NEUROTRANSMITTERS

Neurotransmitters

Neurotranmitters, relatively small molecules which carry information across synapses from a nerve cell to its neighboring cells, are a critical part of the internal machinery controlling animal behavior. Generally speaking, the neurotransmitter is held in membrane-bound vesicles near the synapse. The nerve cell which holds the vesicles is the pre-synaptic cell. When stimulated, the vesicles merge with the cell membrane of the presynaptic cell, and the neurostransmitter is released into the synapse. The neurotransmitter molecules match with receptor molecules in the membrane of the post-synaptic cell. These receptors are crticallly important; they insure that the message transmitted is specific to the neurotransmitter. For each neurotransmitter there are several receptor molecule types. Each neurotransmitter has many different functions in the nervous system, and the receptor type involved in regulating a beahvior often tells us something more about the behavior than just knowing what neurostransmitters are involved.

Dopamine, octompamine, and acetylcholine are examples of neurotransmitters which occur in both vertebrate and invertebrate nervous systems.

For this system to work, the neurotransmitter must be removed from the synapse after the signal is no longer needed. This happens by either cleaving the neurotransmitter to make an inactive form or by taking the neurotransmitter back up into the presynaptic cell. In the case of acetylcholine, a specialized enzyme, acetylcholine esterase, breaks down acetylcholine in the synapse. The parts can then be recycled. In contrast, serotonin is taken up directly by the presynaptic cell. Serotonin levels in synapses can be regulated by drugs called Selective Serotonin Reuptake Inhibitors (SSRI's). Drugs such as Prozac, Zoloft and Lexapro are SSRI's; they alleviate depression in humans by increasing the levels of serotonin in the brain. Serotonin, in combination with dopamine and gamma-amino butyric acid (GABA) regulates mood, and in many ways, personality, in humans. The action of these drugs in regulating behavior is not, however, limited to humans; many mammals respond to SSRI's in ways that are reminiscent of human responses:

Acetylcholine has two critical roles in the nervous system. First, it is the intermediate between the nervous system and the muscular system. Acetylcholine, when released at a nerve/muscle junction, causes depolarization of the membrane surrounding the muscle, triggering the release of calcium ions that then causes the muscle to contract. Second, acetylcholine levels are critical in learning and memory. Drugs which inhibit the activity of acetylcholine esterase can enhance memory functions in humans and animals. Some insecticides, though, act as acetylcholine esterase inhibitors and kill insects by causing an overload of acetylcholine to accumulate in synapses. This leads to spastic muscular movements and ultimately lethal physiological disruption.

Octopamine merits mention because it is often implicated in the regulation of behavior in invertebrates. Examples of behaviors associated with octopamine levels include foraging activity in honey bees (Barron et al. 2002), olfactory responsiveness in fruit flies (Schwaerzel et al. 2003), and firefly flashing (Nathanson 1979).

Barron AB, Schulz DJ, Robinson GE 2002 Octopamine modulates responsiveness to foraging-related stimuli in honey bees (Apis mellifera). JOURNAL OF COMPARATIVE PHYSIOLOGY A-NEUROETHOLOGY SENSORY NEURAL AND BEHAVIORAL PHYSIOLOGY. 188 (8): 603-610
Nathanson, JA 1979 Octopamine receptors, adenosine-3',5'-monophosphate, and neural control of firefly flashing SCIENCE 203 (4375): 65-68
Roeder T, Seifert M, Kahler C, Gewecke M 2003 Tyramine and octopamine: Antagonistic modulators of behavior and metabolism ARCHIVES OF INSECT BIOCHEMISTRY AND PHYSIOLOGY 54 (1): 1-13
Schwaerzel M, Monastirioti M, Scholz H, Friggi-Grelin F, Birman S, Heisenberg M 2003 Dopamine and octopamine differentiate between aversive and appetitive olfactory memories in Drosophila JOURNAL OF NEUROSCIENCE 23 (33): 10495-10502

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