Hippocampal corticosterone receptors and novelty-induced behavioral activity: effect of kainic acid lesion in the hippocampus

Increased Expression of Brain-Derived Neurotrophic Factor Transcripts I and VI, cAMP Response Element Binding, and Glucocorticoid Receptor in the Cortex of Patients with Temporal Lobe Epilepsy

A body of evidence supports a relevant role of brain-derived neurotrophic factor (BDNF) in temporal lobe epilepsy (TLE). Magnetic resonance data reveal that the cerebral atrophy extends to regions that are functionally and anatomically connected with the hippocampus, especially the temporal cortex. We previously reported an increased expression of BDNF messenger for the exon VI in the hippocampus of temporal lobe epilepsy patients compared to an autopsy control group. Altered levels of this particular transcript were also associated with pre-surgical use of certain psychotropic. We extended here our analysis of transcripts I, II, IV, and VI to the temporal cortex since this cerebral region holds intrinsic communication with the hippocampus and is structurally affected in patients with TLE. We also assayed the cyclic adenosine monophosphate response element-binding (CREB) and glucocorticoid receptor (GR) genes as there is experimental evidence of changes in their expression associated with BDNF and epilepsy. TLE and pre-surgical pharmacological treatment were considered as the primary clinical independent variables. Transcripts BDNF I and BDNF VI increased in the temporal cortex of patients with pharmacoresistant TLE. The expression of CREB and GR expression follow the same direction. Pre-surgical use of selective serotonin reuptake inhibitors, carbamazepine (CBZ) and valproate (VPA), was associated with the differential expression of specific BDNF transcripts and CREB and GR genes. These changes could have functional implication in the plasticity mechanisms related to temporal lobe epilepsy.

Role of hippocampal β-adrenergic and glucocorticoid receptors in the novelty-induced enhancement of fear extinction

Fear extinction forms a new memory but does not erase the original fear memory. Exposure to novelty facilitates transfer of short-term extinction memory to long-lasting memory. However, the underlying cellular and molecular mechanisms are still unclear. Using a classical contextual fear-conditioning model, we investigated the effect of novelty on long-lasting extinction memory in rats. We found that exposure to a novel environment but not familiar environment 1 h before or after extinction enhanced extinction long-term memory (LTM) and reduced fear reinstatement. However, exploring novelty 6 h before or after extinction had no such effect. Infusion of the β-adrenergic receptor (βAR) inhibitor propranolol and glucocorticoid receptor (GR) inhibitor RU486 into the CA1 area of the dorsal hippocampus before novelty exposure blocked the effect of novelty on extinction memory. Propranolol prevented activation of the hippocampal PKA-CREB pathway, and RU486 prevented activation of the hippocampal extracellular signal-regulated kinase 1/2 (Erk1/2)-CREB pathway induced by novelty exposure. These results indicate that the hippocampal βAR-PKA-CREB and GR-Erk1/2-CREB pathways mediate the extinction-enhancing effect of novelty exposure. Infusion of RU486 or the Erk1/2 inhibitor U0126, but not propranolol or the PKA inhibitor Rp-cAMPS, into the CA1 before extinction disrupted the formation of extinction LTM, suggesting that hippocampal GR and Erk1/2 but not βAR or PKA play critical roles in this process. These results indicate that novelty promotes extinction memory via hippocampal βAR- and GR-dependent pathways, and Erk1/2 may serve as a behavioral tag of extinction.

Loss of molars early in life develops behavioral lateralization and impairs hippocampus-dependent recognition memory

Background

Using senescence-accelerated mouse prone 8 (SAMP8), we examined whether reduced mastication from a young age affects hippocampal-dependent cognitive function. We anesthetized male SAMP8 mice at 8 weeks of age and extracted all maxillary molar teeth of half the animals. The other animals were treated similarly, except that molar teeth were not extracted. At 12 and 24 weeks of age, their general behavior and their ability to recognize novel objects were tested using the open-field test (OFT) and the object-recognition test (ORT), respectively.

