Nitric oxide (NO) was the first gas to be identified as a neuro-transmitter. Produced by nitric oxide synthetase in the cells of many organs from the amino acid L-arginine, nitric oxide's actions are very different from those of the more familiar nitrous oxide (N2O), or laughing gas, sometimes used as a mild anesthetic in dentistry.
Nitric oxide has a number of different roles in the body. Within blood vessels, it acts as a local tissue regulator that causes the smooth muscles of those vessels to relax, so that the blood vessels dilate. This role will be described in conjunction with the circulatory system in chapter 14. Within macrophages and other cells, nitric oxide helps to kill bacteria. This activity is described in conjunction with the immune system in chapter 15. In addition, nitric oxide is a neurotransmitter of certain neurons in both the PNS and CNS. It diffuses out of the presynaptic axon and into neighboring cells by simply passing through the lipid portion of the cell membranes. Once in the target cells, NO exerts its effects by stimulating the production of cyclic guanosine monophosphate (cGMP), which acts as a second messenger.
In the PNS, nitric oxide is released by some neurons that innervate the gastrointestinal tract, penis, respiratory passages, and cerebral blood vessels. These are autonomic neurons that cause smooth muscle relaxation in their target organs. This can produce, for example, the engorgement of the spongy tissue of the penis with blood. In fact, scientists now believe that erection of the penis results from the action of nitric oxide, and indeed the drug Viagra works by increasing this action of nitric oxide (as described in chapter 20; see fig. 20.23). Nitric oxide is also released as a neurotransmitter in the brain, and has been implicated in the processes of learning and memory. This will be discussed in more detail later in this chapter.
In addition to nitric oxide, another gas—carbon monoxide (CO)—may function as a neurotransmitter. Certain neurons, including those of the cerebellum and olfactory epithelium, have been shown to produce carbon monoxide (derived from the conversion of one pigment molecule, heme, to another, biliverdin; see fig. 18.23). Also, carbon monoxide, like nitric oxide, has been shown to stimulate the production of cGMP within the neurons. Experiments suggest that carbon monoxide may promote odor adaptation in olfactory neurons, contributing to the regulation of olfactory sensitivity. Other physiological functions of neuronal carbon monoxide have also been suggested, including neuroendocrine regulation in the hypothalamus.
Although its importance in the body was recognized only recently, nitric oxide has already been exploited for medical use. The hypotension (low blood pressure) of septic shock, for example, is apparently due to vasodilation caused by nitric oxide and has been successfully treated with drugs that inhibit nitric oxide synthetase. Conversely inhalation of nitric oxide has been used to treat pulmonary hypertension, as well as respiratory distress syndrome (discussed in chapter 16).
Test Yourself Before You Continue
1. Explain the significance of glutamate in the brain and of NMDA receptors.
2. Describe the mechanism of action of glycine and GABA as neurotransmitters and discuss their significance.
3. Give examples of endogenous opioid polypeptides and discuss their significance.
4. Explain how nitric acid is produced in the body and describe its functions.
Fox: Human Physiology, Eighth Edition
7. The Nervous System: Neurons and Synapses
© The McGraw-H Companies, 2003
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.