Age-related changes in medial septal cholinergic and GABAergic projection neurons and hippocampal neurotransmitter receptors: relationship with memory impairment

The hippocampus, which provides cognitive functions, has been shown to become highly vulnerable during aging. One important modulator of the hippocampal neural network is the medial septum (MS). The present study attempts to determine how age-related mnemonic dysfunction is associated with neurochemical changes in the septohippocampal (SH) system, using behavioral and immunochemical experiments performed on young-adult, middle-aged and aged rats. According to these behavioral results, the aged and around 52.8% of middle-aged rats (within the "middle-aged-impaired" sub-group) showed both impaired spatial reference memory in the Morris water maze and habituation in the open field. Immunohistochemical studies revealed a significant decrease in the number of MS choline acetyltransferase immunoreactive cells in the aged and all middle-aged rats, in comparison to the young; however the number of gamma-aminobutyric acid-ergic (GABAergic) parvalbumin immunoreactive cells was higher in middle-aged-impaired and older rats compared to young and middle-aged-unimpaired rats. Western Blot analysis moreover showed a decrease in the level of expression of cholinergic, GABAergic and glutamatergic receptors in the hippocampus of middle-aged-impaired and aged rats in contrast to middle-aged-unimpaired and young rats. The present results demonstrate for the first time that a decrease in the expression level of hippocampal receptors in naturally aged rats with impaired cognitive abilities occurs in parallel with an increase in the number of GABAergic neurons in the MS, and it highlights the particular importance of inhibitory signaling in the SH network for memory function.

Modulation of spatial memory and expression of hippocampal neurotransmitter receptors by selective lesion of medial septal cholinergic and GABAergic neurons

The medial septum (MS) is an important modulator of hippocampal function. The degree of damage in which the particular set of septo-hippocampal projections contributes to the deficits of spatial memory with concomitant changes of hippocampal receptors expression has not been studied till present. Therefore, we investigated spatial memory and the expression level of cholinergic (α7 nACh and M1), GABAergic (α1 subunit of GABAA) and glutamatergic (NR2B subunit of NMDA and GluR 1 subunit of AMPA) receptors in the hippocampus following selective lesions of cholinergic and GABAergic septo-hippocampal projection. Learning process and long-term spatial memory were assessed using a Morris water maze. The obtained results revealed that in contrast to cholinergic lesions, rats with MS GABAergic lesions exhibit a retention deficit in 3 days after training. Western blot analyses revealed the MS cholinergic lesions have significant effect on the expression level of the M1 mACh receptors, while MS GABAergic lesions induce dramatic modulations of hippocampal glutamatergic, cholinergic and GABAergic receptors expression. These results for the first time demonstrated that selective lesions of MS cholinergic and GABAergic neurons differentially affect long-term spatial memory and the memory deficit after MS GABAergic lesion is paralleled with significant changes of hippocampal glutamate, GABA and acetylcholine receptors expression.

Exploratory behavior and recognition memory in medial septal electrolytic, neuro- and immunotoxic lesioned rats

In the present study, the effect of the medial septal (MS) lesions on exploratory activity in the open field and the spatial and object recognition memory has been investigated. This experiment compares three types of MS lesions: electrolytic lesions that destroy cells and fibers of passage, neurotoxic - ibotenic acid lesions that spare fibers of passage but predominantly affect the septal noncholinergic neurons, and immunotoxin - 192 IgG-saporin infusions that only eliminate cholinergic neurons. The main results are: the MS electrolytic lesioned rats were impaired in habituating to the environment in the repeated spatial environment, but rats with immuno- or neurotoxic lesions of the MS did not differ from control ones; the MS electrolytic and ibotenic acid lesioned rats showed an increase in their exploratory activity to the objects and were impaired in habituating to the objects in the repeated spatial environment; rats with immunolesions of the MS did not differ from control rats; electrolytic lesions of the MS disrupt spatial recognition memory; rats with immuno- or neurotoxic lesions of the MS were normal in detecting spatial novelty; all of the MS-lesioned and control rats clearly reacted to the object novelty by exploring the new object more than familiar ones. Results observed across lesion techniques indicate that: (i) the deficits after nonselective damage of MS are limited to a subset of cognitive processes dependent on the hippocampus, (ii) MS is substantial for spatial, but not for object recognition memory - the object recognition memory can be supported outside the septohippocampal system; (iii) the selective loss of septohippocampal cholinergic or noncholinergic projections does not disrupt the function of the hippocampus to a sufficient extent to impair spatial recognition memory; (iv) there is dissociation between the two major components (cholinergic and noncholinergic) of the septohippocampal pathway in exploratory behavior assessed in the open field - the memory exhibited by decrements in exploration of repeated object presentations is affected by either electrolytic or ibotenic lesions, but not saporin.

Effects of chronic memantine treatment on hippocampal extracellular glutamate and GABA levels during spatial alternation testing

These experiments examined the release of glutamate (Glu) and GABA in the hippocampus of memantine (2,5mg/kg, i.p. for four weeks) or saline treated rats prior to, during, and after spontaneous alternation test. Glu and GABA release during the 10 min samples taken at the time of the behavioral testing of memantine or saline treated animals were not different from those seen immediately before and after testing. Similarly, the alternation scores were not significantly different between groups. We found increase in KCl-stimulated glutamate and GABA release in the hippocampus of memantine treated rat compared to the saline treated rat. This difference in KCl response between memantine treated and control rat was statistically significant (p<0,05). Our evaluation of memantine reveals that changes in KCl-stimulated Glu and GABA release after chronic memantine treatment did not affect working memory in adult rats assessed in spontaneous alternation task.