Results

The body weight of molarless mice was reduced significantly compared to that of molar-intact mice after the extraction and did not recover to the weight of age-matched molar-intact mice throughout the experimental period. At 12 weeks of age, molarless mice showed significantly greater locomotor activity in the OFT than molar-intact mice. However, the ability of molarless mice to discriminate a novel object in the ORT was impaired compared to that of molar-intact mice. The ability of both molarless and molar-intact SAMP8 mice to recognize objects was impaired at 24 weeks of age. These results suggest that molarless SAMP8 mice develop a deficit of cognitive function earlier than molar-intact SAMP8 mice. Interestingly, both at 12 and 24 weeks of age, molarless mice showed a lateralized preference of object location in the encoding session of the ORT, in which two identical objects were presented. Their lateralized preference of object location was positively correlated with the rightward turning-direction preference, which reached statistical significance at 24 weeks of age.

Conclusions

Loss of masticatory function in early life causes malnutrition and chronic stress and impairs the ability to recognize novel objects. Hyperactivation and lateralized rotational behavior are commonly observed with dysfunction of the dopaminergic system, therefore, reduced masticatory function may deplete the mesolimbic and mesocorticolimbic dopaminergic systems to impair the cognitive functions of selective attention and recognition memory in the prefrontal cortex and the hippocampus.

Infusions of bicuculline to the ventral tegmental area attenuates sexual, exploratory, and anti-anxiety behavior of proestrous rats

Actions of 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), in the midbrain ventral tegmental area (VTA) modulate sexual receptivity of female rats. Actions of 3alpha,5alpha-THP at GABAergic substrates in the VTA are known to modulate consummatory aspects of sexual behavior among rodents, such as lordosis. However, the extent to which GABA(A) receptors in the VTA are important for appetitive (exploratory, anti-anxiety, social) aspects of sexual receptivity is not well-understood. Proestrous rats were bilaterally-infused with saline or bicuculline (100 ng), a GABA(A) receptor antagonist, to the VTA or missed control sites. Rats were assessed for exploratory/anti-anxiety (open field/elevated plus maze), social (social interaction), and sexual (paced-mating) behavior. Compared to saline or missed site controls, intra-VTA bicuculline significantly reduced the number of central entries in an open field, time spent on the open arms of an elevated plus maze, frequency and intensity of lordosis, anti-aggression towards a male, pacing of sexual contacts, and 3alpha,5alpha-THP concentrations in midbrain and hippocampus. Bicuculline-infused rats also displayed less affiliation with a novel conspecific, fewer sexual solicitations, and had lower 3alpha,5alpha-THP concentrations in diencephalon and cortex, albeit these were not significant differences. Thus, actions at GABA(A) receptors in the midbrain VTA are essential for appetitive and consummatory aspects of sexual receptivity among rats.

Mineralocorticoid and glucocorticoid receptor expressions in astrocytes and microglia in the gerbil hippocampal CA1 region after ischemic insult

In the present study, we observed expression and changes of mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) in the gerbil hippocampal CA1 region, but not in the CA2/3 region, after 5 min of transient forebrain ischemia. In blood, corticosterone levels were increased biphasically at 30 min and 12 h after ischemia/reperfusion, and thereafter its levels were decreased. In the sham-operated group, MR and GR immunoreactivities were weakly detected in the CA1 region. By 3 days after ischemia, MR and GR were not significantly altered in the CA1 region: at 12 h after ischemia, GR was expressed in a few neurons in the CA1 region, whereas MR was not expressed in any neurons after ischemic insult. From 4 days after ischemia, MR and GR immunoreactivities were detected in astrocytes and microglia in the CA1 region, and at 7 days after ischemia, MR and GR immunoreactivities peaked in the hippocampal CA1 region. At this time, 55% of astrocytes and 30% of microglia showed MR immunoreactivity, and 20% of astrocytes and 40% of microglia showed GR immunoreactivity. Western blot analyses showed that the pattern of changes in MR and GR protein levels was similar to the immunohistochemical changes observed after transient forebrain ischemia. From 4 days after ischemia, MR and GR protein levels were increased time-dependently after ischemia. In conclusion, enhanced MR and GR expressions in astrocytes and microglia were detected in the hippocampal CA1 region 4-7 days after ischemia/reperfusion. At this time, GR immunoreactivity was abundant in microglia, whereas MR immunoreactivity was prominent in astrocytes. The specific distribution of corticosteroid receptors in the astrocytes and microglia may be associated with the differences of MR and GR functions against ischemic damage.

