Activated Microglia in the Rat Spinal Cord Following Peripheral Axon Injury Promote Glial and Neuronal Plasticity Which is Necessary for Long-Term Neuronal Survival

Following the transection of peripheral sympathetic preganglionic axons comprising the cervical sympathetic trunk (CST), we observe robust glial and neuronal plasticity at 1 week post-injury in the rat spinal cord intermediolateral cell column (IML), which houses the injured parent neuronal cell bodies. This plasticity contributes to neuroprotection, as no neuronal loss in the IML is present at 16 weeks post-injury. Here, we administered the antibiotic minocycline or vehicle (VEH) daily for 1 week after CST transection to investigate the role of activated microglia in IML glial and neuronal plasticity and subsequent neuronal survival. At 1 week post-injury, minocycline treatment did not alter microglia number in the IML, but led to a dampened microglia activation state. In addition, the increases in oligodendrocyte (OL) lineage cells and activated astrocytes following injury in VEH rats were attenuated in the minocycline-treated rats. Further, the normal downregulation of choline acetyltransferase (ChAT) in the injured neurons was blunted. At 16 weeks post-injury, fewer ChAT+ neurons were present in the minocycline-treated rats, suggesting that activated microglia together with the glial and neuronal plasticity at 1 week post-injury contribute to the long-term survival of the injured neurons. These results provide evidence for beneficial crosstalk between activated microglia and neurons as well as other glial cells in the cord following peripheral axon injury, which ultimately leads to neuroprotection. The influences of microglia activation in promoting neuronal survival should be considered when developing therapies to administer minocycline for the treatment of neurological pathologies.

Elimination of microglia in mouse spinal cord alters the retrograde CNS plasticity observed following peripheral axon injury

Following the transection of peripherally located sympathetic preganglionic axons of the cervical sympathetic trunk (CST), transient retrograde neuronal and glial responses occur in the intermediolateral cell column (IML) of the spinal cord, the location of the parent neuronal cell bodies. The role of microglia in this central response to peripheral axon injury was examined in mice fed the PLX5622 diet containing colony-stimulating factor-1 receptor (CSF-1R) inhibitor for 28 days, which eliminated approximately 90% of spinal cord microglia. Microglia elimination did not impact baseline neurotransmitter expression in the IML neurons, and the typical neuronal plasticity observed following CST transection was unaffected. Oligodendrocyte precursor cells (OPCs) were significantly increased at one week post injury in the IML of mice fed the control diet, with no change in mature oligodendrocytes (OLs). Following microglia elimination, the baseline population of OPCs in the IML was increased, suggesting increased OPC proliferation. Injury in the microglia depleted mice resulted in no additional increase in OPCs. Though baseline astrocyte activation and GFAP protein expression were unaffected, microglia elimination led to increased activation and GFAP protein post injury when compared with mice fed the control diet. These results reveal that microglia regulate the baseline OPC population in the uninjured spinal cord and that activated microglia influence the activities of OL lineage cells as well as astrocytes. The regulatory roles of microglia observed in this study likely contribute to the long term survival of the IML neurons observed following the distal axon injury.

Retrograde influences of SCG axotomy on uninjured preganglionic neurons

There is evidence that neuronal injury can affect uninjured neurons in the same neural circuit. The overall goal of this study was to understand the effects of peripheral nerve injury on uninjured neurons located in the central nervous system (CNS). As a model, we examined whether axotomy (transection of postganglionic axons) of the superior cervical ganglion (SCG) affected the uninjured, preganglionic neurons that innervate the SCG. At 7 days post-injury a reduction in choline acetyltransferase (ChAT) and synaptophysin immunoreactivity in the SCG, both markers for preganglionic axons, was observed, and this reduction persisted at 8 and 12 weeks post-injury. No changes were observed in the number or size of the parent cell bodies in the intermediolateral cell column (IML) of the spinal cord, yet synaptic input to the IML neurons was decreased at both 8 and 12 weeks post-injury. In order to understand the mechanisms underlying these changes, protein levels of brain-derived neurotrophic factor (BDNF) and tyrosine receptor kinase B (TrkB) were examined and reductions were observed at 7 days post-injury in both the SCG and spinal cord. Taken together these results suggest that axotomy of the SCG led to reduced BDNF in the SCG and spinal cord, which in turn influenced ChAT and synaptophysin expression in the SCG and also contributed to the altered synaptic input to the IML neurons. More generally these findings provide evidence that the effects of peripheral injury can cascade into the CNS and affect uninjured neurons.

Increased Cx32 expression in spinal cord TrkB oligodendrocytes following peripheral axon injury

Following injury to motor axons in the periphery, retrograde influences from the injury site lead to glial cell plasticity in the vicinity of the injured neurons. Following the transection of peripherally located preganglionic axons of the cervical sympathetic trunk (CST), a population of oligodendrocyte (OL) lineage cells expressing full length TrkB, the cognate receptor for brain derived neurotrophic factor (BDNF), is significantly increased in number in the spinal cord. Such robust plasticity in OL lineage cells in the spinal cord following peripheral axon transection led to the hypothesis that the gap junction communication protein connexin 32 (Cx32), which is specific to OL lineage cells, was influenced by the injury. Following CST transection, Cx32 expression in the spinal cord intermediolateral cell column (IML), the location of the parent cell bodies, was significantly increased. The increased Cx32 expression was localized specifically to TrkB OLs in the IML, rather than other cell types in the OL cell lineage, with the population of Cx32/TrkB cells increased by 59%. Cx32 expression in association with OPCs was significantly decreased at one week following the injury. The results of this study provide evidence that peripheral axon injury can differentially affect the gap junction protein expression in OL lineage cells in the adult rat spinal cord. We conclude that the retrograde influences originating from the peripheral injury site elicit dramatic changes in the CNS expression of Cx32, which in turn may mediate the plasticity of OL lineage cells observed in the spinal cord following peripheral axon injury.

