Dimemorfan enhances acetylcholine release from rat hippocampal slices

Our previous study reported that an antitussive drug, dimemorfan, attenuates cholinergic dysfunction-induced amnesia in mice and acts like a sigma1 receptor agonist. This study further showed that dimemorfan (30 microM), like the putative sigma1 receptor agonist (+)-SKF-10047 (10 microM), significantly enhanced 25 mM KCl-evoked [3H]acetylcholine release from rat hippocampal but not striatal slices, which was antagonized by a sigma1 receptor antagonist haloperidol (0.3 microM).     

Amino alcohol modulation of hippocampal acetylcholine release.

The synthesis and release of 3H-acetylcholine was measured in hippocampal slices of adult rat brain following acute in vitro exposure to ethanolamine. Evoked release of 3H-acetylcholine was elevated by 60-70% but 3H-acetylcholine synthesis was unaffected. Other amino alcohols were also found to significantly increase evoked 3H-acetylcholine release. The effect may be stereochemically mediated since only one of four possible propanolamine configurations, R-alaninol, was active. The most potent compound tested was R-prolinol which showed an EC50 nearly 10-fold lower than that of either R-alaninol or ethanolamine; S-prolinol was inactive. Slices taken from adult rats which had been fed active compounds for two weeks also exhibited enhancements in evoked 3H-acetylcholine release. These results indicate that amino alcohols modulate acetylcholine release in the rat hippocampus.     

Enhanced hippocampal acetylcholine release in nociceptin-receptor knockout mice

Nociceptin (NOC), an endogenous ligand of the opioid receptor-like 1 receptor, is thought to be involved in learning and memory processes. Since acetylcholine (ACh) is involved in hippocampal function, and the hippocampus plays a critical role on the learning and memory function, hippocampal ACh release in NOC-receptor knockout mice was examined using an in vivo microdialysis method. The release of hippocampal ACh was largely increased in the knockout mice. Furthermore, in the knockout mice, an enhanced hippocampal theta rhythm, which is known to be linked to hippocampal memory function, was also observed. Immunohistochemically, in septum, co-existence of NOC receptor with cholinergic, but not with GABAergic neurons, was verified. The findings demonstrate that the NOC receptor is involved in hippocampal cholinergic function.     

Pregnenolone sulfate increases hippocampal acetylcholine release and spatial recognition

The pregnenolone sulfate is a neurosteroid with promnesic properties. Recently, a correlation between endogenous levels of pregnenolone sulfate in the hippocampus and performance in a spatial memory task has been reported in aged rats. Cholinergic transmission is known to modulate memory processes and to be altered with age. In the present experiment we investigated the effect of increasing doses of pregnenolone sulfate on hippocampal acetylcholine release. Our results show that intracerebroventricular administrations of this neurosteroid induced a dose-dependent increase in acetylcholine release. Administration of 12 and 48 nmol of pregnenolone sulfate induced a short lasting (20 min) enhancement of acetylcholine output with a maximum around 120% over baseline and the administration of 96 and 192 nmol doses induced a long-lasting (80 min) increase that peaked around 300% over baseline. In a second experiment we have observed that the 12 nmol dose enhanced spatial memory performance, whereas the 192 nmol dose was inefficient. These results are consistent with previous work suggesting that, a modest increase in acetylcholine release facilitates memory processes, while elevation beyond an optimal level is ineffective. Nevertheless, neurosteroids may be of value for reinforcing depressed cholinergic transmission in certain age-related memory disorders.     

Dopamine D-1 receptor agonist stimulation of prolactin secretion in man

Summary SKF 38393, a selective D-1 dopamine receptor agonist, elevated plasma prolactin levels in eight patients with various neurological disorders. Growth hormone concentrations were unaffected by SKF 38393 administration. The results suggest that D-1 receptors may be involved in the regulation of prolactin secretion.     

Dopamine and acetylcholine release in thalamus of non-parkinsonian and parkinsonian subjects

A study of neurotransmitter release has been performed on human brain biopsies obtained during the course of a therapeutic neurosurgical procedure. The electrically evoked-release of 3H-dopamine and 3H-acetylcholine was studied on slices from the thalamus of two parkinsonian and two non-parkinsonian subjects. Electrical stimulation of the non-parkinsonian thalamus increased the dopamine release but had no effect on acetylcholine release. This characteristic response had been previously observed in the normal rat thalamus. In contrast, the parkinsonian thalamus responded with a sharp increase in acetylcholine release without concomitant increase in dopamine release indicating the possible existence of an inhibitory control effect action of dopamine on acetylcholine release as seen in rat striatum.     

