Antihypertensive mechanisms underlying a novel salt-sensitive hypertensive model induced by sensory denervation

A novel model of hypertension recently developed in our laboratory shows that neonatal degeneration of capsaicin-sensitive sensory nerves renders a rat responsive to a salt load with a significant rise in blood pressure. To determine the role of the renin-angiotensin system and the sympathetic nervous system in the development of hypertension in this model, newborn Wistar rats were given capsaicin 50 mg/kg SC on the first and second days of life. Control rats were treated with vehicle. After they were weaned, male rats were divided into 6 groups and subjected to the following treatments for 2 weeks: control+high sodium diet (4%) (CON-HS), capsaicin+normal sodium diet (0.5%) (CAP-NS), capsaicin+high sodium diet (CAP-HS), capsaicin+high sodium diet+losartan (10 mg/kg per day) (CAP-HS-LO), capsaicin+high sodium diet+prazosin (3 mg/kg per day) (CAP-HS-PR), and capsaicin+high sodium diet+hydralazine (10 mg/kg per day) (CAP-HS-HY). Levels of calcitonin gene-related peptide in dorsal root ganglia were decreased by capsaicin treatment (P<0.05). Both tail-cuff systolic blood pressure and mean arterial pressure were higher in CAP-HS and CAP-HS-PR than in CON-HS, CAP-NS, CAP-HS-LO, and CAP-HS-HY (P<0.05). The 24-hour urinary volume and sodium excretion were increased when a high sodium diet was given (P<0.05), but they were lower in CAP-HS, CAP-HS-LO, CAP-HS-PR, and CAP-HS-HY than in CON-HS (P<0.05). Urinary potassium excretion was not different among all 6 groups. We conclude that blockade of the angiotensin type 1 receptor with losartan but not antagonism of the alpha1-adrenoreceptor with prazosin prevents the development of salt-sensitive hypertension induced by sensory denervation. Sensory denervation impairs urinary sodium and water excretion in response to a high sodium intake, regardless of blood pressure, suggesting that sensory innervation plays a direct role in regulating the natriuretic response to sodium loading.

A role for nerve growth factor in sympathetic sprouting in rat dorsal root ganglia

The role of nerve growth factor (NGF) and glial-derived neurotrophic factor (GDNF) in sympathetic sprouting within the dorsal root ganglion was investigated. In nerve-intact rats, intrathecal NGF (1 mg/ml, 14 days) but not GDNF (1 mg/ml, 14 days) induced extensive sprouting of tyrosine hydroxylase immunoreactive (TH-IR) fibres and formation of pericellular TH-IR baskets within lumbar DRGs. TH-IR baskets were distributed equally to trkA-expressing and trkA-negative neuronal profiles. Sciatic nerve transection (14-21 days) induced TH-IR baskets within lumbar DRG's around neuronal profiles with both intact and lesioned axons. The percentage of neuronal profiles surrounded by TH-IR baskets following sciatic transection was unaffected following peripheral application of the NGF sequestering antibody, trkA-IgG (1 mg/ml, 14 days). Intracellular responses were recorded from sensory neurons in an in vitro DRG/peripheral nerve preparation following bath application of noradrenaline. In preparations from animals treated 14 days previously with intrathecal NGF, 69% of neurons responded with depolarizing responses whilst 18% of neurons responded to bath applied noradrenaline in tissue prepared from naive animals. Our data indicate that sympathetic neurons sprout into the DRG in response to sciatic nerve injury and intrathecal NGF but not GDNF. Distribution of sympathetic sprouts within the DRG is independent of whether target neurons are injured or express trkA. Sequestration of NGF at the peripheral injury site does not influence basket formation within the DRG. It is likely that functional noradrenergic connections exist between sympathetic sprouts and sensory neuron cell bodies following exogenous NGF.

Effects of (+/-)-kavain on voltage-activated inward currents of dorsal root ganglion cells from neonatal rats

Kava pyrones extracted from pepper Piper methysticum are pharmacologically active compounds. Since kava pyrones exhibit anticonvulsive, analgesic and centrally muscle relaxing properties, the influence of a synthetic kava pyrone, (+/-)-kavain, on voltage-dependent ion channel currents was studied. Effects of (+/-)-kavain on voltage-activated inward currents were analysed in cultured dorsal root ganglion cells derived from neonatal rats. Voltage-activated Ca2+ and Na+ currents were elicited in the whole-cell configuration of the patch clamp technique. Extracellularly applied (+/-)-kavain dissolved in hydrous salt solutions reduced voltage-activated Ca2+ and Na+ channel currents within 3-5 min. As the solubility of (+/-)-kavain in hydrous solutions is low, dimethyl sulfoxide (DMSO) was added to the saline as a solvent for the drug in most experiments. When (+/-)-kavain was dissolved in DMSO, the drug induced a fast and pronounced reduction of both Ca2+ and Na+ currents, which partly recovered within 2-5 min even in the presence of the drug. The present study indicates that (+/-)-kavain reduces currents through voltage-activated Na+ and Ca2+ channels.

