Enver Melik

Effects of ketamine on different types of anxiety/fear and related memory in rats with lesions of the median raphe nucleus

The aim of the present study was to determine the involvement of the median raphe serotonergic system in the effects of ketamine on anxiety behaviours and related memory. The effects of ketamine pretreatment (3 and 10 mg/kg, i.p.) on three types of fear-motivated behaviours, unconditioned one-way escape, conditioned avoidance and freezing were tested. Experiments were performed with the inhibitory avoidance apparatus in rats with ibotenic acid lesions of the median raphe nucleus. It was found that 10 mg/kg ketamine had an anxiogenic-like effect on one-way escape type of fear and anxiolytic-like effect on conditioned freezing-related fear; these effects were unaffected by median raphe lesions. Both ketamine doses impaired freezing-related fear memory. Ketamine (10 mg/kg) also produced an anxiolytic-like effect on avoidance type of fear and impaired avoidance memory. The median raphe lesions attenuated the anxiolytic action of the drug on the avoidance type of fear and prevented ketamine-induced avoidance memory impairment. These results suggest that the anxiolytic-like effect of ketamine on avoidance-type fear is mediated through the median raphe serotonergic system.

Median raphe nucleus mediates forming long-term but not short-term contextual fear conditioning in rats

The brain serotonin is involved in mediation of emotional behaviour including anxiety and related fear conditioning. It is known that the median raphe nucleus (MRN) is the origin of a serotonergic pathway and mainly innervates septo-hippocampal formation which plays an important role in emotional cognition. However, its regulatory role in different types of fear conditioning is still unclear. In the present study, the animals underwent ibotenic acid or sham lesions of the median raphe nucleus and the effects of MRN lesions on immediate and delayed fear conditioning to multiple contextual cues were studied. Freezing behaviour served as a measure of contextual fear. Sham-lesioned animals showed reliable conditional freezing when observed immediately following foot-shock (1.0 mA) for 3-min test and 48 h after the shock for 12-min test. Rats with MRN lesions displayed robust freezing behaviour immediately after the shock, even though they showed a marked deficit in freezing 48 h following the shock. These findings indicate that the MRN-serotonergic septo-hippocampal pathway is involved in the regulation of anxiety related to fear conditioning triggered by contextual cues, suggesting that short-term contextual fear is independent on the MRN while long-term contextual fear depends on the MRN.

Hypofunction of the dorsal hippocampal NMDA receptors impairs retrieval of memory to partially presented foreground context in a single-trial fear conditioning in rats

In the present study, the effects of N-methyl-D-aspartate (NMDA) receptor antagonist, D,L-2-amino-5-phosphonopentanoic acid (AP5) bilaterally infused into the dorsal hippocampus (2.0 microl /5 microg), on the retrieval of fear memory to partial and whole foreground cues were evaluated by using a step-through passive avoidance and Pavlovian fear conditioning. In the both conditioning tasks, following a 30-s preshock exposure period to the shock-associated context, rats received a single shock in a foreground manner for fear memory exhibition by freezing. Rats with AP5 infusion 5 min before the retrieval tests showed profound freezing deficits either immediately or 48 h after the shock in the testing section of the passive avoidance chamber where foreground cues was partially presented. In the Pavlovian conditioning chamber where fear conditioning was tested in the whole of the context that was explicitly paired with the shock, AP5 rats in all infusion schedules exhibited robust freezing responses. These results showed that hypofunction of the hippocampal NMDA receptors impaired the retrieval of fear memory to partial, and not whole, foreground cues. This suggests that NMDA receptors of the hippocampus are involved in the formation of background context representations about foreground events when there is a deficit in perceiving certain sensory properties of the foreground retrieval cues.