Transection of preganglionic axons leads to CNS neuronal plasticity followed by survival and target reinnervation

The goals of the present study were to investigate the changes in sympathetic preganglionic neurons following transection of distal axons in the cervical sympathetic trunk (CST) that innervate the superior cervical ganglion (SCG) and to assess changes in the protein expression of brain derived neurotrophic factor (BDNF) and its receptor TrkB in the thoracic spinal cord. At 1 week, a significant decrease in soma volume and reduced soma expression of choline acetyltransferase (ChAT) in the intermediolateral cell column (IML) of T1 spinal cord were observed, with both ChAT-ir and non-immunoreactive neurons expressing the injury marker activating transcription factor 3. These changes were transient, and at later time points, ChAT expression and soma volume returned to control values and the number of ATF3 neurons declined. No evidence for cell loss or neuronal apoptosis was detected at any time point. Protein levels of BDNF and/or full length TrkB in the spinal cord were increased throughout the survival period. In the SCG, both ChAT-ir axons and ChAT protein remained decreased at 16 weeks, but were increased compared to the 10 week time point. These results suggest that though IML neurons show reduced ChAT expression and cell volume at 1 week following CST transection, at later time points, the neurons recovered and exhibited no significant signs of neurodegeneration. The alterations in BDNF and/or TrkB may have contributed to the survival of the IML neurons and the recovery of ChAT expression, as well as to the reinnervation of the SCG.

Sympathetic reinnervation of peripheral targets following bilateral axotomy of the adult superior cervical ganglion

The ability of adult injured postganglionic axons to reinnervate cerebrovascular targets is unknown, yet these axons can influence cerebral blood flow, particularly during REM sleep. The objective of the present study was to assess quantitatively the sympathetic reinnervation of vascular as well as non-vascular targets following bilateral axotomy of the superior cervical ganglion (SCG) at short term (1 day, 7 day) and long term (8 weeks, 12 weeks) survival time points. The sympathetic innervation of representative extracerebral blood vessels [internal carotid artery (ICA), basilar artery (BA), middle cerebral artery (MCA)], the submandibular gland (SMG), and pineal gland was quantified following injury using an antibody to tyrosine hydroxylase (TH). Changes in TH innervation were related to TH protein content in the SCG. At 7 day following bilateral SCG axotomy, all targets were significantly depleted of TH innervation, and the exact site on the BA where SCG input was lost could be discerned. Complete sympathetic reinnervation of the ICA was observed at long term survival times, yet TH innervation of other vascular targets showed significant decreases even at 12 weeks following axotomy. The SMG was fully reinnervated by 12 weeks, yet TH innervation of the pineal gland remained significantly decreased. TH protein in the SCG was significantly decreased at both short term and long term time points and showed little evidence of recovery. Our data demonstrate a slow reinnervation of most vascular targets following axotomy of the SCG with only minimal recovery of TH protein in the SCG at 12 weeks following injury.

Transient changes in spinal cord glial cells following transection of preganglionic sympathetic axons

Following peripheral nerve injury, retrograde signals originating from the injury site may activate intrinsic factors in the injured neurons, possibly leading to regenerative growth. Retrograde influences from peripheral injury sites may lead to the activation of glial cells in the vicinity of the centrally located cell bodies of the injured neurons. Few studies have examined changes in the spinal cord intermediolateral cell column (IML), which houses sympathetic preganglionic cell bodies, following injury to distal axons in the cervical sympathetic trunk (CST). The goal of the present study was to determine if transection of the CST results in plasticity in glial cells in the IML. At 1 day following injury, changes in the expression of microglial marker Iba1 were observed and the typical oligodendrocyte-neuronal relationship was altered. By 7 days, astrogliosis, microglial aggregation, and increased numbers of oligodendrocytes, as well as enhanced glial-glial and glial-neuronal relationships were present. The majority of cases were similar to controls at 3 weeks following injury and no changes were observed in any cases at 10 weeks following the injury. These results revealed changes in astrocytes, microglia, oligodendrocytes in the spinal cord following transection of preganglionic axons comprising the CST, indicating their ability to respond to distal axonal injury.

Changes in NGF and NT-3 protein species in the superior cervical ganglion following axotomy of postganglionic axons

Mature sympathetic neurons in the superior cervical ganglion (SCG) are regulated by target-derived neurotrophins such as nerve growth factor (NGF) and neurotrophin-3 (NT-3). High molecular weight NGF species and mature NT-3 are the predominant NGF and NT-3 protein isoforms in the SCG, yet it is unknown whether the presence of these species is dependent on intact connection with the target tissues. In an attempt to determine the role of peripheral targets in regulating the neurotrophin species found in the SCG, we investigated the NGF and NT-3 protein species present in the SCG following axotomy (transection) or injury of the post-ganglionic axons. Following a 7 day axotomy, the 22-24 kDa NGF species and the mature 14 kDa NT-3 species in the SCG were significantly reduced by 99% and 66% respectively, suggesting that intact connection with the target is necessary for the expression of these protein species. As expected, tyrosine hydroxylase (TH) protein in the SCG was significantly reduced by 80% at 7 days following axotomy. In order to distinguish between the effects of injury and loss of target connectivity, the SCG was examined following compression injury to the post-ganglionic nerves. Following injury, no reduction in the 22-24 kDa NGF or 14 kDa mature NT-3 species was observed in the SCG. TH protein was slightly, yet significantly, decreased in the SCG following injury. The findings of this study suggest that the presence of the 22-24 kDa NGF and mature 14 kDa NT-3 species in the SCG is dependent on connection with peripheral targets and may influence, at least in part, TH protein expression in adult sympathetic neurons.

Characterization of trkB immunoreactive cells in the intermediolateral cell column of the rat spinal cord

The objective of the present study was to characterize the trkB receptor immunoreactive (-ir) cells in the intermediolateral cell column (IML) of the upper thoracic spinal cord. Small trkB-ir cells (area=56.1+/-4.4 microm(2)) observed in the IML showed characteristics of oligodendrocytes and were frequently observed in close apposition to choline acetyltransferase (ChAT)-ir cell bodies. Large trkB-ir cells (area=209.3+/-25.2 microm(2)) showed immunoreactivity for the neuronal marker NeuN, indicating their neuronal phenotype, as well as for ChAT, a marker for preganglionic neurons. TrkB and ChAT were co-localized in IML neurons primarily in cases that had received in vivo administration of nerve growth factor (NGF). These findings reveal two different cell types, oligodendrocytes and neurons, in the IML of the spinal cord that show trkB immunoreactivity, suggesting their regulation by brain derived neurotrophic factor (BDNF) and/or neurotrophin-4 (NT-4). In addition, there is evidence that NGF may play a role in the regulation of trkB-ir preganglionic neurons in the IML.