Astrocyte-derived kynurenic acid modulates basal and evoked cortical acetylcholine release

We tested the hypothesis that fluctuations in the levels of kynurenic acid (KYNA), an endogenous antagonist of the α7 nicotinic acetylcholine (ACh) receptor, modulate extracellular ACh levels in the medial prefrontal cortex in rats. Decreases in cortical KYNA levels were achieved by local perfusion of S -ESBA, a selective inhibitor of the astrocytic enzyme kynurenine aminotransferase II (KAT II), which catalyses the formation of KYNA from its precursor l -kynurenine. At 5 m m , S -ESBA caused a 30% reduction in extracellular KYNA levels, which was accompanied by a two-threefold increase in basal cortical ACh levels. Co-perfusion of KYNA in the endogenous range (100 n m ), which by itself tended to reduce basal ACh levels, blocked the ability of S -ESBA to raise extracellular ACh levels. KYNA perfusion (100 n m ) also prevented the evoked ACh release caused by d -amphetamine (2.0 mg/kg). This effect was duplicated by the systemic administration of kynurenine (50 mg/kg), which resulted in a significant increase in cortical KYNA formation. Jointly, these data indicate that astrocytes, by producing and releasing KYNA, have the ability to modulate cortical cholinergic neurotransmission under both basal and stimulated conditions. As cortical KYNA levels are elevated in individuals with schizophrenia, and in light of the established role of cortical ACh in executive functions, our findings suggest that drugs capable of attenuating the production of KYNA may be of benefit in the treatment of cognitive deficits in schizophrenia.     

Serotonergic modulation of hippocampal acetylcholine release after long-term neuronal grafting

Adult female rats sustained aspirative fimbria-fornix lesions and, 2 weeks later, received intrahippocampal grafts of fetal septal or mixed septal-raphe cell suspensions. Twenty-four months later, the extracellular concentration of hippocampal acetylcholine (ACh) was determined by microdialysis. Basal ACh levels (5-65 fmol/5 microl sham-operated rats) were strongly reduced after lesioning (3-7 fmol/5 microl). In septally transplanted and septal-raphe co-transplanted rats, hippocampal ACh concentrations were restored to near-normal levels (15-25 fmol/5 microl), indicating long-term functional survival of hippocampal transplants. After administration of citalopram (100 microM by infusion) and fenfluramine (20 mg/kg i.p.), the hippocampal ACh efflux was increased by 2- to 3-fold in all groups of rats. The relative increase of ACh was highest in co-transplanted rats, an effect which was possibly due to functional interactions between grafted raphe and septal neurons.     

Dopamine modulates a voltage-gated calcium channel in rat olfactory receptor neurons

Dopamine D2 receptors exist in the soma of rat olfactory receptor neurons. Actions of dopamine on the voltage-gated Ca(2+) channels in the neurons were investigated using the perforated whole-cell voltage-clamp. In 10 mM Ba(2+) solution, rat olfactory receptor neurons displayed the inward currents elicited by the voltage ramp (167 mV/s) and depolarizing step pulses from a holding potential of -91 mV. The inward Ba(2+) currents were greatly reduced by 10 microM nifedipine (L-type Ca(2+) channel blocker). The Ba(2+) currents were inhibited by the external application of dopamine. The IC(50) for the inhibition was about 1 microM. Quinpirole (10 microM, a D2 dopamine agonist) also inhibited the Ba(2+) currents. Quinpirole did not affect the activation and inactivation kinetics of the Ba(2+) currents. The results suggest that dopamine modulates the L-type Ca(2+) channels in rat olfactory receptor neurons via the mechanism independent of voltage.     

