What are alpha-adrenergic agonists?
These work by stimulating a response from the adrenergic receptors that make up part of the body’s autonomic nervous system, which controls vital bodily functions. The adrenergic receptors, when stimulated, can cause a number of responses, including:
Elevated heart rate
Mobilisation of energy
Diversion of blood flow from non-essential organs to skeletal muscles
Importantly, however, a key response is the reduction in eye fluid production, as well as increased drainage of the fluid from the eye — making alpha-adrenergic agonists a popular choice for treating glaucoma with eye drops.
Types of adrenergic agonist medication
The types of adrenergic agonist drugs that can affect adrenergic receptors can be split into two types: non-selective and selective. Selective drugs target and affect their intended site alone. Non-selective, on the other hand, can affect multiple tissues and organs.
Non-selective types act on one or more adrenergic receptor, binding to them. They can cause different side effects that may vary based on the specific adrenergic agonist as well as the dosage, including:
Raised heart rate (tachycardia)
High blood pressure (hypertension)
Irregular heart rate (arrhythmia)
The common non-selective agonists, also referred to as vasopressors, are noradrenaline, adrenaline, and isoprenaline.
Selective agonists are generally more useful for glaucoma treatment because they can target a specific receptor, as well as their lesser side effects when compared to non-selective types.
Selective types act on a single receptor only. The selective adrenergic agonists are further classified into sub-categories called alpha selective and beta selective. Out of these, it is the alpha-2 (α2) selective adrenergic agonists which are particularly useful in glaucoma treatment.
α2 Adrenergic agonists, such as apraclonidine and brimonidine, have been found particularly helpful primarily for IOP reduction in glaucoma patients.
Apraclonidine hydrochloride is an effective short-term α2 adrenergic agonist commonly used following minor eye surgery.
It works by preventing the release of noradrenaline (a hormone and neurotransmitter) at nerve terminals in the eye, which, as a result, can significantly lower intraocular pressure by reducing aqueous humour (eye fluid) production and increasing the drainage of it.
Brimonidine Tartrate is a highly selective topical α2-adrenergic agonist and has been used for many years in the initial and long-term management of glaucoma.
Brimonidine also lowers intraocular pressure by reducing aqueous humour production and increasing its outflow.
When compared to other glaucoma treatment options, brimonidine has been found to act in a similar way to the beta-blocker timolol, with equal long-term IOP lowering effects. However, although these eye drops effectively reduce IOP, many people find these drops can irritate their eyes. As such, alternatives are often required.
Beyond alpha-adrenergic agonists, there are a whole range of eye drops, such as prostaglandins, that can be beneficial if you have glaucoma. Alongside these, there’s also a host of other treatment options that may work for you. These are the most common forms of treatments, but your ophthalmologist will recommend the right treatment plan for you, based on a variety of factors and your test results.
1. Quigley HA, Broman AT. The number of people with glaucoma worldwide in 2010 and 2020. Br J Ophthalmol. 2006;90(3):262–267. doi:10.1136/bjo.2005.081224
2. Farzam, K. and Lakhkar, A. (2019). Adrenergic Drugs. [online] Ncbi.nlm.nih.gov. Available at: https://www.ncbi.nlm.nih.gov/books/NBK534230/