The 5-hydroxytryptamine (5-HT) system modulates numerous physiological and behavioural processes, and dysfunction within this system underlies many behavioural disorders, such as major depression. The 5-HT1A receptor is the primary somatodendritic autoreceptor that controls the firing rate of 5-HT neurons, but is also coupled to numerous signalling pathways. An understanding of 5-HT1A receptor signalling may lead to the development of antidepressant drugs that selectively target therapeutic pathways in treating depression. The 5-HT1A receptor is coupled to inhibitory G-proteins via its intracellular loops 2 and 3. Point mutations within these loops selectively uncouple receptor signalling pathways. In this thesis, I addressed whether mutant receptors’ uncoupling from signalling pathways is associated with alteration in G-protein interaction and coupling. Using bioluminescence resonance energy transfer (BRET) to monitor receptor-G-protein interactions, we show that both wild-type and mutant receptors demonstrate a saturable interaction with Gαi protein in unstimulated conditions. Addition of 5-HT increased the BRET signal for the wild-type 5-HT1A receptor, and this increase was blocked by a 5-HT1A receptor antagonist and G-protein blocker (pertussis toxin). Mutant receptors that were deficient in Gαi signalling, but not those that still signalled to Gαi, failed to respond to receptor activation with increased receptor-Gαi interaction. Pull down studies verified the basal and agonist-induced interaction of 5-HT1A receptors with Gαi proteins. In conclusion, we have shown that the 5-HT1A receptor interacts with Gαi consistent with a pre-coupled model and that 5-HT-induced activation enhances this interaction and requires specific residues in the intracellular loops.

The 5-hydroxytryptamine (5-HT) system modulates numerous physiological and behavioural processes, and dysfunction within this system underlies many behavioural disorders, such as major depression. The 5-HT1A receptor is the primary somatodendritic autoreceptor that controls the firing rate of 5-HT neurons, but is also coupled to numerous signalling pathways. An understanding of 5-HT1A receptor signalling may lead to the development of antidepressant drugs that selectively target therapeutic pathways in treating depression. The 5-HT1A receptor is coupled to inhibitory G-proteins via its intracellular loops 2 and 3. Point mutations within these loops selectively uncouple receptor signalling pathways. In this thesis, I addressed whether mutant receptors’ uncoupling from signalling pathways is associated with alteration in G-protein interaction and coupling. Using bioluminescence resonance energy transfer (BRET) to monitor receptor-G-protein interactions, we show that both wild-type and mutant receptors demonstrate a saturable interaction with Gαi protein in unstimulated conditions. Addition of 5-HT increased the BRET signal for the wild-type 5-HT1A receptor, and this increase was blocked by a 5-HT1A receptor antagonist and G-protein blocker (pertussis toxin). Mutant receptors that were deficient in Gαi signalling, but not those that still signalled to Gαi, failed to respond to receptor activation with increased receptor-Gαi interaction. Pull down studies verified the basal and agonist-induced interaction of 5-HT1A receptors with Gαi proteins. In conclusion, we have shown that the 5-HT1A receptor interacts with Gαi consistent with a pre-coupled model and that 5-HT-induced activation enhances this interaction and requires specific residues in the intracellular loops.

The 5-hydroxytryptamine (5-HT) system modulates numerous physiological and behavioural processes, and dysfunction within this system underlies many behavioural disorders, such as major depression. The 5-HT1A receptor is the primary somatodendritic autoreceptor that controls the firing rate of 5-HT neurons, but is also coupled to numerous signalling pathways. An understanding of 5-HT1A receptor signalling may lead to the development of antidepressant drugs that selectively target therapeutic pathways in treating depression. The 5-HT1A receptor is coupled to inhibitory G-proteins via its intracellular loops 2 and 3. Point mutations within these loops selectively uncouple receptor signalling pathways. In this thesis, I addressed whether mutant receptors’ uncoupling from signalling pathways is associated with alteration in G-protein interaction and coupling. Using bioluminescence resonance energy transfer (BRET) to monitor receptor-G-protein interactions, we show that both wild-type and mutant receptors demonstrate a saturable interaction with Gαi protein in unstimulated conditions. Addition of 5-HT increased the BRET signal for the wild-type 5-HT1A receptor, and this increase was blocked by a 5-HT1A receptor antagonist and G-protein blocker (pertussis toxin). Mutant receptors that were deficient in Gαi signalling, but not those that still signalled to Gαi, failed to respond to receptor activation with increased receptor-Gαi interaction. Pull down studies verified the basal and agonist-induced interaction of 5-HT1A receptors with Gαi proteins. In conclusion, we have shown that the 5-HT1A receptor interacts with Gαi consistent with a pre-coupled model and that 5-HT-induced activation enhances this interaction and requires specific residues in the intracellular loops.