Dopamine modulates carotid nerve responses induced by acetylcholine on the cat petrosal ganglion in vitro

We have recently reported that application of acetylcholine (ACh) or nicotine to the petrosal ganglion—the sensory ganglion of the glossopharyngeal nerve—elicits a burst of discharges in the carotid nerve branch, innervating the carotid body and sinus, but not in the glossopharyngeal branch, innervating the tongue and pharynx. Thus, the perikarya of sensory neurons for the carotid bifurcation exhibit selective cholinosensitivity. Since dopamine (DA) modulates carotid nerve chemosensory activity, we searched for the presence of DA sensitivity at the perikarya of these neurons in the cat petrosal ganglion superfused in vitro. Applications of DA in doses of up to 5 mg to the ganglion did not modify the rate of spontaneous discharges in the carotid nerve. However, if DA was applied 30 s before ACh injections, ACh-evoked reactions were modified: low doses of DA enhanced the subsequent responses to ACh, while high doses of DA depressed the responses to ACh. This depressant effect of DA on ACh responses was partially antagonized by adding spiroperone to the superfusate. Our results show that the response to ACh of petrosal ganglion neurons projecting through the carotid nerve is modulated by DA acting on D₂ receptors located in the somata of these neurons. Thus, dopaminergic modulation of cholinosensitivity could be shared also by the membranes of peripheral endings and perikarya of primary sensory neurons involved in arterial chemoreception.     

A putative lysophosphatidylinositol receptor GPR55 modulates hippocampal synaptic plasticity

GPR55, an orphan G‐protein coupled receptor, is activated by lysophosphatidylinositol (LPI) and the endocannabinoid anandamide, as well as by other compounds including THC. LPI is a potent endogenous ligand of GPR55 and neither GPR55 nor LPIs' functions in the brain are well understood. While endocannabinoids are well known to modulate brain synaptic plasticity, the potential role LPI could have on brain plasticity has never been demonstrated. Therefore, we examined not only GPR55 expression, but also the role its endogenous ligand could play in long‐term potentiation, a common form of synaptic plasticity. Using quantitative RT‐PCR, electrophysiology, and behavioral assays, we examined hippocampal GPR55 expression and function. qRT‐PCR results indicate that GPR55 is expressed in hippocampi of both rats and mice. Immunohistochemistry and single cell PCR demonstrates GPR55 protein in pyramidal cells of CA1 and CA3 layers in the hippocampus. Application of the GPR55 endogenous agonist LPI to hippocampal slices of GPR55^+/+ mice significantly enhanced CA1 LTP. This effect was absent in GPR55^?/? mice, and blocked by the GPR55 antagonist CID 16020046. We also examined paired‐pulse ratios of GPR55^?/? and GPR55^+/+ mice with or without LPI and noted significant enhancement in paired‐pulse ratios by LPI in GPR55^+/+ mice. Behaviorally, GPR55^?/? and GPR55^+/+ mice did not differ in memory tasks including novel object recognition, radial arm maze, or Morris water maze. However, performance on radial arm maze and elevated plus maze task suggests GPR55^?/? mice have a higher frequency of immobile behavior. This is the first demonstration of LPI involvement in hippocampal synaptic plasticity.     

Dopamine release in human neocortical slices: characterization of inhibitory autoreceptors and of nicotinic acetylcholine receptor-evoked release

The autoinhibitory control of electrically evoked release of [3H]-dopamine and the properties of that induced by nicotinic receptor (nAChR) stimulation were studied in slices of the human neocortex. In both models [3H]-dopamine release was action potential-induced and exocytotic. The selective dopamine D2 receptor agonist (-)-quinpirole reduced electrically evoked release of [3H]-dopamine, yielding IC50 and I(max) values of 23 nM and 76%, respectively. Also, the effects of several other subtype-selective dopamine receptor ligands confirmed that the terminal dopamine autoreceptor belongs to the D2 subtype. The autoinhibitory feedback control was slightly operative under stimulation conditions of 90 pulses and 3 Hz, with a biophase concentration of endogenous dopamine of 3.6 nM, and was enhanced under blockade of dopamine reuptake. [3H]-dopamine release evoked in an identical manner in mouse neocortical slices was not inhibited by (-)-quinpirole, suggesting the absence of dopamine autoreceptors in this tissue and underlining an important species difference. Also, nAChR stimulation-induced release of [3H]-dopamine revealed a species difference: [3H]-dopamine release was evoked in human, but not in rat neocortical slices. The nAChRs inducing [3H]-dopamine release most probably belong to the alpha3/beta2subtype, according to the potencies and efficacies of subtype-selective nAChR ligands. Part of these receptors may be located on glutamatergic neurons.     

