Isolation Rearing Research Articles

Involvement of spinal 5-HT1A receptors in isolation rearing-induced hypoalgesia in mice

Isolation rearing in rodents causes not only abnormal behaviors which resemble the clinical symptoms of schizophrenia but also hypoalgesia in thermal nociception models. However, the mechanism of the hypoalgesia is not known. The present study investigated the effect of isolation rearing on acute pain and the descending pain inhibitory pathways in mice. Rearing in isolation for 6weeks from post-weaning reduced pain sensitivity in the hot plate test and acetic acid-induced writhing test. Isolation rearing also reduced the intraplantar capsaicin-induced licking behavior. Capsaicin increased c-Fos expression, a neuronal activity marker, in the spinal cord and primary somatosensory cortex both in group- and isolation-reared mice, but this effect did not differ between groups. On the other hand, c-Fos expression in the anterior cingulate cortex, periaqueductal gray matter, and rostral ventromedial medulla, but not in the spinal cord or somatosensory cortex, was enhanced by isolation rearing. Systemic administration of WAY100635 (serotonin (5-HT)1A receptor antagonist), but not of ketanserin (5-HT2 receptor antagonist), prazosin (α1-adrenoceptor antagonist), or yohimbine (α2-adrenoceptor antagonist), attenuated isolation rearing-induced hypoalgesia in capsaicin-induced licking behavior. Attenuation of isolation rearing-induced hypoalgesia was also observed following the intrathecal injection of WAY100635. Naloxone, an opioid receptor antagonist, did not affect the hypoalgesia in isolation-reared mice. These findings suggest that isolation rearing causes hypoalgesia in mouse models of acute pain and imply that the spinal 5-HT1A receptor activation probably through descending serotonergic inhibitory pathway is involved in isolation rearing-induced hypoalgesia.

Isolation rearing significantly perturbs brain metabolism in the thalamus and hippocampus

Psychosocial neglect during childhood severely impairs both behavioral and physical health. The isolation rearing model in rodents has been employed by our group and others to study this clinical problem at a basic level. We previously showed that immediate early gene (IEG) expression in the hippocampus and medial prefrontal cortex (mPFC) is decreased in isolation-reared (IR) compared to group-reared (GR) rats. In the current study, we sought to evaluate: (1) whether these changes in IEG expression would be detected by the measurement of brain glucose metabolism using positron emission tomography (PET) with fluorodeoxyglucose (FDG) and (2) whether PET FDG could illuminate other brain regions with different glucose metabolism in IR compared to GR rats. We found that there were significant differences in FDG uptake in the hippocampus that were consistent with our findings for IEG expression (decreased mean FDG uptake in IR rats). In contrast, in the mPFC, the FDG uptake between IR and GR rats did not differ. Finally, we found decreased mean FDG uptake in the thalamus of the IR rats, a region we had not previously examined. The results suggest that PET FDG has the potential to be utilized as a biomarker of molecular changes in the hippocampus. Further, the differences found in thalamic brain FDG uptake suggest that further investigation of this region at the molecular and cellular levels may provide an important insight into the neurobiological basis of the adverse clinical outcomes found in children exposed to psychosocial deprivation.

The Selective Metabotropic Glutamate 2/3 Receptor Agonist MGS0028 Reverses Isolation Rearing–Induced Abnormal Behaviors in Mice

Isolation-induced abnormal behaviors are useful animal models for assessing potential anti-psychotic drugs. This study examined the effect of MGS0028, a selective metabotropic glutamate 2/3 receptor agonist, on abnormal behaviors such as hyperactivity, aggression, and deficits of prepulse inhibition in isolation-reared mice. MGS0028 attenuated hyperactivity and aggressive behaviors in isolation-reared mice. The agonist also reversed isolation rearing–induced deficits of prepulse inhibition. On the other hand, MGS0028 did not affect locomotor activity and prepulse inhibition in group-reared mice. These results suggest that the metabotropic glutamate 2/3 receptor agonist, MGS0028, is a potential compound for the treatment of psychiatric disorders.

Affective stress applied in different periods of brain development induces changes on the expression of APP isoforms and β‐amyloid in the hippocampal formation of rats