Analysis of the neurotrophic effects of GPI-1046 on neuron survival and regeneration in culture and in vivo

The putative neurotrophic effects of the immunophilin ligand GPI-1046 were evaluated in established experimental systems of neuron survival and axon growth in vitro and in vivo. GPI-1046 marginally increased neurite outgrowth of chick dorsal root ganglia in culture under conditions where a very robust effect of nerve growth factor was seen. GPI-1046 failed to protect dopaminergic neurons from 1-methyl-4-phenylpyridinium in culture or to protect cultured cortical neurons from experimentally induced apoptosis in vitro. In adult rats in vivo, daily administration of GPI-1046 (10 mg/kg, s.c.) for three days enhanced the maximal regeneration distance of both motor and large myelinated sensory axons measured using an electrophysiological assay. However, detailed morphometric analysis of these animals failed to provide evidence for an increase in axon numbers in GPI-1046-treated animals. The ability of GPI-1046 to promote the recovery of dopaminergic function following unilateral 6-hydroxydopamine lesions of the substantia nigra was also tested in rats. In the first study, the duration of amphetamine (3 mg/kg, s.c.)-induced circling, but not the maximal number of rotations, was significantly reduced in animals treated with GPI-1046 for five days (10 mg/kg/day). In a second study, testing the effects of delayed GPI-1046 administration, chronic treatment with GPI-1046 (10 mg/kg/day) for two weeks, beginning one month after surgery, did not alter circling responses. Morphometric analysis failed to reveal any changes in either the density of tyrosine hyroxylase-positive fibres in dopaminergic target areas or in cell numbers in the substantia nigra in both experiments. Thus, while GPI-1046 produced marginal effects on neurite outgrowth in dorsal root ganglia cultures and on functional paramaters of nerve regeneration in vivo, we failed to obtain evidence in support of the notion of a general neuroprotective effect of the compound or for an effect on morphologic nerve regeneration in vivo.

Herpes simplex virus type 1 vector-mediated expression of nerve growth factor protects dorsal root ganglion neurons from peroxide toxicity

Nerve growth factor beta subunit (beta-NGF) transgene delivery and expression by herpes simplex virus type 1 (HSV-1) vectors was examined in a cell culture model of neuroprotection from hydrogen peroxide toxicity. Replication-competent (tk- K mutant background) and replication-defective (ICP4(-);tk- S mutant background) vectors were engineered to contain the murine beta-NGF cDNA under transcriptional control of either the human cytomegalovirus immediate-early gene promoter (HCMV IEp) (e.g., KHN and SHN) or the latency-active promoter 2 (LAP2) (e.g., KLN and SLN) within the viral thymidine kinase (tk) locus. Infection of rat B103 and mouse N2A neuronal cell lines, 9L rat glioma cells, and Vero cells with the KHN or SHN vectors resulted in the production of beta-NGF-specific transcripts and beta-NGF protein reaching a maximum at 3 days postinfection (p.i.). NGF protein was released into the culture media in amounts ranging from 10.83 to 352.86 ng/ml, with the highest levels being achieved in B103 cells, and was capable of inducing neurite sprouting of PC-12 cells. The same vectors produced high levels of NGF in primary dorsal root ganglion (DRG) cultures at 3 days. In contrast to HCMV IEp-mediated expression, the LAP2-NGF vectors showed robust expression in primary DRG neurons at 14 days. The neuroprotective effect of vector produced NGF was assessed by its ability to inhibit hydrogen peroxide-induced neuron toxicity in primary DRG cultures. Consistent with the kinetics of vector-mediated NGF expression, HCMV-NGF vectors were effective in abrogating the toxic effects of peroxide at 3 but not 14 days p.i. whereas LAP2-NGF vector transduction inhibited apoptosis in DRG neurons at 14 days p.i. but was ineffective at 3 days p.i. Similar kinetics of NGF expression were observed with the KHN and KLN vectors in latently infected mouse trigeminal ganglia, where high levels of beta-NGF protein expression were detected at 4 wks p.i. only from the LAP2; HCMV-NGF-driven expression peaked at 3 days but could not be detected during HSV latency at 4 weeks. Together, these results indicate that (i) NGF vector-infected cells produce and secrete mature, biologically active beta-NGF; (ii) vector-synthesized NGF was capable of blocking peroxide-induced apoptosis in primary DRG cultures; and (iii) the HCMV-IEp functioned to produce high levels of NGF for several days; but (iv) only the native LAP2 was capable of long-term expression of a therapeutic gene product in latently infected neurons in vivo.

