Choline acetyltransferase activity in the hamster central auditory system and long-term effects of intense tone exposure.

Acoustic trauma often leads to loss of hearing of environmental sounds, tinnitus, in which a monotonous sound not actually present is heard, and/or hyperacusis, in which there is an abnormal sensitivity to sound. Research on hamsters has documented physiological effects of exposure to intense tones, including increased spontaneous neural activity in the dorsal cochlear nucleus. Such physiological changes should be accompanied by chemical changes, and those chemical changes associated with chronic effects should be present at long times after the intense sound exposure. Using a microdissection mapping procedure combined with a radiometric microassay, we have measured activities of choline acetyltransferase (ChAT), the enzyme responsible for synthesis of the neurotransmitter acetylcholine, in the cochlear nucleus, superior olive, inferior colliculus, and auditory cortex of hamsters 5 months after exposure to an intense tone compared with control hamsters of the same age. In control hamsters, ChAT activities in auditory regions were never more than one-tenth of the ChAT activity in the facial nerve root, a bundle of myelinated cholinergic axons, in agreement with a modulatory rather than a dominant role of acetylcholine in hearing. Within auditory regions, relatively higher activities were found in granular regions of the cochlear nucleus, dorsal parts of the superior olive, and auditory cortex. In intense-tone-exposed hamsters, ChAT activities were significantly increased in the anteroventral cochlear nucleus granular region and the lateral superior olivary nucleus. This is consistent with some chronic upregulation of the cholinergic olivocochlear system influence on the cochlear nucleus after acoustic trauma.

Functional identification of sensory mechanisms required for developmental song learning.

A young male zebra finch (Taeniopygia guttata) learns to sing by copying the vocalizations of an older tutor in a process that parallels human speech acquisition. Brain pathways that control song production are well defined, but little is known about the sites and mechanisms of tutor song memorization. Here we test the hypothesis that molecular signaling in a sensory brain area outside of the song system is required for developmental song learning. Using controlled tutoring and a pharmacological inhibitor, we transiently suppressed the extracellular signal-regulated kinase signaling pathway in a portion of the auditory forebrain specifically during tutor song exposure. On maturation, treated birds produced poor copies of tutor song, whereas controls copied the tutor song effectively. Thus the foundation of normal song learning, the formation of a sensory memory of tutor song, requires a conserved molecular pathway in a brain area that is distinct from the circuit for song motor control.

[Metabolic activity of thalamic and telencephalic auditory centers in the pigeon].

Distribution of activity of the mitochondrial oxidative enzyme cytochrome oxidase C was studied in the thalamic (Ov) and telencephalic (field L) auditory centers in pigeons. The CO activity level has been shown to differ in the central (core) and peripheral (belt) subdivisions of these centers: the high CO activity in the former (nCe, L2) and the much lower or absent in the latter (Ovl, Ovm, SPO). Comparison of our data with those of various avian and reptile species confirms the concept of the common plan of rostral auditory centers in sauropsid amniotes by the principle of the center-periphery (core-belt), which is characteristic of the corresponding mammalian centers. The separation of the central and peripheral parts of these centers is better pronounced in birds than in reptiles.

Evidence for increased NADPH-diaphorase-positive neurons in the central auditory system of the aged rat.

CONCLUSIONS: The age-related increase in the production of nitric oxide (NO) suggests that this increase was related to neuron aging. Additional studies may provide information regarding aging-related changes in the central auditory system. OBJECTIVES: Although NO has been associated with aging, it is unclear whether specific areas of the central auditory system are involved. We therefore assayed aging-related changes in NADPH-diaphorase (NADPH-d), a selective histochemical marker for NO, in the neurons of the central auditory system and other brain regions. MATERIALS AND METHODS: The numbers of NADPH-d-stained neurons and the area and staining density of cell bodies were examined in aged (24 months old) and younger (4 months old) Wistar rats. RESULTS: The number of NADPH-d-positive neurons in the inferior colliculus was significantly increased in aged rats (p<0.05), whereas the area of NADPH-d-positive neurons in all areas did not differ significantly between aged and younger rats (p>0.05). The staining densities of NADPH-d-positive neurons in the inferior colliculus, the auditory cortex, and the visual cortex were significantly greater in aged compared with younger rats (p<0.05).

