Wolfgang O. Guldin

No detectable remote lesions following massive intrastriatal injections of ibotenic acid

Behavioral and anatomical consequences of particularly large intrastriatal injections of ibotenic acid are described. Only in the rat with the largest injection, which encompassed almost the entire frontal lobe, were enduring aphagia and adipsia observed; epileptic attacks were, however, not detectable in this or in any other of the rats. In spite of the massiveness of the lesion, neither remote lesions nor damage to passing fibers was observed. It is therefore suggested to substitute kainic acid by ibotenic acid for the production of local, discrete brain lesions.

Epidural kainate, but not ibotenate, produces lesions in local and distant regions of the brain. A comparison of the intracerebral actions of kainic acid and ibotenic acid

Epidural placement of ibotenic acid (Ibo) is described as a new technique for easy and successful lesioning of cortical neurons. The method works reliably without apparent damage to passing or underlying fibers and without causing distant lesion effects. In rats, the favorable effects of epidural placement of Ibo on behavior and on neuronal tissue were compared with and found to contrast with those of kainic acid (KA). Survival periods of 2, 4, 6, and 8 days were allowed for animals of each group. The effects of Ibo from Amanita muscaria were compared to those from Amanita pantherina. Local cortical lesions after epidural placement of KA and Ibo were compared with those after subdural placement. The effects resulting from covering a small surface of the dura with Ibo were compared with those resulting from covering a large surface. Epidural placement of KA and Ibo resulted in neuronal damage of the underlying cortex, 0.4 mg of KA leading to approximately the same lesion extent as 0.5 mg of Ibo. Epidural and subdural lesion effects were found to be similar both for KA and Ibo. The most striking difference between epidural application of KA and Ibo was that KA lesions resulted in a considerable number of damaged areas remote from the local cortical lesion, whereas even 10-times higher doses of Ibo (measured in moles) did not produce any observable neuronal damage outside the local lesion. From these results it is suggested that Ibo has to be favored over KA when local neuronal lesions are desired. Furthermore, epidural placement of Ibo is proposed as an effective technique for producing circumscribed cortical lesions.

Prefrontal Cortex of the Mouse Defined as Cortical Projection Area of the Thalamic Mediodorsal Nucleus

In the mouse small amounts of horseradish peroxidase were injected iontophoretically into different portions of the frontal lobe. Out of a large number of mice, 18 with typical injections which together covered most of the anterior half of the cortex were selected, and their retrogradely labeled thalamic cells were described and illustrated in detail. Special emphasis was laid on the cortical projections of the mediodorsal nucleus which were considered to define locus and extent of the prefrontal cortex. The results reveal that topographically the extent of the mouse, prefrontal cortex closely resembles that of the rat, another species of the rodent order. It includes a small area within and dorsal to the rhinal sulcus and a comparatively larger region within the medial half of the anterior cortex. Furthermore, the dorsal tip of the frontal pole is reached by a considerable number of afferents from the mediodorsal nucleus. Whether this last projection also exists in the rat is still disputed. As in the rat, only the sulcal prefrontal cortex seems to be reached by mediodorsal cells alone, whereas the rest of the prefrontal cortical fields have overlapping projections from the anteromedial nucleus. Though a considerable area within the frontal lobe of the mouse is reached by mediodorsal afferents, the significance and usefulness of this definition of the prefrontal cortex is questioned.

Delayed-alternation performance after selective lesions within the prefrontal cortex of the cat

On the basis of new neuroanatomical findings on relationships between subregions of the mediodorsal thalamic nucleus and the prefrontal cortex of the cat, it was attempted to investigate the relative importance of prefrontal subfields with the aim of obtaining evidence in favor of a functional inequality of different prefrontal subfields. Four areas, named presylvian (PRS), proreal (PR), dorsomedial (DM), and orbito-insular (OI) sectors, were ablated successfully in 30 adult animals. Performance of a 10-sec delayed-alternation task was compared pre- and postoperatively. Furthermore, most of the cats had to learn an extension of this task postoperatively, using a 20-sec delay period, and lastly, these animals were subjected to an extinction test. Significant performance differences were obtained between cats of different groups in all three tasks. Lesions of subregion PR, and even more of subregion PRS, led to severe behavioral deterioration, whereas lesions of subregion OI were without effect, when compared with the behavior of a sham-operated control group. PRS-cats, furthermore, showed motor disturbances during the first postoperative week. The results obtained suggest that it is possible to subdivide the cats prefrontal cortex functionally. In addition, it is hypothesized that behavioral changes in cats of groups PRS and PR are due to an inability to use kinesthetic information properly.

Heterotopic interhemispheric cortical connections in the rat.

In the rat, contrary to other species, interhemispheric cortical connections have been considered to travel largely between homotopical regions only. Based on iontophoretic injections of horseradish peroxidase, the present study reports extensive heterotopic interhemispheric connections between posterior insular (perirhinal) regions of the lateral part of the hemisphere and anterior prefrontal regions of the medial hemisphere and vice versa. Generally, the areas connected interhemispherically are also connected intrahemispherically. The ratio of contralateral projections appears to be less than one third of the ipsilateral ones.

Chapter 24: AEV-insular axis: connectivity

Publisher Summary This chapter discusses the connectivity of anterior ectosylvian visual area (AEV). Comparisons of morphological and functional properties of the insula face the problems of homology as well as of cytological boundaries. Using macroscopical criteria, the structure is well-demarcated in human, catarrhinic and cetacean brains, being covered by the frontal, temporal and parietal opercula within the sylvian fissure. Using the external landmark of the sylvian fissure is possible in gyrified species such as the dog and the cat but it is impossible in lissencephalic animals such as rodents. While there is little controversy as to the extent of the primate insula, the aspects of external morphology and cytoarchitecture become more problematic when considering work on particular species with less gyrification such as the cat. Furthermore, the connections between the insula and the temporal pole and the lateral premotor region again corroborate the frequent finding that adjacent areas have similar connections as well as strong interconnections with each other.

Comparative analysis of delayed alternation learning in cats, mice, and guinea pigs.

The performance of cats, guinea pigs, and mice in a delayed alternation paradigm was compared both during initial learning and following a 10-day retention interval. The testing situation (a modified T-maze), the length of the delay period (10 sec), and the amount of training per session and per week were kept identical for all three species. The results indicated that (1) animals of all three species acquired the task within similar time spans, (2) a considerable variance was apparent in the performance of individual animals independent of their species, and (3) guinea pigs, as a group, appeared to need a somewhat longer time to acquire delayed alternation than did mice and cats. Relearning of the task following the 10-day interval seemed to follow similar laws in all three species.

Activity changes following sulcal, but not medial, ablation of the prefrontal cortex of the guinea pig

Running activity of guinea pigs was compared before and after ablation of the cortical projection areas of the medial and lateral sectors of the thalamic mediodorsal nucleus (sulcal vs. medial prefrontal cortex). Only ablation of the sulcal prefrontal cortex was followed by a significant increase in running activity. This result corresponds to similar observations made in another rodent species, the rat, and also made in nonrodent species. Therefore, it is concluded that, with respect to basic or unlearned functions, interspecies parallels can be drawn for the cortical projection area of the medial portion of the mediodorsal nucleus. At the same time, it is, however, emphasized that the prefrontal cortex cannot be considered to be structurally and/or functionally homogeneous among species.