Biomedical subjects

R W Rhoades

Publications and source records attributed to R W Rhoades.

At least 55 records · Page 3Linked to original sources

PubMed1995

Effects of neonatal and adult enucleation on the synaptic organization of the serotoninergic projection to the superficial gray layer of the hamster's superior colliculus.

Previous experiments from this laboratory have demonstrated that either neonatal or adult enucleation results in a marked increase in the density of the serotoninergic (5-HT) projection to the superficial layers of the hamster's superior colliculus (SC). The present study was undertaken to determine whether this alteration in the density of the 5-HT projections to SC in hamsters that sustained eye removals was associated with any change in the synaptic organization of this pathway. In normal adult hamsters, only 4.0% of 500 5-HT-immunoreactive profiles made synaptic contacts in the stratum griseum superficiale (SGS). Of these, 65% were axoaxonic, 30% were axodendritic, and 5% were axosomatic. In contrast, 22% of 400 5-HT-positive profiles in the neonatally enucleated hamsters and 17.5% of 400 such profiles in hamsters that sustained eye removals in adulthood made synaptic contacts. In the neonatal enucleates, 66% of the contacts were axoaxonic, 33% were axodendritic, and 1% were axosomatic. In the adult enucleates, 73% of the contacts were axoaxonic, 26% were axodendritic, and 1% were axosomatic. Thus, both neonatal and adult enucleation increase the percentage of 5-HT-positive profiles that make synaptic contacts in SC, but neither manipulation significantly changes the distribution of these contacts among different types of postsynaptic targets.

Animals↗

PubMed1995

Fenfluramine depletes serotonin from the developing cortex and alters thalamocortical organization.

A previous experiment from our laboratory showed that neonatal destruction of cortical serotoninergic (5-HT) axons with 5,7-dihydroxytryptamine (5,7-DHT) reduced the size of the clusters of vibrissae-related thalamocortical axons. This result suggested an important role for 5-HT in thalamocortical development, but could be questioned because of potentially direct toxic effects of 5,7-DHT on thalamocortical axons. In the present study, 5-HT was depleted from the cortex using a different method, neonatal administration of +fenfluramine, and vibrissae-related patches of thalamocortical afferents were measured when animals reached 6 days of age. Fenfluramine reduced cortical 5-HT levels to 93.9 +/- 6.0% of normal (P < 0.01) and decreased the average area of vibrissae-related lamina IV patches by 23.8 +/- 4.4% (P < 0.05). Depletion of 5-HT with +fenfluramine did not significantly reduce body, brain, or cortical weight, or the overall dimensions of the somatosensory cortex. Thus, these results extend our previous studies by showing that thalamocortical organization can be altered when 5-HT is depleted without the potential for direct toxic effects on thalamic axons.

Animals↗

PubMed1995

Structure-function relationships in rat brainstem subnucleus interpolaris. XI. Effects of chronic whisker trimming from birth.

Whisker trimming from birth reduces activity and alters receptive fields (RFs) in the barrel cortex and thalamus. To assess whether or not this reflects deprivation effects on trigeminal (V) first- and second-order neurons, 59 primary afferents and 343 cells in V brainstem subnucleus interpolaris (SpVi) were studied in rats whose whiskers were trimmed daily for 6-9 weeks from birth. Deprivation did not effect brainstem somatotopy or primary afferent RFs. However, many SpVi cells had abnormal RFs and higher-order inputs, resembling the changes caused by infraorbital nerve injury. For example, in controls, only 3% of whisker-sensitive local circuit neurons responded to more than one whisker, whereas 35% of the deprived and 41% of the infraorbital nerve cut samples had multiwhisker. RFs. Deprived rats also had higher than normal incidences of cells with split or absent RFs, RFs spanning more than one V division, intermodality convergence, and directional or high-velocity sensitivity. Because these changes mimic those caused by nerve section, deprivation may underlie some nerve injury effects on V brainstem RF size and character. Insofar as cytochrome oxidase, anterograde labeling, and unit recordings revealed normal topography in deprived primary afferents and SpVi cells, RF changes in SpVi cells may reflect altered SpVi circuitry. To test this hypothesis, we assessed the morphology of 32 similarly deprived V primary afferents. In SpVi, deprived fibers had normal numbers of collaterals with normal shapes, transverse arbor areas, and topography. However, the total number of boutons per collateral was significantly reduced. Thus, deprivation effects on V higher-order RFs reflect quantitative changes in V afferent terminals.