Comparative behavioral changes in postpubertal rats after neonatal excitotoxic lesions of the ventral hippocampus and the prefrontal cortex

The neonatal ventral hippocampal (nVH) and the neonatal prefrontal cortex (nPFC) lesions in rats have been used as models to test the hypothesis that early neurodevelopmental abnormalities lead to behavioral changes putatively linked to schizophrenia. We investigated the role of the nVH and the nPFC lesions on behavioral characteristics related to locomotor behaviors, social interaction, and grooming. Bilateral ibotenic acid lesions of the VH, the PFC, or both were made in neonatal Sprague-Dawley rats (postnatal day 7, P7) and their behaviors studied at P35 and P60. No significant differences in any of the behaviors were observed between sham animals and rats with ibotenic acid lesions at P35. Postpubertally (at P60), the spontaneous locomotor activity of nVH-lesioned rats was significantly enhanced compared to the sham controls; however, this hyperactivity was reversed by nVH and nPFC double lesions. Neonatal PFC lesion alone did not alter spontaneous activity, although a trend of increased activity was observed. The duration of grooming was significantly decreased in rats with neonatal lesions of the VH. Similar to the data on locomotion, nVH plus nPFC lesion normalized the grooming behavior. Lesion of the PFC alone was without any significant effect on grooming behavior. Neonatal VH-lesioned animals spent less time in active social interaction, and this effect persisted even in nVH plus nPFC-lesioned animals. By itself, nPFC lesion did not alter social behavior. These data suggest that subtle developmental aberrations within PFC caused by nVH lesions, rather than the lesion of PFC itself, may contribute to some of the behavioral changes seen in the nVH-lesioned rats.

Kainic acid lesions enhance locomotor responses to novelty, saline, amphetamine, and MK-801

Intracerebroventricular (i.c.v.) administration of kainic acid (KA) to rats produces neuronal loss in the hippocampus and other areas of the limbic system. The present study demonstrates that i.c.v. KA enhances the locomotor response to novelty and saline injection, as well as to amphetamine and MK-801. Sixteen to 18 days after i.c.v. administration of KA or vehicle, lesioned and control rats were placed in a novel cage, and locomotor activity and grooming were recorded for 30 min prior to and 60 min following a subcutaneous injection of saline, D-amphetamine, or MK-801. In response to the novel cage and after each injection, KA rats exhibited increased locomotor activity relative to controls. Grooming behavior was found to be elevated in the KA rats when compared to controls, but only in response to the novel cage and saline injection. The possibility that damage to the limbic system disrupts dopaminergic regulation of locomotor behavior is discussed, as well as implications for neuropathology in schizophrenia.

Parallel changes in operant behavioral adaptation and hippocampal corticosterone binding in rats treated with trimethyltin

Rats were given water vehicle or trimethyltin (TMT; 3.0, 6.0 or 7.5 mg/kg, p.o.). Lever responding for food was measured 3 months later, in a test in which the fixed ratio requirement was doubled daily (FR1-128). Response rates for all groups were inverted U-shaped functions of FR values. However, the effect of increasing ratio values was attenuated in the 6.0 mg/kg group, which responded less than controls when control rates were maximal (at FR16 and FR32). In contrast, rats given the high dose responded at higher rates (at FR4 and FR64). [3H]Corticosterone binding to hippocampal cytosolic protein was maximally reduced for the group given 6.0 mg TMT/kg. The greatest reduction in hippocampal weight resulted from injection of 7.5 mg TMT/kg, but a smaller reduction in [3H]corticosterone binding (i.e. 22%) was observed for this group. In the absence of an effect of 3.0 mg TMT/kg upon weight of hippocampus, there also was a reduction in steroid binding, indicating the sensitivity of this parameter for TMT toxicity. The results support the notion that hippocampal corticosteroid receptors are important for behavioral adaptation, and rats given moderate doses of TMT may be useful for studying functions of corticosterone receptors.