Spike generation from dorsal roots and cutaneous afferents by hypoxia or hypercapnia in the rat in vivo

The present study aimed at investigating the responsiveness of different parts of the primary afferent neurones to a brief hypoxia, hypercapnia or ischaemia under in vivo conditions. Action potentials were recorded in separate groups of anaesthetized rats from (i) the peripheral end of the central stump of the cut L3, L4 or L5 dorsal root (dorsal root preparation); (ii) the central end of the peripheral stump of the cut saphenous nerve (saphenous-receptor preparation); (iii) the distal end of a segment of the saphenous nerve cut at both ends (axon preparation). In paralysed animals interruption of artificial ventilation for 20-60 s elicited or increased the frequency of action potentials in both the dorsal root and saphenous-receptor preparations. Activation of these preparations was also achieved by inspiration of gas mixtures containing 10-0% oxygen (mixed with nitrogen) or 20-50% carbon dioxide (mixed with oxygen) which elicited in the blood a decrease in PO2 or an increase in PCO2 with a fall in pH. Occlusion of the femoral artery for 3 min also caused spike generation in the saphenous-receptor preparations with little alteration in blood pressure. All these stimuli failed to evoke action potentials in the axon preparations. Systemic (300 mg kg-1 s.c.) or perineural (2%) capsaicin pretreatment failed to inhibit the effect of hypoxia, hypercapnia or ischaemia, indicating a significant contribution of capsaicin-insensitive neurones to the responses. It is concluded that central and peripheral terminals but not axons of primary afferent neurones are excited by a brief hypoxia or hypercapnia and the peripheral terminals by a short local ischaemia as well. Excitation of central terminals by hypoxia or hypercapnia revealed in this way an antidromic activation of dorsal roots in response to natural chemical stimuli.

Cell type-specific ATP-activated responses in rat dorsal root ganglion neurons

1. The aim of our study is to clarify the relationship between expression pattern of P2X receptors and the cell type of male adult rat (Wistar) dorsal root ganglion (DRG) neurons. We identified the nociceptive cells of acutely dissociated DRG neurons from adult rats type using capsaicin sensitivity. 2. Two types of ATP-activated currents, one with fast, the other with slow desensitization, were found under voltage-clamp conditions. In addition, cells with fast but not slow desensitization responded to capsaicin, indicating that there was a relationship between current kinetics and capsaicin-sensitivity. 3. Both types of neurons were responsive to ATP and alpha, beta methylene-ATP (alpha,betameATP). The concentration of alpha,(beta)meATP producing half-maximal activation (EC50) of neurons with fast desensitization was less (11 microM) than that of neurons with slow desensitization (63 microM), while the Hill coefficients were similar. Suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium (PPADS) antagonized alpha,betameATP-induced currents in both types of neurons. 4. In situ hybridization revealed that small cells of the DRG predominantly expressed mRNAs of P2X3 and medium-sized cells expressed mRNAs of P2X2 and P2X3. In contrast, both of mRNAs were not detected in large cells of the DRG. 5. These results suggest that capsaicin-sensitive, small-sized DRG neurons expressed mainly the homomeric P2X3 subunit and that capsaicin-insensitive, medium-sized DRG neurons expressed the heteromultimeric receptor with P2X2 and P2X3.

Modulation of TTX-R INa by PKC and PKA and their role in PGE2-induced sensitization of rat sensory neurons in vitro

A tetrodotoxin-resistant voltage-gated Na+ current (TTX-R INa) appears to be the current primarily responsible for action potential generation in the cell body and terminals of nociceptive afferents. Although other voltage-gated Na+ currents are modulated by the activation of protein kinase C (PKC), protein kinase A (PKA), or both, the second messenger pathways involved in the modulation of TTX-R INa are still being defined. We have examined the modulation of TTX-R INa in isolated sensory neurons with whole-cell voltage-clamp recording. Activation of either PKC or PKA increased TTX-R INa. PKA activation also produced a leftward shift in the conductance-voltage relationship of TTX-R INa and an increase in the rates of current activation, deactivation, and inactivation. Inhibitors of PKC decreased TTX-R INa, whereas inhibitors of PKA had no effect on the current. Investigating the interaction between PKC and PKA revealed that although inhibitors of PKA had little effect on PKC-induced modulation of TTX-R INa, inhibitors of PKC significantly attenuated PKA-induced modulation of the current. Finally, although PGE2-induced modulation of TTX-R INa was more similar to PKA-induced modulation of the current than to PKC-induced modulation, PGE2-induced effects were inhibited by inhibitors of both PKC and PKA. Thus, although TTX-R INa is a common target for cellular processes involving the activation of either PKA or PKC, PKC activity is necessary to enable subsequent PKA-mediated modulation of TTX-R INa.

