Essential oils are known to have promising potential for the treatment and prevention of various diseases, as well as for the protection and promotion of overall health. However, their application as medicines is limited due to their low water solubility and instability, high volatility, and some side effects (Başer and Buchbauer, 2020). With the development of technology and obtaining “nano” formulations of essential oils, it was possible to overcome these limitations (Saraf, 2010; Flores et. al., 2011; Bilia et al., 2014). Nevertheless, one of the significant approaches that were developed in recent years is the formation of essential oil nanoemulsions (El Asbahani et. al., 2015; Naseema et. al., 2021; Esmaili et. al., 2021; Alshahrani, 2022; Pandey et. al., 2022).

Nanoemulsions are generally defined as stable emulsions containing oil droplets of 1-100 nm in diameter and have many features such as high optical clarity and long-term stability. Such nanoemulsion systems can be spontaneously generated using low-energy emulsification methods by combining oil, surfactant, and water (Mason et al., 2006; Lawrence and Rees, 2012; McClements, 2021). In recent years, the use of emulsions in pharmaceutical, food, and other industrial fields has been of interest to improve the solubility and stability of lipophilic substances. Preparing essential oil nanoemulsions provides many advantages such as controlling drug release, increasing the physical stability of active substances, protecting from interactions with the environment, reducing volatility, increasing biological activities, and reducing toxicity (Saraf, 2010, Bilia et al., 2014; Moazeni et. al., 2021; Karadağ et al., 2022). Also, in vitro biological activity studies, such as antimicrobial effect increase by a combination of essential oils were reported among others (Langeveld et. al, 2014; Göger et. al., 2018; Karaca et. al., 2021). In the recent study by Karaca et al., (2023), oil-in-water (O/W) emulsions of essential oils and diethyl ether fractions were obtained, where in vitro antibacterial, anti-biofilm, antioxidant, and anti-inflammatory effects of emulsion formulations were reported, respectively.

There are many studies on the effects of essential oils on the Central Nervous System (CNS) such as sleep, mood, behavior, perception, anticonvulsive effects, epilepsy treatment, Alzheimer's disease, and Parkinson's disease (Heuberger, 2010, Dobetsberger and Buchbauer, 2011). In this study, rosemary, and coriander essential oils, which are known to be effective on the CNS, were used.

Rosemary (Rosmarinus officinalis L., Lamiaceae) is a medicinal aromatic plant used worldwide. The main components of the essential oil include 1,8-cineole, α-pinene, borneol, and camphor. Antioxidant, antimicrobial, antifungal, anti-carcinogenic, and glucose level lowering effects were shown in previous studies (Borges et al., 2019). It is also reported that rosemary essential oil stimulates the nervous system and could improve memory and concentration abilities (Faixova and Faix, 2008). In a clinical study by Kim et al, (2014), it was reported that smelling 1,8-cineole as main component of rosemary essential oil before operations reduced anxiety. It was discussed in various studies that 1,8-cineole may treat Alzheimer's disease in the early stages by inhibiting acetylcholine esterase (Cai et al., 2021). In addition, plant extracts containing 1,8-cineole showed a higher inhibitory effect than 1,8-cineole alone (Paul et al., 2020).

As it is well known, rosemary preparations are used to stimulate the mind and improve concentration and memory for aging-related CNS diseases, against memory problems. Also, rosemary preparations are used against general circulatory disorders, hypertension, nerve-induced pain, indigestion, neuralgia, fatigue, and old age (Adams et al., 2007).

Coriander (Coriandrum sativum L., Apiaceae) seeds are traditionally used due to their carminative, diaphoretic, diuretic, and stimulating effects. The main components of coriander seed essential oil are linalool, α-pinene, p-cymene, camphor, and geranyl acetate (Mandal and Mandal, 2015). Coriander is known to be a sedative-hypnotic, antidiabetic, hypolipidemic, and hepatoprotective as well. In Europe, sugar-coated coriander seeds are used to strengthen memory and sleep comfort (Adams et al., 2007). Coriander extracts were shown to contribute positively to the memory-enhancing effect (Mani and Parle, 2009, Dobetsberger and Buchbauer, 2011).

The initial aim of this study was to obtain emulsion formulations of rosemary and coriander essential oils as well as their mixture in equal proportions, which are known to be used in CNS diseases. In the second part, the possible effects of stable nanoemulsions on 5-lipoxygenase-induced inflammation and acetylcholinesterase/butyrylcholinesterase-induced destruction of nervous system function seen in CNS diseases were demonstrated by in vitro enzyme inhibition studies. To the best of our knowledge, rosemary and coriander essential oil combinations as well as their nanoemulsification effects on various biological activities is reported here for the first time in the present study.