This study aimed at evaluating the changes induced by isolation in different periods of brain development on the expression of APP695, APP751/770 and Aβ in the hippocampal formation of rats. Isolation rearing (IR): Pups remained with their mothers (6/mother) until weaning (PND21) when were allocated for ten weeks to: 1) grouped, housed 4/cage; 2) isolated, housed individually. Social isolation (SI): Rats (140g) were allocated in the same conditions. Maternal separation (MS): Pups were either separated from their mothers for 180min or left undisturbed from PND1‐21 when were housed 4/cage for 5 weeks. 40‐μm brain sections were used for immunohistochemistry. The immunopositive cells (IC) were quantified in hippocampus and entorhinal cortex (EC). Data were compared by t‐test (p<0.05). MS induced a significant increase in APP695 IC in hilus (35%, p=0.028) while IR induced a significant decrease (41%, p=0.001). No change was induced by SI in hilus but a significant decrease (10%, p=0.02) was found in EC. APP751/770 IC were seen only in CA2 of the hippocampus where a significant decrease was induced only by IR (24%, p=0.005). For Aβ IC a significant decrease was induced by MS (32%, p=0.026) and IR (98%, p=0.001) in hilus. In contrast, a significant increase was found in CA2 (62%, p=0.001) in SI. The isoforms of APP are differently affected in specific areas of the hippocampal formation by affective stress applied in different periods of brain development. These alterations are not necessarily associated to the alterations on Aβ suggesting that several plasticity mechanisms may be triggered by the association of different stages of development and different kinds of affective stress. Financial support: FAPESP and FPA.

Critical period exists in the effects of isolation rearing on sensorimotor gating function but not locomotor activity in rat

Isolation-reared (IR) rats exhibit various cognitive and behavioral abnormalities in adulthood of which locomotor hyperactivity and impaired prepulse inhibition (PPI) of an acoustic startle reflex are the two cardinal characteristics. Using an amended social deprivation-resocialization paradigm, the present study examined the role of the developmental specificity of the effects of IR and its interaction with the non-competitive NMDA receptor antagonist, MK-801. Locomotor activity and PPI were measured in three groups of adult rats: social control, IR throughout life, and IR for the first two weeks after weaning followed by social housing. MK-801 was injected intraperitoneally (i.p.) 30 min prior to testing at doses of 0, 0.02, 0.1, and 0.2 mg/kg. The results revealed that locomotor activity increased only in rats in the IR throughout life group but not in the other two groups. The impairment of PPI was seen in rats in the IR for the first two weeks and the IR throughout life groups. Furthermore, the effect of MK-801 on PPI was bidirectional in rats of the IR for the first two weeks and IR throughout life groups but not in the social control group. These results suggest that the IR-induced behavioral effects are developed distinctively in terms of the critical period hypothesis, which strengthens the developmental hypothesis of schizophrenic-like dysfunctions.

Isolation rearing-induced reduction of brain 5α-reductase expression: Relevance to dopaminergic impairments

Isolation rearing (IR), a well-established rat model of early chronic psychosocial stress, engenders marked behavioral alterations related to changes of dopamine (DA) neurotransmission in cortical and subcortical brain regions. Stress-induced shifts in γ-aminobutyric acid (GABA)-ergic signaling have been implicated in the dysregulation of DA release. The neurosteroid 3α-hydroxy-5α-pregnan-20-one (allopregnanolone/AP), synthesized from progesterone by the action of the rate-limiting enzyme 5α-reductase (5AR), is a potent positive allosteric modulator of GABA A receptor function. Thus, alterations of 5AR activity/expression may impact upon DA neurotransmission. We studied the effects of IR on the 5AR expression/function and extracellular concentrations of DA and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the rat nucleus accumbens (NAcc) and medial prefrontal cortex (mPFC). Immediately after weaning, male rats were subjected to either IR or social rearing (SR) conditions for 5–8 weeks. Compared to SR, IR rats exhibited significantly lower protein expression of 5AR isoforms (1 and 2) in both brain regions and reduced brain, but not plasma, content of AP and allotetrahydrodeoxycorticosterone, the 5α-reduced metabolite of deoxycorticosterone. IR-exposed rats also exhibited higher levels of DA and DOPAC in the NAcc shell, but not in mPFC, when compared to SR rats. The 5AR inhibitor finasteride (FIN, 100 mg/kg, i.p.) enhanced DA and DOPAC content in the NAcc shell of SR, but not IR rats. FIN, however, elicited equivalent increases in DA and DOPAC levels in the mPFC of both groups. These results show that IR induces changes in expression/activity of brain 5AR which, in a brain-region specific manner, may partially underlie the alterations in DA signaling induced by this manipulation. This article is part of a Special Issue entitled ‘Trends in Neuropharmacology: In Memory of Erminio Costa’.