Regulation of NGF and NT-3 protein expression in peripheral targets by sympathetic input

Nerve growth factor (NGF) and neurotrophin-3 (NT-3) are target-derived proteins that regulate innervating sympathetic neurons. Here, we used western blot analysis to investigate changes in NGF and NT-3 protein in several peripheral tissues following loss of sympathetic input. Following removal of the superior cervical ganglion (SCG), large molecular weight (MW) NGF species, including proNGF-A, were increased in distal intracranial SCG targets, such as pineal gland and extracerebral blood vessels (bv). Mature NGF was a minor species in these tissues and unchanged following sympathectomy. Large MW NGF species also were increased when sympathectomy was followed by in vivo NGF administration. Mature NT-3, which was abundant in controls, was significantly decreased in these targets following sympathetic denervation. The decrease in mature NT-3 was enhanced following NGF administration. The trigeminal ganglion, which provides sensory input to these targets, showed increased NGF, but decreased NT-3, in these treatments, demonstrating that decreased NT-3 at the targets did not result from enhanced NT-3 uptake. Unlike pineal gland and extracerebral bv, the external carotid artery, an extracranial proximal SCG target, showed no change in NGF following denervation, and mature NT-3 was significantly increased. Following NGF administration, NT-3 was significantly decreased. We provide evidence for sympathetic regulation of NGF and NT-3 in peripheral targets and that elevated NGF can depress NT-3. The differential response in distal and proximal adult targets is consistent with the idea that neurons innervating proximal and distal targets may serve different roles in regulating neurotrophin protein. In addition, we conclude that previous ELISA results showing increased NGF protein following sympathetic denervation may have resulted from increases in large MW species, rather than an increase in mature NGF.

Estrogen regulation of neurotrophin expression in sympathetic neurons and vascular targets

We hypothesize that estrogen exerts a modulatory effect on sympathetic neurons to reduce neural cardiovascular tone and that these effects are modulated by nerve growth factor (NGF), a neurotrophin that regulates sympathetic neuron survival and maintenance. We examined the effects of estrogen on NGF and tyrosine hydroxylase (TH) protein content in specific vascular targets. Ovariectomized, adult Sprague-Dawley rats were implanted with placebo or 17beta-estradiol (release rate, 0.05 mg/day). Fourteen days later, NGF levels in the superior cervical ganglia (SCG) and its targets, the heart, external carotid artery, and the extracerebral blood vessels, as well as estrogen receptor alpha (ERalpha) content levels in the heart, were determined using semi-quantitative Western blot analysis. TH levels in the SCG and extracerebral blood vessels were determined by Western blotting and immunocytochemistry, respectively. Circulating levels of 17beta-estradiol and prolactin (PRL) were quantified by RIA. Estrogen replacement significantly decreased NGF protein in the SCG and its targets, the external carotid artery, heart and extracerebral blood vessels. TH protein associated with the extracerebral blood vessels was also significantly decreased, but ERalpha levels were significantly increased in the heart following estrogen replacement. These results indicate that estrogen reduces NGF protein content in sympathetic vascular targets, which may lead to decreased sympathetic innervations to these targets, and therefore reduced sympathetic regulation. In addition, the estrogen-induced increase in ERalpha levels in the heart, a target tissue of the SCG, suggests that estrogen may sensitize the heart to further estrogen modulation, and possibly increase vasodilation of the coronary vasculature.

Hypothetical anatomical model to describe the aberrant gag reflex observed in a clinical population of orally deprived children

In this 'clinical conundrum', we propose a hypothetical anatomical model to explain the abnormal gag reflex that is consistently observed in a clinical population of children experiencing feeding delays. This model is based on the presence of 'transient' connections formed during the normal development of autonomic brainstem circuitry involving the nucleus tractus solitarius (NTS). We propose that, as a result of normal feeding and swallowing, the activity of these transient fibers typically diminishes shortly after birth. In children who are orally deprived during infancy, these transient connections persist and the aberrant gag reflex is maintained into childhood. The most critical feature of the proposed model is the idea that swallowing during feeding initiates the retraction of the tactile 'transient' input to NTS. In the NICU feeding clinics, it has been suggested that triggering the gag reflex in neonates by tactile stimulation of non-oral body areas and anterior portions of the mouth directly or indirectly may contribute to oral feeding delays. To the contrary, we propose an anatomical model to suggest that oral feeding delays and lack of swallowing food, when experienced by neonates, actually contribute to the development of the aberrant gag reflex observed in later developmental stages.

Increased NGF proforms in aged sympathetic neurons and their targets

Target-derived neurotrophins such as nerve growth factor (NGF) and neurotrophin-3 (NT-3) regulate sympathetic neuron survival. Here, NGF and NT-3 protein and transcript were examined in sympathetic neurons and targets in order to determine their role in age-related neuronal atrophy. One obvious alteration was a dramatic increase (up to 50-fold) in NGF protein forms, corresponding to proNGF-B, in the superior cervical ganglion (SCG) and targets where sympathetic innervation shows atrophy. In the iris, where sympathetic innervation is protected into old age, proNGF-B was decreased. Alterations in NGF transcript paralleled changes in NGF protein, albeit to a lesser degree. Though significantly increased in aged SCG, NT-3 protein, found primarily as the 'mature' form, showed only minor changes in most tissues, though NT-3 mRNA generally was decreased. In contrast, both NT-3 transcript and NT-3 precursors were increased in iris. The dramatic increases in proNGF, together with minimal changes in NT-3, suggest that alterations in NGF regulation may contribute to the loss of sympathetic innervation observed in many aged peripheral targets.

