Antagonism by citalopram and tianeptine of presynaptic 5-HT1B heteroreceptors inhibiting acetylcholine release

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

During aging, neurotransmission systems such as the cholinergic and serotonergic ones are altered. Using rats aged 3 or 24-26 months, this study investigated whether the well-described 8-OH-DPAT-induced increase of hippocampal acetylcholine release was altered in aged rats and whether it may vary according to the magnitude of age-related cognitive deficits. Long-Evans female rats aged 24-26 months were classified as good or bad performers on the basis of their reference-memory performance in a Morris water-maze task. Subsequently, the efficiency of 5-HT(1A) receptor agonist 8-OH-DPAT (0.5 mg/kg, s.c.) in triggering hippocampal acetylcholine release was evaluated by in vivo microdialysis and high performance liquid chromatography analysis. Besides a reduced baseline release in aged rats and a correlation between the baseline release and probe-trial performance in all rats, the results demonstrated that 8-OH-DPAT produced a significant increase of hippocampal acetylcholine release (peak value) in all rats, whether aged or young. While significant in bad performers (+56%), this increase did not reach significance in good performers (+32%). The results suggest that (i) some aspects of cognitive alterations related to aging might be linked to the baseline release of acetylcholine in the hippocampus, and (ii) the cholinergic innervation of the hippocampus of aged rats responds almost normally to systemic activation of 5-HT(1A) receptors, and (iii) differential alterations of cholinergic/serotonergic interactions assessed by determination of the 8-OH-DPAT-induced release of acetylcholine in the hippocampus could not be linked with clarity to the cognitive status of aged rats.

5,7-Dihydroxytryptamine lesions enhance and serotonergic grafts normalize the evoked overflow of acetylcholine in rat hippocampal slices

Adult rats were subjected to intracerebroventricular injections of 5,7-dihydroxytryptamine (5,7-DHT; 150 micro g) and, 15 days later, to intrahippocampal grafts of fetal raphe cell suspensions. About 11 months later, we assessed baseline and electrically evoked release of tritium ([3H]) in hippocampal slices, preloaded with tritiated ([3H])choline or [3H]serotonin (5-HT), in the presence or absence of the 5-HT1B receptor agonist CP-93,129 and the 5-HT receptor antagonist methiothepine. HPLC determinations of monoamine concentrations were also performed. The lesions reduced the concentration of 5-HT (-90%) and the accumulation (-80%) as well as the evoked release (-90%) of [3H]5-HT. They also decreased the inhibitory effects of CP-93,129 on the evoked release of [3H]5-HT. Most interestingly, they facilitated the evoked release of [3H]acetylcholine (+20%). In slices from rats subjected to lesions and grafts, the responsiveness of the serotonergic autoreceptors (presumably located on the terminals of the grafted neurons) and the release of acetylcholine were close to normal. These results confirm that grafts rich in serotonergic neurons may partially compensate for the dramatic effects of 5,7-DHT lesions on serotonergic hippocampal functions. The lesion-induced reduction of the 5-HT1B autoreceptor-mediated inhibition of evoked 5-HT release may be an adaptation enhancing serotonergic transmission in the (few) remaining terminals. The facilitated release of acetylcholine is probably caused by a reduced serotonergic tone on the inhibitory 5-HT1B heteroreceptors of the cholinergic terminals. When related to data in the literature, this facilitation may be of particular interest in terms of transmitter-based strategies developed to tackle cognitive symptoms related to neurodegenerative diseases.

Local modulation of the 5-HT release in the dorsal striatum of the rat: an in vivo microdialysis study

Using in vivo microdialysis in freely moving rats, we examined the involvement of major striatal transmitters on the local modulation of the 5-HT release. Tetrodotoxin reduced the striatal 5-HT output to 15-20% of baseline. The selective 5-HT(1B) receptor agonist CP 93129 (50 microM) reduced (50%) and the 5-HT(2A/2C) receptor agonist DOI (1-100 microM) increased (220%) the 5-HT output. Neither GABA nor baclofen (100 nM-100 microM) altered the 5-HT output. The glutamate reuptake inhibitor L-trans-PDC (1-4 mM) raised 5-HT to 280% of baseline. This effect was not antagonized by the NMDA receptor antagonist MK-801 (0.5 mg/kg i.p.). Local MK-801 (10-100 microM) did not significantly alter the 5-HT output. Finally, neither carbachol (10-100 microM) nor quipirole (10 microM-1 mM) affected 5-HT. These data suggest that the striatal 5-HT release is influenced by local serotonergic and glutamatergic (but not GABAergic) inputs.

