Neural plasticity and stress induced changes in defense in the rat

We investigated the effects of predator stress on behavior and amygdala afferent and efferent neural transmission in rats. Pathways studied were: ventral angular bundle input to the basolateral amygdala; central and basolateral amygdala output to the periaqueductal gray (PAG). Predator stress was ‘anxiogenic’ in elevated plus maze, light/dark box and acoustic startle tests one week after stress. Lasting changes were also observed in neural transmission. Predator stress appeared to potentiate right and depotentiate left hemisphere afferent amygdala transmission. In contrast, predator stress potentiated amygdala efferent transmission to right and left PAG, depending on the amygdala nucleus stimulated. Paired pulse and intensity series analysis suggests that transmission changes may be postsynaptic or presynaptic, depending on the pathway. Path analysis relating brain and behavioral changes suggests that potentiation and depotentiation in both hemispheres participate jointly in effecting some, but not all, of the behavioral changes produced by predator stress. Potentiation in left hemisphere amygdala afferents and efferents predicts anxiolytic-like effects, while potentiation in the right hemisphere amygdala afferents predicts anxiogenic-like effects. Path analysis also supports the view that changes in different neural systems mediate changes in different behaviors. These findings have their parallel in studies in the cat, but there are species differences.     

The NMDA receptor antagonist CPP blocks the effects of predator stress on pCREB in brain regions involved in fearful and anxious behavior

A 5-min unprotected exposure to a cat produces long-lasting anxiogenic effects on behavior which are NMDA receptor-dependent. Since phosphorylation of CREB is regulated by NMDA receptors and pCREB-like-immunoreactivity (lir) is increased after predator stress, we examined the effects of CPP (3-(2-carboxypiperazin4-yl)propyl-l-phosphonic acid), a competitive NMDA receptor antagonist, on predator stress-induced changes in pCREB-lir in brain areas implicated in fearful and anxious behavior. Areas examined included the amygdala, periqueductal gray (PAG), bed nucleus of the stria terminalis (BNST), anterior cingulate cortex (ACC), and dorsal medial hypothalamus (DMH). CPP blocked the predator stress-induced increase in pCREB-lir in the right lateral PAG and in several amygdala nuclei. CPP also reversed the predator stress-induced suppression of pCREB-lir in the BNST. Importantly, at least in the amygdala and PAG, the pattern of pCREB-lir was hemisphere- and AP plane-dependent. Our results suggest that several amygdala nuclei, the PAG, and the BNST, where predator stress changes pCREB-lir in a NMDA receptor-dependent manner, are candidate areas of neuroplastic change contributing to lasting changes in anxiety-like behaviors.     

Long lasting effects of predator stress on pCREB expression in brain regions involved in fearful and anxious behavior

Predator stress is one animal model of posttraumatic stress disorder (PTSD). Neural plasticity in amygdala afferent and efferent pathways underlies anxiogenic effects of predator stress. Predator stress increases pCREB expression in these pathways 20 min after stress, implicating pCREB in stress-induced neural plasticity. Here we examined impact of predator stress on pCREB expression 6-24 h and 7 days after stress in amygdala pathways and in the supramammillary nucleus (SuM). Patterns of change in pCREB expression were complex, time dependent, column dependent in the periaqueductal gray (PAG), and AP plane dependent in the amygdala. In contrast to past work at 20 min after stress, there were no stress-induced increases in pCREB in the amygdala in the anterior AP plane or in the lateral PAG at 6 h onward after stress. However, dorsal PAG pCREB was increased bilaterally at 24 h and 7 days after stress. In the mid AP plane of all amygdala nuclei there were bilateral stress-induced increases in pCREB at 6 h followed by decreases at 24 h post stress. A similar pattern was observed in the posterior AP plane. In addition, we found a persistent increase (6 h to 7 days after stress) in pCREB expression in the SuM. Further study of this nucleus as a contributor to fear sensitization following predator stress is warranted. Overall, these data highlight persistent neuroplastic changes in key brain areas following traumatic stress. Identification of these changes may aid in understanding the neural mechanisms underlying acquired anxiety disorders such as PTSD.     

