Dendrites caught releasing dopamine

Synaptic transmission is the process by which a nerve cell releases a neurotransmitter substance to cause the excitation of a nearby neuron. Sir Bernard Katz described the details of this process in the neuromuscular synapse long time ago. When it seems that there is nothing new about a process recounted in thousand of textbooks, surprising results come from examining dopaminergic cells in the substantia nigra pars compacta (SNc).

The SNc is not part the striatum; it is located in the midbrain, more caudally. It provides the dopaminergic inputs that modulate the corticostriatal synapse by mechanism that are not completely clear. Results from the group of Laurent Venance in France (Vandercasteele et al., 2008) show surprising finding in the way that dopaminergic cells contact each other. They examined synaptic transmission between dopaminergic cells. This may sound strange, but it happen that dopaminergic cells not only modulate the corticostriatal synapse, but in doing so, they talk to each other. Recording from two cells shows that action potentials in one cell produce a hyperpolarising response in the other cell. The obvious explanation would be that they release GABA. However, when bicuculline was added, the response did not change. The authors added many other things to the pairs. Once they added the D2-receptor blocker raclopride, then the depolarisation vanished. So the transmission is clearly dopaminergic.

The mechanism of action seems to be inhibition of the hyperpolarisation-activated current Ih (also called HCN, the same current that makes the pacemaker in heart cells). The transmission between SNc neurones disappears after adding ZD7288, the classical Ih blocker. In addition, they show that membrane conductance decreases during transmission, and depolarisation of the post-synaptic cell (that would close the HCN channels) also decreases and eventually eliminates the transmission. These new findings support curious early observations (Chéramy et al., 1981, Chéramy et al., 1983) that SNc dendrites can release dopamine.