Gonadectomy before puberty increases the number of neurons and glia in the medial prefrontal cortex of female, but not male, rats.

The human prefrontal cortex, important for executive functions, loses gray matter throughout the adolescent period. In rats, our laboratory demonstrated that a loss of neurons between adolescence and adulthood partially underlies the loss of volume, and this loss is greater in females than males. Here, we examine whether being deprived of gonadal hormones before puberty through adulthood influences the number of neurons in the medial prefrontal cortex (mPFC). Prior to puberty, the testes or ovaries were removed in male and female rats. In adulthood, the number of neurons and glia in the mPFC were quantified using unbiased stereology, and the volume of the frontal white matter was measured. Prepubertal ovariectomy resulted in a higher number of neurons and glia and a larger volume of white matter compared to sham control littermates. Castrated males were not different from sham males on any measure. Thus ovarian hormones secreted after puberty influence the cellular composition of the medial prefrontal cortex.

Lidocaine attenuates anisomycin-induced amnesia and release of norepinephrine in the amygdala.

When administered near the time of training, protein synthesis inhibitors such as anisomycin impair later memory. A common interpretation of these findings is that memory consolidation requires new protein synthesis initiated by training. However, recent findings support an alternative interpretation that abnormally large increases in neurotransmitter release after injections of anisomycin may be responsible for producing amnesia. In the present study, a local anesthetic was administered prior to anisomycin injections in an attempt to mitigate neurotransmitter actions and thereby attenuate the resulting amnesia. Rats received lidocaine and anisomycin injections into the amygdala 130 and 120 min, respectively, prior to inhibitory avoidance training. Memory tests 48 h later revealed that lidocaine attenuated anisomycin-induced amnesia. In other rats, in vivo microdialysis was performed at the site of amygdala infusion of lidocaine and anisomycin. As seen previously, anisomycin injections produced large increases in release of norepinephrine in the amygdala. Lidocaine attenuated the anisomycin-induced increase in release of norepinephrine but did not reverse anisomycin inhibition of protein synthesis, as assessed by c-Fos immunohistochemistry. These findings are consistent with past evidence suggesting that anisomycin causes amnesia by initiating abnormal release of neurotransmitters in response to the inhibition of protein synthesis.

Developmental PCB Exposure Disrupts Synaptic Transmission and Connectivity in the Rat Auditory Cortex, Independent of Its Effects on Peripheral Hearing Threshold.

Polychlorinated biphenyls (PCBs) are enduring environmental toxicants and exposure is associated with neurodevelopmental deficits. The auditory system appears particularly sensitive, as previous work has shown that developmental PCB exposure causes both hearing loss and gross disruptions in the organization of the rat auditory cortex. However, the mechanisms underlying PCB-induced changes are not known, nor is it known whether the central effects of PCBs are a consequence of peripheral hearing loss. Here, we study changes in both peripheral and central auditory function in rats with developmental PCB exposure using a combination of optical and electrophysiological approaches. Female rats were exposed to an environmental PCB mixture in utero and until weaning. At adulthood, auditory brainstem responses (ABRs) were measured, and synaptic currents were recorded in slices from auditory cortex layer 2/3 neurons. Spontaneous IPSCs (sIPSCs) and miniature IPSCs (mIPSCs) were more frequent in PCB-exposed rats compared with controls and the normal relationship between IPSC parameters and peripheral hearing was eliminated in PCB-exposed rats. No changes in spontaneous EPSCs were found. Conversely, when synaptic currents were evoked by laser photostimulation of caged-glutamate, PCB exposure did not affect evoked inhibitory transmission, but increased the total excitatory charge, the number and distance of sites that evoke a significant response. Together, these findings indicate that early developmental exposure to PCBs causes long-lasting changes in both inhibitory and excitatory neurotransmission in the auditory cortex that are independent of peripheral hearing changes, suggesting the effects are because of the direct impact of PCBs on the developing auditory cortex.

Developmental exposure to an environmental PCB mixture delays the propagation of electrical kindling from the amygdala.

Developmental PCB exposure impairs hearing and induces brainstem audiogenic seizures in adult offspring. The degree to which this enhanced susceptibility to seizure is manifest in other brain regions has not been examined. Thus, electrical kindling of the amygdala was used to evaluate the effect of developmental exposure to an environmentally relevant PCB mixture on seizure susceptibility in the rat. Female Long-Evans rats were dosed orally with 0 or 6mg/kg/day of the PCB mixture dissolved in corn oil vehicle 4 weeks prior to mating and continued through gestation and up until postnatal day (PND) 21. On PND 21, pups were weaned, and two males from each litter were randomly selected for the kindling study. As adults, the male rats were implanted bilaterally with electrodes in the basolateral amygdala. For each animal, afterdischarge (AD) thresholds in the amygdala were determined on the first day of testing followed by once daily stimulation at a standard 200µA stimulus intensity until three stage 5 generalized seizures (GS) ensued. Developmental PCB exposure did not affect the AD threshold or total cumulative AD duration, but PCB exposure did increase the latency to behavioral manifestations of seizure propagation. PCB exposed animals required significantly more stimulations to reach stage 2 seizures compared to control animals, indicating attenuated focal (amygdala) excitability. A delay in kindling progression in the amygdala stands in contrast to our previous finding of increased susceptibility to brainstem-mediated audiogenic seizures in PCB-exposed animals in response to a an intense auditory stimulus. These seemingly divergent results are not unexpected given the distinct source, type, and mechanistic underpinnings of these different seizure models. A delay in epileptogenesis following focal amygdala stimulation may reflect a decrease in neuroplasticity following developmental PCB exposure consistent with reductions in use-dependent synaptic plasticity that have been reported in the hippocampus of developmentally PCB exposed animals.

