Imaging Cannabinoid CB1 Receptors in Schizophrenia

Schizophrenia is a debilitating mental disorder with a complex and multifactorial etiology. The exact pathophysiological mechanisms have remained elusive, but a large body of evidence points toward abnormalities in a number of brain neurotransmitter systems: dopamine, glutamate, and gamma-amino butyric acid (GABA). Pharmacological studies have shown that acute exposure to cannabis is able to induce psychotic symptoms in healthy individuals and exacerbate symptoms in patients with an established psychotic illness. In addition, epidemiological studies have established that cannabis use in early adolescence is associated with an increased risk of developing schizophrenia later in life. Together, this evidence suggests that the neural systems targeted by cannabis may be involved in the pathophysiology of schizophrenia.

The brain endocannabinoid (EC) system is a recently discovered brain neurotransmission system, which involves endogenous cannabinoid agents (ECs) that act upon specific receptors (CB1 and CB2). CB1 receptor is abundant in the human brain and acts as an inhibitory modulator of classical neurotransmitters. ECs and CB1 receptors appear to be involved in the pathophysiology of schizophrenia. EC levels are elevated in the cerebrospinal fluid of patients with schizophrenia, and post-mortem studies have shown increased density of radioligand binding to brain CB1 receptors. To what extent CB1 receptors are involved in the pathophysiology of schizophrenia in the living human brain is currently unknown. The lack of suitable methods to reliably quantify CB1 receptors in the living human brain have to date hindered the progress in this field.

In this protocol, we outline studies aiming at elucidating the role of CB1 receptors in schizophrenia by using positron emission tomography (PET) and the recently developed radiotracer for CB1 receptors, [18F]FMPEP-d(2). The aim of this project is to explore CB1 receptor abnormalities in human patients with schizophrenia. The primary hypothesis is that CB1 receptor density is increased in patients with schizophrenia in comparison with healthy subjects. Insight into the role of CB1 receptor function in schizophrenia may help guide future development of pharmacotherapies.