Neural circuit changes mediating lasting brain and behavioral response to predator stress

This paper reviews recent work which points to critical neural circuitry involved in lasting changes in anxiety like behavior following unprotected exposure of rats to cats (predator stress). Predator stress may increase anxiety like behavior in a variety of behavioral tests including: elevated plus maze, light dark box, acoustic startle, and social interaction. Studies of neural transmission in two limbic pathways, combined with path and covariance analysis relating physiology to behavior, suggest long term potentiation like changes in one or both of these pathways in the right hemisphere accounts for stress induced changes in all behaviors changed by predator stress except light dark box and social interaction. Findings will be discussed within the context of what is known about neural substrates activated by predator odor.     

Elevated pCREB in the PAG after exposure to the elevated plus maze in rats previously exposed to a cat

The elevated plus maze (EPM) is an ethologically based test of anxiety-like behavior. In addition, exposure to the maze itself is stressful and anxiogenic. One of the goals of this study was to examine if the stress of EPM exposure increased pCREB-like-immunoreactivity (lir). The second goal of this study was to determine if prior stress impacted expression of pCREB-lir in animals exposed to the EPM. Toward this end, pCREB-lir was examined after exposure to the EPM in young adult male rats that had been exposed to a cat 7 days earlier. Brain areas investigated included the amygdala, periaqueductal gray (PAG), and bed nucleus of the stria terminalis (BNST), all areas considered to be part of the "fear circuit". Results show that there were no pCREB-lir differences between control rats and rats exposed to the EPM only. However, exposure to the EPM in predator stressed rats showed elevated pCREB-lir in the right lateral column of the PAG and bilaterally in the dorsal column of the PAG. In contrast, EPM exposure did not elevate pCREB-lir in the amygdala or BNST in predator stressed rats. Findings suggest mechanisms associated with neuroplasticity may be engaged by relatively mild stresses in animals with a history of severe stress exposure. This may be clinically relevant, as a key feature of posttraumatic stress disorder (PTSD) is the exaggerated reaction to a mild stressor in which the response is more appropriate to the original traumatic situation than the current conditions. If what happens in animals also occurs in humans, the findings of this study suggest that neural mechanisms of prior traumatic stress may interact with subsequent stress to reinforce psychopathology.     

Transmitter systems involved in neural plasticity undelying increased anxiety and defense- Implications for understanding anxiety following traumatic stress

Lasting changes in anxiety-like behavior (ALB) may be produced in several ways. These include partial limbic kindling, injection of the β-carboline FG-7142, and brief, non-injurious, exposure of rodents to cats (predator stress). Both seizures and FG-7142 induce long-term potentiation (LTP) in efferent pathways of the amygdala known to participate in feline defensive behavior. By comparing the behavioral and physiological effects of partial kindling and injection of FG-7142, NMDA-dependent LTP in the right amygdalo-periacqueductal gray (PAG) pathway emerges as being critical to maintained increases in feline ALB. A similar dependence on NMDA-mediated processes is described for lasting increases in rodent ALB following predator stress. The lasting aftereffects of predator stress on a variety of measures parallel many of the symptoms of post-traumatic stress disorder (PTSD). Support is provided for the idea that behavioral changes following FG-7142 and predator stress may model anxiety associated with PTSD. Moreover, it is suggested that both models share mechanisms in common involving the PAG. These mechanisms likely involve initiation of LTP by NMDA receptors, and prolongation of LTP by CCK_B receptors. To the extent that response to the stressors reviewed here mimics the symptoms of PTSD, the data implicate NMDA-mediated processes in the creation of what van der Kolk has called permanent emotional memories in PTSD. Their representation may be in the form of NMDA-dependent LTP of transmission within the amygdala and between the amygdala and its efferents. CCK may play a pivotal role in prolonging limbic LTP and anxiety following traumatic stress. Since block of CCK_B receptors before and after the stressor prevents lasting increases in ALB, pharmacological intervention to block CCK receptors shortly after a traumatic stressor might be efficacious in mitigating the permanence of these emotional memories.     

