W4 CH 4 - Psychopharmacology
Neurotransmitters, Neuromodulators, and Neurohormones [Chemical
Messengers]
Neurotransmitter - Communicates across a synapse.
- Engage in wiring transmission by acting on neurons in their own
immediate vicinity, generally at a synapse. Neurotransmitters are
typically released from an axon terminal and cross the synaptic gap.
Neuromodulator - Communicates with target cells more distant than the
synapse by diffusing away from the point of release.
- Rather than opening ion channels to depolarize or hyperpolarize a
postsynaptic neuron, neuromodulators influence more global
functions, including attention and assessments of threat, saliency,
novelty, and reward.
- Neuromodulators often diffuse away from their site of release to influence diverse
populations of neurons located at some distance from the releasing cell.
- Restricted to interactions with metabotropic receptors. They produce slower and
longer-lasting changes in a neuron's metabolic processing.
- While classic neurotransmitters travel one-way from presynaptic to postsynaptic cells,
neuromodulators can interact with both pre- and postsynaptic cells.
Neurohormone - Communicates with target cells at great distance by traveling through the
circulation.
- Secreted by special neurons into the blood supply. Regardless of the distance traveled,
these chemical messengers will interact only with other cells that have specialized
receptor sites to receive them.
Identifying Neurochemicals
Neurochemicals are substances released by one cell that produce a reaction in a target cell.
Beyond this basic definition, neuroscientists generally agree with the following additional
criteria:
1. The substance must be present within a presynaptic cell.
2. The substance is released in response to presynaptic depolarization.
3. The substance interacts with specific receptors on a postsynaptic cell.
Types of Neurochemicals
Neurochemicals fall into three classes:
- Small molecules - One of a group of neurochemicals that includes amino acids and
amines.
- Neuropeptide - A peptide (chains of amino acids) that acts as a neurotransmitter, a
neuromodulator, or a neurohormone.
- Gasotransmitters - A gas such as nitric oxide (NO) that performs a signaling function.
, The small molecules can be further divided into amino acids [an essential component of
proteins] and amines [derived from amino acids].
Small molecules vs. neuropeptides:
- Small molecules are typically synthesized in the axon terminal, whereas neuropeptides
are synthesized in the cell body and must be transported the length of the axon.
- Vesicles containing neuropeptides are used only once, in contrast to the recycling of
vesicles possible with small molecules.
- Compared to the release of small molecules, the release of neuropeptide vesicles requires
higher levels of calcium, which in turn requires a higher rate of action potentials
reaching the axon terminal.
- Finally, neuropeptides diffuse away from the synapse or are broken down by enzymes,
but unlike the small molecules, they are not deactivated by reuptake.
THE SMALL MOLECULES
Acetylcholine (ACh) - A major small-molecule
neurochemical used at the neuromuscular junction, in the
autonomic nervous system, and in the central nervous
system (CNS).
Neurotransmitters, Neuromodulators, and Neurohormones [Chemical
Messengers]
Neurotransmitter - Communicates across a synapse.
- Engage in wiring transmission by acting on neurons in their own
immediate vicinity, generally at a synapse. Neurotransmitters are
typically released from an axon terminal and cross the synaptic gap.
Neuromodulator - Communicates with target cells more distant than the
synapse by diffusing away from the point of release.
- Rather than opening ion channels to depolarize or hyperpolarize a
postsynaptic neuron, neuromodulators influence more global
functions, including attention and assessments of threat, saliency,
novelty, and reward.
- Neuromodulators often diffuse away from their site of release to influence diverse
populations of neurons located at some distance from the releasing cell.
- Restricted to interactions with metabotropic receptors. They produce slower and
longer-lasting changes in a neuron's metabolic processing.
- While classic neurotransmitters travel one-way from presynaptic to postsynaptic cells,
neuromodulators can interact with both pre- and postsynaptic cells.
Neurohormone - Communicates with target cells at great distance by traveling through the
circulation.
- Secreted by special neurons into the blood supply. Regardless of the distance traveled,
these chemical messengers will interact only with other cells that have specialized
receptor sites to receive them.
Identifying Neurochemicals
Neurochemicals are substances released by one cell that produce a reaction in a target cell.
Beyond this basic definition, neuroscientists generally agree with the following additional
criteria:
1. The substance must be present within a presynaptic cell.
2. The substance is released in response to presynaptic depolarization.
3. The substance interacts with specific receptors on a postsynaptic cell.
Types of Neurochemicals
Neurochemicals fall into three classes:
- Small molecules - One of a group of neurochemicals that includes amino acids and
amines.
- Neuropeptide - A peptide (chains of amino acids) that acts as a neurotransmitter, a
neuromodulator, or a neurohormone.
- Gasotransmitters - A gas such as nitric oxide (NO) that performs a signaling function.
, The small molecules can be further divided into amino acids [an essential component of
proteins] and amines [derived from amino acids].
Small molecules vs. neuropeptides:
- Small molecules are typically synthesized in the axon terminal, whereas neuropeptides
are synthesized in the cell body and must be transported the length of the axon.
- Vesicles containing neuropeptides are used only once, in contrast to the recycling of
vesicles possible with small molecules.
- Compared to the release of small molecules, the release of neuropeptide vesicles requires
higher levels of calcium, which in turn requires a higher rate of action potentials
reaching the axon terminal.
- Finally, neuropeptides diffuse away from the synapse or are broken down by enzymes,
but unlike the small molecules, they are not deactivated by reuptake.
THE SMALL MOLECULES
Acetylcholine (ACh) - A major small-molecule
neurochemical used at the neuromuscular junction, in the
autonomic nervous system, and in the central nervous
system (CNS).
