Intrathecal MK-801 and local nerve anesthesia synergistically reduce nociceptive behaviors in rats with experimental peripheral mononeuropathy.

The hyperalgesia and spontaneous pain that occur following peripheral nerve injury may be related to abnormal peripheral input or altered central activity, or both. The present experiments investigated these possibilities by examining the effects of MK-801 (a non-competitive N-methyl-D-aspartate, NMDA, receptor antagonist) and bupivacaine (a local anesthetic agent) on thermal hyperalgesia and spontaneous nociceptive behaviors in rats with painful peripheral mononeuropathy. Peripheral mononeuropathy was produced by loosely ligating the rat's common sciatic nerve, a procedure which causes chronic constrictive injury (CCI) of the ligated nerve. The resulting hyperalgesia to radiant heat and spontaneous nociceptive behaviors was assessed by using a foot-withdrawal test and a spontaneous pain behavior rating method, respectively. CCI rats receiving 4 daily intraperitoneal (i.p.) MK-801 injections (0.03, 0.1, 0.3 mg/kg) beginning 15 min prior to nerve ligation exhibited less hyperalgesia (i.e., longer foot-withdrawal latencies) on days 3, 5, 7, 10, and 15 after nerve ligation as compared to those receiving saline injections. Thermal hyperalgesia also was reduced when a single MK-801 injection was given intrathecally (i.t.) onto the spinal cord lumbar segments on Day 3 after nerve ligation. This effect of postinjury MK-801 treatment was dose-dependent (2.5-20 nmol) and lasted for at least 48 h after injection. Moreover, i.t. injection of MK-801 (10 nmol) reliably lowered spontaneous pain behavior rating scores in CCI rats compared to those in the saline group. The spinal site of MK-801 action is situated within the caudal (probably lumbar) spinal cord, since i.t. injection of MK-801 (10 nmol) onto the spinal cord thoracic segments did not affect thermal hyperalgesia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intrathecal treatment with dextrorphan or ketamine potently reduces pain-related behaviors in a rat model of peripheral mononeuropathy

The therapeutic effects of dextrorphan and ketamine, two non-competitive N-methyl-d-aspartate (NMDA) receptor antagonists, on neuropathic pain-related behaviors were examined in rats with peripheral mononeuropathy induced by loose ligation of the common sciatic nerve (chronic constrictive injury, CCI). Four daily intrathecal treatments (beginning 1 h after nerve ligation) with dextrorphan or ketamine (12.5–100 nmol) reliably attenuated hyperalgesia to radiant heat and spontaneous pain-related behaviors in CCI rats. Thermal hyperalgesia also was reduced in CCI rats receiving a single intrathecal treatment with either dextrorphan or ketamine (50 and 100 nmol for each compound) on day 3 after nerve ligation when thermal hyperalgesia was well developed. Since both dextrorphan and ketamine are currently utilized in other clinical applications, the results suggest a new therapeutic utility of these ‘old’ compounds in treatment of neuropathic pain syndromes resulting from peripheral nerve injury.     

Intrathecal GM1 ganglioside and local nerve anesthesia reduce nociceptive behaviors in rats with experimental peripheral mononeuropathy.

Our previous experiments demonstrated that systemic treatment with GM1 ganglioside reduces nociceptive behaviors and spinal cord metabolic activity in a rat model of painful peripheral mononeuropathy produced by experimental sciatic nerve ligation (chronic constrictive injury, CCI). In the present study, we examined the effects of intrathecal (i.t.) GM1 treatment on thermal hyperalgesia and spontaneous pain behaviors resulting from nerve ligation in order to determine the locus of GM1 action. In addition, a local anesthetic agent, bupivacaine, given alone or combined with i.t. GM1, was applied to the injured sciatic nerve to determine if peripheral nerve anesthesia would influence post-injury nociceptive behaviors. Thermal hyperalgesia to radiant heat decreased in a dose-dependent manner when GM1 (10-80 nmol, i.t.) was administered once daily onto the lumbar segments of the spinal cord beginning 1 h after experimental nerve injury and continued for the first 9 days after nerve ligation. Moreover, this GM1 (80 nmol) treatment regimen reliably lowered spontaneous pain behavior rating scores in CCI rats suggesting the possible attenuation of spontaneous pain. The central site of i.t. GM1 action is located at the caudal (probably lumbar) spinal cord, since i.t. injection of 20 nmol GM1 onto the cervical spinal cord did not produce any protective effect. A single perinerve injection of a local anesthetic agent, bupivacaine (0.5%, 0.6 ml), on the 3rd day after nerve ligation reduced thermal hyperalgesia for at least 24 h following injection, a duration longer than that of the local anesthetic action of bupivacaine. Neither a single bupivacaine injection nor four daily i.t.(ABSTRACT TRUNCATED AT 250 WORDS)     

