The complexity of neuronal signalling is mediated by neurotransmitters (glutamate, gamma-aminobutyric acid), neuromodulators (e.g. serotonin, dopamine, acetylcholine) (discussed in Section 2.4) and neuropeptides, which are the focus of this section. In contrast to rapid-acting neurotransmitters, which affect the excitability of target neurons by depolarisation or hyperpolarisation (lasting seconds to minutes), neuropeptides exert prolonged actions (lasting hours to days) by changing gene expression and synaptogenesis to mediate a diverse range of physiological and behavioural responses. There are over a hundred different neuropeptides that mediate, for example, eating behaviour, social behaviour, learning, memory, reproduction and analgaesia. Most neurons release classical neurotransmitters or amino acid transmitters as well as neuropeptides. A small minority of neurons release only neuropeptides, including the magnocellular neurons in the hypothalamus, which release peptides such as oxytocin and arginine vasopressin directly into the bloodstream.