Baseline and 8-OH-DPAT-induced release of acetylcholine in the hippocampus of aged rats with different levels of cognitive dysfunction

Effect of Chronic Tibolone Administration on Memory and Choline Acetyltransferase and Tryptophan Hydroxylase Content in Aging Mice

Gonadal steroids exert different effects on the central nervous system (CNS), such as preserving neuronal function and promoting neuronal survival. Estradiol, progesterone, and testosterone reduce neuronal loss in the CNS in animal models of neurodegeneration. However, hormone replacement therapy has been associated with higher rates of endometrial, prostate, and breast cancer. Tibolone (TIB), the metabolites of which show estrogenic and progestogenic effects, is an alternative to reduce this risk. However, the impact of TIB on memory and learning, as well as on choline acetyltransferase (ChAT) and tryptophan hydroxylase (TPH) levels in the hippocampus of aging males, is unknown. We administered TIB to aged C57BL/6J male mice at different doses (0.01 or 1.0 mg/kg per day for 12 weeks) and evaluated its effects on memory and learning and the content of ChAT and TPH. We assessed memory and learning with object recognition and elevated T-maze tasks. Additionally, we determined ChAT and TPH protein levels in the hippocampus by Western blotting. TIB administration increased the percentage of time spent on the novel object in the object recognition task. In addition, the latency of leaving the enclosed arm increased in both TIB groups, suggesting an improvement in fear-based learning. We also observed decreased ChAT content in the group treated with the 0.01 mg/kg TIB dose. In the case of TPH, no changes were observed with either TIB dose. These results show that long-term TIB administration improves memory without affecting locomotor activity and modulates cholinergic but not serotonergic systems in the hippocampus of aged male mice.

Cholinergic connectivity: it's implications for psychiatric disorders

Acetylcholine has been implicated in both the pathophysiology and treatment of a number of psychiatric disorders, with most of the data related to its role and therapeutic potential focusing on schizophrenia. However, there is little thought given to the consequences of the documented changes in the cholinergic system and how they may affect the functioning of the brain. This review looks at the cholinergic system and its interactions with the intrinsic neurotransmitters glutamate and gamma-amino butyric acid as well as those with the projection neurotransmitters most implicated in the pathophysiologies of psychiatric disorders; dopamine and serotonin. In addition, with the recent focus on the role of factors normally associated with inflammation in the pathophysiologies of psychiatric disorders, links between the cholinergic system and these factors will also be examined. These interfaces are put into context, primarily for schizophrenia, by looking at the changes in each of these systems in the disorder and exploring, theoretically, whether the changes are interconnected with those seen in the cholinergic system. Thus, this review will provide a comprehensive overview of the connectivity between the cholinergic system and some of the major areas of research into the pathophysiologies of psychiatric disorders, resulting in a critical appraisal of the potential outcomes of a dysregulated central cholinergic system.

Muscarinic agonists and antagonists in schizophrenia: recent therapeutic advances and future directions

Existing therapies for schizophrenia have limited efficacy, and significant residual positive, negative, and cognitive symptoms remain in many individuals with the disorder even after treatment with the current arsenal of antipsychotic drugs. Preclinical and clinical data suggest that selective activation of the muscarinic cholinergic system may represent novel therapeutic mechanisms for the treatment of schizophrenia. The therapeutic relevance of earlier muscarinic agonists was limited by their lack of receptor selectivity and adverse event profile arising from activation of nontarget muscarinic receptors. Recent advances in developing compounds that are selective to muscarinic receptor subtypes or activate allosteric receptor sites offer tremendous promise for therapeutic targeting of specific muscarinic receptor subtypes in schizophrenia.