Gating deficits in isolation‐reared rats are correlated with alterations in protein expression in nucleus accumbens

The isolation-rearing (IR) paradigm, consisting of the social deprivation for 6-9 weeks after weaning, induces a spectrum of aberrant behaviors in adult rats. Some of these alterations such as sensorimotor gating deficits are reminiscent of the dysfunctions observed in schizophrenia patients. Although gating impairments in IR rats have been linked to impairments in the cortico-mesolimbic system, the specific molecular mechanisms underlying this relation are unclear. To elucidate the neurochemical modifications underlying the gating disturbances exhibited by IR rats, we compared their pre-pulse inhibition (PPI) of the acoustic startle reflex with that of socially reared (SR) controls, and correlated this index to the results of proteomic analyses in prefrontal cortex and nucleus accumbens from both groups. As expected, IR rats exhibited significantly lower startle amplitude and PPI than their SR counterparts. Following behavioral testing, IR and SR rats were killed and protein expression profiles of their brain regions were examined using two-dimensional electrophoresis based proteomics. Image analysis in the Coomassie blue-stained gel revealed that three protein spots were differentially expressed in the nucleus accumbens of IR and SR rats. Mass spectrometry (matrix-assisted laser desorption ionization-time of flight and MS/MS) identified these spots as heat shock protein 60 (HSP60), alpha-synuclein (alpha-syn), and 14-3-3 protein zeta/delta. While accumbal levels of HSP60 was decreased in IR rats, alpha-syn and 14-3-3 proteins were significantly increased in IR in comparison with SR controls. Notably, these two last alterations were significantly correlated with different loudness intensity-specific PPI deficits in IR rats. In view of the role of these proteins in synaptic trafficking and dopaminergic regulation, these findings might provide a neurochemical foundation for the gating alterations and psychotic-like behaviors in IR rats.

The acetylcholine fiber density of the neocortex is altered by isolated rearing and early methamphetamine intoxication in rodents

Alterations in the cholinergic physiology of the brain were the first to be observed when research on environmental influences on postnatal brain development began 35 years ago. Since then, the effects of isolated rearing (IR) or early pharmacological insults have been shown not only on the physiology, but also the anatomy of a variety of transmitter systems. The cholinergic fiber density, however, still remained to be assessed. We therefore used a histochemical procedure to stain cholinergic fibers in the brains of young adult gerbils reared either in groups in enriched environments or isolated in standard makrolon cages. Half of the animals from each rearing condition had received a single high dose of methamphetamine on postnatal day 14. Fiber densities were measured by computerized image analysis in the medial and orbital prefrontal cortex (PFC), dysgranular and granular insular cortex, sensorimotor cortices, and the entorhinal cortex of both hemispheres. Isolation rearing increased the cholinergic fiber densities in the prefrontal cortices of the left hemisphere and in the entorhinal cortex of the right hemisphere by about 10%, with no effect in the respective contralateral side. The early methamphetamine intoxication showed no influence in prefrontal and entorhinal cortices, but diminished the acetylcholine (ACh) innervation of the forelimb area of cortex in both hemispheres in IR gerbils and of the left hemisphere in ER gerbils, and reduced the acetylcholine innervation in the hindlimb area in both sides in both rearing groups. These results demonstrate that (a) cholinergic fiber density is differentially regulated in different cortical areas and (b) the plasticity of the cholinergic system can only be understood in the interplay with other neuromodulatory innervations.

Isolation Rearing Enhances Acquisition in a Conditioned Inhibition Paradigm

Isolation rearing from weaning has been reported to enhance excitatory conditioning. The present study employed a conditioned inhibition procedure to examine whether this result was attributable to locomotor hyperactivity. Rats were raised from 21 days old in isolation or in groups of five. In Phase 1, presentation of stimulus A+ was followed immediately by sucrose availability (excitatory conditioning). In Phase 2, sucrose was again presented after A+ alone, but not after a compound presentation of A+ with a second stimulus, B−. Thus, B is believed to acquire conditioned inhibitory properties, countering the excitatory impact of A, and reducing responding specifically to this stimulus compound. Isolates showed enhanced excitatory conditioning in Phase 1. Furthermore, acquisition of conditioned inhibition in Phase 2 was also facilitated by isolation rearing. In Phase 3, B− was paired with a period of sucrose availability. Isolation rearing initially retarded responding to B−, confirming that this stimulus possessed a greater degree of behavioural inhibition in these animals. Later in training, isolates showed enhanced excitatory conditioning to B−, as shown previously to A+ in Stage 1. These results suggest that isolation rearing enhances the acquisition of appetitive Pavlovian associations, independently of locomotor hyperactivity.

Isolation Rearing in Rats: Pre- and Postsynaptic Changes in Striatal Dopaminergic Systems

