PhD defence: Eva Maria Meier Carlsen

The effect of endocannabinoids on excitatory spinal networks and the participation of spinal and astrocytic cannabinoid receptors in tremor

Abstract
Cannabis has for many years been used to provide symptom relief of motor dysfunction in patients suffering from Multiple Sclerosis or Spinal Cord Injury. But in many instances, cannabis has not been prescribed by a doctor, but obtained illegally. The resentment towards prescribing medical cannabis are rooted in the lack of understanding of the mechanism of action of cannabis. Although cannabis has been used for thousands of years, and although the body itself produces cannabinoids – so called endocannabinoids – there is a major gap in our understanding of the cellular effects of cannabis and endocannabinoids. I therefore decided to examine the cellular effects of endocannabinoids in the spinal cord, as both spasticity associated with MS and SCI are due, at least in part, to damaged spinal networks.

To understand how endocannabinoids decrease spasticity it is necessary to understand their action in the spinal cord. By means of electrophysiological recordings in a slice preparation, I discovered that endocannabinoids are released from spinal neurons after depolarization. The released endocannabinoids in turn inhibit excitatory synaptic transmission. The effect is local but can also spread to neighbor neurons. I found that this spread is not due to diffusion of endocannabinoids, but instead is caused by endocannabinoid mediated activation of astrocytic cannabis receptor 1 (CB1) activation. Once activated, astrocytes release purines via a Ca2+ dependent mechanism, which then activates presynaptic A1 receptors on excitatory synaptic terminals causing a decrease of transmitter release and thus an inhibition of excitatory synaptic transmission.

Besides reducing spasticity, cannabis has in addition been suggested as a therapeutic agent for tremors. I therefore decided to investigate if the cellular effect I uncovered is involved in the beneficial effect of cannabis on tremors. I first investigated if the cannabis analog Win55,212-2 decreases tremors when injected locally in the spinal cord. I found, in a mouse model of essential tremor – that an intrathecal injection of WIN55,212-2 suppresses tremors. For the next step, I started to investigate if CB1 receptors on astrocytes are involved. These experiments are still ongoing. My preliminary data demonstrate that mice selectively lacking CB1 receptors in astrocytes, show an inherent tremor that is absent in wild-type animals. Second, I see a tendency towards an enhanced effect of harmaline-induced tremor in mice lacking astrocytic CB1 receptors. In the future I will test if astrocytic CB1 receptors are necessary for the beneficial effects of WIN55,212-2 on harmaline-induced tremor.

Taken together, my experiments show that endocannabinoids in the spinal cord inhibit excitatory synaptic activity by activation of astrocytes which in turn release purines that act upon presynaptic A1 receptors. Furthermore I show that activation of CB1 receptors in the spinal cord decreases tremor and that the lack of astrocytic CB1 receptors apparently unmask an inherent tremor and enhance harmaline-induced tremor.

Assessment Committee:
Associate Professor Nanna MacAulay (Chairperson)
Professor Abdel El Manira
Associate Professor Gertrudis Perea Parrilla

Supervisor:
Associate Professor Jean-Francois Perrier
Professor Jørn Hounsgaard

Department: 
Neuroscience

Graduate Programme:
Neuroscience

Ask for a copy of the thesis: evacarlsen@gmail.com