'Mature' nerve growth factor is a minor species in most peripheral tissues

The classic neurotrophin hypothesis is based on the idea that innervating neurons derive 'mature' neurotrophin provided by the target for their survival. Yet large precursor forms of the neurotrophin nerve growth factor (NGF) have been reported in both central and peripheral tissues. In the present study, immunoblotting was used to survey peripheral tissues containing NGF-responsive neurons and to characterize various NGF species. These results demonstrate that 'mature' forms of NGF, i.e., the 13 and 16kDa species, are rare in sympathetic and sensory ganglia and in their peripheral targets, and that large molecular weight NGF precursors are abundant. In addition, certain NGF forms predominate in a given tissue, with each tissue exhibiting a characteristic NGF expression pattern. These findings suggest that NGF processing in peripheral tissues and in NGF-responsive ganglia may involve a variety of NGF species.

Reproductive activation of pine voles (Microtus pinetorum): examination of physiological markers

We tested the hypothesis that the presence of an opposite-sex conspecific will result in time-related changes in measures of reproductive activation. We housed male-female pairs of pine voles together for 0, 2, 6, 12, or 24 h before collecting blood, reproductive organs and brains for immunocytochemical analysis of LHRH and c-fos. Control animals were never exposed to an opposite-sex conspecific. Following exposure to a male, there was a significant increase in uterine weight but not in LH levels. In males, there were no changes in peripheral indices of activation, i.e. LH levels, testes and seminal vesicle weights were not altered. Consistent with no change in circulating levels of LH, there was no change in LHRH immunoreactivity at any time. However, c-fos immunoreactivity was significantly greater in both males and females in the cingulate cortex and rostral bed nucleus of the stria terminalis (BNST) at 2 h, and in the caudal BNST at 2, 6 and 12 h. Similarly, c-fos immunoreactivity was increased in the rostral MPOA in both males and females at 2 and 6 h. However, in the caudal MPOA, there was a significant interaction between sex and time due to increased c-fos immunoreactivity in females only at 6 h. These results indicate that, in both male and female pine voles, exposure to an opposite-sex conspecific is sufficient to produce rapid, neural activation in brain areas known to be involved in reproductive activation and sexual behavior. This early activation did not occur in LHRH neurons. It is not known if this activation, particularly at early times, is due to reproductive activation or to the formation of pair bonds.

The effects of deafferentation and exogenous NGF on neurotrophins and neurotrophin receptor mRNA expression in the adult superior cervical ganglion

Levels of nerve growth factor (NGF) and neurotrophin-3 (NT-3) protein and neurotrophin receptor mRNA in adult sympathetic neurons were investigated following surgical removal of preganglionic input and/or in vivo administration of NGF. Expression of trkC and p75, but not trkA, was significantly decreased following a 3-week deafferentation of the superior cervical ganglion (SCG). Protein levels of NGF and NT-3 in the SCG were unchanged by deafferentation. A 2-week intracerebroventricular infusion of NGF without deafferentation resulted in enhanced mRNA levels of trkA, trkC, and p75 as well as significantly increased NGF and NT-3 protein in the SCG. When NGF infusion followed deafferentation, both trkA and p75 showed significant increases while trkC levels were similar to control values. NGF protein was not increased in the SCG when deafferentation preceded exogenous NGF, yet NT-3 was elevated and levels were similar to cases receiving NGF infusion only. These results support a role for preganglionic input in trkC and p75 expression in adult sympathetic neurons. The increased levels of NT-3 protein and trkC gene expression observed following NGF infusion suggest that NGF influences NT-3 regulation in adult sympathetic neurons. In addition, the present findings provide evidence that, when preganglionic input is removed prior to the NGF infusion, NT-3 effectively competes with NGF for trkA binding. Taken together, we propose that NT-3 may play a role in the robust sprouting of sympathetic cerebrovascular axons previously observed following NGF administration, particularly when deafferentation precedes the NGF infusion period.

Sympathetic ingrowth to the trigeminal ganglion following intracerebroventricular infusion of nerve growth factor

The objective of the present study was to examine the remodeling of uninjured sympathetic axons in the adult rat trigeminal ganglion following a 2-week in vivo intracerebroventricular infusion of NGF. The accumulation of infused NGF in the trigeminal was assessed using ELISA and sympathetic fibers were localized immunohistochemically with an antibody to tyrosine hydroxylase (TH). In addition, high performance liquid chromatography coupled with electrochemical detection (HPLC-ECD) allowed for biochemical measurements of the catecholamines norepinephrine (NE) and dopamine (DA). Increased NGF protein in the trigeminal ganglion was paralleled by a significant increase in sympathetic fibers and pericellular plexuses (i.e. baskets) in the cell body regions. Some ganglia showed elevated NE following NGF infusion, yet the 88% increase in mean NE did not reach significance. Following bilateral removal of the sympathetic superior cervical ganglia (SCG), a significant reduction was observed in overall NE levels and in TH-immunoreactive (-ir) fibers in the cell body regions and peripheral branches, suggesting the SCG as the origin of the sympathetic ingrowth. However, mean DA levels as well as TH-ir fibers within the trigeminal central branch were unaffected by NGF infusion or removal of the SCG and likely resulted from intrinsic dopaminergic cell bodies. In conclusion, our data provide evidence that the increased availability of NGF in the young adult rat trigeminal ganglion observed following in vivo NGF infusion enhanced sympathetic associations with the sensory neurons in the trigeminal, supporting a role for NGF in the regulation of sympathosensory interactions.

Evidence that nerve growth factor mediates the formation of sensory pericellular baskets in the rat trigeminal ganglion

A role for nerve growth factor (NGF) in the remodeling of sensory neurons in the trigeminal ganglion was examined. Intracerebroventricular NGF infusion and/or bilateral removal of the sympathetic superior cervical ganglia, both of which are believed to increase the availability of NGF to primary sensory neurons, resulted in a significant increase in the frequency of calcitonin gene-related peptide immunoreactive pericellular baskets. The results of this study suggest that increased NGF is sufficient to enhance the formation of sensory baskets in this ganglion, and provide evidence that NGF may mediate the formation of sensory baskets in the sensory ganglia following injury.

