Selective immunolesion of cholinergic neurons leads to long-term changes in 5-HT2A receptor levels in hippocampus and frontal cortex

Although loss of cholinergic neurons in the basal forebrain is considered a key initial feature in Alzheimer's disease (AD), changes in other transmitter systems, including serotonin and 5-HT_2A receptors, are also associated with early AD. The aim of this study was to investigate whether elimination of the cholinergic neurons in the basal forebrain directly affects 5-HT_2A receptor levels. For this purpose intraventricular injection of the selective immunotoxin 192 IgG-Saporin was given to rats in doses of either 2.5 or 5 μg. The rats were sacrificed after 1, 2, 4 and 20 weeks. 5-HT_2A protein levels were determined by western techniques in frontal cortex and hippocampus. A significant 70% downregulation in frontal cortex and a 100% upregulation in hippocampus of 5-HT_2A receptor levels were observed 20 weeks after the cholinergic lesion when using the highest dose of 192 IgG-Saporin. Our results show that cholinergic deafferentation leads to decreased frontal cortex and increased hippocampal 5-HT_2A receptor levels. This is probably a consequence of the interaction between the serotonergic and the cholinergic system that may vary depending on the brain region.     

Changes in 5-HT2A-mediated behavior and 5-HT2A- and 5-HT1A receptor binding and expression in conditional brain-derived neurotrophic factor knock-out mice

Changes in brain-derived neurotrophic factor (BDNF) expression have been implicated in the etiology of psychiatric disorders. To investigate pathological mechanisms elicited by perturbed BDNF signaling, we examined mutant mice with central depletion of BDNF (BDNF^2L/2LCk-cre). A severe impairment specific for the serotonin 2A receptor (5-HT_2AR) in prefrontal cortex was described previously in these mice. This is of much interest, as 5-HT_2ARs have been linked to neuropsychiatric disorders and anxiety-related behavior. Here we further characterized the serotonin receptor alterations triggered by BDNF depletion. 5-HT_2A ([³H]-MDL100907) and 5-HT_1A ([³H]-WAY100635) receptor autoradiography revealed site-specific alterations in BDNF mutant mice. They exhibited lower 5-HT_2A receptor binding in frontal cortex but increased binding in hippocampus. Additionally, 5-HT_1A receptor binding was decreased in hippocampus of BDNF mutants, but unchanged in frontal cortex. Molecular analysis indicated corresponding changes in 5-HT_2A and 5-HT_1A mRNA expression but normal 5-HT_2C content in these brain regions in BDNF^2L/2LCk-cre mice. We investigated whether the reduction in frontal 5-HT_2AR binding was reflected in reduced functional output in two 5-HT_2A-receptor mediated behavioral tests, the head-twitch response (HTR) and the ear-scratch response (ESR). BDNF^2L/2LCk-cre mutants treated with the 5-HT_2A receptor agonist (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) showed a clearly diminished ESR but no differences in HTR compared to wildtypes. These findings illustrate the context-dependent effects of deficient BDNF signaling on the 5-HT receptor system and 5-HT_2A-receptor functional output.     

Regional brain serotonin receptor changes in portacaval shunted rats

The pathogenesis of hepatic encephalopathy is unknown, but metabolic perturbations, including hyperammonaemia and increased brain turnover of serotonin (5-HT), have been identified. Possible alterations of 5-HT receptors in the brain have been rudimentarily studied. We therefore investigated the 5-HT_{1 A}, 5-HT_{1 B} and 5-HT_{2 A} receptor density in 18–22 different regions in the brain of portacaval shunted rats by means of radioligand binding with autoradiographical evaluation. The results revealed a decreased 5-HT_{1 A} receptor binding in seven serotonergic projection areas of the brain, and an increase in the nucleus accumbens, hypothalamus and subiculum. No changes in the raphe nuclei were observed. An increased 5-HT_{1 B} receptor binding was seen in five brain regions: basal ganglia, olfactorial regions, hippocampus, mid brain and thalamus. However, decreased binding was seen in three regions of cortical areas and hippocampus. The 5-HT_{2 A} receptor binding site density was essentially unaltered. These findings suggest that perturbations in the central serotonergic neurotransmission may play a functional role in chronic hepatic encephalopathy.     

