Gustatory cortex in the rat. II. Thalamocortical projections

The thalamic relay for lingual tactile, thermal, and gustatory sensibility was defined electrophysiologically in the rat. Subsequently, injections of tritiated leucine were centered in these functionally defined locations in separate series of rats. Following suitable survival periods, the brains were processed for autoradiographic tracing of axonal projections. After injections confined to the thalamic gustatory relay, labeled fibers terminated in agranular insular cortex. These results provide support for our previous experiments correlating neurophysiological localization of rat gustatory cortex and regional cytoarchitecture, and contrast with the traditional assignation of gustatory cortex to the granular insular area.     

Gradual changes in the structure of the barrels during maturation of the primary somatosensory cortex in the rat

The maturation of the barrel field in the primary somatosensory cortex was observed in Nissl-stained preparations from rats ranging in age from 12 days to 1.5 years postpartum. Prior to the 20th day, the barrels in the rat resemble those of the mouse and have distinct cell-sparse hollows that are surrounded by cell-dense sides. They span the full thickness of layer IV. Between the 20th and 34th days, the barrels in only the posteromedial part of the barrel field gradually change and the distinction between the hollows and sides is lost throughout all but the deepest part of layer IV. The resulting mature barrels are relatively indistinct and have a uniformly high cell density that extends well into the supragranular layers. In contrast, the barrels in the anterolateral part of the barrel field remain essentially unchanged. The remodeling apparently is not due passively to cortical growth because, by P20, the thicknesses of the cortical layers and the dimensions of the barrels are virtually the same as in the adult. Several mechanisms are considered that may account for the changes. These include a redistribution of the neurons that originally were in barrel sides; a reduction in the neuropil between the neurons that originally were within hollows; and differential growth of layer IV dendrites. The changes in the barrel structure may be related to the differentiation and quantity of innervation in the hairy skin between the vibrissae.     

Thalamocortical connections in newborn mice

Thalamocortical axons reach the developing neocortex and become distributed within the cortical subplate during the third week of gestation. The present study is an analysis of the organization of connections that link thalamus and cortical subplate (corresponding to future layers V and VI) at birth. This age antedates the ascent of thalamic axons to contact cells of the supragranular layers, their principal targets in the adult cortex. At birth thalamic nuclear subdivisions are explicit; field-characteristic cytoarchitectonic features, relating principally to the infragranular layers, delineate the majority of neocortical fields. The projection of principal relay nuclei upon the majority of fields of the cerebral convexity has been mapped by means of retrograde transport of HRP. Nucleus-to-field interrelationships as well as topologic order of the overall thalamic projection prove to be identical to that in the adult animal. The neonatal projection appears to be somewhat more divergent than that of the adult.     

Electrophysiological study of the topography of the vibrissae projections to the tactile thalamus and cerebral cortex in mutant mice with hair defects

Projections of the mystacial vibrissae to the tactile thalamus and somatosensory neocortex were studied in mutant mice with hair defects, of the Mottled ($\text{Mo}\text{Mo}$), Hairless ($\text{hr}\text{hr}$), and Nude ($\text{Nu}\text{Nu}$) types. Results show that Mottled mice have projections of the same kind as normal mice. In Hairless mice there are silent zones between the projections of intact vibrissae. In Nude mice which lack hairs since birth, the most important change is that the vibrissae project to regions of the thalamus and the SI cerebral cortex ordinarily innervated from the common fur of the muzzle. Thus Nude mice seem to compensate for the absence of hairs of the common fur. We hypothesize that this change represents the counterpart of changes observed in normal mice whose vibrissae follicles were destroyed at birth. Nude mice lack hairs since birth and their vibrissae project to the fur regions. In normal mice whose vibrissae were destroyed at birth, common hairs of the mystacial pad project to the vibrissa region.     