Evidence for reduced accumulation of exogenous neurotrophin by aged sympathetic neurons

The present study investigated the potential for neurotrophin uptake by cerebrovascular axons and subsequent accumulation in the aged superior cervical ganglion (SCG) following a two week intracerebroventricular infusion of nerve growth factor (NGF). In the SCG from aged rats, NGF protein levels declined significantly compared with the SCG from young adult rats. Following NGF infusion, perivascular axons from both young adult and aged rats showed intense NGF immunostaining. In addition, significant increases in NGF protein were shown using enzyme-linked immunosorbent assay (ELISA) and in counts of NGF immunopositive cell bodies in the SCG when compared with age-matched controls. NGF accumulation in ganglia from aged rats, however, was significantly less when compared with ganglia from young adult rats. The results of the present study suggest that NGF protein is significantly reduced in aged ganglia with the neurons retaining some capacity to take up and transport exogenous neurotrophin. Even so, the potential for NGF accumulation is dramatically reduced in aged rats when compared with that of young adult rats. While previous results have shown robust NGF-induced neurotransmitter responses by sympathetic neurons from the aged animal, the present finding of reduced accumulation of NGF in aged sympathetic neurons suggests an age-related difference in the utilization or transport of NGF.

Plasticity of aged perivascular axons following exogenous NGF: analysis of catecholamines

The present study investigated the atrophy of aged perivascular sympathetic axons and the response of these cerebrovascular neurons to the neurotrophin nerve growth factor (NGF). Using high performance liquid chromatography coupled with electrochemical detection (HPLC-ECD) to quantify catecholamines and immunohistochemical methods to quantify the density of TH immunoreactive fibers, we found a significant decrease in norepinephrine (NE) and TH in aged sympathetic axons. However, following in vivo administration of exogenous neurotrophin, aged neurons exhibited a robust response to NGF that was similar to the young adult, suggesting little decline in the capability of aged neurons to utilize exogenous neurotrophin. These results suggest that the age-related atrophy of aged sympathetic axons may result primarily from reduced availability of target-derived neurotrophin rather than from intrinsic alterations of neuronal function.

Statistical comparison of axon-scaled neurochemical production

Treatments designed to increase neurochemical levels may also result in increases in the numbers of axons that produce the neurochemicals of interest. A natural research question is how does one compare the average neurochemical production per axon between two (or more) experimental groups. Two statistical methods are proposed for this problem. The first method utilizes a delta-method approximation to the variance of a function of random variables while the second method is based on the bootstrap. These methods are illustrated with data obtained from perivascular norepinephrine following intracerebroventricular infusion of neurotrophin nerve growth factor in adult rats and are studied in a small simulation experiment. The delta-method confidence intervals exhibited better coverage properties than the bootstrap alternative.

Remodeling of adult sensory axons in the superior cervical ganglion in response to exogenous nerve growth factor

In previous studies, we found that a 2-week in vivo intracerebroventricular infusion of nerve growth factor (NGF) elicited a sprouting response by sympathetic perivascular axons associated with the intradural segment of the internal carotid artery. We hypothesized that NGF infused into the ventricular system would be internalized by responsive sympathetic cerebrovascular axons, retrogradely transported to parent cell bodies in the superior cervical ganglion (SCG), and subsequently released into the local ganglionic environment. Because fibers exhibiting immunoreactivity for calcitonin gene related peptide (CGRP) have been localized in the SCG, we used immunohistochemical methods to investigate whether a response by CGRP-immunoreactive axons in the SCG occurred following the proposed transport to and release of exogenous NGF in the ganglion. In consecutive tissue sections of the SCG stained for either CGRP or NGF, we found CGRP pericellular 'baskets' surrounding identified NGF-immunoreactive cell bodies. Nerve growth factor infusion resulted in a significant increase both in the number of CGRP pericellular baskets and in NGF-immunoreactive cell bodies. A significant positive correlation (r=0.95, P<0.05) between the pericellular baskets and NGF-immunoreactive cell bodies was observed, suggesting that intracranial projection neurons in the SCG released infused NGF (or possibly a converted signal) into the local ganglionic environment to elicit remodeling of CGRP fibers to form pericellular baskets. These findings were confirmed in sections double labeled for NGF and CGRP immunoreactivity. This remodeling suggests that exogenous NGF may mediate retrograde transneuronal plasticity, allowing for future in vivo examinations of the mechanisms involved in neurotrophin transport and release.

Uninjured aged sympathetic neurons sprout in response to exogenous NGF in vivo

The extent to which the loss of plasticity by aged neurons is due to changes in the neuronal environment or to a loss of growth potential of the neurons has not been determined. In previous studies we observed that young adult cerebrovascular axons undergo a sprouting response following a 2-week intracerebroventricular infusion of nerve growth factor (15 microg; NGF). The present study used electron microscopy to examine the innervation of the intradural segment of the internal carotid artery of the aged rat and to determine whether aged sympathetic perivascular axons would respond to in vivo infusion of NGF. Young adult and aged Fischer 344 female rats received a 2-week intracranial infusion of NGF (15 microg) or vehicle (VEH) and were perfused for electron microscopy. Although there was no change in the total number of perivascular axons associated in aged VEH when compared with young adult VEH, a significant reduction was observed in aged VEH when total axons and sympathetic axons were expressed per microm2 vascular wall, reflecting an age-related increase in blood vessel size. Following NGF infusion, aged sympathetic axons were significantly increased by 192% compared with aged VEH cases. These results suggest that there is a proportional reduction in sympathetic cerebrovascular neurons with aging but that they exhibit robust sprouting in response to an exogenous neurotrophin.

Dose-dependent response of mature cerebrovascular axons in vivo following intracranial infusion of nerve growth factor

The perivascular axons associated with the intradural segment of the internal carotid artery undergo a sprouting response following intracerebroventricular infusion of nerve growth factor (NGF). The objective of the present study was to determine whether a relationship exists between the number of sprouted axons and the amount of NGF infused into the ventricular system. Using regression analysis, we observed a significant log-log relationship between the dose of NGF and number of axons. No significant relationship was observed for the control (VEH) group. A significant increase in axonal number was observed following infusion of 3.0 micrograms NGF and higher when compared with VEH treatment of similar concentration. Results of this analysis suggest that a maximal response to NGF is approximated at doses of 15 micrograms or higher. These findings suggest a dose-dependent relationship between the response of mature sympathetic cerebrovascular axons in vivo and the dose of exogenous NGF.