The 5-hydroxytryptamine (5-HT) system modulates numerous physiological and behavioural processes, and dysfunction within this system underlies many behavioural disorders, such as major depression. The 5-HT1A receptor is the primary somatodendritic autoreceptor that controls the firing rate of 5-HT neurons, but is also coupled to numerous signalling pathways. An understanding of 5-HT1A receptor signalling may lead to the development of antidepressant drugs that selectively target therapeutic pathways in treating depression. The 5-HT1A receptor is coupled to inhibitory G-proteins via its intracellular loops 2 and 3. Point mutations within these loops selectively uncouple receptor signalling pathways. In this thesis, I addressed whether mutant receptors’ uncoupling from signalling pathways is associated with alteration in G-protein interaction and coupling. Using bioluminescence resonance energy transfer (BRET) to monitor receptor-G-protein interactions, we show that both wild-type and mutant receptors demonstrate a saturable interaction with Gαi protein in unstimulated conditions. Addition of 5-HT increased the BRET signal for the wild-type 5-HT1A receptor, and this increase was blocked by a 5-HT1A receptor antagonist and G-protein blocker (pertussis toxin). Mutant receptors that were deficient in Gαi signalling, but not those that still signalled to Gαi, failed to respond to receptor activation with increased receptor-Gαi interaction. Pull down studies verified the basal and agonist-induced interaction of 5-HT1A receptors with Gαi proteins. In conclusion, we have shown that the 5-HT1A receptor interacts with Gαi consistent with a pre-coupled model and that 5-HT-induced activation enhances this interaction and requires specific residues in the intracellular loops.

The serotonin system, tightly controlled by negative regulation by 5-HT1A autoreceptors, is involved in MDD since manipulating different aspects of the 5-HT systems produces behavioral changes akin to MDD symptoms. Deaf1 knockout mice, which have shown increased 5-HT1A autoreceptor levels, were examined. Conversely, the effect of loss of 5-HT1A autoreceptors in adulthood was studied in 5-HT1AautoKO mice engineered for inducible knockout of the 5-HT1A autoreceptor. Anxiety and depression behavioural assays were performed on the Deaf1KO and 5-HT1AautoKO models. Deaf1KO mice displayed an overall increase in anxious behaviour, yet no change in depressive behaviour relative to controls. Conditional 5-HT1A mice displayed no changes in anxiety or depressive behaviour. Compensation from remaining aspects of the serotonin system and/or other neurological systems may account for some of the behavioural results (or lack thereof). Additional testing that includes a stress paradigm is warranted in order to reveal any stress-dependent vulnerabilities to anxiety and depression.

Selective Serotonin Reuptake Inhibitors (SSRIs) are the first-line treatment for major depression, but require several weeks to elicit a clinical effect. One mechanism that may underlie this delay in SSRI action implicates the gradual desensitization of 5-HT1A autoreceptors, leading to enhanced firing and increasing serotonin (5-HT) at the synapse. I hypothesized that in absence of 5-HT1A autoreceptors, fluoxetine (FLX) would improve behavior faster and more effectively. To specifically knock out 5-HT1A receptors in 5-HT neurons, we crossed TPH2-CREERT2 and flx5HT1A-YFP mice to generate the flx1A mice, a tamoxifen-inducible conditional knockout. Tamoxifen-induced recombination in adult flx1A-/- mice induced YFP expression and reduced 5-HT1A receptor levels by over 90%, specifically in TPH-positive cells of the raphe. To test the response to sub-chronic SSRI treatment, the mice were treated for 9 days with SSRIs FLX or escitalopram and examined for anxiety and depression-like behavior using multiple tests, including the novelty suppressed feeding test (NSF) that responds to chronic but not acute SSRI treatment. Subchronic FLX treatment had no effect in flx1A +/+ control mice, but resulted in an unexpected anxiogenic effect in flx1A -/- littermates, in both the NSF and elevated plus maze tests. Subchronic treatment with escitalopram also increased anxiety-like behavior in the NSF in flx1A-/-, but not wild-type mice. To determine whether FLX treatment differentially affected brain activity in these mice, the number of FosB-stained cells was determined as an index of chronic activation. In flx1A -/- vs. +/+ mice, the number of FosB-positive cells was reduced in several brain regions linked to anxiety and depression including hippocampus and entorhinal cortex, and FLX III treatment reduced activation in the flx1A +/+ compared to -/- mice these areas and in 5-HT neurons of the median raphe nucleus. These results suggest that in the absence of 5-HT1A autoreceptors, SSRIs have a pro-anxiety effect that may involve reduced inactivation anxiety-related brain regions.