Regional variations of cytochrome oxidase activity in the central auditory system of Relnrl-Orl (reeler) mutant mice.

Despite preserved cell differentiation, the Reln(rl-Orl) phenotype comprises laminar abnormalities of cell position in auditory cortex and dorsal cochlear nucleus. The metabolic consequences of the cell ectopias were determined by estimating cytochrome oxidase (CO) activity, a marker of neuronal activity. CO activity increased in the granular cell layer of dorsal cochlear nucleus, trapezoid body nucleus, intermediate lateral lemniscus, central and external inferior colliculus, and pyramidal cell layer of primary auditory cortex. On the contrary, CO activity decreased in the superficial molecular layer of dorsal cochlear nucleus as well as in the medioventral periolivary nucleus. These metabolic variations are discussed in terms of their possible relation to morphologic anomalies observed in the mutant.

[Effect of Electroacupuncture on Expression of Catechol-O-methyltransferase in the Inferior Colliculus and Auditory Cortex in Age-related Hearing Loss Guinea Pigs].

OBJECTIVE: To observe the expression of catechol-O-methyltransferase (COMT) in inferior colliculus and auditory cortex of guinea pigs with age-related hearing loss(AHL) induced by D-galactose, so as to explore the possible mechanism of electroacupuncture(EA) underlying preventing AHL. METHODS: Thirty 3-month-old guinea pigs were randomly divided into control group, model group and EA group(n=10 in each group), and ten 18-month-old guinea pigs were allocated as elderly group. The AHL model was established by subcutaneous injection of D-galactose. EA was applied to bilateral "Yifeng"(SJ 17) and "Tinggong"(SI 19) for 15 min in the EA group while modeling, once daily for 6 weeks. After treatment, the latency of auditory brainstem response(ABR) Ⅲ wave was measured by a brain-stem evoked potentiometer. The expressions of COMT in the inferior colliculus and auditory cortex were detected by Western blot. RESULTS: Compared with the control group, the latencies of ABR Ⅲ wave were significantly prolonged and the expressions of COMT in the inferior colliculus and auditory cortex were significantly decreased in the model group and the elderly group(P<0.05). After the treatment, the latency of ABR Ⅲ wave was significantly shortened and the expressions of COMT in the inferior colliculus and auditory cortex were significantly increased in the EA group in comparison with the model group (P<0.05). CONCLUSIONS: EA at "Yifeng" (SJ 17) and "Tinggong" (SI 19) can improve the hearing of age-related deafness in guinea pigs, which may contribute to its effect in up-regulating the expression of COMT in the inferior colliculus and auditory cortex.

Age-related loss of the GABA synthetic enzyme glutamic acid decarboxylase in rat primary auditory cortex.

Age-related changes within the auditory brainstem typically include alterations in inhibitory neurotransmission and coding mediated by GABA and glycinergic circuits. As part of an effort to evaluate the impact of aging on neurotransmission in the higher auditory centers, the present study examined age-related changes in the GABA synthetic enzyme, glutamic acid decarboxylase (GAD), in rat primary auditory cortex (AI), which contains a vast network of intrinsic and extrinsic GABAergic circuits throughout its layers. Message levels of the two GAD isoforms found in brain, GAD(65) and GAD(67), and GAD(67) protein levels were compared in young adult, middle-aged and aged rats using in situ hybridization and quantitative immunocytochemistry, respectively. For comparison, age-related GAD changes were also assessed in the parietal cortex and hippocampus. Significant age-related decreases in GAD(65&67) messages were observed in AI layers II-VI of aged rats relative to their young adult cohorts. The largest changes were identified in layer II (GAD(65): -26.6% and GAD(67): -40.1%). GAD(67) protein expression decreased significantly in parallel with mRNA decreases in all layers of AI. Adjacent regions of parietal cortex showed no significant GAD(67) protein changes among the age groups, except in layer IV. As previously described, GAD(67) message and protein levels in selected hippocampal regions were significantly reduced in aged rats. Age-related GAD reductions likely reflect decreases in both metabolic and pre-synaptic GABA levels suggesting a plastic down-regulation of normal adult inhibitory GABA neurotransmission. Consistent with the present findings, functional studies in primate visual cortex and preliminary studies in AI find coding changes suggestive of altered inhibitory processing in aged animals. An age-related loss of normal adult GABA neurotransmission in AI would likely alter temporal coding properties and could contribute to the loss in speech understanding observed in the elderly.

