Hypocretins, also known as orexins, are hypothalamic peptides that have been implicated in a variety of behaviors, e.g. food-seeking and feeding, arousal and sleep, reaction to stress. Disturbances in the central hypocretin neurotransmission are believed to underlie narcolepsy. It has also been postulated that hypocretins play an important role in the regulation of hypothalamus-pituitary-adrenal (HPA) axis, energy homeostasis, acquisition and learning of reward system-stimulating signals.

Cyclic adenosine 3’:5’-monophosphate and cyclic guanosine 3’:5’-monophosphate, common- ly know as cAMP and cGMP, are key second messengers in living organisms ranging from bacteria to Homo sapiens. Cyclic AMP and cyclic GMP are produced from ATP and GTP by the action of adenylyl cyclase and guanylyl cylase and are hydrolyzed to AMP and GMP by the cyclic nucleotide phosphodie- sterase. The natural occurrence of cyclic nucleotides in higher plants is now established, as is the presence of enzymes involved in their metabolism.

Activating polypeptide Pituitary adenylate cyclase ( PACAP ) and vasoactive intestinal peptide (VIP) belong to a family of polypeptide hormones which include representatives glucagon, secretin and growth hormone releasing hormone . PACAP and VIP are widely distributed in both the central nervous system and in peripheral tissues of various vertebrates . PACAP exists in two forms : parent , 38 - amino acid form ( PACAP38 ) and shorter 27 - amino acid form ( PACAP27 ) . PACAP27 has 68 % sequence identity with VIP consisting of 28 amino acids.