Hypothalamic serotonin-1A receptor binding measured by PET predicts the plasma level of dehydroepiandrosterone sulfate in healthy women

Serotonin modulates the activity of the hypothalamic–pituitary–adrenal (HPA) axis particularly via the serotonin-1A receptor (5-HT_1A). Therefore, the rationale of this positron emission tomography (PET) study was to investigate the influence of the 5-HT_1A receptor distribution in the human brain on plasma levels of dehydroepiandrosterone sulfate (DHEAS) and cortisol in vivo. Eighteen healthy female were measured with PET and the selective 5-HT_1A receptor radioligand [carbonyl-¹¹C]WAY-100635. Nine a priori defined brain regions (hypothalamus, orbitofrontal cortex, amygdala, hippocampus, anterior and posterior cingulate cortices, dorsal raphe nucleus, retrosplenial cortex, and insula) and the cerebellum (reference region) were delineated on coregistered MR images. DHEAS and cortisol plasma levels were collected by blood sampling in the morning of the PET day. Linear regression analysis of DHEAS plasma level as dependent variable and hypothalamic 5-HT_1A receptor binding potential (BP) as independent variable showed a highly significant association (r = .691, p = .002). The hypothalamic 5-HT_1A BP predicted 47.7% of the variability in DHEAS plasma levels. Regressions were borderline significant (p < .01, Bonferroni corrected threshold <.0056) between 5-HT_1A BP in the anterior cingulate and orbitofrontal cortices and free cortisol levels. No significant associations between DHEAS or cortisol and the 5-HT_1A receptor BP in other investigated brain regions were found. In conclusion, the serotonergic system may influence the DHEAS plasma level by modulating CRH and ACTH release via hypothalamic 5-HT_1A receptors as reported for cortisol before. As disturbances of the HPA axis as well as changes of the 5-HT_1A receptor distribution have been reported in affective disorders, future studies should focus on these interactions.     

Effect of chronic activation of 5-HT3 receptors on 5-HT3, 5-HT1A and 5-HT2A receptors functional activity and expression of key genes of the brain serotonin system

Among serotonin (5-HT) receptors, the 5-HT₃ receptor is the only ligand-gated ion-channel. Little is known about the interaction between the 5-HT₃ receptor and other 5-HT receptors and influence of 5-HT₃ chronic activation on other 5-HT receptors and the expression of key genes of 5-HT system. Chronic activation of 5-HT₃ receptor with intracerebroventricularly administrated selective agonist 1-(3-chlorophenyl)biguanide hydrochloride (m-CPBG) (14 days, 40 nmol, i.c.v.) produced significant desensitization of 5-HT₃ and 5-HT_1A receptors. The hypothermic responses produced by acute administration of selective agonist of 5-HT₃ receptor (m-CPBG, 40 nmol, i.c.v.) or selective agonist of 5-HT_1A receptor (8-hydroxy-2-(di-n-propylamino)tetralin) (8-OH-DPAT, 1 mg/kg, i.p.) was significantly lower in m-CPBG treated mice compared with the mice of control groups. Chronic m-CPBG administration failed to induce any significant change in the 5-HT_2A receptor functional activity and in the expression of the gene encoding 5-HT_2A receptor. Chronic activation of 5-HT₃ receptor produced no considerable effect on the expression on 5-HT₃, 5-HT_1A, and 5-HT transporter (5-HTT) and tryptophan hydroxylase-2 (TPH-2) genes – the key genes of brain 5-HT system, in the midbrain, frontal cortex and hippocampus. In conclusion, chronic activation of ionotropic 5-HT₃ receptor produced significant desensitization of 5-HT₃ and postsynaptic 5-HT_1A receptors but caused no considerable changes in the expression of key genes of the brain 5-HT system.     

Laminar distribution of neurotransmitter receptors in different reeler mouse brain regions