Effect of ketamine on exploratory behaviour in BALB/C and C57BL/6 mice

In this study, we evaluated the effect of ketamine on exploratory locomotion behaviours in the Balb/c and C57BL/6 strains of mice, which differ in their locomotion behaviours. Intraperitoneal administration of ketamine at three different doses (1, 5 or 10 mg/kg, 0.1 ml/10 gr body weight) was performed on adult male Balb/c and C57BL/6 mice. The same volume of saline was applied to the control group. The open-field and elevated plus maze apparatus were used to evaluate exploratory locomotion. In the open-field test, Balb/c mice less spend time in the centre of the field and was decreased locomotor activity compared to C57BL/6 mice (p<0.01). Ketamine treatment of Balb/c mice at 10 mg/kg dose caused an increase in locomotor activity and an increase in the amount of time spent in the centre in the open-field test, compared to the control group (p<0.05). In C57BL/6 mice, ketamine treatment (1 and 10 mg/kg) decreased locomotor activity (p<0.05). In C57BL/6 mice, the three different doses of ketamine application each caused a decrease in the frequency of centre crossing (p<0.001) and the spent time in the centre (p<0.05). In the elevated plus maze, the number of open-arm entries, the percentage of open-arm time and total arm entries were decreased in Balb/c mice compared to C57BL/6 mice (p<0.001). Ketamine treatment of Balb/c mice at 10 mg/kg dose caused an increase in the open-arm activity (p<0.001). Ketamine application (10 mg/kg) decreased the open-arm activity in C57BL/6 mice (p<0.05). A subanaesthetic dose of ketamine increased exploratory locomotion in Balb/c mice. In contrast, a subanaesthetic dose of ketamine decreased exploratory locomotion in C57BL/6 mice. In conclusion, hereditary factors may play an important role in ketamine-induced responses.

The investigation of neonatal MK-801 administration and physical environmental enrichment on emotional and cognitive functions in adult Balb/c mice.

N-methyl-D-aspartate (NMDA) receptors play an important role in brain maturation and developmental processes. It is known that growing up in an enriched environment has effects on emotional and cognitive performance. In our study, we evaluated the effects of physically enriched environment on the emotional and cognitive functions of the adult brain in the setting of previous NMDA receptor hypoactivity during the critical developmental period of the nervous system. In this study, NMDA receptor blockade was induced 5-10 days postnatally (PD5-10) using MK-801 in mice Balb/c (twice a day 0.25 mg/kg, for 5 days, intraperitoneal). MK-801 was given to developing mice living in a standard (SE) and an enrichment environment (EE) and once the animals reached adulthood, emotional behaviors were evaluated using an open field test (OF) and an elevated plus maze (EPM) test whereas cognitive processes were evaluated using the Morris water-maze (MWM). The EE group showed decreased locomotor activity (p<0.05) in the OF and increased exploratory behaviour (p<0.01) and decreased fear of heights/anxiety-like behaviour (p<0.05) in the EPM test. The EE had positive effects on spatial learning in the MWM (p<0.05). Blockade of the NMDA receptor increased the fear of height (p<0.05), decreased exploratory behaviour and locomotor activity (p<0.001). Also, it led to decreased spatial learning (p<0.05). The decreases in spatial learning and exploratory behaviours and the increase in fear of heights/anxiety-like behaviour with NMDA receptor blockade was not reversed by EE. NMDA receptor blockade during the critical period of development led to deterioration in the emotional and cognitive processes during adulthood. An enriched environmental did not reverse the deleterious effects of the NMDA receptor blockade on emotional and cognitive functions.

Effects of excitotoxic median raphe lesion on working memory deficits produced by the dorsal hippocampal muscarinic receptor blockade in the inhibitory avoidance in rats.

The experiments investigated the interactions between median raphe nucleus (MRN) serotonergic and septo-hippocampal muscarinic cholinergic systems in the modulation of forming and storing performances of working memory. Rats with ibotenic acid-induced MRN-lesion bilaterally received scopolamine (2-4 microg/each side) infusion into the dentate gyrus of the dorsal hippocampus and were tested in a single trial step-through inhibitory avoidance. Initial preference to the dark compartment (escape latency) was taken as the measure of non-mnemonic behaviours and response latency to enter the dark compartment immediately after the foot-shock was used to measure working memory. The high-dose scopolamine infusion 10 min before the training decreased escape latencies in the sham-lesioned rats, whereas had no effect in the MRN-lesioned rats. Although MRN lesion per se did not alter response latency, it alleviated pre-training scopolamine-induced decrease, but aggravated post-training scopolamine-induced reduction in this parameter. These results suggest that the antagonistic interactive processes between MRN-serotonergic and hippocampal cholinergic systems modulate non-mnemonic component of working memory formation, whereas the storing performance of working memory is modulated by the synergistic interactions between these systems in the hippocampus, mainly in the dentate gyrus.

Enriched environment has limited capacity for the correction of hippocampal memory-dependent schizoid behaviors in rats with early postnatal NMDAR dysfunction.

