Neurotransmitters (The Neuron) Part 5

Imidazole amines

Imidazole amines consist of an imidazole ring with an attached amino group. Imidazole consists of a five-membered ring containing two nitrogen atoms with a hydrogen atom located on either of the two nitrogen atoms. Histamine consists of an imidazole ring and an amino group connected by two methylene groups.

Histamine

Synthesis and Removal. In the periphery, histamine is synthesized in mast cells. Histamine circulating in the blood does not cross the blood-brain barrier. Brain cells synthesize their own histamine from histidine, which enters the brain by active transport. Histidine is then decarboxylated by histidine decarboxylase to form histamine. Histamine is metabolized by two enzymes—histamine methyltransferase and diamine oxidase (histaminase)—to organic aldehydes and acids.

Distribution. The highest density of histamine-containing neurons has been found in the median eminence and premammillary regions of the hypothalamus. These neurons send projections to almost all areas of the brain and spinal cord.

Physiological and Clinical Considerations. Histamine has been implicated as a transmitter in the regulation of food and water intake as well as in thermoregulation and autonomic functions.

Purines

Recently, ATP (adenosine triphosphate) has been implicated as a neurotransmitter. Since it contains a purine ring, it has been included in a new class of neurotrans-mitters called purines. Purinergic transmission has been demonstrated in autonomic neurons innervating the bladder, intestinal smooth muscles, and vas deferens. ATP has also been implicated in pain mechanisms. For example, when ATP is released by tissue damage, it excites the peripheral nerve endings of the dorsal root ganglion cells via a subtype of ionotropic purine receptor. Subsequently, ATP is released at the terminal of the central axon of the dorsal root ganglion cell, and neurons in the dorsal horn of the spinal cord are activated via another subtype of ionotropic purine receptor. Ade-nosine is also considered to be a purinergic neurotrans-mitter. However, it is not a classical neurotransmitter in the sense that it is not stored in presynaptic vesicles and is not released in a Ca2+-dependent manner. It is generated by degradation of ATP by extracellular enzymes.

Neuroactive Peptides

More than 100 pharmacologically active peptides have been identified in neurons and implicated as neurotrans-mitters. However, representatives of only two groups will be discussed in this topic because some of their functions have been delineated.

Opioid Peptides

There are three endogenous opioid peptide families: P-endorphins, enkephalins, and dynorphins.

1. b-endorphin: The pre-propeptide, pre-proopiomelano-cortin, is synthesized in rough endo-plasmic reticulum of neurons in the anterior pituitary, in the intermediate lobe of pituitary, and in the arcuate nucleus of hypotha-lamus. Removal of a signal peptide from the pre-propeptide within the rough endoplasmic reticulum results in the generation of the propeptide proopi-omelanocortin, which is then transported to the Golgi apparatus where it is packaged into vesicles. These vesicles are transportevd to the axon terminal by fast axonal transport. Further proteolytic processing in the terminal results in the generation of the active peptides, adrenocorticotropic hormone (ACTH) and P-lipotropin. P-lipotropin is further cleaved into y-lipotropin and P-endorphin (31-amino acid C-terminal fragment). The term "endorphin" refers to a substance that has morphine-like properties. P-endorphin-containing neurons are located in the hypothalamus and send projections to periaqueductal gray (PAG) and noradrenergic cells in the brainstem.

2. Enkephalins: The pre-propeptide, pre-proenkephalin A, is synthesized in the rough endoplasmic reticulum of neurons in the hindbrain. Removal of the signal pep-tide from the pre-propeptide within the rough endo-plasmic reticulum results in the generation of the propeptide, proenkephalin A, which is then transported to the Golgi apparatus where it is packaged into vesicles. The vesicles are transported to the axon terminal by fast axonal transport. Further proteolytic processing in the terminal results in the generation of the active peptides, methionine and leucine enkephalin. Both of these peptides are pentapeptides. Met-enkephalin contains methionine, and leu-enkephalin contains leucine at position 5. Enkephalins are then packaged into dense-core vesicles and are released by exocytosis as transmitters.

3. Dynorphin (1-13): This peptide can be isolated from the pituitary and consists of C-terminally extended form of Leu5-enkephalin.

In the CNS, the action of most peptides is terminated by their degradation due to the presence of peptidases.

Physiological and Clinical Considerations

Blood-borne peptides do not cross the blood-brain barrier. Intracerebroventricular injection of opioid peptides (e.g., P-endorphin) produces only transient analgesia. However, P-endorphin is much more potent than morphine in these tests. A search for better analgesics, devoid of side-effects like addiction, is going on in many laboratories. Enkephalinergic interneurons in the spinal cord have been shown to play a role in the modulation of pain sensation.Opioid peptides have been implicated in regulating blood pressure, temperature, feeding, aggression, and sexual behavior. Analgesics (e.g., morphine) produce their therapeutic effects via opiate receptors.

Tachykinins: Substance P

Substance P will be discussed as a representative of tachykinins. This substance is an undecapeptide (it is composed of 11 amino acids). The dorsal root ganglia projecting to the substantia gelatinosa of the spinal cord are rich in substance P-containing neurons. These neurons have been called nociceptors because they transmit information regarding tissue damage to the pain-processing areas located in the CNS. The sensation of pain is initiated at the peripheral terminals of these sensory neurons. These terminals are stimulated by noxious chemical, thermal, and mechanical stimuli. The central terminals of these sensory neurons release substance P in the substantia gelatinosa. Substance P has been implicated as one of the neurotransmitters in mediating pain sensation.