Sympathetic response to intracranial NGF infusion in the absence of afferent input: axonal sprouting without neurotransmitter production

The anatomical relationships between postganglionic sympathetic neurons, their targets, and their afferent inputs provide an opportunity to experimentally distinguish between the anterograde and the retrograde influences on neuronal responsivity to growth factors. In the present study, the effect of decentralization of the superior cervical ganglion (SCG) on the NGF-induced sympathetic sprouting response by mature cerebrovascular axons was assessed in young adult rats. Growth by the perivascular axons associated with the intradural segment of the internal carotid artery was quantified using electron microscopy and changes in norepinephrine (NE) levels were monitored using high-performance liquid chromatography coupled with electrochemical detection. The mean number of perivascular axons observed in the treatment group receiving decentralization of the SCG prior to NGF infusion was not significantly different from that in NGF-infused cases, suggesting that central input was not required for axonal growth of intact sympathetic neurons. However, decentralization prevented the typical NGF-induced increase in perivascular NE associated with the ICA, indicating that afferent input was necessary for the neurotransmitter increase to occur. Thus, afferent input appears to play a role in the regulation of neurotransmitter expression of the sprouted axons but is not required for trophic factor-induced axonal growth.

Increased perivascular norepinephrine following intracerebroventricular infusion of NGF into adult rats

In the present study, we used high performance liquid chromatography coupled with electrochemical detection to examine perivascular catecholamines associated with the intradural segment of the internal carotid artery following a 2-week in vivo intracerebroventricular infusion of the neurotrophin nerve growth factor (NGF). Following administration of NGF, a significant increase (87.3%) in perivascular norepinephrine (NE; microgram/g) was observed when compared with vehicle-infused controls, suggesting that increased sympathetic neurotransmitter accompanies the NGF-induced sprouting response by sympathetic perivascular axons previously observed using electron microscopy (13, 15). The biochemical quantification of perivascular NE in the present study taken together with our previous morphological quantification of perivascular sprouts at the ultrastructural level reveal that the increase in NE is not proportional to the increase in the number of axons. Thus, when compared with controls, the relative amount of norepinephrine per axon apparently is reduced following NGF infusion. The apparent decrease in NE per axon following NGF infusion suggests that, during the 2-week infusion period, exogenous NGF did not stimulate the biosynthesis of perivascular NE beyond that necessary to accommodate the newly sprouted axons. These results extend our morphological findings and provide evidence for plasticity of neurotransmitter expression by adult sympathetic perivascular axons in vivo. In addition, we provide evidence that the increased perivascular catecholamine histofluorescence previously observed following NGF infusion results from an increase in the number of perivascular axons associated with the vessel rather than from an increase in the amount of NE within individual axons.

Plasticity of mature sensory cerebrovascular axons following intracranial infusion of nerve growth factor

Mature perivascular sympathetic axons associated with the intradural segment of the internal carotid artery (ICA) of the adult rat respond by sprouting following a two week infusion of nerve growth factor (NGF) into the lateral ventricle of the brain. Because nonsympathetic axons such as those comprising the sensory and parasympathetic population have been shown to respond to NGF, the present study was carried out to determine whether mature sensory axons respond to in vivo NGF infusion and whether competitive interactions between the innervating populations might affect the responsiveness of these axons to NGF. Standard electron microscopic techniques as well as calcitonin-gene-related peptide (CGRP) immunohistochemistry at the light microscopic level were used to examine the effects of intracerebroventricular NGF infusion on mature perivascular fibers with and without prior sympathetic denervation (i.e., bilateral superior cervical ganglionectomy). Following NGF infusion, CGRP-immunoreactive fibers appeared thicker and more numerous in the longitudinal plane when compared with vehicle controls. However, at the ultrastructural level, a significant increase in the total number of axons was not observed, although there was an increase in the number of large granular vesicles, suggesting that the CGRP fibers responded to exogenous NGF with an increase in neurotransmitter content, but not by sprouting. Sympathetic denervation, on the other hand, resulted in a significant increase in the number of fibers passing in the circumferential plane. The most dramatic change in CGRP immunoreactivity was observed following combined sympathetic denervation and subsequent NGF infusion, where, in addition to the presence of thicker immunoreactive fibers, a significant increase in the perivascular density of immunoreactive fibers associated with the intradural blood vessels was observed. These findings suggest that exogenous NGF has different effects on mature sympathetic and nonsympathetic fibers that innervate intradural blood vessels. The former exhibit robust sprouting, whereas the latter do not sprout in response to NGF but show evidence for increased neuropeptide content. In addition, the heightened response by sensory axons following denervation and subsequent NGF infusion provides support for the idea that sensory and sympathetic axons normally compete for target space and/or target-derived neurotrophic factors.

The duration of sprouted cerebrovascular axons following intracranial infusion of nerve growth factor

We reported previously that a 2-week infusion of the trophic protein nerve growth factor (NGF) into the lateral ventricle of the adult rat brain elicits a sprouting response by perivascular axons associated with the intradural segment of the internal carotid artery. In the present study, we used electron microscopy to determine whether the sprouted axons persist following cessation of NGF delivery and, if not, to determine the time course of their disappearance. Our results demonstrate that NGF-induced sprouted axons do not persist following cessation of NGF delivery. The total number of axons at 1 week following the end of the NGF infusion was elevated compared to control values, but significantly reduced compared with NGF cases sacrificed immediately following the infusion period. Three weeks following the end of the NGF infusion, the total number of axons was similar to controls although there were no signs of axonal degeneration. These results suggest that continued elevation of NGF levels is necessary to maintain the sprouted axons and that endogenous levels of NGF, or other factors produced by the vascular target tissue, are not sufficient to maintain the newly formed axons. The demonstration that mature perivascular axons proliferate and disappear as a function of exogenous NGF exposure supports the accumulating evidence for continued plasticity in the mature nervous system.