Afferent Pathways↗

PubMed1995

Lesion-induced reorganization in the brainstem is not completely expressed in somatosensory cortex.

Electrophysiological and neuroanatomical methods were used to determine the extent to which neonatal forelimb removal altered the organization of the cuneate nucleus and representations of the fore- and hindlimbs in the primary somatosensory cortex of adult rats. Neonatal forelimb removal resulted in invasion of the cuneate nucleus by sciatic nerve primary afferents and development of cuneothalamic projection neurons with split receptive fields that included both the hindlimb and forelimb stump. Mapping in the primary somatosensory cortex of the neonatally manipulated adult rats demonstrated abnormalities, but the major change observed in the cuneate nucleus was demonstrable at only a few (5%) cortical recording sites in the remaining stump representation and there were none at all in the hindlimb representation. These results suggest that lesion-induced brainstem reorganization may be functionally suppressed at either the thalamic or cortical level.

Animals↗

PubMed1995

Patterning of local intracortical projections within the vibrissae representation of rat primary somatosensory cortex.

Anterograde and retrograde tracing with biotinylated dextran amine and Phaseolus vulgaris leukoagglutinin was used to assess projection patterns within the vibrissae representation of the rat's primary somatosensory cortex (S-I). Large and small injections of either tracer into the center of the vibrissae representation yielded dense anterograde and retrograde labelling throughout much of the tangential extent of the vibrissae representation within S-I. In all layers, the pattern and extent of retrograde and anterograde label was in rough congruence. The organization of this labelling varied across cortical layers. In layers II and III, labelled fibers extended away from injection sites in all directions and yielded a uniform pattern, which decreased in density with increasing distance from the tracer injection. There was a tendency for labelling to be more extensive along the representation of the row of vibrissae follicles that included the injection site than across rows. There was also a tendency for anterograde labelling to be more extensive in the direction of the representation of follicles more rostral on the face than that injected. In lamina IV, both labelled fibers and cells were restricted for the most part to the septa regions between the barrels. However, a small number of retrogradely labelled neurons were also located in the barrels (approximately one-ninth of the number found in the septa). The pattern observed in laminae II-III was repeated in layers V and VI. In these laminae, there was no evidence of a pattern of intracortical connections related to the vibrissae representation in overlying lamina IV.

Animals↗

PubMed1995

Neonatal infraorbital nerve transection and blockade of axoplasmic transport reduce expression of acetylcholinesterase by thalamocortical axons.

Acetylcholinesterase (AChE) is transiently expressed by ventrobasal thalamic neurons and their axons in the primary somatosensory cortex and this enzyme has been used as a marker for these axons in studies concerned with cortical development and plasticity. The present experiment evaluated the effects of both peripheral nerve transection and blockade of axoplasmic transport upon the expression of AChE in thalamic axon terminals in the primary somatosensory cortex. Both manipulations resulted in marked reductions in the density of AChE in thalamocortical axons and less dramatic decreases in the density of this enzyme in thalamic neurons. These results indicate that AChE may not invariably provide unequivocal information about the distribution of somatosensory thalamocortical axon terminals in developing rodents.

Acetylcholinesterase↗

PubMed1995

Birth dates and survival after axotomy of neurochemically defined subsets of trigeminal ganglion cells.

Trigeminal (V) ganglion cells with different neurochemical phenotypes or different birth dates are affected differently by neonatal axonal transection. The aim of the present study was to determine if V ganglion cell birth date and neurochemical phenotype were correlated and if these two variables could be related to responses to neonatal axonal transection. Immunocytochemistry, histochemistry, and [3H]thymidine labelling were used to determine the birth dates of V ganglion cells recognized by antibodies directed against neurofilament protein (NF), calcitonin gene-related peptide (CGRP), and substance P (SP) and those that bound the lectin Bandierea simplicifolia-I (BS-I). All V ganglion cells were born between embryonic days (E-) 9.5 and 14.5. All ganglion cells were born between E-9.5 and E-14.5. In a normalized population (percentages normalized to equal 100%), over 90% of NF-positive V ganglion cells were born between E-10.5 and E-12.5. The majority of CGRP-positive and SP-positive ganglion cells (> 90%) were generated from E-13.5 to E-14.5 and E-12.5 through E-14.5, respectively. Almost 85% of BS-I-positive ganglion cells were generated on E-12.5 through E-14.5. Previous results and additional data from this study indicated that NF- and BS-I-positive ganglion cells are proportionally more likely to be lost after neonatal axotomy and that SP-positive cells are more likely to remain. The percentage of CGRP-positive cells in the V ganglion was not significantly altered by neonatal infraorbital nerve transection. Overall, these findings do not indicate a strong relationship between cell birth date and the probability of survival after neonatal axonal damage for all V ganglion cell phenotypes.

