PQQ (pyrroloquinoline quinone) is an active anti-oxidant that acts as an inhibitor of neuronal oxidative stress. It is a precursor of nerve growth factor (NGF). NGF is important in the maintenance of nerve plasticity. This process allows nerve cells to communicate and form connections.

PQQ was shown to improve memory learning ability in aged mice. It has also been found to be beneficial for the treatment of memory impairment in schizophrenic rats.

In addition, PQQ may have protective effects against arthritis and diabetes. It also has a positive effect on insulin resistance. Besides, it has been reported to reduce the risk of traumatic brain injury.

IPQ is similar to PQQ in its biological activity. However, its toxicity is lower than PQQ.

The protective effects of IPQ and PQQ are different depending on the cell type. SK-N-SH cells have a lower sensitivity to both substances. Both PQ and IPQ showed a concentration-dependent effect on cell proliferation. But the protective effect of IPQ was weaker.

Neuropeptides from ENS have an essential role in the regulation of neuronal survival. They have been reported to be effective in a variety of inflammatory and stress situations. For instance, neuropeptide Y (NPY) assists in the repair of neurons. Nerve growth factor is especially important for the maintenance of neuroplasticity.

The effects of IPQ and PQQ are dependent on the quinone moiety. In general, both substances have a low toxicity.

To determine the effects of IPQ and PQQ, the step-through passive avoidance test was conducted. Compared to the controls, the time taken to enter a dark room was longer for mice administered IPQ.