Dopamine D5 receptor localization on cholinergic neurons of the rat forebrain and diencephalon: a potential neuroanatomical substrate involved in mediating dopaminergic influences on acetylcholine release

The study of dopaminergic influences on acetylcholine release is especially useful for the understanding of a wide range of brain functions and neurological disorders, including schizophrenia, Parkinson's disease, Alzheimer's disease, and drug addiction. These disorders are characterized by a neurochemical imbalance of a variety of neurotransmitter systems, including the dopamine and acetylcholine systems. Dopamine modulates acetylcholine levels in the brain by binding to dopamine receptors located directly on cholinergic cells. The dopamine D5 receptor, a D1-class receptor subtype, potentiates acetylcholine release and has been investigated as a possible substrate underlying a variety of brain functions and clinical disorders. This receptor subtype, therefore, may prove to be a putative target for pharmacotherapeutic strategies and cognitive-behavioral treatments aimed at treating a variety of neurological disorders. The present study investigated whether cholinergic cells in the dopamine targeted areas of the cerebral cortex, striatum, basal forebrain, and diencephalon express the dopamine D5 receptor. These receptors were localized on cholinergic neurons with dual labeling immunoperoxidase or immunofluorescence procedures using antibodies directed against choline acetyltransferase (ChAT) and the dopamine D5 receptor. Results from this study support previous findings indicating that striatal cholinergic interneurons express the dopamine D5 receptor. In addition, cholinergic neurons in other critical brain areas also show dopamine D5 receptor expression. Dopamine D5 receptors were localized on the somata, dendrites, and axons of cholinergic cells in each of the brain areas examined. These findings support the functional importance of the dopamine D5 receptor in the modulation of acetylcholine release throughout the brain.     

Nicotinic acetylcholine receptor antagonist effect of fluoxetine in rat hippocampal slices

We compared the effect of mecamylamine and fluoxetine on the hippocampal noradrenaline (NA) release evoked by nicotine in vitro. Nicotine (100 μM) increased the basal release of [³H]NA from rat hippocampal slices. This effect was blocked by the potent nicotinic antagonist mecamylamine in a dose-dependent manner (IC₅₀=0.19 μM). The selective serotonin reuptake inhibitor (SSRI) fluoxetine also antagonised the response to nicotine in a dose-dependent manner with a similar strength (IC₅₀=0.57 μM). Our data indicate that fluoxetine has nicotinic acetylcholine receptor antagonist effect in the central nervous system. The possible clinical significance of this finding is discussed.     

PACAP/PAC1R signaling modulates acetylcholine release at neuronal nicotinic synapses

Neuropeptides collaborate with conventional neurotransmitters to regulate synaptic output. Pituitary adenylate cyclase-activating polypeptide (PACAP) co-localizes with acetylcholine in presynaptic nerve terminals, is released by stimulation, and enhances nicotinic acetylcholine receptor- (nAChR-) mediated responses. Such findings implicate PACAP in modulating nicotinic neurotransmission, but relevant synaptic mechanisms have not been explored. We show here that PACAP acts via selective high-affinity G-protein coupled receptors (PAC₁Rs) to enhance transmission at nicotinic synapses on parasympathetic ciliary ganglion (CG) neurons by rapidly and persistently increasing the frequency and amplitude of spontaneous, impulse-dependent nicotinic excitatory postsynaptic currents (sEPSCs). Of the canonical adenylate cyclase (AC) and phospholipase-C (PLC) transduction cascades stimulated by PACAP/PAC₁R signaling, only AC-generated signals are critical for synaptic modulation since the increases in sEPSC frequency and amplitude were mimicked by 8-Bromo-cAMP, blocked by inhibiting AC or cAMP-dependent protein kinase (PKA), and unaffected by inhibiting PLC. Despite its ability to increase agonist-induced nAChR currents, PACAP failed to influence nAChR-mediated impulse-independent miniature EPSC amplitudes (quantal size). Instead, evoked transmission assays reveal that PACAP/PAC₁R signaling increased quantal content, indicating that it modulates synaptic function by increasing vesicular ACh release from presynaptic terminals. Lastly, signals generated by the retrograde messenger, nitric oxide- (NO-) are critical for the synaptic modulation since the PACAP-induced increases in spontaneous EPSC frequency, amplitude and quantal content were mimicked by NO donor and absent after inhibiting NO synthase (NOS). These results indicate that PACAP/PAC₁R activation recruits AC-dependent signaling that stimulates NOS to increase NO production and control presynaptic transmitter output at neuronal nicotinic synapses.     