Animals↗

PubMed1995

The formation of a cortical somatotopic map.

The primary somatosensory cortex of small rodents is an isomorphic representation of the body surface. Similar representations are characteristic of the subcortical pathways, leading from the periphery to the cortex, and these representations develop in a sequence that begins at the periphery, and that ends in the cortex. Furthermore, central representations at all levels of the neural axis are altered by perinatal perturbations of the peripheral surface. This has led to the hypothesis that the periphery plays an instructional role in the formation of central neuronal structures. The morphology of this discrete organization has been examined thoroughly during the development of the thalamocortical projections. The mechanism(s) that underlies the formation of these representations remains unclear although some recent evidence suggests the involvement of activity-dependent processes that are modulated by 5-HT.

Animals↗

PubMed1995

Long-term age-related consequences of forelimb damage upon expression of primary afferent phenotypes in the cervical dorsal horn.

Rats that sustained forelimb removal on either embryonic day 16 (E-16) or the day of birth (P-0), or transection of the brachial plexus in adulthood, had sections through the cervical dorsal horn stained for galanin, calcitonin gene-related peptide (CGRP), or the plant lectin Bandieria simplicifolia-I (BS-I) 35-50 days after these lesions. The results of these experiments demonstrated age-related differences in the effects of peripheral nerve damage upon the distributions of each of these three primary afferent markers in the dorsal horn. Damage to the brachial plexus in adulthood caused a significant increase in the density of galanin immunoreactivity in the medial portion of layers I and II and the appearance of galanin immunoreactivity in layers III and IV of the cervical dorsal horn. Such lesions resulted in significant reductions in the density of CGRP immunoreactivity in layers I and II and of BS-I binding in lamina II. Forelimb removal on the day of birth resulted in no significant change in the density of galanin immunoreactivity in layers I and II, but in the appearance of galanin-immunoreactive fibers in layers III-V. Neonatal forelimb removal resulted in no significant change in the density of CGRP immunoreactivity in layers I and II, but in a significant reduction in the density of BS-I binding in the medial portion of lamina II. Removal of the forelimb on E-16 caused a significant increase in the density of galanin immunoreactivity in layers III-V, but had no significant effect on the density or distribution of either CGRP immunoreactivity or BS-I binding in the cervical dorsal horn. These results suggest that peripheral nerve damage at all ages may cause an up-regulation of galanin in a wider distribution of ganglion cell types than was previously thought to be the case, and that there are different sensitive periods for lesion-induced, long-term changes in the innervation of the dorsal horn by CGRP- and BS-I-positive primary afferent axons.

Afferent Pathways↗

PubMed1994

Evidence for survival of the central arbors of trigeminal primary afferents after peripheral neonatal axotomy: experiments with galanin immunocytochemistry and Di-I labelling.

Studies employing axoplasmic transport techniques have suggested that the central arbors of vibrissae-related primary afferents are rapidly and permanently lost from the trigeminal (V) brainstem complex after transection of the intraorbital nerve (ION). The present study reexamined this issue using immunocytochemistry for galanin (GAL) and anterograde labelling with Di-I to evaluate V brainstem organization in rats that sustained damage to the ION or individual vibrissae follicles in infancy or adulthood. After adult nerve damage, GAL-positive fibers are increased in layers I and II of V subnucleus caudalis (SpC). This was apparent by 3 days after the lesion. In rats that sustained nerve damage at birth (P0), GAL immunoreactivity (IR) appeared throughout the V brainstem complex and had a patchy distribution similar to that of vibrissae-related V primary afferents in normal rats. Increased GAL-IR in rostral portions of the V brainstem complex was observed in rats that sustained ION damage as late as P14. Additional experiments in which nerve damage was followed by destruction of the V ganglion demonstrated that this GAL-IR was contained in primary afferents. Damage to single vibrissa follicles or to a row of follicles produced a single patch or row of GAL-IR terminals in the somatotopically appropriate portion of the ipsilateral V brainstem complex. Di-I labelling in neonatally nerve-damaged rats demonstrated that primary afferent axons filled the central territory normally innervated by this nerve and that their terminal distribution was patchy. These results suggest that the V ganglion cells that survive neonatal axotomy may retain somatotopically organized projections to the V brainstem complex for at least a limited postnatal period.

