Hormones are chemical compounds inside our body that regulate the function of the various organs in our body. Most Hormone secretions from the endocrine glands are regulated by negative feedback loops except most Hormones that are secreted by the pituitary gland. These hormones are regulated by a positive feedback from hormones in the hypothalamus. An exception to this rule is the hormone prolactin which is regulated by a negative feedback from a hormone in the hypothalamus that is called dopamine. Hormones can be divided into water soluble and lipid soluble hormones. The cellular membrane is not freely permeable to chemicals but is rather a selective entity due to the existence of the phopholipid bilayer which allows for the diffusion of lipophilic compounds only. For this reason the two types of hormones do not have the same mechanism of signaling the cell.
Lipid soluble hormones such as cortisol and testosterone are capable of penetrating the cellular membrane. Therefor they make their effect by attaching to the nuclear membrane inside the cell and thus controlling the rate of transcription and hence the translation of proteins and enzymes. The duration of action of this type of signaling is longer than the signaling with water soluble hormones. Other lipid soluble hormones that function in this mechanism are thyroxine and aldosterone.
By virtue of their inability to penetrate the lipid bilayer, water soluble hormones make their effect by attaching to receptors on the cellular membrane. These receptors are called G-proteins. They in turn activate adenylate cyclase which is an enzyme responsible for the generation of cyclic adenosine monophosphate or cAMP from ATP. CAMP in turn activates protein kinase which activates another enzyme by phosphorylation. This then makes its final action on the target cell. This type of signaling lasts shorter than that of the lipid soluble hormones. A type of receptor that uses G-protein is acetylcholine receptor.
Examples of water soluble hormones are catechol amines such as serotonin and dopamine or peptide hormones such as anti diuretic hormone and oxytocin.
A class of hormones that are specific for the nervous system are called neurotransmitters. An example of a neurotransmitter is epinephrine and norepinephrine which function by stimulating the sympathetic branch of the nervous system. Another hormone in this class is dopamine which is an inhibitory neurotransmitter that its excess or deficiency is manifested as a clinical disease. Another hormone in this class is serotonin which has importance in the case of depression