Post-injury treatment with GM1 ganglioside reduces nociceptive behaviors and spinal cord metabolic activity in rats with experimental peripheral mononeuropathy

In a rat model of painful peripheral mononeuropathy, this study examined the effects of post-injury treatment with a monosialoganglioside, GM1, on abnormal nociceptive behaviors and spinal cord neural activity resulting from loose ligation of the rat common sciatic nerve (chronic constrictive injury, CCI). Thermal hyperalgesia and spontaneous pain behaviors of CCI rats were assessed by measuring foot-withdrawal latencies to radiant heat and by rating spontaneous hind paw guarding positions, respectively. Neural activity within different regions of the spinal cord was inferred in both CCI and sham-operated rats by employing the [14C]-2-deoxyglucose (2-DG) autoradiographic technique to measure spinal cord glucose metabolism. Intraperitoneal (i.p.) GM1 treatment (10 mg/kg) initiated 1 h or 24 h after injury and once daily for the first 9 post-injury days reduced thermal hyperalgesia of the hind paw ipsilateral to nerve ligation and lowered spontaneous pain behavior rating scores in CCI rats. Sciatic nerve ligation reliably increased basal 2-DG metabolic activity of CCI rats in all four sampled regions (laminae I-IV, V-VI, VII, VIII-IX) of spinal cord lumbar segments (L2-L5) both ipsilateral and contralateral to nerve ligation 10 days after injury. Consistent with the drug's effects on spontaneous pain behaviors, 10 daily GM1 treatments (10 mg/kg, i.p.) initiated 1 h after nerve ligation reduced spinal cord 2-DG metabolic activity in laminae V-VI and VII ipsilateral to nerve ligation and in all four sampled regions contralateral to nerve ligation. This attenuation of the increased spinal cord glucose utilization that occurs in the absence of overt peripheral stimulation may reflect an influence of GM1 on increased neural activity contributing to spontaneous pain. Since gangliosides are thought to protect neurons from excitotoxic effects of excitatory amino acids, these results suggest that ganglioside treatment may result in attenuation of excitatory neurotoxicity that may occur following peripheral nerve injury. Thus, ganglioside treatment could provide a new approach to the clinical management of neuropathic pain syndromes following peripheral nerve injury.     

Effect of post-injury NMDA antagonist treatment on long-term Fos expression and hyperalgesia in a model of chronic neuropathic pain

Chronic constriction injury (CCI) of the sciatic nerve results in persistent mechanical hyperalgesia together with Fos protein expression in the lumbar spinal cord. We have examined the relationship between mechanical hyperalgesia and Fos expression within the lumbar spinal cord on days 14, 35 and 55 after either CCI or sham operation. To determine the role of NMDA receptor mechanisms in the maintenance of hyperalgesia and Fos expression, the NMDA antagonist MK-801 (0.3 mg kg-1 s.c.) was administered daily on days 28 to 34 after operation. CCI animals developed unilateral hind limb hyperalgesia that persisted unchanged from days 14 to 55 of the study. MK-801 treatment reduced hyperalgesia by 57% (p=0.02) on day 35 in CCI animals but did influence hyperalgesia at day 55. In the spinal cord, Fos positive cells were present bilaterally throughout laminae 3-10 at all time points examined in both CCI and sham group animals. Fos counts ipsilateral to the side of injury in laminae 3-10 correlated significantly with hyperalgesia scores in the CCI but not sham animals. MK-801 treatment resulted in a suppression of Fos expression in ipsilateral laminae 3-4 (p=0.0017) and laminae 5-10 (p=0.0026) of CCI animals on day 35. Fos expression in sham group animals was not inhibited by MK-801 treatment at day 35. These results indicate that Fos expression is maintained by differing mechanisms following nerve injury or sham operation. The functional consequences of Fos expression following nerve injury and sham operation are discussed.     

