Constitutive expression of calmodulin-binding phosphoprotein GAP-43 in rat serotonergic and noradrenergic cell groups which project to the spinal cord

Protocadherin-alpha family is required for serotonergic projections to appropriately innervate target brain areas

Serotonergic axons from the raphe nuclei in the brainstem project to every region of the brain, where they make connections through their extensive terminal arborizations. This serotonergic innervation contributes to various normal behaviors and psychiatric disorders. The protocadherin-alpha (Pcdha) family of clustered protocadherins consists of 14 cadherin-related molecules generated from a single gene cluster. We found that the Pcdhas were strongly expressed in the serotonergic neurons. To elucidate their roles, we examined serotonergic fibers in a mouse mutant (Pcdha(Delta CR/Delta CR)) lacking the Pcdha cytoplasmic region-encoding exons, which are common to the gene cluster. In the first week after birth, the distribution pattern of serotonergic fibers in Pcdha(Delta CR/Delta CR) mice was similar to wild-type, but by 3 weeks of age, when the serotonergic axonal termini complete their arborizations, the distribution of the projections was abnormal. In some target regions, notably the globus pallidus and substantia nigra, the normally even distribution of serotonin axonal terminals was, in the mutants, dense at the periphery of each region, but sparse in the center. In the stratum lacunosum-molecular of the hippocampus, the mutants showed denser serotonergic innervation than in wild-type, and in the dentate gyrus of the hippocampus and the caudate-putamen, the innervation was sparser. Together, the abnormalities suggested that Pcdha proteins are important in the late-stage maturation of serotonergic projections. Further examination of alternatively spliced exons encoding the cytoplasmic tail showed that the A-type (but not the B-type) cytoplasmic tail was essential for the normal development of serotonergic projections.

Proximal colon distension induces Fos expression in oxytocin-, vasopressin-, CRF- and catecholamines-containing neurons in rat brain

Little is known about the chemical coding of the brain neuronal circuitry activated by nociceptive signals of visceral origin. We characterized brain nuclei activated during isovolumetric phasic distension of the proximal colon (10 ml, 30 s on/off for 10 min) in conscious male rats, using Fos as a marker of neuronal activation and dual immunohistochemistry to visualize co-localization of Fos expression and oxytocin (OT), arginine-vasopressin (AVP), corticotrophin-releasing factor (CRF) or tyrosine hydroxylase (TH). Proximal colon distension, compared with sham distension, induced a robust increase in Fos-like immunoreactive (IR) neurons in the paraventricular nucleus (PVN), supraoptic nucleus (SON) and accessory neurosecretory nuclei of the hypothalamus, nucleus of the solitary tract (NTS) and ventrolateral medulla (VLM), and to a lower extent, in the locus coeruleus (LC) and Barrington nucleus. Fos-IR neurons in the PVN after colon distension were identified in 81% of OT-IR, 18% AVP-IR and 16% CRF-IR neurons, while in the SON it represented 36% of OT-IR and 16% AVP-IR. Catecholaminergic cell groups in the pons (LC) and medulla (VLM, NTS) were also activated by proximal colon distension. Of the TH-IR neurons in VLM and NTS, 74% and 42% respectively were double labeled. These results indicate that colon distension stimulates OT-, AVP- and CRF-containing hypothalamic neurons, likely involved in the integration of colonic sensory information to modulate autonomic outflow and pain-related responses. Activation of medullary catecholaminergic centers might reflect the afferent and efferent limbs of the functional responses associated to visceral pain.

Differential calmodulin gene expression in the nuclei of the rat midbrain-brain stem region

We investigated the expression patterns of the three calmodulin (CaM) genes, using in situ hybridization techniques, to detect gene-specific [(35)S]- and digoxigenin-labeled cRNA probes complementary to the multiple CaM mRNAs in the nuclei of the midbrain-brain stem region of the adult rat. The distinct CaM genes were widely expressed throughout this region with moderate intensities. In spite of the similar general pattern, significant differences in the distributions of the multiple CaM mRNA species were found in certain areas. In general, the CaM III mRNAs were most abundant, followed by the CaM I and CaM II mRNA populations. Most of the transcripts were found in the neuronal somata comprising the medullar nuclei, while much less label was detected in the neuropil. The CaM III mRNAs were more than 2.5 times more abundant than the CaM II mRNAs in the nucleus of the trapezoid body, and more than two times more abundant in the motor trigeminal nucleus, the principal sensory trigeminal nucleus and the olivary nucleus. The CaM III mRNAs were less dominant in the medial lemniscus, the inferior colliculus and the pontine reticular nucleus than those of the other CaM gene-specific transcripts. The CaM mRNA levels were low to moderate, without significant differences, in the mesencephalic trigeminal nucleus. The differential control of the expression of the CaM genes may contribute to the regulation of the multiple neuronal functions linked to this complex brain region and regulated by different CaM-dependent mechanisms via its target proteins.

