Human dermal fibroblasts in psychiatry research

In order to decipher the disease etiology, progression and treatment of multifactorial human brain diseases we utilize a host of different experimental models. Recently, patient-derived human dermal fibroblast (HDF) cultures have re-emerged as promising in vitro functional system for examining various cellular, molecular, metabolic and (patho)physiological states and traits of psychiatric disorders. HDF studies serve as a powerful complement to postmortem and animal studies, and often appear to be informative about the altered homeostasis in neural tissue. Studies of HDFs from patients with schizophrenia (SZ), depression, bipolar disorder (BD), autism, attention deficit and hyperactivity disorder and other psychiatric disorders have significantly advanced our understanding of these devastating diseases. These reports unequivocally prove that signal transduction, redox homeostasis, circadian rhythms and gene*environment (G*E) interactions are all amenable for assessment by the HDF model. Furthermore, the reported findings suggest that this underutilized patient biomaterial, combined with modern molecular biology techniques, may have both diagnostic and prognostic value, including prediction of response to therapeutic agents.

Expression analysis in a rat psychosis model identifies novel candidate genes validated in a large case-control sample of schizophrenia

Antagonists of the N-methyl-D-aspartate (NMDA)-type glutamate receptor induce psychosis in healthy individuals and exacerbate schizophrenia symptoms in patients. In this study we have produced an animal model of NMDA receptor hypofunction by chronically treating rats with low doses of the NMDA receptor antagonist MK-801. Subsequently, we performed an expression study and identified 20 genes showing altered expression in the brain of these rats compared with untreated animals. We then explored whether the human orthologs of these genes are associated with schizophrenia in the largest schizophrenia genome-wide association study published to date, and found evidence for association for 4 out of the 20 genes: SF3B1, FOXP1, DLG2 and VGLL4. Interestingly, three of these genes, FOXP1, SF3B1 and DLG2, have previously been implicated in neurodevelopmental disorders.

Effects of maternal immune activation on gene expression patterns in the fetal brain

We are exploring the mechanisms underlying how maternal infection increases the risk for schizophrenia and autism in the offspring. Several mouse models of maternal immune activation (MIA) were used to examine the immediate effects of MIA induced by influenza virus, poly(I:C) and interleukin IL-6 on the fetal brain transcriptome. Our results indicate that all three MIA treatments lead to strong and common gene expression changes in the embryonic brain. Most notably, there is an acute and transient upregulation of the α, β and γ crystallin gene family. Furthermore, levels of crystallin gene expression are correlated with the severity of MIA as assessed by placental weight. The overall gene expression changes suggest that the response to MIA is a neuroprotective attempt by the developing brain to counteract environmental stress, but at a cost of disrupting typical neuronal differentiation and axonal growth. We propose that this cascade of events might parallel the mechanisms by which environmental insults contribute to the risk of neurodevelopmental disorders such as schizophrenia and autism.

Involvement of the PRKCB1 gene in autistic disorder: significant genetic association and reduced neocortical gene expression

Protein kinase C enzymes play an important role in signal transduction, regulation of gene expression and control of cell division and differentiation. The fsI and betaII isoenzymes result from the alternative splicing of the PKCbeta gene (PRKCB1), previously found to be associated with autism. We performed a family-based association study in 229 simplex and 5 multiplex families, and a postmortem study of PRKCB1 gene expression in temporocortical gray matter (BA41/42) of 11 autistic patients and controls. PRKCB1 gene haplotypes are significantly associated with autism (P<0.05) and have the autistic endophenotype of enhanced oligopeptiduria (P<0.05). Temporocortical PRKCB1 gene expression was reduced on average by 35 and 31% for the PRKCB1-1 and PRKCB1-2 isoforms (P<0.01 and <0.05, respectively) according to qPCR. Protein amounts measured for the PKCbetaII isoform were similarly decreased by 35% (P=0.05). Decreased gene expression characterized patients carrying the 'normal' PRKCB1 alleles, whereas patients homozygous for the autism-associated alleles displayed mRNA levels comparable to those of controls. Whole genome expression analysis unveiled a partial disruption in the coordinated expression of PKCbeta-driven genes, including several cytokines. These results confirm the association between autism and PRKCB1 gene variants, point toward PKCbeta roles in altered epithelial permeability, demonstrate a significant downregulation of brain PRKCB1 gene expression in autism and suggest that it could represent a compensatory adjustment aimed at limiting an ongoing dysreactive immune process. Altogether, these data underscore potential PKCbeta roles in autism pathogenesis and spur interest in the identification and functional characterization of PRKCB1 gene variants conferring autism vulnerability.

