Down-regulation - when excessive numbers of transmitter molecules are available to the receptor over a period of time, a decrease in the receptor sites can be counted. This is called down-regulation. It accounts for tolerance e.g., when heroine consistently occupies opiod binding sites. The desensitized post-synaptic neuron will not respond to average amounts of heroine, but increased amounts are required to obtain and opiod effect. This in turn induces greater levels of tolerance.
Up-regulation - the reverse occurs when decreased number of transmitter molecules are available at post-synaptic receptors. This leads to up-regulation with increased number of sensitized receptors. Such a condition would occur during a receptor blockade. e.g., the anti-psychotic medication thorazine bocks the dopamine receptor inducing up-regulation. As a result, additional dopamine receptors appear on the post-synaptic membrane. At this point the post-synaptic neuron is hypersensitive, and if the blockade is ended, even average amounts of dopamine can cause the movement disorder: Tardive Dyskinesia. This condition may be treated by administration of dopamine agonists or by reestablishing the dopamine blockade.
Saturday, October 13, 2012
How to break a smoking habit…
