In this model, the potent vasoconstrictor Endothelin-1 (ET-1) will be administered to the infra-orbital nerve (IoN) of male Sprague-Dawley rats. The infra-orbital nerve of the rat is indeed extremely suited for the investigation of neuropathic pain syndromes because the IoN has a entirely sensoric function, without any motor function. This offers a great advantage compared to the often used sciatic nerve, which consists of a combination of motor and sensory components. Furthermore, clinical manifestation of a IoN injury induced pain syndrome can be fairly easy measured and investigated in a laboratory setting.
Application of ET-1 will probably result in occurrence of spontaneous and evoked neuropathic pain symptoms in the facial area of the rats. After application of ET-1 to the IoN of the rat, animals will be tested for the presence of spontaneous pain (determination of spontaneous face-grooming behavior), as well as evoked pain sensations (after mechanical and thermal stimulation). Pain symptoms after innocuous mechanical stimulation (mechanical allodynia) will be induced by stimulating the facial area (zone I-II-III) with increasing sizes of von Frey hairs. For the investigation of the development of thermal allodynia we will use an entirely new method of investigation. Indeed, until now, thermal stimulation was almost never applied in laboratory settings due to the lack of standardization and reproducibility. In recent months we particpated in the development of system to apply short and well-defined thermal stimuli by remote control (in order to avoid the occurrence of sensitization on repeated stimulation in the animals). Hereby, we can apply a very precise repeatable stimulus to the animal. We can do this in two possible ways. Option 1: Apply a set amount of heat for a set amount of time. This can be set by the user and changed at any time. Option 2: Increasing controlled amount of heat. The user can set a starting temperature, a cutoff temperature and the ramp time. We can set a cutoff temperature to reduce the risk of tissue and nerve damage. Once the animal reacts the user can stop the stimulus and the temperature and time would be recorded automatically. In option 1 the one thing I would like to mention is that the heat source would need a very short amount of time to reach its set temperature, (something less than one second). The time of the stimulus can also be changed at anytime. The reaction of the animal will then be determined by the experimenter. The implementation of this method will mean that for the very first time rats can be objectively tested for the presence of thermal hyperalgesia and/or allodynia! Our lab will be the first lab in the world to be able to perform this sophisticated way of performing thermal testing in rats. The possibility of being able to investigate the presence and extent of thermal allodynia has to be considered as of extreme value since thermal allodynia is one of the most important characteristics of (common) clinical neuropathic pain syndromes.
In addition, the infra-orbital nerve allows the use of several different methods of application of ET-1, which will provide us with extremely important ways of mimicking all kinds of both acute and chronic clinical conditions. More precisely, we will be able to perform extraneural application of ET-1 in acute (one single application of ET-1) and chronic conditions (implantation of osmotic pump in the surroundings of the IoN with continuous long-term application of very low doses of ET-1, mimicking the clinical 'ie. oncological- conditions). Also, intraneural application of ET-1 will be performed, as a model of intraneural invasion of tumor cells in clinical conditions. Finally, subcutaneous injection of ET-1 is possible in the facial area of the rat, which allows (electrophysiologic, morphological and biochemical) research into the pathophysiology of cutaneous hyperalgesia and allodynia (a condition seen in many oncological,