Carl A. McCandlish

Relationship between the organization of the forepaw barrel subfield and the representation of the forepaw in layer IV of rat somatosensory cortex.

We studied the organization of the forepaw barrel subfield (FBS) in layer IV of adult rat somatosensory cortex using the mitochondrial marker cytochrome oxidase and related this organization to the representation of the forepaw. The FBS is an ovoid structure consisting of barrels and barrel-like structures, the most conspicuous of which form four centrally located medio lateral running bands. Each band contains three to four barrels. These centrally located bands are bordered along their entire lateral side by a nebulous zone of undifferentiated labeling. At the anterior border, two small barrels are located laterally and one or two larger barrels are located medially. Medial to the central zone are three well-defined barrels. The posterior border consists of a nebulous field of labeling and occasional barrel-like structures. The results from our electrophysiological recording and mapping revealed that the forepaw representation was topographically organized into a single map and that the forepaw map matches almost precisely with individual barrels and barrel-like structures in the FBS. Each of the four central bands is associated with the representation of a single glabrous digit. Digit two (D2) is represented anteriorly and followed posteriorly by D3 through D5. Within each digit band the digit is somatotopically organized, with the skin over the distal phalanx represented in the two lateral barrels and the middle and proximal phalanges represented in the medial barrel(s). The dorsal hairy digit skin and dorsal hand are represented in the lateral zone. D1 is represented by two small anteriorly located barrels. Medial to the representation of the glabrous digits is the representation of the palmar pads. The representation of these pads, in turn, lies between the representations of the thenar (located anteriorly) and hypothenar (located posteriorly) pads. Posterior to the hypothenar pad representation lie the representations of the wrist and forearm. While the present results support the conclusion that individual barrels are associated with discrete locations on the forepaw, examples were found where the recording site was not precisely located within the predicted barrel. Some of these errors may be accounted for by limitations in the mapping techniques; nevertheless, the FBS offers an excellent model system to study relationships between cortical structure and function.

Early development of the SI cortical barrel field representation in neonatal rats follows a lateral-to-medial gradient: an electrophysiological study

SummaryDevelopment of the barrel field in layer IV of SI cortex of neonatal rats was studied in vivo using electrophysiological recording techniques. This study was designed to determine (a) the earliest time SI cortex is responsive to peripheral mechanical and/or electrical stimulation and (b) whether the development of the SI cortical barrel field map of the body surface follows a differential pattern of development similar to the pattern previously demonstrated using peanut agglutinin (PNA) binding (McCandlish et al. 1989). Carbon fiber microelectrodes were used to record evoked responses from within the depth of the cortex in neonatal rats between postnatal day 1 (PND-1), defined as the day of birth, and PND-14. Evoked responses were first recorded approximately 12 h after birth. These responses in the youngest animals were of low amplitude, monophasic waveshape, and long latency, with long interstimulus intervals necessary to drive the cortex. Increases in amplitude and complexity of waveshape and decreases in latency were observed over subsequent postnatal days. The earliest responses recorded on middle PND-1 were evoked by stimulation of the face and/or mystacial vibrissae. The next responses were evoked approximately 24 h after birth (late PND-1) by stimulation of the forelimb. The last responses were evoked approximately 36 h after birth (middle PND-2), by stimulation of the hindlimb. The physiological map of the representation of the body surface follows a developmental gradient similar to the gradient observed using PNA histochemistry; however, the lectin-generated morphological map lagged approximately 48 h behind the physiological map. The representation of the body surface appears to be topographically organized as early as PND-2. Our results suggest that thalamocortical afferents have reached the developing cortical plate and are functional before glial cells are first detected. These results do not sit well with a theory of barrel field development based entirely on the role of glia in pattern formation.

Peer Reviewed: Organic SIMS of Biologic Tissue

Instrumental developments over the past 20 years now make it possible to identify and map some biologically important molecules by SIMS.

