6.6.2 Hibernation-induction trigger
Researchers have devoted much effort to the search for a possible blood-borne chemical messenger that might communicate a signal within the brain and to other body tissues, causing entry to hibernation. Serum from hibernating animals such as the woodchuck (Marmota monax; Figure 8), when injected into active animals, can induce torpor. Partly purified serum extracts are also able to induce hibernation-like behavioural changes in a variety of mammalian species. Chemical analysis of the serum extracts reveals two components, one of high molecular mass (M r = 88 000) which has a structure closely resembling a natural protease inhibitor, and the other of low molecular mass (M r < 5000) which is a member of the family of natural morphine-like or ‘opioid’ compounds. Pure samples of an opioid called enkephalin also induce hibernation in Colombian ground squirrels (Spermophilus columbianus). Dynorphin A is another opioid present in the ground squirrel brain. It acts at so-called delta receptor sites similar to those for enkephalin and rises during hibernation to a level 15 times higher than in non-hibernating euthermia, reaching an intermediate level in euthermia between bouts of hibernation. The origin of the different hibernation-inducing trigger components is not certain, but these observations point to the possibility that the blood circulation does contribute to the transmission of the signal to enter hibernation from one tissue to another. Opioids also have remarkable properties in inducing survival of cells in the brain and other organs under anoxic conditions. This discovery has led to an interest in the chemical induction of hibernation amongst medical researchers seeking ways in which to prolong tissue survival after trauma, or during and after transplantion.