Increased behavioral and neuronal responses to a hallucinogenic drug in PACAP heterozygous mutant mice.

Accumulating evidence from human genetic studies implicates the pituitary adenylate cyclase-activating polypeptide (PACAP) gene as a risk factor for psychiatric disorders, including schizophrenia and stress-related diseases. Mice with homozygous disruption of the PACAP gene display profound behavioral and neurological abnormalities that are ameliorated with the atypical antipsychotic and dopamine D2 and serotonin (5-HT)2 antagonist risperidone and the 5-HT2 receptor antagonist ritanserin; however, the underlying mechanisms remain unknown. Here, we investigated if PACAP heterozygous mutant (PACAP(+/-)) mice, which appear behaviorally normal, are vulnerable to aversive stimuli. PACAP(+/-) mice were administered a 5-HT2 receptor agonist, (±)-2,5-dimethoxy-4-iodoamphetamine (DOI), a hallucinogenic drug, and their responses were compared with the littermate wild-type mice. After DOI injection, PACAP(+/-) mice showed increased head-twitch responses, while their behavior was normal after saline. DOI induced deficits in sensorimotor gating, as determined by prepulse inhibition, specifically in PACAP(+/-) mice. However, other 5-HT2 receptor-dependent responses, such as corticosterone release and hypothermia, were similarly observed in PACAP(+/-) and wild-type mice. c-Fos expression analysis, performed in various brain regions, revealed that the DOI-induced increase in the number of c-Fos-positive cells was more pronounced in 5-HT2A receptor-negative cells in the somatosensory cortex in PACAP(+/-) mice compared with wild-type mice. These results indicate that PACAP(+/-) mice exhibit specific vulnerability to DOI-induced deficits in cortical sensory function, such as exaggerated head-twitch responses and sensorimotor gating deficits. Our findings provide insight into the neural mechanisms underlying impaired behavioral responses in which 5-HT2 receptors are implicated.

Inhibitory effects of osemozotan, a serotonin 1A-receptor agonist, on methamphetamine-induced c-Fos expression in prefrontal cortical neurons.

Psychostimulants induce hyperlocomotion in normal subjects, although, they are effective in producing a calming effect in hyperactive subjects. This paradoxical effect has been related to changes in serotonin (5-HT) neurotransmission in hyperactive dopamine transporter-knockout mice. In addition, we observed that hyperlocomotion in mice lacking pituitary adenylate cyclase-activating polypeptide was attenuated by amphetamine dependent on 5-HT(1A) receptor signaling and that amphetamine, when co-administered with a 5-HT(1A) agonist, produced a calming effect in wild-type mice. Here, in an attempt to address how 5-HT(1A) receptor signaling exerts the calming action of psychostimulants, we examined c-Fos expression in several brain regions after administration of methamphetamine and osemozotan, a selective 5-HT(1A) receptor agonist. The number of c-Fos-positive cells was increased in the medial prefrontal cortex, striatum and nucleus accumbens in methamphetamine (3 mg/kg body weight)-injected mice. Osemozotan (1 mg/kg) significantly reduced the methamphetamine-induced c-Fos expression in the medial prefrontal cortex and striatum, but not in the nucleus accumbens. This osemozotan action was completely blocked by the 5-HT(1A) receptor antagonist WAY-100635 (1 mg/kg). As the prefrontal cortex is considered to be involved in the beneficial actions of psychostimulant medications for attention-deficit/hyperactivity disorder, the present result showing 5-HT(1A)-mediated inhibition of corticostriatal activity may partly be related to this psychostimulant action.