ACTA NEUROBIOL. EXP. 1977, 37: G3-81

A STEREOTAXIC ATLAS OF THE PREFRONTAL CORTEX OF THE CAT

Hans J. MARKOWITSCH and Monika PRITZEL

Department of Psychology, University of Konstanz Konstanz, FRG

Abstract. A survey of existing atlases of the cat's brain has revealed a lack of coronal sections on the levels of the prefrontal cortex. On the other hand, neurophysiological and behavioral studies of this region have increased greatly in recent years. As the extent of coronal sections through the prefrontal cortex was seen to differ markedly even at separa- tions of 1 mm, a stereotaxic atlas was made on the basis of brain sections of 16 mongrel cats. Brains were cut with the use of the paraffin or freezing method,,and stained with cresyl violet, Lux01 fast blue, or Kluver-Barrera's combination. Statistical methods were used to yield representative coronal outlines of sections from +20 to +30 mm anterior in 1 mm steps. A comparison with Reinoso-Suarez' five coronal sections within this range showed a marked congruence between the two atlases.

INTRODUCTION

The domestic cat is an animal most extensively studied by neuro- physiologists (5, 39). Various parts of its brain are described in more than a dozen atlases (3, 4, 10, 13, 16-19, 22, 23, 26, 33, 35, 12, 45, 49, 53). Nevertheless, only one of these atlases includes the prefrontal cortex of the adult cat (33), and that one shows only five coronal plates between f 20 and +30 (at 4-20, 4-22, +24, +27, t 3 0 ) .

Though the monkey is still a prevalent laboratory animal in research concerning the frontal lobes (e.g., 47), in recent years the prefrontal regions of nonprimate species, like carnivores (11, 12, 21, 51, 52, 54) and rodents (20, 24, 29, 37) have also become subjects of studies attempting

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to solve "the riddle of frontal lobe function" (46). Especially the pre- frontal cortex of the cat has become an area of increasing interest in recent years to anatomically (e.g., 2, 50), behaviorally (e.g., 12, 28, 31, 38), and electrophysiologically oriented workers (e.g., 27, 41).

The absence of an atlas that would include exact coronal sections in a close sequence is a serious disadvantage, especially as the gyral and sulcal patterns of the frontal cortical areas vary to a substantial degree even within 1 mm. As the area of the cat's cortex which is said to be homologous to the primate's frontal granular cortex is usually delimited merely by reference to sulcal boundaries (1, 28, 36, 50, 51), exact stereo- taxic locations are needed to facilitate the comparability of studies dealing with the prefrontal cortex, as well as to increase the usefulness of coronal frontal brain sections which up to now have frequently been presented without stereotaxic references.

The area of prefrontal cortex is rather small in cats as compared to most primates. Only 3.1°/o (2) to 3.4% (8) of the cat's cortical surface correspond to the primate's frontal lobes. For the definition of this region, Rose and Woolsey's (36) suggestion is generally accepted that the prefrontal cortex is the projection area of the mediodorsal nucleus (1, 48). Though there is no final agreement as to the exact borders of the cat's prefrontal area (28), Fig. 1 presents a traditional illustration of its extent. It can be seen that the dorsal border of the prefrontal area is limited by the cruciate sulcus, a fissure more likely to be homologous to the primate sulcus praecentralis superior than to the central sulcus

Fig. 1. The extent of the prefrontal cortex of the cat .(dashed; after Markowitsch and Pritzel, 28). A, frontal, B, anterolateral, C, medial view. Abbreviations: bo, bulbus olfactorius; cc, corpus callosum; gc, gyrus coronalis; gci, gyrus cinguli; gf, gyrus frontalis; go, gyrus orbitalis; gp, gyrus proreus; gr, gyrus rectus; gsa, gyrus sigmoideus anterior; gsp, gyrus sigmoideus posterior; sc, sulcus cruciatus; sp, sulcus praesylvius. Nomenclature after Nomina anatomica (3rd ed.) (30) and after (33). The dotted line in the medial view represents the approximate boundary

between g. frontalis and cinguli.