Mapping of multiple receptors of neurotransmitters provides insight into the spatial distribution of neurotransmission-relevant molecules in the cerebral cortex. During development, lack of reelin leads to impaired migration, disturbed lamination of the hippocampus and inverted neocortical layering. In the adult, reelin may regulate synaptic plasticity by modulating neurotransmitter receptor function. Using quantitative in vitro receptor autoradiography, different receptors, in particular, the binding site densities and laminar distribution of various glutamate, GABA, muscarinic and nicotinic acetylcholine, serotonin, dopamine and adenosine receptors, were analyzed in cortical and subcortical structures of reeler and wild-type brains. Differential changes in the laminar distribution, maximum binding capacity (B _max) and regional density of neurotransmitter receptors were found in the reeler brain. A decrease of whole brain B _max was found for adenosine A₁ and GABA_A receptors. In the forebrain, several binding sites were differentially up- or down-regulated (kainate, A₁, benzodiazepine, 5-HT₁, M₂, α₁ and α₂). In the hippocampus, a significant decrease of GABA_B, 5-HT₁ and \textA₁^￠ {\text{A}}_1^{\prime} receptors were observed. The density of M₂ receptors increased, while other receptors remained unchanged. In the neocortex, some receptors demonstrated an obviously inverted laminar distribution (AMPA, kainate, NMDA, GABA_B, 5-HT₁, M₁, M₃, nAch), while the distribution of others (A₁, GABA_A, benzodiazepine, 5-HT₂, muscarinic M₂, adrenergic α₁, α₂) seemed to be less affected. Thus, the laminar receptor distribution is modulated by the developmental impairment and suggests and reflects partially the laminar inversion in reeler mice.     

Regulation of cortical and striatal 5-HT1A receptors in the MPTP-lesioned macaque

Serotonergic 1A (5-HT_1A) receptor agonists reduce L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia in Parkinson's disease (PD), though the mechanism(s) and site(s) of action remain unclear. We employed [³H]-WAY 100,635 autoradiographic receptor binding to measure 5-HT_1A receptor levels in 4 groups of macaques: normal (vehicle-vehicle); 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned, without exposure to L-DOPA, i.e., untreated parkinsonian (MPTP-vehicle); MPTP-lesioned, receiving a single administration of L-DOPA to alleviate parkinsonism (MPTP-L-DOPA-acute); and MPTP-lesioned, chronically treated with L-DOPA, parkinsonism alleviated but exhibiting dyskinesia (MPTP-L-DOPA-chronic). We demonstrate that 5-HT_1A receptor binding decreases (by 10%-20%, p < 0.05) in the external layers, but increases (by 80%-100%, p < 0.05) in the middle layers, of the premotor and motor cortex of all MPTP-lesioned macaques. In the striosomes of the caudate nucleus, 5-HT_1A receptor binding increases in MPTP-vehicle macaques (by 50%, p < 0.05), compared with normal macaques. While 5-HT_1A receptor binding is low in the matrix of the caudate nucleus in normal macaques, it increases (by 200%, p < 0.05) in MPTP-L-DOPA-chronic macaques. These data suggest that 5-HT_1A receptors are involved in the pathophysiology of both parkinsonism and complications of L-DOPA therapy.     

5-HT2A receptor levels increase in MPTP-lesioned macaques treated chronically with L-DOPA

Long-term L-3,4-dihydroxyphenylalanine (L-DOPA) treatment in Parkinson's disease (PD) is associated with motor complications such as dyskinesia. There are clear functional interactions between dopaminergic and serotonergic type 2A receptors (5-HT_2A)-mediated neurotransmission. Moreover, 5-HT_2A receptor antagonists can reduce L-DOPA-induced dyskinesia (LID). We hypothesized that enhanced 5-HT_2A-mediated neurotransmission may be involved in the genesis of L-DOPA-induced dyskinesia. Radioligand binding autoradiography, using [³H]-ketanserin, was performed to define 5-HT_2A receptor levels in brain tissue from macaques: 6 normal; 5 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned, parkinsonian macaques, without exposure to L-DOPA; 6 MPTP-lesioned, parkinsonian macaques, receiving a single administration of L-DOPA, and exhibiting no dyskinesia; and 6 MPTP-lesioned, parkinsonian, macaques chronically treated with L-DOPA, and exhibiting dyskinesia. 5-HT_2A receptor binding was increased in the caudate, putamen, and middle layers of the motor cortex in chronically L-DOPA-treated animals, by 50%, 50%, and 45% respectively, compared with normal macaques. 5-HT_2A binding was not significantly altered in parkinsonian, untreated, or parkinsonian, single treatment, nondyskinetic macaques, compared with normal. These data provide an anatomical basis for mechanisms to explain the efficacy of 5-HT_2A antagonists against dyskinesia.     