In recent years, a topical cream containing capsaicin has been used as an analgesic in the treatment of painful disorders, such as viral neuropathies (e.g., shingles) and arthritic conditions (e.g., osteoarthritis and rheumatoid arthritis). Capsaicin, the pungent substance present in hot chili peppers, mediates its actions via vanilloid receptors, which are present exclusively on the membranes of primary afferent neurons. The mechanism by which capsai-cin acts as an analgesic is not fully understood. Initially, it causes a burning sensation, which is consistent with activation of peripheral terminals of primary afferent neurons. With repeated applications, the vanilloid receptors may become desensitized, thus reducing pain sensations. With prolonged use, capsaicin causes death of primary afferent neurons as a consequence of increased intracellular Ca2+ concentrations. Reduction in the population of primary afferent neurons results in a reduction in the release of one of the primary neurotransmitters (substance P) mediating pain sensation. This mechanism may provide a novel approach for designing topical analgesics with fewer side effects.

Gaseous Neurotransmitters

This is a new class of neurotransmitters. Nitric oxide (NO) and carbon monoxide (CO) are two important members of gaseous neurotransmitters. In this topic, NO is discussed as a representative of this group because relatively more information is available for this neuro-transmitter.

Nitric Oxide

In isolated vascular smooth muscle preparations, Ach and other vasodilators release a short-acting substance from the endothelial cells that relaxes blood vessels. This relaxing factor was named endothelium-derived relaxing factor (EDRF). Subsequent studies have shown that EDRF and NO are the same molecule. It is now well recognized that NO plays an important role in mediating vasodilation. NO is also produced in many other cells, including neurons, where it has been implicated as a neurotransmitter.

Differences From Other Transmitters

NO does not satisfy some of the criteria formulated for classical transmitters (small-molecule and peptide transmitters), and it differs from conventional neurotransmitters in the following respects:

1. It is not stored in vesicles and is generated when it is needed.

2. It is not released by calcium-dependent exocytosis from a presynaptic terminal. NO is an uncharged molecule; it diffuses freely across cell membranes and modifies the activity of other cells.

3. Inactivation of NO is passive (there is no active process that terminates its action). It decays spontaneously and is converted to nitrites, nitrates, oxygen, and water.

4. It does not interact with receptors on target cells. Its sphere of action depends on the extent to which it diffuses, which may include several target cells. Therefore, the action of NO is not confined to the conventional presynaptic-postsynaptic direction.

5. NO acts as a retrograde messenger and regulates the function of axon terminals presynaptic to the neuron in which it is synthesized.

Synthesis and Removal

In the CNS, the enzyme needed for the synthesis of nitric oxide, nitric oxide synthase (NOS), is present in discrete populations of neurons, but it is not present in glia. Three isoforms of NOS have been cloned; isoform I (nNOS or cNOS) is found in neurons and epithelial cells, isoform II (iNOS) is induced by cytokines and is found in macrophages and smooth muscle cells, and isoform III (eNOS) is found in endothelial cells lining blood vessels (Table 8-2). All three isoforms require tetrahydrobiopterin as a cofac-tor and nicotinamide adenine dinucleotide phosphate (NADPH) as a coenzyme. Isoforms I and III of NOS are activated by the influx of extracellular calcium into the cell. Isoform II (iNOS), the inducible form of NOS, is activated by cytokines. The steps involved in the activation of constitutive forms of NOS (cNOS and eNOS) are as follows (Fig. 8-16).

1. Glutamate is released from a presynaptic neuron.

2. Glutamate acts on NMDA receptors located on the post-synaptic neuron, and Ca2+ enters the postsynaptic neuron and binds with calmodulin (calcium-binding protein).

3. Ca2+-calmodulin complex activates NOS.

4. Activation of NOS results in the formation of NO and citrulline from L-arginine.

TABLE 8-2 Isoforms of Nitric Oxide Synthase

Property	Isoform I	Isoform II	Isoform III
Name	cNOS or nNOS	iNOS	eNOS
Expression	Constitutive"	Inducible by cytokines’	Constitutive
Calcium dependence	Yes	No	Yes
Tissue	Neurons, epithelial cells	Macrophages, smooth muscle cells	Endothelial cells

NOS = nitric oxide synthase.

"A constitutive enzyme is constantly produced in the cell regardless of the condition of growth. bAn inducible enzyme can be detected by addition of a particular substance, in this case cytokines.

FIGURE 8-16 Steps involved in the synthesis of nitric oxide ([NO] see text for details). cGMP = cyclic guanosine monophosphate; NMDA = N-methyl-D-aspartic acid; nNOS = nitric oxide synthase isoform I.

5. Once generated, NO interacts with the heme moiety of soluble guanylate cyclase; this results in an allosteric transformation and activation of this enzyme.

6. The stimulation of soluble guanylate cyclase results in the formation of cyclic guanosine monophosphate (cGMP) from guanosine triphosphate (GTP) in the postsynaptic neuron. Increased levels of cGMP in the postsynaptic neuron result in a physiological response.

7. NO generated in the postsynaptic neuron can diffuse out to the presynaptic terminal. Diffusion of NO to the presynaptic terminal suggests that it serves as a retrograde messenger. This action is believed to result in enhanced and prolonged transmitter release from the presynaptic neuron.

8. NO can also diffuse out to the adjacent neuron and (9) adjacent glial cells. In each of these sites, NO stimulates soluble guanylate cyclase and increases cGMP levels, which then brings about a response.

As mentioned earlier, NO decays spontaneously and is converted to nitrites, nitrates, oxygen, and water.

Physiological and Clinical Considerations

In the CNS, the role of NO as a transmitter is still under investigation.