Differential effect of peripheral glutamate (NMDA,non-NMDA) receptor antagonists on bee venom-induced spontaneous nociception and sensitization

This study aimed to investigate the role of peripheral N-methyl-d-aspartate (NMDA) and non-NMDA receptor on (1). spontaneous nociception and (2). on sensitization induced by subcutaneous (s.c.) injection of bee venom (0.2mg/50 micro l) in rats. Peripheral s.c. administration of the competitive NMDA receptor antagonist dl-2-amino-5-phosphonovaleric acid (AP5), the non-competitive NMDA receptor channel blocker MK-801, and the competitive non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were performed before (pre-treatment) and after (post-treatment) bee venom-induced inflammation. Pre-treatment with AP5 (10mM, 50 micro l) and both pre-treatment and post-treatment with MK-801 (2mM, 50 micro l) into the same area of the bee venom injection site markedly reduced the bee venom-increased spontaneous responses of wide-dynamic range (WDR) neuron of the spinal cord. Post-treatment with the same dose of AP5 as well as pre-treatment and post-treatment with CNQX (5mM, 50 micro l) did not produce any inhibitory effects. Additionally, the role of peripheral NMDA and non-NMDA receptors on bee venom-induced mechanical allodynia and hyperalgesia were investigated and assessed by the paw withdrawal reflex to the innocuous and noxious mechanical stimulation. Peripheral administration of AP5, but not CNQX, reduced mechanical allodynia and hyperalgesia. The data suggest that the peripheral NMDA receptor, but not non-NMDA receptor, plays a pivotal role in the bee venom-induced persistent nociception and hyperexcitability.     

MK-801 blocks the development of thermal hyperalgesia in a rat model of experimental painful neuropathy

Loose ligation of the sciatic nerve in the rat can produce behavioral signs of hyperalgesia in the hindpaw. This study examined the effect of an NMDA (N-methyl-D-aspartate) receptor antagonist (MK-801) on the development of hyperalgesia in this model. Rats received i.p. injections of saline or MK-801 (1.0 mg/kg) prior to and then for 7 days after a unilateral sciatic nerve ligation. Testing of each hindpaw for latency to withdrawal from a standardized thermal stimulus was performed prior to ligation and then at 10, 12, 17, 27, and 37 days postoperatively. Hyperalgesia of the operated hindpaw developed in saline-treated animals as measured by a decrease in withdrawal latency. Hyperalgesia did not develop in animals treated with MK-801. MK-801 may therefore prevent the development of hyperalgesia following experimental nerve injury, possibly through an NMDA receptor-mediated effect.     

Cell death in the superficial dorsal horn in a model of neuropathic pain

The aims of this study were to investigate the occurrence of apoptotic cell death in the dorsal horn of the adult rat spinal cord following chronic constriction injury (CCI) to the sciatic nerve and to correlate this with behavioural responses. Six groups of six rats were used as follows: 1) CCI, 2) CCI, 3) MK801 + CCI, 4) axotomy, 5) sham, and 6) naive. Group 1 animals were behaviourally tested for thermal hyperalgesia 8 days following surgery and sacrificed and the spinal cords removed and frozen. The rest of the groups underwent the same procedure 14 days following surgery. The lumbar region of the spinal cord was cryosectioned and the incidence of apoptotic cells investigated using the TUNEL technique plus Hoechst double labelling. By 8 days post-CCI, hyperalgesia had developed in the ipsilateral paw, which was still present 14 days after the injury compared to the contralateral paw and naive and sham animals. Preemptive MK-801 prevented the onset of hyperalgesia. Significant numbers of apoptotic cells were present in the ipsilateral dorsal horn of the spinal cord 8 and 14 days following CCI compared to the contralateral side and to naive and sham animals. Preemptive treatment with MK-801 reduced the extent of apoptosis resulting from CCI to the level seen in control animals. This study demonstrates that cells undergo apoptosis as a result of CCI simultaneous with the occurrence of hyperalgesia. Furthermore, MK-801 prevents the onset of hyperalgesia and reduces the extent of apoptotic cell death, suggesting, perhaps, that apoptosis contributes to the initiation/maintenance of hyperalgesia.     

