Hippocampal Cholinergic Receptors Research Articles

Hippocampal cholinergic receptors and the mTOR participation in fear-motivated inhibitory avoidance extinction memory

Evidence has demonstrated the hippocampal cholinergic system and the mammalian target of rapamycin (mTOR) participation during the memory formation of aversive events. This study assessed the role of these systems in the hippocampus for the extinction memory process by submitting male Wistar rats to fear-motivated step-down inhibitory avoidance (IA). The post-extinction session administration of the nicotinic and muscarinic cholinergic receptor antagonists, mecamylamine and scopolamine, respectively, both at doses of 2 µg/µl/side, and rapamycin, an mTOR inhibitor (0.02 µg/µl/side), into the CA1 region of the dorsal hippocampus, impaired the IA extinction memory. Furthermore, the nicotinic and muscarinic cholinergic receptor agonists, nicotine and muscarine, respectively, had a dose-dependent effect on the IA extinction memory when administered intra-CA1, immediately after the extinction session. Nicotine (0.6 µg/µl/side) and muscarine (0.02 µg/µl/side), respectively, had no effect, while the higher doses (6 and 2 µg/µl/side, respectively) impaired the IA extinction memory. Interestingly, the co-administration of muscarine at the lower dose blocked the impairment that was induced by rapamycin. This effect was not observed when nicotine at the lower dose was co-administered. These results have demonstrated the participation of the cholinergic receptors and mTOR in the hippocampus for IA extinction, and that the cholinergic agonists had a dose-dependent effect on the IA extinction memory. This study provides insights related to the behavioural aspects and the neurobiological properties underlying the early stage of fear-motivated IA extinction memory consolidation and suggests that there is hippocampal muscarinic receptor participation independent of mTOR in this memory process.

Neurochemical Effects of Nicotine on Albino Rat’s Brain

Background: In the recent era, smoking has become fashionable. Seventy percent of the Indian population is continuously being exposed to smoking. Cigarette smoking produces various quantities of nicotine that is readily absorbed by the physiological system of smokers. Objectives: Adolescence is the period in which smokers typically begin their nicotine addiction. To date, no attempt has been made to estimate changes in cholinergic and DA-D2 receptors (Dopaminergic D2 receptors) following nicotine administration, although literature review has revealed available publications regarding changes in dopamine concentration following nicotine. Hence the present experiment was conducted to study the alteration of cholinergic and DA-D2 receptors in hippocampus and corpus striatum respectively following nicotine exposure by using neurochemical receptor binding techniques. Materials and Methods: In the present study an experiment was done on male albino rats. Nicotine was administered for eight weeks orally via a cannula, using dose rate (5 mg/d, 10 mg/d). The results were compared with control adult rats, given vehicle in an identical manner. After exposure, we assessed [3H] Quinuclidinyl Benzilate as a specific ligand for cholinergic receptors in the hippocampus and DA-D2 receptors binding in corpus striatum using [3H] Spiperone as a specific ligand for DA-D2 receptor. Results: Chronic nicotine treatment induced the up-regulation of cholinergic receptors in the hippocampus and down-regulation of DAD2 receptors in the striatum. Conclusions: Such knowledge is important for understanding nicotine dependence and the consequences of nicotine administration for the treatment of neurological disorders. These effects may underlie long-term behavioral changes associated with adolescent nicotine exposure.

The long‐term effects of neonatal morphine administration on the pentylenetetrazol seizure model in rats: The role of hippocampal cholinergic receptors in adulthood