5-HT-moduline, a 5-HT(1B/1D) receptor endogenous modulator, interacts with dopamine release measured in vivo by microdialysis

5-Hydroxytryptamine-moduline (5-HT-moduline) is an endogenous tetrapeptide (Leu-Ser-Ala-Leu) recently isolated and characterized from mammalian brain. This compound interacts with 5-HT1B receptors as a non-competitive, high-affinity antagonist and has the properties of an allosteric modulator. 5-HT-moduline could play an important role in the regulation of serotonergic transmission and also, through heteroreceptors, dopaminergic transmission. The aim of this work was to examine the potential ability of 5-HT-moduline to modify the basal extracellular concentration of dopamine and its metabolites (3-methoxytyramine, dihydroxyphenylacetic acid and homovanillic acid), in the rat striatum and to determine its potential interaction with the stimulating activity of a specific 5-HT1B receptor agonist, 3-(1,2,5,6-tetrahydropyrid-4-yl) pyrrolo [3,2-b] pyrid-5-one (CP-93,129), on the release of dopamine. The technique is based on in vivo microdialysis using probes implanted in the striatum of the conscious rat. Results showed that the perfusion of 5-HT-moduline directly into this structure (1.25 mM) increased the striatal level of dopamine by two-fold (104% of the absolute basal release values, P = 0.0015) and that of 3-methoxytyramine by 3-fold (293%, P = 0.0001) without any change in the terminal metabolite concentrations. The intrastriatal administration of CP-93,129 induced a statistically significant, dose-dependent increase of dopamine levels (P < 0.0001). Coperfusion of 5-HT-moduline did not significantly alter the effect of CP-93,129 at 0.1 and 0.5 mM, but appeared to have an additive effect on the lowest dose (P = 0.0406). The results obtained show that 5-HT-moduline directly administered into the striatum increases the release of dopamine in this area. Presumably, this effect results from the desensitization of 5-HT1B receptors located on dopamine terminals. However, the fact that a 5-HT1B receptor agonist (CP-93,129) also increased the release of dopamine in the striatum and that 5-HT-moduline exhibited a slight additive effect with that of a low concentration of CP-93,129 suggests that the two substances interact with different mechanisms.

The fimbria-fornix/cingular bundle pathways: a review of neurochemical and behavioural approaches using lesions and transplantation techniques

Extensive lesions of the fimbria-fornix pathways and the cingular bundle deprive the hippocampus of a substantial part of its cholinergic, noradrenergic and serotonergic afferents and, among several other behavioural alterations, induce lasting impairment of spatial learning and memory capabilities. After a brief presentation of the neuroanatomical organization of the hippocampus and the connections relevant to the topic of this article, studies which have contributed to characterize the neurochemical and behavioural aspects of the fimbria-fornix lesion "syndrome" with lesion techniques differing by the extent, the location or the specificity of the damage produced, are reviewed. Furthermore, several compensatory changes that may occur as a reaction to hippocampal denervation (sprouting changes in receptor sensitivity and modifications of neurotransmitter turnover in spared fibres) are described and discussed in relation with their capacity (or incapacity) to foster recovery from the lesion-induced deficits. According to this background, experiments using intrahippocampal or "parahippocampal" grafts to substitute for missing cholinergic, noradrenergic or serotonergic afferents are considered according to whether the reported findings concern neurochemical and/or behavioural effects. Taken together, these experiments suggest that appropriately chosen fetal neurons (or other cells such as for instance, genetically-modified fibroblasts) implanted into or close to the denervated hippocampus may substitute, at least partially, for missing hippocampal afferents with a neurochemical specificity that closely depends on the neurochemical identity of the grafted neurons. Thereby, such grafts are able not only to restore some functions as they can be detected locally, namely within the hippocampus, but also to attenuate some of the behavioural (and other types of) disturbances resulting from the lesions. In some respects, also these graft-induced behavioural effects might be considered as occurring with a neurochemically-defined specificity. Nevertheless, if a graft-induced recovery of neurochemical markers in the hippocampus seems to be a prerequisite for also behavioural recovery to be observed, this neurochemical recovery is neither the one and only condition for behavioural effects to be expressed, nor is it the one and only mechanism to account for the latter effects.

