Exploring the role of 5-HT1A receptors in the regulation of prepulse inhibition in mice: implications for cross-species comparisons

Effects of combined 5-HT2A and cannabinoid receptor modulation on a schizophrenia-related prepulse inhibition deficit in mice

RATIONALE

Prepulse inhibition of the startle reflex (PPI) is disrupted in several psychiatric disorders including schizophrenia. Understanding PPI pharmacology may help elucidate the pathophysiology of these disorders and lead to better treatments. Given the advantages of multi-target approaches for complex mental illnesses treatment, we have investigated the interaction between receptors known to modulate PPI (5-HT_1A and 5-HT_2A) and the neuromodulatory endocannabinoid system.

OBJECTIVES

To investigate serotonin and cannabinoid receptor (CBR) co-modulation in a model of PPI disruption relevant to schizophrenia METHODS: Male Swiss mice were pretreated with WIN 55,212-2 (CBR agonist), rimonabant (CB1R inverse agonist), 8-OH-DPAT (5-HT_1A/7 agonist), and volinanserin (5-HT_2A antagonist) or with a combination of a cannabinoid and a serotonergic drug. PPI disruption was induced by acute administration of MK-801.

RESULTS

WIN 55,212-2 and rimonabant did not change PPI nor block MK-801-induced deficits. 8-OH-DPAT increased PPI in control mice and, in a higher dose, inhibited MK-801-induced impairments. Volinanserin also increased PPI in control and MK-801-treated mice, presenting an inverted U-shaped dose-response curve. Co-administration of either cannabinoid ligand with 8-OH-DPAT did not change PPI; however, the combination of volinanserin with rimonabant increased PPI in both control and MK-801-exposed mice.

CONCLUSIONS

WIN 55,212-2 and rimonabant had similar effects in PPI. Moreover, serotonin and cannabinoid receptors interact to modulate PPI. While co-modulation of CBR and 5-HT_1A receptors did not change PPI, a beneficial effect of 5-HT_2A and CB1R antagonist combination was detected, possibly mediated through potentiation of 5-HT_2A blockade effects by concomitant CB1R blockade.

Interaction of Brain-Derived Neurotrophic Factor Val66Met genotype and history of stress in regulation of prepulse inhibition in mice

The Brain-Derived Neurotrophic Factor (BDNF) Val66Met polymorphism results in reduced activity-dependent BDNF release and has been implicated in schizophrenia. However, effects of the polymorphism on functional dopaminergic and N-methyl-d-aspartate (NMDA) receptor-associated activity remain unclear. We used prepulse inhibition, a measure of sensorimotor gating which is disrupted in schizophrenia, and assessed the effects of acute treatment with the dopamine receptor agonist, apomorphine (APO), and the NMDA receptor antagonist, MK-801. We used adult humanized hBDNF^Val66Met 'knockin' mice which express either the Val/Val, Val/Met or Met/Met genotype. An interaction of BDNF with stress was modelled by chronic young-adult treatment with corticosterone (CORT). At 1 or 3mg/kg, APO had no effect in Val/Val mice but significantly reduced PPI at the 100ms inter-stimulus interval (ISI) in Val/Met and Met/Met mice. However, after CORT pretreatment, APO significantly reduced PPI in all genotypes similarly. At 0.1 or 0.25mg/kg, MK-801 significantly disrupted PPI at the 100ms ISI independent of genotype or CORT pretreatment. There were differential effects of APO and MK-801 on PPI at the 30ms ISI and startle between the genotypes, irrespective of CORT pretreatment. These results show that the BDNF Val66Met Val/Met and Met/Met genotypes are more sensitive than the Val/Val genotype to the effect of APO on PPI. A history of stress, here modelled by chronic CORT administration, increases effects of APO in Val/Val mice.

Adult vitamin D deficiency exacerbates impairments caused by social stress in BALB/c and C57BL/6 mice

Vitamin D deficiency is prevalent in adults throughout the world. Epidemiological studies have shown significant associations between vitamin D deficiency and an increased risk of various neuropsychiatric and neurodegenerative disorders, such as schizophrenia, depression, Alzheimer's disease and cognitive impairment. However, studies based on observational epidemiology cannot address questions of causality; they cannot determine if vitamin D deficiency is a causal factor leading to the adverse health outcome. The main aim of this study was to determine if AVD deficiency would exacerbate the effects of a secondary exposure, in this case social stress, in BALB/c mice and in the more resilient C57BL/6 mice. Ten-week old male BALB/c and C57BL/6 mice were fed a control or vitamin D deficient diet for 10 weeks, and the mice were further separated into one of two groups for social treatment, either Separated (SEP) or Social Defeat (DEF). SEP mice were placed two per cage with a perforated Plexiglas divider, whereas the DEF mice underwent 10days of social defeat prior to behavioural testing. We found that AVD-deficient mice were more vulnerable to the effects of social stress using a social avoidance test, and this was dependent on strain. These results support the hypothesis that vitamin D deficiency may exacerbate behavioural outcomes in mice vulnerable to stress, a finding that can help guide future studies. Importantly, these discoveries support the epidemiological link between vitamin D deficiency and neuropsychiatric and neurodegenerative disorders; and has provided clues that can guide future studies related to unravelling the mechanisms of action linking adult vitamin D deficiency and adverse brain related outcomes.