Early life exposure to opiates may affect neuropathological conditions, such as epilepsy, during adulthood. We investigated whether neonatal morphine exposure affects pentylenetetrazol (PTZ)-induced seizures in adulthood. Male rats were subcutaneously injected with morphine or saline on postnatal days 8-14. During adulthood, each rat was assigned to 1 of the following 10 sub-groups: saline, nicotine (0.1, 0.5, or 1 μg), atropine (0.25 or 1 μg), oxotremorine M (0.1 or 1 μg), or mecamylamine (2 or 8 μg). An intrahippocampal infusion of the indicated compound was administered 30 min before seizure induction (80 mg/kg PTZ). Compared with the saline/oxotremorine (1 μg), saline/saline, and morphine/saline groups, the morphine/oxotremorine (1 μg) group showed a significantly increased latency to the first epileptic behavior. The duration of tonic-clonic seizures was significantly lower in the morphine/oxotremorine (1 μg) group compared to the saline/saline and morphine/saline groups. The severity of seizure was significantly decreased in the morphine/atropine (1 μg) group than in the saline/atropine (1 μg). Seizure severity was also decreased in the morphine/mecamylamine (2 μg) group than in the saline/mecamylamine (2 μg) group. Latency for death was significantly lower in the morphine/mecamylamine (2 μg) group compared with the saline/mecamylamine (2 μg) group. Mortality rates in the morphine/atropine (1 μg) and morphine/mecamylamine (2 μg) groups were significantly lower than those in the saline/atropine (1 μg) and saline/mecamylamine (2 μg) groups, respectively. Chronic neonatal morphine administration attenuated PTZ-induced seizures, reduced the mortality rate, and decreased the impact of the hippocampal cholinergic system on seizures and mortality rate in adult rats. Neonatal morphine exposure induces changes to μ-receptors that may lead to activation of GABAergic neurons in the hippocampus. This pathway may explain the anti-convulsant effects of morphine observed in our study.

Neuroprotective efficacy of curcumin in arsenic induced cholinergic dysfunctions in rats

Our recent studies have shown that curcumin protects arsenic induced neurotoxicity by modulating oxidative stress, neurotransmitter levels and dopaminergic system in rats. As chronic exposure to arsenic has been associated with cognitive deficits in humans, the present study has been carried out to implore the neuroprotective potential of curcumin in arsenic induced cholinergic dysfunctions in rats. Rats treated with arsenic (sodium arsenite, 20mg/kg body weight, p.o., 28 days) exhibited a significant decrease in the learning activity, assessed by passive avoidance response associated with decreased binding of 3H-QNB, known to label muscarinic–cholinergic receptors in hippocampus (54%) and frontal cortex (27%) as compared to controls. Decrease in the activity of acetylcholinesterase in hippocampus (46%) and frontal cortex (33%), staining of Nissl body, immunoreactivity of choline acetyltransferase (ChAT) and expression of ChAT protein in hippocampal region was also observed in arsenic treated rats as compared to controls. Simultaneous treatment with arsenic and curcumin (100mg/kg body weight, p.o., 28 days) increased learning and memory performance associated with increased binding of 3H-QNB in hippocampus (54%), frontal cortex (25%) and activity of acetylcholinesterase in hippocampus (41%) and frontal cortex (29%) as compared to arsenic treated rats. Increase in the expression of ChAT protein, immunoreactivity of ChAT and staining of Nissl body in hippocampal region was also observed in rats simultaneously treated with arsenic and curcumin as compared to those treated with arsenic alone. The results of the present study suggest that curcumin significantly modulates arsenic induced cholinergic dysfunctions in brain and also exhibits neuroprotective efficacy of curcumin.

Early changes of cholinergic receptors in hippocampus and parietotemporal cortex of the rat model of sporadic Alzheimer's disease

disease (AD) and neurodegeneration. VitaminD has been shown to down-regulate the L-type voltage-sensitive calcium channels, LVSCC-A1C and LVSCC-A1D, and up-regulate (nerve growth factor) NGF. However, expression of these proteins when VDR is repressed is unknown. The aim of this study is to investigate LVSCC-A1C, LVSCC-A1D, calbindin-D28k, inducible nitric oxide synthase (iNOS) expressions and NGF release in VDR-silenced primary cortical neurons. Methods: qRT-PCR and western blots were performed to determine VDR, LVSCC-A1C, LVSCC-A1D, calbindin-D28k and iNOS expression levels. NGF and cytotoxicity levels were determined by ELISA. Apoptosis was determined by TUNEL. Results: Our findings illustrate that LVSCC-A1C and iNOS expressions increased rapidly in cortical neurons when VDR is down-regulated, whereas, LVSCC-A1D levels did not change, NGF release calbindin-D28k expression decreased in response to VDR down-regulation. Although vitamin D regulates LVSCC-A1C through VDR, it may not regulate LVSCC-A1D through VDR. Conclusions: Our results indicate that suppression of VDR or vitamin D-VDR pathway disruption disrupts LVSCC-A1C, calbindin-D28k and NGF production and increase oxidative stress. Thus it can make neurons vulnerable to aging and neurodegeneration, and when combined with As toxicity, it is possible to explain some of the events that occur during neurodegeneration.