Ibogaine and cocaine abuse: pharmacological interactions at dopamine and serotonin receptors

Ibogaine is an indole alkaloid that has been of interest in recent years due to its putative efficacy in the treatment of drug dependence. For the most part, animal data have shown attenuation of some of the effects of stimulant drugs, for example, motor stimulation and self-administration. The mechanism of this inhibition of drug-induced behavior seems to suggest the action of the dopamine, serotonin, NMDA, kappa, and/or sigma receptor sites, as indicated by the affinity of ibogaine to receptor selective ligands in binding competition studies. However, affinity for receptors does not in itself indicate their involvement. In vitro perfusion studies have proven a useful model to study the effect of ibogaine on neurotransmitter systems and the functional effects of such interactions. This review summarizes these data and the support of multiple effects of ibogaine, and the potential importance of its action on serotonergic modulation of dopamine release.

Involvement of GABA systems in acetylcholine release induced by 5-HT3 receptor blockade in slices from rat entorhinal cortex

The aim of the present study was to examine the role of 5-HT3 receptors in spontaneous and K(+)-evoked acetylcholine (ACh) release from rat entorhinal cortex and striatal slices. The 5-HT3 receptor antagonists ondansetron and granisetron (0.01-10 microM) produced a concentration-dependent increase in both spontaneous and K(+)-evoked [3H]ACh release in the two brain regions studied. The release of ACh was Ca(2+)-dependent and tetrodotoxin-sensitive. 5-HT3 receptor agonists, such as 2-methyl-5-HT and 1-phenylbiguanide, at concentrations up to 1 microM, did not show any intrinsic effect on [3H]ACh release in both rat brain regions. However, 2-methyl-5-HT, 1 microM, fully blocked the ondansetron-induced enhancement in both basal and K(+)-evoked ACh release, suggesting that 5-HT, through 5-HT3 receptor activation, tonically inhibits ACh release. The possible implication of interposed inhibitory systems in ACh release after 5-HT3 receptor blockade was subsequently analyzed. While the effect of ondansetron was not modified by haloperidol or naloxone, the GABAA receptor antagonist bicuculline produced a marked potentiation of ACh release in the entorhinal cortex but not in the striatum. The results suggest that in this cortical area 5-HT activates 5-HT3 receptors located on GABAergic neurons which in turn inhibit cholinergic function.

Isolation and characterization of an endogenous peptide from rat brain interacting specifically with the serotonergic 1B receptor subtypes

The existence of endogenous compounds interacting with the serotonergic system was previously postulated. In the present work, rat brain tissues were extracted by acidic and organic procedures. The resulting extract was tested for its capacity to interact with the binding of [3H]5-hydroxytryptamine ([3H]5-HT) to 5-HT1 receptors. Compounds responsible for the observed inhibitory activities were isolated and purified by high pressure liquid chromatography. A tetrapeptide corresponding to a novel amino acid sequence Leu-Ser-Ala-Leu (LSAL) was identified. It reduces the binding of [3H]5-HT to 5-HT1 receptors at low concentration (IC50 = 10(-10) M). This effect corresponds to a specific interaction at 5-HT1B receptors since LSAL does not significantly affect other neurotransmitter bindings. LSAL appears heterogeneously distributed throughout the brain (hippocampus > cerebellum > striatum > brain stem) and in peripheral tissues (kidney > lung > stomach > blood > liver > spleen). Two other peptides, Leu-Ser (LS) and Ala-Leu (AL), were also purified. They hardly affected [3H]5-HT binding compared with LSAL. They presumably represent degradation products of the functional peptide LSAL. The fact that LSAL interacts specifically with 5-HT1B receptors that inhibit the release of neurotransmitters and particularly that of 5-HT itself suggests that this peptide may be involved in mechanisms controlling 5-HT neurotransmission and, accordingly, may play an important role in pathophysiological functions related to 5-HT activity.

