Effects of ketamine on pro-inflammatory mediators in equine models

Abstract

Ketamine is frequently used in both human and veterinary anaesthesia. Beside its anaesthetic and analgesic effects, ketamine has been demonstrated to possess anti-inflammatory properties in rodents and humans. To date, no data are available on the anti-inflammatory effects of ketamine in horses. This thesis summarises experimental work conducted into the anti-inflammatory effects of ketamine in different in vitro and in vivo equine models. In an equine bone-marrow-derived macrophage cell line (e-CAS cells), ketamine significantly suppressed lipopolysaccharide (LPS)-induced tumour necrosis factor (TNF)-alpha and interleukin (IL)-6 concentrations in a concentration-dependent manner (0-36 microM). Characterisation of the molecular mechanism underlying this cytokine-modulating activity revealed that ketamine inhibited LPS-induced cytokine production by directly inhibiting the nuclear transcription factor nuclear factor-kappa B (NF-kappa B). Next to cytokines, NF-kappa B is known to be involved in the transcription of enzymes such as cyclooxygenase (COX)-2 and inducible nitric oxide synthase (iNOS), which account for many of the clinical symptoms associated with inflammatory responses. Although ketamine significantly inhibited LPS-induced NF-kappa B expression, no effect on COX-2 and iNOS expression could be demonstrated. Inhibiton experiments showed that this lack of effect is most likely contributed to the fact that LPS-induced COX-2 and iNOS expression are mainly regulated by another signal transduction pathway, the mitogen-activated protein kinase (MAPK) pathway, which itself is not affected by ketamine. In contrast to these results, ketamine significantly reduced the production of reactive oxygen species (ROS) in e-CAS cells following co-stimulation with LPS and phorbol myristate acetate (PMA). The finding that ketamine reduced cytokine production and ROS formation in LPS-treated e-CAS cells indicates that ketamine has cytokine-modulating effects in vivo. Considering the concentration-dependency and the finding that elevated plasma cytokine levels have been detected in horses for several hours after onset of inflammation, it was hypothesised that ketamine may need to be administered for a prolonged period of time and at relatively high dose rates to be clinically effective in modulating inflammatory mediators. A pharmacodynamic study showed that a ketamine CRI of 1.5 mg/kg/h can safely be administered to conscious healthy horses for at least 6 h. To evaluate the cytokine-modulating effects of this ketamine CRI in vivo, a tissue chamber model was used. In LPS-inoculated tissue chambers in Shetland ponies, ketamine did not exhibit cytokine-modulating or antioxidative properties, which can be explained by the rapid biotransformation and elimination of ketamine as well as the limited affinity to proteins, thus preventing adequate accumulation of ketamine in the inflamed tissue chambers. However, the observed anti-inflammatory properties of ketamine in vitro still justify further research into the anti-inflammatory effects of ketamine in horses suffering from inflammatory disorders.