Afferent Pathways↗

PubMed1994

Patterning of the neocortical projections from the raphe nuclei in perinatal rats: investigation of potential organizational mechanisms.

Serotoninergic (5-HT) fibers in the cerebral cortex of perinatal rats have a pattern that coincides with the boundaries of primary sensory areas and within the primary somatosensory cortex form the rattunculus. This patterned immunoreactivity (IR) appears about 60 hours after birth and disappears between postnatal days (P-) 12 and 15. Three experiments were carried out to evaluate mechanisms that might underlie the precise patterning of the 5-HT-IR. Retrograde labelling with fluorescent tracers in perinatal rats revealed only a coarse rostrocaudal topography in the raphe-cortical projection and the existence of raphe cells projecting to multiple cortical locations. Thus, a precise point-to-point, raphe-cortical projection does not underlie the patterned cortical 5-HT-IR. Ablation of the thalamus prior to the age at which patterned 5-HT-IR could be seen in the developing cortex caused a complete loss of patterned immunoreactivity. This suggests that 5-HT fibers may require the presence of thalamocortical axons to achieve the pattern observed in normal animals. Serotoninergic raphe neurons transplanted to the cortices of newborn rats exhibited extensive axonal outgrowth, but did not form a somatotopic pattern. This result also suggests that specific spatiotemporal interactions between growing 5-HT and thalamocortical axons may be necessary for the somatotopic patterning of the former fibers.

Aging↗

PubMed1994

Evidence for prenatal competition among the central arbors of trigeminal primary afferent neurons: single axon analysis.

Previous studies from this laboratory have demonstrated that prenatal damage to vibrissae follicles results in significant increases in the brainstem representations of the remaining vibrissae as demonstrated by staining for the mitochondrial enzyme cytochrome oxidase (CO). Because CO is primarily a postsynaptic marker, these results do not directly address the question of whether there were changes in the projections of primary afferent fibers. To address this issue, we made intra-axonal recordings from individual vibrissa-related primary afferents in rats that sustained damage to vibrissae follicles on embryonic day 17, and then injected horseradish peroxidase (HRP) into these axons to visualize their terminal arbors in the brainstem at the level of trigeminal subnucleus interpolaris (SpI). All vibrissae-related primary afferents responded to deflection of one and only one vibrissa, and the terminal arbors of axons (N = 47) recovered from animals that sustained fetal peripheral lesions were significantly larger than those (N = 23) from normal rats. Fibers from fetally damaged animals had increased total fiber lengths and numbers of branch points. These results indicate that reduced competition among primary afferent axons results in increases in the terminal arbors that remain. These increases occur without any significant alteration in their peripheral receptive fields.

Animals↗

PubMed1994

Effects of postnatal blockage of cortical activity with tetrodotoxin upon lesion-induced reorganization of vibrissae-related patterns in the somatosensory cortex of rat.

Previous studies have shown that postnatal blockade of thalamocortical activity with either tetrodotoxin (TTX) or the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (APV) does not prevent the formation of vibrissae-related patterns. In the present study, blockade of cortical activity with TTX was combined with ablation of a row of vibrissae follicles or transection of the infraorbital nerve (ION, the trigeminal nerve branch that supplies the vibrissae follicles) to determine whether the cortical reorganization that follows these lesions in otherwise untreated animals was dependent upon neuronal activity that could be blocked with TTX. The results demonstrated that cortical TTX implants had no quantitative or qualitative effects upon the cortical reorganization that followed either vibrissae follicle cauterization or ION transection.

2-Amino-5-phosphonovalerate↗

PubMed1994

Ultrastructural organization of the noradrenergic innervation of the superficial gray layer of the hamster's superior colliculus.