Alterations in GABA-related transcriptome in the dorsolateral prefrontal cortex of subjects with schizophrenia

In subjects with schizophrenia, impairments in working memory are associated with dysfunction of the dorsolateral prefrontal cortex (DLPFC). This dysfunction appears to be due, at least in part, to abnormalities in gamma-aminobutyric acid (GABA)-mediated inhibitory circuitry. To test the hypothesis that altered GABA-mediated circuitry in the DLPFC of subjects with schizophrenia reflects expression changes of genes that encode selective presynaptic and postsynaptic components of GABA neurotransmission, we conducted a systematic expression analysis of GABA-related transcripts in the DLPFC of 14 pairs of schizophrenia and age-, sex- and post-mortem interval-matched control subjects using a customized DNA microarray with enhanced sensitivity and specificity. Subjects with schizophrenia exhibited expression deficits in GABA-related transcripts encoding (1) presynaptic regulators of GABA neurotransmission (67 kDa isoform of glutamic acid decarboxylase (GAD(67)) and GABA transporter 1), (2) neuropeptides (somatostatin (SST), neuropeptide Y (NPY) and cholecystokinin (CCK)) and (3) GABA(A) receptor subunits (alpha1, alpha4, beta3, gamma2 and delta). Real-time qPCR and/or in situ hybridization confirmed the deficits for six representative transcripts tested in the same pairs and in an extended cohort, respectively. In contrast, GAD(67), SST and alpha1 subunit mRNA levels, as assessed by in situ hybridization, were not altered in the DLPFC of monkeys chronically exposed to antipsychotic medications. These findings suggest that schizophrenia is associated with alterations in inhibitory inputs from SST/NPY-containing and CCK-containing subpopulations of GABA neurons and in the signaling via certain GABA(A) receptors that mediate synaptic (phasic) or extrasynaptic (tonic) inhibition. In concert with previous findings, these data suggest that working memory dysfunction in schizophrenia is mediated by altered GABA neurotransmission in certain DLPFC microcircuits.

DNA pooling: a comprehensive, multi-stage association analysis of ACSL6 and SIRT5 polymorphisms in schizophrenia

Many candidate gene association studies have evaluated incomplete, unrepresentative sets of single nucleotide polymorphisms (SNPs), producing non-significant results that are difficult to interpret. Using a rapid, efficient strategy designed to investigate all common SNPs, we tested associations between schizophrenia and two positional candidate genes: ACSL6 (Acyl-Coenzyme A synthetase long-chain family member 6) and SIRT5 (silent mating type information regulation 2 homologue 5). We initially evaluated the utility of DNA sequencing traces to estimate SNP allele frequencies in pooled DNA samples. The mean variances for the DNA sequencing estimates were acceptable and were comparable to other published methods (mean variance: 0.0008, range 0-0.0119). Using pooled DNA samples from cases with schizophrenia/schizoaffective disorder (Diagnostic and Statistical Manual of Mental Disorders edition IV criteria) and controls (n=200, each group), we next sequenced all exons, introns and flanking upstream/downstream sequences for ACSL6 and SIRT5. Among 69 identified SNPs, case-control allele frequency comparisons revealed nine suggestive associations (P<0.2). Each of these SNPs was next genotyped in the individual samples composing the pools. A suggestive association with rs 11743803 at ACSL6 remained (allele-wise P=0.02), with diminished evidence in an extended sample (448 cases, 554 controls, P=0.062). In conclusion, we propose a multi-stage method for comprehensive, rapid, efficient and economical genetic association analysis that enables simultaneous SNP detection and allele frequency estimation in large samples. This strategy may be particularly useful for research groups lacking access to high throughput genotyping facilities. Our analyses did not yield convincing evidence for associations of schizophrenia with ACSL6 or SIRT5.

