4.2.4 Psychotropics

Psychotropics and Relation to Psychopharmacology

Psychotropics are a class of drugs that are used to treat psychiatric and neurological disorders. They are the cornerstone of modern psychopharmacology and have revolutionized the treatment of mental illness over the past several decades. This class of drugs includes antidepressants, antipsychotics, anxiolytics, and mood stabilizers, among others.

Psychopharmacology is the study of the effects of drugs on the mind and behaviour. It is a rapidly evolving field that integrates knowledge from neuroscience, pharmacology, and psychiatry. The relationship between psychotropics and psychopharmacology is complex, but can be understood as the application of psychopharmacological principles to the design, development, and use of psychotropics.

Psychotropics act on specific neurotransmitter systems in the brain, including serotonin, dopamine, and norepinephrine. They are thought to modify the levels of these neurotransmitters or the way they interact with their receptors, leading to changes in mood, cognition, and behaviour. For example, antidepressants are thought to increase the availability of serotonin in the brain, while antipsychotics block dopamine receptors.

The use of psychotropics is not without its challenges. Many drugs in this class have side effects, including sedation, weight gain, and sexual dysfunction. They can also interact with other drugs and may have long-term effects on the brain and behaviour. As a result, the use of psychotropics is carefully monitored and regulated by healthcare professionals and regulatory bodies.

Despite these challenges, psychotropics have been shown to be effective in treating a range of psychiatric and neurological conditions, including depression, anxiety, schizophrenia, and bipolar disorder. They have helped millions of people around the world manage their mental health and improve their quality of life (Adams, 2018).

Lipophilicity vs hydrophilicity:

Lipophilicity and hydrophilicity are terms used to describe the solubility of a substance in lipids (fats) and water, respectively.

Lipophilic substances are soluble in lipids and tend to dissolve in organic solvents, such as hexane, ether, or chloroform. They are characterized by a high affinity for fatty tissues and cell membranes and tend to accumulate in the body. This can be beneficial for drugs that need to cross the blood-brain barrier, but it can also lead to toxicity and increased side effects.

Hydrophilic substances are soluble in water and tend to dissolve in aqueous solutions. They have low affinities for fatty tissues and cell membranes and tend to be rapidly excreted from the body. This can be beneficial for drugs that need to be quickly cleared from the body, but it can also reduce their efficacy and bioavailability.

Lipophilicity and hydrophilicity are important concepts in pharmacology, as they can affect the pharmacokinetics (i.e. absorption, distribution, metabolism, and excretion) of a drug. A drug’s lipophilicity or hydrophilicity can influence its ability to penetrate the blood-brain barrier, its distribution to different tissues and organs, its metabolism by the liver and other enzymes, and its excretion by the kidneys and other organs.

The blood-brain barrier – structure and function especially in relation to psychopharmacology:

The blood-brain barrier (BBB) is a selectively permeable membrane that separates the blood from the central nervous system (CNS), including the brain and spinal cord. The BBB is composed of a layer of tightly packed cells, known as endothelial cells, which form a continuous barrier around the capillaries of the brain. The tight junctions between the endothelial cells are crucial for the formation of the BBB and regulate the passage of substances between the blood and the CNS.

The structure of the BBB also includes a basement membrane and astrocyte end-feet, which provide additional support and physical barriers to substance entry. The BBB also contains active transport mechanisms, such as pumps and carriers, which regulate the transport of essential nutrients and neurotransmitters into the brain while excluding harmful substances.

However, there are certain regions in the brain where the BBB is not as well-developed or is absent, such as the area postrema and the circumventricular organs, which are involved in the regulation of homeostasis. These regions are considered to be “leaky” and allow for the entry of substances into the CNS without regulation by the BBB. Additionally, the BBB can become disrupted or damaged during disease or injury, leading to an increase in the permeability of the BBB and an influx of harmful substances into the CNS.

While the BBB is generally effective in excluding harmful substances, there are certain regions where the BBB is weak or absent, and it can become disrupted or damaged during disease or injury (Ballabh, 2004)

The blood-brain barrier (BBB) plays a crucial role in maintaining the homeostasis of the central nervous system (CNS) by regulating the passage of substances between the blood and the CNS. The BBB is composed of tightly packed endothelial cells, a basement membrane, and active transport mechanisms, which work together to protect the brain and spinal cord from harmful substances and toxins in the blood.

In terms of pharmacokinetics, the BBB affects the pharmacokinetics of drugs that target the CNS, including psychotropics. The BBB is selective in what it allows to pass through, and most drugs have difficulty crossing the BBB and entering the CNS. Lipophilic drugs tend to penetrate the BBB more easily due to their ability to dissolve in fat, while hydrophilic drugs have a more difficult time crossing the BBB.

Additionally, the BBB contains active transport mechanisms, such as pumps and carriers, which regulate the transport of essential nutrients and neurotransmitters into the brain while excluding harmful substances. This regulation helps to ensure that the pharmacokinetics of drugs are altered as little as possible so that they can effectively target the CNS (Abbott, 2017).

References:

(1) Abbott, N. J., & Reiner, P. B. (2017). The blood-brain barrier: an overview: structure, regulation, and clinical implications. Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 42(1), 31-40. doi: 10.1038/npp.2016.147

(2) Adams, J. D., & Fullerton, M. (2018). Psychotropics. In The Oxford Textbook of Psychopharmacology (pp. 13-22). Oxford University Press.

(3) Ballabh, P., Braun, A. and Nedergaard, M. (2004). The blood-brain barrier: an overview: structure, regulation, and clinical implications. Neurobiology of disease, [online] 16(1), pp.1–13. doi:10.1016/j.nbd.2003.12.016.