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Fast synaptic transmission is crucial for real-time functioning of the brain. All the receptor molecules that mediate fast transmission events are also ligand-gated ion channels, i.e., they are ion channels that undergo allosteric structural changes on binding a particular neurotransmitter molecule, resulting in the opening of the channel, the entry of selected ions into the neuron and subsequent signalling events. Their primary function is to receive signal input at postsynaptic membranes, where some also play central roles in synaptic plasticity. However, they are also found in postsynaptic membranes outside synapses, and in presynaptic terminals, where they are involved in the control of transmitter release. This chapter presents an overview of current knowledge of the molecular biology of these receptors.

The sense of smell begins with the action of smell molecules on the receptor molecules in our nose, which in turn are a critical part of the smell and flavor system in the brain. This chapter examines how the critical and fascinating meeting between the molecules that are in our food and those that are in our receptor cells occurs. It discusses the discovery of olfactory receptors and considers evidence supporting the hypothesis that odor determinants interact within a binding pocket in the olfactory receptors. This binding pocket is similar to that of other G-protein coupled receptors (GPCRs), but with a set of critical sites that varies with different receptors and can show graded interactions with the determinants of different odor molecules.

Fish, and other vertebrates, possess both a neural control system and a hormonal one dependent on the circulation. Fish hormones may be secreted by well-defined endocrine glands or by diffuse tissue and can have profound effects on the function of target tissues or organs which possess receptor molecules recognized by specific hormones. Some endocrine activities in fish are not relevant to mammals, and vice versa. Thus fish can have chromatophores controlled hormonally to change skin colour, the same hormones having different roles in mammals. Likewise, ‘prolactin’ in fish cannot regulate lactation. Individual endocrine glands, tissues and cells of fish are described in detail and the possible roles of their hormones are discussed. Fish endocrinology is becoming increasingly important with the realization of its potential applications in aquaculture and with the recognition of the consequences of pollutant endocrine disruption.