Embryonic Tissue Induces Growth of Adult Axons from Myelinated Fiber Tracts

Suspensions of late embryonic hippocampal tissue were microinjected so as to be completely enclosed within the myelinated fiber bundles of the adult rat fimbria. Previous studies have shown that the axons from such transplanted neurons readily cross the graft/host interface and extend rapidly through the host fiber tract. The present study shows that the adult axons from the host fiber tract can also cross this interface in the opposite direction and enter the transplants. Biotin dextran tracing shows that the adult host fimbrial axons traverse the embryonic grafts and also form terminal arborizations within the transplants. Electron microscopy of orthograde electron-dense degeneration confirms that these host axons form synaptic terminals accounting for at least 6.6% of the synapses in the neuropil of the transplant. Thus, contact with embryonic nervous tissue can induce elongative growth by the adult fibers in a myelinated central tract.     

Nerve Growth Factor Promotes Regeneration of Sensory Axons into Adult Rat Spinal Cord

Injured adult mammalian axons are unable to regenerate spontaneously in the central nervous tissue. This study investigated in two adult rat models the effects of nerve growth factor (NGF) on the capacity of central primary sensory axons to regenerate back into the spinal cord. Sensory fibers were conditioned by transection of the peripheral nerve 1 week prior to the experiment and identified by anterograde tracing with cholera toxin B subunit injected in the sciatic nerve. In the first model, a predegenerated autologous peripheral nerve graft was implanted as a bridge for the transected sensory fibers into a resection gap in the dorsal columns at the tenth thoracic (T10) spinal cord segment. Vehicle or vehicle with purified mouse or recombinant human NGF was continuously infused for 2 weeks directly into the dorsal column at T9, 3 mm from the rostral border of the nerve graft. With vehicle infusion many ascending sensory axons had grown across the nerve bridge, but essentially none had grown back into the rostral cord. In sharp contrast, NGF promoted the reentry into the denervated dorsal columns of 51% of the sensory axons that had reached the rostral level of the nerve graft. Twenty-six percent had grown 2 mm into the spinal tissue and 10% had reached the NGF-infusion site at 3 mm from the nerve graft. A few fibers were found circling around, but not beyond, the infusion site, perhaps due to the chemoattractant action of NGF. In a second model, the fourth lumbar (L4) dorsal root was crushed 2 mm from its insertion point into the spinal cord and the dorsal roots L2, L3, L5, and L6 were transected. Vehicle or vehicle with purified mouse NGF was infused for 2 weeks directly into the lumbar spinal cord, 2.5 mm rostral to the transition zone of the crushed L4 root. With vehicle, only 6% of the regenerating fibers at the transition zone had crossed the root–spinal cord barrier, but not farther than 0.5 mm into the spinal tissue. With NGF, 18% of the fibers at the transition zone were found at 0.5 mm, 9% at 1.5 mm, and 5% at 2.5 mm (the infusion site) from the transition zone. The present results demonstrate that NGF can promote the regeneration of adult sensory fibers into the otherwise nonpermissive spinal cord white matter.     

Autoradiographic Localization of Receptors in the Cochlear Nucleus of the Mouse

Light microscopic autoradiography of bound radiolabeled ligands was used to describe the distribution of six receptor types in the dorsal and ventral mouse cochlear nuclei: Glycine receptor ([³H]strychnine); GABA receptor (³H]muscimol); benzodiazepine receptor ([³H]flunitrazepam); adenosine receptor ([³H]cyclohexyladenosine); muscarinic ACh receptor ([³H]quinuclidinyl benzilate); histamine receptor ([³H]mepyramine). The most intense [³H]strychnine labeling was observed in the deep region of the dorsal cochlear nucleus (DCN), with slightly lower levels in the molecular and pyramidal layers. Highest density of [³H]muscimol binding sites was observed in the granule cell layer of the posterior ventral nucleus (PVCN) and in the pyramidal layer of the DCN. Diffuse [³H]flunitrazepam labeling was distributed over all laminar regions of the DCN; the highest grain density was observed over the granule cell layer of the PVCN. Intense [³H]cyclohexyladenosine labeling was seen over the molecular layer, possibly extending into the pyramidal layer, of the DCN. The granule cell layer of the PVCN was also densely labeled. High concentrations of [³H]qiuinuclidinyl benzilate sites were seen in the molecular layer, possibly extending into the pyramidal layer, of the DCN. A thin band of high grain density was also visible over the granule cell layer of the PVCN. Moderate, diffuse [³H]mepyramine labeling was visible throughout the DCN, with slightly higher grain density over the molecular and possibly the pyramidal layers, than over the deep region of the DCN.     