A soluble Nogo receptor differentially affects plasticity of spinally projecting axons

In the central nervous system, regeneration of injured axons and sprouting of intact axons are suppressed by myelin-derived molecules that bind to the Nogo receptor (NgR). We used a soluble form of the NgR (sNgR), constructed as an IgG of the human NgR extracellular domain, to manipulate plasticity of uninjured primary afferent and descending monoaminergic projections to the rat spinal cord following dorsal rhizotomy. Rats with quadruple dorsal rhizotomies were treated with intrathecal sNgR or saline, or were left untreated for 2 weeks. Rhizotomy alone resulted in sprouting of serotonergic axons and to a lesser extent, tyrosine-hydroxylase (TH)-expressing axons, while axons expressing dopamine-beta-hydroxylase (DbetaH) were unaffected. Human IgG immunohistochemistry revealed that sNgR infused into the intrathecal space penetrated approximately 300 microm into spinal white and grey matter. Separate axonal populations differed in their responses to intrathecal sNgR: TH-expressing and DbetaH-expressing axons responded most and least vigorously, respectively. Serotonergic axons were identified by serotonin (5-HT) or serotonin transporter (SERT) immunohistochemistry. Interestingly, a large increase in 5-HT compared to SERT-positive axons density in both saline and sNgR-treated rats indicated that serotonergic axons both sprouted and increased their transmitter content in response to rhizotomy and sNgR treatment. Calcitonin gene-related peptide-positive axons were largely depleted ipsilaterally by rhizotomy, and sNgR increased axon density only in deeper contralateral laminae (III-V). GAP-43 immunohistochemistry revealed a small increase in axon density following dorsal rhizotomy that was further augmented by sNgR treatment. These results reveal a differential effect of myelin antagonism on distinct populations of spinally projecting axons.

GAP-43 is critical for normal development of the serotonergic innervation in forebrain

Serotonergic (5-HT) axons from the raphe nuclei are among the earliest afferents to innervate the developing forebrain. The present study examined whether GAP-43, a growth-associated protein expressed on growing 5-HT axons, is necessary for normal 5-HT axonal outgrowth and terminal arborization during the perinatal period. We found a nearly complete failure of 5-HT immunoreactive axons to innervate the cortex and hippocampus in GAP-43-null (GAP43-/-) mice. Abnormal ingrowth of 5-HT axons was apparent on postnatal day 0 (P0); quantitative analysis of P7 brains revealed significant reductions in the density of 5-HT axons in the cortex and hippocampus of GAP43-/- mice relative to wild-type (WT) controls. In contrast, 5-HT axon density was normal in the striatum, septum, and amygdala and dramatically higher than normal in the thalamus of GAP43-/- mice. Concentrations of serotonin and its metabolite, 5-hydroxyindolacetic acid, and norepinephrine were decreased markedly in the anterior and posterior cerebrum but increased in the brainstem of GAP43-/- mice. Cell loss could not account for these abnormalities, because unbiased stereological analysis showed no significant difference in the number of 5-HT dorsal raphe neurons in P7 GAP43-/- versus WT mice. The aberrant 5-HT innervation pattern persisted at P21, indicating a long-term alteration of 5-HT projections to forebrain in the absence of GAP-43. In heterozygotes, the density and morphology of 5-HT axons was intermediate between WT and homozygous GAP43-/- mice. These results suggest that GAP-43 is a key regulator in normal pathfinding and arborization of 5-HT axons during early brain development.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Spatiotemporal distribution of GAP-43 in the developing rat spinal cord: a histological and quantitative immunofluorescence study

In the rat spinal cord we studied developmental changes in spatiotemporal expression of the growth-associated protein GAP-43, which is known to play an important role in neural development, axonal regeneration, and modulation of synaptic function. GAP-43 was expressed predominantly in the white matter at embryonic day 13 to postnatal day 7, evenly in the white and gray matter at the 2nd to the 3rd postnatal week, and predominantly in the gray matter after the 5th postnatal week. The shifting of predominance was quantitatively assessed. On the basis of histological findings and quantitative assessment of GAP-43 immunoreactivity, it appears likely that the development proceeds from the phase of mostly axonal elongation during the embryonic period and the 1st postnatal week, via the phase of axonal elongation and formation of end arbors and synaptic organization during the 2nd to the 4th postnatal week, to the phase of final maturation of synaptic organization. GAP-43 was continuously expressed through adulthood in neuropil of the gray matter, the pyramidal tract, and the dorsal portion of the lateral funiculus that was identified as serotonergic by confocal laser scanning microscopic studies. The continuous expression may imply perpetual remodeling in these structures even in adulthood.