Diadenosine polyphosphates facilitate the evoked release of acetylcholine from rat hippocampal nerve terminals

Diadenosine polyphosphates are present in synaptic vesicles, are released upon nerve stimulation and possess membrane receptors, namely in presynaptic terminals. However, the role of diadenosine polyphosphates to control neurotransmitter release in the CNS is not known. We now show that diadenosine pentaphosphate (Ap(5)A, 3-100 microM) facilitated in a concentration dependent manner the evoked release of acetylcholine from hippocampal nerve terminals, with a maximal facilitatory effect of 116% obtained with 30 microM Ap(5)A. The selective diadenosine polyphosphate receptor antagonist, diinosine pentaphosphate (Ip(5)I, 1 microM), inhibited by 75% the facilitatory effect of Ap(5)A (30 microM), whereas the P(2) receptor antagonists, suramin (100 microM) and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 10 microM) only caused a 18-24% inhibition, the adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (20 nM), caused a 36% inhibition and the adenosine A(2A) receptor antagonist, 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo [2,3-a][1,3, 5]triazin-5-ylamino]ethyl)phenol (ZM 241385, 20 nM), was devoid of effect. These results show that diadenosine polyphosphates act as neuromodulators in the CNS, facilitating the evoked release of acetylcholine mainly through activation of diadenosine polyphosphate receptors.     

The nitric oxide system modulates the in vivo release of acetylcholine in the nucleus accumbens induced by stimulation of the hippocampal fornix/fimbria-projection

Nerve signals from the hippocampus to the nucleus accumbens (NAc) are transmitted through a glutamatergic pathway via the fornix/fimbria fibres. The aim of the present study was to investigate whether cholinergic neurons are activated by this projection and whether the nitric oxide (NO) system is also involved in the signal transduction within this nucleus. For this purpose, the NAc of urethane-anaesthetized rats was superfused, by the push-pull technique, with compounds that influence the NO system while the fornix/fimbria was electrically stimulated for short periods. The amount of acetylcholine (ACh) released in the superfusate was then determined. Electrical stimulation of the fornix/fimbria increased the ACh output in the NAc. This effect was abolished by superfusion with tetrodotoxin and decreased by superfusion with the glutamate receptor antagonists AP-5 and DNQX indicating the involvement of action potentials and glutamate. Superfusion with the inhibitor of neuronal NO synthase, NS 2028 also diminished stimulation-evoked ACh release. The NO donor PAPA/NO increased basal release. Simultaneous application of PAPA/NO and electrical stimulation led to an over-additive increase of ACh release. The effect of PAPA/NO on stimulation-evoked release was also abolished by NS 2028. The selective inhibitor of phosphodiesterase type 5 (PDE 5), 5-[2-ethoxy-5-(morpholinylacetyl)phenyl]-1,6-dihydro-1-methyl-3-propyl-7H-pyrazolo[4,3-d]pyrimidin-7-one methanesulphanate monohydrate also enhanced stimulation-induced release of ACh. Our findings indicate, that action potentials propagated by the fornix/fimbria to the NAc release glutamate which increases ACh release predominantly via NMDA receptors. In addition, nitrergic neurons are activated to enhance NO synthesis. The released NO seems to exert, via cGMP, a potent facilitatory role in the transduction and processing of signals from the hippocampus within the NAc, while the PDE 5 decreases the effects of NO.     

Inhibition of hippocampal acetylcholine release after acute and repeated Δ-tetrahydrocannabinol in rats

The effects of acute and repeated administration of Δ⁹-tetrahydrocannabinol (Δ⁹-THC), the psychoactive principle of marijuana, on acetylcholine release in the hippocampus was studied in freely moving rats by microdialysis. The acute intraperitoneal (i.p.) administration of Δ⁹-THC at the doses of 2.5 and 5 mg/kg reduced acetylcholine release by about 25% and 45%, respectively. A dose of 7.5 mg/kg produced no further reduction. Δ⁹-THC effects were antagonized by the cannabinoid CB₁ antagonist SR141716A at the i.p. dose of 1 mg/kg, per se ineffective in modifying acetylcholine concentrations. After a repeated exposure (twice daily for up to seven days) to Δ⁹-THC (7.5 mg/kg, i.p.) or vehicle (0.3 ml/kg, i.p.), the inhibitory effect of Δ⁹-THC (2.5 and 5 mg/kg, i.p) on acetylcholine release was not reduced. The results confirm previous observations that cannabinoids inhibit acetylcholine release through cannabinoid CB₁ receptors, and indicate that no tolerance to this effects develops after a repeated Δ⁹-THC administration.