Journal of Neuroscience, Vol 2, 463-469, Copyright © 1982 by Society for Neuroscience
Pseudopodial interdigitations between abutted nerve terminals: diffusion traps which occur in several nuclei of the rat limbic system
AF Boyne and SB Tarrant
Stimulation of the Torpedine ray electric organ can cause the loss of
synaptic vesicles and the growth of pseudopodia from the nerve terminals
(Boyne, A. F., and S. McLeod (1979) Neuroscience 4: 615-624). The latter
embed themselves in corresponding indentations in abutted terminals. The
geometry of these pseudopodial indentations (PSIs) can vary: (i) in length,
(ii) in the extent of constriction of the base, and (iii) through a
compound interaction between different pseudopodia extending in opposite
directions. Examination of six rat brain nuclei in the limbic system has
shown that their neuropil can be categorized according to the prevalence of
either (i) nerve terminals indented by nerve terminal outgrowths (i.e.
PSIs) or (ii) nerve terminals indented by dendritic outgrowths: these have
been previously termed spinules. Clusters of simple PSIs were seen in the
central nucleus of the amygdala, while base-constricted and compound forms
were found in the globus pallidus and substantia nigra. Dendritic spinules
were prevalent in the nucleus accumbens and the molecular layer of the
hippocampus. In the CA4 hilar region of the hippocampus, large nerve
terminals containing PSIs were found. The caudate neuropil appeared to be
of mixed character in that the small terminals often had spinules but
occasionally showed PSIs. Spinules have been recognized for many years and
the possibility of their plasticity has been raised previously (Tarrant, S.
B, and A. Routtenberg (1977) Tissue Cell 9: 461-473). The present report
appears to be first detailed description of an alternative form of invasion
which is known to be plastic in the elasmobranch electric organ. It is
suggested that the extracellular space between the partners of a PSI could
act as variable diffusion traps. If the involved boutons carry action
potentials, then nonsynaptic release and accumulation of substances such as
potassium, amino acids, and nucleotides may be expected during stimulation.
Consequent direct or receptor-mediated effects on the membrane potential
could influence transmission through adjacent synapses.