Abnormalities in 5-HT1A and 5-HT1B receptor binding in severe-seizure genetically epilepsy-prone rats (GEPR-9s)

The present study was designed to determine whether abnormalities in serotonin receptor binding co-exist with the presynaptic serotonergic deficits that have previously been identified in the genetically epilepsy-prone rat (GEPR) brain. In vitro binding of [3H]8-OH-DPAT (0.16-10.3 nM) to 5-HT1A receptor sites was found to be decreased in the hippocampus of severe seizure GEPRs (GEPR-9s) when compared to nonepileptic control rats, while no difference in [3H]8-OH-DPAT binding was observed in the GEPR-9 corpora quadrigemina or midbrain tegmentum. The decreased binding of [3H]8-OH-DPAT to hippocampal membranes was due to a decrease in Bmax (P < 0.001), rather than to a change in the Kd. Conversely, in vitro binding of [125I]cyanopindolol (2-400 pM) to 5-HT1B receptor sites was increased in the GEPR-9 hippocampus, corpora quadrigemina and midbrain tegmentum when compared to nonepileptic control rats. The increased binding of [125I]cyanopindolol in all three regions resulted from an increase in the Bmax (P < 0.05), rather than a change in the Kd. These finding suggest that in addition to the innate reduction in 5-HT presynaptic markers, GEPR-9s also exhibit abnormalities in the density of 5-HT1A and 5-HT1B receptors in some regions of the brain. Inasmuch as serotonin acts to attenuate audiogenic seizures in GEPRs, these abnormalities in 5-HT receptor binding may contribute to the seizure susceptibility exhibited by these animals.

Downregulation of muscarinic- and 5-HT1B-mediated modulation of [3H]acetylcholine release in hippocampal slices of rats with fimbria-fornix lesions and intrahippocampal grafts of septal origin

Adult Long-Evans female rats sustained electrolytic fimbria-fornix lesions and, two weeks later, received intrahippocampal suspension grafts of fetal septal tissue. Sham-operated and lesion-only rats served as controls. Between 6.5 and 8 months after grafting, both the [3H]choline accumulation and the electrically evoked [3H]acetylcholine ([3H]ACh) release were assessed in hippocampal slices. The release of [3H]ACh was measured in presence of atropine (muscarinic antagonist, 1 microM), physostigmine (acetylcholinesterase inhibitor, 0.1 microM), oxotremorine (muscarinic agonist, 0.01 microM-10 microM), mecamylamine (nicotinic antagonist, 10 microM), methiothepin (mixed 5-HT1/5-HT2 antagonist, 10 microM), 8-OH-DPAT (5-HT1A agonist, 1 microM), 2-methyl-serotonin (5-HT3 agonist, 1 microM) and CP 93129 (5-HT1B agonist, 0.1 microM-100 microM), or without any drug application as a control. In lesion-only rats, the specific accumulation of [3H]choline was reduced to 46% of normal and the release of [3H]ACh to 32% (nCi) and 43% (% of tissue tritium content). In the grafted rats, these parameters were significantly increased to 63%, 98% and 116% of control, respectively. Physostigmine reduced the evoked [3H]ACh release and was significantly more effective in grafted (-70%) than in sham-operated (-56%) or lesion-only (-54%) rats. When physostigmine was superfused throughout, mecamylamine had no effect. Conversely, atropine induced a significant increase of [3H]ACh release in all groups, but this increase was significantly larger in sham-operated rats (+209%) than in the other groups (lesioned: +80%; grafted: +117%). Oxotremorine dose-dependently decreased the [3H]ACh release, but in lesion-only rats, this effect was significantly lower than in sham-operated rats. Whatever group was considered, 8-OH-DPAT, methiothepin and 2-methyl-serotonin failed to induce any significant effect on [3H]ACh release. In contrast, CP 93129 dose-dependently decreased [3H]ACh release. This effect was significantly weaker in grafted rats than in the rats of the two other groups. Our data confirm that cholinergic terminals in the intact hippocampus possess inhibitory muscarinic autoreceptors and serotonin heteroreceptors of the 5-HT1B subtype. They also show that both types of receptors are still operative in the cholinergic terminals which survived the lesions and in the grafted cholinergic neurons. However, the muscarinic receptors in both lesioned and grafted rats, as well as the 5-HT1B receptors in grafted rats show a sensitivity which seems to be downregulated in comparison to that found in sham-operated rats. In the grafted rats, both types of downregulations might contribute to (or reflect) an increased cholinergic function that results from a reduction of the inhibitory tonus which ACh and serotonin exert at the level of the cholinergic terminal.