Synaptic vesicle proteins and neuronal plasticity in adrenergic neurons

The neurons in the superior cervical ganglion are active in plasticity and re-modelling in order to adapt to requirements. However, so far, only a few studies dealing with synaptic vesicle related proteins during adaptive processes have been published. In the present paper, changes in content and expression of the synaptic vesicle related proteins in the neurons after decentralization (cutting the cervical sympathetic trunk) or axotomy (cutting the internal and external carotid nerves) were studied. Immunofluorescence studies were carried out using antibodies and antisera against integral membrane proteins, vesicle associated proteins, NPY, and the enzymes TH and PNMT. For colocalization studies, the sections were simultaneously double labelled. Confocal laser scanning microscopy was used for colocalization studies as well as for semi-quantification analysis, using the computer software. Westen blot analysis, in situ 3'-end DNA labelling, and in situ hybridization were also employed. After decentralization of the ganglia several of the synaptic vesicle proteins (synaptotagmin I, synaptophysin, SNAP-25, CLC and GAP-43) were increased in the iris nerve terminal network, but with different time patterns, while TH-immunoreactivity had clearly decreased. In the ganglia, these proteins had decreased at 1 day after decentralization, probably due to degeneration of the pre-ganglionic nerve fibres and terminals. At later intervals, these proteins, except SNAP-25, had increased in the nerve fibre bundles and re-appeared in nerve fibres outlining the principal neurons.

trkA, trkB, and trkC messenger RNA expression by bulbospinal cells of the rat

Previous research has shown that corticospinal as well as rubrospinal neurons express the high-affinity trkB and trkC receptors but not the high-affinity trkA receptor. To determine if bulbospinal neurons in other brainstem areas show the same pattern of trk receptor expression, bulbospinal cells were labelled via the injection of the retrograde tracer FluoroGold into the spinal cord. Brainstem sections were then processed for in situ hybridization using oligonucleotide probes to the trkA, trkB, and trkC receptors. The results indicated that, although trkA expression occurred in brainstem areas that contain bulbospinal neurons (e.g., the vestibular nuclei, and the pontine reticular formation), very few FluoroGold-labelled cells expressed the trkA receptor. In contrast, at least 90% of bulbospinal cells in each brainstem area examined expressed the trkB receptor. Quantitative analysis indicated differences in the level of trkB labelling between bulbospinal cells in different brainstem areas, with the highest levels seen in the locus coeruleus and magnocellular portion of the red nucleus, and the lowest levels seen in the medial and superior vestibular nuclei and the raphe obscurus. With the exception of the accessory trigeminal nucleus, over 84% of bulbospinal cells in each brainstem area also expressed the trkC receptor. TrkC receptor expression was greatest in the locus coeruleus and subcoeruleus and lowest in the accessory trigeminal nucleus, the raphe magnus, and the vestibular nuclei. Results indicate that, as with other descending pathways, virtually all bulbospinal pathways should be amenable to treatment with brain-derived neurotrophic factor, neurotrophin-4/5 or neurotrophin-3, but not nerve growth factor, following spinal cord damage.

Co-localization of substance P and dopamine beta-hydroxylase with growth-associated protein-43 is lost caudal to a spinal cord transection