Long Fibre Growth by Axons of Embryonic Mouse Hippocampal Neurons Microtransplanted into the Adult Rat Fimbria

We have described a method for the microtransplantation of a suspension of a few thousand cells from mid to late embryonic mouse hippocampi into the fimbria of immunosuppressed adult rat hosts. There was close graft-to-host contact, across a non-scarred interface. The transplanted cells included CA3 type pyramids, and were enclosed within the host myelinated fibre tract, whose glial framework was largely undisturbed. Immunohistochemistry of two species-specific markers (M6 and Thy-1.2) showed that the donor mouse neurons grew fine (<0.5 μm diameter) axons which extended singly or in fascicles through the rat host fimbria for a maximum distance of at least 10 mm. The donor axons were intimately integrated among and closely aligned to the host tract axons and to the interfascicular glial rows of the host tract. The axons travelled (i) laterally through the ipsilateral fimbria, (ii) medially across the midline in the ventral hippocampal commissure to reach the contralateral fimbria and alveus, and (iii) rostro-medially to the septum. On approaching the terminal fields appropriate to hippocampal CA3 pyramidal cell axons, the transplant axons gave rise to fine preterminal branches which were continuous with a reticular or amorphous immunoreactivity in the stratum oriens and stratum pyramidale of the ipsilateral hippocampus, and in the lateral and triangular septal nuclei. The donor axons extended along the host fimbria at a rate of ∼ 1 mm per day, reaching their terminal field destinations by ∼1–2 weeks. At 7 weeks the projections were maintained, but with little further extension. These observations indicate that the microenvironment of myelinated adult fibre tracts is permissive for an abundant and rapid growth of axons from transplanted embryonic cell suspensions. These axons can leave host tracts to invade appropriate terminal fields.     

Noradrenalin Enhances the Activity of Cochlear Nucleus Neurons in the Rat

The cochlear nucleus of rats is heavily innervated by noradrenergic fibres from the locus coeruleus. The physiological meaning of this innervation is poorly understood. Therefore, iontophoretically applied noradrenalin was tested on single neurons of the cochlear nucleus in urethane-anaesthetized rats. Iontophoresis of noradrenalin had a dual effect. During application noradrenalin led to moderate inhibition of tone-evoked activity in 37% of the tested neurons. In contrast, ∼20-30 s after the onset of iontophoresis a long-lasting increase in discharge activity was found in most neurons. Data from iontophoresis of the α₁-receptor agonist phenylephrine and the α₂-receptor agonist clonidine suggest that the fast moderate inhibition is mediated by α₂-receptors while the pronounced long-lasting elevated neuronal firing is mediated by α₁-receptors. However, these data do not exclude the possibility that part of the response to noradrenalin is also mediated by β-receptors. Electrical stimulation of the locus coeruleus resulted in an increase in discharge activity comparable with iontophoresis of noradrenalin or phenylephrine. Thus, activation of the locus coeruleus predominantly increases spontaneous and tone-evoked neuronal firing in the cochlear nucleus of the rat. This α₁-receptor-mediated enhanced discharge activity may serve to increase the sensitivity of acoustic processing mechanisms or to lower the threshold for short-latency acoustic reflexes.     

