Neurotransmitters, chemical signals

Of the several classes of receptors for endogenous chemical signals [3], two are used as postsynaptic receptors in synaptic transmission ligand-gated ion channels (LGICs) and G protein-coupled receptors (GPCRs Fig. 1). Due to the large number of transmitters and the existence of several receptor types for almost all, postsynaptic receptor activation is the most diversified step of synaptic transmission. Table 1 shows selected neurotransmitter receptors. 

Figure 3.2 Normally, when the stomach is full it sends a signal to the brain telling the body to stop eating. This message is carried by neurotransmitters (chemical messengers in the brain) to the hunger center in the hypothalamus (a part of the brain). Examples of neurotransmitters that carry this message are norepinephrine, serotonin, and dopamine. Many diet pills increase these same neurotransmitters that signal the brain that the stomach is full. These diet pills, called appetite suppressants, trick the brain into thinking the stomach is full and therefore decrease hunger.

According to the International Union of Pharmacology Committee, a receptor is a cellular molecule, or an assembly of macromolecules, that is concerned directly and specifically in chemical signaling between and within cells. Combination of a hormone, neurotransmitter, drug, or intracellular messenger with its receptor(s) initiates a change in cell function. 

Signaling across synapses may be electrical or chemical. The chemical signal carriers are known as neurotransmitters. They are released from the presynaptic side and recognized by their receptors on the postsynaptic side. 

Neurotransmitter a substance that communicates a chemical signal across a synapse. 

Neurotransmitter/Receptor Binding. At this point, the neurotransmitter chemical is free in the synapse (extracellular fluid) and drifts (diffuses) in all directions. Some of the neurotransmitter molecules float across the synapse and bind to receptors on the surface of the adjacent nerve cell. Each neurotransmitter has its own unique three-dimensional shape and binds with certain receptors but not others. The binding between a neurotransmitter and a receptor is similar to fitting a key into a lock. When the neurotransmitter binds the receptor, the signal has been passed to the neighboring nerve cell. This is the process of neurotransmission. 

During neurotransmission, the axon of one neuron sends a chemical signal (a neurotransmitter) to receptors on the dendrite of another neuron across the synaptic cleft. 

The hippocampus has innumerable afferent and efferent connections to other brain structures both within the limbic system and beyond. There are receptors for many different chemical signals ranging from the "classical neurotransmitters such as acetylcholine to steroid hormones and neurotrophic factors. Some of these receptors are located in the synapses that form the intrinsic hippocampal circuits and others are the targets of specific projection pathways from other brain areas. A comprehensive review of all neurotransmitter interactions relevant to function is not within the scope of this chapter. There are detailed reviews of modulation of neurochemical systems on place learning in the watermaze (McNamara and Skelton, 1993) or other limbic-system dependent tasks (Izquierdo and Medina, 1995) in animals. The effects of key neurochemical, other than NMDA channel-mediated, and environmental influences are discussed below. 

The chemical connection between brain cells is by means of neurotransmitters—chemicals such as dopamine that convey nerve signals. In normal brain connections a neuron fires (discharges electrical potential). This in turn releases a small quantity of the neurotransmitter, which attaches itself to a receptor molecule in an adjacent neuron, causing it to fire in turn. Eventually the dopamine is returned to the cells until the ne.xt signal comes along. 

Neurons send electrical impulses from one part of the cell to another part of the same cell via their axons, but these electrical impulses do not jump directly to other neurons. Neurons communicate by one neuron hurling a chemical messenger, or neurotransmitter, at the receptors of a second neuron. This happens frequently, but not exclusively, at the sites of synaptic connections between them (Fig. 1 — 3). Communication between neurons is therefore chemical, not electrical. That is, an electrical impulse in the first neuron is converted to a chemical signal at the synapse between it and a second neuron, in a process known as chemical neurotransmission. This occurs predominantly in one direction, from the presynaptic axon terminal, to any of a variety of sites on a second postsynaptic neuron. However, it is increasingly apparent that the postsynaptic neuron can also talk back to the presynaptic neuron with chemical messengers of its own, perhaps such as the neurotransmitter nitric oxide. The frequency and extent of such cross-communication may determine how... 

These neurons know where to go because of a series of remarkable chemical signals, different from neurotransmitters, called adhesion molecules (Table 1—4). First, glial cells form a cellular matrix. Neurons can trace glial fibers like a trail through the brain to their destinations. Later, neurons can follow the axons of other neurons... 

The axon is specialized to react to changes in membrane potential. When the cell s membrane potential reaches a certain threshold the axon responds with an action potential that rapidly transmits an electrical signal from the cell body to its terminals. Finally, the nerve terminal is specialized to convert the electrical signal of the action potential back into a chemical signal. It responds to depolarization by releasing a neurotransmitter that acts either upon the soma or dendritic membranes of the next neuron or, in the PNS, on an effector site (Figure 11.5). The specialized membrane is essential to the electrochemical properties of neurons. 

Each neuron is a distinct anatomic unit, and no structural continuity exists between most neurons. Communication between nerve cells— and between nerve cells and effector organs—occurs through the release of specific chemical signals, called neurotransmitters, from the nerve terminals. This release depends on processes that are triggered by Ca++ uptake and regulated by phosphorylation of synaptic proteins. The neurotransmitters rapidly diffuse across the synaptic cleft or gap (synapse) between nerve endings and combine with specific receptors on the postsynaptic (target) cell (see pp. 37 and 57). 

---