Ultrastructural localization of the GTP-binding protein Go in neurons.

The ultrastructural localization of Go, a GTP-binding protein (G protein) highly expressed in nervous tissues, was performed in cultured fetal and adult murine neurons, using affinity-purified polyclonal antibodies against the alpha subunit of the Go protein (Go alpha). These antibodies recognized denatured Go alpha and both the native Go alpha-subunit and the Go alpha beta gamma heterotrimer. At the ultrastructural level, the positive immunoreactivity detected in cultured cells as well as in thin frozen sections, showed that Go was largely distributed in cell bodies and neuritic cytoplasm. Labelling was principally noted on the cytoplasmic face of the plasma membrane lining the cell body and the neurites, especially in 'cell-cell' contacts, but also in the cytoplasmic matrix, between endoplasmic reticulum and Golgi cisternae. No immunoreactivity was observed on the inner face of the pre- or postsynaptic membranes in both adult brain and in cultured neurons. This last finding strongly suggests that the Go protein is not involved in transducing chemical signals at the level of synapses, but more probably modulates the synaptic functions by controlling the activity of effectors localized outside of the synaptic densities.

Cloning, expression and pharmacology of the mouse 5-HT(4L) receptor.

Since most of our knowledge on pharmacological properties of brain 5-HT4 receptors have been discussed for mouse colliculi neurons, we cloned the corresponding receptor using the RT-PCR approach. As expected, the homology with the already cloned rat 5-HT(4L) receptor was high, revealing only 16 differences at the amino-acid level. One of the differences, proline75 in mouse, alanine75 in the already published rat sequences was not confirmed. Therefore this proline is part of the consensus sequence present in all 5-HT receptor transmembrane domain II (LVMP). Comparing the affinities of 11 agonists and five antagonists for the cloned mouse receptor (5-HT(4L))expressed in LLCPK1 and the corresponding receptor in mouse colliculi shows an excellent correlation. The transfected mouse 5-HT(4L) receptor stimulated cAMP production. When expressed at high density, it exhibited intrinsic activity. In contrast to the previously described distribution, we found that mRNA encoding for both the short (5-HT(4S))and the long form (5-HT(4L)) of 5-HT4 receptors are expressed in all mouse and rat brain areas.

Regional distribution and ontogeny of 5-HT4 binding sites in rodent brain.

We have investigated the regional distribution of 5-hydroxytryptamine4 (5-HT4) receptor binding sites in the adult guinea pig, rat and mouse brain using the specific 5-HT4 antagonist [3H]GR113808 as a radioligand. The developmental changes in the expression of these binding sites were also investigated quantitatively in the rat brain (gestational days 16 and 19; postnatal days 1, 3, 7, 9, 12 and 21). In order to compare previously obtained data on primary cultures, semi-quantitative analysis was also performed during mouse brain ontogeny (postnatal days 1, 7 and 11). The main finding of this study is that 5-HT4 receptors have comparable, wide and heterogeneous distributions in the adult brain of the species investigated, with densities reaching adult levels between the second and third postnatal week in most regions of the rat and mouse brain. In contrast, a progressive loss of 5-HT4 binding sites is observed in the pons, whereas a transient peak of receptor expression is seen during the second postnatal week in the globus pallidus and substantia nigra pars lateralis. The developmental pattern of 5-HT4 receptor distribution suggests, except in latter regions, that these receptors probably exert a minor role in developmental processes. In the adult, high densities of [3H]GR113808 binding sites are present in various regions belonging to limbic system (islands of Calleja, olfactory tubercle, fundus striati, ventral pallidum, septal region, hippocampus, amygdala), or known to be components of different pathways, such as the hippocampo-habenulo-interpeduncular and the striato-nigro-tectal pathways. While the regional distributions of [3H]GR113808 binding sites were identical in the mouse and rat, some differences were observed in the guinea-pig, in particular in the globus pallidus, substantia nigra and interpeduncular nucleus. The expression of 5-HT4 receptors in limbic areas is highly suggestive of a role for these receptors in emotional processes, whereas their expression in the striato-nigral-tectal pathway might be indicative of a role in the control of visuo-motor activity.

The 5-HT4 receptor subtype inhibits K+ current in colliculi neurones via activation of a cyclic AMP-dependent protein kinase.