Thalamocortical connections of the rostral intralaminar nuclei: an autoradiographic analysis in the cat

In this study the pattern of projections from the rostral intralaminar thalamic nuclei to the cerebral cortex was examined in the cat by autoradiography. Injections of tritiated proline and leucine were placed into the central lateral, paracentral, central medial, and para-stria medullaris nuclei. After injections into the central lateral nucleus, label is present on the lateral side within the presylvian sulcus, in most of the suprasylvian gyrus, including the adjacent lateral and suprasylvian sulci, and in the posterior corner of the ectosylvian gyrus. On the medial side, label is present in the orbitofrontal (Of), precentral agranular (Prag), anterior limbic (La), retrosplenial (Rs), and postsubicular (Ps) areas, as defined by Rose and Woolsey ('48a). The cingulate gyrus also contains label throughout (part of which was defined as the "cingular area," Cg, by Rose and Woolsey, '48a). Label is also found on both banks of the splenial and cruciate sulci. In addition, label is present within the lateral gyrus, on both its lateral and medial sides. The paracentral projections are similar to the central lateral input. On the lateral side, label is found within the presylvian sulcus, suprasylvian gyrus and adjacent lateral and suprasylvian sulci, and posterior ectosylvian gyrus. Medially, label is present in the Of, Prag, La, Cg, Rs, and Ps areas, and within the cruciate and splenial sulci, and in portions of the lateral gyrus. Following injections of the central medial nucleus, label is present in the presylvian sulcus; but in contrast to the central lateral and paracentral projections, the suprasylvian gyrus is labeled only in its posterior part. The central medial nucleus also projects to the posterior lateral gyrus, both laterally and medially. Also, the central medial nucleus projects heavily to rostral cortical zones, which include the Of, Prag and La areas, cruciate sulcus, and the rostral cingulate gyrus. The para-stria medullaris nucleus projects only to the presylvian sulcus and orbitofrontal cortex laterally, but, medially, has an extensive input similar to the central lateral and paracentral projections in that label is present in the Of, Prag, La, Cg, Rs, and Ps areas, in the cruciate and splenial sulci, and in the posterior lateral gyrus. The laminar distribution of label is as follows: the central lateral, paracentral and para-stria medullaris nuclei project primarily to layers I and III, whereas the central medial nucleus projects to layers I and VI. In addition, the central lateral projection has a patchy appearance in the retrosplenial and postsubicular cortices.     

Thalamocortical and thalamo-amygdaloid projections from the parvicellular division of the posteromedial ventral nucleus in the cat

Projections from the parvicellular division of the posteromedial ventral thalamic nucleus (VPMpc) of the cat were examined. After injection of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) into the VPMpc, both anterogradely labeled axon terminals and retrogradely labeled neuronal cell bodies were found ipsilaterally in three discrete regions of the cerebral cortex, i.e., in the orbital cortex, caudoventral part of the infralimbic cortex, and medial part of the fundus of the posterior rhinal sulcus (perirhinal area); in the subcortical regions, anterogradely labeled axon terminals were seen ipsilaterally in the rostrodorsal part of the lateral amygdaloid nucleus. Neuronal connections between these VPMpc-recipient regions were further verified by injecting WGA-HRP into each of the three cortical and the lateral amygdaloid regions. After injection of WGA-HRP into each of the three cortical regions, labeled neuronal cell bodies and axon terminals were seen ipsilaterally in the VPMpc, especially in its medial part, and in the other two of the three VPMpc-recipient cortical regions. In the rostrodorsal part of the lateral amygdaloid nucleus, both axon terminals and neuronal cell bodies were labeled after WGA-HRP injection into the perirhinal area, and only axon terminals were labeled after WGA-HRP injection into the orbital cortex, but no labeling was observed after WGA-HRP injection into the infralimbic cortex. After injection of WGA-HRP into the rostrodorsal portion of the lateral amygdaloid nucleus, both axon terminals and neuronal cell bodies were labeled ipsilaterally in the perirhinal area and the ectorhinal area, and only neuronal cell bodies were labeled ipsilaterally in the VPMpc (especially in its medial part) and orbital cortical region; no labeling was observed in the infralimbic cortex. The present results indicate that the VPMpc of the cat is connected reciprocally with the orbital, infralimbic, and perirhinal cortical regions on the ipsilateral side, that the three VPMpc-recipient cortical regions are reciprocally connected with each other, that the VPMpc sends fibers ipsilaterally to the rostrodorsal part of the lateral amygdaloid nucleus, which may relay information from the VPMpc to the perirhinal cortical area, and that the VPMpc-recipient area in the lateral amygdaloid nucleus receives cortical fibers from the orbital and perirhinal cortical regions.     

