Homeostasis negative feedback mechanisms

The receptor-mediated, presynaptic, negative-feedback mechanisms are rapidly activated after administration of tricyclic antidepressants. By limiting synaptic availability of NE, such mechanisms normally tend to maintain functional homeostasis. However, with repeated drug exposure, Qj-receptor responses eventually are diminished, possibly from desensitization secondary to increased exposure to the endogenous agonist (NE) or from prolonged occnpation of the NE transporter itself via an allosteric effect. Over a period of days to weeks, this adaptation allows the presynaptic production and release of NE to return to, or even exceed, baseline levels. However, long-term treatment eventually can reduce the expression of tyrosine hydroxylase as well as the NE transporter. 

When self-regulating physiological systems (generally controlled by negative feedback systems, e.g. endocrine, cardiovascular) are subject to interference, their control mechanisms respond to minimise the effects of the interference and to restore the previous steady state or rhythm this is homeostasis. The previous state may be a normal function, e.g. ovulation (a rare example of a positive feedback mechanism), or an abnormal function, e.g. 

Many biological processes rely on negative feedback. As the population of a species expands, so its food supply per individual is diminished the result is that the population then begins to fall. Many biochemical processes are controlled by feedback inhibition. Feedback mechanisms play an important role in maintaining a state of equilibrium within an organism (see homeostasis). 

Homeostasis A tendency of stability in the normal body states (internal environment) of the organism, achieved via a system of control mechanisms activated by positive and negative feedback. 

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