Early development of the representation of the body surface in SI cortex barrel field in neonatal rats as demonstrated with peanut agglutinin binding: evidence for differential development within the rattunculus

SummaryPhysiological studies have demonstrated a highly organized somatotopic representation of the body surface in SI cortex of rat. This representation is correlated morphologically with the presence of barrel-shaped structures in layer IV. Conventional staining techniques reveal barrels in the latter part of the first postnatal week. Recently, the peroxidase conjugates of lectins, which recognize glycosylated molecules, have been used to study barrel field formation. Con A, for example, has been shown to bind primarily to prospective barrel sides and septa as early as postnatal day 3 (PND-3) in mouse. To date, investigations of SI cortex using the lectin (Arachis hypogaea) peanut agglutinin (PNA) have been confined to the study of the barrel field representation of the face and mystacial vibrissae in the mouse. In the present study we extend these findings to the development of the representation of the entire body surface called the rattunculus. Rats ranging from PND-1 (first 24 h after birth) to PND-12 were anesthetized with Nembutal and perfused with 4% paraformaldehyde and 2% glutaraldehyde in 0.2 M sodium cacodylate buffer. Brains were removed, flattened tangentially, and sectioned on a vibratome at 30–120 microns. Sections were blocked in TRIS-buffered saline (TBS) plus 2% bovine serum albumin and incubated in peanut lectin at 4° C. Following incubation, sections were washed with TBS and processed using peroxidase histochemistry. Lectin binding in the prospective forelimb representation was apparent by PND-5 whereas lectin binding to the prospective face-mystacial vibrissae representation occurred before PND-4. These results suggest that body part representations show individual variations during early pattern formation. In rat, the representation of the limbs may lag behind the representation of the face-mystacial vibrissae during early postnatal development. This developmental gradient within the cortex may reflect a differential expression of lectin receptors.

Identification and Mapping of Phosphocholine in Animal Tissue by Static Secondary Ion Mass Spectrometry and Tandem Mass Spectrometry

Secondary ion mass spectra and images were obtained from animal tissue samples using less than 10(13) primary ions/cm2. The mass spectra showed abundant peaks at m/z 184 and m/z 86. Tandem mass spectrometry (MS/MS) was used to identify the source of these ions as phosphocholine. Secondary ion images obtained using MS/MS were used to show that m/z 86 is an abundant gas-phase fragment ion derived from m/z 184. These results are discussed in terms of the physiology of the samples investigated.

Digit removal leads to discrepancies between the structural and functional organization of the forepaw barrel subfield in layer IV of rat primary somatosensory cortex

The physiological representation of the forepaw in rat primary somatosensory cortex (SI) is topographically organized. This representation is associated with the unique arrangement of barrels in layer IV of the forepaw barrel subfield (FBS) in SI and provides an example of a relationship between cortical structure and function. It has been reported that removal of peripheral afferent input to the FBS prior to postnatal day 5 or 6 results in a disorganized FBS, while deafferentation at later times produces little or no alteration of the FBS. Therefore, restricted deafferentations of individual digits in adult rats should result in little, if any, disruption of the FBS, while at the same time eliminating afferent input to the FBS from a localized region of the periphery. This manipulation is likely to create a mismatch between structure and function and offer insight into what barrels actually represent in the adult deafferent. In the present study, we amputated digit three (D3) in eight adult rats, allowed a 1-month survival time, physiologically mapped the representation of D2, D4, and the stump, and compared this physiological map to the underlying barrels in the FBS. Our results showed that FBS barrels formerly associated with the representation of D3 were now associated with the representation of surrounding digits D2 and D4, as well as the remaining stump. By superimposing the morphological and physiological map upon one another, it was clear that the D2 and D4 representations expanded into the former D3 barrel territory and septae between the barrels. The reorganized physiological map was somatotopically organized, even though the general configuration of the morphological map remained unaltered, as visualized with cytochrome oxidase staining. These results suggest that in the deafferent, neurons within FBS barrels previously associated with the representation of punctate regions of skin become associated with neighboring regions of skin. A morphological substrate to account for this cortical reorganization is described.