Relationship of the predatory attack experience to neural plasticity, pCREB expression and neuroendocrine response

Aggression takes at least two, an attacker and a target. This paper will address the lasting consequences of being a target of aggression. We review the lasting impact of predatory attack on brain and behavior in rodents. A single brief unprotected exposure of a rat to a cat lastingly alters affective responses of rats in a variety of contexts. Alterations of these behaviors resembles both generalized anxiety comorbid with post traumatic stress disorder (PTSD), and the hyper arousal expressed in enhanced startle in PTSD. Examination of neural transmission and neural plasticity in limbic circuits implicates changes in transmission in two connecting pathways in many but not all of the behavioral changes. Quantification of the predator encounter reveals that both the behavior of the predator and the reaction of the rat to attack are highly predictive of the effects of predatory attack on molecular biological (pCREB expression) and electrophysiological measures of limbic neuroplastic change. Moreover, a case will be made that the pattern of change of corticosteroid level over three hours after the predator encounter, in interaction with the predatory experience, plays an important part in initiation of lasting changes in brain and behavior.     

Does long term potentiation in periacqueductal gray (PAG) mediate lasting changes in rodent anxiety-like behavior (ALB) produced by predator stress?--Effects of low frequency stimulation (LFS) of PAG on place preference and changes in ALB produced by predator stress

The effects on rodent behavior of low frequency bilateral stimulation (LFS, 900 pulses at 1 Hz) of periacqueducatal gray (PAG) was investigated. The first experiment examined aversive qualities of LFS in a place preference paradigm. There was no evidence of a place preference after 1 or 7 applications of LFS. After the first LFS, rats showed longer latencies to leave the conditioned chamber, suggesting a positively reinforcing effect of LFS. Latency differences were not accounted for by freezing or immobility prior to leaving. Rats with electrodes outside the PAG did not show these effects. After repeated LFS, stimulated rats did not differ from controls in place preference or in anxiety-like behavior (ALB). Experiment 2 studied the effects of predator stress in unimplanted rats on an extended battery of measures of ALB in hole board, plus maze and light/dark box tests of rodent anxiety. Effects of electrode damage in the PAG on ALB was also examined. In addition, the effect of 7 applications of bilateral LFS of PAG on ALB following a 5 min unprotected exposure of rats to a cat (predator stress) was examined. Predator stress lastingly changed a wide variety of behaviors in the plus maze, [Rodgers, Behav. Pharmacol. 8 (1997) 477] replicating and extending previous reports. A new finding is an increase in light avoidance in the light/dark box test. Moreover, factor analysis revealed open arm avoidance, risk assessment, light avoidance and cautious exploration loaded on independent factors, replicating and extending previous findings. Bilateral, but not unilateral, damage specific to PAG was also found to be anxiolytic in plus maze measures of ALB. Bilateral implants in the PAG seemed to prevent many of the effects of predator stress on ALB measured 8 days later. Nevertheless, predator stress did decrease head dips in the open arm and LFS reversed this effect. Light avoidance also increased following predator stress and LFS reversed this increase. These findings suggest the PAG occupies an important position in the final common path of substrate changes mediating effects of predator stress on a range of behaviors in the rodent. The fact that LFS in the PAG can reverse stress induced changes in behavior supports the idea that LTP in PAG mediates stress induced increases in anxiety in rodents, as it does in the cat [Adamec, Neurosci. Biobevav. Rev. 21(6) (1997) 755; Adamec, J. Psychopharmacol. 2000 (in press); Adamec, J. Psychopharmacol. 2000 (in press); Adamec, J. Psychopharmacol. 12(2) (1998) 129; Adamec, J. Psychopharmacol. 12(13) (1998) 227].     