The Effects of Hypothyroidism on 5-HT1A and 5-HT2A Receptors and the Serotonin Transporter Protein in the Rat Brain

The effects of hypothyroidism on 5-HT_1A and 5-HT_2A receptors and the serotonin transporter protein were studied in thyroidectomized male Wistar rats in two experimental groups: 1) animals kept on an iodine-free diet (hypothyroid rats) and 2) animals kept on thyroxine (15 g/kg) for 21 days (giving normal thyroid hormone levels, euthyroid animals). Sham-operated rats served as controls. Binding of [³H]8-OH-DPAT with 5-HT_1A receptors and [³H]citalopram with the transporter protein in the hippocampus and midbrain showed no changes in hypothyroid rats as compared with controls. Conversely, there were significant decreases in [³H]ketanserin binding to 5-HT_2A receptors in the frontal cortex in hypothyroid rats as compared with controls; this decrease was reversed by thyroxine treatment. Thus, losses of cortical 5-HT_2A receptors appears to be the main consequence of hypothyroidism at the level of the serotonin system of the brain.     

Receptor-genes cross-talk: effect of chronic 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin treatment on the expression of key genes in brain serotonin system and on behavior

Dysfunction in brain serotonin (5-HT) system has been implicated in the psychopathology of anxiety, depression, drug addiction, and schizophrenia. The 5-HT_1A receptors play a central role in the control of 5-HTergic neurotransmission. There are some scarce data showing cross-regulation between 5-HT receptors. Here, we investigated whether interaction exists between 5-HT_1A receptor and genes encoding key members in brain 5-HT system. Chronic treatment with selective agonist of 5-HT_1A receptor 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (1.0 mg/kg i.p., 14 days) produced considerable decrease in hypothermic response to acute administration of 8-OH-DPAT in CBA/Lac mice indicating desensitization of 5-HT_1A receptors. The decrease in 5-HT_1A gene expression as well as decrease in the expression of gene encoding key enzyme in 5-HT synthesis, tryptophan hydroxylase-2 (TPH-2) in the midbrain, and the expression of the gene encoding 5-HT_2A receptor in the frontal cortex was shown. There were no significant changes in 5-HT transporter mRNA level in the midbrain. Despite considerable decrease in the expression of the genes encoding tryptophan hydroxylase-2, 5-HT_1A and 5-HT_2A receptors, chronic 8-OH-DPAT treatment failed to produce significant changes in 5-HT_1A-linked behavior (intermale aggression, open-field behavior, light-dark box, and pinch-induced catalepsy), suggesting compensatory and adaptive effect of genes suppression. The obtained data on the effect of 8-OH-DPAT-induced desensitization of 5-HT_1A receptors on 5-HT_1A, 5-HT_2A and TPH-2 gene expression demonstrated the role of 5-HT_1A receptor as indirect regulator of gene expression. The results provide the first evidence of receptor-key genes interaction in brain 5-HT system and may have profound implications in understanding the functioning of the brain neurotransmitter systems.     

Functional Characteristics of Serotonin 5-HT<Subscript>2A</Subscript> and 5-HT<Subscript>2C</Subscript> Receptors in the Brain and the Expression of the 5-HT<Subscript>2A</Subscript> and 5-HT<Subscript>2C</Subscript> Receptor Genes in Aggressive and Non-Aggressive Rats

The functional activity of serotonin 5-HT_2A and 5-HT_2C receptors and the expression of the genes encoding them were studied in Norway rats bred for 60 generations for the presence and absence of high levels of stress-evoked aggression to humans. There were no significant differences in the levels of 5-HT_2A receptor mRNA in the midbrain, frontal cortex, and hippocampus and the extents of head twitching evoked by the 5-HT_2A agonist DOI in rats with and without genetically determined high levels of aggression. Administration of the selective 5-HT_2C agonist MK-212 weakened reflex startle in response to an acoustic signal (the acoustic startle response) in non-aggressive animals but had no significant effects on the response in aggressive animals. Increases in the level of 5-HT_2C receptor mRNA were seen in the frontal cortex and hippocampus in non-aggressive rats as compared with aggressive animals. Increases in the expression of the 5-HT_2C receptor gene and the functional state of 5-HT_2C receptors were seen in the brains of non-aggressive rats, without any changes in the 5-HT_2A receptor mRNA level or receptor sensitivity; this is evidence for the involvement of 5-HT_2C receptors in the mechanisms inhibiting fear-evoked aggressive behavior.     