Depressor responses to L-proline microinjected into the rat ventrolateral medulla are mediated by ionotropic excitatory amino acid receptors

The essential amino acid L-proline produces a depressor response when microinjected into the caudal ventrolateral medulla (CVLM) of anesthetized rats. L-proline may activate some excitatory amino acid (EAA) receptors. The present study tested this hypothesis by investigating the effects of two ionotropic excitatory amino acid receptor antagonists on the depressor response to L-proline in the CVLM: the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptor-selective antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the N-methyl-D-aspartate (NMDA) receptor-selective antagonist MK801. Urethane-anesthetized rats received arterial catheters and their ventrolateral medulla surface was exposed. Injections of the antagonists CNQX and MK801 (2 mM, 68 nl in each case) into the CVLM completely blocked depressor responses to subsequent administration of AMPA (2 pmol/34 nl) and NMDA (2 pmol/34 nl), respectively. The depressor response to L-proline (3.4 nmol/34 nl) was strongly inhibited by prior injection of CNQX (2 mM, 68 nl) and significantly attenuated by prior injection of a high dose (20 mM, 68 nl), but not a low dose (2 mM, 68 nl), of MK801. The results indicate that the depressor response to L-proline in the CVLM includes mechanisms of ionotropic excitatory amino acid receptors.     

Role of glutamate receptors in the dorsal reticular nucleus in formalin‐induced secondary allodynia

The role of glutamate receptors present in the medullary dorsal reticular nucleus (DRt) in the formalin test and formalin‐induced secondary nociception was studied in rats. Secondary mechanical allodynia was assessed with von Frey filaments applied to the rat's hindpaw, and secondary thermal hyperalgesia was evaluated with the tail‐immersion test. The selective glutamate receptor antagonists MK801 (N‐methyl‐d ‐aspartate receptor antagonist), 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) (AMPA/KA receptor antagonist) and A841720 (metabotropic glutamate 1 receptor antagonist) were injected into the DRt before or 6 days after formalin injection in the rat. In the formalin test, the three antagonists significantly reduced the number of flinches in both phases of the test. DRt microinjection of MK801 or A841720, but not of CNQX, reduced both secondary nociceptive behaviors. Moreover, pre‐treatment with the three antagonists injected into the DRt prevented the development of secondary mechanical allodynia and secondary thermal hyperalgesia. Similarly, in these rats, the number of c‐Fos‐like immunoreactive neurons were markedly reduced in both the superficial and deep lamina of the dorsal horn. Our findings support the role of DRt as a pain facilitator in acute and chronic pain states, and suggest a key role of glutamate receptors during the development and maintenance of formalin‐induced secondary allodynia.     

In vitro and in vivo effects of a novel dimeric inhibitor of PSD‐95 on excitotoxicity and functional recovery after experimental traumatic brain injury

PSD‐95 inhibitors have been shown to be neuroprotective in stroke, but have only to a very limited extent been evaluated in the treatment of traumatic brain injury (TBI) that has pathophysiological mechanisms in common with stroke. The aims of the current study were to assess the effects of a novel dimeric inhibitor of PSD‐95, UCCB01‐147, on histopathology and long‐term cognitive outcome after controlled cortical impact (CCI) in rats. As excitotoxic cell death is thought to be a prominent part of the pathophysiology of TBI, we also investigated the neuroprotective effects of UCCB01‐147 and related compounds on NMDA‐induced cell death in cultured cortical neurons. Anesthetized rats were given a CCI or sham injury, and were randomized to receive an injection of either UCCB01‐147 (10 mg/kg), the non‐competitive NMDAR‐receptor antagonist MK‐801 (1 mg/kg) or saline immediately after injury. At 2 and 4 weeks post‐trauma, spatial learning and memory were assessed in a water maze, and at 3 months, brains were removed for estimation of lesion volumes. Overall, neither treatment with UCCB01‐147 nor MK‐801 resulted in significant improvements of cognition and histopathology after CCI. Although MK‐801 provided robust neuroprotection against NMDA‐induced toxicity in cultured cortical neurons, UCCB01‐147 failed to reduce cell death and became neurotoxic at high doses. The data suggest potential differential effects of PSD‐95 inhibition in stroke and TBI that should be investigated further in future studies taking important experimental factors such as timing of treatment, dosage, and anesthesia into consideration.     