Protein synthesis and the mechanisms of lasting change in anxiety induced by severe stress

Brief, unprotected exposure of rats to cats (predator stress) may be lastingly anxiogenic in a variety of tests of rodent anxiety. Recent findings suggest that predator stress induced plasticity in neural circuitry implicated in fear learning underlies some of these anxiogenic effects. In addition, recent work implicates a consolidation-like process in the impact of predator stress on anxiety in that effects of predator stress may be interrupted by immediate post stressor pharmacological interventions. The present study tested whether "consolidation" of the anxiogenic effects of predator stress were dependent on protein synthesis. In addition, the study examined whether a protein synthesis dependent reconsolidation-like process was at work when rats were exposed to a cat twice. Anisomycin (210 mg/kg) or vehicle (Tween 80 in saline) was injected subcutaneously 1 min after a single cat exposure (consolidation test paradigm) or a 1 min after a second cat exposure (reconsolidation test paradigm) and behavior tested 7-8 days after predator stress. In the consolidation test paradigm, anisomycin blocked the anxiogenic effects of predator stress in the elevated plus maze (EPM) measured with open arm exploration. Moreover, anisomycin blocked the potentiation of startle by predator stress when rats were startled in the light, but not when startled in the dark. In contrast, the delay of habituation of startle produced by predator stress was unaffected by anisomycin. Suppression of risk assessment in the EPM by predator stress was not affected by anisomycin either. In startle testing, vehicle injection 1 min after predator stress led to a lasting suppression, rather than enhancement of startle response. Vehicle plus predator stress enhanced and prolonged corticosterone level changes sampled over 30-180 min after treatment when compared to handled or predator stressed only rats. In addition, predator stress plus vehicle suppression of startle was blocked by a benzodiazepine anxiolytic (chloradiazepoxide) or the glucorticoid receptor (GR) blocker RU486. Both drugs returned startle to the predator stressed only heightened levels. It is argued that an added anxiogenic effect of vehicle injection plus predator stress leads to a suppression, rather than enhancement of startle. Startle suppression appears to be mediated, in part, by activation of GR by corticosterone which engages a protein synthesis dependent process, since anisomycin blocked the startle suppressive effects of vehicle. Startle suppression also appeared to be independent of the startle enhancing effect of predator stress and in competition with it. Since predator stress may model aspects of hyperarousal associated with post traumatic stress disorder (PTSD), implications of these findings for understanding of mechanisms of initiation of the disorder and for treatment are discussed.     

Evidence that limbic neural plasticity in the right hemisphere mediates partial kindling induced lasting increases in anxiety-like behavior: effects of low frequency stimulation (quenching?) on long term potentiation of amygdala efferents and behavior following kindling