Activation of basal forebrain cholinergic neurons differentially regulates brain-derived neurotrophic factor mRNA expression in different projection areas.

Afferent cholinergic pathways from the basal forebrain were activated by injections of the glutamate analog quisqualate either into the nucleus basalis or into the medial septal nucleus. Nucleus basalis injections had no effect on the expression of brain-derived neurotrophic factor (BDNF) mRNA in its neocortical projection areas as measured by in situ hybridization. In contrast, 7 h after an injection into the septum the level of BDNF mRNA increased 3- to 5-fold in the dentate gyrus, throughout CA1 to CA3 in the hippocampus and in the piriform cortex.     

Regional central serotonin receptor binding in rats treated chronically with high-dose 5-hydroxy-L-tryptophan

Summary The serotonin (5-HT) precursor 5-hydroxy-L-tryptophan (L-5-HTP) exerted differential regional effects on central 5-HT receptors in rats treated chronically by intraperitoneal injections of large incremental doses (50–200 mg/kg). There were significant reductions in B_max of agonist-labelled (-35%) and antagonist-labelled (-20%) 5-HT₂ sites in cortex but no changes in brainstem. K_d and n_H were unaffected by L-5-HTP. B_max of 5-HT₁ sites (unsubtyped) was reduced 16% in cortex and 18% in spinal cord, but the changes were not significant. Brainstem 5-HT₁ sites were unaffected. Studies at a single isotope concentration in other regions revealed significant reductions of antagonist-labelled 5-HT₂ specific binding in diencephalon (-26%) but not septum, and of 5-HT₁ binding in diencephalon (-25%) and cerebellum (-30%) but not in hippocampus or striatum. Absence of L-5-HTP-evoked changes in 5-HT receptors in brainstem may have implications for L-5-HTP-responsive and brainstem-mediated human myoclonic disorders.     

Redistribution of CB1 cannabinoid receptors during evolution of cholinergic basal forebrain territories and their cortical projection areas: a comparison between the gray mouse lemur (Microcebus murinus, primates) and rat

Endocannabinoid signaling, mediated by presynaptic CB1 cannabinoid receptors on neurons, is fundamental for the maintenance of synaptic plasticity by modulating neurotransmitter release from axon terminals. In the rodent basal forebrain, CB1 cannabinoid receptor-like immunoreactivity is only harbored by a subpopulation of cholinergic projection neurons. However, endocannabinoid control of cholinergic output from the substantia innominata, coincident target innervation of cholinergic and CB1 cannabinoid receptor-containing afferents, and cholinergic regulation of endocannabinoid synthesis in the hippocampus suggest a significant cholinergic-endocannabinergic interplay. Given the functional importance of the cholinergic modulation of endocannabinoid signaling, here we studied CB1 cannabinoid receptor distribution in cholinergic basal forebrain territories and their cortical projection areas in a prosimian primate, the gray mouse lemur. Perisomatic CB1 cannabinoid receptor immunoreactivity was unequivocally present in non-cholinergic neurons of the olfactory tubercule, and in cholecystokinin-containing interneurons in layers 2/3 of the neocortex. Significantly, CB1 cannabinoid receptor-like immunoreactivity was localized to cholinergic perikarya in the magnocellular basal nucleus. However, cortical cholinergic terminals lacked detectable CB1 cannabinoid receptor levels. A dichotomy of CB1 cannabinoid receptor distribution in frontal (suprasylvian) and parietotemporal (subsylvian) cortices was apparent. In the frontal cortex, CB1 cannabinoid receptor-containing axons concentrated in layers 2/3 and layer 6, while layer 4 and layer 5 were essentially devoid of CB1 cannabinoid receptor immunoreactivity. In contrast, CB1 cannabinoid receptors decorated axons in all layers of the parietotemporal cortex with peak densities in layer 2 and layer 4. In the hippocampus, CB1 cannabinoid receptor-containing terminals concentrated around pyramidal cell somata and proximal dendrites in the CA1-CA3 areas, and granule cell dendrites in the molecular layer of the dentate gyrus. CB1 cannabinoid receptors frequently localized to inhibitory GABAergic terminals while leaving glutamatergic boutons unlabeled. Aging did not affect either the density or layer-specific distribution of CB1 cannabinoid receptor-immunoreactive processes. We concluded that organizing principles of CB1 cannabinoid receptor-containing neurons and their terminal fields within the basal forebrain are evolutionarily conserved between rodents and prosimian primates. In contrast, the areal expansion and cytoarchitectonic differentiation of neocortical subfields in primates is associated with differential cortical patterning of CB1 cannabinoid receptor-containing subcortical and intracortical afferents.     

