Hippocampal nicotinic cholinergic mechanisms mediate spontaneous alternation and fear during ontogenesis but not later in the rat

Postnatal exposure to fluoxetine led to cognitive-emotional alterations and decreased parvalbumin positive neurons in the hippocampus of juvenile Wistar rats

The exposure to selective serotonin reuptake inhibitors (SSRIs) during development results in behavioural impairment in adulthood in humans and animal models. Indeed, serotonergic overexpression in early life leads to structural and functional changes in brain circuits that control cognition and emotion. However, the effects of developmental exposure to these substances on the behaviour of adolescent rats are conflicting and remain poorly characterised. We performed a behavioural screening to investigate the effects of postnatal exposure to fluoxetine on memory and behaviours related to anxiety, anhedonia, and depression, as well we evaluate the parvalbumin expression in hippocampus of juvenile (~PND45) female and male rats. Fluoxetine (daily 20 mg/kg s.c. injections from PND7-PND21)- or vehicle-treated adolescent rats went through several behavioural tasks (from PND 38 to PND52) and were subject to transcardial perfusion and brain removal for immunohistochemical analysis (PND53). We found that postnatal exposure to fluoxetine increased anxiety- and depression-like behaviours in the open field and sucrose preference and forced swimming tests, respectively. In addition, this treatment induced working memory and short-term (but not long-term) recognition memory impairments, and reduced parvalbumin-positive interneurons in the hippocampus. In addition, the results revealed developmental sex-dependent effects of fluoxetine postnatal treatment on adolescent rats' behaviour. These outcomes indicate that affective disorders and mnemonic alterations caused by SSRIs perinatal exposure can be present at adolescence.

Beta-amyloid pathology in the entorhinal cortex of rats induces memory deficits: implications for Alzheimer's disease

Alzheimer's disease is characterized by the presence of senile plaques in the brain, composed mainly of aggregated amyloid-beta peptide (Abeta), which plays a central role in the pathogenesis of Alzheimer's disease and is a potential target for therapeutic intervention. Amyloid plaques occur in an increasing number of brain structures during the progression of the disease, with a heavy load in regions of the temporal cortex in the early phases. Here, we investigated the cognitive deficits specifically associated with amyloid pathology in the entorhinal cortex. The amyloid peptide Abeta(1-42) was injected bilaterally into the entorhinal cortex of rats and behavioral performance was assessed between 10 and 17 days after injection. We found that parameters of motor behavior in an open-field as well as spatial working memory tested in an alternation task were normal. In contrast, compared with naive rats or control rats injected with saline, rats injected with Abeta(1-42) showed impaired recognition memory in an object recognition task and delayed acquisition in a spatial reference memory task in a water-maze, despite improved performance with training in this task and normal spatial memory in a probe test given 24 h after training. This profile of behavioral deficits after injection of Abeta(1-42) into the entorhinal cortex was similar to that observed in another group of rats injected with the excitotoxic drug, N-methyl-d-aspartate. Immunohistochemical analysis after behavioral testing revealed that Abeta(1-42) injection induced a reactive astroglial response and plaque-like deposits in the entorhinal cortex. These results show that experimentally-induced amyloid pathology in the entorhinal cortex induces selective cognitive deficits, resembling those observed in early phases of Alzheimer's disease. Therefore, injection of protofibrillar-fibrillar Abeta(1-42) into the entorhinal cortex constitutes a promising animal model for investigating selective aspects of Alzheimer's disease and for screening drug candidates designed against Abeta pathology.

Sex differences on spontaneous alternation in prepubertal rats: implications for an animal model of obsessive-compulsive disorder

Sex differences in the prevalence of obsessive-compulsive disorder (OCD) in prepubertal children have been described. Deficits on spontaneous alternation behavior (SAB) have been proposed as an animal model of OCD.

OBJECTIVES

To explore possible sex differences in the ontogeny of SAB and in the effect of the 5-HT1A agonist, 8-OH-hydroxy-2 (di-n-propylamino)-tetralin (8-OH-DPAT) in an animal model of OCD.

METHODS

The ontogeny of SAB and the perseveration produced by 8-OH-DPAT were compared between male and female prepubertal rats.

RESULTS

Males alternated their arm choose from postnatal day 32 onwards, while females perseverated in the chosen arm until postnatal day 38. The mean number of repetitive choices remained close to 1 in males from postnatal day 23 onwards, but females showed a mean number of repetitive choices higher than 1.5 until the end of the test. The 8-OH-DPAT (0.125, 0.5 and 2.0 mg/kg, 15 min) produced perseveration in males but not in females.

CONCLUSIONS

These data show important sex differences in the ontogeny of SAB and the effect of 8-OH-DPAT in a model of OCD. Such differences could be relevant for the sex differences in the prevalence of childhood OCD.

Behavioral patterns under cholinergic control during development: lessons learned from the selective immunotoxin 192 IgG saporin

The immunotoxin 192 IgG saporin (192 IgG-sap) offers a valuable tool to investigate the role of the developing basal forebrain cholinergic system in modulating behavioral functions in developing, as well as adult rats. After neonatal 192 IgG-sap lesions, rats display reduced ultrasonic vocalizations as neonates, deficits in passive avoidance learning as juveniles, and altered reactions to spatial novelty as adults. These data suggest that neonatal cholinergic depletion affects cognitive performance in juvenile and adult rats. Additionally, neonatal cholinergic depletion alters ultrasonic vocalizations, which could then alter establishing normal mother-infant relationships, and thus compound the pup's cognitive deficits. These findings underscore the importance of assessing behavior during ontogeny, as well as in adulthood.

