Neuromodulation

A neuromodulator can be conceptualized as a neurotransmitter that is not reabsorbed by the pre-synaptic neuron or broken down into a metabolite. Such neuromodulators end up spending a significant amount of time in the cerebrospinal fluid (CSF), influencing (or 'modulating') the activity of several other neurons in the brain. For this reason, some neurotransmitters are also considered to be neuromodulators, such as serotonin and acetylcholine. Neuromodulation is often contrasted with classical fast synaptic transmission. In both cases the transmitter acts on local postsynaptic receptors, but in neuromodulation, the receptors are typically G-protein coupled receptors while in classical chemical neurotransmission, they are ligand-gated ion channels. Neurotransmission that involves metabotropic receptors (like G-protein linked receptors) often also involves voltage-gated ion channels, and is relatively slow. Conversely, neurotransmission that involves exclusively ligand-gated ion channels is much faster.A related distinction is also sometimes drawn between modulator and driver synaptic inputs to a neuron, but here the emphasis is on modulating ongoing neuronal spiking versus causing that spiking. Neuromodulators may alter the output of a physiological system by acting on the associated inputs (for instance, central pattern generators). However, modeling work suggests that this alone is insufficient, because the neuromuscular transformation from neural input to muscular output may be tuned for particular ranges of input. Stern et al. (2007) suggest that neuromodulators must act not only on the input system but must change the transformation itself to produce the proper contractions of muscles as output. Neurotransmitter systems are systems of neurons in the brain expressing certain types of neurotransmitters, and thus form distinct systems. Activation of the system causes effects in large volumes of the brain, called volume transmission. Volume transmission is the diffusion of neurotransmitters through the brain extracellular fluid released at points that may be remote from the target cells with the resulting activation of extrasynaptic receptors, and with a longer time course than for transmission at a single synapse. Such prolonged transmitter action is called tonic transmission, in contrast to the phasic transmission that occurs rapidly at single synapses. The major neurotransmitter systems are the noradrenaline (norepinephrine) system, the dopamine system, the serotonin system, and the cholinergic system. Drugs targeting the neurotransmitter of such systems affects the whole system, and explains the mode of action of many drugs. Most other neurotransmitters, on the other hand, e.g. glutamate, GABA and glycine, are used very generally throughout the central nervous system. The noradrenaline system consists of just 1500 neurons on each side of the brain, primarily in the locus coeruleus. This is diminutive compared to the more than 100 billion neurons in the brain. As with dopaminergic neurons in the substantia nigra, neurons in the locus coeruleus tend to be melanin-pigmented. Noradrenaline is released from the neurons, and acts on adrenergic receptors. Noradrenaline is often released steadily so that it can prepare the supporting glial cells for calibrated responses. Despite containing a relatively small number of neurons, when activated, the noradrenaline system plays major roles in the brain including involvement in suppression of the neuroinflammatory response, stimulation of neuronal plasticity through LTP, regulation of glutamate uptake by astrocytes and LTD, and consolidation of memory. The dopamine or dopaminergic system consists of several pathways, originating from the ventral tegmentum or substantia nigra as examples. It acts on dopamine receptors.