Synaptic features dendritic release of dopamine and electrical synapses

SYNAPTIC FEATURES DENDRITIC RELEASE OF DOPAMINE AND ELECTRICAL SYNAPSES 

It is now well ascertained that dendrites are capable of propagating action potentials not only in distal to proximal direction, but also in the reverse direction by back-propagation after initiation at the cell body (Ludwig and Pittman, 2003). The so-called law of dynamic polarization enunciated by Cajal (see Berlucchi, 1999) was aimed at stating the unidirectional propagation of excitations within the nervous system, and assumed that nerve impulses are conducted from the dendrite or soma to axon terminals. This dogma is now being reconsidered, not only in view of the evidence of dendrodendritic synapses, but also in view of the existence of electrical synapses in which the flow of information can be bidirectional. 

Since the description in the vertebrate olfactory bulb (Rail et al., 1966), the occurrence of presynaptic dendrites has been reported in a variety of central nervous system (CNS) regions. The SN was one of the structures in which dendrodendritic contacts were first observed (Bjorklund and Lindvall, 1975), and demonstrated at the electron microscopic level (Hajdu et al, 1973 Wilson et al., 1977 Groves and Linder, 1983). DA was one the first neuroactive substances shown to be released from dendrites (Groves et al., 1975 Geffen et al., 1976), and, as reviewed by Cheramy et al., (1981), local dendritic release of DA in the SN was firmly established since the initial studies on this neurotransmitter. 

For many years dendritic release of the neurotransmitter was considered a peculiarity of the midbrain dopaminergic neurons. However, this mechanism of release has now been ascertained for other neuroactive substances as well, including other neurotransmitters. 

The exocytotic machinery of dendritic release, which differs from that of axon terminals, is at present the subject of extensive investigation (Ludwig and Pittman, 2003). 

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