Deranged spontaneous behaviour and decrease in cholinergic muscarinic receptors in hippocampus in the adult rat, after neonatal exposure to the brominated flame-retardant, 2,2′,4,4′,5-pentabromodiphenyl ether (PBDE 99)

Polybrominated diphenyl ethers (PBDEs), which are used as flame-retardants, have recently been shown to be increasing in the environment and in human milk. We have recently reported that neonatal exposure to PBDEs, including 2,2′,4,4′,5-pentaBDE (PBDE 99), can induce persistent aberrations in spontaneous behaviour and also affect learning and memory functions and nicotinic cholinergic receptors in the adult mice. The present study indicates that spontaneous behaviour, along with the cholinergic system during its developing stage, can be targets for PBDEs in the rat as well. Neonatal oral exposure of male Sprague–Dawley rats, on postnatal day 10, to 0.8, 8.0, and 16 mg PBDE 99/kg body weight, was shown to disrupt normal spontaneous behaviour in a dose–response related manner at 2 months of age. Also, the animals of the present study showed a decrease in density of muscarinic cholinergic receptors in hippocampus, at an adult age. These findings show similarities to observations made from neonatal exposure of mice to PBDE 99, 2,2′,4,4′,5,5′-hexaBDE (PBDE 153) and certain PCBs, compounds shown to affect both spontaneous behaviour and the cholinergic system.

Neonatal exposure to polybrominated diphenyl ether (PBDE 153) disrupts spontaneous behaviour, impairs learning and memory, and decreases hippocampal cholinergic receptors in adult mice

Neonatal exposure to polybrominated diphenyl ether (PBDE 153) disrupts spontaneous behaviour, impairs learning and memory, and decreases hippocampal cholinergic receptors in adult mice. Flame retardants are used to suppress or inhibit combustion processes in an effort to reduce the risk of fire. One class of flame retardants, polybrominated diphenyl ethers (PBDEs), are present and increasing in the environment and in human milk. The present study shows that neonatal exposure to 2,2′,4,4′,5,5′-hexaBDE (PBDE 153), a PBDE persistent both in environment and in human milk, can induce developmental neurotoxic effects, such as changes in spontaneous behaviour (hyperactivity), impairments in learning and memory, and reduced amounts of nicotinic receptors, effects that get worse with age. Neonatal NMRI male mice were orally exposed on day 10 to 0.45, 0.9, or 9.0 mg of PBDE 153/kg of body weight. Spontaneous behaviour (locomotion, rearing, and total activity) was observed in 2−, 4−, and 6-month-old mice, Morris water maze at an age of 6 months. The behaviour tests showed that the effects were dose-response and time-response related. Animals showing defects in learning and memory also showed significantly reduced amounts of nicotinic receptors in hippocampus, using α-bungarotoxin binding assay. The observed developmental neurotoxic effects seen for PBDE 153 are similar to those seen for PBDE 99 and for certain PCBs. Furthermore, PBDEs appear to as potent as the PCBs.

Lesion with the neurotoxin AF64A alters hippocampal cholinergic receptor function

The effect of selective lesion of cholinergic inputs to the hippocampus on the function of hippocampal cholinergic receptors was examined. Hippocampal cholinergic neurons were lesioned in the rat by administration of the selective cholinergic neurotoxin AF64A (ethylcholine mustard azirtdinium). Cholinergic receptor function was examined by assessing the ability of cholinergic agonists and antagonists to modulate the evoked release of radiolabelled acetylcholine (ACh) from hippocampal slices. Nicotine enhanced release, with a bell-shaped dose-response curve. The dose-response curve and EC 50 for nicotine was shifted 10-fold to the left in lesioned rats, suggesting an increased sensitivity to nicotine. However, there were no differences in either the number or affinity of nicotinic receptors as determined with binding studies. The muscarinic agonist oxotremorine inhibited the evoked release of ACh in control tissues, but had much less effect in AF64A-lesioned tissues. Binding to the M1 receptor subtype was not changed. However, the K d for binding to the high affinity subtype of the M2 receptor was increased 10-fold, suggesting that the receptor has become less sensitive to stimulation. Loss of M2 function may allow an increase in the effect of stimulating nicotinic receptors that modulate ACh release.

