Which method is appropriate for killing rodents up to 500g?

The method of dislocation of the neck is appropriate for killing rodents up to 500g as it is one of the most humane and effective techniques for euthanizing small animals. This method involves a swift and decisive action that quickly severs the spinal cord, leading to rapid loss of consciousness and death. It minimizes suffering and is consistent with ethical guidelines for the humane treatment of animals.

Utilizing dislocation is generally preferred in laboratory settings due to its efficiency and the control it affords the operator. It is crucial for ensuring that the action is performed correctly to avoid any unnecessary pain or distress to the animal, making it a widely accepted practice within veterinary and animal research communities.

The other methods, while they might have their specific applications, may not be as suitable in terms of ensuring a rapid and humane death. Overdose of anaesthetic, for instance, requires proper dosing and administration by trained personnel and could lead to a prolonged death if done improperly. Cooling in a fixative item may induce distress before death occurs, and concussion of the brain, though potentially quick, can be technically difficult and should only be performed by trained individuals to ensure effectiveness and minimize suffering.