Unpacking the cognitive map: the parallel map theory of hippocampal function

In the parallel map theory, the hippocampus encodes space with 2 mapping systems. The bearing map is constructed primarily in the dentate gyrus from directional cues such as stimulus gradients. The sketch map is constructed within the hippocampus proper from positional cues. The integrated map emerges when data from the bearing and sketch maps are combined. Because the component maps work in parallel, the impairment of one can reveal residual learning by the other. Such parallel function may explain paradoxes of spatial learning, such as learning after partial hippocampal lesions, taxonomic and sex differences in spatial learning, and the function of hippocampal neurogenesis. By integrating evidence from physiology to phylogeny, the parallel map theory offers a unified explanation for hippocampal function.

Cultured rat hippocampal neural progenitors generate spontaneously active neural networks

We previously demonstrated that the neural cell adhesion molecule (N-CAM) inhibited the proliferation of cultured rat hippocampal progenitor cells and increased the number of neurons generated. We demonstrate here that the continued presence of fibroblast growth factor 2 along with N-CAM or brain-derived neurotrophic factor over 12 days of culture greatly increased the number of both progenitors and neurons. These progenitor-derived neurons expressed neurotransmitters, neurotransmitter receptors, and synaptic proteins in vitro consistent with those expressed in the mature hippocampus. Progenitor cells cultured on microelectrode plates formed elaborate neural networks that exhibited spontaneously generated action potentials after 21 days. This activity was observed only in cultures grown in the presence of fibroblast growth factor 2 and either N-CAM or brain-derived neurotrophic factor. Analysis of neuronal activity after various pharmacological treatments indicated that the networks formed functional GABAergic and glutamatergic synapses. We conclude that mitogenic growth factors can synergize with N-CAM or neurotrophins to generate spontaneously active neural networks from neural progenitors.

Serotonin involvement in the spontaneous alternation ability: a behavioral study in tryptophan-restricted rats

Spontaneous alternation (SA) is controlled by septal cholinergic terminals in the hippocampus. Serotoninergic terminals end on cholinergic nerve endings in the hippocampus, and their possible role in SA was investigated in rats fed with a tryptophan-deficient diet, from weaning to 60 days of age. A T-maze was used for the test. At the age of 40 days, an increase in SA occurred in the tryptophan deficient rats, although this effect disappeared by 60 days of age. A modulatory role of serotonin in the psychoneural control of SA is suggested, and it may be through presynaptic inhibition of hippocampal cholinergic terminals.

Age-dependent effects of hippocampal muscarinic receptor blockade on memory-based learning in the developing rat

The effects of ventral intrahippocampal injections of atropine sulfate on patterned single alternation (PSA), a discrimination task that requires intact short-to-intermediate-term memory, were examined in the developing rat at 16-17 and 28-32 days of age. Atropine treatment disrupted simple acquisition in some 16- to 17-day-old pups by interfering with approach to the goal, but did not eliminate PSA at either 8- or 15-s intertrial intervals when approach was normal. In the older rats, atropine treatment delayed the onset and reduced the magnitude of PSA, indicating a reduced memory-based discrimination. These results provide additional support for an increasing role of muscarinic receptors in learning and memory as this system matures in the developing rat, and suggest different mechanisms for PSA at the two ages.

Behavioral and biochemical effects of early postnatal cholinergic lesion in the hippocampus

The effects of early postnatal (PD 8) intracerebroventricular injection of ethylcholine mustard aziridinium ion (AF64A) on development of open-field and cognitive behaviors and cholinergic markers in several brain areas were examined in the rat. The cholinotoxin was bilaterally administered in a dose range of 0.25 to 2.0 nmol. In the open-field tests, the cholinergic lesion caused a dose-dependent increase in activity at 20 days of age, while it resulted in lengthened latency to initiate exploration and decreased rearing activity at adulthood. Hole-board spatial learning was severely inhibited in adult age. The biochemical activity of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) in the hippocampus was markedly decreased in a dose-dependent manner, but was unchanged in the neocortex and striatum. Histochemical staining of AChE-positive fibers revealed a severe cholinergic denervation of the granular and pyramidal cell layers of the hippocampus. The results showed that a selective cholinergic deafferentation of the hippocampus at a critical stage of development leads to long-lasting abnormal open-field and spatial learning behaviors.

Spatial and non-spatial spontaneous alternation and hippocampal mossy fibre distribution in nine inbred mouse strains

Ten male mice from each one of nine inbred strains were tested for spontaneous alternation in a T-maze, which was placed in a spatially richly structured room. Each test consisted of two trials, the first choice to be made was forced, the second one free. By turning the maze 180 degrees between choices during 8 of the 16 tests, 3 variables could be measured: total alternation, spatial alternation, and non-spatial alternation. After Timm's staining, the sizes of the intra- and infrapyramidal mossy fibre terminal fields (iip-MF) were measured. Significant strain differences were found for all variables, but none of the behavioural variables correlated with hippocampal variation. These results disagree with earlier experiments, where large correlations between the iip-MF and spatial working memory were found. Some hypotheses to explain this discrepancy are presented.