Muscarinic cholinergic receptors in the hippocampus of aged rats: influence of choline alphoscerate treatment

The present study was designed to investigate age-dependent changes of muscarinic M 1 and M 2 cholinergic receptors in the rat hippocampus using radioreceptor assay and autoradiographic techniques with [ 3H]pirenzepine and [ 3H]AF-DX 116 as ligands. The analysis was performed on 2-, 12- and 27-month-old male Wistar rats, considered young, adult and old, respectively. Moreover, the influence of a 6-month treatment with choline alphoscerate on the density and pattern of M 1 and M 2 cholinergic receptors was assessed. Choline alphoscerate ( l-α-glyceryl phosphorylcholine) is a precursor in the biosynthesis of several brain phospholipids which increases the availability of acetylcholine in various tissues. Muscarinic M 1 cholinergic receptors were significantly decreased with increasing age whereas M 2 cholinergic receptors did not show changes. Choline alphoscerate treatment countered, in part, the loss of muscarinic M 1 receptor sites in old rats. Light microscope autoradiography revealed a loss of silver grains developed after exposure of sections of hippocampus to [ 3H]pirenzepine in the stratum oriens of CA 1 and CA 3 fields in rats of 12 and 27 months in comparison with young animals. Choline alphoscerate restored, in part, the decrease of silver grains noted in old rats. Quantitative analysis of the density of silver grains developed in the cell body of pyramidal neurons of CA 1 and CA 3 fields processed for the demonstration of muscarinic M 1 receptor sites revealed a decrease of these grains in rats of 27 months in comparison with younger cohorts. These findings suggest that the reduction in muscarinic M 1 sites noticeable between 2- and 12-month rats is probably dependent on the loss of nerve cells and/or terminals in these hippocampal fields rather than to a reduction of their density per neuron. Treatment with choline alphoscerate increased the expression of muscarinic M 1 cholinergic receptors within the cell body of pyramidal neurons of CA 1 and CA 3 fields compared to age-matched control old rats. Consistent with radioreceptor assay data, no changes in the density of muscarinic M 2 cholinergic receptors in the animal groups examined were demonstrated by light microscope autoradiography. The possible pharmacological relevance of the increased expression of muscarinic M 1 cholinergic receptors elicited by choline alphoscerate in the hippocampus of aged rats is discussed.

Alterations in hippocampal cholinergic receptors and hippocampal behaviors after early exposure to nicotine

Mice were exposed to nicotine prenatally by injecting the mother with 1.5 mg/kg nicotine SC twice daily on gestation days 9–18 (PreN mice) or neonatally by daily SC injections of 1.5 mg/kg nicotine on postnatal days 2–21 (NeoN mice). At age 50 days, hippocampal muscarinic receptors S max of PreN and NeoN mice were 58% and 79% above control, respectively ( p < 0.01); K d was unaffected by early nicotine exposure. Eight-arm maze performance of nicotine-exposed animals fell behind control level. Both PreN and NeoN made approximately 10% less correct responses in the first eight trials than controls throughout the test period ( p < 0.01). By the last day of testing, PreN needed 23% and NeoN 31% more trials than controls to enter all arms ( p < 0.001). In addition, PreN needed 35 and NeoN 42% more days than controls to reach criterion ( p < 0.05). Similarly, while 61% of controls reached criterion by day 6 only 17% of PreN and 25% of NeoN reached criterion ( p < 0.01). In the Morris maze, PreN needed from 43–119% more time to reach the platform ( p < 0.001). In the spatial probe test, PreN animals made 35% fewer crosses over the area of the missing platform ( p < 0.001). The study suggests that nicotine administered to the fetus or neonate alters septohippocampal chemistry and induces deficits in hippocampus-related behaviors. The possible reversal of the behavioral changes by manipulating the cholinergic innervations should be the subject of future investigations.

The status of hippocampal cholinergic receptors in human cognitive disorders

The status of hippocampal cholinergic receptors in human cognitive disorders