Feedback action and tonic influence of corticosteroids on brain function: a concept arising from the heterogeneity of brain receptor systems

Two types of corticosteroid receptors can be distinguished in rat brain. The type 1 receptor resembles the kidney mineralocorticoid receptor and has two functional expressions in brain, i.e. type 1 corticosterone (CORT) preferring sites (CR) and mineralocorticoid receptors (MR). The type 2 receptor is similar to the liver glucocorticoid receptor (GR). CORT binds to both CR and GR. The localization, binding specificity, and capacity of the receptor systems have served as criteria to evaluate steroid dependent events in brain biochemistry and behaviour. The GR is widely distributed in neurons and glial cells, with the highest density in frontal brain regions. The GR becomes occupied concomitant with rising plasma CORT levels after stress and as part of the circadian rhythm. The GR mediates the feedback action of CORT on stress-activated brain processes. The CR has its predominant localization in neurons of the septo-hippocampal complex and has a ten-fold higher affinity for CORT than that of the GR. The CR is, at all times of intact adrenocortical secretion, 90% or more occupied by endogenous hormone. The CR mediates a tonic influence exerted with stringent specificity by CORT on hippocampus-associated functions, e.g. cognition, mood, and affect. CORT, via the CR, thus contributes to hippocampus function in interpretation of sensory information, leading to appropriate neuroendocrine and behavioural responses, which are themselves subsequently subject to feedback action via the GR. The MR mediates the mineralocorticoid effect on salt and water balance and its behavioural corollary of salt appetite. The anatomical localization of the MR system is as yet ill-defined, although functional studies suggest circumventricular organs as mineralocorticoid target sites. The CR and the MR have in common the high affinity for mineralocorticoids, but the CR is defined by its exclusive responsiveness to CORT as its agonist. The CR and MR probably represent the same chemical receptor modality (type 1), which is expressed differentially depending on the presence of extravascular corticosteroid binding globulin (CBG) in the vicinity of the receptor. GR capacity is subject to autoregulation. Chronic stress, senescence, and chronic CORT administration reduce GR number, with, as a consequence, a less efficient feedback signal. The CR number seems not to be under the control of corticosteroids, probably since the receptor sites are extensively occupied by endogenous hormones. The CR number displays a circadian rhythm and is reduced during senescence.(ABSTRACT TRUNCATED AT 400 WORDS)

Function and plasticity of brain corticosteroid receptor systems: action of neuropeptides

Two types of corticosteroid receptors may be distinguished in rat brain. Type 1 resembles the kidney mineralocorticoid receptor and Type 2 is similar to the liver glucocorticoid receptor (GR). Type 1 receptor system displays two functional expressions, i.e. Type 1 corticosterone (CORT)-preferring sites (CR) and Type 1 mineralocorticoid receptors (MR). MR occurs in circumventricular organs and mediates behaviors such as salt appetite. CR has its principal localization in neurons of the hippocampus, and mediates tonic influences of CORT on hippocampus-associated functions. CR responds with stringent specificity to CORT. Differentiation between CR and MR is due to a different accessibility of the receptor by CORT and ALDO, which seems dependent on the presence of extravascular corticosteroid binding globulin (CBG). GR has a wide distribution in brain, occurs in neurons and glial cells and has its principal localization in such regions as the paraventricular nucleus and the n. tractus solitarii (site of CRF synthesis and of blood pressure regulation, respectively). GR mediates the feedback action of CORT on stress-activated brain processes. GR is subject to autoregulation by CORT. Chronic stress, senescence, and chronic CORT administration reduce receptor number, while GR capacity is increased after adrenalectomy. Reduced GR receptor number results in a less-efficient feedback action. The CORT signal via CR probably can only be altered via changes in CR density evoked rather by neural factors than by autoregulation. CR density is reduced at senescence, but is increased to receptor number of young control animals after chronic treatment with a behaviorally potent ACTH-(4-9) analogue, Org 2766. CR plasticity is prominent for the hippocampus, which is a structure involved in cognition, emotional state and subtle regulation of pituitary-adrenal function.