Specificity and timing of neocortical transcriptome changes in response to BDNF gene ablation during embryogenesis or adulthood

Brain-derived neurotrophic factor (BDNF) has been reported to be critical for the development of cortical inhibitory neurons. However, the effect of BDNF on the expression of transcripts whose protein products are involved in gamma amino butric acid (GABA) neurotransmission has not been assessed. In this study, gene expression profiling using oligonucleotide microarrays was performed in prefrontal cortical tissue from mice with inducible deletions of BDNF. Both embryonic and adulthood ablation of BDNF gave rise to many shared transcriptome changes. BDNF appeared to be required to maintain gene expression in the SST-NPY-TAC1 subclass of GABA neurons, although the absence of BDNF did not alter their general phenotype as inhibitory neurons. Furthermore, we observed expression alterations in genes encoding early-immediate genes (ARC, EGR1, EGR2, FOS, DUSP1, DUSP6) and critical cellular signaling systems (CDKN1c, CCND2, CAMK1g, RGS4). These BDNF-dependent gene expression changes may illuminate the biological basis for transcriptome changes observed in certain human brain disorders.

DNA microarray profiling of developing PS1-deficient mouse brain reveals complex and coregulated expression changes

Presenilin 1 (PS1) plays a critical role in the nervous system development and PS1 mutations have been associated with familial Alzheimer's disease. PS1-deficient mice exhibit alterations in neural and vascular development and die in late embryogenesis. The present study was aimed at uncovering transcript networks that depend on intact PS1 function in the developing brain. To achieve this, we analyzed the brains of PS1-deficient and control animals at embryonic ages E12.5 and E14.5 using MG_U74Av2 oligonucleotide microarrays by Affymetrix. Based on the microarray data, overall molecular brain development appeared to be comparable between the E12.5 and E14.5 PS1-deficient and control embryos. However, in brains of PS1-deficient mice, we observed significant differences in the expression of genes encoding molecules that are associated with neural differentiation, extracellular matrix, vascular development, Notch-related signaling and lipid metabolism. Many of the expression differences between wild-type and PS1-deficient animals were present at both E12.5 and E14.5, whereas other transcript alterations were characteristic of only one developmental stage. The results suggest that the role of PS1 in development includes influences on a highly co-regulated transcript network; some of the genes participating in this expression network may contribute to the pathophysiology of Alzheimer's disease.

Analysis of complex brain disorders with gene expression microarrays: schizophrenia as a disease of the synapse

The level of cellular and molecular complexity of the nervous system creates unique problems for the neuroscientist in the design and implementation of functional genomic studies. Microarray technologies can be powerful, with limitations, when applied to the analysis of human brain disorders. Recently, using cDNA microarrays, altered gene expression patterns between subjects with schizophrenia and controls were shown. Functional data mining led to two novel discoveries: a consistent decrease in the group of transcripts encoding proteins that regulate presynaptic function; and the most changed gene, which has never been previously associated with schizophrenia, regulator of G-protein signaling 4. From these and other findings, a hypothesis has been formulated to suggest that schizophrenia is a disease of the synapse. In the context of a neurodevelopmental model, it is proposed that impaired mechanics of synaptic transmission in specific neural circuits during childhood and adolescence ultimately results in altered synapse formation or pruning, or both, which manifest in the clinical onset of the disease.

Microarrays in brain research: the good, the bad and the ugly

Making sense of microarray data is a complex process, in which the interpretation of findings will depend on the overall experimental design and judgement of the investigator performing the analysis. As a result, differences in tissue harvesting, microarray types, sample labelling and data analysis procedures make post hoc sharing of microarray data a great challenge. To ensure rapid and meaningful data exchange, we need to create some order out of the existing chaos. In these ground-breaking microarray standardization and data sharing efforts, NIH agencies should take a leading role

Disease-specific changes in regulator of G-protein signaling 4 (RGS4) expression in schizophrenia