1. The aim of the present study was to examine the effect of 5-hydroxytryptamine (5-HT) on K+ current in primary culture of mouse colliculi neurones and to identify the 5-HT receptor subtype that could be involved in this effect. 2. The voltage-activated K+ current of the neurones was partially blocked by 8-bromo adenosine 3':5'-cyclic monophosphate (8-bromo-cyclic AMP). This effect was mimicked by 5-HT and the action of 5-HT could be antagonized by H7, a non specific protein kinase inhibitor, and by PKI, the specific cyclic AMP-dependent protein kinase blocker. 3. A similar cyclic AMP-dependent blockade of the K+ current was found with renzapride (BRL 24,924) and other 5-HT4 receptor agonists such as cisapride, BIMU 8, zacopride and 5-methoxytryptamine (5-MeOT). ICS 205,930, the classical 5-HT4 receptor blocker, could not be used in this study because it inhibited the studied K+ current by itself. However, the novel 5-HT4 receptor antagonist, DAU 6285 blocked the effects of 5-HT and renzapride on the K+ current. 4. The current was insensitive to the 5-HT1 and 5-HT3 receptor agonists (8-hydroxy-2-(di-n-propylamino) tetralin, RU 24,969, carboxamidotryptamine, 2-CH3-5-HT) as well as to 5-HT1, 5-HT2 and 5-HT3 antagonists (methiothepin, ketanserin, ondansetron [GR 38,032]). Moreover, these antagonists did not affect the actions of the tested 5-HT4 receptor agonists. 5. The present results show that part of the voltage-activated K+ current in mouse colliculi neurones is cyclic AMP-sensitive and the blockade of the current by 5-HT involves the 5-HT4 receptor subtype.The putative implication of 5-HT4 receptors in neuronal plasticity, via a blockade of K+ channels, is discussed.

5-HT4 receptors: gene, transduction and effects on olfactory memory.

In this paper we discuss 1) the primary structures, pharmacology, and brain distribution of cloned 5-HT4 receptors; 2) the chromosomal localization of the h5-HT4 receptor; 3) whether benzamides are full or partial agonists because of a species or a coupling difference; 4) the intrinsic activity of 5-HT4 receptors and inverse agonism of GR125487 in COS-7 cells but not in colliculi neurons; 5) the modulation of 5-HT4 receptor binding and activity; and 6) the long-term blockade of K+ channels by 5-HT4 agonists and its effect on olfactory memory. We conclude that 1) the cloning of 5-HT4 receptors in different species using RT-PCR from different tissues reveals the presence of several splice variants for 5-HT4 receptors differing in the C-terminal part, downstream from the amino acid L358; 2) the pharmacological properties of 5-HT4 receptors are dependent on the cellular context in which they are expressed; and 3) 5-HT4 agonists can be added to the list of compounds having pro-cognitive properties.

5-HT4 receptors: cloning and expression of new splice variants.

On the basis of differences in the potencies and intrinsic activity of 5-HT4 receptor agonists in different biological models it has been suggested that there is heterogeneity among 5-HT4 receptors. Here, we report the molecular cloning of several 5-HT4 receptor splice variants in mouse, rat, and human brain. Our data suggest that the differences in efficacy of 5-HT4 ligands on 5-HT4 receptor-mediated responses in several tissues is due to differences in coupling efficiency rather than to the presence of different 5-HT4 receptor isoforms.

5-HT6 receptors positively coupled to adenylyl cyclase in striatal neurones in culture.

5-HT receptor positively coupled to adenylyl cyclase in striatal neurones in culture does not correspond to the 5-HT4 receptor. 5-HT induces an increase in cAMP level with an EC50 of 125 nM. 5-HT agonists displayed the following rank order of potencies 5-HT > LSD > 5-MeOT > 5-CT. 8-OH-DPAT, RU 24969 and cisapride were inactive. The most efficacious antagonists were methiothepin and tricyclic antipsychotic drugs (clozapine, amitriptyline and nortryptyline). The pharmacological profile defined by both functional studies (cAMP level) and binding experiments ([125I]-LSD binding), and its localization in striatal neurones are in favour of the presence of the recently cloned 5-HT6 receptor in these cells.

Cloning and expression of human 5-HT4S receptors. Effect of receptor density on their coupling to adenylyl cyclase.