Transient retinal axon collaterals to visual and somatosensory thalamus in neonatal hamsters

We have studied the postnatal development of individual axons in the optic tract and thalamus of the Syrian hamster, concentrating attention on retinal ganglion cell axons that make a transient projection to the main somatosensory nucleus, the ventrobasal complex. We bulk-filled axons with horseradish peroxidase in hemithalami maintained en bloc, in vitro. After processing and reaction with diaminobenzidine, we reconstructed individual axons from serial sections. In hamsters and other rodents, the optic tract is composed of superficial and internal components, either or both being possible sources of the retino-ventrobasal projection. Both project to the midbrain, but in normal adults only the superficial optic tract maintains collaterals in the thalamus. We found that the axons of the internal component bear numerous transient thalamic collaterals on postnatal days 0, 1, and 2, and some of these extend into the ventrobasal complex. Axons in the superficial optic tract also bear collaterals on days 0 to 2, but these are confined to the superficial half of the dorsal lateral geniculate nucleus. Thus the transient retino-ventrobasal projection comprises solely transient collaterals originating from axon trunks in the internal optic tract. On days 1 and 2, some collaterals from the superficial optic tract appear to have begun to arborize in the lateral geniculate nucleus. In contrast, collaterals from internal optic tract axons to the ventrobasal complex branch little if at all as they traverse the lateral geniculate nucleus, and at no time prior to their elimination do they develop an appreciable terminal arbor. These long collaterals often terminate in growth cones that include lamellopodia. Our HRP-impregnation method also revealed some transient non-retinofugal axons that pass medially from the ventral lateral geniculate nucleus to the ventrobasal complex but then return without terminating or branching. By day 4, they are absent, as are collaterals from the internal optic tract to the ventrobasal complex.     

Protracted maturation of forebrain afferent connections of the ventral tegmental area in the rat

The mesocorticolimbic dopamine system has long attracted the interest of researchers concerned with the unique gamut of behavioral and mental health vulnerabilities associated with adolescence. Accordingly, the development of the mesocorticolimbic system has been studied extensively, but almost exclusively with regard to dopaminergic output, particularly in the nucleus accumbens and medial prefrontal cortex. To the contrary, the ontogeny of inputs to the ventral tegmental area (VTA), the source of mesocorticolimbic dopamine, has been neglected. This is not a trivial oversight, as the activity of VTA neurons, which reflects their capacity to transmit information about salient events, is sensitively modulated by inputs. Here, we assessed the development of VTA afferent connections using the β subunit of cholera toxin (Ctβ) as a retrograde axonal tracer in adolescent (postnatal day 39) and early adult (8–9‐week‐old) rats. After intra‐VTA injections of Ctβ, adolescent and early adult animals exhibited qualitatively similar distributions of retrogradely labeled neurons in the sense that VTA‐projecting neurons were present at all of the same rostrocaudal levels in all of the same structures in both age groups. However, quantitation of retrogradely labeled neurons revealed that adolescent brains, compared with early adult brains, had significantly fewer VTA‐projecting neurons preferentially within an interconnected network of cortical and striatopallidal forebrain structures. These findings provide a novel perspective on the development of the mesocorticolimbic dopamine system and may have important implications for age‐dependent specificity in the function of this system, particularly with regard to adolescent impulsivity and mental health vulnerabilities. J. Comp. Neurol. 522:1031–1047, 2014. © 2013 Wiley Periodicals, Inc.     

Thalamic connections of three representations of the body surface in somatosensory cortex of gray squirrels