Protein kinase C in central auditory pathways of the rat.

Protein kinase C is an important intracellular signaling molecule. Many of its ten isoforms are highly expressed in brain, and protein kinase C has been implicated in the regulation of the activity of receptors of several major neurotransmitters, including glutamate, acetylcholine, glycine, and gamma-aminobutyric acid. These neurotransmitters and their receptors are present in central auditory pathways, suggesting their role in auditory signal processing. Although they may be important modulators of the function of these neurotransmitter receptors, the distribution of protein kinase C isoforms in central auditory systems has not been well characterized. By using immunocytochemistry with specific antibodies, we studied the distribution of immunoreactivity of four isoforms of protein kinase C, betaI, betaII, gamma, and gamma, in central auditory systems of rat brain. Each of these protein kinase C isoforms was found to have a unique distribution in the auditory brainstem and cortex, supporting a role for these isoforms of protein kinase C in different aspects of auditory sensory processing.

Tonotopic organization, architectonic fields, and connections of auditory cortex in macaque monkeys.

Microelectrode recordings were used to investigate the tonotopic organization of auditory cortex of macaque monkeys and guide the placement of injections of wheat germ agglutinin-horse radish peroxidase (WGA-HRP) and fluorescent dyes. Anatomical and physiological results were later related to histological distinctions in the same brains after sections were processed for cytoarchitecture, myeloarchitecture, acetylcholinesterase (AchE), or cytochrome oxidase (CO). The experiments produced several major findings. (1) Neurons throughout a broad expanse of cortex were highly responsive to pure tones, and best frequencies could be determined for neurons in arrays of recording sites. (2) The microelectrode recordings revealed two systematic representations of tone frequencies, the primary area (AI) and a primary-like rostral field (R) as previously described. The representation of high to low frequency tones in A1 was largely caudorostral along the plane of the sulcus. A reversal of the order of representation of frequencies occurred in R. (3) AI and R together were coextensive with a koniocellular, densely myelinated zone that expressed high levels of AchE and CO. These architectonic features were somewhat less pronounced in R than AI, but a clear border between the two areas was not apparent. (4) Cortex bordering AI and R was less responsive to tones, but when best frequencies for neurons could be determined, they matched those for adjoining parts of AI and R. (5) Architectonically distinct regions were apparent within some of the cortex bordering AI and R. (6) The major ipsilateral cortical connections of AI were with R and cortex immediately lateral and medial to AI. (7) Callosal connections of AI were predominantly with matched locations in the opposite AI, but they also included adjoining fields. (8) Neurons in the ventral (MGV), medial (MGM), and dorsal (MGD) nuclei of the medial geniculate complex projected to AI and cortex lateral to AI. (9) Injections in cortex responsive to high frequency tones labeled more dorsal parts of MGV than injections in cortex responsive to low frequency tones.

Laminar distribution and neuronal targets of GABAergic axon terminals in cat primary auditory cortex (AI).