We have isolated a cDNA encoding the 5-HT4S receptor by RT-PCR on poly (A)+ RNA from both human heart and brain. The sequence homology with the rat and mouse 5-HT4 receptors was high: 93.8% of identity in the amino acid sequence. None of the 24 amino acid substitutions observed between rat and human receptors are at positions likely to modify their pharmacology. Comparing the pharmacological properties of six agonists and five antagonists on rat and human 5-HT4S receptors revealed no significant differences. We have analyzed the behavior of renzapride, a full and a partial agonist on mouse colliculi neurons and human heart biological responses respectively. The coupling efficiency of renzapride was two-fold lower than that of 5-HT for the stimulation of 5-HT4S receptors transfected in two different cell lines (LLC-PK1 and COS-7), but increasing the receptor density suppressed the partial agonist effect of renzapride.

[3H]-GR113808 labels 5-HT4 receptors in the human and guinea-pig brain.

A tritiated specific and highly potent 5-HT4 antagonist: [3H]-GR113808 was used to label specific binding sites in human brain and for comparison in guinea-pig brain. [3H]-GR113808 association and dissociation were rapid, the binding was saturable and displaced by various serotoninergic agents with an affinity corresponding to their previously reported 5-HT4 receptor mediated effects. In vitro ligand binding autoradiography was used to investigate the distribution of [3H]-GR113808 recognition sites on human brain sections. The highest density was observed in the striato-nigral system. Low densities of binding sites were also found in hippocampus, neocortex and colliculus. This distribution of [3H]-GR113808 binding sites is similar to that found in the guinea-pig brain.

Vasoactive intestinal peptide actions on cyclic AMP levels in cultured striatal neurons.

The actions of vasoactive intestinal peptide (VIP) on intracellular cyclic AMP, in primary cultures of striatal neurons, were examined. VIP stimulated cyclic AMP formation five-fold over basal levels in neurons after 6 days in vitro (DIV); half maximal activation (EC50) was obtained with 10 nM of the peptide. VIP stimulation was both more potent and effective than those due to adrenocorticotropin (ACTH), dopamine (DA) or serotonin (5-HT). VIP efficacy was augmented to 15-20-fold in the presence of 0.1 microM forskolin, which had virtually no effect on cyclic AMP production alone; VIP potency was unaffected. At saturating concentrations of VIP (0.1-1.0 microM), no other agonist can further activate cyclic AMP production. Under these conditions, the interaction with opiate, DA D2 and 5-HT1 receptors, whose activation results in the inhibition of cyclic AMP production, was shown. During the differentiation of striatal neurons, VIP stimulation of cyclic AMP over basal levels, in the presence of 0.1 microM forskolin, decreases progressively from 30-fold after 3 DIV to 11-fold after 10-13 DIV.

The metabotropic glutamate receptor (MGR): pharmacology and subcellular location.

A pharmacological characterization of the metabotropic glutamate receptor (MGR) was performed in striatal neurons. Among the excitatory amino acid receptor antagonists tested, only D, L-2-amino-3-phosphonopropionate (D, L-AP3) inhibited QA-induced inositol phosphate (InsP) formation in a competitive manner (mean pKi = 4.45 +/- 0.43, n = 4). However, this drug was a partial agonist of MGR since it stimulated the inositol-phosphate formation. We found that D, L-AP3 also inhibited NMDA-induced calcium increase, in a competitive manner (mean pIC50 = 4.34 +/- 0.22, n = 8, and mean pKi = 3.7 +/- 0.11 n = 5). 1 mM of the ionotropic agonists alpha-amino-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate (KA) or domoate (DO) (100 microM or higher) induced a significant InsP formation in striatal neurons. The InsP responses induced by all these agonists were totally blocked by the phorbol ester phorbol-12,13-dibutyrate (PdBu), but not by atropine or prazosin. Agonist-induced increases of intracellular calcium concentrations ([Ca2+]i) were insensitive to PdBu, suggesting that all these substances were able to stimulate the MGR in striatal neurons. Trans-1-amino-cyclopentyl-1,3-dicarboxylate (trans-ACPD) evoked dose-dependent inositol phosphate formations with an EC50 of 29 microM but had no significant effect on NMDA or AMPA receptors, as measured by the patch clamp technique. In the presence of 30 microM of AMPA, trans-ACPD induced a significant release of arachidonic acid (AA) in striatal neurons. No important AA release was observed by any of these agonists alone. 56 mM K+ did not mimic AMPA in this associative ionotropic/metabotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional distribution and ontogeny of 5-HT4 binding sites in rat brain.

