Lack of feedback inhibition on rat basolateral amygdala following stress or withdrawal from sedative-hypnotic drugs.

Previous research has demonstrated that suppression of inhibition in projection neurons of the basolateral complex of the amygdala (BLA) represents an essential mechanism underlying the emergence of negative emotional responses, including exaggerated fear and anxiety. The present work evaluates inhibitory postsynaptic potentials (IPSPs) in pyramidal projection neurons of the BLA in rats subjected to either diazepam or ethanol withdrawal or uncontrollable stress. These are experimental paradigms conducive to a negative emotional state. In slices containing the BLA, IPSPs were studied using whole-cell patch clamp. In control animals, a small IPSP was evoked by sub-threshold stimulation of the external capsule. When an action potential (AP) was evoked by supra-threshold stimuli, IPSPs were considerably larger; these IPSPs were sensitive to blockade of GABA(A) receptors by picrotoxin. However, IPSPs were clearly reduced in diazepam- or ethanol-withdrawn and in stressed rats. Firing of an AP by a depolarizing pulse applied through the patch pipette consistently evoked an inhibitory postsynaptic current (IPSC) in the pyramidal neurons of control animals from all three experimental models; these IPSCs were mediated by GABA(A) receptor activation and were blocked after suppression of glutamatergic transmission. In contrast, no IPSCs were observed in slices from diazepam- or ethanol-withdrawn or stressed animals, although the depolarizing pulse regularly evoked an AP in pyramidal neurons. It is concluded that, in withdrawn or stressed rats, GABAergic disinhibition occurs due to attenuation or suppression of feedback inhibition.

Prenatal stress and early adoption effects on benzodiazepine receptors and anxiogenic behavior in the adult rat brain.

Chronic maternal stress during pregnancy has been associated with behavioral alterations that persist into adulthood. Moreover, adoption procedures performed immediately after birth can reverse these alterations. In this study, we examined the effects of prenatal restraint stress and adoption at birth (cross-fostering) on the behavioral response to an anxiety-provoking situation and on the adult male offspring expression of benzodiazepine (BDZ) receptors in selected brain areas. Adult offspring of rats stressed during the last week of pregnancy exhibited higher levels of anxiety than control rats. The anxiogenic behavior found at the elevated plus maze (EPM) has been related to the reduced levels of BDZ receptor levels in specific brain areas. Adult offspring of rats stressed during pregnancy exhibited a decrease in the number of BDZ receptors binding sites in the central amygdaloid nucleus (Ce), CA1, CA3, and the dentate gyrus regions of the hippocampus when compared to controls. Regarding the adoption procedure, control pups raised by a foster gestationally stressed mother showed similar levels of anxiety as stressed groups. Stressed offspring raised by a foster control mother showed reduced anxiety levels compared to that of the control groups. Adoption per se showed no difference in time spent, neither in the open arms of the plus maze nor in BDZ receptor levels, when compared to the corresponding control and stressed groups. Stressed offspring raised by a foster control mother reverted BDZ receptor levels to control values. However, control pups raised by a gestationally stressed foster mother showed similar values compared to the control offspring in hippocampus, in spite of showing an anxiogenic behavior in the EPM. We found a significant increase of Ce BDZ receptor levels in control offspring raised by a foster stressed mother that could be explained as a compensatory effect to a GABA receptor desensitization. In summary, the behavioral outcome of the adult offspring is vulnerable both to the stress experience during the late prenatal period as well as to possible variations in care during lactation by mothers subjected to chronic stress during gestation. There seems to be a direct correlation between anxiety state and BDZ receptor levels in the adult offspring raised by their biological mothers. However, the mechanism of BDZ regulation leading to an anxious behavior might be different if the insult is received only postnatally as opposed to both pre and postnatally.

Antidepressants attenuate both the enhanced ethanol intake and ethanol-induced anxiolytic effects in diazepam withdrawn rats.