Cannabidiol (CBD), a non-psychoactive cannabinoid found in cannabis suppresses vomiting in shrews (Suncus murinus, Parker et al., 2004), and conditioned gaping in rats (a selective measure of nausea-like behaviour, Parker et al., 2002). CBD‘s anti-emetic/anti-nausea mechanism of action is unknown. However, evidence suggests that CBD may act as a somatodendritic 5-hydroxytryptamine 1A (5-HT1A) autoreceptor agonist in the dorsal raphe nucleus (DRN), because the anxiolytic (Campos and Guimaraes, 2008a) and neuroprotectant (Mishima et al., 2005) properties of CBD are 5-HT1A-mediated. Therefore, here we investigated if administration of 5-HT1A receptor antagonists, (WAY100135 or WAY100635) would block CBD‘s anti-emetic/anti-nausea-like effects. Systemic administration of WAY100135 prevented the anti-emetic effect of CBD in shrews, and WAY100135 and WAY100635 attenuated the anti-nausea-like effect of CBD in rats. The effect of CBD on conditioned gaping reactions was most likely the result of its action on somatodendritic 5-HT1A receptors in the DRN, because the anti-nausea-like action of systemic CBD was reversed by intra-DRN administration of WAY100635. As well, when administered into the DRN, CBD suppressed conditioned gaping, an effect that was blocked by systemic WAY100635. In vitro studies revealed that CBD enhanced the ability of 8-OH-DPAT to stimulate [35S]GTPS binding and in vivo studies revealed that systemic subthreshold doses of combined CBD and 8-OH-DPAT synergistically suppressed conditioned gaping. These results suggest that CBD produces its anti-emetic/anti-nausea-like effects by indirect receptor agonism of DRN somatodendritic 5-HT1A autoreceptors. CBD‘s mechanism of action was explored further, by examining its interaction with cannabigerol (CBG), another cannabinoid, which acts in vitro as a 5-HT1A receptor antagonist (Cascio et al., 2010). CBG blocked the systemic CBD-, and 8-OH-DPAT-induced suppression of gaping in rats, as well as the systemic CBD-induced suppression of vomiting in shrews. Therefore, CBG and CBD may be in opposition at the 5-HT1A receptor. These findings shed light on the mechanism of action of non-psychoactive cannabinoids in the cannabis plant, and their effect on nausea and vomiting. These results suggest CBD alone may be an effective treatment in reducing nausea and vomiting. / This research was supported by research grants from the Natural Sciences and Engineering Research Council (NSERC) of Canada to Dr. Parker, a scholarship from NSERC to Erin Rock, and a grant from NIDA to Dr. Mechoulam and Dr. Pertwee.