Immunocytochemistry with an antibody-directed tyrosine hydroxylase (TH) was combined with electron microscopy and serial-section analysis to examine the synaptic organization of the catecholaminergic projection to the stratum griseum superficiale (SGS) of the hamster's superior colliculus (SC). A total of 250 TH-immunoreactive profiles within SGS were examined. Of these, 114 (45.6%) made synaptic contacts; 81 (71.1%) were axodendritic, and the remainder (33, 28.9%) were axo-axonic. Serial-section analysis was employed to evaluate the presence or absence of synaptic contacts for 26 profiles. Overall, 19 (73.1%) of the profiles followed through serial sections exhibited synaptic contacts. Double staining of single sections with antibodies directed against TH and dopamine-beta-hydroxylase (D beta H) and examination in the light microscope indicated that virtually all TH-positive fibers also contained D beta H. This indicated that the fibers examined at the electron microscopic level were noradrenergic rather than dopaminergic. These results suggest that norepinephrine may have both pre- and postsynaptic actions in the hamster's SC and that at least some of these effects are mediated by conventional synapses.

Animals↗

PubMed1994

Modulation of retinotectal transmission by presynaptic 5-HT1B receptors in the superior colliculus of the adult hamster.

1. Radioligand binding with [125I]-cyanopindolol in the presence of isoproterenol was used to define the distribution of 5-HT1B receptors in the superior colliculus (SC) of adult hamsters. There was a high density of these receptors in the stratum griseum superficiale (SGS), and they were much less dense in other SC laminae. Enucleation of one eye produced a marked reduction in the density of these receptors in the contralateral SGS, suggesting that they are located primarily on retinotectal axon terminals. 2. Intracellular recording techniques were used to evaluate the effects of serotonin (5-HT) on the excitatory postsynaptic potentials (EPSPs) evoked in SC cells of adult hamsters by stimulation of the optic tract (OT) in vitro. Application of 5-HT produced a reduction of > or = 50% in OT-evoked EPSPs in 79% of the 67 cells tested. The average EPSP amplitude was 7.8 +/- 2.1 (SD) mV under control conditions and 2.7 +/- 1.9 mV in the presence of 5-HT (P < 0.01). For most of these neurons, application of 5-HT had little effect on their membrane potential or input resistance. The average percent change in membrane potential for cells tested with 5-HT was 0.5 +/- 6.0% and the average percent change in input resistance was 0.6 +/- 22.9%. 3. For four of six cells tested, application of 5-HT had no significant effects on the responses evoked by application of glutamate, either under normal bathing conditions or when the medium included low Ca2+ and high Mg2+. 4. Pharmacologic experiments indicated that the effects of 5-HT on retinotectal transmission were mimicked by the 5-HT1B agonists 1-[3-(trifluoromethyl)phenyl]-piperazine and 7-trifluoromethyl-4(4-methyl-1-piperazinyl) [1,2-a]-quinoxaline maleate and antagonized by the 5-HT1A/1B antagonists (-)-pindolol and methiothepin. The effects of 5-HT on the OT-evoked EPSP were not antagonized by either spiperone, ketanserin, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]-piperazine HBr, or [1-H-3 alpha-5 alpha-tropan-3-yl]-3,5-dichlorobenzoate. 5. Both the anatomic and physiological results are consistent with the conclusion that 5-HT presynaptically inhibits retinotectal transmission and that this effect is mediated by the 5-HT1B receptor.

Animals↗

PubMed1994

Effects of 5-HT on thalamocortical synaptic transmission in the developing rat.