Thalamic afferents of area 4 and 6 in the dog: a multiple retrograde fluorescent dye study

In the present study, we compared the distribution of thalamocortical afferents of cortical area 4 to that of cortical area 6 in the dog, using fluorescent tracers. Multiple injections of combinations of two dyes (diamidino yellow dihydrochloride, Evans blue, fast blue, granular blue) were made into either the anterior and posterior sigmoid gyri or into the medial and lateral regions of the anterior sigmoid gyrus in the anesthetized dog. We found that the thalamic afferents of areas 4 and 6 arise from topographically organized bands of cells that traverse several thalamic nuclei and extend throughout the rostrocaudal extent of the thalamus. The most medial band included area 6-projecting neurons in the anterior nuclei, the rhomboid nucleus, the ventral anterior nucleus (VA), ventromedial nucleus (VM) and mediodorsal nucleus (MD). Within this band, cells projecting to medial area 6 a alpha tended to be more numerous in the anterior nuclei, anterior parts of VA and VM and anterior and caudal parts of MD. Fewer cells in MD but more cells in caudal parts of VA and VM projected to lateral area 6 a beta. Lateral bands of cells in central through lateral parts of VA and VL projected topographically to lateral area 4 on the anterior sigmoid gyrus and lateral through medial parts of postcruciate area 4. The most lateral band of cells in VL continued ventrally into the zona incerta. Area 4 also received input from VM and the central lateral (CL) and centrum medianum (CM) nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve growth factor-induced sprouting of mature, uninjured sympathetic axons

The infusion of nerve growth factor (NGF) into the lateral ventricle of the mature rat brain elicits a sprouting response from axons associated with the intradural segment of the internal carotid artery. Using electron microscopic techniques, we observed a three-fold increase in the total number of perivascular axons. This NGF-elicited response is characterized by a dramatic reduction in glial cell ensheathment similar to that observed during development and by the presence of profiles devoid of organelles that may represent newly formed sprouts. In spite of the increase in axon number, no significant changes in the percentage of small, medium, or large axons were observed. The three-fold increase in the total number of axons was accompanied by an increase in the number of axons/fascicle but no change in the number of fascicles. This, along with the observation that a majority of sprouted axons were associated with other axons, supports the idea that the sprouted axons tend to associate preferentially with other axons. Bilateral superior cervical ganglionectomies following cytochrome C infusion indicate that approximately 60% of the axons associated with the internal carotid artery arise from the superior cervical ganglion and that the majority of axons contacting the smooth muscle layer arise from this ganglion. Sympathectomy following NGF infusion resulted in a 79% reduction in the total number of perivascular axons, demonstrating overwhelmingly that the majority of sprouted axons are sympathetic fibers. These results demonstrate that infusion of NGF into the mature rat brain results in the preferential sprouting of sympathetic axons associated with the internal carotid artery. These findings are consistent with the hypothesis that NGF normally plays a role in the regulation of autonomic cerebrovascular innervation in the adult animal and that mature, uninjured sympathetic neurons remain responsive to NGF.

Trichloroethylene affects learning and decreases myelin in the rat hippocampus

The goal of the present study was to relate previously observed behavioral effects with changes in myelin in the hippocampus following exposure to the industrial solvent 1,1,2-trichloroethylene (TCE). Young adult rats exposed to TCE via their drinking water underwent tests to evaluate their ability to perform spatial navigational tasks, and their brains were examined for changes in myelin in the hippocampus. Exposure to an average daily load of 5.5 mg TCE first for 4 weeks and then to 8.5 mg for an additional 2 weeks (separated by a 2-week interval) resulted in an increased level of performance in spatial navigational tasks. Examination of the brains from these animals revealed a significant decrease in the amount of myelin in one layer of the hippocampus, the stratum lacunosum-moleculare. No increase in performance was observed in rats exposed to an average daily load of 5.5 mg TCE for 4 weeks only. A reduction in myelin was observed, however, in the stratum lacunosum-moleculare of these animals. This decrease was not as severe as that seen in the twice-exposed animals. The results of this study suggest that exposure to TCE results in a reduction of hippocampal myelin, and that this reduction may be related to the increased level of performance observed following a second exposure to TCE.

Intracerebral NGF infusion induces hyperinnervation of cerebral blood vessels

The use of intracerebral NGF (nerve growth factor) infusions as a therapeutic tool to prevent the degeneration of cholinergic neurons in humans suffering from Alzheimer's disease has recently been suggested. In the present study, intracerebroventricular infusion of nerve growth factor into the adult rat brain was found to induce axonal sprouting of mature, uninjured axons associated with the intradural segment of the internal carotid artery. Following NGF infusion, a three-fold increase in the total number of axons associated with the vessel wall was observed when compared with vehicle-infused animals. This vascular hyperinnervation might also occur in humans. Before NGF infusion therapy is initiated, more research is necessary concerning the specificity, mechanisms, and functional significance of the sprouting response observed in this study.

Maternal exposure to 1,1,2-trichloroethylene affects myelin in the hippocampal formation of the developing rat

The effect of 1,1,2-trichloroethylene (TCE), an industrial solvent, on myelin in the dorsal hippocampus of the developing rat was investigated. Rat pups were exposed to TCE via their dams' drinking water while in utero and until they were sacrificed at 21 days of age. Frozen coronal sections through the dorsal hippocampus were stained for the presence of myelin using a modification of the Heidenhain procedure developed for frozen sections. A significant decrease in myelinated fibers was found in the stratum lacunosum-moleculare, an area comprised of distal dendritic profiles of CA1 pyramidal neurons which receive input from the entorhinal cortex. These findings suggest that the reduction in myelin in the hippocampus may be responsible, in part, for behavioral effects observed following TCE exposure.

