UV filters are potentially harmful to marine organisms. Given their worldwide dissemination and the scarcity of studies on marine fish, we evaluated the toxicity of an organic (oxybenzone) and an inorganic (titanium dioxide nanoparticles) UV filter, individually and in a binary mixture, in the turbot (Scophthalmus maximus). Fish were intraperitoneally injected and a multi-level assessment was carried out 3 and 7 days later. Oxybenzone and titanium dioxide nanoparticles induced mild effects on turbot, both isolated and in mixture. Neither oxidative stress (intestine, liver and kidney) nor neurotoxicity (brain) was found. However, liver metabolic function was altered after 7 days, suggesting the impairment of the aerobic metabolism. An increased motility rate in oxybenzone treatment was the only behavioural alteration (day 7). The intestine and liver were preferentially targeted, while kidney and brain were unaffected. Both infra- and supra-additive interactions were perceived, with a toxicodynamic nature, resulting either in favourable or unfavourable toxicological outcomes, which were markedly dependent on the organ, parameter and post-injection time. The combined exposure to the UV filters did not show a consistent increment in toxicity in comparison with the isolated exposures, which is an ecologically relevant finding providing key information towards the formulation of environmentally safe sunscreen products.

Adrenaline (Ad) and glucose released into the bloodstream during stress may strengthen contextual fear memory. However, no previous studies have detached the effects of glucose from Ad in this paradigm. Using Ad-deficient mice, we aimed to evaluate the effect of glucose on contextual fear memory when endogenous Ad is absent. Fear conditioning was performed in wild-type (WT) and Ad-deficient mice (129 × 1/SvJ) administered with glucose (30 or 10 mg/kg; i.p.) or/and Ad (0.01 mg/kg; i.p.) or vehicle (0.9% NaCl; i.p.). Catecholamines were quantified using HPLC-ED. Real-time qPCR was used to assess mRNA expression of hippocampal genes. WT and Ad-deficient mice display increased contextual fear memory when administered with glucose both in acquisition and context days when compared to vehicle. Also, Nr4a3 and Bdnf mRNA expression increased in glucose-administered Ad-deficient mice. Sub-effective doses of glucose plus Ad administered simultaneously to Ad-deficient mice increased contextual fear memory, contrary to independent sub-effective doses. Concluding, glucose may be an important part of the peripheral to central pathway involved in the retrieval and reconsolidation of fear contextual memories independently of Ad, possibly due to increased hippocampal Nr4a3 and Bdnf gene expression. Furthermore, Ad and glucose may act synergically to strengthen contextual fear memory.

Adrenaline (Ad) strengthens contextual fear memory by increasing blood glucose, possibly enhancing hippocampus acetylcholine synthesis. Nevertheless, it is unclear if peripheral Ad influences the cholinergic system, contributing to contextual fear memory strengthening. We aimed to evaluate whether peripheral Ad alters muscarinic receptor expression and if the cholinergic system is involved in peripheral Ad contextual fear memory strengthening effect. Wild-type (WT) and Ad-deficient male mice (129 × 1/SvJ) underwent a fear conditioning procedure followed by intraperitoneal pre-training and pre-context administration of Ad (0.1 mg/kg), atropine (10 mg/kg), methylatropine (0.5 mg/kg), Ad (0.1 mg/kg) plus atropine (10 mg/kg) or vehicle (NaCl, 0.9%). Shock responsiveness and freezing behaviour were accessed. Hippocampal M₁, M₂, and M₄ mRNA expression were evaluated. Ad-deficient mice presented decreased hippocampal muscarinic M₄ subtype receptor mRNA expression compared to WT mice. In Ad-administered Ad-deficient mice, hippocampal muscarinic M₄ subtype receptor mRNA expression increased compared with vehicle-administered Ad-deficient mice. On the context day, atropine-administered WT mice presented decreased freezing behaviour compared to vehicle or methylatropine-administered WT mice. Moreover, Ad plus atropine-administered Ad-deficient mice led to decreased freezing behaviour compared to Ad-administered Ad-deficient mice. In conclusion, Ad-deficient mice's contextual fear memory impairment was associated with hippocampal muscarinic M₄ subtype receptor down expression, which was reversed by Ad. This may be related to contextual fear memory consolidation or retrieval induced by peripheral Ad. Furthermore, the effect of Ad contextual fear memory might be due to increased hippocampus muscarinic subtype M₄ expression, which may contribute to increased cholinergic activity in the central nervous system.

Stress-induced physiological changes are crucial for adaptive responses to threats. Catecholamines, specifically epinephrine, activate peripheral β-adrenergic receptors, facilitating contextual fear memory consolidation. The vagus nerve is thought to be essential in transmitting physiological signals to the brain, influencing emotional memory consolidation. We aimed to evaluate the vagus nerve's role in contextual fear memory strengthening induced by epinephrine. Male 129 × 1/SvJ mice underwent bilateral subdiaphragmatic vagotomy, right or left cervical vagotomy, or sham surgeries. After a seven-day recovery, mice were subjected to the fear conditioning paradigm, receiving three foot shocks on conditioning day, followed by contextual reminder exposure. Epinephrine (0.1 mg/kg, intraperitoneal, i.p.) was administered to a subset of left cervically vagotomized mice 3 min before each session. Freezing behavior was evaluated. Catecholamines were quantified by reverse-phase high-performance liquid chromatography (HPLC) with electrochemical detection. Subdiaphragmatic vagotomy did not alter freezing behavior on both days. In cervical vagotomies, while no differences were observed on the conditioning day, a significant decrease in freezing behavior was observed on the context day. In left cervically vagotomized mice, epinephrine administration significantly increased freezing behavior. In conclusion, unilateral left or right cervical vagotomy impaired contextual fear memory, contrary to subdiaphragmatic vagotomy. Impaired contextual fear memory in left cervical vagotomy was reversed by exogenous epinephrine, possibly due to the intact right cervical vagus. We suggest that stress-induced catecholamine release may activate thoracic peripheral β-adrenergic receptors and elevate systolic blood pressure, signals that are transmitted through the vagus nerve, influencing the hippocampus and facilitating contextual fear memory consolidation.