Parabrachial Origin of Calcitonin Gene-Related Peptide-Immunoreactive Axons Innervating Meynert's Basal Nucleus

Meynert's basal nucleus is innervated by calcitonin gene-related peptide (CGRP)-immunoreactive axons synapsing with cholinergic principal cells. Origin of CGRP-immunopositive axons was studied in the albino rat. Since beaded axons containing the nicotinic acetylcholine receptor (nAChR) are also present in the basal nucleus, the microstructural arrangement raises the question whether or not an interaction between CGRP and nAChR exists like in the neuromuscular junction. We found that electrolytic lesion of the parabrachial nucleus results in degeneration of CGRP-immunoreactive axons in the ipsilateral nucleus basalis and induces shrinkage of principal cholinergic neurons while the contralateral nucleus basalis remains intact. Electrolytic lesions in the thalamus, caudate-putamen, and hippocampus did not induce alterations in Meynert's basal nucleus. Disappearance of CGRP after lesions of the parabrachial nucleus does not impair presynaptic nAChR in the basal nucleus, suggesting that, unlike in the neuromuscular junction, CGRP is not involved in the maintenance of nAChR in the basal forebrain. It is concluded that the parabrachial nucleus is involved in the activation of the nucleus basalis-prefrontal cortex system, essential in gnostic and mnemonic functions.     

Immunocytochemical Evidence that Glutamate is a Neurotransmitter in the Cochlear Nerve: A Quantitative Study in the Guinea-pig Anteroventral Cochlear Nucleus

The large so-called type I afferents of the cochlear nerve carry the majority of the auditory input from the cochlea to the cochlear nuclei in the brainstem. These fibres are excitatory and previous studies have suggested they may use glutamate as their neurotransmitter. In the present investigation therefore, antibodies to glutamate and to the glutamate precursor, glutamine, were applied to resin sections of perfusion-fixed brains and of in vitro brain slices subjected to depolarizing levels of potassium before fixation to study glutamate handling and synaptic release. Ultrathin sections were labelled by the immunogold technique, and the immunoreactivity was quantified by recording the density of gold particles over the various tissue profiles. Non-primary, presumably inhibitory, terminals and glial processes were used as reference structures. The cochlear primary terminals proved to be strongly immunoreactive for glutamate. The density of glutamate labelling was higher in primary terminals than in non-primary ones, and lowest in glial processes. The ratio between the mean glutamate and glutamine labelling densities was also higher in primary terminals than in non-primary ones, and lowest in glial processes in each case. In the primary terminals, the glutamate immunoreactivity was higher over vesicle-containing regions than over vesicle-free regions, whilst glutamine was evenly distributed throughout. The in vitro brain slices showed a potassium-induced, partly calcium-dependent depletion of glutamate from the primary terminals but not from the non-primary ones. These observations strongly support the conclusion that glutamate is a neurotransmitter of type I cochlear afferents.     

Radial Glia-Like Cells in the Supraoptic Nucleus of the Adult Rat

Conventional light and confocal microscopy of thick vibratome sections of the hypothalamus of adult male and female rats immunostained for the astrocytic marker glial fibrillary acidic protein (GFAP) revealed that the supraoptic nucleus (SON) contains two morphologically distinct types of astrocytes. One has a stellate form, similar to that of most astrocytes in the adult CMS. The other has a morphology reminiscent of radial glia in the developing CNS: from their cell bodies, located along the ventral glia lamina (VGL), arise one long thick process that spans the SON in the coronal plane, several horizontally-oriented processes that form a dense network in the VGL, and a short process oriented towards the pia. The latter astrocytes are immunoreactive for vimentin, an intermediate filament protein of immature glial cells and a marker for radial glia. The stellate astrocytes showed no vimentin immunoreactivity. The functional significance of each type of supraoptic astrocyte is at present unknown but the presence of radial glia-like cells in this hypothalamic region suggests that the SON retains a certain degree of immaturity during adulthood, that may be linked to its well known capacity to undergo neuronal-glial plasticity under physiological and experimental stimulation.     