Behavioral and physiological effects of partial kindling of the right ventral hippocampus by perforant path (PP) stimulation were investigated in the cat. Partial kindling produced lasting changes in affect (increased defensive response to rats) and predatory attack (decreased pawing and biting attack). Partial kindling also induced long term potentiation (LTP) of amygdala efferent transmission to ventromedial hypothalamus (VMH) and periaqueductal gray (PAG) in left and right hemispheres. LTP of field population spikes evoked in area CA3 by PP stimulation was also observed. LTP was detected using evoked potential methods. These findings parallel previous studies of left PP-CA3 partial kindling. Analysis of covariance removing effects of LTP from behavioral changes suggests that initiation of increased defensiveness at 2 days after completion of partial kindling depended on LTP of left and right amygdalo–VMH and right amygdalo–PAG transmission. From 6 days after kindling onward, increased defensiveness depended on LTP of right amygdalo–PAG transmission. Depotentiation of amygdala efferent LTP by bilateral low frequency amygdala stimulation (LFS) (900 pulses at 1 Hz, once daily for 7 days) selectively reduced LTP in right amygdala efferents. At the same time, defensive, but not predatory attack behavior, was returned to levels seen prior to partial kindling. Both depotentiation and reduction of defensiveness were transient. Defensiveness increased to post-kindling levels by 76 days after LFS. At the same time, LTP was restored in the right amygdalo–PAG pathway. In contrast LTP in the right amygdalo–VMH pathway remained depotentiated. Effects of LFS were not due to damage, as thresholds to evoke amygdala efferent response were unchanged. These findings suggest that lasting change in affect following partial hippocampal kindling depends on LTP of right amygdala efferent transmission to PAG. The findings parallel studies of non-convulsant pharmacological induction of lasting increases in defensiveness and amygdalo–PAG LTP with FG-7142. The parallel between the present findings and the FG-7142 experiments suggests that lasting changes in defensive response are dependent on LTP of right amygdala efferents to the PAG, however produced. The findings suggest further that the spectrum of behavioral changes produced by partial kindling are dependent on changes in a variety of neural circuits, and that amygdala efferent transmission changes are responsible for changes in defensive behavior, but not predatory attack behavior. Clinical implications are discussed.     

The role of the read through variant of acetylcholinesterase in anxiogenic effects of predator stress in mice

This study examined the role of the read through variant of acetylcholinesterase (AChE-R) in lasting changes in murine affective behavior produced by a brief predator stress. AChE-R is elevated by stress in limbic cholinergic circuits implicated in anxiogenic effects of predator stress. The expression of AChE-R was blocked with a systemically administered central acting antisense oligonucleotide for AChE-R (EN101). EN101 was injected at multiple points prior to and after a predator stress in male C57 mice. Seven days after the last injection, behavior was tested. Predator stress caused a significant increase in startle amplitude, which EN101 blocked. This effect was specific to EN101, as the negative control inactive form of EN101, INVEN101 was without effect on stress effects on startle. Neither EN101 nor INVEN101 altered the anxiogenic effects of predator stress on behavior in the elevated plus maze, and both drugs partially reduced stress suppression of time active in the hole board. In the light dark box test, INVEN101 exhibited a weak block of stress effects on behavior for reasons which are unclear. Taken together, findings support the view that multiple neural systems are responsible for the different changes in behavior produced by predator stress. Present findings also suggest a role for AChE-R in specific anxiogenic (hyperarousal) effects following predator stress. Since AChE-R manipulations took place starting 23 h prior to predator stress and continued 48 h after predator stress, further research is necessary to determine the role of AChE-R in initiation and/or consolidation of hyperarousal effects of predator stress.     

Vulnerability to mild predator stress in serotonin transporter knockout mice

Effect of predator stress on rat and mouse anxiety-like behavior may model aspects of post traumatic stress disorder (PTSD). A single cat exposure of wild type (C57, CFW) mice can produce lasting anxiety-like effects in the elevated plus maze, light/dark box tests and startle. In addition, female but not male C57 mice are made more anxious in the plus maze by exposure to predator odors alone, suggesting differential vulnerability to predator stressors of differing intensity. There is a link between genetic variation in the serotonin (5-HT) transporter (SERT) and anxiety in humans. This prompted the generation of SERT knockout mice [see Holmes A, Murphy DL, Crawley, JN. Biol Psychiatry 2003;54(10):953-9]. Present work used these mice to determine if there was a link between vulnerability to the anxiogenic effects of predator odors and abnormalities of 5-HT transmission induced by a life long reduction in 5-HT reuptake. Wild type (WT, C57 background), heterozygous (SERT +/-, HET) mice and homozygous knockout (SERT -/-, KO) were assigned to handled control groups or groups exposed for 10 min to a large testing room rich in cat odor. One week after handling or room exposure, anxiety testing took place in the dark phase of the light/dark cycle, in red light. Predator odor exposure was selectively anxiogenic in the plus maze and light/dark box tests in SERT -/- mice. Exposure to predator odor did not potentiate startle. Findings suggest a role for abnormalities in 5-HT transmission in vulnerability to some of the lasting anxiogenic effects of species relevant stressors and possibly in vulnerability to PTSD.     