The form, density, and neuronal targets of presumptive axon terminals (puncta) that were immunoreactive for gamma-aminobutyric acid (GABA) or its synthesizing enzyme, glutamic acid decarboxylase (GAD), were studied in cat primary auditory cortex (AI) in the light microscope. High-resolution, plastic-embedded material and frozen sections were used. The chief results were: 1) There was a three-tiered numerical distribution of puncta, with the highest density in layer Ia, an intermediate number in layers Ib-IVb, and the lowest concentration in layers V and VI, respectively. 2) Each layer had a particular arrangement: layer I puncta were fine and granular (less than 1 micron in diameter), endings in layers II-IV were coarser and more globular (larger than 1 micron), and layer V and VI puncta were mixed in size and predominantly small. 3) The form and density of puncta in every layer were distinctive. 4) Immunonegative neurons received, in general, many more axosomatic puncta than immunopositive cells, with the exception of the large multipolar (presumptive basket) cells, which invariably had many puncta in layers II-VI. 5) The number of puncta on the perikarya of GABAergic neurons was sometimes related to the number of puncta in the layer, and in other instances it was independent of the layer. Thus, while layer V had a proportion of GABAergic neurons similar to layer IV, it had only a fraction of the number of puncta; perhaps the intrinsic projections of supragranular GABAergic cells are directed toward layer IV, as those of infragranular GABAergic neurons may be. Since puncta are believed to be the light microscopic correlate of synaptic terminals, they can suggest how inhibitory circuits are organized. Even within an area, the laminar puncta patterns may reflect different inhibitory arrangements. Thus, in layer I the fine, granular endings could contact preferentially the distal dendrites of pyramidal cells in deeper layers. The remoteness of such terminals from the spike initiation zone contrasts with the many puncta on all pyramidal cell perikarya and the large globular endings on basket cell somata. Basket cells might receive feed-forward disinhibition, pyramidal cells feed-forward inhibition, and GABAergic non-basket cells would be the target of only sparse inhibitory axosomatic input. Such arrangements imply that the actions of GABA on AI neurons are neither singular nor simple and that the architectonic locus, laminar position, and morphological identity of a particular neuron must be integrated for a more refined view of its role in cortical circuitry.

Choline acetyltransferase-immunoreactive cochlear efferent neurons in the chick auditory brainstem.

Cholinergic neurons in the chick auditory brainstem were studied with the aid of an antiserum to choline acetyltransferase (ChAT), the biosynthetic enzyme for acetylcholine. ChAT-immunoreactive (ChAT-I) neurons were found in a ventrolateral and a dorsomedial cell group. The ventrolateral group is a rostrocaudally directed column of cells that surround the superior olive (SO), are ventromedial to the ventral facial nucleus (VIIv), and are lateral to the nucleus pontis lateralis (PL) as far rostrally as the nucleus subceruleus ventralis. Cells in the dorsomedial group were found in the pontine reticular formation medial to the dorsal facial nucleus and lateral to the abducens nerve root. Occasionally, small ChAT-I cells were found in the crossed dorsal cochlear tract and in the medial vestibular nucleus near the dorsal border of the caudal nucleus magnocellularis (NM). No ChAT-I neurons or fibers were observed in NM, nucleus angularis, nucleus laminaris, in the nuclei of the lateral lemniscus, or in the nucleus mesencephalicus lateralis pars dorsalis. To determine which cholinergic neurons project to the cochlea, a double-labeling technique was used combining ChAT-I and the retrograde transport of biotinylated dextran amine (BDA) from the inner ear. Double-labeled cells were found bilaterally in both the ventrolateral and dorsomedial cell groups, with the exception of large ChAT-I cells dorsal to the SO, which do not appear to project to the cochlea. Cholinergic cells that project to the cochlea were classified into three morphological groups: multipolar, elongate, and round-to-oval. Both the ventrolateral and the dorsomedial cell groups appear to have a mixture of these different cell types. The average somal area of cholinergic cochlear efferents was 246 microns 2. Only about 70% of the cochlear efferent neurons, however, are cholinergic.

Architectonic identification of the core region in auditory cortex of macaques, chimpanzees, and humans.