Cholinergic innervation of canine thalamostriatal projection neurons: an ultrastructural study combining choline acetyltransferase immunocytochemistry and WGA-HRP retrograde labeling

Choline acetyltransferase (ChAT) immunocytochemistry and lectin-conjugated horseradish peroxidase (WGA-HRP) histochemistry were combined at the electron microscopic level to examine the morphology of cholinergic terminals in the canine centrum medianum-parafascicular complex (CM-Pf) and to localize cholinergic terminals making synaptic contact with retrogradely labeled CM-Pf thalamostriatal projection neurons. Following WGA-HRP injections into the caudate nucleus, CM-Pf neurons were heavily labeled with WGA-HRP reaction product. Examination with the electron microscope revealed retrogradely labeled neurons characterized by a large nucleus with deep infoldings of the nuclear envelope. ChAT-positive terminals were observed arising from small-diameter nonmyelinated axonal profiles. These terminals varied in size from 0.5 to 1.4 micron in long diameter. The smaller terminals (0.5-0.7 micron) were seen most frequently and established symmetrical or slightly asymmetrical synaptic contacts with small dendritic profiles. The larger ChAT-positive terminals (1.0-1.4 micron) were less frequently observed, contained several mitochondria and small clusters of pleomorphic vesicles, and contacted large dendritic shafts and cell somata. Some of the postsynaptic targets of both smaller and larger ChAT-positive terminals were identified as belonging to retrogradely HRP-labeled thalamostriatal neurons. These observations indicate that at least some thalamostriatal neurons within the CM-Pf complex are innervated by cholinergic terminals which probably arise from ChAT-positive cell bodies located within the pontomesencephalic tegmentum, particularly within the nucleus tegmenti pedunculopontinus and the laterodorsal tegmental nucleus. These findings provide evidence for direct influence by cholinergic brainstem nuclei over activities of thalamostriatal neurons.

Thalamostriatal projections from the ventral anterior nucleus in the dog

Thalamostriatal projections from the ventral anterior nucleus (VA) were mapped by using autoradiographic and horseradish peroxidase techniques in the dog. Injections of tritiated leucine and proline into the lateral, central, and medial parts of VA resulted in anterograde label over the dorsolateral, midlateral, and dorsal parts of the head of the caudate nucleus, respectively. The dorsolateral and midlateral parts of the caudate contained the heaviest label. No silver grains were located over the medial or ventral parts of the caudate. Light to moderate label was located over the most dorsal part of the putamen. After injections of lectin-conjugated horseradish peroxidase (WGA-HRP) into the dorsolateral or intermediate areas of the head of the caudate, retrogradely labeled cells were present in the lateral and central parts of VA, respectively. In cases with dorsolateral caudate injections, labeled cells formed a narrow dorsoventrally oriented band located in the lateral part of VA whereas in the case with a larger injection into midcaudate, large numbers of labeled neurons were scattered throughout the central area of VA. Retrogradely labeled cells were also found in the rostral part of the ventral lateral nucleus (VL). Injections of WGA-HRP into the medial part of the caudate resulted in only a few labeled cells located in the dorsomedial part of VA. Combining these data with those from other studies mapping neostriatal afferents from the cerebral cortex in the dog, it is apparent that the midlateral part of the caudate receiving input from VA also receives afferents from cortical area 6. Furthermore, the dorsolateral part of the caudate that receives input from the lateral part of VA also receives afferents from cortical area 4. These results indicate that the dorsal and lateral parts of the canine caudate nucleus may constitute important links in the transmission and integration of information related to complex motor activities.

Cholinergic and non-cholinergic projections from the canine pontomesencephalic tegmentum (Ch5 area) to the caudal intralaminar thalamic nuclei

The distribution and morphology of cholinergic and non-cholinergic neurons projecting to the caudal intralaminar thalamic nuclei from the Ch5 area in the dog were examined using a technique combining horseradish peroxidase (HRP) retrograde labeling with choline acetyltransferase (ChAT) immunocytochemistry. After processing for ChAT, cholinergic neurons were found primarily within the nucleus tegmenti pedunculopontinus (PPN) and the central tegmental tract (ctt). ChAT positive neurons were also located in the nucleus cuneiformis and among the fibers of the lateral lemniscus and medial longitudinal fasciculus. On the basis of immunocytochemical and cytoarchitectonic data, PPN was divided into two distinct cell groups - a compact cell group located dorsolateral to the brachium conjunctivum and a diffuse cell group intermingled among the fibers of the brachium conjunctivum. Tissue processed for WGA-HRP and ChAT following injections of lectin-conjugated horseradish peroxidase into either the centrum medianum (CM) or parafascicular (Pf) nucleus resulted in double labeled cholinergic projection neurons in both PPN and ctt. Injections which involved CM and the caudal part of the central lateral thalamic nucleus (CL) resulted in more retrogradely labeled neurons than did those injections involving Pf. Injections of CM and CL also resulted in more double labeled cells in the dorsolateral compact portion of PPN than did injections confined to Pf. In all cases a small number of cholinergic neurons located in the contralateral PPN were retrogradely labeled as well. A substantial number of retrogradely labeled neurons were not ChAT positive, and in some cases, comprised up to 27% of the total population of projection neurons. Measurements of cell soma areas indicated that cells comprising the general cholinergic population were mostly medium (300-600 micrograms2) or large (greater than 600 micrograms2) in size. The majority of cholinergic projection neurons fell within the medium size category while the noncholinergic projection neurons were significantly smaller than their cholinergic counterparts. The results of this study suggest that in the dog, Ch5 cholinergic neurons which project to the caudal intralaminar thalamic nuclei are medium in size and are located primarily within PPN and ctt. In addition, a parallel projection to the caudal intralaminar nuclei exists which originates from smaller, non-cholinergic neurons in these same regions. Based on the results of this study, it appears that cholinergic projections to intralaminar thalamic nuclei which in turn project to the neostriatum may be one of the pathways over which PPN can affect basal ganglia activity.

Combined immunocytochemistry and autoradiographic retrograde axonal tracing for identification of transmitters of projection neurons

A double labeling method is described which combines immunocytochemistry for identification of the neurotransmitter serotonin with autoradiographic retrograde axonal tracing using wheat germ agglutinin (WGA), N-[acetyl-3H]. The permanence, sensitivity, and distinctness of the two labels provide a valuable means for analyzing transmitter-identified projection neurons in the central nervous system. Combined immunohistochemical/autoradiographic preparations, following injections of WGA, N-[acetyl-3H] in the caudate-putamen of mice, revealed large numbers of serotonergic and fewer non-serotonergic raphe-striatal projection neurons.