Agonists

Nerve cells, called neurons, are one of the building blocks of behavior. It is estimated that there are between 10 and 100 billion neurons in the nervous system and that neurons make 13 trillion connections with each other. The neurons send electrochemical messages to the brain so that people can respond to stimuli—either from the environment or from internal changes in the body.

The process by which these messages are sent is called neurotransmission.  The electrical impulse that travels along the body of the neuron is called an action potential. When an action potential travels down the axon of the neuron, it releases neurotransmitters that are stored in the neuron’s terminal buttons. The neurotransmitters are then released into the gap between the neurons – called the synapse. The synaptic gap is an incredible one-millionth of a centimeter!

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Neurotransmitters are the body’s natural chemical messengers that transmit information from one neuron to another. After crossing the synapse, the neurotransmitters fit into receptor sites on the post-synaptic membrane, like a key in a lock. Once the message is passed on, the neurotransmitters are either broken down by an enzyme or reabsorbed by the terminal buttons, in a process called reuptake.

Neurotransmitters have been shown to have a range of different effects on human behavior. In fact, neurotransmission underlies behavior as varied as mood, sleep, learning and memory, sexual arousal, and mental illness. The table below, which highlights just a few neurotransmitters, gives an idea of the variety of behaviors that are influenced by these neurochemicals.When discussing the process of neurotransmission, biologists refer to chemicals as agonists or antagonists, depending on the effect they have on the post-synaptic receptor sites.

All neurotransmitters are agonists for receptor sites.  They are referred to as endogenous agonists since they are biologically already part of our nervous system.  So, acetylcholine is an agonist for ACh receptor sites. 

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Drugs can also be agonists.  Since they are external to our system, they are referred to as exogenous agonists.  For example, nicotine is an agonist for ACh receptor sites and in the short term appears to have some positive effects on memory. (It should be noted, however, that long-term use of nicotine has a negative effect on memory!)

Antagonists are drugs that block the receptor site and do not allow the neurotransmitter to do its job, so no action potential is sent down the neuron. For example, scopolamine is an antagonist for ACh. 

The researchers used a sample of twenty healthy male adults, with a mean age of 28 years old. The study used a double-blind procedure and participants were randomly allocated to one of two conditions. They were injected with either Scopolamine or a placebo 70 - 90 minutes before taking part in the experimental task.

The participants were then put into an fMRI where they were scanned while playing the "Arena task."  This is a rather complex virtual reality game in which the researchers are observing how well the participants can create spatial memories.  The goal is for the participants to navigate around an "arena" to reach a pole. After they have learned where the pole is located, the screen would go blank for 30 seconds.  During this time, the participants were told to actively rehearse how to get to the pole in the arena. When the arena reappeared, the participant was now at a new starting point in the arena. The participants would have to use their spatial memory to determine how to get to the location of the pole.

The participants were first trained in the game to make sure that they were comfortable with the use of the joystick and that they understood the rules of the game.  After they were trained, they then took part in the experiment. The participants' brain activity was measured for six trials.

The participants returned between 3 - 4 weeks later and redid the test - receiving the opposite treatment to the original study.  In other words, the study was a repeated measures design.

The researchers found that when participants were injected with scopolamine, they demonstrated a significant reduction in the activation of the hippocampus compared to when they received a placebo. It appears that acetylcholine could play a key role in the encoding of spatial memories in humans, as well as in rats.