The goal of the present study was to determine whether the architectonic criteria used to identify the core region in macaque monkeys (Macaca mulatta, M. nemestrina) could be used to identify a homologous region in chimpanzees (Pan troglodytes) and humans (Homo sapiens). Current models of auditory cortical organization in primates describe a centrally located core region containing two or three subdivisions including the primary auditory area (AI), a surrounding belt of cortex with perhaps seven divisions, and a lateral parabelt region comprised of at least two fields. In monkeys the core region can be identified on the basis of specific anatomical and physiological features. In this study, the core was identified from serial sets of adjacent sections processed for cytoarchitecture, myeloarchitecture, acetylcholinesterase, and cytochrome oxidase. Qualitative and quantitative criteria were used to identify the borders of the core region in individual sections. Serial reconstructions of each brain were made showing the location of the core with respect to gross anatomical landmarks. The position of the core with respect to major sulci and gyri in the superior temporal region varied most in the chimpanzee and human specimens. Although the architectonic appearance of the core areas did vary in certain respects across taxonomic groups, the numerous similarities made it possible to identify unambiguously a homologous cortical region in macaques, chimpanzees, and humans.

Cytochrome oxidase activity in the auditory system of the mouse: a qualitative and quantitative histochemical study.

Detailed qualitative and quantitative determinations of cytochrome oxidase activity in the central auditory system of BALB/cJ mice were obtained at the light microscopic level. Cytochrome oxidase activity was determined using quantitative densitometry calibrated with standards of spectrophotometrically assayed enzymatic activity. This was done together with a cobalt-intensified histochemical procedure using fresh-frozen brains without perfusion-fixation. The resulting method showed improved sensitivity and allowed quantification of histochemical labeling as actual enzyme activity units. Adjacent sections were processed for either Nissl, fiber or Golgi stains to correlate the histochemical labeling with tissue morphology. The more peripheral auditory nuclei showed primarily somatic labeling with specific cell types showing predominant reactivity. However, higher auditory structures, including the inferior colliculus, medial geniculate and auditory cortex, showed predominantly neuropil reactivity. Comparison of mean cytochrome oxidase activities for the 27 auditory regions quantified revealed a trend for decreasing activity from the brainstem to the forebrain in central lemniscal structures. The extra-lemniscal auditory regions at each level showed lower activity than the corresponding lemniscal regions. The regions with the higher activity values showed around 10 times the labeling density of the white matter, indicating the high sensitivity of the method. The darkly labeling auditory structures were clearly delineated from surrounding neural regions, supporting the concept that basal levels of oxidative metabolic capacity are larger for the auditory system. It was concluded that the quantitative approach to cytochrome oxidase histochemistry may be applied successfully to the mouse brain. The normative data presented may be used as a starting point for other investigations of the effects of experimental manipulations on the metabolic activity of the auditory system.

NADPH-diaphorase-positive neurons in the auditory cortex of young and old rats.

Age-related changes in NADPH-diaphorase (NADPH-d)-positive neurons were examined in the auditory cortex of young (3 months old) and very old (36 months old) rats (strain Long Evans). In very old rats a significant reduction was found in the thickness of the auditory cortex, to 54% of that in young animals, as well as changes in the shape and configuration of nerve cell bodies and dendrites. Quantitative analysis demonstrated an age-related increase in the number of dendritic segments and dendritic branching points. The length of dendrites in NADPH-d-positive neurons and their density increased in very old rats. The total number of NADPH-d-positive neurons within the Te 1 and Te 3 fields was 13% lower in the old rats than in the young.

An investigation of sensory deficits underlying the aphasia-like behavior of macaques with auditory cortex lesions.

Bilateral auditory cortex lesions in Japanese macaques result in an aphasia-like deficit in which the animals are unable to discriminate two forms of their coo vocalizations. To determine whether this deficit is sensory in nature, two monkeys with bilateral lesions were tested for their ability to discriminate frequency and frequency change. The results indicated that although the animals were able to discriminate between sounds of different frequencies, they were unable to determine whether a sound was changing in frequency. Because the animals' coo vocalizations differ primarily in the predominant direction of their frequency change and not in their absolute frequency content, the aphasia-like deficit of animals with bilateral auditory cortex lesions appears to be a sensory disorder.

