Two methods were used to study the effect of cannabinoids on presynaptic axon terminals. The more frequently used electrophysiological approach measures neurotransmission. In brain slices or neuronal cultures, electrical currents in post-synaptic neurons are recorded with patch-clamp or microelectrode techniques. Presynaptic axon terminals are electrically stimulated and the postsynaptic current resulting from stimulation of ligand-gated ion channels of postsynaptic neurons by the released transmitter is determined. The change in the postsynaptic current amplitude is a measure of the change in synaptic transmission.
In the other method, the release of endogenous or radiolabelled neurotrans-mitters from presynaptic axon terminals is determined chemically. Although this latter method shows directly what happens at the level of axon terminals, it does not measure "neurotransmission".
In electrophysiological experiments, cannabinoids inhibited neurotransmission. The inhibition was always due to inhibition of transmitter release from axon terminals and never to interference of cannabinoids with the postsynaptic effects of the neurotransmitters. The experiments in which transmitter release was determined neurochemically also indicated that cannabinoids inhibit transmitter release from axon terminals. In most instances the presynaptic cannabinoid receptors can be classified as CB1 receptors (but some exceptions are given in Tables 1 and 2). Effects of cannabinoids on the release of individual transmitters are discussed below. Effects of cannabinoids on neurotransmission have also been reviewed by Schlicker and Kathmann (2001).
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