Membrane Currents Influencing Action Potential Latency in Granule Neurons of the Rat Cochlear Nucleus

Granule cells are the most numerous neurons in the cochlear nucleus, but, because of their small size, little information on their membrane properties and ionic currents is available. We used an in vitro slice preparation of the rat ventral cochlear nucleus to make whole-cell recordings from these cells. Under current clamp, some granule neurons fired spontaneous action potentials and all generated a train of action potentials on depolarization (threshold current, 10–35 pA). Hyperpolarization increased the latency to the first action potential evoked during a subsequent depolarization. We examined which voltage-gated currents might underlie this latency shift. In addition to a fast inward Na⁺ current, depolarization activated two outward potassium currents. A transient current was rapidly inactivated by membrane potentials positive to -60 mV, while a second, more slowly inactivating current was observed following the decay of the transient current. No hyperpolarization-activated conductances were observed in these cells. Modelling of the currents suggests that removal of inactivation on hyperpolarization accounts for the increased action potential latency in granule cells. Such a mechanism could account for the 'pauser'-type firing patterns of the fusiform cells which receive a prominent projection from the granule cells in the dorsal cochlear nucleus.     

Regeneration of Cut Adult Axons Fails Even in the Presence of Continuous Aligned Glial Pathways

The present study tests whether lesions small enough to allow the rapid reestablishment of a normally aligned tract glial framework would provide a permissive environment for the regeneration of cut adult CNS axons. We made penetrating microlesions which cut a narrow beam of axons in the adult rat cingulum, but caused minimal damage to the tract glial framework and no cavitation. The proximal tips of cut axons were identified by enhanced immunoreactivity for low affinity neurotrophin receptor, p75. From 1 day they became expanded into large growth-cone-like structures. At later times some axons turned back and extended in the reverse direction. Up to 14 days (after which time p75 could no longer be used as a marker), no axons advanced beyond the line of the lesion. From 1 to 2 days, OX42 immunostaining and electron microscopy showed that the lesion site was densely infiltrated by macrophages, which disappeared by 3 to 4 days. This was followed by a local hypertrophy of the OX42 immunoreactive resident tract microglial cells and an increase in both GFAP and vimentin immunoreactivity of the tract astrocytes. These responses were greatly reduced by 8 days, when the longitudinal alignment of glial processes across the lesion site was similar to that of an undamaged tract. The large growth-cone-like structures formed at the ends of the cut axons resemble those of developing axons exposed to chemorepulsive factors. This suggests that cellular elements in adult tract lesions may also exert chemorepulsive influences blocking regeneration of axons even in an apparently “open” tract framework.     

Glycine Receptors in Adult Guinea Pig Brain Stem Auditory Nuclei: Regulation after Unilateral Cochlear Ablation

In young adult guinea pigs, the effects of unilateral cochlear ablation were determined on the specific binding of [³H]strychnine measured in subdivisions of the cochlear nucleus (CN), the superior olivary complex, and the auditory midbrain, after 2, 7, 31, 60, and 147 postlesion days. Changes in binding relative to that in age-matched controls were interpreted as altered activity and/or expression of synaptic glycine receptors. Postlesion binding declined ipsilaterally in most of the ventral CN and in the lateral superior olive (LSO). Binding was modestly deficient in the ipsilateral dorsal CN and in the anterior part of the contralateral anteroventral CN. Binding was elevated in the contralateral LSO. Transient changes also occurred. Binding was elevated transiently, between 2 and 31 days, contralaterally in parts of the anteroventral CN, bilaterally in the medial superior olive (MSO), and bilaterally in most of the midbrain nuclei. Binding was deficient transiently, at 60 days, in most of the contralateral CN and bilaterally in the midbrain nuclei. The present findings, together with previously reported postlesion changes in glycine release, were consistent with persistently weakened glycinergic inhibitory transmission ipsilaterally in the ventral CN and the LSO and bilaterally in the dorsal CN. Glycinergic inhibitory transmission was strengthened in the contralateral LSO and transiently strengthened in the MSO bilaterally. A hypothetical model of the findings suggested that glycine receptor regulation may depend on excitatory and glycinergic input to auditory neurons. The present changes in glycine receptor activity may contribute to altered auditory functions, which often accompany hearing loss.     