1. Recent immunocytochemical and receptor binding data have demonstrated a transient somatotopic patterning of serotonin (5-HT)-immunoreactive fibers in the primary somatosensory cortex of developing rats and a transient expression of 5-HT1B receptors on thalamocortical axons from the ventral posteromedial thalamic nucleus (VPM). 2. These results suggest that 5-HT should strongly modulate thalamocortical synaptic transmission for a limited time during postnatal development. This hypothesis was tested in intracellular recording experiments carried out in thalamocortical slice preparations that included VPM, the thalamic radiations, and the primary somatosensory cortex. Effects of 5-HT and analogues were monitored on membrane potentials and input resistances of cortical neurons and on the amplitude of the synaptic potentials evoked in them by stimulation of VPM. 3. Results obtained from cortical neurons in slices taken from rats during the first 2 wk of life indicated that 5-HT strongly inhibited the VPM-evoked excitatory postsynaptic potential (EPSP) recorded from cortical neurons in a dose-dependent manner. In contrast, 5-HT had no significant effects on membrane potential, input resistance, or depolarizations induced by direct application of glutamic acid to cortical cells. 4. The effects of 5-HT were mimicked by the 5-HT1B receptor agonists 1-[3-(trifluoromethyl)phenyl]-piperazine (TFMPP) and 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]-quinoxaline maleate and antagonized by the 5-HT1B receptor antagonist (-)-pindolol. The 5-HT1A agonist [(+/-)8-hydroxydipropylaminotetralin HBr] (8-OH-DPAT) had less effect on the VPM-elicited EPSP, and the effects of 5-HT upon this response were generally not antagonized by either 1-(2-methoxyphenyl)-4-[4-(2- phthalimmido)butyl]piperazine HBr (a 5-HT1A antagonist) or ketanserine (a 5-HT2 antagonist) or spiperone (a 5-HT1A and 2 antagonist). 5. The ability of 5-HT to inhibit the VPM-evoked EPSP in cortical neurons was significantly reduced in slices from animals > 2 wk of age. The effectiveness of TFMPP in such animals was even more attenuated than that of 5-HT, and the effectiveness of 8-OH-DPAT was unchanged with age. These results are consistent with the disappearance of 5-HT1B receptors from thalamocortical axons after the second postnatal week and the maintenance of 5-HT1A receptors on some neurons. 6. All of the results obtained in this study are consistent with the conclusion that 5-HT has a profound, but developmentally transient, presynaptic inhibitory effect upon thalamocortical transmission in the rat's somatosensory cortex.

Afferent Pathways↗

PubMed1994

Alterations in brainstem and cortical organization of rats sustaining prenatal vibrissa follicle lesions.

Vibrissa follicles were cauterized in late fetal or newborn rats to determine whether the relationships between brainstem and cortical changes observed after neonatal peripheral damage would also be obtained when vibrissa follicles were cauterized earlier in development. Vibrissa follicles were cauterized between embryonic day 15 (E-15) and the day of birth (P-0). The vibrissa-related representation in the brainstem was examined with cytochrome oxidase histochemistry, and that in the cortex was evaluated with either serotonin immunocytochemistry or anterograde labeling with Di-I when animals reached 6-8 days of age. There was a significant relationship between the ages at which lesions were carried out and the extent to which the representations of undamaged vibrissa follicles were altered in the brainstem and cortex. Peripheral lesions carried out between E-15 and E-18 resulted in significant increases in the cross-sectional areas of the patches corresponding to the undamaged vibrissa follicles in both the brainstem and cortex. Lesions at later ages resulted in significant increases only in the cortex. In some animals that sustained peripheral damage on E-20 and all of those that received lesions on P-0, there were aggregates of labeling in cortex that had no counterpart in the brainstem. Prenatal, but not postnatal, vibrissa follicle damage also reduced the overall dimensions of the cortical region devoted to the representation of these receptor organelles. Finally, there was a strong negative correlation between the magnitude of peripheral lesions (i.e., the number of vibrissa follicles ablated) and the extent of the reorganization in the brainstem and cortex.

Afferent Pathways↗

PubMed1994

Effects of postnatal blockade of cortical activity with tetrodotoxin upon the development and plasticity of vibrissa-related patterns in the somatosensory cortex of hamsters.

Several previous studies have shown that postnatal blockade of thalamocortical activity with either tetrodotoxin (TTX) or the N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonovalerate (APV) does not prevent the formation of vibrissa-related patterns in the primary somatosensory cortex of rats. One limitation of these studies is that this pattern forms very shortly after birth in rats, and there may be only a very limited time over which it may be influenced by activity blockade. In the present study, the effect of activity blockade was evaluated in a more altricial rodent, the hamster. The present study showed that a pattern of thalamocortical afferents corresponding to the vibrissae is not observed until the fourth postnatal day in hamsters. Nevertheless, application of TTX-impregnated implants to the cortices of newborn hamsters had no qualitative or quantitative effect upon vibrissa-related patterns in the primary somatosensory cortices of these animals. Moreover, TTX implants did not prevent the changes in patterns that followed cauterization of a row of vibrissa follicles.

Afferent Pathways↗