The main aim of the presented project will be to examine the concentration of selected receptors (CB1, CB2, TRPV-1) showing affinity for binding to cannabidol (CBD) and cannabinol (CBN) in blood samples collected from people with chronic pain in the masticatory muscles and from people without this pain. Moreover, study participants will be examined by a qualified doctor, according to the validated and standardized international DC/TMD protocol (Diagnostic Criteria for Temporomandibular Disorders) and the criteria included in ICOP-1 (International Classification of Orofacial Pain, 1st edition). The nature and duration of pain, psychoemotional state and central sensitization (the phenomenon of increased reactivity of the nervous system to stimuli) will be determined using validated questionnaires. The obtained results will be subjected to statistical analysis in order to check the relationships between the examined parameters.

Background: Orofacial pain is a common phenomenon in adult patients in Poland. According to available scientific research, it occurs in over half of the population, i.e. 55.9% In its chronic form, orofacial pain may lead to reduced sleep quality, reduced life satisfaction and psychoemotional disorders, including depressive disorders. A popular trend among patients suffering from chronic orofacial pain is the abuse of non-steroidal anti-inflammatory drugs, which in most cases demonstrate low, short-term effectiveness and disproportionate to the side effects. There are scientific reports suggesting that substances such as cannabidol (CBD) and cannabinol (CBN) may be effective in the treatment of chronic nociceptive and neuropathic pain, but there is a clear lack of research on the importance of the concentration of receptors on which they can potentially affect exogenous cannabinoids in the risk of occurrence and development of chronic pain. Of the many receptors to which cannabinoids have affinity, the present study selected those that are involved in neurotransmission related to pain, in particular orofacial pain. The concentrations of CB1, CB2, TRPV-1 receptors will be examined, below is a brief description of each of them to explain their selection.

CB1 and CB2 receptors are G protein-coupled metabotropic receptors and belong to the 7-transmembrane receptor family. CB1 has been detected in central nervous system synapses and sensory neurons. The effect of high CB1 expression in areas closely related to the regulation of pain sensation is the relief of migraine and neuropathic pain. However, CB2 receptors can be found in T and B lymphocytes, spleen, pancreas and tonsils. They work to modulate the release of cytokines and the migration of immune cells in a way that appears to reduce inflammation and some types of pain.

The TRPV-1 receptor is the so-called transient potential 1 vanilloid receptor. They occur primarily at the peripheral endings of nerve fibers, where, as a result of the stimuli that stimulate them, they may participate in the development and conduction of pain. The receptor mediates peripheral pain sensation, able to lower the activation threshold and delay the response of nociceptors. Secondary analgesia following ligand-dependent activation is used in the treatment of some difficult-to-control types of pain, e.g. neuropathic pain. TRPV1 can be activated by endogenous substances, e.g. anandamide, or exogenous substances, e.g. cannabinoids contained in cannabis. It is also worth adding that CBD has the property of blocking FAAH (the enzyme fatty acid amide hydrolase), protecting anandamide against degradation, which means that CBD is also an agonist towards TRPV-1 and protects the natural agonist against degradation by FAAH.

Aim: The aim of this study is to answer how the concentration of the above receptors may influence the occurrence of chronic pain in the masticatory muscles in the study population.

It should be mentioned that CB1, CB2 and TRPV-1 receptors have not yet been included in this research perspective and there are no publications or scientific reports on this subject.

Material and methods:

The study included a study group of 51 people suffering from chronic pain in the masticatory muscles and a control group of 51 healthy people, without pain in the masticatory muscles. Patients will be seen at the Clinic of the University Dental Center in Wrocław. The patient will be qualified based on the standardized and validated international DC/TMD protocol (Diagnostic Criteria for Temporomandibular Disorders) and the criteria included in ICOP-1 (International Classification of Orofacial Pain, 1st edition). The patient's examination will be performed by a qualified dentist.

The control group will include people without chronic pain in the masticatory muscles. The exclusion criteria remain the same as for the study group.

As mentioned above, each study participant will initially undergo an extraoral and intraoral examination based on the validated international DC/TMD (Diagnostic Criteria for Temporomandibular Disorders) protocol. The examinations will be performed by trained dentists.

Patients from both groups will be asked to complete the following validated questionnaires:

• regarding pain - Graded Chronic Pain Scale and McGill Pain Questionnaire
• regarding headache - The Migraine Disability Assessment Test (MIDAS)
• regarding the scale of perceived stress - Perceived Stress Scale - 10 (PSS-10)
• regarding generalized anxiety disorder - Generalized Anxiety Disorder - 7 (GAD-7)
• regarding the scale of somatic symptoms - The Somatic Symptom Scale -8 (SSS8)

The last stage includes collecting a venous blood sample from patients in the study and control groups and determining the concentration of CB1, CB2 and TRPV-1 receptors. The receptors will be determined using specialized ELISA kits.

The collected data will then be subjected to statistical analysis in order to verify the research hypothesis and capture statistically significant relationships.

Inclusion Criteria:

• age over 18, under 80
• the presence of chronic pain in the muscles of the masticatory system, which lasts for more than 3 months in accordance with the international criteria included in ICOP-1.

Exclusion Criteria:

• people under 18 years of age
• pregnant or breastfeeding women,
• obese people,
• people with active cancer,
• people with severe systemic diseases, including genetic and neurological diseases,
• people with severe mental illnesses and taking psychoactive substances, people who are taking or have taken antidepressants and/or myorelaxants and/or other drugs that affect neuromuscular activity in the last 12 months
• people addicted to alcohol, drugs and other psychoactive substances
• condition after craniofacial injuries

Patients with suspicion of Chronic Orofacial Pain of Muscle Origin referred to the Wroclaw Medical University. Healthy controls.