Transient postnatal thyroxine treatment leads to an increased number of cholinergic neurons in the medial septum and to a higher density of cholinergic fibers in hippocampal CA3 in rats

Changes in acetylcholinesterase, Na+,K+-ATPase, and Mg2+-ATPase activities in the frontal cortex and the hippocampus of hyper- and hypothyroid adult rats

The thyroid hormones (THs) are crucial determinants of normal development and metabolism, especially in the central nervous system. The metabolic rate is known to increase in hyperthyroidism and decrease in hypothyroidism. The aim of this work was to investigate how changes in metabolism induced by THs could affect the activities of acetylcholinesterase (AChE), (Na+,K+)- and Mg2+-adenosinetriphosphatase (ATPase) in the frontal cortex and the hippocampus of adult rats. Hyperthyroidism was induced by subcutaneous administration of thyroxine (25 microg/100 g body weight) once daily for 14 days, and hypothyroidism was induced by oral administration of propylthiouracil (0.05%) for 21 days. All enzyme activities were evaluated spectrophotometrically in the homogenated brain regions of 10 three-animal pools. A region-specific behavior was observed concerning the examined enzyme activities in hyper- and hypothyroidism. In hyperthyroidism, AChE activity was significantly increased only in the hippocampus (+22%), whereas Na+,K+-ATPase activity was significantly decreased in the hyperthyroid rat hippocampus (-47%) and remained unchanged in the frontal cortex. In hypothyroidism, AChE activity was significantly decreased in the frontal cortex (-23%) and increased in the hippocampus (+21%). Na+,K+-ATPase activity was significantly decreased in both the frontal cortex (-35%) and the hippocampus (-43%) of hypothyroid rats. Mg2+-ATPase remained unchanged in the regions of both hyper- and hypothyroid rat brains. Our data revealed that THs affect the examined adult rat brain parameters in a region- and state-specific way. The TH-reduced Na+,K+-ATPase activity may increase the synaptic acetylcholine release and, thus, modulate AChE activity. Moreover, the above TH-induced changes may affect the monoamine neurotransmitter systems in the examined brain regions.

Transient early postnatal corticosterone treatment of rats leads to accelerated aquisition of a spatial radial maze task and morphological changes in the septohippocampal region

In the present study new-born rats were treated with corticosterone (CORT) between postnatal days 1 and 12. At the age of 16-20 weeks, these animals were tested for spatial learning capacity using an eight-arm radial maze. After behavioral testing, density of cholinergic fibers and sizes of the mossy fiber terminal fields in the hippocampus and number of cholinergic and GABAergic neurons in the septal area were quantified. In the radial arm maze CORT-treated animals initially showed better working memory performance than controls. However, control animals showed a significant improvement of spatial working memory in the last trials and reached similar working memory scores as compared to treated animals. At neither day of training differences in reference memory errors were found between groups. In the diagonal band of Broca, both numbers of cholinergic and GABAergic neurons were increased after corticosterone treatment. The fiber systems in hippocampus showed no significant differences between groups. In conclusion, early postnatal stress induced by CORT administration in neonatal rats results in mild, yet significant morphological and behavioral changes in later life.

Early postnatal corticosterone administration regulates neurotrophins and their receptors in septum and hippocampus of the rat

The principal glucocorticoid in rats, corticosterone, interacts with neurons in the limbic system and leads to morphological and behavioral changes. Putative corticosterone-triggered mediators are neurotrophins. In the present study we investigated the effects of early postnatal corticosterone treatment in rats on neurotrophic factors of the nerve growth factor (NGF) family and their receptors. Newborn rats were treated with corticosterone-containing polymers until postnatal day 12. The mRNA and protein levels of the neurotrophins of the NGF family (NGF, BDNF, NT-3 and NT-4/5) and their receptors (trkA, trkB, trkC and p75) were quantified in septum and hippocampus using RT-PCR. In the septal region, we found an unchanged mRNA expression after corticosterone treatment, whereas in the hippocampus there was a general increase in mRNA. Particularly, the gene expression of NGF, NT-3, and the high affinity receptors trkA, trkB and trkC increased significantly. Quantification of the neurotrophin protein levels using an ELISA revealed significant treatment effects for NGF and NT-4/5 in the hippocampus. The present study of corticosterone treatment in young rats demonstrates interactions of steroid hormones with neurotrophic factors and their receptors in the septo-hippocampal system during the first two postnatal weeks.