Identification of Respiratory Neurons Regenerating Axons into Peripheral Nerve Grafts in Adult Rats

The present aimed to identify the origin of medullary and upper cervical respiratory neurons regenerating their axons into the peripheral nerve grafts in adult rats. We employed an antidromic activation technique and a retrograde horseradish peroxidase (HRP) tracing method for determining the origins of the regenerating axons. Autologous segments of the common peroneal nerve were successfully implanted ventrolaterally into the proximal cut end of the C2 spinal cord hemisection (n = 24). Two to 5 months after implantation, spontaneous multi-unit discharges were recorded in all grafts; respiratory-related discharges in 21 (87.5%) grafts; and non-respiratory discharges in 3 (12.5%) grafts. The respiratory discharge patterns were similar to those for normal respiratory efferent neurons in rats. After the recordings were completed, 5 of 182 respiratory units explored in the medullary ventral respiratory cell group (VRG) neurons were antidromically activated by electrical stimulation of the grafts (n = 15). The estimated axonal conduction velocities ranged from 5.6 to 7.4 (mean 6.7) m/s. Retrograde horseradish peroxidase (HRP) labelings applied to the distal cut end of the grafts (n = 13) revealed that HRP-labeled cells were located, predominantly ipsilaterally, in the brainstem up to 5-10 mm distant from the implanted site. Some HRP-labeled cells were observed in the region of the nucleus ambiguus where many respiratory neurons exist. These results demonstrate that peripheral nerve grafts implanted in the C2 segment can induce axonal regeneration of medullary VRG neurons conveying funcional efferent signals.     

Nerve Growth Factor Receptor-immunoreactive Fibres Innervate the Reticular Thalamic Nucleus: Modulation by Nerve Growth Factor Treatment in Neonate,Adult and Aged Rats

Terminal arborizations expressing nerve growth factor receptor (NGF-R) have been detected with immunohistochemistry in the reticular thalamic nucleus of neonate, adult and aged rats. Intracerebroventricular administration of nerve growth factor (NGF) resulted in a dramatic increase in NGF-R immunoreactivity throughout the lifespan. This effect was paralleled by a concomitant increase in NGF-R immunopositivity in the neurons of the basal forebrain, which was here demonstrated also in aged animals, thus indicating that the NGF-R immunoreactivity within the reticular thalamic nucleus derives in all likelihood from cholinergic neuronal cell bodies of the basal forebrain. Our results demonstrate a prominent ability of NGF to up-regulate its receptors within fibres innervating the reticular thalamic nucleus, and show that this up-regulation of NGF-R is maintained throughout the lifetime. Altogether this indicates that the reticular thalamic nucleus may represent a new, important site of action of endogenous NGF or NGF-like molecules within the brain. In view of the crucial role played by the reticular thalamic nucleus in gating thalamocortical information, the autoregulation of NGF-R in this structure may have important concomitants in both physiological and pathological conditions.     

Retino-geniculate Axons Regenerating in Adult Hamsters Are Able to Form Morphologically Distinct Terminals

Adult mammalian retinal ganglion cells (RGCs) can regenerate through peripheral nerve (PN) grafts and innervate central nervous system (CNS) targets. Previous studies have demonstrated a basic level of differentiation of such projections. To further assess the specificity and functionality of regenerated CNS connections we have developed a new model in which RGCs are directed through a PN graft into the dorsolateral geniculate body (LGBd) while preserving the visual cortex and radiations. We also describe by light microscopy that regenerating RGC axons which enter the LGBd differentiate and form subtypes of RGC terminals reminiscent of those seen in the normal LGBd. Thus the adult CNS contains cues that permit phenotypic differentiation of terminal types during regeneration.     