An immunohistochemical study of the serotonin 1A receptor in the hippocampus of subjects with Alzheimer's disease

Alzheimer's disease (AD) is associated with neuronal degeneration, synaptic loss and deficits in multiple neurotransmitter systems. Alterations in the serotonin 1A (5-HT1A) receptor can contribute to impaired cognitive function in AD, and both in vitro binding and Positron emission tomography (PET) imaging studies have demonstrated that 5-HT1A receptors in the hippocampus/medial temporal cortex are affected early in AD. This neuropathological study examined the localization and immunoreaction intensity of 5-HT1A receptor protein in AD hippocampus with the goal to determine whether neuronal receptor levels are influenced by the severity of NFT severity defined by Braaks' pathological staging and to provide immunohistochemical confirmation of the binding assays and PET imaging studies. Subjects included AD patients and non-AD controls (NC) stratified into three Braaks' stages (Braak 0-II, NC; Braak III/IV and V/VI, AD). In the Braak 0-II group, 5-HT1A-immunoreactivity (ir) was prominent in the neuropil of the CA1 and subiculum, moderate in the dentate gyrus molecular layer (DGml), and low in the CA3 and CA4. No changes in 5-HT1A-ir were observed in the hippocampus of AD subjects in the Braak III/IV group. Hippocampal 5-HT1A-ir intensity was markedly decreased in the CA1 region in 6/11 (54.5%) subjects in the Braak V/VI group. Across all three groups combined, there was a statistically significant association between reduced 5HT1A-ir and neuronal loss in the CA1, but not in the CA3. The present data demonstrate that hippocampal 5-HT1A receptors are mainly preserved until the end-stage of NFT progression in AD. Thus, the utility of PET imaging using a 5-HT1A-specific radiolabeled probe as a marker of hippocampal neuronal loss may be limited to the CA1 field in advanced stage AD cases.

Evaluation of a combined therapeutic regimen of 8-OH-DPAT and environmental enrichment after experimental traumatic brain injury

When provided individually, both the serotonin (5-HT(1A))-receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and environmental enrichment (EE) enhance behavioral outcome and reduce histopathology after experimental traumatic brain injury (TBI). The aim of this study was to determine whether combining these therapies would yield greater benefit than either used alone. Anesthetized adult male rats received a cortical impact or sham injury and then were randomly assigned to enriched or standard (STD) housing, where either 8-OH-DPAT (0.1 mg/kg) or vehicle (1.0 mL/kg) was administered intraperitoneally once daily for 3 weeks. Motor and cognitive assessments were conducted on post-injury days 1-5 and 14-19, respectively. CA1/CA3 neurons and choline acetyltransferase-positive (ChAT(+)) medial septal cells were quantified at 3 weeks. 8-OH-DPAT and EE attenuated CA3 and ChAT(+) cell loss. Both therapies also enhanced motor recovery, acquisition of spatial learning, and memory retention, as verified by reduced times to traverse the beam and to locate an escape platform in the water maze, and a greater percentage of time spent searching in the target quadrant during a probe trial in the TBI + STD + 8-OH-DPAT, TBI + EE + 8-OH-DPAT, and TBI + EE + vehicle groups versus the TBI + STD + vehicle group (p ≤ 0.0016). No statistical distinctions were revealed between the TBI + EE + 8-OH-DPAT and TBI + EE + vehicle groups in functional outcome or CA1/CA3 cell survival, but there were significantly more ChAT(+) cells in the former (p = 0.003). These data suggest that a combined therapeutic regimen of 8-OH-DPAT and EE reduces TBI-induced ChAT(+) cell loss, but does not enhance hippocampal cell survival or neurobehavioral performance beyond that of either treatment alone. The findings underscore the complexity of combinational therapies and of elucidating potential targets for TBI.

Expression of GDNF transgene in astrocytes improves cognitive deficits in aged rats

Glial cell line-derived neurotrophic factor (GDNF) was assayed for its neurotrophic effects against the neuronal atrophy that causes cognitive deficits in old age. Aged Fisher 344 rats with impairment in the Morris water maze received intrahippocampal injections at the dorsal CA1 area of either a lentiviral vector encoding human GDNF or the same vector encoding human green fluorescent protein as a control. Recombinant lentiviral vectors constructed with human cytomegalovirus promotor and pseudotyped with lyssavirus Mokola glycoprotein specifically transduced the astrocytes in vivo. Astrocyte-secreted GDNF enhanced neuron function as shown by local increases in synthesis of the neurotransmitters acetylcholine, dopamine and serotonin. This neurotrophic effect led to cognitive improvement of the rats as early as 2 weeks after gene transduction. Spatial learning and memory testing showed a significant gain in cognitive abilities due to GDNF exposure, whereas control-transduced rats kept their performance at the chance level. These results confirm the broad spectrum of the neurotrophic action of GDNF and open new gene therapy possibilities for reducing age-related neurodegeneration.