[Regularities of the egocentric spatial memory development in children aged 24-60 months]

In order to assess development of the egocentric system of the spatial short-term memory in children (n=66) of different ages (24-60 months) the Inverted Delayed Reaction test has been used. It was found that in the children aged 24-36 months regularities of performance of the Inverted Delayed Reaction test significantly differ in conditions of different loads onto the mechanisms of dead reckoning; the children aged 36-60 months do not show sensitivity to different loads. In children aged 42+/-4 months functional elimination of any of the sensory system (visual, kinesthetic, vestibular) during rotation significantly deteriorated results of the Inverted Delayed Reaction test performance, while in children aged 60+/-4 months number of correct responses decreased if two or three sensory systems were eliminated simultaneously. The data obtained permit to conclude that the Inverted Delayed Reaction test is sufficiently sensitive for evaluation development of the egocentric spatial memory system in children and that formation of the dead reckoning mechanisms starts in an age of 24 months and in the period of 24-60 months its further upgrading does occur.

Effects of the selective lesions of cholinergic septohippocampal neurons on different forms of memory and learning process

The aim of this study was to investigate the modulation of the hippocampal function by the cholinergic cells of the septum and the role of the septo-hippocampal cholinergic system in learning and memory. Immunotoxin 192 IgG-saporin was used to produce selective lesions of cholinergic septohippocampal neurons. Hippocampal AChE was used as a quantitative measure of lesion extent. A total of 16 male outbred albino rats were used in the present study to investigate the ability of sham-operated and medial septal (MS) immunotoxin lesioned rats to learn the location of a visible, as well as submerged platform in a water maze. The rats' responses on the competition test were classified as either cue or place, based on the swim path for those trials. Examination of the AChE stained sections showed that after injections of 192 IgG saporin into the MS, animals exhibited significantly less AChE staining in hippocampus as compared to sections obtained from sham-operated animals. Differences in the platform reaching latency between the animals of different groups for the training trials were not found. Data from both competition trials for each group show that the sham-operated rats in 13 trials out of 16 competition test trial used place strategy and MS-lesioned ones used this strategy in 11 trials. Decreased place-bias in MS-lesioned rats compared to the sham-operated rats was not significant. These findings suggest that the septo-hippocampal cholinergic system is not essential for all types of hippocampal-dependent memory and deficits observed after septal electrolytic lesions cannot be accounted solely to the loss of hippocampal ACh.

[Concerning a conflict nature of the "spatial delayed response" test]

Neuropsychological analysis of rats' performance of the spatial delayed response (SDR) in different testing conditions revealed a conflict nature of the indirect variation of the SDR task. It was found that the execution of the response based on the image short-term memory interferes with the response differentiation acquired during learning the rule of indirect SDR performance, i.e., during acquisition of the spatial discrimination. It is evident that the maximization of conditions, which promote the acquisition of response differentiation (additional training of animals for spatial discrimination), makes it difficult to perform the indirect variation of the SDR task, while the minimization of these conditions facilitates the correct task performance.

[Formation of spatial short-term memory in 18-54-month old children]

The development of the spatial short-term memory (SSTM) in children aged 18-54-months was assessed by performance of the inverted delayed-response task (IDR). A child perceived the localization of a target object in one of the two containers (left or right) situated on a table in front of the child. The distance between the containers was 30 cm. Then the child was escorted to the opposite side of the table so that the target object that was earlier in the left container seemed to be in the right one. After the rotation, in 74% of cases children at the age of 18-24-months failed to localize the object correctly despite the presence of various reference objects in the room. Percent of correct responses increased with age. Children aged 24-54-months performed the IDR better in the presence of reference objects than in their absence. The preschool children at the age of 48-54-months correctly localized the object in 70% of cases even in the absence of the experimental reference objects. The results of this experiment demonstrate the stages of the SSTM development in children.

Significance of emotional excitation in the mechanisms of spatial short-term memory in lower apes

Correlates of variation in the level of emotional tension are studied in long-term experiments on adult monkeys. The simultaneous appearance of the electrodermal response and of a change in the heart rate is shown to reflect more accurately a rise in the level of emotional tension. Continuous recording of autonomic indexes reveals an increase in tension level during the presentation of conditioned signals and the performance of the motor response in the spatial delayed response test program. The emotiogenic structures are thought to influence the specific mechanisms of short-term memory during the presentation of conditioned signals and the performance of food-procuring instrumental behavior.

Characteristics of neuronal activity in prefrontal cortex during performance of spatial delayed reactions in monkeys

Several types of neurons were differentiated on the basis of a study of neuronal activity in various parts of the cortex near the sulcus principalis during the execution of spatial delayed reactions by monkeys. It was established that the different types of neurons are represented in different numbers in different parts of the cortex near the sulcus principalis. The determination of several factors influencing the activity of these neurons and the comparison of data on their quantitative representation in the anterior, middle, and posterior parts of the cortex near the sulcus principalis with the existing behavioral data obtained after local ablations of identical regions of the brain made it possible to postulate that neurons belonging to the different types are involved in the analysis of different processes and represent different functional units.

[Characteristics of the activity of prefrontal cortex neurons during spatially delayed reactions in the monkey]

On the basis of unit activity in the cortex around sulcus principalis during the spatial delayed response, several types of neurons were distinguished in monkeys. The number of different neurons varied in different parts of the cortex. Determination of some factors affecting the activity of these neurons and comparison of data on their quantitative distribution in the anterior, medial and posterior parts of the cortex around sulcus principalis with the available behavioral data obtained with restricted removal of the same brain areas, lend support to the assumption that the neurons attributed to different types are responsible for a variety of processes and represent various functional units.