Comparative neurotoxicity of oxaliplatin, cisplatin, and ormaplatin in a Wistar rat model

Oxaliplatin (4 mg/kg), cisplatin (2 mg/kg with 20 mg/kg mannitol) and ormaplatin (2 mg/kg) were administered i.p. twice weekly for 4.5 weeks. Lactose injections (0.9%) were used as a control for oxaliplatin and 0.9% saline injections were used as a control for cisplatin and ormaplatin. Morphometric changes to dorsal root ganglia L4-L6 were quantitated as a measure of neurotoxicity. Drug treatment resulted in a decrease in cell and nuclear area and an increase in the percentage of cells with eccentric nucleoli for neuronal cell bodies in the DRG. Immediately following treatment the order of morphometric changes was ormaplatin > cisplatin > or = oxaliplatin. The accumulation of platinum in the DRG was measured by inductively coupled plasma mass spectrometry. The order of accumulation was cisplatin > oxaliplatin > ormaplatin. Following an 8-week recovery period the order of morphometric changes to the DRG was ormaplatin approximately equal to oxaliplatin > cisplatin. This correlated with a greater retention of platinum by the DRG for ormaplatin and oxaliplatin than for cisplatin. The results suggest that ormaplatin is uniquely neurotoxic immediately following treatment in the Wistar rat model. However, following an 8-week recovery period both ormaplatin and oxaliplatin are more neurotoxic than cisplatin and this neurotoxicity correlates with a greater retention of platinum by the DRG.

Effect of peripheral nerve injury on cGMP and nitric oxide synthase levels in rat dorsal root ganglia: time course and coexistence

Using the indirect immunofluorescence method, the distribution of cyclic GMP (cGMP) and nitric oxide synthase (NOS) was investigated in lumbar 5 dorsal root ganglia (DRGs) of untreated rats 1, 3 and 7 days following sciatic nerve section (axotomy). Untreated and axotomized (7 days) rats were also studied after perfusion with the NO donor sodium nitroprusside (SNP). Moreover, rats were injected with carrageenan lambda into the unilateral hindpaw and studied after 6 h, 1 day or 2 days. An increase in the number of cGMP-positive satellite cell profiles was found in axotomized DRGs at 3 days with lower numbers after 7 days. In contrast, no change in cGMP-like immunoreactivity (LI) in satellite cell profiles was detected 1 day after axotomy or 6h, 1 day or 2 days after inflammation, as compared to controls. Axotomy induced a marked increase in the percentage of NOS-immunoreactive (IR) neuron profiles in the ipsilateral DRGs as follows: 3.0% at 1 day, 15% at 3 days and 25% at 7 days, whereas no significant change was found in the expression of NOS-LI in the inflamed DRGs as compared to untreated DRGs. Between 15 and 20% of all NOS-positive neuron profiles were surrounded by, or in partial contact with, cGMP-IR satellite cells in controls 1 and 3 days after axotomy, whereas the corresponding figure was around 5% after 7 days. After SNP perfusion 60-70% of all DRG neuron profiles were partly or totally associated with cGMP-positive satellite cell profiles, with no significant difference between untreated and axotomized ganglia. The nerve injury-induced, parallel upregulation of NOS in DRG neurons and cGMP in satellite cells in the initial phase after axotomy suggests an involvement of NO as a signalling molecule between neurons and satellite cells in DRGs, especially after peripheral nerve injury, perhaps exerting a survival effect as recently proposed by Thippeswamy and Morris (1997).

Neurotoxicity of tetraphenylporphinesulfonate (TPPS4) and a hematoporphyrin derivative (Photosan) in organotypic cultures of chick embryonic dorsal root ganglia

The neurotoxic effect of tetraphenylporphinesulfonate (TPPS4) and a hematoporphyrin derivative (HPD, Photosan) has been studied in organotypic cultures of chick dorsal root ganglia maintained in a semi-solid culture medium. The changes in two characteristics of neurite outgrowth, the mean radial length of neurites growing out from the ganglia and the area of neurite outgrowths, are used as parameters to evaluate the toxic effect. The porphyrins are tested over the concentration range 10-160 micrograms ml-1. TPPS4 is slightly more toxic than the HPD Photosan. The median inhibitory concentration (IC50) for TPPS4 is 45-50 micrograms ml-1 and for the HPD Photosan 50-60 micrograms ml-1, respectively. Nevertheless, the toxicity of the two drugs is relatively low compared to that of commonly used anticancer drugs, such as cisplatin or taxol.