Age-dependent effects of serotonin-1A receptor gene deletion in spatial learning abilities in mice

The serotonin (5-hydroxytryptamine, 5-HT) receptor 1A is involved in many physiological functions, including the regulation of learning and memory by acting either as an autoreceptor located on 5-HT neurons (raphe nuclei) or as a heteroreceptor on non-5-HT neurons, mainly in the hippocampal formation. To investigate whether the effects of 5-HT via 5-HT1A receptors on learning are age-sensitive, we evaluated the performance of young-adult (3 months old) and aged (22 months old) 5-HT1A knockout (KO) mice and their homologous wild types (WT) in the hippocampal-dependent spatial reference memory version of the Morris water maze. We demonstrated that young-adult 5-HT1AKO mice exhibit an impairment in learning and retention of the spatial task, as compared to WT mice, without showing any sign of change in their sensori-motor and locomotor abilities or motivation. This genotype effect does not persist during aging. In fact, aged 5-HT1AKO mice seem to be slightly facilitated during the early stages of learning. These results are consistent with a possible prevalence of 5-HT1A raphe functions in learning and memory abilities of young-adult animals, since the effects of the mutation on mice performance (impairment) are opposite to those found after intra-raphe injection of 5-HT1A agonists (facilitation), and with data showing increased activity of 5-HT neurons in 5-HT1AKO mice. The reduced effect of the mutation in aged animals possibly reflects the lower efficacy of autoreceptors due to aging and/or a prevalence of hippocampal heteroreceptors.

Psychopharmacological profile of the alkaloid psychollatine as a 5HT2A/C serotonin modulator

Behavioral effects of psychollatine, a new glycoside indole monoterpene alkaloid isolated from Psychotria umbellata, was investigated in models of anxiety, depression, memory, tremor, and sedation related to 5-HT and/or GABA neurotransmission. The GABA antagonist picrotoxin and the 5-HT2 antagonist ritanserin were used to examine the role of GABA and 5-HT2 receptors in psychollatine-induced effects. In the light/dark and hole-board models of anxiety, diazepam (0.75 mg/kg) and psychollatine (7.5 and 15 mg/kg) showed anxiolytic-like effect at doses that do not increase sleeping time nor alter spontaneous locomotor activity. The anxiolytic effect of psychollatine was prevented by prior administration of ritanserin, but not of picrotoxin, indicating that 5-HT2 but not GABA receptors are implicated. In the forced swimming model of depression, psychollatine (3 and 7.5 mg/kg) effects were comparable to the antidepressants imipramine (15 mg/kg) and fluoxetine (20 mg/kg). Psychollatine suppressed oxotremorine-induced tremors in all doses. In the step-down learning paradigm, diazepam (0.85 mg/kg), MK-801 (0.15 mg/kg), and psychollatine 100 mg/kg impaired the acquisition of learning and memory consolidation, without interfering with retrieval. It is concluded that the effects of psychollatine at the central nervous system involve serotonergic 5HT2(A/C) receptors.

Localization of m2 muscarinic receptor protein in parvalbumin and calretinin containing cells of the adult rat entorhinal cortex using two complementary methods

We investigated parvalbumin (PV) and calretinin (CR) containing interneurons in the rat entorhinal cortex. RNA amplification following single cell dissection of immunohistochemically labeled cells from layers II to VI revealed that PV cells, in contrast to CR cells, express the m2 muscarinic receptor (M2AchR) protein. Double immunostaining to confirm the results of RNA amplification indicated that the majority of PV cells contain M2AchR protein, whereas only a small proportion of CR cells do. In contrast, a large number of layer I CR cells, which are mostly Cajal-Retzius cells, were positive for M2AchR. RNA amplification following dissection of these cells also revealed that they contain the M2AchR protein. These findings emphasize that there are significant differences in the expression of different proteins, even among similar neuronal types in the same brain region. This highlights the importance of accurately collecting single cells, and knowledge of anatomical details in molecular biological studies.