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Pattern separation is a hippocampal process in which highly similar stimuli are recognized as separate representations, and deficits could lead to memory impairments in neuropsychiatric disorders such as schizophrenia. The 5-HT1A receptor (5-HT1AR) is believed to be involved in these hippocampal pattern separation processes. However, in the dorsal raphe nucleus (DRN), the 5-HT1AR is expressed as a somatodendritic autoreceptor, negatively regulates serotonergic signaling, and could thereby counteract the effects of hippocampal postsynaptic 5-HT1A receptors. Therefore, this study aims to identify how pre- and post-synaptic 5-HT1AR activity affects pattern separation. Object pattern separation (OPS) performance was measured in male Wistar rats after both acute and chronic treatment (i.p.) with 5-HT1AR biased agonists F13714 (0.0025 mg/kg acutely, 0.02 mg/kg/day chronically) or NLX-101 (0.08 mg/kg acutely, 0.32 mg/kg/day chronically), which preferentially activate autoreceptors or postsynaptic receptors respectively, for 14 days. Body temperature - a functional correlate of hypothalamic 5-HT1AR stimulation - was measured daily. Additionally, 5-HT1AR density (DRN) and plasticity markers (hippocampus) were assessed. Acute treatment with F13714 impaired OPS performance, whereas chronic treatment normalized this, and a drop in body temperature was found from day 4 onwards. NLX-101 enhanced OPS performance acutely and chronically, and caused an acute drop in body temperature. Chronic NLX-101 treatment increased doublecortin positive neurons in the dorsal hippocampus, while chronic treatment with F13714 resulted in a downregulation of 5-HT1A autoreceptors, which likely reversed the acute impairment in OPS performance. Chronic treatment with NLX-101 appears to have therapeutic potential to improve brain plasticity and OPS performance.

Soluble guanylate cyclase (sGC) - cyclic guanosine monophosphate (cGMP) signalling is important for healthy memory function and a healthy vascular system. Targeting sGC-cGMP signalling can therefore be a potential strategy to enhance memory processes. sGC can be targeted by using agonists, such as sGC stimulator riociguat. Therefore, this study aimed to target sGC using riociguat to investigate its acute effects on memory function and neuronal plasticity in mice. The effects of riociguat on long-term memory and a biperiden-induced memory deficit model for assessing short-term memory were tested in the object location task, and working memory was tested in the Y-maze continuous alternation task. Pharmacokinetic measurements were performed within brain tissue of mice, and hippocampal plasticity measures were assessed using western blotting. Acute oral administration with a low dose of 0.03 mg/kg riociguat was able to enhance working-, short-, and long-term spatial memory. Under cerebral vasoconstriction higher doses of riociguat were still effective on memory. Pharmacokinetic measurements revealed poor brain penetration of riociguat and its metabolite M-1. Increased activation of VASP was found, while no effects were found on other memory-related hippocampal plasticity measures. Memory enhancing effects of riociguat are most likely regulated by vascular peripheral effects on cGMP signalling. Yet, further research is needed to investigate the possible contribution of hemodynamic or metabolic effects of sGC stimulators on memory performance.

Pattern separation is a hippocampal process in which highly similar stimuli are recognized as separate representations, and deficits could lead to memory impairments in neuropsychiatric disorders such as schizophrenia. The 5-HT

The xylazine/ketamine anesthesia test is widely used as a predictor of the emetic potential of pharmacological compounds in rats. An emetic reflex is usually triggered by the emetic center, which is populated with many different chemoreceptors. Inhibition of the alpha 2 adrenergic receptor (alpha 2 receptor) is involved in the initiation of the emetic reflex, and this is the key mechanism behind the xylazine/ketamine anesthesia test. In this study, we attempt to validate this test as a predictor of the emetic potential of pharmacological compounds. Furthermore, it was investigated whether an anti-emetic potential of pharmacological compounds could be assessed within this test as well. Rats were anesthetized with a combination of low doses of ketamine and xylazine, and subsequently treated with PDE4 inhibitor rolipram, alpha 2 receptor antagonist yohimbine, alpha 2 receptor agonist clonidine, tricyclic antidepressant imipramine, D2-receptor antagonist haloperidol, or 5-HT3 receptor antagonist (and anti-emetic drug) ondansetron. We were able to successfully reproduce the reduction in anesthesia time after rolipram or yohimbine treatment, as found in previous studies and has been suggested to be indicative of emetic properties of these treatments is humans. Furthermore, clonidine shortened anesthesia duration whereas imipramine and haloperidol lengthened anesthesia duration. Ondansetron was unable to rescue the reduction in duration of anesthesia induced by either rolipram or yohimbine. Altogether, the xylazine/ketamine anesthesia test is a reliable measure for alpha 2 receptor antagonism. However, it may not be appropriate to assess emesis independent of this mechanism.

Stress during pregnancy increases the risk to develop psychological disorders such as depression during pregnancy or in the postpartum period. According to the neurotrophin hypothesis of depression, the pathophysiology of depression is caused by reduced neurotrophic activity in the brain. However, most studies only focus on the molecular changes happening to the offspring upon gestational stress. To gain insight into the potential molecular changes happening in the stressed dams, C57B16/J mice were stressed during their first week of gestation. At 28 days postpartum, the hippocampus and nucleus accumbens core of the dams, two brain regions heavily implicated in depression, were evaluated using immunohistochemistry to detect changes in the neurotrophin system. Gestational stress decreased the weight of the dams, increased the chance for spontaneous abortion and increased the weight of offspring. Litter size, survival rates and sex distribution were not altered as a consequence of gestational stress. Hippocampal brain-derived neurotrophic factor (BDNF) decreased following exposure to stress during pregnancy. Hippocampal protein levels of p75(NTR), a low-affinity receptor for BDNF which can induce apoptosis, were increased following exposure to stress. Protein levels of p11, of which the expression is regulated by BDNF, were decreased in the hippocampus. No changes were found for TrkB immunostaining or apoptosis. Taken together, this shows that stress during pregnancy negatively affects the neurotrophin system in the hippocampus of the dams, thereby reducing hippocampal plasticity. These data confirm that gestational stress has a negative impact on pregnancy. Open Access funded by VSNU

It is well known that stress affects memory performance. However, there still appears to be inconstancy in literature about how acute stress affects the different stages of memory: acquisition, consolidation and retrieval. In this study, we exposed rats to acute stress and measured the effect on memory performance in the object recognition task as a measure for episodic memory. Stress was induced 30 min prior to the learning phase to affect acquisition, directly after the learning phase to affect consolidation, or 30 min before the retrieval phase to affect retrieval. Additionally, we induced stress both 30 min prior to the learning phase and 30 min prior to the retrieval phase to test whether the effects were related to state-dependency. As expected, we found that acute stress did not affect acquisition but had a negative impact on retrieval. To our knowledge, we are the first to show that early consolidation was negatively affected by acute stress. We also show that stress does not have a state-dependent effect on memory.