Diversas evidências indicam que o núcleo dorsal da rafe (NDR), principal fonte de inervação serotonérgica para estruturas límbicas, é uma estrutura heterogênea composta por populações de neurônios serotonérgicos anatômica e funcionalmente distintas, as quais têm sido diferentemente implicadas na fisiopatologia dos transtornos de ansiedade, como o transtorno do pânico. Fármacos antidepressivos são a primeira escolha para o tratamento de tais transtornos e o atraso para a manifestação dos efeitos terapêuticos desses fármacos tem sido, consistentemente, associado a alterações na neurotransmissão serotonérgica. Entretanto, ainda é desconhecido se estas alterações podem ser heterogêneas entre as diferentes sub-regiões do NDR e é este o foco de investigação do presente estudo. Inicialmente, foi investigado o perfil de ativação neuronal das diferentes sub-regiões do NDR, através da avaliação da expressão de proteína Fos, em ratos submetidos à tarefa de fuga no modelo do labirinto em T elevado, após administração aguda ou crônica com a fluoxetina ou a imipramina. Nestes mesmos animais, foram também avaliado o número de células triptofano hidroxilase positivas. Para fins comparativos, o efeito do tratamento com a imipramina em animais submetidos à esquiva inibitória também foi avaliado. A análise imunoistoquímica indicou que a tarefa de fuga promoveu um aumento na expressão da proteína Fos em neurônios nãoserotonérgicos localizados na sub-região das asas laterais do NDR. Efeito este que não foi observado em animais tratados cronicamente com a fluoxetina e a imipramina. Além disso, o tratamento prolongado com a fluoxetina, mas não com a imipramina, foi capaz de aumentar a ativação de neurônios serotonérgicos nesta mesma sub-região. Diferente do que foi observado na tarefa de fuga, a tarefa de esquiva inibitória promoveu um aumento na ativação de neurônios serotonérgicos nas sub-regiões DRC, DRD e DRV. Perfil que não foi observado nos grupos aguda e cronicamente tratados com imipramina. Além disso, somente o tratamento prolongado com a imipramina promoveu uma diminuição no recrutamento de neurônios não-serotonérgicos em diversas sub-regiões do NDR. Diante do resultado imunoistoquímico observado no experimento com a fluoxetina, avaliamos a participação dos autorreceptores 5-HT1A das asas laterais no efeito observado. Através da técnica whole-cell patch clamp, em uma linhagem de camundongos transgênicos que apresenta neurônios serotonérgicos fluorescentes, foi avaliada a responsividade destes receptores após o tratamento com a fluoxetina. Os resultados indicam que os animais tratados cronicamente com fluoxetina apresentam aumento na excitabilidade basal com diminuída sensibilidade ao agonista de receptores 5-HT1A, 8-OHDPAT. Assim, é possível sugerir que o efeito antipânico da fluoxetina administrada cronicamente parece estar relacionado com uma redução na ativação de neurônios não-serotonérgicos e um aumento no recrutamento de neurônios serotonérgicos localizados nas asas laterais, sendo que esta última pode ser explicada pela dessensibilização dos autorreceptores 5-HT1A. Com relação à imipramina, é possível sugerir que o efeito panicolítico promovido pelo tratamento prolongado pode ser devido à diminuição no recrutamento de neurônios não-serotonérgicos das asas laterais. Já para o efeito ansiolítico, tanto a diminuição no recrutamento de neurônios não-serotonérgicos em diferentes sub-regiões do NDR, quanto a diminuição na ativação de neurônios serotonérgicos no DRC, DRD e DRV parecem estar envolvidas. / A wealth of evidence indicates that the dorsal raphe nucleus (DR) is a heterogeneous structure, composed of anatomically and functionally distinct populations of serotonergic neurons, which have been differently implicated in the pathophysiology of anxiety, such as panic disorder. Antidepressant drugs are the first choice in treatment of anxiety disorders, and the delay for the therapeutic effect have consistently been associated with changes in serotonergic neurotransmission within the DR. However, it is unknown whether these alterations can be heterogeneous among the different subregions of the DR, and this is the focus of investigation of the present study. First, it was investigated the profile of neuronal activation of different subregions of the DR - by using the evaluation of Fos protein expression of rats exposed to the escape task in the elevated T-maze test, after acute or chronic administration of fluoxetine or imipramine. In the same animals, it was also investigated the number of positive triptophan hidroxylasis cells. For comparative reasons, it was evaluated the behavioral and immunohistochemical effects of imipramine treatment on inhibitory avoidance acquisition in the elevated T-maze, a response associated with anxiety. The results of the immunohistochemical analysis indicated that animals exposed to escape behavior exhibited higher expression of Fos protein in non-serotonergic neurons in the DR lateral wings. This effect was not observed in fluoxetine or imipramine chronically treated animals. Moreover, chronic treatment with fluoxetine, but not imipramine, was able to increase the activation of serotonergic neurons on this subregion. On the other hand, the inhibitory avoidance task promoted an increase in the activation of serotonergic neurons in the sub-regions DRC, DRD, and DRV. This profile was not observed after acute or chronic treatment with imipramine. Additionally, only the long-term treatment with imipramine showed a decrease in the activation of non-serotonergic neurons in different subregions of the DR. Based on the results obtained with fluoxetine experiment, we evaluated the role of the 5-HT1A autoreceptors located in the lateral wings. For this, we used the whole-cell patch clamp technique in a transgenic mouse line, which exhibit fluorescence in serotonergic neurons. The results indicate that the animals treated chronically with fluoxetine presented an increase in the basal excitability, with lower responsivity to the 5-HT1A agonist - 8-OH-DPAT. Altogether, the results suggest that the anti-panic effect caused by chronic fluoxetine treatment is associated with a reduction in the activation of non-serotonergic neurons, and an increase in the recruitment of non-serotonergic neurons in lateral wings. This last observation seems to be related to a 5-HT1A autoreceptor desensitization in the lateral wings. Regarding imipramine, this panicolytic effect caused by chronic administration of this drug seems to be related to a decreased activation of the non-serotonergic neurons in the lateral wings. Finally, the anxiolytic effect, of imipramine may be associated to a decrease in the recruitment of non-serotonergic neurons in different DR subregions, allied with a decreased activation of serotonergic neurons in the DRC, DRD and DRV.