Developmental PCB Exposure Increases Audiogenic Seizures and Decreases Glutamic Acid Decarboxylase in the Inferior Colliculus.

Previously, we observed that developmental polychlorinated biphenyl (PCB) exposure resulted in an increase in audiogenic seizures (AGSs) in rats. However, the rats were exposed to loud noise in adulthood, and were not tested for AGS until after 1 year of age, either of which could have interacted with early PCB exposure to increase AGS susceptibility. This study assessed susceptibility to AGS in young adult rats following developmental PCB exposure alone (without loud noise exposure) and investigated whether there was a decrease in GABA inhibitory neurotransmission in the inferior colliculus (IC) that could potentially explain this effect. Female Long-Evans rats were dosed orally with 0 or 6 mg/kg/day of an environmentally relevant PCB mixture from 28 days prior to breeding until the pups were weaned at postnatal day 21. One male-female pair from each litter was retained for the AGS study whilst another was retained for Western blot analysis of glutamic acid decarboxylase (GAD) and GABAAα1 receptor in the IC, the site in the auditory midbrain where AGS are initiated. There was a significant increase in the number and severity of AGSs in the PCB groups, with females somewhat more affected than males. GAD65 was decreased but there was no change in GAD67 or GABAAα1 in the IC indicating decreased inhibitory regulation in the PCB group. These results confirm that developmental PCB exposure alone is sufficient to increase susceptibility to AGS, and provide the first evidence for a possible mechanism of action at the level of the IC.

Long-term effects of adolescent exposure to bisphenol A on neuron and glia number in the rat prefrontal cortex: Differences between the sexes and cell type.

Bisphenol A (BPA), an endocrine disruptor used in a variety of consumer products, has been found to alter the number of neurons in multiple brain areas in rats following exposure in perinatal development. Both the number of neurons and glia also change in the medial prefrontal cortex (mPFC) during adolescence, and this process is known to be influenced by gonadal hormones which could be altered by BPA. In the current study, we examined Long-Evans male and female rats that were administered BPA (0, 4, 40, or 400µg/kg/day) during adolescent development (postnatal days 27-46). In adulthood (postnatal day 150), the number of neurons and glia in the mPFC were stereologically assessed in methylene blue/azure II stained sections. There were no changes in the number of neurons, but there was a significant dose by sex interaction in number of glia in the mPFC. Pairwise comparisons between controls and each dose showed a significant increase in the number of glia between 0 and 40µg/kg/day in females, and a significant decrease in the number of glia between 0 and 4µg/kg/day in males. In order to determine the type of glial cells that were changing in these groups in response to adolescent BPA administration, adjacent sections were labelled with S100ß (astrocytes) and IBA-1 (microglia) in the mPFC of the groups that differed. The number of microglia was significantly higher in females exposed to 40µg/kg/day than controls and lower in males exposed to 4µg/kg/day than controls. There were no significant effects of adolescent exposure to BPA on the number of astrocytes in male or females. Thus, adolescent exposure to BPA produced long-term alterations in the number of microglia in the mPFC of rats, the functional implications of which need to be explored.

Developmental exposure to PCBs alters the activation of the auditory cortex in response to GABAA antagonism.

Developmental exposure of rats to polychlorinated biphenyls (PCBs) causes impairments in hearing and in the functioning of peripheral and central auditory structures. Additionally, recent work from our laboratory has demonstrated an increase in audiogenic seizures. The current study aimed to further characterize the effects of PCBs on auditory brain structures by investigating whether developmental exposure altered the magnitude of activation in the auditory cortex (AC) in response to electrical stimulation of thalamocortical afferents. Long-Evans female rats were fed cookies containing either 0 or 6mg/kg of an environmental PCB mixture daily from 4 weeks prior to breeding until postnatal day 21. Brain slices containing projections from the thalamus to the AC were collected from adult female offspring and were bathed in artificial cerebrospinal fluid (aCSF) alone, aCSF containing a gamma-aminobutyric acid (GABA) receptor antagonist (200nM SR95531), and aCSF containing an and N-methyl-d-aspartate (NMDA) receptor antagonist (50µM AP5). During each of these drug conditions, electrical stimulations ranging from 25 to 600µA were delivered to the thalamocortical afferents. Activation of the AC was measured using flavoprotein autofluorescence imaging. Although there were no differences seen between treatment groups in the aCSF condition, there were significant increases in the ratio of aCSF/SR95531 activation in slices from PCB-exposed animals compared to control animals. This effect was seen in both the upper and lower layers of the AC. No differences in activation were noted between treatment groups when slices were exposed to AP5. These data suggest that developmental PCB exposure leads to increased sensitivity to antagonism of GABAA receptors in the AC without a change in NMDA-mediated intrinsic excitability.

