flows through the AMPA channels which are permeable to Na+ only producing a resting membrane potential (Fig 22). Magnesium blockade of the NMDA channel is still present during the resting membrane potential, but disappears as soon as the cell is strongly depolarised due to excess Na+ entering the cell. Due to the voltage gated dependency of the NMDA gate depolarisation results in a release of the magnesium ion from it’s channel allowing current to flow through (Fig. 22). In comparison with AMPA, NMDA channels are far more permeable to calcium, this is an important second messenger and is essential for the establishment of LTP (Fig. 23). It can be deduced from this description that the NMDA receptor subtype acts like a molecular “AND” gate, the channel opens only when glutamate is bound to NMDA receptors and the cell is depolarised by glutamate bound AMPA allowing LTP to occur. Therefore if a weak stimulus is present glutamate will be released but depolarisation will not occur. (PURVES et al 1997)Figure 23 Influx of Calcium mediating LTP In the late 1970’s the platelet 2 adrenergic receptor was first radiolabelled, this peripheral receptor has been utilised as a marker for central 2 receptors. Radioligand binding techniques have been used to investigate 2 adrenergic receptors. Some studies have suggested an increase in total 2 adrenergic receptor numbers and others, and demonstrated increased affinity states or changes in ratio of affinity states, affinity of ligand, or direct demonstration of an altered receptor effector efficiency in various mental illness states. Taken as a whole these studies do suggest some altered peripheral 2 receptor/effector functioning in affective disorders. Appropriate binding studies have been designed to facilitate the complexity of 2 adrenergic receptor/effector systems and their regulation. The platelet 2 adrenergic recepto...
