The capacity of synaptic networks to express activity-dependent changes in strength and connectivity is essential for learning and memory processes. In recent years, glial cells (most notably astrocytes) have been recognized as active participants in the modulation of synaptic transmission and synaptic plasticity, implicating these electrically nonexcitable cells in information processing in the brain. While the concept of bidirectional communication between neurons and glia and the mechanisms by which gliotransmission can modulate neuronal function are well established, less attention has been focussed on the computational potential of neuron-glial transmission itself. In particular, whether neuron-glial transmission is itself subject to activity-dependent plasticity and what the computational properties of such plasticity might be has not been explored in detail. In this review, we summarize current examples of plasticity in neuron-glial transmission, in many brain regions and neurotransmitter pathways. We argue that induction of glial plasticity typically requires repetitive neuronal firing over long time periods (minutes-hours) rather than the short-lived, stereotyped trigger typical of canonical long-term potentiation. We speculate that this equips glia with a mechanism for monitoring average firing rates in the synaptic network, which is suited to the longer term roles proposed for astrocytes in neurophysiology. Plasticity as the Cellular Basis of Learning and Memory in the Central Nervous System At a high level of abstraction, the brain is essentially an organ that detects environmental stimuli, processes the received sensory information, and initiates an appropriate motor response. From this perspective, the primary role of the brain is information processing, and the computational processes associated with transforming input to output are centred on the network of trillions of synapses through which the signals are relayed. The train of action potentials initiated in sensory neurons must be transduced by the central synaptic networks in such a way as to reliably trigger a pattern of action potentials in the motor neurons that effect the necessary coordinated activation of muscles needed to evoke a behavioural response. It is thus widely accepted that, despite defying human comprehension, there must be a particular spatiotemporal pattern of network activity reliably associated with generating a given response to a given external cue. To cope with a complex and changing environment, the synaptic network must also be adaptable, such that experience can refine and reorganize the spatiotemporal patterns of network activity in response to, for example, injurious stimuli. This adaptability requires controlled alteration of synaptic strength, a phenomenon termed synaptic plasticity The forms and mechanisms of synaptic plasticity have been extensively studied for many decades in many brain region