Developmental PCB exposure increases susceptibility to audiogenic seizures in adulthood.

Developmental exposure to polychlorinated biphenyls (PCBs) causes auditory deficits. Thus, we recently conducted a study to investigate if developmental PCB exposure would exacerbate noise-induced hearing loss in adulthood. Unexpectedly, some PCB-exposed rats exhibited seizure-like behaviors when exposed to loud noise. Therefore, we conducted the current experiment to determine if adult rats perinatally exposed to PCBs are more susceptible to audiogenic seizures when tested in a standard audiogenic seizure paradigm. Adult male and female rats exposed to PCBs during gestation and lactation (0, 1, 3 or 6 mg/kg/day) and previously tested in the noise-induced hearing loss study were presented with a 100 dB noise stimulus. If they did not exhibit clonus in response to the 100 dB noise, they were exposed to a 105 dB stimulus 24-48 h later. This was followed by an 110 dB stimulus 24-48 h later if they did not exhibit clonus at 105 dB. Female and male rats exposed to either 3 or 6 mg/kg PCBs exhibited a significantly higher incidence of audiogenic seizures, shorter latency to onset of seizures, and greater severity of seizures compared to controls. Thyroxine measured in littermates at weaning was significantly lower in all PCB groups compared to controls, suggesting a potential mechanism for the increased incidence of audiogenic seizures. This is the first study to show that developmental PCB exposure increases the susceptibility to audiogenic seizures in adulthood.

Effects of perinatal bisphenol A exposure during early development on radial arm maze behavior in adult male and female rats.

Previous work has shown that exposure to bisphenol A (BPA) can affect anxiety behavior. However, no studies have examined whether administration of this endocrine disruptor during the perinatal period has the potential to induce alterations in cognitive behavior in both adult males and females as assessed in an appetitive task. The goal of the current study was to determine whether exposure to different doses of BPA during early development alters performance on the 17-arm radial maze in adulthood in Long-Evans rats. Oral administration of corn oil (vehicle), 4 µg/kg, 40 µg/kg, or 400 µg/kg BPA to the dams occurred daily throughout pregnancy, and the pups received direct oral administration of BPA between postnatal days 1-9. Blood was collected from offspring at weaning age to determine levels of several hormones (thyroxine, thyroid stimulating hormone, follicle stimulating hormone, luteinizing hormone). One male and one female from each litter were evaluated on the 17-arm radial maze, a working/reference memory task, in adulthood. Results indicated that after exposure to BPA at both 4 and 400 µg/kg/day, rats of both sexes had decreased levels of FSH at weaning. There were no significant effects of BPA on performance on the radial arm maze in males or females. In conclusion, exposure to BPA during early development had modest effects on circulating hormones but did not affect performance on a spatial learning and memory task.

Effects of stress, corticosterone, and epinephrine administration on learning in place and response tasks.

These experiments examined the effects of prior stress, corticosterone, or epinephrine on learning in mazes that can be solved efficiently using either place or response strategies. In a repeated stress condition, rats received restraint stress for 6h/day for 21 days, ending 24h before food-motivated maze training. In two single stress conditions, rats received a 1-h episode of restraint stress ending 30 min or 24h prior to training. Single stress ending 30 min prior to training resulted in a significant interaction of stress and learning on the two tasks, with significant enhancement of learning in the response task and non-significant impairment in the place task. Neither acute nor chronic stress significantly altered learning in either task when the stress ended 24h before training. Thus, the anterograde effects of stress on maze learning ended within a single day. Two stress-related hormones, corticosterone and epinephrine, were tested for effects on learning parallel to those of acute stress. When administered 30 min prior to training, a corticosterone dose (40 mg/kg) that enhanced memory on a spontaneous alternation task did not significantly enhance or impair learning in either task. Two doses of epinephrine that modulate memory in other settings were used to test the effects of epinephrine on learning. Pre-training injections of 0.03 mg/kg epinephrine impaired place learning, while 0.1mg/kg epinephrine impaired response learning. The epinephrine results mimicked those seen with acute stress on the place task, but were opposite those seen after acute stress on the response task. Thus, corticosterone does not appear to be a major factor mediating the effects of acute stress on place and response learning and epinephrine is, at most, a partial contributor to these effects.