Immunohistochemical detection of raf protein kinase in cerebral cortical areas of adult guinea pigs and rats.

The raf protooncogenes are the cellular counterparts of the v-raf oncogene expressed by a murine sarcoma virus. The raf protooncogenes encode cytoplasmic serine/threonine-specific protein kinases which can be activated from different growth factor receptors by phosphorylation. The mRNAs of raf protooncogenes are found in a large variety of normal adult tissues, including the central nervous system. As concerns the distribution and localization of their protein products (the raf kinases), very few data are to be found in the literature. This is the first detailed description of their light microscopic localization in neocortical and allocortical areas of rodents. Preembedding immunohistochemical studies were performed on vibratome sections from the brains of adult guinea pigs and albino rats. The localizations of two isoenzymes, raf-1 kinase and B-raf kinase, were studied with the help of isoenzyme-specific polyclonal antibodies. Both of the antibodies detected raf protein-like immunoreactivity in many neurons and scattered glial cells of the sensory neocortex, and the cingular, pyriform, perirhinal and entorhinal allocortical areas. Pyramidal and non-pyramidal cells of Ammon's horn, granule cells of the dentate fascia and the large neurons in the hilar region were immunoreactive, too. The findings indicated that B-raf protein kinase and raf-1 kinase are present almost ubiquitously in the neurons of the investigated cortical structures. The intensity of staining obtained with serial dilutions of the antibodies indicated that the cytoplasmic concentration of B-raf kinase is tended to be higher than that of raf-1 kinase. The present findings suggested that the raf kinases are localized in postsynaptic structures, mainly in dendrites and cell bodies. Their cytosolic localization and their ability to undergo intracellular translocation during activation and phosphorylation raise the possibility that they play a pivotal role in the intracellular signaling of neurons.

Organization of the mouse cerebral cortex: a histochemical study using glycogen phosphorylase.

The postmortem phosphorylase a activity in cryostat sections taken from the brains of mice killed without any prior period of maintained anaesthesia ('unanaesthetized' mice) was compared histochemically with the activity in sections taken from mice anaesthetized with pentobarbitone prior to decapitation ('anaesthetized' mice). This study provides evidence for the existence of two separate overlapping modular subdivisions of the somatosensory cortex. The modules observed in the 'unanaesthetized' mice were restricted to layer IV and corresponded to the hollows of the whisker barrels where the thalamic afferents terminate. In contrast, the modules observed in the 'anaesthetized' mice extended from layer I to layer V, and formed a mosaic of cylinders that was out of register with the whisker barrels. These cylinders may correspond to the terminal fields of corticocortical afferents. In a few of the 'unanaesthetized' mice bands of high phosphorylase a activity are evident in the visual areas 17 and 18. This suggests that in the mouse, as in higher mammals, the thalamic input to the visual cortex gives rise to a columnar organization. In the 'anaesthetized' mice the mosaic of modules observed in the somatosensory cortex is also present in the auditory and motor areas. The modules in both groups of mice have similar diameters of between 200 and 350 microns.

Interaction of LSD and other hallucinogens with dopamine-sensitive adenylate cyclase in primate brain: regional differences.