The tritiated specific antagonist [3H]GR 113808 was used to study the brain regional distribution of 5-HT4 binding sites in rat during development. In adult brain, high densities of binding sites were found in the limbic system (islands of Calleja, olfactory tubercle, fundus striati, ventral pallidum, septum, hippocampus and amygdala) and in the striato-nigro-tectal pathway. Only low densities of binding sites were found in the prenatal brain, except in the brainstem. Marked density increases were observed during the second and third postnatal week in most forebrain regions. This ontogenic pattern indicates that 5-HT4 receptors probably exert a minor role in developmental processes, while, in the adult, the expression of 5-HT4 receptors in limbic areas strongly suggests a role for these receptors in emotional functions. The synchronous appearance of 5-HT4 receptors and cholinergic markers also indicates that these receptors might control acetylcholine release. This hypothesis has been confirmed by functional evidence and further supports the view that 5-HT4 drugs could improve cognitive functions in diseases with impaired cholinergic transmission (e.g., Alzheimer's disease).

BRL 24924: a potent agonist at a non-classical 5-HT receptor positively coupled with adenylate cyclase in colliculi neurons.

A non-classical 5-hydroxytryptamine (5-HT) receptor that we have previously proposed to call 5-HT4 and which mediates stimulation of adenylate cyclase activity in mouse embryo colliculi neurons in primary culture was also stimulated by substituted benzamide derivatives such as metoclopramide and BRL 24924 ([ (+/-)-(endo)]-4-amino-5-chloro-2-methoxy-N-(1-azabicyclo-[3.3.1]-non- 4-yl)-benzamide hydrochloride). The non-additivity of the effects of 5-HT and BRL 24924 on cAMP formation and the inhibition by ICS 205 930, a potent 5-HT3 antagonist, suggest that 5-HT and BRL 24924 act on the same receptor. In light of these results, we think that a similarity may exist between the non-classical 5-HT receptor, coupled with an adenylate cyclase in colliculi neurons, and the non-classical 5-HT receptor, involved in gastric and ileum motility which is specifically stimulated by substituted benzamide derivatives in the same order of potency (metoclopramide, BRL 24924).

Pharmacological comparison between [3H]GR 113808 binding sites and functional 5-HT4 receptors in neurons.

5-HT4 receptors positively coupled to adenylyl cyclase and possessing unique pharmacological properties were first described in mouse colliculi neurons using functional studies. The recent introduction of a radiolabeled 5-HT4 receptor antagonist, [3H]GR 113808 [1-[2-(methylsulphonylamino)ethyl]4-piperidinyl]methyl-1-methyl-in dole-3 carboxylate] having high specificity and affinity allowed the pharmacological comparison between the specific binding sites identified with this compound and the functional 5-HT4 receptors in the same preparation, the colliculi neurons. We show here that [3H]GR 113808 binding is saturable in this preparation and reveals a homogeneous population of sites with a pKd value of 9.5 +/- 0.2 and a Bmax of 75 +/- 23 fmol/mg protein. Seventeen agonists and six antagonists with molecules structurally related either to indoles, benzamides or benzimidazolones and previously known as 5-HT4 receptor ligands, were tested for their ability to compete with [3H]GR 113808 binding sites and to stimulate or inhibit 5-HT-stimulated adenylyl cyclase activity. Highly significant correlations were obtained between the affinities of either agonists or antagonists for [3H]GR 113808 binding sites and their potencies for functional 5-HT4 receptors (r = 0.87 and 0.99, respectively). In addition, we also found good correlations between the Kd of several 5-HT4 receptor ligands determined in cell membranes of mouse colliculi neurons and their Kd determined in previous studies in guinea-pig striatum (0.95) and in human caudate (0.97). [3H]GR 113808 binding studies demonstrated that the 50% decrease in 5-HT-stimulated cAMP accumulation which followed a 5 min exposure period with 5-HT (10 microM) was not accompanied by any significant decrease in the number of binding sites. Longer exposure periods with 5-HT resulted in a decrease in [3H]GR 113808 binding sites which started to be significant after 30 min.