The anatomical tracer, wheat germ agglutinin, was used to determine the connections of electrophysiologically identified locations in three architectonically distinct representations of the body surface in the somatosensory cortex of gray squirrels. Injections in the first somatosensory area, S-I, revealed reciprocal connections with the ventroposterior nucleus (VP), a portion of the thalamus just dorsomedial to VP, the posterior medial nucleus, Pom, and sometimes the ventroposterior inferior nucleus (VPI). As expected, injections in the representation of the face in S-I resulted in label in ventroposterior medial (VPM), the medial subnucleus of VP, whereas injections in the representation of the body labeled ventroposterior lateral (VPL), the lateral subnucleus of VP. Furthermore, there was evidence from connections that the caudal face and head are represented dorsolaterally in VPM, and the forelimb is represented centrally and medially in VPL. The results also support the conclusion that a representation paralleling that in VP exists in Pom, so that the ventrolateral part of Pom represents the face and the dorsomedial part of Pom is devoted to the body. Because connections with VPI were not consistently revealed, the possibility exists that only some parts or functional modules of S-I are interconnected with VPI. Two separate small representations of the body surface adjoin the caudoventral border of S-I. Both resemble the second somatosensory area, S-II, enough to be identified as S-II in the absence of evidence for the other. We term the more dorsal of the two fields S-II because it was previously defined as S-II in squirrels (Nelson et al., '79), and because it more closely resembles the S-II identified in most other mammals. We refer to the other field as the parietal ventral area, PV (Krubitzer et al, '86). Injections in S-II revealed reciprocal connections with VP, Pom, and a thalamic region lateral and caudal to Pom and dorsal to VP, the posterior lateral nucleus, Pol. Whereas major interconnections between S-II and VPI have been reported for cats, raccoons, and monkeys, no such interconnections were found for S-II in squirrels. The parietal ventral area, PV, was found to have prominent reciprocal interconnections with VP, VPI, and the internal (magnocellular) division of the medial geniculate complex (MGi). The pattern of connections conforms to the established somatotopic organization of VP and suggests a crude parallel somatotopic organization in VPI. Less prominent interconnections were with Pol.(ABSTRACT TRUNCATED AT 400 WORDS)     

Vibrissal stimulation affects glucose utilization in the trigeminal/somatosensory system of normal rats and rats prenatally exposed to ethanol

The effect of gestational ethanol exposure on stimulus-induced sensory activity in the trigeminal/somatosensory System was determined. The mature offspring of mothers fed an ethanol-containing diet (Et) or pair-fed a nutritionally matched control diet (Ct) were examined. The C-row mystacial whiskers were stimulated. Glucose utilization in the principal sensory nucleus of the trigeminal nerve (PSN), ventrobasal thalamus, and somatosensory cortex was determined with [¹⁴C]2-deoxyglucose autoradiography. In Ct- and Et-treated rats, whisker stimulation increased glucose utilization in C-row barrel(oid)s in the left PSN, the right ventrobasal thalamus, and the right somatosensory cortex. The rate of glucose utilization in the C-row barrel(oid)s and in nonstimulated regions was lower in the Et-treated rats than in controls. In the cortices of Ct-treated rats, the activity in the C-row barrels on the right side was greater than in the right nonbarrel somatosensory cortex. Et-treated rats also exhibited an increase in glucose utilization, albeit smaller than that in the Ct-treated rats. In contrast, the glucose utilization in the left B- and C-row barrels of Ct-treated rats was decreased. No such decrease was evident in the left cortices of Et-treated rats. Thus, stroking whiskers stimulates the activity of sites in the trigeminal/somatosensory system. In cortex, the definition of these sites is emphasized by depressed activity, i.e., “surround” inhibition, in sites connected via callosal or corticocortical projections. Prenatal exposure to ethanol depresses the metabolic activity regardless of the physiological state; however, the “surround” inhibition of cortical activity is eliminated by prenatal exposure to ethanol through an exuberant projection. © 1993 Wiley-Liss, Inc.     

The differentiation of the olfactory placode in Xenopus laevis: a light and electron microscope study

