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I have selected problems that focus on innovative approaches rather than reviewing existing mechanisms and their well known limitations. Small molecules capable of blocking neuropeptide receptors have the potential to become novel antidepressants. Likely candidates have been corticotropin releasing factor CRF antagonists and neurokinin-1 receptor antagonists. However, the effi cacy of this treatment was not confi rmed in clinical phase III trials. Preclinical data from α2,3-subunit selective GABA-A receptor agonists suggest that these targets may yield anxioselective agents. However, the α1 subtype mediates a substantial proportion of the anxiolytic actions of benzodiazepines. Considering that GABA-A1 receptors are expressed in higher densities than GABA-A2 or -A3 receptors and that there is substantial overlap of their localization, it may remain necessary to have a residual degree of their activation to achieve clinically signifi cant anxiolysis. The discovery of novel molecules designed to regulate the specifi c aspects of neurogenesis may lead to improved CNS therapeutics, including antidepressant drugs. The majority of antidepressant drugs currently prescribed for the treatment of depression inhibits the reuptake of serotonin and/or norepinephrine. The strategy for developing triple reuptake inhibitors (TRIs) is based on preclinical and clinical studies indicating that adding a dopaminergic component will improve these therapies. Metabotropic glutamate receptors mGlu2,3 and mGlu5, and the cannabinoid receptors CB1 and CB2 are also attractive therapeutic targets. It is concluded, that a deeper understanding of the breadth of neuromodulators structural diversity as well as the components required for their stimulation/inhibition will guide a rational drug development.
The individual differences in the response to aversive stimuli could be important predictor for anxiety and affective disorder. Recently, in our laboratory we evaluated model of high (HR) and low anxiety rats (LR), selected according to their behaviour in the contextual fear test (i.e., the duration of a freezing response was used as a discriminating variable) to examine the neurochemical background of differences in the individual responses to conditioned aversive stimuli. Both groups had different behavioural and biochemical profiles. During test session of conditioned freezing test, LR, had higher c-Fos activity and stronger 5-HT and CRF related immunostaining in the M2 (secondary motor cortex) and higher c-Fos in the DG of hippocampus in comparison to HR. LR vocalised more in the aversive band (22 Hz) during test session, and had higher serum levels of corticosterone and higher GABA levels in BLA. HR showed also an increase in c-Fos activity and CRF related immunostaining in BLA. We found that HR rats showed a significant decrease in the conditioned fear response over the course of two extinction sessions. Upon re-testing (24 h after the conditioned fear re-training), the fear-controlled freezing behaviour of HR rats partially returned at levels below the pre-extinction value. The behaviour of the LR group remained unchanged at each stage of the experiment. The re-exposure to conditioned fear on re-test activates the prefrontal cortex and limbic areas (increased expression of c-Fos, glucocorticoid receptor, alpha-2 subunit of GABA-A receptor, gephyrin and NR2B subunit of NMDA receptor) in HR rats.
The aim of the study was to examine the neurochemical background of differences in the individual responses to conditioned aversive stimuli, using the strength of a rat conditioned freezing response (the contextual fear test), as a discriminating variable. It was shown that high responder (HR), i.e. rats with duration of a freezing response one standard error, or more, above the mean value, had a higher c-Fos activity in the FrA and PrL prefrontal cortical areas, and stronger 5HT immunostaining in the FrA. However, these animals had lower CRF immunostaining in the same cortical areas in comparison with low responder (LR), i.e. rats with the duration of a freezing response one standard error, or more, below the mean value. The LR group vocalized more during test session in the aversive band, and had higher serum levels of corticosterone, examined 10 min after test session. It was shown that different natural patterns of responding to conditioned aversive stimuli are associated with different expression of CRF and serotonergic- innervation of prefrontal cortical areas.
It might seem obvious that pain sensitivity would predict individual, inborn susceptibilities to aversive stimuli and the strength of fear-conditioned responses. Such relationships are based on the assumption that there is a close association between fear-evoked behavioral reactions and the responses to painful, aversive stimuli. However, this problem has not been systematically studied. To this end, we investigated the relationship between pain sensitivity in two pain tests (the ‘tail-flick’ and ‘flinch-jump’ tests) and a conditioned, fear-evoked, freezing response in rats. The results show that there was no correlation between: (1) the conditioned (associative) and the novelty-evoked (non-specific stress-related) fear response and (2) individual differences in pain threshold and fear responses. Furthermore, factor analysis did not group freezing in the conditioned fear test, individual footshock sensibility, or ‘tail-flick’ reaction to painful stimuli together. These results indicate that pain sensitivity and conditioned emotional responses to pain are not directly correlated.
We examined the effects of midazolam and D-cycloserine on the release of GABA in the basolateral amygdala (BLA) of high (HR) and low (LR) anxiety rats during extinction session of a conditioned fear test. HR and LR anxiety rats were selected according to their behaviour in the contextual fear test (i.e., the duration of a freezing response was used as a discriminating variable). Administration of D-cycloserine (15 mg/kg, i.p.), significantly enhanced the inhibition of an aversive context-induced freezing response observed during the extinction session in HR and LR rats 7 days after contextual fear test. It was also found that midazolam and D-cycloserine facilitated the GABA release in HR rats under the influence of conditioned fear. HR rats pretreated with saline had higher expression of alpha-2 subunits of GABA-A receptor in BLA compared to LR rats. Administration of D-cycloserine and midazolam increased the expression of alpha-2 subunits in the BLA of HR rats compared to HR rats pretreated with saline, and to drug administered LR rats. Moreover, D-cycloserine enhanced the expression of alpha-2 subunits and gephyrin in the prefrontal cortex of HR rats. Together, these findings suggest that animals that are more vulnerable to stress differ in the expression of alpha-2 subunits of GABA-A receptor in amygdala and prefrontal cortex which is involved in the control of emotional behaviour. These animals might have innate deficits in forebrain systems that control the activity of the limbic structures including GABAergic innervation of the BLA. These data indicate also possible neurochemical mechanisms for individual differences observed in response to anxiolytic drugs among patients with anxiety disorders. The current results also suggest that activation of glutamatergic function can initiate neural changes which improve fear extinction and provide preclinical evidence in support of the clinical use of NMDA(R) modulators for the treatment of anxiety-related disorders.