We have recently shown that the abrupt discontinuation of chronic diazepam (DZM) administration facilitated ethanol consumption and enhanced the anxiolytic properties of ethanol. Tricyclic antidepressants such as desipramine and the selective serotonin reuptake inhibitor fluoxetine have been shown to reduce alcohol intake in rodent models of alcoholism and in alcoholics who are depressed. In the present study, we tested whether desipramine (1.25; 2.5 and 5 mg/kg, i.p.) and fluoxetine (5 mg/kg, i.p.) treatment affect both ethanol intake in a free-choice test and the anxiolytic effect induced by ethanol in DZM withdrawn rats. Adult male Wistar rats were submitted to a chronic DZM treatment (2 mg/kg per day) or vehicle (VEH) for 21 days. Twenty-four hours after the last DZM injection, rats were subjected to a free-choice paradigm between water and increasing ethanol concentrations with or without concurrent desipramine or fluoxetine administration (ethanol concentration (v/v) was increased every 4 days as follows: 2, 4, 6, 8 and 10% for the final 8 days). Chronic treatment with desipramine (24 days, twice a day, 2.5 and 5 mg/kg, i.p.) and fluoxetine (24 days, once a day; 5 mg/kg, i.p.) significantly reduced the amount of ethanol intake in DZM withdrawn rats. Furthermore, subchronic treatments with desipramine (4 days, twice a day, 2.5 and 5 mg/kg) and fluoxetine (4 days, once a day, 5 mg/kg, i.p.) blocked the anxiolytic-like behavior in the elevated plus maze induced by ethanol (1 g/kg; i.p.) in DZM withdrawn rats at day 5 of withdrawal. The present findings suggest that desipramine and fluoxetine could be effective pharmacological tools to prevent the subsequent development of ethanol dependence in rats previously exposed to DZM withdrawal.

Increased fear learning coincides with neuronal dysinhibition and facilitated LTP in the basolateral amygdala following benzodiazepine withdrawal in rats.

Animals chronically administered with diazepam (DZM -- 2 mg/kg/day i.p.) or vehicle (VEH) for 21 days were tested in a fear-conditioning paradigm 4 days after the last administration. Increased freezing was observed in DZM withdrawn rats as compared to VEH injected control rats in both associative and nonassociative context and this increase was greatest for the DZM withdrawal group in the paired context. In animals anesthetized with urethane, single pulses in the medial prefrontal cortex evoked a field potential including a population spike (PS) in the basolateral complex of the amygdala (BLA) of control animals, whereas in DZM withdrawn animals multiple PSs were evoked. In brain slices from control rats, stimulation of the external capsule evoked a field potential including a PS in the BLA, whereas in DZM withdrawn rats multiple PSs were evoked. The amplitude of the PS was smaller in slices obtained from DZM withdrawn rats than from control rats, and paired pulse inhibition was significantly less in the former. Perfusion with DZM 2 microM of slices obtained from DZM withdrawn rats eliminated repetitive spiking. GABAergic blockade with 50 microM picrotoxin in control rats resulted in the appearance of multiple secondary PSs. In slices from DZM withdrawn rats high-frequency stimulation induced a highly significant potentiation that lasted at least 2 h (LTP), whereas in control rats the same stimulation did not induce LTP. Neuronal hyperexcitability leading to facilitated LTP observed in BLA of DZM withdrawn rats could be due to depressed GABAergic activity (dysinhibition). The increased synaptic plasticity may be at the root of the increased fear learning observed in withdrawn animals.

Early adoption modifies the effects of prenatal stress on dopamine and glutamate receptors in adult rat brain.

Stressful stimuli during pregnancy induce complex effects that influence the development of offspring. These effects can be prevented by environmental manipulations during the early postnatal period. Repeated restraint during the last week of pregnancy was used as a model of prenatal stress, and adoption at birth was used to change the postnatal environment. No differences were found in various physical landmarks, except for testis descent, for which all prenatally stressed pups showed a 1-day delay in comparison with control rats, regardless of the postnatal adoption procedure. Levels of dopamine (DA) D(2) and glutamate (Glu) N-methyl-D-aspartate (NMDA) receptors were differentially regulated in different forebrain regions of cross-fostered adult offspring. Increased concentrations of cortical D(2) receptors detected in stressed pups, raised by a gestationally stressed biological mother, were not detected when the pups were raised by a control mother. Control pups raised by a foster mother whether gestationally stressed or not had higher levels of NMDA receptors in cortical areas. These findings suggest that the normal expression of DA and Glu receptors is influenced by in utero experience and by lactation. The complex pattern of receptor changes reflects the high vulnerability of DA and Glu systems to variations both in prenatal and in postnatal environment, particularly for cortical D(2) receptors and NMDA receptors in cerebral cortex and nucleus accumbens. In contrast, testis descent appears to be more susceptible to prenatal than to postnatal environmental events.