Corticothalamic projections from postcruciate area 4, located on the rostral part of the posterior sigmoid gyms, were traced with the autora-diographic technique in the dog. Injections of tritiated amino acids were made into the lateral and medial parts of area 4 in regions corresponding to the forelimb and hindlimb areas of the primary motor cortex, respectively. In cases with injections placed in the lateral part of areas, dense accu-mulations of label were present in the lateral part of the ventral anterior nucleus (VA), the central part of the ventral lateral nucleus (VL), the ventral half of the ventral posterior inferior nucleus (VPI), the caudal part of the central lateral nucleus (CL), and the centrum medianum (CM). Lighter label was also present in the lateral part of the cytoarchitectonically distinct VL region bordering the ventrobasal complex (VB), as well as in the ventro-lateral part of the mediodorsal nucleus (MD), and in the lateral posterior nucleus (LP). In one case in which the injection site involved an adjacent part of area 3a, label was also seen ventrally in the medial division of the posterior nuclear group (POm). However, no detectable differences in VL, MD, or intralaminar labeling patterns were noted between this case and the four other cases with injections confined to the lateral part of area 4. In two cases with injections restricted to the medial part of area 4, dense label was present in the lateralmost part of VL, the ventral part of VPI, the caudal part of CL, and CM. Lighter label was also present in the VL region bordering the dorsolateral edge of VB and in LP. An additional case in which the injection also involved the rostral border of area 3a showed a similar pattern cf thalamic labeling. Projections from both the lateral and medial parts of area 4 were also noted in the subthalamic nucleus, zona incerta, and nucleus of Darkschewitsch. These results suggest that Corticothalamic projections from postcruciate area 4 to VL are organized topographically such that projections from the lateral part of area 4 project centrally within VL while those from the medial part of area 4 project more laterally. Both parts of area 4 also project top-ographically to a cytoarchitectonically distinct region of VL located im-mediately adjacent to VB, In contrast, the projections to the intralaminar nuclei do not appear to be topographically organized. The data from cases involving spread of the injection into area 3a suggest that projection pat-terns from area 3a to ventral, intralaminar, and medial thalamic nuclei are similar to those from area 4. However, it appears that at least the lateral part of area 3a also projects to POm.     

Inhibition of thalamic ventrobasal complex neurons by glutamate infusion into the thalamic reticular nucleus in rats

In urethane-anesthetized rats a 0.36-mm metallic cannula for infusion was positioned in the somatosensory component of the thalamic reticular nucleus (sTR), where movement of the vibrissae evoked neuronal discharge. Infusion there of 0.125-0.5 microliter of a 50 mM solution of glutamate over a 1-min period suppressed both spontaneous and evoked discharge of neurons in the ventrobasal complex (VB), but only for those which also responded to vibrissal stimulation. VB neurons activated by somatosensory stimuli at other locations were unaffected. Thus, excitation of neurons in sTR inhibits those in VB, but the effect appears to be highly coordinated somatotopically.     

FMR1基因敲除鼠感觉皮质功能柱树突棘修剪发育的研究

目的研究脆性X智力低下蛋白（FMRP）缺失对感觉皮质功能柱神经可塑性的影响。方法从纯合子鼠血液提取DNA，对FMR1基因片段进行扩增，电泳观察结果。并选用子代幼龄FMR1基因敲除及野生型小鼠共16只，按基因型和年龄分为1周龄FMR1基因敲除型组（KO^1），1周龄野生型组（WT^1），4周龄FMR1基因敲除型组（KO^4），4周龄野生型组（WT^4），每组各4只。采用快速Golgi染色法分别观察不同发育时间段感觉皮质功能柱区域神经元的功能柱中心和周边两个方向上树突分支、树突棘密度和长度。结果与对照组相比，幼龄FMR1基因敲除鼠树突棘密度显著增高（P〈0．05）而长度无变化，感觉皮质功能柱区域中心朝向的树突棘密度无显著性差异，而周边朝向的树突棘密度KO^1、KO^4分别高于WT^1、WT^4，并且主要表现在距胞体10～80μm段的密度异常（P〈0．01）。结论FMRP缺失导致树突棘密度增高和感觉皮质功能柱区域树突棘修剪异常，并且树突棘修剪和树突修剪的机制不同。     

Development and lesion induced reorganization of the cortical representation of the rat's body surface as revealed by immunocytochemistry for serotonin

