Sympatholytics: Alpha-2 agonists



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Alex Aranda

Central anti-adrenergics are a class of medications that’s not very commonly used these days. Their mechanism of action is to target the adrenergic neurons in the central nervous system, and prevent them from effectively releasing the catecholamines: norepinephrine and epinephrine.

So, the nervous system is divided into the central nervous system, so the brain and spinal cord; and the peripheral nervous system, which includes all the nerves that connect the central nervous system to the muscles and organs. The peripheral nervous system can be divided into the somatic nervous system, which controls voluntary movement of our skeletal muscles; and the autonomic nervous system, which controls the involuntary movement of smooth muscles and glands of our organs.

Now, the autonomic nervous system - which includes both the sympathetic and parasympathetic nervous systems - is made up of a relay that includes two neurons. We’ll focus on just the sympathetic nervous system. Signals for the autonomic nervous system start in the hypothalamus, at the base of the brain. Hypothalamic neurons have really long axons that carry signals all the way down to the thoracic and lumbar spinal cord nuclei, where they synapse with preganglionic neuron cell bodies. Here, they release the neurotransmitter norepinephrine, which causes the preganglionic neurons to transmit the signals down their relatively short axon, which exits the central nervous system via the spinal cord. These short nerve fibers reach the nearby sympathetic ganglion, which consists of many postganglionic neuron cell bodies. The postganglionic neurons are also called adrenergic neurons, because they release the neurotransmitter norepinephrine, which is also called noradrenaline; and to a much lesser degree, epinephrine, or adrenaline. These two catecholamines activate the adrenergic receptors on many different organs, which allow the sympathetic nervous system to trigger the fight or flight response that increases the heart rate and blood pressure, as well as slowing digestion. All of this maximizes blood flow to the muscles and brain, and can help you either run away from a threat, or fight it, which is why it’s also called the “fight or flight response.”

Alright, so let’s zoom into the synapse between the hypothalamic neurons and the preganglionic neurons, which can be found throughout the brainstem and spinal cord. The presynaptic terminal contains loads of tiny synaptic vesicles, each of which stores thousands of norepinephrine molecules. But for norepinephrine to be there in the first place, a precursor amino acid, called tyrosine, is taken up by the adrenergic neuron and gets converted to L-dihydroxyphenylalanine, or L-DOPA for short, by an enzyme called tyrosine hydroxylase. Next, L-DOPA is converted by an enzyme called DOPA decarboxylase to dopamine, which is then packaged into the synaptic vesicles. The remaining dopamine will be broken down by a class of enzymes called monoamine oxidases, or MAOs for short. Okay, now once inside the vesicles, dopamine get converted into norepinephrine. And then, whenever the appropriate signal travels down the axon to the axon terminal, these vesicles fuse with the presynaptic membrane in order for norepinephrine to get released (or exocytosed) into the synaptic cleft and take action on the adrenergic receptors of the postsynaptic neuronal membrane. Okay, but this release of norepinephrine is controlled through negative feedback inhibition. So when a pre-synaptic nerve terminal is stimulated to release a bunch of norepinephrine in the synapse, some of it will bind to a special type of receptor called an alpha-2 adrenergic receptor, located on the presynaptic membrane. These alpha-2 receptors then inhibit further release of norepinephrine into the synapse, so the postsynaptic neuron doesn’t get over stimulated.

Alright, so medications that act on adrenergic neurons of the brainstem to inhibit adrenergic signal transmission are called central anti-adrenergics. What these do, is collectively oppose the effects of the sympathetic nervous system. So overall, the heart rate and blood pressure decrease, digestive processes speed up, and the fight-or-flight response gets blocked.

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