成年S-D大鼠动眼神经核的显微解剖学研究

目的研究成年S-D大鼠动眼神经核的显微解剖学特征。方法成年S-D大鼠30只,经右眼球结膜入路行单眼外肌肌鞘内注射辣根过氧化物酶(HRP),逆行标记中脑动眼神经运动神经元,48小时后取中脑组织切片还原显色(TMB法),并在光镜和电镜下观察。结果动眼神经核的运动神经元直径平均为(19.20±1.15)μm,单侧核内的运动神经元总数为(1467±57.55)个,其中支配内直肌(424±41.39)个,支配上直肌(338±19.90)个,支配下直肌(238±20.01)个,支配下斜肌(276±16.72)个,支配提上睑肌(191±8.51)个。各组神经元有相对固定的解剖位置,其中下斜肌神经元位于核的背侧中间部,下直肌神经元位于核的背内侧,内直肌神经元位于核的腹内侧,上直肌神经元位于核的腹外侧,提上睑肌神经元位于核的外侧中部。结论动眼神经核内的运动神经元按功能不同集中分布(形成各个亚核),各亚核之间存在不同程度重叠,单一眼外肌均有来自双侧的运动神经元支配。     

Adult Retinofugal Axons Regenerating Through Peripheral Nerve Grafts Can Restore the Light-induced Pupilloconstriction Reflex

To investigate the ability of mature regenerating retinal axons to form functional connections within central targets, severed axons were guided into the primary visual centres which subserve the pupillary constriction reflex in response to light. The ocular stump of the transected optic nerve of adult rats was connected by means of an autologous peripheral nerve graft with the pretectal region which contains the relay nucleus of pupillary constriction, the olivary pretectal nucleus. This nucleus is efferently connected with preganglionic neurons in the oculomotor nuclear complex which innervates parasympathetically the muscle constrictors of the iris. Six to sixteen weeks after optic nerve transection and peripheral nerve transplantation, brisk responses were observed in the pupils upon illumination of the transplanted eye. Recovery of the pupil responses indicated that retinal neurons used the peripheral nerve 'bridge' to access the pretectum, in which they established synaptic contacts in sufficient density and with appropriate specificity to reconstitute the function of the traumatically interrupted neuronal circuitry.     

Development of Signals Influencing the Growth and Termination of Thalamocortical Axons in Organotypic Culture

Explants of embryonic or postnatal rat cortex, organotypically cultured in serum-free medium, maintain their structural integrity and their upper layers continue to mature. Coculture of portions of embryonic thalamus with cortical slices taken at different ages reveals a temporal cascade of cortical signals. (1) Slices of occipital cortex taken at E19 or earlier stimulate axonal outgrowth from explants of embryonic lateral geniculate nucleus but do not allow the fibers to invade. (2) In cortical slices taken after E19 but before P2, thalamic axons enter the slice, from any direction, and extend radially across the entire depth of the cortical plate without branching or terminating. (3) In slices taken after P2, fibers slow down, arborize, and terminate in the maturing layer 4 of the cortex. If the thalamic explant is placed against the pial surface of the cortical slice, axons still enter and branch in the same layer. These findings imply that the developing cortex expresses a diffusible growth-promoting factor and then itself becomes growth permissive, and finally the maturing layer 4 expresses a "stop signal." In triple cocultures of one thalamic explant with a "choice" of two neighboring slices, thalamic axons will not invade slices of cerebellum but behave indistinguishably in response to slices from any region of the hemisphere. Thus the initial tangential distribution of the thalamic projection in vivo (which is achieved by about E16) is unlikely to be controlled by regional variation in signals produced by the cortex. When cortical slices were precultured alone for 7-14 days before the addition of an explant of embryonic thalamus for 4 further days of coculture, the pattern of innervation was more appropriate to the chronological age of the slice than the age at which it was first taken. Thus the timing of the cascade of cortical properties is at least partly intrinsically determined. This sequence of expression of these signals suggests that they play a part in vivo in controlling the outgrowth of thalamic fibers, their accumulation under the cortical plate, their invasion of the plate, and their arborization in layer 4.     

