Neuromodulation 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 through the brain extracellular fluid of neurotransmitters 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.
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 caeruleus tend to be melanin-pigmented. In spite of their small number, when activated, the system plays major roles in the brain, as seen in table above. Noradrenaline is released from the neurons, and acts on adrenergic receptors.
Parkinson's disease is at least in part related to dropping out of dopaminergic cells in deep-brain nuclei, primarily the melanin-pigmented neurons in the substantia nigra but secondarily the noradrenergic neurons of the locus coeruleus. Treatments potentiating the effect of dopamine precursors have been proposed and effected, with moderate success.
Prozac or fluoxetine, a selective serotonin reuptake inhibitor (SSRI), is a widely used antidepressant that blocks the reuptake of serotonin. Although changes in neurochemistry are found immediately after taking an antidepressant, symptoms will not begin to improve until 4 to 6 weeks after administration.
Monoamine oxidase inhibitors are thought to change the rate of oxidation of biogenic amines within the brain. A lack of oxidation means that more neurotransmitters (specifically monoamines such as dopamine or serotonin) are available for release into synapses. MOAIs take several weeks to alleviate the symptoms of depression.
Tricyclic antidepressants block the reuptake of biogenic amines from the synapse, back into the neuron. They typically take 4 to 6 weeks to alleviate any symptoms of depression. They are considered to have immediate and long-term effects.
Opioid peptides - a large family of endogenous neuropeptides that are widely distributed throughout the central and peripheral nervous system. Opiate drugs such as heroin and morphine act at the receptors of these neurotransmitters.
Neuromodulation also refers to an emerging class of medical therapies that target the nervous system for restoration of function (such as in cochlear implants), relief of pain, or control of symptoms, such as tremor seen in movement disorders like Parkinson's disease. The therapies consist primarily of targeted electrical stimulation, or infusion of medications into the cerebrospinal fluid using intrathecal drug delivery, such as baclofen for spasticity. Electrical stimulation devices include deep brain stimulation systems (DBS), colloquially referred to as "brain pacemakers", spinal cord stimulators (SCS), which are implanted using minimally invasive procedures, or transcutaneous electrical nerve stimulation devices, which are fully external, among others.
^Castaneda-Hernandez, Gilberto C.; Bach-y-Rita, Paul (2003). "Volume Transmission and Pain Perception". The Scientific World JOURNAL3: 677–683. doi:10.1100/tsw.2003.53.
^ abcUnless else specified in boxes, then ref is: Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone. pp. 474 for noradrenaline system, page 476 for dopamine system, page 480 for serotonin system and page 483 for cholinergic system. ISBN0-443-07145-4.
^ abcdWoolf NJ, Butcher LL (1986). "Cholinergic systems in the rat brain: III. Projections from the pontomesencephalic tegmentum to the thalamus, tectum, basal ganglia, and basal forebrain". Brain Res Bull.16 (5): 603–37. doi:10.1016/0361-9230(86)90134-6. PMID3742247.