Serotonin 5-HT1 receptors mediate inhibition of cyclic AMP production in neurons.

In purified striatal and cortical neurons in primary culture, serotonin (5-HT) stimulated basal cyclic AMP production (EC50, 0.5 microM) 2.5- and 1.5-fold, respectively. The 5-HT1 selective agonists, RU 24969 and 8-hydroxy-2-(di-n-propylamino)tetralin (PAT), did not stimulate cyclic AMP production. However, 5-HT, RU 24969 and PAT inhibited VIP-stimulated cyclic AMP formation in a dose-dependent manner. The actions of selective agonists and antagonists at 5-HT receptors mediating attenuation of cyclic AMP production suggest that they may be of the 5-HT1 subtype.

On concanavalin A-treated striatal neurons quisqualate clearly behaves as a partial agonist of a receptor fully activated by kainate.

In cultured striatal neurons, maximal [3H]GABA release stimulated by quisqualate (QA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) was 10-20 times smaller than that stimulated by kainate (KA), and we have previously reported that QA or AMPA competitively inhibited KA-evoked GABA release. Since the lectin concanavalin A (Con A) has been shown to inhibit QA receptor desensitization, the interaction between QA and KA was further studied in Con A-treated neurons. Con A dose-dependently and specifically potentiated QA- or AMPA-evoked [3H]GABA release, so that maximal responses of QA or AMPA were half of that of KA. The responses of these agonists were inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) with similar apparent Ki values, indicating that they resulted from non-NMDA receptor activation. In Con A-treated neurons, QA and AMPA competitively inhibited the KA-induced GABA release. The apparent affinities of QA and AMPA in inhibiting the KA response were identical to their affinities in stimulating GABA release. Moreover, the maximal KA response measured in the presence of QA or AMPA was identical to that measured with KA alone. These results clearly indicate that to stimulate GABA release from Con A-treated striatal neurons, QA and AMPA behave as partial agonists of a receptor fully activated by KA. These results further support the hypothesis that QA, AMPA and KA act on a common receptor type in striatal neurons.

Establishment of a long-term primary culture of striatal neurons.

A new method of obtaining long-term primary cultures (lasting more than 8 weeks) of striatal neurons is described in this paper. The originality of the method consists of: (1) starting the culture for 3 days in a serum-free medium which allows attachment and neurite proliferation of neurons as well as the death of non-neuronal cells (mainly consisting of astrocytes); (2) introducing a limited amount of fetal calf serum (FCS) (2-5%) after 3 days in vitro (3 DIV), which likely provides optimal neuronal survival and attachment factors, and a limited amount of astrocyte proliferating factors. The period of introduction of serum, as well as the amount of serum introduced are critical factors. By phase contrast and transmission electron microscopy, we observed that neurons continued to develop neurite extensions, synaptic vesicles and synapse formations up to 50 DIV. Neuronal membranes, and synaptic contacts were particularly healthy up to 50 DIV. Interestingly, the number of astrocytes was constant between 30-50 DIV and limited to about 10%. We therefore obtained an equilibrium between neuronal and astrocyte differentiation and proliferation. It is likely that the small population of astrocytes, plus the low percentage of FCS added, provide essential factors for neuronal survival and differentiation, whereas a high density of differentiated neurons inhibited astrocyte cell proliferation. The clear-cut stability of these neuronal cultures goes in parallel with the stability of the pharmacological responses studied here: the coupling of carbachol and quisqualate receptors with the inositol phosphate production system. The culture method described here could be of particular interest to pursue biochemical, pharmacological and biological studies on neurons as well as on reciprocal interactions between neurons and astrocytes.

The gastrointestinal prokinetic benzamide derivatives are agonists at the non-classical 5-HT receptor (5-HT4) positively coupled to adenylate cyclase in neurons.