Effect of early thyroxine treatment on brain-derived neurotrophic factor mRNA expression and protein amount in the rat medial septum/diagonal band of Broca

There is evidence that morphological alterations concerning deficiency or abundance of thyroid hormones (TH) may be mediated by brain-derived neurotrophic factor (BDNF). It has been demonstrated that the mRNA-expression of BDNF is increased after TH-treatment during the first postnatal weeks. After transient treatment of newborn rats with thyroxine mRNA expression, protein concentration and number and size of BDNF-immunopositive neurons were quantified in the medial septum/vertical diagonal Band of Broca (MS/vDB). The number and size of BDNF-immunopositive neurons were estimated in young (P10) and adult (4 months). The amount of mRNA and protein are significantly increased in TH-treated rats at P10 compared to control animals. TH-treated animals showed a significant decrease of BDNF-immunopositive cell numbers in the adulthood. The results demonstrate a correlated increase of BDNF mRNA and protein in the septum at P10 which is an important stage of differentiation processes in the septohippocampal system. These results provide further evidence that BDNF is a possible candidate for the mediation the TH effects in the MS/vDB.

Transient postnatal thyroxine treatment leads to variation in transmitter binding site densities in the hippocampus of rats

Newborn rats were treated daily with thyroid hormone (TH) until postnatal day 12. In the adult animals we measured the densities of glutamatergic (N-methyl-D-aspartate (NMDA), amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), kainic acid (KA)), cholinergic (muscarinic subtype 1 and 2 (M1, M2)), gamma-aminobutyric acid (GABA)ergic (GABA(A)) and serotonergic (5-hydroxytryptamine (5-HT(1A))) binding sites using quantitative receptor-autoradiography with tritiated ligands. In the TH-treated rats the KA binding site density was increased in the stratum oriens of cornu ammonis (CA)3, the terminal field of the infrapyramidal mossy fibers (IPMF). Densities of M2 and 5-HT(1A) were increased in the CA1 region. In contrast, binding site densities for NMDA in the entire hippocampus and for AMPA in the dentate gyrus were reduced, whereas binding site densities for M2 and GABA(A) remained unchanged. From this study we conclude that concomitant with the increase of the IPMF zone the density of the KA binding sites is specifically enhanced. In addition, we found a general shift from binding sites receiving cortical to those receiving subcortical input in the hippocampus.

Density of choline acetyltransferase-immunoreactive terminals in the rat dentate gyrus after entorhinal cortex lesions: a quantitative light microscope study

Lesion of the entorhinal cortex in the adult rat is a model for Alzheimer's disease and produces a marked increase in acetylcholinesterase (AChE) activity in the outer molecular layer (OML) of the dentate gyrus. This has been attributed to the sprouting of cholinergic axons terminals in response to denervation of the OML. The aim of this study was to investigate the density changes of cholinergic terminals in the OML at the light microscope level by using choline acetyltransferase (ChAT) immunohistochemistry and quantitative analysis. The results showed that between days 10 and 33 after an entorhinal cortex lesion, there was a measurable increase in the density of ChAT-positive boutons in the OML of the ipsilateral dentate gyrus (x1.2-1.6 of contralateral). However, when shrinkage of the ipsilateral OML (x0.5-0.75 of contralateral) was taken into account, the apparent increase in ChAT terminal density was entirely accounted for by shrinkage of the OML. Thus ChAT immunohistochemistry at the light microscope level provides no positive evidence for a proliferation of cholinergic terminals in the entorhinal cortex lesion model. This is in agreement with previous biochemical assays that have shown no change of total ChAT activity in the dentate gyrus after entorhinal cortex lesions.

Genetic variation in the morphology of the septo-hippocampal cholinergic and GABAergic systems in mice: II. Morpho-behavioral correlations

We investigated the contribution of the septo-hippocampal cholinergic and GABAergic system to spatial and nonspatial aspects of learning and memory that had previously been found to correlate with the extent of the hippocampal intra- and infrapyramidal mossy fiber projection in different inbred mouse strains. The following cholinergic and GABAergic markers were measured in the septi and hippocampi of male mice: the number of cholinergic and parvalbumin-containing neurons in the medial septum/vertical limb of the diagonal band of Broca (MS/vDB), the number of septo-hippocampal cholinergic and GABAergic projection neurons, the density of cholinergic fibers in different hippocampal subfields, and the density of muscarinic receptors (predominantly M1 and M2) in the hippocampus. In addition, animals were behaviorally tested for spatially dependent and activity-dependent variables in a water maze and spatial and nonspatial working and reference memory in different experimental set-ups in an eight-arm radial maze. Using only those variables for which significant strain differences were obtained, we looked for covariations between behavior and neuroanatomy. The density of cholinergic fibers in the dentate gyrus was significantly correlated with activity-dependent learning in the water maze, whereas the number of septo-hippocampal cholinergic projection neurons correlated with spatial and, to a lesser extent, also with nonspatial aspects of radial maze learning. Only weak correlations were found between receptor densities and behavioral traits. From these data we conclude that variations in the septo-hippocampal cholinergic system, like variations in the mossy fiber projection, entail functional consequences for different types of maze learning in mice.