Changes in S100B cerebrospinal fluid levels of rats subjected to predator stress

Predator stress is a type of psychogenic stress induced by an innate recognition of threat. S100B, a calcium-binding protein secreted by astrocytes, has been associated with neurotrophic or neurotoxic action in several neuropsychiatric disorders. It has been recently demonstrated that serum S100B levels in rats are increased after stress by immobilization [S. Scaccianoce, P. Del Bianco, G. Pannitteri, F. Passarelli, Relationship between stress and circulating levels of S100B protein, Brain Res. 1004 (2004) 208-11]. This study aimed to measure cerebrospinal fluid (CSF) S100B in rats after an acute stress situation, which is induced by exposure to a predator. S100B was measured in CSF and in hippocampal and cortical slices by ELISA. Forty-three male Wistar rats, aged 70 days, were randomly assigned to handled (control) or stressed groups (exposed to a cat for 5 min). CSF and brain tissue were removed 1 or 24 h after the procedures. Rats exposed to the cat demonstrated a biphasic change in CSF S100B levels. An increase was observed at 1 h after cat exposure, and a decrease was observed 24 h later, although this was not accompanied by changes in S100B content in hippocampus or cerebral cortex. The effectiveness of the stressor used was confirmed by increased freezing response (during cat exposure) and increased anxiety in the plus maze test (1 h after cat exposure). These results indicate that CSF S100B is changed by stress, reinforcing the possibility that this protein is involved in the adaptive response to stress and/or in secondary neuropsychiatric disorders.     

Predatory hunting and exposure to a live predator induce opposite patterns of Fos immunoreactivity in the PAG

Considering the periaqueductal gray's (PAG) general roles in mediating motivational responses, in the present study, we compared the Fos expression pattern in the PAG induced by innate behaviors underlain by opposite motivational drivers, in rats, namely, insect predation and defensive behavior evoked by the confrontation with a live predator (a cat). Exposure to the predator was associated with a striking Fos expression in the PAG, where, at rostral levels, an intense Fos expression was found largely distributed in the dorsomedial and dorsolateral regions, whereas, at caudal levels, Fos-labeled cells tended to be mostly found in the lateral and ventrolateral columns, as well as in the dorsal raphe nucleus. Quite the opposite, insect predation was associated with increased Fos expression predominantly in the rostral two thirds of the lateral PAG, where the majority of the Fos-immunoreactive cells were found at the oculomotor nucleus levels. Remarkably, both exposure to the cat and insect predation upregulated Fos expression in the supraoculomotor region and the laterodorsal tegmental nucleus. Overall, the present results clearly suggest that the PAG activation pattern appears to reflect, at least partly, the animal's motivational status. It is well established that the PAG is critical for the expression of defensive responses, and, considering the present findings, it will be important to investigate how the PAG contributes to the expression of the predatory behavior, as well.     

Estrous cycle stage influences on neuronal responsiveness to repeated anxiogenic stress in female rats

