fed), an inhibition of LH response to acute stressors quite similar to that found in chronic IMO rats was observed. These data indicate that chronic stress-induced inhibition of LH release caused by short-term exposure to acute stressors was located above the pituitary and was mainly due to anorexia accompanying daily exposure to the stressor.

Purkinje cell age-distribution in fissures and in foliar crowns: a comparative study in the weaver cerebellum

Generation and settling of Purkinje cells (PCs) are investigated in the weaver mouse cerebellum in order to determine possible relationships with the fissuration pattern. Tritiated thymidine was supplied to pregnant females at the time that these neurons were being produced. Autoradiography was then applied on brain sections obtained from control and weaver offspring at postnatal (P) day 90. This makes it possible to assess the differential survival of neurons born at distinct embryonic times on the basis of the proportion of labeled cells located at the two foliar compartments: fissures and foliar crowns. Our data show that throughout the surface contour of the vermal lobes, generative programs of PCs were close between wild type and homozygous weaver. Similar data were found in the lobules of the lateral hemisphere. On the other hand, the loss of PCs in weaver cerebella can be related to foliar concavities or convexities depending on the vermal lobe or the hemispheric lobule studied. Lastly, we have obtained evidence that late-generated PCs of both normal and mutant mice were preferentially located in fissures. These quantitative relationships lead us to propose a model in which the final distribution of PCs through the vermal contour would be coupled to two factors: the cortical fissuration patterning and a “time-sequential effect” of weaver mutation.