Isolation rearing of rats produces a behavioral syndrome indicative of altered dopamine (DA) function in the nucleus accumbens (NAC). The present experiments extend these findings by investigating: (a) interactions between isolation rearing and repeated handling/testing on presynaptic DA function in the NAC using in vivo microdialysis; (b) the dose–response curve for the effects of d-amphetamine, and the responses elicited by high potassium, using in vivo microdialysis; and (c) postsynaptic function in isolates as indexed by DA receptor-linked cAMP production. Experiment 1 showed that both isolation rearing and repeated handling/testing had effects on monoamine function in the NAC. However, while both manipulations enhanced DA release evoked by d-amphetamine, only isolated rats had elevated basal DA levels. Opposite neurochemical changes were observed with respect to the serotonin metabolite 5-HIAA, isolates having lower, and repeatedly handled/tested animals having higher, extracellular levels. Experiment 2 provided evidence for enhanced d-amphetamine–evoked DA release in isolated animals, while potassium-evoked DA release was reduced. Experiment 3 provided evidence that the isolation rearing induced changes in presynaptic DA function were accompanied by postsynaptic changes. Specifically, the inhibitory influence of the D 2 receptor on D 1 receptor-stimulated cAMP production was attenuated in ventral striatal slices taken from isolates, suggesting a functional downregulation of D 2 receptors.

Isolation Rearing Enhances the Rate of Acquisition of a Discriminative Approach Task but Does Not Affect the Efficacy of a Conditioned Reward

Harmer, C. J. and Phillips, G. D.Isolation rearing enhances the rate of acquisition of a discriminative approach task but does not affect the efficacy of a conditioned reward.PHYSIOL BEHAV 63(2) 177–184, 1998.—Male Lister hooded rats were reared from weaning either alone (isolation reared) or in groups of five (socially reared controls). Experiments began at 18-weeks postweaning. Subjects were trained to associate an arbitrary stimulus with 10% sucrose reward. Trials (VT30sec) consisted of a 5-s light stimulus (houselight off, wall lights on), followed by a 5-s period of access to the sucrose reward. Alcove approach between trials delayed the next trial by the duration of approach plus 3 s. Activity not associated directly with approach behaviour was also recorded. Isolation rearing enhanced the rate of acquisition of the discriminative approach response. Acquisition of both conditioned approach behaviour during trials and conditioned avoidance between trials was more rapid in isolates. In general, effects were most evident early in training, with asymptotic performance least affected. Horizontal and vertical activity extraneous to approach behaviour was enhanced in isolates during the first training session and increased further relative to social controls after several training sessions. Subsequently, two novel levers were presented; a response on one lever resulted in a 0.5-s presentation of the conditioned stimulus (CS) (probability 0.5), whereas the second lever had no programmed consequences. Sucrose reward was not available at any time. Both groups of animals showed a preference for the CS-associated lever, although rates of response by isolates were higher than social controls on both levers. The proportion of responses emitted upon the active lever, by comparison with the total number of responses recorded upon both levers, was, however, unaffected by isolation rearing.

Effects of Isolation Rearing and Mirror Exposure on Social and Asocial Discrimination Performance

Four experiments examined the effects of rearing in isolation on rats performance on discrimination-based and social learning tasks. After demonstrating that the rearing procedures produced similar results in an open field task to those previously established (Experiment 1), rats were subjected to two discrimination tasks: an instrumental occasion setting procedure (Experiment 3) and a nonspatial win-stay/lose-shift versus win-shift/lose-stay procedure (Experiment 4). Deficits in acquisition of the necessary discriminations were noted in the rats raised in isolation, but there were no differences between isolation-reared and socially reared subjects in response acquisition per se. In Experiment 2, rats were presented with an observational learning task using the bidirectional control procedure. Socially reared rats had a tendency to imitate the behavior they had observed, but rats raised in isolation performed the opposite behavior to that observed, indicating a failure to use a conspecific as a reference point in the task. The presence of a mirror during rearing in isolation was also investigated, but was found to have little effect in attenuating the above deficits in behavior.

The Effect of Isolation Rearing and Stress on Monoamines in Forebrain Nigrostriatal, Mesolimbic, and Mesocortical Dopamine Systems

The Effect of Isolation Rearing and Stress on Monoamines in Forebrain Nigrostriatal, Mesolimbic, and Mesocortical Dopamine Systems

The Effects of Isolation Rearing on Behavioural Inhibition in the Rat

The possibility that isolation-rearing in the rat affects the development of inhibitory mechanisms was studied in a series of experiments. It was found that socially-isolated rats were (1) slower to learn both a lever-panel alternation, and a two-lever alternation schedule of reinforcement, (2) more persistent than controls in pressing a lever for food when a supply of identical “free food” was introduced into the operant chamber, but (3) similar to control rats in their response to preloading with food, a procedure which inhibited lever pressing to the same extent in the two groups. Finally, it was shown in a separate experiment that the effects of increased food deprivation on lever pressing in the presence of free food were qualitatively different from the effects of social isolation, and therefore the social/isolate difference cannot be interpreted as motivational. The possible contributions of neophobia to the difference are discussed. It is concluded that isolates may well suffer from a disinhibitory defect, but that there are probably other effects of isolation in addition.

The persistence of abnormal behaviour in monkeys following isolation rearing.

The persistence of abnormal behaviour in monkeys following isolation rearing.