I want to use one extract, but have no idea which solvent (saline, methanol, ethanol) will be better to use and which route of administration have to select in rats.
You can use either of the solvent and it depends on the hypothesis that you want to test and what’s the scientific question you want to ask after the administration of the extract in rat. Some of the compounds are soluble in Saline but others are soluble in fat and volatile solvents such as ethanol or methanol. So, if you would like to see the acute effect of the extract after administration in the rat, then you might need to administer more concentrated active compound of the extract, on the other hand you can use less concentration for the chronic treatments. It also depends of on the solubility of your extract; if it dissolved readily in water then you can use water/0.9%saline as a solvent. You can administer the extract orally or i.p. for oral administration it should be diluted enough to neutralize the taste of extract, if the extract has the taste that rat not like, then it will be difficult to get the amount that you might need to get administer as rats stop drinking the water or reduce the intake. If you have concentrated active compound in the extract then you need to find its solubility in saline, methanol and ethanol or in other solvents. This will give you an idea about which solvent is best for your experiment based on how much you want to administer. Also if it has not already standardized, then you might want to dilute it in 3-5 different concentrations of the extract on log scale and standardize the dose of extract for your rats. If your extract has both polar and non-polar compounds then it can be dissolve in small volume of DMSO and once it get dissolved completely then dilute it with saline to the desired concentration and can be administered either i.p or subcutaneously based on your experiment.