Differential effects of wortmannin on the release of substance P and amino acids from the isolated spinal cord of the neonatal rat

1. Effects of wortmannin, an inhibitor of myosin light chain kinase, on the release of substance P and amino acids, GABA and glutamate, were investigated in the isolated spinal cord preparation of the neonatal rat. 2. Wortmannin at 0.5 - 10 microM depressed the release of substance P evoked by high-K+ (90 mM) medium from the spinal cord (IC50 = 1.1 microM). Wortmannin also depressed the high-K+ (70 mM)-evoked release of substance P from cultured dorsal root ganglion neurons of neonatal rats. In contrast, the high-K+ (90 mM)-evoked release of GABA and glutamate from the spinal cord was not affected by wortmannin (0.1 - 10 microM). 3. Upon stimulation of a dorsal root, a monosynaptic reflex and a subsequent slow ventral root depolarization were evoked in the ipsilateral ventral root of the same segment in the isolated spinal cord preparation. The magnitude of the slow ventral root depolarization was depressed gradually to about 70% of the control during the course of 30 min under wortmannin (1 microM). In contrast, the monosynaptic reflex was unaffected by wortmannin. 4. Immunofluorescent staining revealed that immunoreactivities of substance P and myosin II were colocalized at presynaptic terminals in the dorsal horn of the neonatal rat spinal cord. 5. The present results suggest that myosin phosphorylation by myosin light chain kinase may play a crucial role in the release of substance P, but not in the release of GABA and glutamate in the neonatal rat spinal cord. This may reflect a difference in the exocytic mechanisms of substance P-containing large dense core vesicles and amino acid-containing small clear vesicles.

[The protective effect of the nerve growth factor in exposing a nerve tissue culture to diphtheria toxin]

Diphtheria toxin (1.10(-1)-1.10(-6) Lf/ml) was found to inhibit neurite extension in chick embryo dorsal root ganglia in vitro. If the nerve growth factor (60 ng/ml) was added with toxin in culture media the diphtheria toxin effect was decreased and the neurite outgrowth was compared with control. Protective effect of nerve growth factor by influence of diphtheria toxin may be used in new principles of diphtheria treatment.

Rapid and long-lasting suppression of the ATF-2 transcription factor is a common response to neuronal injury

The activating transcription factor 2 (ATF-2) protein, a neuronal constitutively expressed CRE-binding transcription factor, is essential for the intact development of the mammalian brain. ATF-2 is activated by c-Jun N-terminal kinases and modulates both the induction of the c-jun gene and the function of the c-Jun protein, a mediator of neuronal death and survival. Here we show by immunocytochemistry and Western blotting that ATF-2 is rapidly suppressed in neurons within 1-4 h following neuronal stress such as transient focal ischemia by occlusion of the medial cerebral artery, mechanical injury of the neuroparenchym, stimulation of adult dorsal root ganglion neurons in vitro by doxorubicin as well as within 24 h following nerve fiber transection. ATF-2 reappears and regains basal levels between 12 h and 72 h following ischemia, between 50 and 100 days following axotomy, but remains absent around the site of mechanical injury during the process of degeneration. Following ischemia and tissue injury, ATF-2-IR also disappeared in areas remote from the affected brain compartments indicating the regulation of its expression by diffusible molecules. These findings demonstrate that the rapid and persistent down-regulation of ATF-2 is a constituent of the long-term neuronal stress response and that the reappearance of ATF-2 after weeks is a marker for the normalization of neuronal gene transcription following brain injury.

Differential sensitivities of TTX-resistant and TTX-sensitive sodium channels to anesthetic concentrations of ethanol in rat sensory neurons

Ethanol at concentration of 200 mM induces anesthesia in experimental animals and depresses neurotransmission in isolated spinal cords. To determine whether actions on primary afferent nerve terminals contribute to ethanol's depressant effects on spinal cord, a study was undertaken to test whether ethanol blocks sodium currents (I(Na)) in dorsal root ganglion neurons (DRGn). Whole-cell patch clamp was used to examine I(Na) in DRGn isolated from 1- to 15-day-old rats. At a holding potential of -80 mV ethanol (200 mM) decreased peak tetrodotoxin-resistant (TTX-R) and tetrodotoxin-sensitive (TTX-S) I(Na) by 19.0% +/- 2.7 (mean +/- SEM) and 8.5% +/- 2.2, respectively. Maximal available I(Na) was reduced to 82 +/- 4% (TTX-R) and 93 +/- 1% (TTX-S) of control. Steady-state inactivation curves were shifted in the hyperpolarizing direction by 2.1 +/- 0.2 mV (TTX-R) and 1.1 +/- 0.1 mV (TTX-S). At prepulse potentials of -30 mV (TTX-R) and -70 mV (TTX-S), these shifts contributed an additional 17 +/- 1% (TTX-R) and 7 +/- 1% (TTX-S) reduction in available I(Na). Ethanol thus selectively induced both voltage-independent and voltage-dependent block of TTX-R I(Na) in DRGn. Because DRGn TTX-R sodium channels are associated with small-diameter primary afferent fibers, these results are consistent with a role for ethanol actions on sodium channels in depression of nociceptive-related neurotransmission in spinal cord.