Complex defects in neuronal signaling may underlie the dysfunctions that characterize schizophrenia. Using cDNA microarrays, we discovered that the transcript encoding regulator of G-protein signaling 4 (RGS4) was the most consistently and significantly decreased in the prefrontal cortex of all schizophrenic subjects examined. The expression levels of ten other RGS family members represented on the microarrays were unchanged and hierarchical data analysis revealed that as a group, 274 genes associated with G-protein signaling were unchanged. Quantitative in situ hybridization verified the microarray RGS4 data, and demonstrated highly correlated decreases in RGS4 expression across three cortical areas of ten subjects with schizophrenia. RGS4 expression was not altered in the prefrontal cortex of subjects with major depressive disorder or in monkeys treated chronically with haloperidol. Interestingly, targets for 70 genes mapped to the major schizophrenia susceptibility locus 1q21--22 were present on the microarrays, of which only RGS4 gene expression was consistently altered. The combined data indicate that a decrease in RGS4 expression may be a common and specific feature of schizophrenia, which could be due either to genetic factors or a disease- specific adaptation, both of which could affect neuronal signaling.

Molecular characterization of schizophrenia viewed by microarray analysis of gene expression in prefrontal cortex

Microarray expression profiling of prefrontal cortex from matched pairs of schizophrenic and control subjects and hierarchical data analysis revealed that transcripts encoding proteins involved in the regulation of presynaptic function (PSYN) were decreased in all subjects with schizophrenia. Genes of the PSYN group showed a different combination of decreased expression across subjects. Over 250 other gene groups did not show altered expression. Selected PSYN microarray observations were verified by in situ hybridization. Two of the most consistently changed transcripts in the PSYN functional gene group, N-ethylmaleimide sensitive factor and synapsin II, were decreased in ten of ten and nine of ten subjects with schizophrenia, respectively. The combined data suggest that subjects with schizophrenia share a common abnormality in presynaptic function. We set forth a predictive, testable model.

Ultrastructural analysis of ectopic synaptic boutons arising from peripherally regenerated primary afferent fibers

The central axons of peripherally regenerated Abeta primary sensory neurons were impaled in the dorsal columns of alpha-chloralose-anesthetized cats 9-12 mo after axotomy. The adequate peripheral stimulus was determined, and the afferent fibers intracellularly stimulated while simultaneously recording the resulting cord dorsum potentials (CDPs). Fibers that successfully had reinnervated the skin responded to light tactile stimulation, and evoked CDPs that suggested dorsally located boutons were stained intracellularly with horseradish peroxidase (HRP). Two HRP-stained regenerated Abeta afferent fibers were recovered that supported large numbers of axon collaterals and swellings in laminae I, IIo, and IIi. Sections containing the ectopic collateral fibers and terminals in the superficial dorsal horn were embedded in plastic. Analyses of serial ultrathin sections revealed that ectopic projections from both regenerated fibers supported numerous synaptic boutons filled with clear round vesicles, a few large dense core vesicles (LDCVs) and several mitochondria (>3). All profiles examined in serial sections (19) formed one to three asymmetric axo-dendritic contacts. Unmyelinated portions of ectopic fibers giving rise to en passant and terminal boutons often contained numerous clear round vesicles. Several boutons (47%) received asymmetric contacts from axon terminals containing pleomorphic vesicles. These results strongly suggest that regenerated Abeta fibers activated by light tactile stimuli support functional connections in the superficial dorsal horn that have distinct ultrastructural features. In addition, the appearance of LDCVs suggests that primary sensory neurons are capable of changing their neurochemical phenotype.

Properties of individual embryonic primary afferents and their spinal projections in the rat

