Neurotransmitters and neuromodulators
The traditional models for the study of neurotransmitter release are either the neuromuscular junction of the frog, crayfish, and rat or the giant synapse of the squid. These synapses are relatively simple in their structure, with a single axon terminal forming an identifiable synapse at the postsynaptic membrane of a muscle fibre or neuron. Recordings can be obtained from these single-synaptic junctions in response to the release of a single neurotransmitter. At neurons of the central nervous system, on the other hand, the situation is more complex. Each central neuron has several synapses with other neurons at various locations, such as on the dendrites, soma, and initial segment of the axon. Several neurotransmitters, therefore—some excitatory and others inhibitory—may be involved in the final integrated response of a central neuron, making their identities difficult to determine.
Further complicating neurotransmitter action is the presence not only of multiple transmitter substances but also of neuromodulators. Neuromodulators are substances that do not directly activate ion-channel receptors but that, acting together with neurotransmitters, enhance the excitatory or inhibitory responses of the receptors. It is often impossible to determine, in the presence of many substances, which are transmitters and which are modulators. Such is the case with many of the neuropeptides (see the section Neuroactive peptides).
In addition to the multiplicity of transmitters and modulators there is a multiplicity of receptors. Some receptors directly open ion channels, while others activate the second-messenger system, any of a number of reactions that take place in the cytoplasm or plasma membrane and indirectly act upon the ion channels. One second-messenger system involves the activation by receptor proteins of linking proteins, which move across the membrane, bind to channel proteins, and open the channels. Another system is the cyclic adenosine monophosphate (cAMP) system. In this chain reaction, receptor proteins activate linking proteins, which then activate the enzymes that synthesize cAMP. The cAMP molecules activate other enzymes that, in turn, activate ion channels.
Whether they activate channels directly or through a second-messenger system, neurotransmitters are considered to be primary messengers. Described below are the principal proved or suggested neurotransmitters of the mammalian nervous system and their corresponding receptors.
Acetylcholine
Although early studies of acetylcholine were undertaken at neuromuscular junctions, where it is especially concentrated, the concept leading to the identification of the substance as a neurotransmitter of the central nervous system is a landmark in neuroscience. The concept is called Dale’s principle after Sir Henry Dale, a British physiologist who, in 1935, stated that a neurotransmitter released at one axon terminal of a neuron can be presumed to be released at other axon terminals of the same neuron. (Dale’s principle refers only to the presynaptic neuron, as the responses of different postsynaptic receptors to a single neurotransmitter can vary in the same or different neurons.) The first application of Dale’s principle was at the mammalian spinal cord, from which motor neurons send their axons to striated muscles, where the terminals are observed to release acetylcholine. According to Dale’s principle, all the branches of a single motor neuron axon should release acetylcholine—including the terminals in the spinal cord. In fact, it was found that some collateral branches leave the motor axons and reenter the gray matter of the spinal cord, where they synapse onto spinal interneurons. The neurotransmitter released at these terminals is acetylcholine. High concentrations of the acetylcholine-synthesizing enzyme, choline acetyltransferase, and the enzyme for its breakdown, acetylcholinesterase, are also found in motor neuron regions of the spinal cord.
Acetylcholine receptors (also called cholinergic receptors) appear in clusters on muscle-cell membranes opposite the active zones of presynaptic terminals. Their density at these receptor regions is between 7,000 and 30,000 sites per square micrometre (micron; millionth of a metre). The number drops drastically even a few nanometres (billionths of a metre) away from the receptor region, so that sensitivity to acetylcholine is about 50 to 100 times less one millimetre from the receptor region than it is at the receptor site itself. Cholinergic receptors also exist on the presynaptic terminals of neurons that release acetylcholine as well as on terminals that release other neurotransmitters. These receptors are called autoreceptors, and they probably regulate the release of neurotransmitter at the terminal.
There are two main categories of cholinergic receptor, nicotinic and muscarinic. The nicotinic receptor is a channel protein that, upon binding by acetylcholine, opens to allow diffusion of cations. The muscarinic receptor, on the other hand, is a membrane protein; upon stimulation by neurotransmitter, it causes the opening of ion channels indirectly, through a second messenger. For this reason, the action of a muscarinic synapse is relatively slow. Muscarinic receptors predominate at higher levels of the central nervous system, while nicotinic receptors, which are much faster acting, are more prevalent at neurons of the spinal cord and at neuromuscular junctions in skeletal muscle.
The nicotinic receptor channel is a glycoprotein composed of five subunits (see the ). Two alpha- (α-) subunits contain the two acetylcholine-binding sites associated with the channel. Three other subunits—a beta- (β-) subunit, a gamma- (γ-) subunit, and a delta- (δ-) subunit—complete the protein. High-resolution electron microscopy with optical image reconstruction, as well as freeze-fracture electron microscopy, reveal a highly symmetrical structure, looking from the top somewhat like a life belt, with the presumed channel in the centre. About one-third of the protein protrudes from the plasma membrane, while the rest is embedded in the membrane or protruding into the cell.
Patch-clamp techniques give information on single channel currents and, therefore, on the conductance and kinetics of the cholinergic receptor channel. At the neuromuscular junction, approximately 20,000 univalent ions carry the charge across a single activated channel, and a quantum of acetylcholine activates about 1,500 channels. The time constant for the decay of the MEPP is the same as that for channel closing. The time constant for channel closing is voltage dependent, with depolarization shortening the duration of open channels and hyperpolarization lengthening the duration.
Studies show that nicotinic acetylcholine-activated channels allow cations to permeate the membrane with no specificity—that is, all cations can diffuse through the channels indiscriminately. Because the resting membrane is already near the equilibrium potential of K+, this means that much more Na+ and Ca2+ diffuse into the cell than K+ out, causing depolarization and excitation of the neuron or muscle cell. However, in certain molluscan neurons, nicotinic acetylcholine receptors can also activate Cl− channels, causing hyperpolarization of the postsynaptic membrane and inhibition of excitability. With respect to muscarinic receptors, the situation is not clear. Second messengers may be involved, and potassium channels may be activated.
Epinephrine and norepinephrine
These related hormones, also called adrenaline (epinephrine) and noradrenaline (norepinephrine), act to increase the heart rate, blood pressure, and levels of sugar and fat in the blood. They are secreted into the bloodstream by the adrenal glands in response to stress, but they are also synthesized and released as neurotransmitters by axon terminals in the central nervous system and in sympathetic fibres of the autonomic nervous system.
Receptors sensitive to norepinephrine and epinephrine are called adrenergic receptors. They are divided into two types, α and β. These are further classified into subtypes α1, α2, β1, and β2.
Both types of adrenergic receptors produce changes in the postsynaptic membrane potential by acting upon ion channels specific to K+ and Ca2+. They differ in the mechanisms that, upon stimulation by neurotransmitter, they employ to activate those channels. Stimulated β1 receptors bind to linking proteins that in turn bind to calcium channels, changing their shape and altering their permeability to the cation. More important, the linking proteins stimulate the synthesis of cAMP, which, through another series of reactions, opens potassium channels. The efflux of K+ tends to hyperpolarize the postsynaptic membrane, inhibiting the generation of a nerve impulse. The β2 receptor has been found on glial cells.
The α2 receptor activates potassium channels in both the postsynaptic and presynaptic membranes, probably via linking proteins and the synthesis of cAMP. The α1 receptor acts on calcium channels through a series of reactions linked to the lipid molecules of the plasma membrane.
Both epinephrine and norepinephrine are terminated by uptake back into the presynaptic terminals, where they are enzymatically degraded or inactivated.