What's the dopaminergic system?
The system by which dopamine exerts its influence (the -ergic part of the word. This sums it up pretty well, taken from
Dopamine: a potential substrate for synaptic plasticity and memory mechanisms Thérèse M. Jay
Progress in Neurobiology
Volume 69, Issue 6 , April 2003, Pages 375-390
In the field of catecholamine transmitters, dopamine (DA) pathways have then been recognized to play a critical role in cognition and emotion, and the last decade has seen a large increase in the experimental evidence for a role of DA in both synaptic plasticity and memory processes.
Since the discovery of LTP in the hippocampus ([Bliss and Lomo, 1973]), synapses that undergo plastic changes have been described in various parts of the brain and particularly in brain regions that receive DA innervations. It is now well established that the strength of synaptic transmission can be modified on a long-term basis by specific patterns of activation such as high frequency trains that produce LTP, and also by the action of endogenous modulators such as DA.
Different approaches from unit recording to lesion and pharmacological studies have demonstrated that DA plays a critical role in the modulation of neuronal activities that are related to different forms of learning and memory.
Converging evidence indicates that DA innervation of the prefrontal cortex plays a major role in working memory, a form of memory that allows to maintain and manipulate active representations of information that are held temporarily in mind. DA loss in the prefrontal cortex can lead to a great deficit in the performance of a working memory task in monkeys ([Brozoski et al]) as does lesion of the VTA in the rat ( [Simon et al]).
Synaptic plasticity induced in the different regions examined (hippocampus, striatum and prefrontal cortex) does not appear to recruit the DA systems in similar manners. It is conceivable that a local regulation of these plastic events is specific to the region where the synapses are activated. Although a comparable DA modulation appears to be present in the hippocampus and the prefrontal cortex, only the consolidation of LTP in the hippocampus is dependent on D1 receptors whereas a potential permissive and facilitatory effect of DA and D1 agonists is observed at an early stage of LTP and required for the initiation of this plastic event in the prefrontal cortex.
On top of a different subcellular DA innervation and spatial distribution of D1 and D5 receptors in the two regions that could explain these discrepancies, a distinct temporal combination between the glutamate and the modulatory DA pathways might also be at the origin, for functional purposes, of the immediate versus delayed recruitment of DA systems. Indeed, DA neurons are differentially activated depending on cognitive demands. Thus, the heterosynaptic influence of the DA signal could gain its selectivity from the activity patterns that are initiated in the VTA during changes in behavioral states and that are present in the DA inputs to the post-synaptic sites in the hippocampus or the prefrontal cortex. Whether the mesohippocampal or mesocortical DA systems are involved in different memory processes would differentially alter the relative strength of the synapses in the two regions. This mode of synaptic plasticity and modulation by a DA tone demonstrates a dynamic and specific regulation of synapses that may be important for memory.
The future will define more precisely at the cellular level which specific target cells of DA terminals and which receptors influence intrinsic and extrinsic circuits and at the molecular level which proteins are specifically involved in these DA/glutamate interactions relevant to plasticity and memory processes. Increased understanding of these mechanisms may also be relevant to the pathophysiology of DA-related psychiatric disorders.