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.

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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.

Development 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.(ABSTRACT TRUNCATED AT 250 WORDS)

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.