Embryonic (E19-E20) and early postnatal (P2) spinal cords with intact saphenous and sciatic nerves were isolated and placed in aerated artificial cerebral spinal fluid (CSF). Intracellular recordings were made from cells in the L2-L6 dorsal root ganglia using microelectrodes filled with 3 M potassium acetate or 5% neurobiotin (NB) in 1 M potassium acetate. Several physiological properties of adequately impaled cells were measured, including peripheral conduction velocity, action potential (AP) amplitude and duration, duration of afterhyperpolarization (AHP), input impedance, rheobase, presence of inward rectifying current, and maximum somal firing frequency. The extent to which these properties are correlated also was determined. One cell per ganglion was injected with NB. Stained somata and their central projections in the spinal cord were visualized in serial 50 microm sections. Cell size was determined and the central morphology of the central projections examined. Although some fibers were in the process of growing into the spinal cord, others had established projections over several millimeters in the dorsal columns. Although most of these fibers supported projections in the gray matter, 22% only maintained fibers in the dorsal columns. Fibers with projections in the dorsal horn exhibited three types of morphology: projections confined to the superficial dorsal horn (laminae I, II); terminals confined to laminae III-V; and projections spanning laminae II-V. In addition, some embryonic fibers maintained projections to the dorsal horn that extended over five lumbar segments. Somal APs could be divided into two groups: broad spikes with inflections on their falling phase and narrow spikes without inflections. On average, cells with broad spikes (BS) had the following characteristics: slower peripheral conduction velocity, larger amplitude, higher rheobase and input impedance, longer AHP duration, and lower maximum firing frequency. There were significant correlations between conduction velocity and several of the physiological properties. Conduction velocity was negatively correlated with AP duration, rheobase, and input impedance and positively correlated with maximum firing frequency. Comparisons between spike shape and central morphology revealed that cells lacking collaterals in the gray matter and those with projections in the superficial dorsal horn always had broad somal spikes with inflections. Those with projections confined to laminae III-V always had narrow somal spikes (NS).

Plasticity of dorsal horn cell receptive fields after peripheral nerve regeneration

1. The tibial and sural nerves were transected and repaired in nine adult cats. The receptive field (RF) properties of dorsal horn neurons were examined at three different intervals (5-6, 9, or 12 mo) after axotomy. The properties examined included RF location, area, and modality convergence. In some cases, discrete areas of the cell's RF were stimulated electrically while the evoked cord dorsum potentials (CDPs) and any intracellularly recorded responses were simultaneously recorded. 2. At the shortest interval following reinnervation, the somatotopic organization in the affected areas of the dorsal horn was lost. Dorsal horn cells that received input primarily from regenerated fibers had large, low-threshold excitatory RFs that contained much of the reinnervated skin. Those cells with RFs restricted to a fraction of the reinnervated skin had significant components of their RFs on the foot dorsum supplied by intact fibers (i.e., superficial peroneal nerve). 3. At longer intervals the somatotopic organization remained scrambled. Dorsal horn cell low-threshold RFs were significantly reduced in size. Many cells exhibited large areas of excitatory subliminal fringe and concise inhibitory RFs. In addition, those cells that responded to peripheral stimuli across a wide range of stimulus intensities (wide-dynamic-range cells) also exhibited plasticity in the relative sizes of their low- and high-threshold RFs. 4. At the shortest recovery time, focal electrical stimulation of the skin within the RF of an impaled cell and simultaneous recordings of the evoked CDPs and postsynaptic potentials revealed that at numerous locations within the initial large RFs, single fibers or small groups of fibers could be electrically activated that were not connected to the dorsal horn cell. At the longer recovery times there was a much higher incidence of connectivity. 5. These results suggest that mechanisms affecting both synaptic efficacy of afferent fiber connections and/or the establishment of afferent-driven inhibitory inputs may effect the reshaping of dorsal horn cell RFs after reinnervation. These results are discussed in relation to their potential contribution to previously observed cortical plasticity and functional recovery following similar lesions.

Morphology of functional long-ranging primary afferent projections in the cat spinal cord