Ameroid rings for gradual chronic constriction of the sciatic nerve in rats: contribution of different nerves to neuropathic pain

Mononeuropathy was induced by placing an ameroid ring around the sciatic nerve and was compared with chronic constriction injury (CCI) of the sciatic nerve [Pain 33 (1988) 87] in rats. Mechanical allodynia was assessed and the role of sciatic and saphenous afferents (Adelta and C) in thermal hyperalgesia investigated. A shorter duration of mechanical allodynia in ameroid rats as compared to CCI rats was observed. Thermal hyperalgesia was observed in the saphenous innervated skin of the hindpaw for Adelta and C nociceptors in ameroid and for Adelta nociceptors only in CCI rats, respectively. The sciatic innervated skin showed a thermal hypoalgesia with a fast onset for Adelta afferents and a slower onset for C afferents in CCI and ameroid rats. The duration of both thermal hypo- and hyperalgesia was longer in ameroid rats. We conclude that ameroid rings are a useful tool for the investigation of long-duration hyperalgesic effects of nerve injury, as the effects were more stable and seen for a longer time (>8 weeks) as compared to the CCI model. The uninjured saphenous afferents, in particular C fibers, mediate thermal hyperalgesia after chronic constriction of the sciatic nerve using an ameroid ring.     

Chronic dizocilpine or apomorphine and development of neuropathy in two rat models I: behavioral effects and role of nucleus accumbens

Dopaminergic and glutamatergic inputs converge on nucleus accumbens (NAC) and affect the neuropathic pain. We tested the effects of daily systemic administration of dizocilpine (MK-801), a N-methyl-d-Aspartate (NMDA) noncompetitive receptor antagonist, or apomorphine (APO), a dopamine (DA) D1 and D2 receptor agonist, on neuropathic manifestations in the chronic constriction injury (CCI) and the spared nerve injury (SNI) models of mononeuropathy in rats. Six groups of rats were subjected to CCI or SNI neuropathy and 5–7 days later received daily intraperitoneal (ip) injections of saline, MK-801, or APO for two weeks. Tests for nociception and motor behaviors were performed at regular intervals. Tactile and cold allodynia were assessed using von Frey hairs or acetone drops, respectively. Heat hyperalgesia was assessed by the paw withdrawal test. Tests were performed before administering the daily injections. Another four groups of rats were subjected to SNI surgery, and then had their NAC (contralateral to the lesioned paw) perfused for two weeks with MK-801, saline, APO + ascorbic acid, or ascorbic acid alone using mini-osmotic pumps. Behavioral manifestations were assessed as above. Systemic daily injections of MK-801 and APO markedly attenuated the neuropathic manifestations in the CCI and SNI models with a minimal effect on cold allodynia. The same results were seen in the SNI model with chronic perfusion of NAC. Our results suggest that daily systemic administration of DA agonists and NMDA antagonists can attenuate neuropathic pain manifestations and that the NAC is involved in the modulation of neuropathic-like behaviors.     

Nerve growth factor alleviates a painful peripheral neuropathy in rats

Nerve growth factor (NGF) reverses some effects of axotomy and prevents toxic neuropathy in adult rodents. We tested the effect of NGF on behavioral hyperalgesia resulting from a chronic constriction injury (CCI) of the sciatic nerve in the rat [5]. CCI rats exhibit thermal hyperalgesia as demonstrated by a reduction of paw withdrawal latency to a noxious thermal stimulus applied to the paw on the side of injury. The mechanical sensitivity of the ipsilateral hindpaw, assessed with von Frey filaments, was also significantly increased. There were no significant changes in nociceptive thresholds on the contralateral side. When NGF was infused directly on the ligated nerve via an osmotic pump (0.5 μg/μl/h for 7 days) immediately after the ligation, thermal hyperalgesia was abolished from postoperative days 5 up to at least two weeks. The CCI-induced decrease in mechanical threshold was also abolished by NGF. However, NGF had only a minor effect on the abnormally long response duration, a second measure of mechanical sensitivity, to the mechanical stimulus. Delayed infusion of NGF four days after the ligation failed to block hyperalgesia. Infusion of NGF on the sciatic nerve of rats that had no CCI had no significant effect on paw withdrawal latency. Infusion of anti-NGF antiserum did not enhance hyperalgesia in CCI rats. These results suggest that alterations in neurotrophic factor(s) contribute to the development of behavioral hyperalgesia in an animal model of neuropathy and that NGF may have therapeutic value in the treatment of neuropathic pain in humans.     