Neurotrophic Effects of Testosterone on the Medial Nucleus of the Amygdala in Adult Male Rats

Our previous reports of major sex differences in the substance P-immunoreactive (SPir) innervation of the medial posterior divisions of the bed nucleus of the stria terminalis (BST) and medial nucleus of the amygdala in rats raised the question of the hormonal regulation of this innervation. We now report the results of two experiments which examined the effects of castration of adult males on the SPir innervation of these regions. In experiment 2 we asked whether castration might also alter the cytoarchitecture of these regions. In experiment 1 three groups; sham operated (Sham), castrated (C) and castrated plus testosterone (C + T) were examined at each of the three survival periods (2, 4 and 8 weeks) post castration. Animals of the C + T groups each received a 45 mm silastic implant of testosterone sc at the time of castration to maintain testosterone levels postoperatively. Castration produced a consistent and highly significant decrease in the area of dense SPir fiber staining in the posterior medial amygdala which became greater with increasing survival. By 8 weeks the area of staining was 42% smaller in group C as compared to the matched sham-operated group. Smaller decreases were seen in the size of the dense field of SPir fibers in the posterior part of the dorsomedial BST. Testosterone implants maintained the size of the SPir fields of fibers in both the medial amygdala and BST, as the areas of staining in the C + T groups were not significantly different from those in the Sham groups at any of the 3 survival times. In experiment 2 we measured the area and optical density of SPir fiber staining in the medial amygdala and medial BST at 8 weeks post-castration. In addition, we measured the size of the cell groups within these regions using cresyl-stained sections. As in experiment 1, at 8 wks following castration there was a marked decrease in the area of dense SPir staining in both the BST and medial amygdala. The sizes of the dense fields of fibers were reduced by approximately 23% in the BST and by 40% in the posterior medial amygdala. Castration also significantly reduced the optical density of staining within the medial amygdala. The major finding of experiment 2 is that castration affects the cytoarchitecture as well as the SPir staining in these areas. In the BST, the cell group BSTMPM receives most of the dense SPir innervation. Gonadectomy reduced the size of BSTMPM by approximately 28%. In the amygdala, the cell group MePD receives most of the dense SPir innervation. Gonadectomy reduced the size of MePD by approximately 27%, while its neighbour MePV was reduced by a similar degree (26%). These atrophic changes are at least somewhat specific, as other features such as brain weight, the overall size of the forebrain as estimated from whole coronal sections, and the size of the suprachiasmatic nucleus were unchanged. The atrophic changes in the cytoarchitecture of the posterior medial amygdala and BST suggest that the changes in SPir staining seen in experiments 1 and 2 may be secondary to structural atrophy, including reduced axonal and dendritic branching, of hormone responsive SP-containing neurons. The time course of the response to castration demonstrated in experiment 1 suggests that these changes in SP-containing neurons are relevant to the gradual decline in male sexual behavior which follows castration.     

Using Extracochlear Multichannel Electrical Stimulation to Relieve Tinnitus and Reverse Tinnitus-Related Auditory-Somatosensory Plasticity in the Cochlear Nucleus

ObjectivesTinnitus has no reliable cure but may be significantly relieved by the usage of cochlear implants. However, not all tinnitus patients necessitate cochlear implantation that can impair hearing. This study was to investigate whether a novel extracochlear electrical stimulation (EES) strategy could relieve tinnitus of guinea pigs without hearing impairment, and the roles of auditory-somatosensory plasticity in the cochlear nucleus in the tinnitus relief.Materials and MethodsWe used a novel four-electrode extracochlear implant to electrically stimulate the cochlea of tinnitus guinea pigs. Tinnitus was assessed by the gap-prepulse inhibition of the acoustic startle reflex (GPIAS) ratios and the tinnitus index. The plasticity of auditory and somatosensory innervation in the different subdivisions of cochlear nucleus was evaluated by immunostaining of vesicular glutamate transporter 1 (VGLUT1) and VGLUT2, respectively.ResultsThe EES induced significant decreases of GPIAS ratios and the tinnitus index of tinnitus guinea pigs, indicating reductions of tinnitus behavioral manifestations. Meanwhile, the EES reversed the abnormal auditory-somatosensory innervation in the cochlear nucleus of tinnitus animals but did not change the hearing and the numbers of inner hair cell synapses.ConclusionsThis study demonstrated that the novel EES strategy could effectively relieve tinnitus without impairment to hearing and cochlear structure of tinnitus animals. The reversal of tinnitus-related auditory-somatosensory plasticity in the cochlear nucleus was correlated with the tinnitus relief induced by the EES.