(7, 8). The lateral limitation extends a bit beyond the presylvian sulcus (1, 8). Posteromedially the anterior limbic region borders on the pre- frontal cortex (1).

Figures 2 to 12 form an atlas including coronal sections of the pre- frontal area defined in Fig. 1. The planes of the figures differ by 1 mm - a separation consistent with the intention of this atlas to give as many clues as possible to the identification of a given brain section on the one hand, but also to take interindividual variation of cat brains into account on the other. This was found to be of about 0.5 mm for sections of the area under consideration (see Observations and Discussion).

MATERIALS AND METHODS

Sixteen adult mongrel cats of both sexes (weights: 2.4-4.9 kg) were used for experiment. Under deep narcosis the animals were transcardially perfused with saline and 1O0/o buffered formalin in saline, following the instructions of Clark (9), Frontera (14), and Humason (15). The animals were then decapitated and the heads stored in 10°/o sucrose-formalin (9), renewed frequently.

To calculate a possible shrinkage factor, the brains of two cats were treated with methods similar to those used by Brawer et al. (6) and Palkovits and Jacobowitz (32). Prior to perfusion both anesthetized cats were rigidly mounted in a stereotaxic instrument (La Precision CinCma- tographique). Two holes, separated 20 mm sagittally, were drilled in the skull of one of the cats over both hemispheres a t the same lateral level, and insect pins (size 00) were inserted into the brain to establish an "electrode track". Distances between the tracks could be follwed during the sectioning procedure. The hemispheres of the other cat received small transverse cuts (with a razor blade) in 5 mm intervals from anterior 0 to 4-20 after the overlying skull areas had been opened.

All cats were subjected to the following precedures 1 to 2 days after perfusion: First, the heads of the cats were fixed in a stereotaxic instru- ment. Thereafter, superficial coronal cuts were made with a razor blade

0, a t the levels 0 and +15. After removal of the bony tentorium two thin glass tubes were pushed through two-thirds of the brains in dorso-caudal direction. The brains were then removed and embedded in a box filled with warm agar-agar in a horizontal level established with the help of two glass tubes. After removing the glass tubes, the brains were cut into three blocks a t levels 0 and +15. The other brains were cut in situ with a specially prepared razor blade a t the same levels. The blocks were then stored again in 10°/o formalin-saline for another fortnight. All brains were carefully measured.

In five cats, in addition to these procedures, thin glass tubes or insect pins were driven into the brain a t various horizontal, vertical, and lateral distances from the zero-zero stereotaxic coordinates, to establish markings for three-dimensional coordinates which later were compared with corresponding contour lines in existing atlases. The brains were then prepared for sectioning. For this purpose one hemisphere of each brain was dehydrated and embedded in paraffin (13, 34) before sectioning, while the other was either cut immediately or after embed- ding in gelatin, using a Leitz 1,400 K freezing microtome. Paraffin sec- tions were cut a t 25 ym, frozen sections at 50 pm intervals. Every 10th (5th) section was mounted and stained with cresyl violet for nuclear details, or with Lux01 fast blue (fibers), or with Kliiver-Barrera's combination (both). According to Smith et al. (44) these stains give the best details in photographs. All sections of corresponding levels between brains were compared, independently by the two authors. In addition to establishing the longitudinal level of each section, further reference points or features - such as the first appearance of the dent of the s. coronalis, of the s. cruciatus, of the s. lateralis, and of the s. ectosyl- vius anterior, among others - were used to identify each section. The resulting values were a t first compared by both authors, and then, after having achieved final concordance, they were correlated statistically, by computing means, standard deviations, and by applying the Spearman rank correlation coefficient (43). Only sections within a narrow limit of correspondence were used for further procedures.