The influence of physical structure and charge on neurite extension in a 3D hydrogel scaffold

Understanding neural cell differentiation and neurite extension in three-dimensional scaffolds is critical for neural tissue engineering. This study explores the structure-function relationship between a 3D hydrogel scaffold and neural cell process extension and examines the role of ambient charge on neurite extension in 3D scaffolds. A range of agarose hydrogel concentrations was used to generate varied gel physical structures and the corresponding neurite extension was examined. Agarose gel concentration and the corresponding pore radius are important physical properties that influence neural cell function. The average pore radii of the gels were determined while the gel was in the hydrated state and in two different dehydrated states. As the gel concentration was increased, the average pore radius decreased exponentially. Similarly, the length of neurites extended by E9 chick DRGs cultured in agarose gels depends on gel concentration. The polycationic polysaccharide chitosan and the polyanionic polysaccharide alginate were used to incorporate charge into the 3D hydrogel scaffold, and neural cell response to charge was studied. Chitosan and alginate were covalently bound to the agarose hydrogel backbone using the bi-functional coupling agent 1,1'-carbonyldiimidazole. DRGs cultured in chitosan-coupled agarose gel exhibited a significant increase in neurite length compared to the unmodified agarose control. Conversely, the alginate-coupled agarose gels significantly inhibited neurite extension. This study demonstrates a strong, correlation between the ability of sensory ganglia to extend neurites in 3D gels and the hydrogel pore radius. In addition, our results demonstrate that charged biopolymers influence neurite extension in a polarity dependent manner.

Developmental regulation of apoptosis in dorsal root ganglion neurons

The survival of dorsal root ganglion (DRG) neurons, both in vivo and in vitro, is dependent on the availability of nerve growth factor (NGF) for a transient period early in development after which these neurons become independent of NGF for survival. The precise molecular mechanism by which developing DRG neurons gain independence from NGF has not been determined. We used an in vitro model of DRG neuronal development to test hypotheses that independence from NGF in mature DRG neurons could be caused by developmental regulation of either elements of the NGF withdrawal signal transduction pathway or of proteins important for activation of the apoptosis output pathway. Interruption of phosphotidylinositol-3 kinase activation, by treatment with the specific inhibitor LY294002, resulted in apoptosis in immature but not mature DRG neurons in a manner similar to that observed with NGF withdrawal. Further downstream along the signal transduction pathway, c-JUN phosphorylation occurred in both immature and mature DRG neurons after NGF withdrawal or treatment with LY294002, despite the fact that the older neurons did not undergo apoptosis. In contrast, the ratio of expression of the proapoptotic gene bax to antiapoptotic gene bcl-xL was many times higher in immature than mature neurons, both in vivo and in vitro. Taken together, these results strongly suggest that developmental regulation of NGF withdrawal-induced apoptosis in DRG occurs via control of the relative level of expression of members of the bcl-2 gene family.

The expression of P2X3 purinoreceptors in sensory neurons: effects of axotomy and glial-derived neurotrophic factor

We have studied the distribution and regulation of the P2X3 receptor (a ligand-gated ion channel activated by ATP) in adult dorsal root ganglion (DRG) neurons using a polyclonal antibody. P2X3 receptor immunoreactivity was normally present in about 35% of L4/5 DRG neurons, virtually all small in diameter. In the dorsal horn, P2X3 receptor expression was restricted to the terminals of sensory neurons terminating in lamina IIinner. P2X3 receptors were expressed in approximately equal numbers of sensory neurons projecting to skin and viscera but in very few of those innervating skeletal muscle. P2X3 receptors were found mostly in sensory neurons that bind the lectin IB4. After sciatic nerve axotomy, P2X3 receptor expression dropped by more than 50% in L4/5 DRG. Glial cell line-derived neurotrophic factor (GDNF), delivered intrathecally, completely reversed axotomy-induced down-regulation of the P2X3 receptor. We conclude that P2X3 receptors are normally expressed in nociceptive primary sensory neurons, predominantly the nonpeptidergic nociceptors. P2X3 receptors are down-regulated following peripheral nerve injury and their expression can be regulated by GDNF.