Corticosteroid receptor plasticity and recovery of a deficient hippocampus-associated behavior after unilateral (dorsal) hippocampectomy

Unilateral ablation of the right dorsal hippocampus (HCX) produced changes in maximal corticosteroid binding capacity (Bmax) in the contralateral hippocampal lobe of the rat with time. The mechanism by which this time course of changes was produced seemed to involve the pituitary-adrenal system, since a certain difference in corticosteroid receptor binding pattern was noted between chronic adrenalectomized (ADX) rats and rats which remained intact during postlesion survival. In the presence of endogenous adrenal hormones the HCX-induced changes in corticosteroid receptor binding relative to that observed in rats with the overlying neocortex ablated (control) were the following: a 26% decrease at 5 days after HCX; an increase the following 3 weeks with a maximum of 46% at 20 days postsurgery; and recovery towards control values after longer survival times. After discrimination of corticosteroid binding into two corticosterone (CORT) binding receptor populations, e.g. glucocorticoid receptors (GR) and mineralocorticoid-like or CORT receptors (CR), the lesion-induced effect was more pronounced in GR than in CR. A 72% increase over controls was measured at 20 days postsurgery. In the absence of the adrenals, however, the Bmax of corticosteroid binding was not decreased at 5 days after HCX. The relative increase in Bmax reached a maximum of 39% over control levels at 30 days postsurgery and recovery towards control values after longer survival did not occur. The increase in corticosteroid receptor capacity after HCX, therefore, is transient in the presence of adrenocortical secretion and permanent in its absence.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of corticosterone synthesis alters the behavior of hippocampally lesioned rats

The suppression of corticosterone synthesis with metyrapone (25 mg/kg) reduced the hyperactivity and altered the exploratory activity of hippocampally lesioned animals (HPC) in the open field to the level of cortical and sham controls (Experiment 1). In a second experiment, corticosterone (600 micrograms/kg) pretreatment 2 h, but not 1 h, before metyrapone partially restored the hyperactivity of HPC animals that had been decreased by the corticosteroid-suppressant drug. Alterations in exploratory behavior induced by metyrapone were also prevented by corticosterone pretreatment. The results suggest that the suppression of corticosterone in hippocampally lesioned animals produces a normalization of behavior that can be prevented by pretreatment with corticosterone.

Spatial learning and the hippocampal corticosterone receptor system of old rats: effect of the ACTH4-9 analogue ORG 2766

Old (26 months) and young (6 months) male Wistar rats were treated chronically for 2 weeks with ORG 2766 or with vehicle, delivered via subcutaneously implanted minipumps (0.5 microgram peptide/0.5 microliter/h). Learning of a spatial task was not impaired in the old animals, except for one measure, i.e. the latency to find the goal box. In neither age group did ORG 2766 influence behavioral performance. The number of corticosterone receptor sites was decreased in the hippocampus of senescent rats, but restored to the level observed in young rats following ORG 2766 treatment. It is concluded that the number of hippocampal corticosterone receptor sites is a sensitive index of brain aging and effectiveness of ORG 2766.

Adrenal steroids as modulators of nerve cell function

Adrenal steroids modulate the function of nerve cells. Some, but not all actions of these steroids take place after binding to intracellular receptor systems and translocation of the steroid-receptor complex into the cell nucleus. Studies on the rat brain revealed heterogeneity of receptors. One population of receptor sites is present in abundance in extrahypothalamic limbic brain regions, e.g. neurons of the hippocampus, septum and amygdala. This neuronal receptor system displays a stringent binding specificity towards corticosterone, which is the naturally occurring glucocorticoid of the rat. Focussing the studies on the corticosterone receptor system in hippocampal neurons has provided further insight in the understanding of some of the actions of the steroid. Certain hippocampus-associated behaviors and indices of neurotransmission (serotonin) were disturbed after removal of the adrenals, but selectively restored after replacement with a low dose of corticosterone. The specificity, localization and dose-dependency of the corticosterone action on behavior and neurotransmission corresponds to the properties of its receptor system. The responsiveness to corticosterone is altered after changes in number of receptor sites. Chronic stress or high doses of exogenous corticosterone cause a long-term reduction. Other factors involved in regulation of receptor number are the neurotransmitter serotonin and neuropeptides related to ACTH and vasopressin. These substances restore changes in number of hippocampal corticosterone receptor sites due to aging, endocrine or neural deficiencies. Our results show that the number of corticosterone receptors is a sensitive index for brain functioning. Thus, the receptor system mediates some of the modulatory actions of corticosterone on nerve cell function and it may adjust its capacity under the influence of neural and endocrine factors.