Satisfactory sections were processed further in three ways. First, they were enlarged exactly 7.5 times using a Kodak projector, and outlines of both the section as a whole and of the fiberlcell contours were drawn. Second, the same sections were photographed and magnified 7.5 times. Third, the sections were inspected while magnified 6 to 40 times, using a Zeiss operation microscope, to further detect fiber/cell borders.

Drawn and photographed outlines were compare and used - together with the information from microscope inspection - for the preparation of each of the final figures.

OBSERVATIONS AND DISCUSSION

The procedures employed yielded brain sections of marked uniformity both between the cats used and between our drawings and the frontal sections of the atlas of Reinoso-Suarez (33). The statistical measurements revealed an average section standard deviation of i0.497 mm across

sections. This is less than the 1 mm tolerance stated in Reinoso-Suarez' atlas. The deviation between the two tolerances may be explained by the different references used: while Reinoso-Suarez' value holds as an average for the whole brain, ours holds only for its anterior fifth.

The shrinkage factor calculated from the two specially prepared brains turned out to be nearly negligible (5-7OIo; frozen material) for the pre- frontal sections not of importance. Nevertheless it was taken into account in the scales of Fig. 2 to 12. Each of these Figures represents the best fitting Gestalt of a section a t each of the given coronal levels. The atlas begins a t the level +20, because the prefrontal cortex ends at least a t this level, and because several other atlases present coronal sections-at 1-mm intervals posterior to this level.

Figures of this atlas were compared with those of Reinoso-Suarez and with various figures in other published studies. Figures of given corres- ponding levels in the atlas of Reinoso-Suarez and in this atlas correspond to a high degree. The only remarkable differences concern the extent of the lateral ventricle and of the caudate nucleus, both of which are more prominent in Reinoso-Suarez at level 4-20 than in our corresponding fi- gure. Even that difference, however, smaller than 1 mm.

Although most surveyed studies which include prefrontal sections do not specify the levels of their sections, there are some that do so. For instance Schlag and Schlag-Rey (40) describe eight section outlines of their Fig. 2 as being less than 1 mm from each other. Siege1 et al. (41) include maps representing medial parts of coronal sections a t +23, +24.5, and +26. Their level classification seems to correspond both with that of Reinoso-Suarez and of this atlas. On the other, hand Schlag and Schlag-Rey (40) reported anterior-posterior variations of up to 5.7 mm between the position of the cruciate sulcus in 14 cats weighing 3 to 4 kg. They explained the differences by interindividual variations among ani- mals, referring to observations of Loewenthal and Altman (25). But variations of this magnitude were neither reported in the conventional atlases nor detected in this one; 2 mm are the maximum deviation men- tioned between the location of some nuclei and stereotaxic zero, confirm- ing the emphasis of Jasper and Ajmone-Marsan (18) on the relatively constant size and shape of the cat's brain.

We hope that this atlas, presenting coronal sections a t separations of 1 mm and with tolerances of about 0.5 mm between cats, will provide a reliable reference guide for prefrontal investigations.

The authors thank Prof. R. B. Freeman, Jr. for his continued support and for many helpful comments on this paper.

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Accepted 15 November 1976

Hans J. MARKOWITSCH and Monika PRITZEL, Department of Psychology, University of Konstanz, Postbox 7733, D-7150 Konstanz, FRG.

Fig. 2-12. Drawings of coronal sections through the prefrontal cortex of the cat. The level of each section is stated in millimeters anterior to the vertical zero. Horizontal zero is 10 mm dorsal to the auditory meatus. Fibers are hatched, scales are in mm. Abbreviations: bo, bulbus olfactorius; nc, nucleus caudatus; sco, sulcus coronalis; scr, sulcus cruciatus; sea, sulcus ectosylvius anterior; sl, sulcus lateralis; sp, sulcus praesylvius; sra, sulcus rhinalis anterior; to, tractus olfactorius; vl, ventriculus lateralis. Nomenclature after Nomina anatomica (3rd ed., ref. 30) and

after (33).

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