1. A beta-cutaneous primary afferent fibers were impaled in the dorsal columns of alpha-chloralose-anesthetized cats. Penetrations were made with the use of electrodes filled with 2 or 5% N-(2-aminoethyl) biotinamide hydrochloride (Neurobiotin, NB) in 0.1 or 1 M KCl. After determining its adequate stimulus, each fiber was activated by current pulses (18 Hz) injected via the microelectrode. The resulting cord dorsum potential (CDP) was recorded at four locations. NB was then injected into the fiber with the use of positive current pulses (11-22 nA) and a 75% duty cycle. 2. After allowing 2-8 h for diffusion, animals were perfused with saline (37 degrees C) followed by 4% paraformaldehyde (4 degrees C). Frozen 50-microns sections were cut in either the transverse or sagittal plane, processed on slides with the use of standard avidin-biotin protocols, and visualized by the nickel-enhanced diaminobenzidine (DAB) reaction. 3. A total of 15 A beta-cutaneous afferents innervating both rapidly (RA) and slowly adapting (SA) receptors were adequately stained and their central projections recovered. For selected fibers the rostrocaudal and laminar bouton distributions were determined and compared with the distribution of monosynaptic CDP amplitudes recorded at the four surface locations. 4. The rostrocaudal extent of a single A beta-afferent fiber bouton distribution visualized with NB ranged from 8 to 17.5 mm (14.4 +/- 2.4 mm, mean +/- SD), or two to three times greater than that previously shown with the use of horseradish peroxidase (HRP). 5. The strong correlation between the rostrocaudal distribution of boutons and monosynaptic CDP amplitudes, and the improved agreement between modeled and observed CDP amplitudes over that seen previously with the use of HRP (mean percent error, HRP = 23 +/- 2.9%; NB = 9 +/- 2.3%), suggest that boutons along the entire length of the visualized distribution contribute to the recorded potentials. 6. Taken together, these findings suggest that inputs from a given point on the skin can directly influence sensory information processing over a much greater rostrocaudal extent than predicted by dorsal horn somatotopic maps. These findings are discussed in terms of their implications for spinal cord plasticity.

Prenatal development of rat primary afferent fibers: I. Peripheral projections

Development of the peripheral innervation patterns of the L1-S1 lumbosacral ganglia and motor segments in embryonic day 12-17 (E12-17) rat embryos was examined using carbocyanine dyes. Individual dorsal root ganglia (DRGs) and/or isolated ventral horn (VH) segments, or individual peripheral nerves, were isolated in rat embryos fixed at different stages and filled with one of three carbocyanine dyes; DiI, DiA, and DiO. Individual experimental preparations included labeling of 1) single DRGs; 2) multiple DRGs with alternating dyes, DiO, DiI, and DiA; 3) single isolated VH segments; 4) multiple VH segments with alternating dyes; 5) single VH segments and the corresponding segmental DRGs with different dyes; and 6) two or more individual peripheral nerves labeled with different dyes. Results from these preparations have shown that the first fibers exited the lumbar ventral horn and DRGs at E12. At E13 major nerve trunks (e.g., femoral and sciatic) were visible as they exited the plexus region. By E14 afferent fibers were present in the epidermis of the proximal hindlimb, and the major nerve trunks extended into the leg. Fibers originating from L3 to L5 (DRG and VH) reached the paw by E14.5-E15, and the epidermis of the most distal toes was innervated by E16-E16.5. While afferent fibers and motor axons of the same segmental origin mixed extensively in the spinal nerve, fibers of different segmental origin combined in the plexus and major nerve trunks with little or no interfascicular mixing. Dermatomes observed at E14 were in general spotty and non-contiguous. However, by E16 the dermatomes resembled mature forms with substantial overlap only between adjacent ones. Thus the adult pattern of spatial relationships between cutaneous afferent fibers in the periphery is established early in development.

Prenatal development of rat primary afferent fibers: II. Central projections

These studies were designed to determine the pattern of initial afferent fiber ingrowth into the prenatal spinal gray matter and the establishment of the topographic organization of the presynaptic neuropil in the dorsal horn. A total of 113 lumbar dorsal root ganglia were labeled with carbocyanine fluorescent dye DiI or DiA in 67 rat embryos and neonatal pups aged embryonic day 13 to postnatal day 0 (E13-P0). The initial fiber penetration of the lumbar spinal gray began at E15 and was restricted to the segments of entry. Subsequent growth of fibers into gray matter of adjacent segments began approximately one day later, and this delay was continued, about one day for each successive segment. A second wave of ingrowth of putative small-diameter afferents into the substantia gelatinosa began at E19 and also displayed the same rostrocaudal delay. Fiber ingrowth was specific and occupied the somatotopic area appropriate for the adult, from the earliest stages (E18) in which dorsal horn laminae could be adequately defined. The somatotopic organization of the presynaptic neuropil in laminae III and IV did not change significantly throughout embryonic development as the amount of overlap between adjacent and non-adjacent ganglion projections remained constant throughout embryonic development. In addition, it was found that fibers innervating the proximal and distal hindlimb entered the spinal gray simultaneously at E15 before the innervation of the distal toes was established. The results of these studies indicate that the somatotopic organization of the presynaptic neuropil is established very early in development and requires little refinement to match that seen in the adult. The simultaneous penetration of the fibers originating from the proximal and distal areas of the limb before innervation is complete suggests that this ingrowth may be independent of the establishment of specific peripheral connections.