Effects of amyloid-β peptides on the serotoninergic 5-HT1A receptors in the rat hippocampus

A recent [¹⁸F]MPPF-positron emission tomography study has highlighted an overexpression of 5-HT_1A receptors in the hippocampus of patients with mild cognitive impairment compared to a decrease in those with Alzheimer's disease (AD) [Truchot, L., Costes, S.N., Zimmer, L., Laurent, B., Le Bars, D., Thomas-Antérion, C., Croisile, B., Mercier, B., Hermier, M., Vighetto, A., Krolak-Salmon, P., 2007. Up-regulation of hippocampal serotonin metabolism in mild cognitive impairment. Neurology 69 (10), 1012-1017]. We used in vivo and in vitro neuroimaging to evaluate the longitudinal effects of injecting amyloid-β (Aβ) peptides (1-40) into the dorsal hippocampus of rats. In vivo microPET imaging showed no significant change in [¹⁸F]MPPF binding in the dorsal hippocampus over time, perhaps due to spatial resolution. However, in vitro autoradiography with [¹⁸F]MPPF (which is antagonist) displayed a transient increase in 5-HT_1A receptor density 7 days after Aβ injection, whereas [¹⁸F]F15599 (a radiolabelled 5-HT_1A agonist) binding was unchanged suggesting that the overexpressed 5-HT_1A receptors were in a non-functional state. Complementary histology revealed a loss of glutamatergic neurons and an intense astroglial reaction at the injection site. Although a neurogenesis process cannot be excluded, we propose that Aβ injection leads to a transient astroglial overexpression of 5-HT_1A receptors in compensation for the local neuronal loss. Exploration of the functional consequences of these serotoninergic modifications during the neurodegenerative process may have an impact on therapeutics targeting 5-HT_1A receptors in AD.     

Effects of neonatal handling on the basal forebrain cholinergic system of adult male and female rats

Neonatal handling is an early experience which results in improved function of the hypothalamic-pituitary-adrenal axis, increased adaptability and coping as a response to stress, as well as better cognitive abilities. In the present study, we investigated the effect of neonatal handling on the basal forebrain cholinergic system, since this system is known to play an important role in cognitive processes. We report that neonatal handling results in increased number of choline-acetyl transferase immunopositive cells in the septum/diagonal band, in both sexes, while no such effect was observed in the other cholinergic nuclei, such as the magnocellular preoptic nucleus and the nucleus basalis of Meynert. In addition, neonatal handling resulted in increased M1 and M2 muscarinic receptor binding sites in the cingulate and piriform cortex of both male and female rats. A handling-induced increase in M1 muscarinic receptor binding sites was also observed in the CA3 and CA4 (fields 3 and 4 of Ammon's horn) areas of the hippocampus. Furthermore, a handling-induced increase in acetylcholinesterase staining was found only in the hippocampus of females. Our results thus show that neonatal handling acts in a sexually dimorphic manner on one of the cholinergic parameters, and has a beneficial effect on BFCS function, which could be related to the more efficient and adaptive stress response and the superior cognitive abilities of handled animals.     

Effects of cholinergic lesions produced by infusions of 192 IgG-saporin on glucocorticoid receptor mRNA expression in hippocampus and medial prefrontal cortex of the rat

Principal neurons in the hippocampus and prefrontal cortex of the rat have been identified as targets for glucocorticoids involved in the hypothalamic-pituitary-adrenocortical stress response. Alterations in mRNA expression for glucocorticoid receptors in each of these regions have been shown to affect the negative feedback response to corticosterone following an acute stressor. Both decreases in forebrain glucocorticoid receptors and in the efficiency of adrenocortical feedback have been observed in normal aging, and have been selectively induced with experimental lesions or manipulations in neurotransmitter systems. The current study investigated the possibility that a loss of cholinergic support from cells in the basal forebrain, a hallmark of aging, contributes to the selective age-related loss of glucocorticoid receptor mRNA expression at cholinoceptive target sites that include the hippocampus and medial prefrontal cortex. Lesions of the basal forebrain cholinergic system in young adult rats were made by microinjections of the immunotoxin 192 IgG-saporin into the medial septum/vertical limb of the diagonal band and substantia innominata/nucleus basalis. Basal levels of circulating glucocorticoids were unaffected by the lesions. Analysis of both mineralocorticoid and glucocorticoid receptor mRNA expression revealed a significant decrease in glucocorticoid receptor mRNA in the hippocampus and medial prefrontal cortex, with spared expression at subcortical sites and no detectable change in mineralocorticoid receptor mRNA in any of the examined regions. Thus, rats with lesions of the basal forebrain cholinergic system recapitulate some of the detrimental effects of aging associated with glucocorticoid-mediated stress pathways in the brain.     