A role of excitatory amino acids in the activation of locus coeruleus neurons following cutaneous thermal stimuli

Previous electrophysiological experiments have shown that brain noradrenaline neurons in the locus ceruleus are activated by thermal cutaneous stimuli. In the present study a putative involvement of excitatory amino acids (EAA) in cutaneous LC activation was analyzed. Intraventricular administration of kynurenic acid (1 mumol), a broad spectrum EAA antagonist, or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 0.1 mumol) as well as subcutaneous administration of the specific NMDA antagonist MK 801 (2 mg/kg) almost totally abolished the response of LC neurons to both non-noxious and noxious cutaneous sensory stimuli. We propose that the activation of LC neurons following thermal cutaneous stimuli is mediated via release of EAA from nerve terminals emanating from nucleus paragigantocellularis (PGi).     

MK-801, an NMDA receptor antagonist, in the rostroventromedial medulla attenuates development of neuropathic symptoms in the rat.

Segmental ligation of spinal nerves in the rat induces a long-lasting hyperalgesia and allodynia that mimicks neuropathic conditions in humans. In the present study we attempted to determine whether supraspinal NMDA receptors contribute to the induction of the long-lasting hypersensitivity to noxious and innocuous mechanical stimulation following segmental ligation of spinal nerves in the rat. MK-801, an NMDA receptor antagonist, was microinjected into the rostroventromedial medulla (RVM) 15 min before or 25 min after the ligation of spinal nerves and mechanical hypersensitivity was assessed at various time points following surgery by determining the hindlimb withdrawal threshold to noxious and innocuous mechanical stimulation. A single dose of MK-801 administered prior to nerve ligation into the RVM significantly attenuated the development of mechanical hypersensitivity throughout the 2 week postoperative observation period, whereas corresponding administration of MK-801 immediately after the nerve ligation attenuated the development of mechanical hypersensitivity only during the first postoperative day but not later. The results indicate that NMDA receptors in the RVM are involved in triggering the enhanced sensitivity to mechanical stimulation induced by a nerve injury.     

Cardiorespiratory responses to glutamatergic antagonists in the caudal ventrolateral medulla of rats.

The role of caudal ventrolateral medullary (CVLM) depressor neurons in influencing arterial pressure and ventilation as well as the baroreflex control of arterial pressure was investigated, and the part played by excitatory N-methyl-D-aspartate (NMDA) and non-NMDA receptors in mediating the responses was determined. In urethane-anesthetized, spontaneously breathing rats unilateral microinjections into the caudal depressor area of the broad-band glutamatergic antagonist kynurenic acid (KYN, 5 nmol or 1.58 nmol), or NMDA antagonist 2-amino-5-phosphonovaleric acid (2-APV, 2.7 nmol), or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 0.257 nmol) caused a respiratory arrest within 4 min and the animals had to be artificially ventilated. Respiratory frequency increased on injecting KYN and CNQX while it did not change significantly with 2-APV. Apnea resulted from progressive decrease in tidal volume. During the apnea ventilation with 5% CO2 did not revive breathing. Mean arterial pressure (MAP) increased significantly with KYN and 2-APV injections but not with CNQX. The baroreflex decrease of MAP, elicited by left or right aortic depressor nerve stimulation, was significantly reduced or abolished after bilateral microinjections of all 3 antagonists. Ventilation as well as the baroreflex usually recovered after 1-1.5 h. Microinjections of the same doses of antagonists into the facial nucleus, as well as application of KYN (25 nmol) to the ventral medullary surface above the hypoglossal rootlets, had no significant effect. The results support previous findings that the CVLM neurons of the rat inhibit sympathetic neurons providing the vasomotor tone, and that an intact CVLM is obligatory for mediating the baroreflex decrease of arterial pressure. The results also indicate that: (1) the CVLM is essential for sustaining ventilation in the rat; (2) only NMDA receptors are involved in maintaining baseline blood pressure while both NMDA and non-NMDA receptors mediate the baroreceptor depressor reflex; and (3) both NMDA and non-NMDA receptor activation is necessary to sustain ventilation.     

Antagonism of non-NMDA receptors augments the neuroprotective effect of NMDA receptor blockade in cortical cultures subjected to prolonged deprivation of oxygen and glucose.