Investigating the Role of Serotonin in Methamphetamine Psychosis: Unaltered Behavioral Effects of Chronic Methamphetamine in 5-HT1A Knockout Mice

Methamphetamine (Meth) is a widely abused stimulant drug, but this abuse is associated with an increased risk of developing psychosis. In addition to its well-known action on brain dopamine, Meth also affects serotonergic (5-HT) neurons. The aim of this study was to investigate this role in mice, which lack one of the main serotonin receptors, the 5-HT_1A receptor, which has been implicated in both schizophrenia and Meth-induced psychosis. Male and female wild-type or 5-HT_1A knockout (KO) mice received daily treatment with increasing doses of methamphetamine from 6 to 9 weeks of age (1-4 mg/kg/day twice a day). At least 2 weeks after the last injection, the mice underwent a battery of behavioral tests focusing on psychosis-related behaviors, including Meth-induced hyperactivity, prepulse inhibition (PPI), social interaction, elevated plus maze (EPM), and Y-maze. Meth pretreatment resulted in significantly increased hyperlocomotion in response to an acute Meth challenge, but this effect was independent of genotype. Chronic Meth treatment resulted in decreased levels of anxiety in the EPM in both sexes, as well as increased startle responses in female mice only, again independent of genotype. 5-HT_1A KO mice showed an increased locomotor response to acute Meth in both sexes, as well as increased PPI and decreased startle responses in female mice only, independent of Meth pretreatment. In conclusion, the effects of chronic Meth appear unaffected by the absence of the 5-HT_1A receptor. These results do not support a role of the 5-HT_1A receptor in Meth-induced psychosis.

Cholinergic and serotonergic modulations differentially affect large-scale functional networks in the mouse brain

Resting-state functional MRI (rsfMRI) is a widely implemented technique used to investigate large-scale topology in the human brain during health and disease. Studies in mice provide additional advantages, including the possibility to flexibly modulate the brain by pharmacological or genetic manipulations in combination with high-throughput functional connectivity (FC) investigations. Pharmacological modulations that target specific neurotransmitter systems, partly mimicking the effect of pathological events, could allow discriminating the effect of specific systems on functional network disruptions. The current study investigated the effect of cholinergic and serotonergic antagonists on large-scale brain networks in mice. The cholinergic system is involved in cognitive functions and is impaired in, e.g., Alzheimer's disease, while the serotonergic system is involved in emotional and introspective functions and is impaired in, e.g., Alzheimer's disease, depression and autism. Specific interest goes to the default-mode-network (DMN), which is studied extensively in humans and is affected in many neurological disorders. The results show that both cholinergic and serotonergic antagonists impaired the mouse DMN-like network similarly, except that cholinergic modulation additionally affected the retrosplenial cortex. This suggests that both neurotransmitter systems are involved in maintaining integrity of FC within the DMN-like network in mice. Cholinergic and serotonergic modulations also affected other functional networks, however, serotonergic modulation impaired the frontal and thalamus networks more extensively. In conclusion, this study demonstrates the utility of pharmacological rsfMRI in animal models to provide insights into the role of specific neurotransmitter systems on functional networks in neurological disorders.

Serotonin-1A receptor stimulation mediates effects of a metabotropic glutamate 2/3 receptor antagonist, 2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoic acid (LY341495), and an N-methyl-D-aspartate receptor antagonist, ketamine, in the novelty-suppressed feeding test

RATIONALE

α-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor stimulation has been proposed to be a common neural mechanism of metabotropic glutamate 2/3 (mGlu2/3) receptor antagonists and an N-methyl-D-aspartate receptor antagonist, ketamine, exerting antidepressant effects in animal models. AMPA receptor stimulation has also been shown to mediate an increase in the extracellular level of serotonin (5-HT) in the medial prefrontal cortex by an mGlu2/3 receptor antagonist in rats. However, involvement of the serotonergic system in the actions of mGlu2/3 receptor antagonists and ketamine is not well understood.

OBJECTIVES

We investigated involvement of the serotonergic system in the effects of an mGlu2/3 receptor antagonist, 2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoic acid (LY341495), and ketamine in a novelty-suppressed feeding (NSF) test in mice.

RESULTS

The intraperitoneal administration of LY341495 or ketamine at 30 min prior to the test significantly shortened latency to feed, which was attenuated by an AMPA receptor antagonist, 2,3-dioxo-6-nitro-1,2,3,4-tetrahydr-obenzo[f]quinoxaline-7-sulfonamide (NBQX). The effects of LY341495 and ketamine were no longer observed in mice pretreated with a tryptophan hydroxylase inhibitor, para-chlorophenylalanine (PCPA). Moreover, the effects of LY341495 and ketamine were blocked by a 5-HT1A receptor antagonist, N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridynyl) cyclohexane-carboxamide (WAY100635), but not by a 5-HT2A/2C receptor antagonist, ritanserin. Likewise, an AMPA receptor potentiator, 2,3-dihydro-1,4-benzodioxin-7-yl-(1-piperidyl)methanone (CX546), shortened latency to feed in the NSF test, which was prevented by depletion of 5-HT and blockade of 5-HT1A receptor.

CONCLUSIONS

These results suggest that AMPA receptor-dependent 5-HT release and subsequent 5-HT1A receptor stimulation may be involved in the actions of an mGlu2/3 receptor antagonist and ketamine in the NSF test.