Rats were given 14 “daily” (6 doses/week) s.c. doses of 0.9% NaCl (Sal), morphine (Mor), or methadone (Met), then left untreated for 14 days, and fi nally challenged with Mor, except that half each of the Sal groups were given Sal instead. All the rats were then tested for open fi eld behavior, and were sacrifi ced 2 h post-challenge. Striatal (CPu), accumbal (Acb), sensorimotor cortex (S1) and prefrontal cortex contents of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3 MT, assessed only in the CPu and Acb) were measured by HPLC. Analysis of total distance covered by the rats showed Mor and Met-induced sensitization to Mor in the WHP rats only, whereas the distance covered in the central part of the test arena was signifi cantly affected (increased) only by the Mor pretreatment and only in the WHP rats. There was no signifi cant between-line or treatment-related difference in DA content, and no signifi cant between-line difference in baseline DA metabolite contents, except that S1 HVA content was signifi cantly higher in drug-naive WLP rats than in drug-naive WHP rats. HVA and – to a lesser extent – DOPAC contents were, in general, higher in the Mor-challenged rats than in the respective Sal-treated controls, but the differences were more pronounced in the WLP rats than in their WHP counterparts, whereas the opioid -pretreated WHP rats showed higher CPu and Acb 3 MT contents than their WLP counterparts.
In the present study, the c-Fos expression was used to map brain structures recruited during the evolution of seizures after repeated, administration of pentylenetetrazol at the subconvlusive dose (35 mg/kg, i.p.), in rats. It has been found that the earliest expression of cFos, at the stage 1,2 of kindling, appeared in the nucleus accumbensshell, the piriform cortex, the prefrontal cortex and striatum. On the 3rd stage of kindling, the central amygdala nucleus, the entorhinal cortex, and the lateral septal nucleus (LSV), showed an enhanced expression of c-Fos. On the 4th stage of kindling, c-Fos was increased in the basolateral amygdala, and CA1 area of the hippocampus. Finally, c-Fos labelling was enhanced in the dentate gyrus of the hippocampus, only when the stage 5 of kindling, i.e. the clonic-tonic convulsions, appeared. The most potent changes in c-Fos (in a descending order) were shown in the dentate gyrus, piriform cortex, CA1 area, the LSV, basolateral amygdala, central amygdala nuclei, and prefrontal cortex. It appeared, that there are important similarities in the structures recruited at the beginning and at the end of electrically and chemically-induced kindling, i.e. the piriform cortex and the hippocampal dentate gyrus, respectively. On the other hand, the differences gradually disappear at the later stages of kindling, followed by the symmetrical propagation of epileptic activity from the limbic system to the neocortex, during the generalized seizures.
Despite its widespread use, the mechanisms of valproic acid (VPA) action are not fully understood. In the current study, we have examined the peripheral and central effects of VPA administration on the metabolic pathway of tryptophan (TRP): concentration of its centrally active metabolites, kynurenine (KYN) and kynurenic acid (KYNA). Moreover, the role of a displacement of TRP from serum albumin binding sites, and changes in the peripheral and central concentration of amino acid including glutamate (GLU), GABA, alanine (ALA), glutamine (GLTM), glycine (GLY), aspartate (ASP), were also studied. We found that VPA administration produced a progressive and strong increase in the central concentration of KYNA, KYN and TRP. Simultaneously, TRP concentration in plasma declined while the peripheral increase of KYNA in plasma was weaker and occurred earlier than in the hippocampus. We also observed that administration of ibuprofen to rats, a prototypic drug used to study drug binding to serum albumin, strongly increased the amount of a free serum and hippocampal TRP concentration, to a degree similar to the effect of VPA. Moreover, we found that the most pronounced changes in the concentration of amino acids caused by administration of VPA include an increase of GLU and a decrease of ALA in the plasma as well as a decrease of ASP and an increase of GABA in the brain. The factor analysis revealed that the changes in the concentrations of TRP, determined both in the plasma and in the hippocampus grouped strongly with the changes in the plasma concentrations of GLU and the central concentration of ASP. Our results showed that administration of VPA strongly modifies the activity of the kynurenine pathway with significant changes in TRP, KYN and KYNA levels in the CNS. The reason for this may be a strong VPA-induced displacement of TRP from its binding sites to plasma albumin. It appears also that the changes in TRP evoked by VPA administration due to competition for transport into the brain, may result in a shift in the central and peripheral balance between branched-chain (BCAA) and aromatic amino acids (AAA). This may lead to a decrease in BCAA transport to the brain, leading to a deficit of BCAAs as a donor of amino groups to the process of GLU resynthesis from pyruvate. Changes in the BCAA/ AAA ratio, arising as a consequence of changes in the TRP level, could explain an observed increase in the plasma concentrations of GLU and a decrease in the ASP concentrations in the brain that occurred after administration of VPA. In sum, given the neuroprotective role of KYNA, the current study suggests that stimulation of the kynurenine pathway may also apply to the central and peripheral concentration of amino acids. The modification of the activity of the kynurenine pathway may at least in part contribute to the related antiepileptic and neuroprotective mechanisms of VPA action.
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