Fig. 1.

Pathways for intranasal transport. Proteins and peptides delivered into the nasal cavity come into contact with the cribriform plate. The substrates reach the brain via direct and indirect pathways. The direct pathway can occur through the olfactory bulbs or trigeminal nerve. The indirect pathway transports the substrates into the lymphatic system, which then drains into the circulation. The proteins and peptides can then enter the brain if able to navigate the BBB. Once in the brain, the substances are transported to various regions by extranueronal, intraneuronal, or transneuronal transport