Immunocytochemistry with an antiserum directed against serotonin (5-HT) was used to assess the development of the representation of the body surface in the rat's primary somatosensory cortex (S-I). Within 1 hour of birth (P-O), 5-HT-positive fibers were present in the marginal zone, the cortical plate, and developing layers V and VI. Immunoreactivity in the marginal zone consisted of a thin band of coarse fibers oriented parallel to the pia. Only a small number of isolated fibers were visible in the cortical plate. A denser network of both coarse and fine fibers could be seen in presumptive layers V and VI. By the first hour of P-1, 5-HT-positive axons in the deeper cortical plate were organized into a crude representation of the rat's body surface. At this age, aggregates of fibers corresponding to the head, lower jaw, trunk, and forepaw could be clearly distinguished. These regions of dense 5-HT immunoreactivity consisted primarily of fine caliber axons that had invaded the lower part of the cortical plate. Dense aggregates of fine caliber axons were also visible in developing layers V and VI. Coarse 5-HT-positive fibers were visible in all layers, but they did not appear to contribute to the pattern that corresponded to the body surface. By the first hour of P-2, the map of the body surface in S-I was more refined and a row-related organization of 5-HT-immunoreactive fibers was visible in the portion of the cortex representing the vibrissa pad. The laminar distributions of coarse and fine caliber serotoninergic axons at this age were essentially the same as on P-1. By P-2.5 (60 hours after birth), patches of 5-HT-positive fibers corresponding to individual vibrissa follicles were clearly evident. These consisted of dense aggregates of fine caliber axons that were centered in presumptive layer IV, but which also extended above and below this lamina. Over the next 3 days, the pattern continued to mature. By P-4, dense 5-HT labelling was also visible in the secondary somatosensory cortex (S-II). By the beginning of P-5, clusters of fibers corresponding to more rostral facial hairs and individual digits within the forepaw representation could also be discerned. By P-12, the differential distribution of 5-HT fibers in S-I was no longer visible. Thus, immunocytochemistry for serotonin showed a representation in S-I homeomorphic with the body surface prior to the age at which it can be discerned with other methods thought to reveal thalamocortical axons. Transection of the infraorbital nerve (ION) on the day of birth altered the organization of the vibrissal representation in the contralateral cortex from the earliest age at which it could be detected by 5-HT immunocytochemistry in normal animals. Hawever, the departure from the normal organization was gradual. Row-related organization was clearly visible in the cortices of rats sacrificed on P-3, but not in those of rats that were killed on P-5. These results suggested that the organization of the 5-HT innervation of the cortex may be guided by thalamic afferents and further that some aspects of this guidance persist. albeit temporarily, after ION transection on P-0. The 5-HT immunoreactivity that we observed in the developing somatosensory cortex was not contained in thalamocortical axons. Unilateral electrocautery of the ventrobasal thalamus on P-4 did not reduce the density or alter the pattern of the 5-HT innervation of the cortex in rats that were examined on P-6.     

The maturation of frequency selectivity in C57BL/6J mice studied with auditory evoked response tuning curves

The present study was designed to examine the ontogeny of frequency selectivity in the neonatal auditory system. Mice were tested between 12 and 65 days of age. At each age two measures of auditory sensitivity were made from cochlear nucleus evoked responses. Tone-burst evoked-response thresholds in the quiet were determined for frequencies between 1.0 and 39.0 kHz. A two-tone simultaneous masking procedure was then used to obtain evoked response tuning curves. The frequency selectivity of the tuning curves was quantified by calculating a Q ratio. The results show that tuning is poor in neonates but rapidly improves to adult-like level swithin 5–16 days after the inception of auditory function. The data also indicate that the development of frequency selectivity varies directly with the maturation of threshold sensitivity.     

Reduced plasticity of cortical whisker representation in adult tenascin-C-deficient mice after vibrissectomy

The effect of the extracellular matrix recognition molecule tenascin-C on cerebral plasticity induced by vibrissectomy was investigated with 2-deoxyglucose (2DG) brain mapping in tenascin-C-deficient mice. Unilateral vibrissectomy sparing row C of vibrissae was performed in young adult mice. Two months later, cortical representations of spared row C vibrissae and control row C on the other side of the snout were visualized by [(14)C]2DG autoradiography. In both wild-type and tenascin-C-deficient mice, cortical representation of the spared row was expanded in all layers of the barrel cortex. However, the effect was significantly more extensive in wild-type animals than in the mutant. Elimination of tenascin-C by genetic manipulation thus reduces the effect of vibrissectomy observed in the somatosensory cortex. No increase in number of fibres in the vibrissal nerve of spared vibrissae was seen, and occurrence of additional nerve to the spared follicle was very rare. Thus, in tenascin-C-deficient mice functional plasticity seems to be impaired within the CNS.     