A 30-60 min period of oxygen and glucose deprivation induced widespread degeneration of cultured murine neocortical neurons. Neuronal degeneration could be blocked by adding the selective NMDA antagonist MK-801 to the bathing medium; however, if the deprivation period was prolonged to 90-105 min, the neuroprotective effect of MK-801 was overcome. The non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) at 1-100 microM concentrations also failed to protect neurons against this prolonged insult, but the combination of CNQX with either MK-801 or D-APV produced marked neuroprotection. This synergistic action of CNQX was not due to enhanced blockade of NMDA receptors, as it was not mimicked by combining MK-801 with D-APV or 7-chlorokynurenate. These observations support the idea that combined NMDA and non-NMDA receptor blockade may have value in ameliorating the neuronal loss associated with prolonged ischemic insults in vivo.     

Chronic dizocilpine or apomorphine and development of neuropathy in two animal models II: effects on brain cytokines and neurotrophins

Dopaminergic and glutamatergic mechanisms are involved in the development and modulation of neuropathy. Cytokines and neurotrophins can be also involved in the supraspinal maintenance of neuropathic pain. We assessed the effects of chronic intraperitoneal (ip) injection of dizocilpine (MK-801), a N-methyl-d-Aspartate (NMDA) noncompetitive receptor antagonist, or apomorphine (APO), a dopamine (DA) D1 and D2 receptor agonist, on neuropathic manifestations in the chronic constriction injury (CCI) and the spared nerve injury (SNI) models of neuropathy in rats. Six groups of rats were subjected to SNI or CCI (3 groups each) neuropathy and 5–7 days later received daily ip injections of saline, MK-801, or APO for two weeks. An additional control group was subjected to sham surgery without nerve lesion or injections. Rats were then sacrificed, and levels of IL-1β, IL-6, NGF, BDNF and GDNF were determined in the cingulum, striatum, and hippocampus. In both models, the neuropathy seen in the saline group was associated with decreased BDNF and an increase in IL-1β, IL-6, NGF and GDNF in most brain regions when compared to sham group. Chronic systemic MK-801 or APO injections decreased the neuropathic manifestations in both models, increased the BDNF level and modulated the other cytokines and neurotrophins. This modulation depended on the neuropathy model and the region/side of the brain studied. Our results showed that the changes in surpraspinal cytokines and neurotrophins could parallel neuropathic manifestations. These changes and the observed hyperalgesia can be modulated by chronic systemic injections of NMDA antagonists or DA agonists.     

Differential effects of (NMDA) and non-NMDA receptor antagonists on spinal release of amino acids after development of acute arthritis in rats

Following induction of acute knee joint arthritis in rats, an increase in the release of amino acids in the spinal dorsal horn occurs in two phases: (1) at the time of injection for all amino acids tested; and (2) a late prolonged phase for aspartate (Asp) and glutamate (Glu) (3.5–8 h). In the present study, the increased late phase release of Glu was reversed by posttreatment of the spinal cord with the (NMDA) receptor antagonist, AP7, but not with the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Asp late phase release in arthritic animals was unaffected by posttreatment of the spinal cord with either AP7 or CNQX. Arthritic animals became hyperalgesic to radiant heat stimuli by 4 h and this hyperalgesia was reversed by both CNQX and AP7. During the paw withdrawal latency (PWL) test for heat hyperalgesia, there was an increase in the glycine (Gly) and serine (Ser) concentrations in the dorsal horn. This increase in Gly and Ser was blocked by both CNOX and AP7. Indications of inflammation in arthritic animals posttreated with AP7, including increased joint circumference and temperature, were similar to animals that did not receive antagonists. Arthritic animals posttreated with CNQX, however, showed a reduction in the degree of joint swelling. Thus, both non-NMDA and NMDA receptors appear to play a role in the processing of the information evoked by stimuli in the periphery. The arthritis-induced release of Gly and Ser during the PWL test for heat hyperalgesia appears to be dependent on activation of both non-NMDA and NMDA receptors. In contrast, the data indicate that continued increased release of Glu in the late phase, normally observed following induction of arthritis, is dependent on NMDA but not on non-NMDA receptor activation. The continued increased release of Asp, however, appears to be independent of non-NMDA and NMDA receptor activation 5 h following initiation of events activated by induction of arthritis.