Effects of suicide transport lesions of the striatopallidal or striatonigral pathways on striatal ultrastructure

Neither ibotenic acid nor volkensin lesions of the nucleus accumbens shell affect the expression of cocaine sensitization

Studies have shown that the nucleus accumbens shell plays an integral role in the expression of psychostimulant-induced behavioural sensitization. Dopaminergic regulation of excitatory amino acid inputs in this region of the brain could be a key factor in the neural influence of this phenomenon. Alterations in the dopaminergic innervation patterns in the shell have been demonstrated in rats that received repeated cocaine injections. Furthermore, lesions of brain regions that send projections to the shell alter psychostimulant-induced locomotion, both acutely and in sensitization paradigms. A previous study from our laboratory demonstrated that lesions of the shell before repeated cocaine treatment decrease the locomotor response to cocaine during the induction phase of behavioural sensitization. To better understand the role of this brain region during the expression phase of behavioural sensitization, the present study examined the effects of two forms of cytotoxic lesions of the shell. Rats received a sensitization-inducing regimen of cocaine (bi-daily injections of 15 mg/kg i.p. for 5 consecutive days). Two days after the last injection, rats demonstrating behavioural sensitization received one of three bilateral microinjections into the shell: (i) 0.5 micro L 0.9% saline; (ii) 2.5 micro g/0.5 micro L ibotenic acid (which lesions the cell bodies at the injection site); or (iii), 0.5 ng/0.2 micro L of volkensin (a retrograde suicide transport lectin). Upon challenge with cocaine (15 mg/kg) 12 days after surgery, neither ibotenic acid- nor volkensin-lesioned rats showed any difference in their locomotor response compared with sham controls. These data indicate that bilateral shell lesions do not affect the long-term expression of behavioural sensitization in cocaine-sensitized rats.

Neuronal lesioning with axonally transported toxins

Axonally transported toxins can be used to make selective lesions of the nervous system. Collectively, these techniques are termed 'molecular neurosurgery' because they exploit the surface molecular identity of neurons to selectively destroy specific types of neurons. Suicide transport, is anatomically selective but not type-selective. The most widely used suicide transport agents are the toxic lectins (ricin, volkensin) and the immunotoxin, OX7-saporin. The toxic lectins and saporin are ribosome inactivating proteins that irreversibly inhibit protein synthesis. The toxic lectins have binding subunits but saporin requires a targeting vector to gain entrance into cells. Immunolesioning uses monoclonal anti-neuronal antibodies to deliver saporin selectively into neurons that express a particular target surface antigen. Neuropeptide-saporin conjugates selectively destroy neurons expressing the appropriate peptide receptors. Notable experimental uses of these agents include analysis of the function of the cholinergic basal forebrain (192-saporin) and pain research (anti-DBH-saporin, substance P-saporin). It is likely that more immunolesioning and neuropeptide-toxin conjugates will be developed in the near future.

Synaptic organization of the human striatum: a postmortem ultrastructural study

The goal of this study was to characterize the synaptic organization of the normal human adult striatum for comparison with other species and with the diseased human striatum. Samples of striatal tissue from the Maryland Brain Collection obtained at autopsy with postmortem intervals of less than 4 hours were prepared for electron microscopic analysis according to standard techniques. The caudate nucleus and the putamen were similar in terms of the proportions of synaptic subtypes, the lengths of synaptic subtypes, and the area of most types of axon terminals. The proportions of major striatal synaptic subdivisions, such as axospinous synapses (83.5%) and asymmetric synapses (77.5%), were similar to that of the monkey (82% and 77%, respectively) but slightly lower than found in the rat (90% and 89%, respectively). Interestingly, the proportion of synapses with perforated postsynaptic densities (23%), a type of synapse thought to represent synaptic plasticity, was much higher in humans than in rats (5-8%). The lengths of asymmetric synapses (0.697 micron) were significantly longer than that of symmetric synapses (0.423 microns), a relationship found in other mammals. Also, the areas of terminals forming asymmetric synapses (0.707 micron2) were larger than those forming symmetric synapses (0.401 micron2), also consistent with data from other species. The length of axospinous synapses (0.656 micron) and the area of the terminals forming them (0.611 micron2) were not significantly different from the length of axodendritic synapses (0.523 micron) or the area of terminals forming them (0.602 micron2). This study is the first quantitative study on synaptic organization in human postmortem striatum. The results indicate that the synaptic organization of the human striatum is similar, but not identical, to that of other mammalian species.