Properties of regenerated primary afferents and their functional connections

1. The tibial and sural nerves in cats were cut, anastomosed to their distal stumps, and allowed to regenerate for 3-17.5 mo. In the terminal acute experiment, individual afferents were impaled in the dorsal root ganglion to study their receptive field properties, somatic spike parameters, and spinal projections using cord dorsum potential (CDP) measurements. Properties of the CDPs provided evidence on whether the afferent fibers were originally proprioceptive or cutaneous (rapidly or slowly adapting). 2. Fibers with the largest conduction velocity were selectively slowly adapting, suggesting that large muscle afferents maintained their adaptation properties regardless of the peripheral structure innervated. Similarly, the relationship of somatic spike configuration to mechanical threshold was largely normal. Cells with narrow spikes innervated low-threshold mechanoreceptors, whereas cells with broad spikes and an inflection on the descending limb innervated high-threshold mechanoreceptors. 3. Spikes with intermediate properties were observed in some cells that innervated low-threshold mechanoreceptors in the periphery. These were classified as "hybrid" spikes. 4. The largest CDPs were evoked by afferents classified as having originally been cutaneous fibers, regardless of whether they had reinnervated cutaneous or subcutaneous receptors. Fibers classified as has having originally been proprioceptive afferents produced much smaller CDPs; however, these afferents never produce CDPs in intact preparations. Afferents nonresponsive to peripheral stimulation, classified putatively as having been cutaneous, also evoked small CDPs. 5. Fibers classified as putatively cutaneous or proprioceptive could reinnervate foreign target tissue (subcutaneous tissue or skin, respectively), but a propensity to reinnervate the original target tissue was observed. 6. Among putative cutaneous afferents, those with rostrocaudal CDP distributions somatotopically correct for the location of their receptive fields evoked the largest CDPs regardless of the peripheral tissue innervated. 7. We conclude that receptive field properties (adaptation, mechanical threshold) of regenerated afferents are well matched with the electrophysiological properties of the soma and axon. The properties of the central projections of these afferents are not as well matched with their peripheral receptor properties. This is discussed in terms of the plasticity of the central projections of axotomized afferents.

Central sprouting and functional plasticity of regenerated primary afferents

A combination of neuroanatomical and electrophysiological techniques was used to study the effects of peripheral axotomy and regeneration of primary afferents on their central projections in the spinal cord. Individual regenerated afferent fibers were impaled with HRP-filled electrodes in the dorsal columns of alpha-chloralose-anesthetized cats and activated by current pulses delivered via the intracellular electrode. The resulting cord dorsum potentials (CDPs) were recorded at four rostrocaudal locations and HRP was iontophoretically injected into the fiber. Central distributions of boutons and CDPs were compared with peripheral receptor type to determine the accuracy of peripheral regeneration and the effects of central-peripheral mismatches. Reconstruction of the central projections of 13 individual afferents for which the adequate stimulus and CDPs had been recorded revealed many abnormalities. For example, unlike controls, four group I and II afferents with central projections typical of proprioceptors (concentrated in laminae V, VI, and VII) innervating either cutaneous or noncutaneous targets evoked measurable CDPs. Three other group II or A beta afferents innervating low-threshold mechanoreceptors with central terminations confined to the dorsal horn exhibited extensive collateralization in laminae I and II in addition to large numbers of terminals in laminae III-IV. These fibers activated central networks whose adaptation behavior was identical to those evoked by high-threshold mechanoreceptive afferents in controls. These results suggest that primary afferents and their central connections are capable of significant modifications following axotomy and regeneration. In addition, the anatomical studies indicate some reorganization in the laminar distribution of boutons as well as in bouton size.