Early development of SI cortical barrel subfield representation of forelimb in normal and deafferented neonatal rat as delineated by peroxidase conjugated lectin, peanut agglutinin (PNA)

SummaryDevelopment of the barrel field representation of the forelimb in the primary somatosensory cortex (SI) was studied in normal and deafferented neonatal rat pups by means of the peroxidase conjugated lectin peanut agglutinin (PNA), which most likely binds to radial glial cells within barrel boundaries. 1. Alterations in lectin binding were seen in animals sacrificed on postnatal day 8 (PND-8) if deafferentation took place on PND-1 (day of birth) through PND-6. 2. Deafferentation on PND-5 or on PND-6 had the least effect on lectin binding. In these animals, lectin binding was reduced, although the prospective representation was intact. 3. Deafferentation on PND-2, 3, and 4 had the greatest effect on lectin binding. In these animals, lectin binding was reduced and the prospective cortical representation was disrupted. 4. Deafferentation on PND-1 resulted in reduced lectin binding, however the prospective cortical representation was only slightly impaired compared to that in animals deafferented on PND-2, 3, and 4. 5. These results suggest that SI barrel field boundaries are important to plasticity and that a sensitive period for predevelopment of the forelimb barrels consists of postnatal days 1 through 6. Furthermore, the formation of normal SI barrel field boundaries requires an ongoing interaction between incoming afferents and radial glial cells.

Secondary ion images of the rodent brain.

Sections of biologic tissue obtained from laboratory rodents are prepared and analyzed by secondary ion mass spectrometry. The intensity of phosphocholine secondary ions is used to identify anatomical features of the brain from secondary ion images and to evaluate the effectiveness of procedures developed. Secondary ion emission of phosphocholine (m/z 184), is found to be abundant and its intensity is heterogeneous. Effects of sample thickness are addressed. Correspondence between conventional optical images of stained tissue and secondary ion images shows that successive ion images may be used to produce a three-dimensional map of the brain, i.e., an atlas.

Large unresponsive zones appear in cat somatosensory cortex immediately after ulnar nerve cut.

The organization of the primary somatosensory cortex innervated by the ulnar nerve was studied before and immediately after ulnar nerve transection in 11 cats electrophysiologically mapped under Nembutal or Ketamine anesthesia. The cortex was reexamined a second time beginning 42 hr after nerve transection in four cats anesthetized with Nembutal. One additional sham-operated control was also mapped. The region of cortex formerly served by the ulnar nerve remained largely unresponsive to somatic stimulation independent of the type of anesthetic used during recording. Nonetheless, animals anesthetized with Ketamine had more new responsive sites in deafferented cortex following nerve cut than cats anesthetized with Nembutal. New responses, when observed, were evoked by stimulation of a region of skin adjacent to the region served by the ulnar nerve. These findings suggest that the immediate response to deafferentation of somatosensory cortex is a limited acquisition of novel responses restricted to a region immediately adjacent to cortex containing normal afferent input.

Electrical stimulation of a forepaw digit increases the physiological representation of that digit in layer IV of SI cortex in rat

We studied the physiological representation of digit three (D3) in rat somatosensory cortex (SI) before and immediately after electrical stimulation (1.5x threshold for 2 h) of the glabrous tip of D3 in anesthetized animals (n = 6). Measurements of D3 representation were also made in anesthetized non-stimulated control animals (n = 2). The post-stimulation areal measurements of D3 representation in experimental animals were statistically significantly larger than both pre-stimulation measurements in experimental animals and post-stimulation measurements in control animals. Our results suggest that short-term electrical stimulation is sufficient to expand the D3 representation in each of the experimental animals, while the maps in non-stimulated controls showed little variation. The fact that these studies were carried out in anesthetized animals suggests that the results are independent of the state of the animal. The present findings emphasize the importance of afferent input in modulating cortical organization.