Glycerol gangliotomy of the second dorsal cervical root in rats: an experimental study to evaluate a minimal invasive approach for the treatment of the chronic cervicogenic headache

Glycerol is a known agent in the therapy of chronic tic douloureux. It has been used for about 20 years in percutaneous, retrogasserian minimal-invasive rhizotomy, although the pharmacological mechanism of the pain relief involved remains unclear. To investigate glycerol treatment as a possible replacement for invasive approaches in the therapy of chronic cervicogenic headaches, we performed an experimental study on the pathomorphologic action of anhydrous glycerol injection into the second upper cervical dorsal root ganglion (DRG) of rats. Glycerol injections into the second cervical ganglion were investigated light- and electron-microscopically in a series of 40 rats for survival times of up to 30 days. We detected an unspecific overall effect on sensory neurons and satellite cells, as well as on myelinated and unmyelinated axons and Schwann cells. This could be detected after 5 days and sometimes led to degeneration of most of the neurons. Contralateral saline injections as a control showed no morphological effects. The loss of afferent fiber connections to the posterior horn of the myelon could be detected by immunohistochemical labeling of reactive astrocytes. Our results show a glycerol-induced deterioration of the cytoarchitecture of the neurons and their glial satellite cells. The effects on the ganglion cells appear to have been mediated by membrane disturbances and loss of glial integrity. These observations are contrary to previously reported results indicating the specific effect of glycerol on thin myelinated sensory axons.

Neuronal signals are required for estrogen-mediated induction of progesterone receptor in cultured rat Schwann cells

Rat glial cells from the central nervous system (CNS) and the peripheral nervous system (PNS) express steroid hormone receptors. Whereas progestin receptors (PR) in cultured glial cells of the CNS are estrogen-inducible, similar increase of PR in cultured Schwann cells, the glial cells of the PNS, prepared from newborn rat sciatic nerves, could not be demonstrated. In the present work we have used fetal dorsal root ganglion cultures to study the effect of estrogen and its antagonist ICI 164,384 on the expression of PR in rat Schwann cells. The PR levels were measured by hormone binding in whole cell assays or in cell cytosol, 18 h after excision of the ganglion from the cultures. Treatment of DRG-Schwann cell cultures with estradiol (E2) increased PR levels from about 60 to 160 fmol per mg cytosol protein, in untreated and estrogen-treated cells, respectively. This increase was dose-dependent; maximal induction was obtained at 50 nM E2-concentration. Treatment of the cultures with the antagonist ICI 164,384 completely inhibited the estrogen-induction of PR, whereas ICI alone did not influence receptor levels in Schwann cells. The estrogen-induction of PR was dependent on the presence of dorsal root ganglion during the period of estrogen treatment. Excision of the neuronal mass from the cultures caused a rapid decrease and disappearance of estrogen-inducible progestin receptors, whereas the concentration of non-inducible PR-binding sites remained unchanged. Estradiol had no influence on DRG-Schwann cell proliferation, only replated secondary Schwann cells showed a slightly higher level of proliferation in presence of 100 nM E2 and 5 microM forskolin. Receptors for estrogen (ER) were also demonstrated in DRG-Schwann cells by ligand binding experiments. Specific ER-binding was 36 +/- 8 fmol bound estradiol per mg cytosol protein. Finally, both PR and ER were visualized in Schwann cells by indirect immunofluorescence staining using specific anti-receptor antibodies. These findings suggest that the expression of estrogen-inducible progestin receptors in cultured glial cells of the PNS is mediated via intracellular estrogen receptors and that it requires the presence of neuronal signal(s).

Nerve growth factor increases sensitivity to bradykinin, mediated through B2 receptors, in capsaicin-sensitive small neurons cultured from rat dorsal root ganglia

To examine the effects of nerve growth factor (NGF) on the response to bradykinin (BK) of primary afferent neurons, intracellular recordings were obtained from small (< 30 microm) and large (> or = 35 microm) neurons in rat dorsal root ganglia (DRG). The response to BK in the small neurons was tested in 23 freshly dissociated neurons (dissociated group), 37 neurons cultured in the absence of NGF (no-NGF group) and 117 neurons in the presence of NGF (NGF group). Application of BK (10(-7) or 10(-5) M) induced a depolarization in a small number of neurons in the freshly dissociated (13%) and the no-NGF (11%) groups. After cultivation with NGF, the percentage of neurons that were depolarized by BK significantly increased to 46% after 2 days of cultivation. In the NGF group, the percentage of neurons sensitive to BK was significantly greater among capsaicin (CAP)-sensitive than among CAP-insensitive neurons (48 vs 20%). This BK-induced depolarization was completely blocked by a B2 receptor antagonist, but not a B1 receptor antagonist. With large neurons, in contrast, NGF did not increase the percentage that were BK-sensitive (9% in the dissociated group vs 0% after being cultured 2 days with NGF). These results demonstrate that NGF increases sensitivity to BK, mediated through B2 receptors only, in capsaicin-sensitive small neurons cultured from rat DRGs.