Experiments were carried out to investigate (i) whether estrous cycle stage influences nociceptive responsiveness to anxiogenic stress and (ii) whether prior experience of the stressor modifies the response. Exposure to mild anxiogenic vibration stress evoked hyperalgesia, reflected by a decrease in tail flick latency, only in animals in the late diestrus phase. Stress evoked hyperalgesia in late diestrus regardless of whether the rat was experiencing the stress for the first time or had been exposed to the stress previously, when in another cycle stage. Whilst the behavioral response to stress appeared to be determined exclusively by estrous cycle stage, the stress-evoked pattern of Fos expression in the periaqueductal grey matter (PAG) depended not only on cycle stage but also on whether the rat had previous experience of the stress. First exposure to stress in late diestrus evoked a 50% decrease in Fos expression compared to non-stressed controls, particularly in the lateral and dorsolateral sectors of the rostral PAG. In contrast, in experienced rats in late diestrus the pattern of Fos expression increased up to 4-fold, particularly in the ventral half of the caudal PAG but also in the lateral and dorsolateral parts. At other cycle stages Fos expression was not changed except for an increase in rats in proestrus. The results suggest that in females, changes in gonadal hormone levels during the estrous cycle impact significantly on the processing of fear-inducing stimuli by the PAG. These hormonal influences may also influence how the PAG responds to a subsequent anxiogenic challenge.     

Parallel circuits mediating distinct emotional coping reactions to different types of stress

All animals, including humans, react with distinct emotional coping strategies to different types of stress. Active coping strategies (e.g. confrontation, fight, escape) are evoked if the stressor is controllable or escapable. Passive coping strategies (e.g. quiescence, immobility, decreased responsiveness to the environment) are usually elicited if the stressor is inescapable and help to facilitate recovery and healing. Neural substrates mediating active versus passive emotional coping have been identified within distinct, longitudinal neuronal columns of the midbrain periaqueductal gray (PAG) region. Active coping is evoked by activation of either the dorsolateral or lateral columns of the PAG; whereas passive coping is triggered by activation of the ventrolateral PAG. Recent anatomical studies indicate that each PAG column receives a distinctive set of ascending (spinal and medullary) and descending (prefrontal cortical and hypothalamic) afferents. Consistent with the anatomy, functional studies using immediate early gene expression (c-fos) as a marker of neuronal activation have revealed that the preferential activation of a specific PAG column reflects (i) the type of emotional coping reaction triggered, and (ii) whether a physical or psychological stressor was used.     

Estrogen-induced dendritic spine elimination on female rat ventromedial hypothalamic neurons that project to the periaqueductal gray

Neurons of the ventromedial hypothalamic nucleus (VMH) that project to the periaqueductal gray (PAG) form a crucial segment of the motor pathway that produces the lordosis posture, the hallmark of female rat sexual behavior. One suggested mechanism through which estrogen facilitates lordosis is by remodeling synaptic connectivity within the VMH. For instance, estrogen alters VMH dendritic spine density. Little is known, however, about the local VMH microcircuitry governing lordosis nor how estrogen alters synaptic connectivity within this local circuit to facilitate sexual behavior. The goal of this study was to define better the neuron types within the VMH microcircuitry and to examine whether estrogen alters synaptic connectivity, as measured by dendritic spine density, on VMH projection neurons. A retrograde tracer was injected into the PAG of ovariectomized rats treated with vehicle or estradiol. Retrogradely labeled VMH neurons were filled with Lucifer yellow, then immunostained for estrogen receptor-alpha (ER alpha). VMH neurons that project to the PAG had more dendrites than functionally unidentified neurons. Additionally, VMH projection neurons could be subdivided into those located within the cluster of ER alpha-containing neurons and those medial to the cluster. Estrogen decreased spine density by 57% on the long primary dendrites of VMH projection neurons located within the ER alpha cluster but not on projection neurons medial to the cluster. Only 4% of the VMH projection neurons expressed ER alpha. These results suggest that estrogen may facilitate sexual behavior by decreasing spines selectively, via an indirect mechanism, on a subset of VMH neurons that project to the PAG.     