Presynaptic 5-HT auto- and heteroreceptors in the human central and peripheral nervous system

In view of the potential pathophysiological and therapeutic implications, presynaptic 5-HT auto- and heteroreceptors have been identified and characterized in isolated human tissues and their functional role has been determined. Such investigations have been carried out in different laboratories including that of the authors. Basic evidence for the involvement of inhibitory 5-HT receptors in modulation of 5-HT release in the cerebral cortex was obtained in slices: exogenous 5-HT inhibited 5-HT release in a manner susceptible to blockade by methiothepin, which given alone facilitated 5-HT release, probably by preventing endogenous 5-HT from activating the inhibitory receptors. The latter receptors are located on the 5-HT nerve terminals themselves, since 5-HT (and sumatriptan) also inhibited 5-HT release from cortical synaptosomes. Their pharmacological properties conform to those of the 5-HT1D class. Subclassification (5-HT1D alpha or 5-HT1D beta) has been tried with ketanserin which has an at least 60 times higher affinity for 5-HT1D alpha (pki = 7.1) than 5-HT1D beta receptors. Since ketanserin (0.32 microM) did not affect the concentration-response curve for 5-carboxamidotryptamine (5-CT), the presynaptic 5-HT autoreceptor may belong to the 5-HT1D beta rather than the 5-HT1D alpha subtype. The sympathetic nerve terminals of the human saphenous vein are endowed with inhibitory 5-HT1D beta heteroreceptors, as indicated by the potency ratio of several 5-HT receptor agonists in inhibiting noradrenaline release in strips of this blood vessels and by the ability of methiothepin, but not of ketanserin 0.3 microM, to act as an antagonist. Noradrenergic nerves in the dura mater, which probably innervate its microvasculature, may also be endowed with inhibitory 5-HT receptors, since 5-HT inhibited noradrenaline release from this tissue. In strips of atrial appendages, 5-HT receptor agonists (e.g. 5-HT, 5-CT and sumatriptan) inhibited noradrenaline release at potencies which are correlated with their ki values at 5-HT1D alpha and 5-HT1D beta receptors. Since this inhibitory effect was antagonized by ketanserin (0.3 but not 0.03 microM) and methiothepin, the presynaptic 5-HT receptor in this tissue may belong to the 5-HT1D alpha subtype. However, this conclusion needs further confirmation by experiments with more potent and subtype-selective antagonists of 5-HT1D alpha and 5-HT1D beta receptors.

Serotonergic modulation of cholinergic function in the central nervous system: cognitive implications

Accumulating evidence suggests that serotonin may modulate cholinergic function in several regions of the mammalian brain and that these serotonergic/cholinergic interactions influence cognition. The first part of this review is an overview of histological, electrophysiological and pharmacological (in vitro, in vivo) data indicating that, in several brain regions (e.g., hippocampus, cortex and striatum), there are neuroanatomical substrates for a serotonergic/cholinergic interaction, and that alterations in serotonergic activity may induce functional changes in cholinergic neurons. In the second part, the review focuses on experimental approaches showing or suggesting that central cholinergic and serotonergic mechanisms are cooperating in the regulation of cognitive functions. These arguments are based on lesion, intracerebral grafting and pharmacological techniques. It is concluded that not all mnesic perturbations induced by concurrent manipulations of the serotonergic and cholinergic systems can be attributed to a serotonergic modification of the cholinergic system. The cognitive faculties of an organism arise from interactions among several neurotransmitter systems within brain structures such as, for instance, the hippocampus or the cortex, but also from influences on memory of other general functions that may involve cerebral substrates different from those classically related to mnesic functions (e.g., attention, arousal, sensory accuracy, etc.).

Effect of chronic antidepressant treatment on 5-HT1B presynaptic heteroreceptors inhibiting acetylcholine release

The effect of long term treatment with two tricyclic antidepressants on the sensitivity of 5-HT1B presynaptic heteroreceptors inhibiting acetylcholine (ACh) release was investigated. Groups of male rats received during 14 days either saline, citalopram (20 mg/kg), a serotonin (5-HT) uptake blocker, or tianeptine (2 x 10 mg/kg), an antidepressant that enhances 5-HT uptake. The efficacy of the 5-HT1B selective agonist 7-trifluoromethyl-4-(4-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline (CGS 12066B) in reducing K(+)-evoked [3H]acetylcholine release from hippocampal synaptosomes was determined 24 hr after the last administration. The chronic treatment with citalopram or tianeptine modified neither the basal nor the K(+)-evoked release of [3H]acetylcholine. In contrast, these treatments significantly reduced the efficacy of CGS 12066B to inhibit the release of [3H]acetylcholine induced by K+ depolarization. These data suggest that chronic antidepressant treatment desensitizes 5-HT1B presynaptic heteroreceptors through a mechanism which seems to be independent of the synaptic availability of 5-HT.