**Question:** Which of the following is not permeable through the Blood Brain Barrier:

A. Glucose
B. Oxygen
C. Water
D. Amitriptyline

**Core Concept:** The Blood Brain Barrier (BBB) is a selective semipermeable barrier that regulates the transport of substances between the blood and the central nervous system (CNS). It is primarily composed of brain capillary endothelial cells, tight junctions, and astrocytic end-feet. The BBB plays a crucial role in protecting the brain from harmful substances while maintaining essential neurotransmitters and nutrients.

**Why the Correct Answer is Right:** Amitriptyline is a tricyclic antidepressant drug that is not lipophilic, meaning it does not readily cross the BBB due to its polar structure. Lipophilic substances, such as glucose and oxygen, can easily diffuse across the BBB through passive diffusion and the carrier-mediated transport system, respectively. Water molecules can passively diffuse across the BBB due to osmosis.

**Why Each Wrong Option is Incorrect:**

A. Glucose is a necessary substrate for the brain and crosses the BBB through facilitated diffusion. It is lipophilic, allowing it to easily diffuse through the barrier.

B. Oxygen is essential for brain function and crosses the BBB through passive diffusion along the concentration gradient. Oxygen is a gaseous molecule, which makes it lipophilic and easily permeable to the BBB.

C. Water molecules passively diffuse across the BBB through the process of osmosis due to their polar nature. This polarity prevents water from being lipophilic, making it permeable to the BBB.

**Clinical Pearl:** Understanding the permeability characteristics of the Blood Brain Barrier is crucial in assessing the pharmacokinetics of drugs targeting the CNS. It is essential to consider the lipophilicity of a compound when designing drugs for CNS effects or determining the potential for drug-drug interactions involving the BBB.