CNR - Institute of Neuroscience CNR
Institute of Neuroscience


Neurobiological bases of chronic stress in the rat

Effects of voluntary ethanol consumption on emotional state and stress responsiveness in socially isolated rats

Adverse life experiences, family influences, and alcohol accessibility are the most common environmental factors implicated in increased risk for alcohol abuse in humans. Animals have been used to model the effects of adverse life experiences on the development of drinking behavior. Repeated maternal separation stress during early development or repeated episodes of social defeat stress in adulthood have been shown to increase alcohol abuse. Moreover, separation of rats from their peers during adolescence or adulthood increases voluntary ethanol consumption.

Rats deprived of social contact with other rats at a young age experience a form of prolonged stress that leads to long-lasting alterations in their behavioral profile. Isolated rodents are aggressive, neophobic, and highly reactive to human handling. They display aggressive behaviour when transferred to groups of animals and more responsive to novelty relative to rats housed socially, as indexed by increased locomotors activity and enhanced preference for novel environments. We previously showed that rats subjected to social isolation at weaning for one month, exhibit reduced brain levels of neuroactive steroids, manifest anxiety-like behavior as adults. These findings, together with the observations that social isolation enhances the stimulatory effect of acute ethanol administration on brain steroidogenesis, GABAA receptor structure and function and associated behavior, suggest that this chronic social stress may induce plastic adaptation of neuronal systems that contribute to vulnerability to alcohol abuse. The final goal of this part of the research is to examine whether social isolation stress affects alcohol consumption, and whether voluntary ethanol consumption modifies the effects of isolation on neuroactive steroid concentrations, anxiety-like behavior, and GABAA receptor gene expression. We then investigated the effects of ethanol voluntary consumption brain plasticity by measuring the expression of BDNF and Arc in the brain of isolated rats.

Ethanol intake in socially isolated rats


Voluntary ethanol consumption by group-housed or isolated rats increased slightly as the concentration of the ethanol solution increased during the first 3 weeks from 2.5 to 5.0 to 7.5%, and it then approximately doubled during the 4th week when the ethanol concentration was 10% . During the 1st and 2nd weeks, the amount of ethanol drank by isolated animals was smaller (p = 0.71; p = 0.90, respectively) than that consumed by group-housed rats. Although the mean daily intake of ethanol did not differ significantly between the two rearing conditions, isolated rats showed a significant reduction (–31%) in ethanol preference compared with group-housed animals This difference was due to a significant increase (+21%) in water consumption by isolated rats such an effect was also observed in isolated rats provided with only water.

Effects of voluntary ethanol consumption and foot shock on the abundance of allopregnanolone in socially isolated rats


Consistent with our previous data, social isolation for 30 days in the absence of any other stressor induced significant decreases in the cerebrocortical and hippocampal concentrations of allopregnanolone compared with the corresponding values for group-housed animals. Voluntary ethanol consumption throughout the isolation period tended to reduce the cerebrocortical level of allopregnanolone in both socially isolated and group-housed rats, although this effect was not significant. As expected, the percentage increase in the cerebrocortical concentration of allopregnanolone induced by foot shock, used as a novel acute stressor, was markedly greater in isolated rats than in group-housed animals (p<0.05). Voluntary ethanol consumption greatly attenuated this effect of foot shock stress in both isolated and group-housed animals.

Effect of voluntary ethanol consumption on anxiety-like behavior in socially isolated rats


Consistent with our previous finding, rats isolated for 30 days immediately after weaning exhibited an anxiety-like profile in the elevated plus-maze test, as revealed by significant decreases in both the percentage entries into and time spent in the open arms of the maze compared with the corresponding values for group-housed rats. Voluntary ethanol consumption throughout the isolation period significantly increased both the percentage of time spent and entries in the open arms of the maze in socially isolated rats (+151% and +173%, respectively, relative to isolated water-drinking rats, p<0.05). However these two parameters were still markedly reduced compared with the corresponding values for group-housed animals with access to ethanol or to water alone.


Effects of voluntary ethanol consumption on hippocampal BDNF and Arc levels

Voluntary ethanol consumption did not significantly affect the down-regulation of BDNF or Arc expression apparent in the hippocampus of socially isolated rats. Ethanol consumption slightly reduced the hippocampal abundance of these proteins in group-housed animals, but this effect was not statistically significant.

Effects of voluntary ethanol consumption on expression of α4 and δ subunits of the GABAA receptor in the hippocampus of isolated or group-housed rats


Expression of the α4 and δ subunits of the GABAA receptor in the hippocampus of isolated or group-housed rats after voluntary ethanol consumption was examined by immunohistochemistry with specific antibodies generated in response to extracellular epitopes of these proteins. The antibodies recognized single proteins of ~70 and 54 κDa for the α4 and δ subunits, respectively, in immunoblot analysis of a crude membrane fraction prepared from rat hippocampal neurons, and the immunoreactive bands were not detected when blots were incubated with the antibodies in the presence of the corresponding peptide antigen.


Consistent with our previous observations, the levels of α4 and δ subunit immunoreactivity were increased throughout the hippocampus of socially isolated rats compared with those in the hippocampus of group-housed rats. Whereas voluntary ethanol consumption did not affect the expression of the α4 subunit in the hippocampus of either group-housed or socially isolated animals, it increased that of the δ subunit in the CA1 and CA3 regions of the hippocampus as well as in the dentate gyrus of animals raised alone or in groups.


  • Morgan ML, Rapkin AJ, Biggio G, Serra M, Pisu MG, Rasgon N (2010) Neuroactive steroids after estrogen exposure in depressed postmenopausal women treated with sertraline and asymptomatic postmenopausal women. Arch Womens Ment Health 13:91-8.
  • Pisu MG, Mostallino MC, Dore R, Mura ML, Maciocco E, Russo E, De Sarro G, Serra M (2008) Neuroactive steroids and GABAA receptor plasticity in the brain of the WAG/Rij rat, a model of absence epilepsy. J. Neurochem. 106:2502-14.
  • Serra M, Pisu MG, Mostallino MC, Sanna E, Biggio G (2008) Changes in neuroactive steroid content during social isolation stress modulate GABAA receptor plasticity and function. 57:520-30.
  • Serra M, Mostallino MC, Talani G, Pisu MG, Carta M, Mura ML, Floris I, Maciocco E, Sanna E, Biggio G (2006) Social isolation-induced increase in alpha and delta subunit gene expression is associated with a greater efficacy of ethanol on steroidogenesis and GABA receptor function. J. Neurochem. 98:122-33.


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Maria Giuseppina Pisu

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