Laminar distribution and co-distribution of neurotransmitter receptors in early human visual cortex

The laminar distributions of 16 neurotransmitter receptor binding sites were analysed in visual cortical areas V1–V3 by quantitative in vitro receptor autoradiography. For each receptor (glutamatergic: AMPA, kainate, NMDA; cholinergic: M₁, M₂, M₃, nicotinic; GABAergic: GABA_A, GABA_B, benzodiazepine binding-sites; adrenergic: α₁, α₂; serotoninergic: 5-HT_1A, 5-HT₂; dopaminergic: D₁; Adenosine: A₁), density profiles extracted perpendicular to the cortical surface were compared to cyto- and myeloarchitectonic profiles sampled at corresponding cortical sites. When testing for differences in laminar distribution patterns, all receptor-density profiles differed significantly from the cyto- and myeloarchitectonic ones. These results indicate that receptor distribution is an independent feature of the cortical architecture not predictable by densities of cell bodies or myelinated fibres. Receptor co-distribution was studied by cluster analyses, revealing several groups of receptors, which showed similar laminar distribution patterns across all analysed areas (V1–V3). Other receptors were co-distributed in extrastriate but not primary visual cortex. Finally, some receptors were not co-distributed with any of the analysed other ones. A comparison of the laminar patterns of receptor binding sites in the human visual cortex with those reported for non-human primates and other mammals showed that the laminar distributions of cholinergic and glutamatergic receptors seem largely preserved, while serotoninergic and adrenergic receptors appear to be more variable between different species.     

Activation of the 5-HT6 receptor attenuates long-term potentiation and facilitates GABAergic neurotransmission in rat hippocampus

The 5-HT₆ receptor is predominantly expressed in the CNS and has been implicated in the regulation of cognitive function. Antagonists of the 5-HT₆ receptor improve cognitive performance in a number of preclinical models and have recently been found to be effective in Alzheimer's disease patients. Systemic administration of 5-HT₆ antagonists increases the release of acetylcholine and glutamate in the frontal cortex and dorsal hippocampus. In contrast, the selective 5-HT₆ agonist, WAY-181187, can elicit robust increases in extracellular levels of GABA. The reported behavioral and neurochemical effects of 5-HT₆ receptor ligands raise the possibility that the 5-HT₆ receptor may modulate synaptic plasticity in the hippocampus. In the present study, selective pharmacological tools were employed to determine the effect of 5-HT₆ receptor activation on long-term potentiation (LTP) in brain slices containing area CA1 of the hippocampus. While having no effect on baseline synaptic transmission, the results demonstrate that the selective 5-HT₆ agonist, WAY-181187, attenuated LTP over a narrow dose range (100–300 nM). The increase in the slope of the field excitatory post synaptic potential (fEPSP) caused by theta burst stimulation in brain slices treated with the most efficacious dose of WAY-181187 (200 nM) was 80.1±4.0% of that observed in controls. This effect was dose-dependently blocked by the selective 5-HT₆ antagonist, SB-399885. WAY-181187 also increased the frequency of spontaneous GABA release in area CA1. As assessed by measuring and evaluating spontaneous inhibitory postsynaptic currents (sIPSCs), 200 nM WAY-181187 increased sIPSC frequency by 3.4±0.9 Hz. This increase in GABA sIPSCs was prevented by the selective 5-HT₆ antagonist SB-399885 (300 nM). Taken together, these results suggest that the 5-HT₆ receptor plays a role in the modulation of synaptic plasticity in hippocampal area CA1 and that the regulation of GABAergic interneuron activity may underlie the cognition enhancing effects of 5-HT₆ antagonists.