Enantioselective blockade of T-type Ca2+ current in adult rat sensory neurons by a steroid that lacks gamma-aminobutyric acid-modulatory activity

A number of steroids seem to have anesthetic effects resulting primarily from their ability to potentiate currents gated by gamma-aminobutyric acidA (GABAA) receptor activation. One such compound is (3alpha,5alpha, 17beta)-3-hydroxyandrostane-17-carbonitrile [(+)-ACN]. We were interested in whether carbonitrile substitution at other ring positions might result in other pharmacological consequences. Here we examine effects of (3beta,5alpha, 17beta)-17-hydroxyestrane-3-carbonitrile [(+)-ECN] on GABAA receptors and Ca2+ channels. In contrast to (+)-ACN, (+)-ECN does not potentiate GABAA-receptor activated currents, nor does it directly gate GABAA-receptor mediated currents. However, both steroids produce an enantioselective reduction of T-type current. (+)-ECN blocked T current with an IC50 value of 0.3 microM with a maximal block of 41%. (+)-ACN produced a partial block of T current (44% maximal block) with an IC50 value of 0.4 microM. Block of T current showed mild use- and voltage-dependence. The (-)-ECN enantiomer was about 33 times less potent than (+)-ECN, with an IC50 value of 10 microM and an amount of maximal block comparable to (+)-ECN. (+)-ECN was less effective at blocking high-voltage-activated Ca2+ current in DRG neurons (IC50 value of 9. 3 microM with maximal block of about 27%) and hippocampal neurons. (+)-ECN (10 microM) had minimal effects on voltage-gated sodium and potassium currents in rat chromaffin cells. The results identify a steroid with no effects on GABAA receptors that produces a partial inhibition of T-type Ca2+ current with reasonably high affinity and selectivity. Further study of steroid actions on T currents may lead to even more selective and potent agents.

Dieldrin and picrotoxinin modulation of GABA(A) receptor single channels

The insecticide dieldrin is known to suppress the GABA(A) receptor-channel complex in a manner similar to that of picrotoxin. To elucidate the more detailed mechanisms of dieldrin and picrotoxin interactions with the GABA system, single-channel patch clamp experiments were performed using rat dorsal root ganglion neurons in primary culture. Dieldrin did not alter the open time distribution and mean current amplitude or the distribution of burst duration and the mean burst duration. However, the mean closed time was prolonged indicating that dieldrin decreased the channel open probability. Previous studies have demonstrated that dieldrin and picrotoxin share the common binding site on the GABA receptor. Thus, the effects of picrotoxinin on the GABA(A) receptor single channels were also examined. Dieldrin and picrotoxinin had similar effects at the single-channel level. These changes of single-channel parameters explain the suppressive effects of these chemicals on GABA-induced whole-cell currents.

Are there functional P2X receptors on cell bodies in intact dorsal root ganglia of rats?

P2X purinoceptors have been suggested to participate in transduction of painful stimuli in nociceptive neurons. In the current experiments, ATP (1-10 mM), alpha,beta-methylene-ATP (10-30 microM) and capsaicin (10 nM-1 microM) were applied to neurons impaled with high resistance microelectrodes in rat dorsal root ganglia (L4 and L5) isolated in vitro together with the sciatic nerve and dorsal roots. The agonists were either bath applied or focally applied using a picospritzer. GABA (100 microM) and 40-80 mM K+ solutions gave brisk responses when applied by either technique. Only three of 22 neurons with slowly conducting axons (C cells) showed evidence of P2X-purinoceptor-mediated responses. Only two of 13 cells which responded to capsaicin (putative nociceptors), and none of 29 cells with rapidly conducting axons (A cells), responded to the purinergic agonists. When acutely dissociated dorsal root ganglion cells were studied using patch-clamp techniques, all but four of 30 cells of all sizes responded with an inward current to either ATP or alpha,beta-methylene-ATP (both 100 microM). Our data suggest that few sensory cell bodies in intact dorsal root ganglia express functional purinoceptors.

Antagonism of neuronal kainate receptors by lanthanum and gadolinium

The effects of lanthanum and gadolinium on currents evoked by excitatory amino acids were studied in cultured rat hippocampal and cortical neurons, in freshly dissociated dorsal root ganglion neurons, and in human embryonic kidney 293 cells expressing the GluR6 kainate receptor subunit. In all of these cells, currents mediated by kainate-preferring receptors were antagonized by low micromolar concentrations of the trivalent ions. At negative holding potentials, the IC50 values for inhibition in DRG cells were 2.8 microM for La and 2.3 microM for Gd. Kainate receptor-mediated currents in hippocampal neurons and in 293 cells expressing GluR6 were blocked by La with IC50 values of 2.1 and 4.4 microM, respectively. Steady-state inhibition by the lanthanides showed very slight dependence on membrane potential, however, we were not able to resolve any systematic variation with membrane potential in the kinetics of block onset or recovery. Inhibition was not use-dependent and was not overcome by increasing the concentration of agonist. These results indicate that lanthanides probably do not bind deep within the ion pore or directly compete for the agonist binding site. In contrast to neuronal AMPA receptors, which require more than 100 microM lanthanides for half-maximal blockade, the inhibition of neuronal and recombinant kainate receptors by these ions displays significantly higher potency.