Neuroplasticity in specific limbic system circuits may mediate specific kindling induced changes in animal affect-implications for understanding anxiety associated with epilepsy

In two complementary experiments, we studied the effects of low frequency stimulation (LFS) of the amygdala on behavioral effects of kindling in rats and cats. These studies tested the hypothesis that kindling induced long term potentiation (KLTP) in amygdala circuits underlies interictal behavioral change. Since LFS can depotentiate LTP, it was predicted that LFS should both depotentiate KLTP and reverse behavioral effects of kindling. In cats, the effects of LFS on KLTP of amygdala efferents was studied, and related to behavioral effects. Partial ventral hippocampal kindling in cats and right amygdala kindling in rodents lastingly increased defensive response to rats in cats, and anxiety-like behavior (ALB) in the elevated plus-maze in rats. In addition, partial kindling reduced predatory attack behavior in cats independent of its effects on defensive response. Partial kindling also induced KLTP of amygdala efferent transmission to ventromedial hypothalamus (VMH) and periaqueductal gray (PAG) in left and right hemispheres. Depotentiation of amygdala efferent KLTP by bilateral amygdala LFS selectively reduced KLTP in right amygdala efferents. At the same time, defensive behavior, but not attack behavior, was returned to levels seen prior to partial kindling. Defensiveness returned to post kindling levels between 44 and 76days after LFS. At the same time, LTP was restored in the right Amygdalo-PAG pathway only. These findings suggest that lasting change in affect produced by kindling depends on LTP of right amygdala efferent transmission to PAG, replicating studies of the effects of FG-7142 on brain and behavior in the cat. The findings suggest further that the spectrum of behavioral changes produced by partial kindling are dependent on changes in a variety of neural circuits, and that amygdala efferent transmission changes are responsible for changes in defensive behavior, but not attack behavior. Effects of LFS were not due to damage, as thresholds to evoke amygdala efferent response were unchanged. Other data suggest KLTP and depotentiation in right Amygdalo-PAG may reflect changes in glutamate receptor density/synapse number. Kindling effects on rat ALB persisted for at least 1month. Bilateral but not unilateral amygdala LFS reversed the effects of kindling on risk assessment in the plus maze for at least 3weeks. Bilateral LFS also reversed the effects of kindling on open arm exploration, but effects were shorter lived, appearing at 1day but not 3weeks after kindling and LFS. These findings are consistent with other studies which suggest that amygdala neuroplasticity in separable amygdala circuits mediates lasting changes in open arm avoidance and risk assessment. Taken together, the findings of both studies support the hypothesis that a form of LTP of specific amygdala circuits underlies lasting changes in affect produced by limbic kindling. Clinical implications of these findings are discussed.     

Prenatal exposure to restraint or predator stresses attenuates field excitatory postsynaptic potentials in infant rats

Exposure to stress is known to change synaptic plasticity and results in long-term depression; further, this stress precipitates seizures. In the study described here, the prenatal restraint and predator stress models were used to test the hypothesis that indirect prenatal stresses influence hippocampal synaptic potentiation and may affect seizures susceptibility in infant rats. Pregnant female Wistar rats were divided into 3 groups: control, restraint-stressed, and predator-stressed groups. Both stressed groups were exposed to the stressor on gestation days 15, 16, and 17. The restraint stress involved 1-h sessions twice daily in a Plexiglas tube and the predator stress involved 2-h sessions once daily in a cage placed within the visual range of a caged cat. Blood corticosterone (COS) levels were measured in different time points. Hippocampal slices were prepared and field excitatory postsynaptic potentials (fEPSP) were studied on postnatal day 15. Pilocarpine was administered on postnatal day 25 and mortality rates were measured after 2 and 24h. Restraint and predator stresses resulted in significantly elevated COS blood levels in dams and pups. Both the amplitude and slope of fEPSP in the CA1 area decreased significantly in the stressed groups as compared to the control. Prenatal restraint and predator stresses significantly increased the fatal effect of pilocarpine at 24h after injection. Exposure to prenatal stresses and COS blood levels elevation reduce hippocampal synaptic potentiation and increase mortality rate of seizure in infant rats and may affect on later seizure susceptibility and prognosis.