Sharpening of topographical projections and maturation of geniculocortical axon arbors in the hamster

During specification of orderly neural maps, axons correctly navigate to their targets and form terminal arbors in topographically correct positions. To learn more about this mapping process, the patterns of geniculocortical topography were correlated with growth of axon arbors in the hamster visual cortex. Topography was studied by retrograde transport of WGA-HRP from area 17 to the dorsal lateral geniculate nucleus (LGd) and visualized with TMB histochemistry. In separate experiments, geniculocortical axon arbors were filled with HRP deposited extracellularly into the optic radiations and stained with cobalt-intensified DAB. On the day of birth (P0) and on P1-2, a crude topography was detected in the geniculocortical system. At these ages, geniculocortical axons coursed in the embryonic white matter of the visual cortex, parallel to the pia. During their passage, multiple short collaterals, with no terminal arbors, were extended into the subplate and deeper portions of the cortical plate. By P3-5, the topography was more precise and simple axonal arbors had now begun to be formed on some branches within the cortical plate. During the second postnatal week, branches in the white matter without terminals were eliminated and the ramifications of branches in the gray matter became more elaborate. The arbors continued to increase in complexity and resembled adult forms by P24.     

Developmental profile of GABAA-mediated synaptic transmission in pyramidal cells of the somatosensory cortex

Inhibitory synaptic transmission mediated by γ-aminobutyric acid (GABA)_A receptors is involved in regulation of experience-dependent cortical plasticity. However, little information is available on their presynaptic and postsynaptic developmental profiles. The present study aims to investigate the developmental changes of miniature and unitary inhibitory postsynaptic currents (mIPSCs and uIPSCs) in mouse barrel cortex. mIPSCs recorded from supragranular pyramidal neurons showed a gradual increase in frequency during postnatal days 6–15 (PD6–15) followed by a marked increase at PD16–20, and mIPSCs frequency reached a plateau at about PD21–30. The amplitude of mIPSCs showed a transient decrease at PD10–12 followed by an increase during PD13–30, and reached a plateau at about PD30. Their decay time constant progressively decreased during the first 30 days postnatally, and reached a steady level at about PD30. Paired recordings from interneurons and synaptically coupled target pyramidal cells revealed that uIPSC amplitude increased with age up to PD30. In contrast, failure rate and coefficient of variation decreased during PD7–15 and showed little change at a later stage. Short-term depression induced by presynaptic stimulation at 33 Hz progressively decreased during PD6–20, and was stabilized at about PD21–30. Quantal analysis revealed that the number of release sites increased with age up to PD30, while the release probability increased during PD6–12 and then reached a plateau level. These results suggest that the number of release sites and release probability of GABA and GABA_A-mediated IPSC kinetics show distinct developmental profiles, which could play roles in regulating the onset and offset of critical periods for experience-dependent cortical plasticity.     

The resting‐state neurovascular coupling relationship: rapid changes in spontaneous neural activity in the somatosensory cortex are associated with haemodynamic fluctuations that resemble stimulus‐evoked haemodynamics

Although promise exists for patterns of resting‐state blood oxygen level‐dependent (BOLD) functional magnetic resonance imaging (fMRI) brain connectivity to be used as biomarkers of early brain pathology, a full understanding of the nature of the relationship between neural activity and spontaneous fMRI BOLD fluctuations is required before such data can be correctly interpreted. To investigate this issue, we combined electrophysiological recordings of rapid changes in multi‐laminar local field potentials from the somatosensory cortex of anaesthetized rats with concurrent two‐dimensional optical imaging spectroscopy measurements of resting‐state haemodynamics that underlie fluctuations in the BOLD fMRI signal. After neural ‘events’ were identified, their time points served to indicate the start of an epoch in the accompanying haemodynamic fluctuations. Multiple epochs for both neural ‘events’ and the accompanying haemodynamic fluctuations were averaged. We found that the averaged epochs of resting‐state haemodynamic fluctuations taken after neural ‘events’ closely resembled the temporal profile of stimulus‐evoked cortical haemodynamics. Furthermore, we were able to demonstrate that averaged epochs of resting‐state haemodynamic fluctuations resembling the temporal profile of stimulus‐evoked haemodynamics could also be found after peaks in neural activity filtered into specific electroencephalographic frequency bands (theta, alpha, beta, and gamma). This technique allows investigation of resting‐state neurovascular coupling using methodologies that are directly comparable to that developed for investigating stimulus‐evoked neurovascular responses.