The influence of D-lysergic acid diethylamide (LSD) and mescaline on adenylate cyclase activity was studied in homogenates of Cebus and rhesus monkey anterior limbic cortex (ALC), frontal cortex (FC), caudate nucleus and retina. Previous studies have shown these tissues to contain dopamine-stimulated adenylate cyclase (AC). In addition, we are now reporting the presence of a dopamine-sensitive adenylate cyclase in the auditory cortex. AC of ALC and auditory cortex was stimulated by LSD and mescaline, whereas activity of FC, caudate nucleus and retina was not stimulated by the same agents. In contrast to regional specificity for stimulation, LSD was capable of antagonizing dopamine-stimulated activity in all brain regions examined. LSD and mescaline produced similar maximal stimulation (about 70%) of AC of ALC homogenates, but the EC50 for LSD (0.43 micrometer) was about one-tenth that for mescaline (4.5 micrometer). Similar relative potencies were also observed for the auditory cortex enzyme. Although much weaker than LSD, methamphetamine also produced a dose-dependent stimulation of ALC AC. Both agonist and antagonist effects of the hallucinogens appear to involve interaction with dopamine receptors; LSD- or methamphetamine-stimulated activity in ALC was blocked by haloperidol and fluphenazine, which are dopamine antagonists, but not by phentolamine, an alpha-receptor blocker. Antagonism of dopamine by LSD in both ALC and FC was found to be competitive and mescaline was an effective but weaker antagonist than was LSD. In addition, neither histamine--nor Gpp(NH)p--stimulated activity of FC was inhibited by LSD. It is proposed that the occurrence of dopamine agonistic action of hallucinogens in only certain regions of primate brain may provide a basis for at least some of the behavioral effects of LSD, mescaline and methamphetamine in primates.

Primary auditory cortex in the rat: transient expression of acetylcholinesterase activity in developing geniculocortical projections.

A characteristic pattern of acetylcholinesterase (AChE) activity is expressed transiently in primary auditory cortex (cortical area 41) of developing laboratory rats during early postnatal life. This AChE activity occurs as a dense plexus in cortical layer IV and the deep part of layer III. This transient band of AChE activity is first detected by histochemical techniques on postnatal day (P) 3, reaches peak intensity at approximately P8-10, and declines to form the adult pattern by P23. The ventral nucleus of the medial geniculate body of the thalamus also displays prominent, and transient, staining for AChE. This intense staining for AChE, found within neuronal somata and neuropil, is detected at the time of birth, reaches peak intensity around P8, and declines to adult levels by P16. The areal and laminar patterns of the transient band of AChE activity in temporal cortex correspond to the patterns of anterograde transneuronal labeling of geniculocortical terminals following injection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the inferior colliculus. Placement of lesions that include the medial geniculate nucleus or the geniculocortical axons results in a marked decrease in AChE staining in thalamorecipient layers of auditory cortex. Placement of lesions that include the medial globus pallidus reduce AChE staining of some axons in temporal cortex of developing rats, but the dense band of AChE in layers III and IV remains. Placement of lesions in the inferior colliculus in newborn animals results in marked decrease in AChE staining in cells of the ipsilateral ventral medial geniculate nucleus and in ipsilateral auditory cortex of developing pups. These data indicate that transiently expressed AChE activity is characteristic of geniculocortical neurons, including their somata in the medial geniculate body and their terminal axons in primary auditory cortex. This AChE activity is expressed early in postnatal development, probably during the time when thalamocortical axons are proliferating in cortical layer IV and forming synaptic contacts with cortical neurons.

Chemoarchitectonics of axonal and perikaryal acetylcholinesterase along information processing systems of the human cerebral cortex.

The distribution of axonal and perikaryal acetylcholinesterase (AChE) was studied in whole-brain sections. All cytoarchitectonic sectors and cortical layers of the human cerebral cortex contained AChE-rich axons. These axons displayed multiple varicosities which appeared to come in contact with AChE-rich and AChE-poor cortical perikarya. The upper layers of cortex tended to contain the highest density of AChE-rich axons. The AChE-rich axons were more dense in limbic-paralimbic areas of cortex than in primary sensory-motor and association areas. Within unimodal sensory association areas, the parasensory (upstream) sectors had a slightly lesser density of AChE-rich axons than the downstream sectors. Within paralimbic areas, the nonisocortical sectors displayed a distinctly higher density of AChE-rich axons than the more differentiated isocortical sectors. These observations indicate that the distribution of AChE-rich axons displays orderly variations that obey the organization of information processing systems in the cerebral cortex.