Diversas evidências indicam que o núcleo dorsal da rafe (NDR), principal fonte de inervação serotonérgica para estruturas límbicas, é uma estrutura heterogênea composta por populações de neurônios serotonérgicos anatômica e funcionalmente distintas, as quais têm sido diferentemente implicadas na fisiopatologia dos transtornos de ansiedade, como o transtorno do pânico. Fármacos antidepressivos são a primeira escolha para o tratamento de tais transtornos e o atraso para a manifestação dos efeitos terapêuticos desses fármacos tem sido, consistentemente, associado a alterações na neurotransmissão serotonérgica. Entretanto, ainda é desconhecido se estas alterações podem ser heterogêneas entre as diferentes sub-regiões do NDR e é este o foco de investigação do presente estudo. Inicialmente, foi investigado o perfil de ativação neuronal das diferentes sub-regiões do NDR, através da avaliação da expressão de proteína Fos, em ratos submetidos à tarefa de fuga no modelo do labirinto em T elevado, após administração aguda ou crônica com a fluoxetina ou a imipramina. Nestes mesmos animais, foram também avaliado o número de células triptofano hidroxilase positivas. Para fins comparativos, o efeito do tratamento com a imipramina em animais submetidos à esquiva inibitória também foi avaliado. A análise imunoistoquímica indicou que a tarefa de fuga promoveu um aumento na expressão da proteína Fos em neurônios nãoserotonérgicos localizados na sub-região das asas laterais do NDR. Efeito este que não foi observado em animais tratados cronicamente com a fluoxetina e a imipramina. Além disso, o tratamento prolongado com a fluoxetina, mas não com a imipramina, foi capaz de aumentar a ativação de neurônios serotonérgicos nesta mesma sub-região. Diferente do que foi observado na tarefa de fuga, a tarefa de esquiva inibitória promoveu um aumento na ativação de neurônios serotonérgicos nas sub-regiões DRC, DRD e DRV. Perfil que não foi observado nos grupos aguda e cronicamente tratados com imipramina. Além disso, somente o tratamento prolongado com a imipramina promoveu uma diminuição no recrutamento de neurônios não-serotonérgicos em diversas sub-regiões do NDR. Diante do resultado imunoistoquímico observado no experimento com a fluoxetina, avaliamos a participação dos autorreceptores 5-HT1A das asas laterais no efeito observado. Através da técnica whole-cell patch clamp, em uma linhagem de camundongos transgênicos que apresenta neurônios serotonérgicos fluorescentes, foi avaliada a responsividade destes receptores após o tratamento com a fluoxetina. Os resultados indicam que os animais tratados cronicamente com fluoxetina apresentam aumento na excitabilidade basal com diminuída sensibilidade ao agonista de receptores 5-HT1A, 8-OHDPAT. Assim, é possível sugerir que o efeito antipânico da fluoxetina administrada cronicamente parece estar relacionado com uma redução na ativação de neurônios não-serotonérgicos e um aumento no recrutamento de neurônios serotonérgicos localizados nas asas laterais, sendo que esta última pode ser explicada pela dessensibilização dos autorreceptores 5-HT1A. Com relação à imipramina, é possível sugerir que o efeito panicolítico promovido pelo tratamento prolongado pode ser devido à diminuição no recrutamento de neurônios não-serotonérgicos das asas laterais. Já para o efeito ansiolítico, tanto a diminuição no recrutamento de neurônios não-serotonérgicos em diferentes sub-regiões do NDR, quanto a diminuição na ativação de neurônios serotonérgicos no DRC, DRD e DRV parecem estar envolvidas. / A wealth of evidence indicates that the dorsal raphe nucleus (DR) is a heterogeneous structure, composed of anatomically and functionally distinct populations of serotonergic neurons, which have been differently implicated in the pathophysiology of anxiety, such as panic disorder. Antidepressant drugs are the first choice in treatment of anxiety disorders, and the delay for the therapeutic effect have consistently been associated with changes in serotonergic neurotransmission within the DR. However, it is unknown whether these alterations can be heterogeneous among the different subregions of the DR, and this is the focus of investigation of the present study. First, it was investigated the profile of neuronal activation of different subregions of the DR - by using the evaluation of Fos protein