Pre‐ and early postnatal stress can cause dysfunction of the N‐methyl‐d‐aspartate receptor (NMDAR) and thereby promote the development of hippocampus memory‐dependent schizoid abnormalities of navigation in space, time, and knowledge. An enriched environment improves mental abilities in humans and animals. Whether an enriched environment can prevent the development of schizoid symptoms induced by neonatal NMDAR dysfunction was the central question of our paper. The experimental animals were Wistar rats. Early postnatal NMDAR dysfunction was created by systemic treatment of rat pups with the NMDAR antagonist MK‐801 at PD10–20 days. During the development period (PD21–90 days), the rats were reared in cognitively and physically enriched cages. Adult age rats were tested on navigation based on pattern separation and episodic memory in the open field and on auto‐hetero‐associations based on episodic and semantic memory in a step‐through passive avoidance task. The results showed that postnatal NMDAR antagonism caused abnormal behaviors in both tests. An enriched environment prevented deficits in the development of navigation in space based on pattern separation and hetero‐associations based on semantic memory. However, an enriched environment was unable to rescue navigation in space and auto‐associations based on episodic memory. These data may contribute to the understanding that an enriched environment has a limited capacity for therapeutic interventions in protecting the development of schizoid syndromes in children and adolescents.

EFFECTS OF EXCITOTOXIC MEDIAN RAPHE LESIONS ON SCOPOLAMINE-INDUCED WORKING MEMORY DEFICITS IN INHIBITORY AVOIDANCE

The aim of the present study was to investigate the effects of excitotoxic damage of the serotonergic cell bodies in the median raphe nucleus (MRN) on the scopolamine-induced working memory deficits in a single-trial light/dark inhibitory avoidance task. Rats were given 1 mg/kg of scopolamine hydrobromide (intraperitonal, IP) or saline before the inhibitory avoidance training, in which initial preference to the dark compartment (escape latency) was used to measure nonmnemonic behaviors, and response latency to enter the dark compartment immediately after the shock was used to measure working memory. It was found that scopolamine significantly reduced escape latencies in sham-lesioned rats, whereas it had no effect in the rats with MRN lesions. Although MRN lesion per se did not alter response latency, it prevented scopolamine-induced decrease in this parameter. These results suggest that the antagonistic interactive processes between serotonergic projections of the MRN and the muscarinic cholinergic system modulate nonmnemonic attentional component of working memory formation in the inhibitory avoidance.

The Effects of Maternal Deprivation on the Hippocampal Structure in Adult Rats

OBJECTIVES To examine the ultrastructural effects of maternal deprivation during developmental periods of limbic-hypothalamo-pituitary-adrenal system on hippocampal dendritic structures in adult rats. METHODS The experiments were carried out with male and female Wistar rats in our department. The rats were mated and, after birth, the pups were divided into four groups. The first group (control group) pups remained undisturbed with their dam until postweaning day 22. Maternal deprived groups were separated from their dams for 24 hours at postnatal day 4, 9 and 18. The subjects were provided with food and water ad libitum until 3-months-of-age. At the third month, the rats were transcardially perfused, samples were taken from CA1 and CA3 regions of the hippocampus. Tissues were prepared for electron microscopy. RESULTS When the data were analyzed, there were no differences between male and female rats in both ultrastructure and semiquantitative analysis of axodendritic synapses. The ultrastructure of Group 1 was seen as normal while in the second Group some neurons nuclear envelope made deep invagination into the nucleus. Additionally, axodendritic synapses were found normal. In Group 3, micrographs and axodendritic synapses were showed normal structure. However, in Group 4 in some neurons invaginations were seen similar to Group 2. Axodendritic synapses were found to be normal. CONCLUSION These experiments establish that MD in rats produces slight ultrastructural changes and decreases the number of synapses in CA1 and CA3 subregions of the hippocampus.

A comparative study on the influence of estrous cycle on cognitive and coping behaviors in rats

Materials and methods The present study was investigate the influence of estrous cycle in adult female Wistar rats on the responses to emergency novelty by using an open field (OF) and on the recognition of fearful partial or whole cues presented during testing in the passive avoidance (PA) apparatus. In conditioning task, rats recieved a single shock (1 mA) following a 30-s preshock exposure period to the shock associated context of the PA. Estrous cycle phases were determined by vaginal lavage.