After spinal cord injury, abnormal responses of spinal cord neurons to sensory input lead to conditions such as autonomic dysreflexia, urinary bladder dyssynergia, muscle spasticity and chronic pain syndromes. These responses suggest that the spinal cord undergoes marked reorganization after an injury. In previous studies, we demonstrated changes in individual patterns of immunoreactivity for growth-associated protein-43, dopamine beta-hydroxylase and substance P that suggest growth and/or changes in expression of neurotransmitter enzymes and peptides in the cord caudal to a transection injury. In the present study we determined whether (i) growth-associated protein-43 and dopamine beta-hydroxylase or substance P were co-expressed in the same neurons prior to cord injury, and (ii) these patterns of expression changed after injury. A change in co-localization patterns caudal to an injury would suggest diversity in responses of different populations of spinal neurons. We used double-labelling immunocytochemistry to determine whether either dopamine beta-hydroxylase or substance P was co-localized with growth-associated protein-43 in control rats and in rats one, two or six weeks after spinal cord transection. We focused on the intermediate gray matter, especially the sympathetic intermediolateral cell column. In control rats, fibres travelling in a stereotyped ladder-like pattern in the thoracic gray matter contained growth-associated protein-43 co-localized with dopamine beta-hydroxylase or substance P. In spinal rats, such co-localization was also observed in spinal cord segments rostral to the cord transection. In contrast, caudal to the transection, substance P and growth-associated protein-43 were found in separate reticular networks. Immunoreactivity for dopamine beta-hydroxylase disappeared in fibres during this time, but was clearly present in somata. Immunoreactivity for growth-associated protein-43 was also found in somata, but never co-localized with that for dopamine beta-hydroxylase. These observations demonstrated co-localization of growth-associated protein-43 with dopamine beta-hydroxylase and substance P in descending spinal cord pathways. Caudal to a cord transection, this co-localization was no longer found, although each substance was present either in an abundant neural network or in somata. One population of spinal neurons responded to cord injury by expressing the growth-associated protein, whereas two others changed in the intensity of their expression of neurotransmitter peptides or enzymes or in the abundance of fibres expressing them. Thus, three populations of spinal neurons had distinct responses to cord injury, two of them increasing their potential input to spinal sensory, sympathetic or motor neurons. Such responses would enhance transmission through spinal pathways after cord injury.

The 21-aminosteroid U-74389F attenuates hyperexpression of GAP-43 and NADPH-diaphorase in the spinal cord of wobbler mouse, a model for amyotrophic lateral sclerosis

The wobbler mouse suffers an autosomal recessive mutation producing severe neurodegeneration and astrogliosis in spinal cord. It has been considered a model for amyotrophic lateral sclerosis. We have studied in these animals the expression of two proteins, the growth-associated protein (GAP-43) and the NADPH-diaphorase, the nitric oxide synthesizing enzyme, employing immunocytochemistry and histochemistry. We found higher expression of GAP-43 immunoreactivity in dorsal horn, Lamina X, corticospinal tract and ventral horn motoneurons in wobbler mice compared to controls. Weak NADPH-diaphorase activity was present in control motoneurons, in contrast to intense labeling of the wobbler group. No differences in diaphorase activity was measured in the rest of the spinal cord between control and mutant mice. A group of animals received subcutaneously for 4 days a 50 mg pellet of U-74389F, a glucocorticoid-derived 21-aminosteroid with antioxidant properties but without glucocorticoid activity. U-74389F slightly attenuated GAP-43 immunostaining in dorsal regions of the spinal cord from wobblers but not in controls. However, in motoneurons of wobbler mice number of GAP-43 immunopositive neurons, cell processes and reaction intensity were reduced by U-74389F. The aminosteroid reduced by 50% motoneuron NADPH-diaphorase activity. Hyperexpression of GAP-43 immunoreactivity in wobbler mice may represent an exaggerated neuronal response to advancing degeneration or muscle denervation. It may also be linked to increased nitric oxide levels. U-74389F may stop neurodegeneration and/or increase muscle trophism and stop oxidative stress, consequently GAP-43 hyperexpression was attenuated. Wobbler mice may be important models to evaluate the use of antioxidant steroid therapy with a view to its use in human motoneuron disease.

Growth associated protein 43 (GAP-43) mRNA is upregulated in the rat superior cervical ganglia after preganglionic transection

Growth-associated protein 43 (GAP-43) is a growth-associated protein which is synthesised in high amounts in neurons during neuronal outgrowth. In a previous study we have shown that GAP-43 immunoreactivity is increased in neurons in superior cervical ganglia (SCG) and in nerve terminals in the irides after preganglionic denervation. We have now examined changes in GAP-43 mRNA using in situ hybridisation. GAP-43 mRNA was seen to be constitutively expressed by principal neurons of the rat superior cervical ganglion. Expression was increased further by section of the cervical sympathetic trunk, reaching a maximum (increased by about 30%) 3 days after decentralisation. The increased GAP-43 protein seen after decentralisation thus appears to be due to an upregulation of GAP-43 mRNA in the adrenergic neurons. The results imply that GAP-43 expression in the SCG is under presynaptic control, acting at least partly by control of mRNA levels.