expression of rats exposed to the escape task in the elevated T-maze test, after acute or chronic administration of fluoxetine or imipramine. In the same animals, it was also investigated the number of positive triptophan hidroxylasis cells. For comparative reasons, it was evaluated the behavioral and immunohistochemical effects of imipramine treatment on inhibitory avoidance acquisition in the elevated T-maze, a response associated with anxiety. The results of the immunohistochemical analysis indicated that animals exposed to escape behavior exhibited higher expression of Fos protein in non-serotonergic neurons in the DR lateral wings. This effect was not observed in fluoxetine or imipramine chronically treated animals. Moreover, chronic treatment with fluoxetine, but not imipramine, was able to increase the activation of serotonergic neurons on this subregion. On the other hand, the inhibitory avoidance task promoted an increase in the activation of serotonergic neurons in the sub-regions DRC, DRD, and DRV. This profile was not observed after acute or chronic treatment with imipramine. Additionally, only the long-term treatment with imipramine showed a decrease in the activation of non-serotonergic neurons in different subregions of the DR. Based on the results obtained with fluoxetine experiment, we evaluated the role of the 5-HT1A autoreceptors located in the lateral wings. For this, we used the whole-cell patch clamp technique in a transgenic mouse line, which exhibit fluorescence in serotonergic neurons. The results indicate that the animals treated chronically with fluoxetine presented an increase in the basal excitability, with lower responsivity to the 5-HT1A agonist - 8-OH-DPAT. Altogether, the results suggest that the anti-panic effect caused by chronic fluoxetine treatment is associated with a reduction in the activation of non-serotonergic neurons, and an increase in the recruitment of non-serotonergic neurons in lateral wings. This last observation seems to be related to a 5-HT1A autoreceptor desensitization in the lateral wings. Regarding imipramine, this panicolytic effect caused by chronic administration of this drug seems to be related to a decreased activation of the non-serotonergic neurons in the lateral wings. Finally, the anxiolytic effect, of imipramine may be associated to a decrease in the recruitment of non-serotonergic neurons in different DR subregions, allied with a decreased activation of serotonergic neurons in the DRC, DRD and DRV.

No / Recent data has suggested that the 5-HT1A receptor is involved in cognitive processing. A novel 5- HT1A receptor antagonist, 4-cyano-N- [(2R)-[4- (2,3-dihydrobenzo [1,4] dioxin-5-yl) piperazin-1-yl] propyl]-N-pyridin-2-yl-benzamide hydrochloride (lecozotan), which has been characterized in multiple in vitro and in vivo pharmacologic assays as a drug to treat cognitive dysfunction, is reported. In vitro binding and intrinsic activity determinations demonstrated that lecozotan is a potent and selective 5-HT1A receptor antagonist. Using in vivo microdialysis, lecozotan (0.3 mg/kg sc) antagonized the decrease in hippocampal extracellular 5-HT induced by a challenge dose (0.3 mg/kg sc) of 8 OH-DPAT and had no effects alone at doses 10-fold higher. Lecozotan significantly potentiated the potassium chloride-stimulated release of glutamate and acetylcholine in the dentate gyrus of the hippocampus. Chronic administration of lecozotan did not induce 5-HT1A receptor tolerance or desensitization in a behavioral model indicative of 5- HT1A receptor function. In drug discrimination studies, lecozotan (0.01-1 mg/kg im) did not substitute for 8-OH-DPAT and produced a dose-related blockade of the 5-HT1A agonist discriminative stimulus cue. In aged rhesus monkeys, lecozotan produced a significant improvement in task performance efficiency at an optimal dose (1 mg/kg po). Learning deficits induced by the glutamatergic antagonist MK-801 (assessed by perceptually complex and visual spatial discrimination) and by specific cholinergic lesions of the hippocampus (assessed by visual spatial discrimination) were reversed by lecozotan (2 mg/kg im) in marmosets. The heterosynaptic nature of the effects of lecozotan imbues this compound with a novel mechanism of action directed at the biochemical pathologies underlying cognitive loss in AD.