We have previusly shown that a non-classical 5-hydroxytryptamine (5-HT4) receptor mediates the stimulation of adenylate cyclase activity in mouse embryo colliculi neurons in primary culture. The pharmacological characteristics of this receptor exclude the possibility that it belongs to the known 5-HT1, 5-HT2 or 5-HT3 receptor types. Here we report that this 5-HT receptor can be stimulated by 4-amino-5-chloro-2-methoxy substituted benzamide derivatives. All these compounds have been reported to be potent stimulants of gastrointestinal motility and some of them are 5-HT3 receptor antagonists. The rank order of potency of these substituted benzamide derivatives in stimulating cAMP formation was: cisapride greater than BRL 24924 greater than 5-HT greater than zacopride greater than BRL 20627 greater than metoclopramide. The non-additivity of benzamide and 5-HT activities suggests that 5-HT and the substituted benzamide derivatives act on the same receptor. Only ICS 205930, a recognized 5-HT3 receptor antagonist, competitively antagonized the stimulatory effect of cisapride, zacopride and BRL 24924. However, its pKi (6-6.3) for this new receptor was very different from its pKi for 5-HT3 receptors (pKi = 8-10). Other selective 5-HT3 receptor antagonists with an indole group (BRL 43694 and GR 38032F), with a benzoate group (cocaïne, MDL 72222) or with a piperazine group (quipazine) were ineffective in reversing the stimulatory effect of benzamide derivatives. Exposure of neuronal cells to potent agonists at this receptor such as BRL 24924 rapidly reduces its capacity to stimulate cAMP production.(ABSTRACT TRUNCATED AT 250 WORDS)

Piperazine derivatives including the putative anxiolytic drugs, buspirone and ipsapirone, are agonists at 5-HT1A receptors negatively coupled with adenylate cyclase in hippocampal neurons.

Two putative anxiolytic drugs [ipsapirone (TVXQ 7821) and buspirone], structurally unrelated to benzodiazepines, have negligible ataxic and sedative side effects. These drugs are piperazine analogs which interact at 5-HT1 binding sites. It is demonstrated here that these drugs and two other piperazine derivatives, trifluoromethylphenylpiperazine (TFMPP) and m-chlorophenylpiperazine (mCPP), are agonists at 5-HT1A receptors, a subclass of the 5-HT1 receptor, mediating inhibition of forskolin (100 microM) stimulated adenylate cyclase in particulate fractions of guinea pig hippocampus as well as inhibition of the formation of cyclic AMP promoted by vasoactive intestinal polypeptide (0.1 microM) plus forskolin (1 microM) in mouse hippocampal neurons in primary culture. This study demonstrates that these piperazine based drugs act in both brain homogenate preparations and in intact neurons in a similar manner. The biochemical models described here may aid in the development of even more active drugs in this class.

Azabicycloalkyl benzimidazolone derivatives as a novel class of potent agonists at the 5-HT4 receptor positively coupled to adenylate cyclase in brain.

Recent experimental evidence indicates that central 5-HT4 receptors which are positively coupled to adenylate cyclase, are stimulated by a family of 2-methoxy-4-amino-5-chloro substituted benzamide derivatives. These compounds are also potent stimulants of the gastro-intestinal motility. In this study the ability of three azabicycloalkyl benzimidazolone derivatives, BIMU 1, BIMU 8, and DAU 6215 (structural formulas are given in the text), to stimulate cAMP formation in colliculi neurons in primary culture have been tested. Two of the compounds, BIMU 1 and BIMU 8, which show prokinetic activity in various animal models, were also good agonists at the 5-HT4 receptors, whereas DAU 6215, a drug devoid of prokinetic activity, was only a weak, partial agonist at 5-HT4 receptors. The rank order of their potencies as compared with those of 5-HT and cisapride was as follows: BIMU 8 = cisapride greater than 5-HT greater than BIMU 1 greater than DAU 6215. The efficacies of BIMU 8 and cisapride were comparable (133 +/- 9% and 124 +/- 8% of the maximal 5-HT efficacy, respectively), whereas BIMU 1 and DAU 6215 elicited, respectively, only 72 +/- 11% and 16 +/- 4% of the maximal 5-HT effect. The activities of the azabicycloalkyl benzimidazolone derivatives and 5-HT on cAMP formation were not additive and ICS 205-930 antagonized the stimulatory effect of these compounds with low potency (pKi = 6.1-6.4), further strengthening the notion of interaction with 5-HT4 receptors. In addition, cross desensitization between the effects of 5-HT and the azabicycloalkyl benzimidazolones on adenylate cyclase was noted, another argument in favor of an interaction of these drugs on 5-HT4 receptors.