Shripad B. Deshpande

Indian red scorpion (Buthus tamulus) venom-induced augmentation of cardiac reflexes is mediated through the involvement of peripheral 5-HT3 and central 5-HT1A receptor subtypes.

The present study was undertaken to identify 5-HT receptor subtypes involved in Buthus tamulus (BT) venom-induced augmentation of cardiac reflexes elicited by phenyldiguanide (PDG). Intravenous injection of PDG (10 microg/kg) produced parallel decrease in mean arterial pressure (MAP) and heart rate (HR) in urethane anaesthetized rats (r=0.82; p < 0.001). Injection of PDG (1-40 microg/kg, i.v.) produced concentration-dependent decrease in time-response area of the HR. After BT venom (20 microg/kg) the concentration-response curve was shifted to the left. Further, fall of MAP and HR in response to submaximal concentration of PDG (10 microg/kg) were augmented significantly. Pretreatment with 5-HT3 receptor antagonist (ondansetron; 10 microg/kg) intravenously, blocked the BT venom-induced augmentation of PDG reflex but spiperone (100 microg/kg; 5-HT1A/5-HT2 antagonist) or ketanserin (300 microg/kg; 5-HT2 antagonist) failed to do so. Afferent discharges elicited by PDG (10 microg/kg) in vagus nerve were doubled after exposure to BT venom. Ondansetron (100 microg/kg, i.v.) totally abolished the discharges after exposure to BT venom but not by spiperone or ketanserin. Intracerebroventricular injection of spiperone (100 microg/kg) but not ketanserin or ondansetron, blocked the BT venom-induced augmentation of PDG reflex. Results show that the BT venom-induced augmentation of reflex elicited by PDG is mediated through the involvement of 5-HT3 receptors peripherally and 5-HT1A type of receptors centrally.

Indian red scorpion (Buthus tamulus) venom-induced augmentation of cardiac reflexes is mediated through the mechanisms involving kinins in urethane anaesthetized rats.

The mechanism underlying the action of Indian red scorpion (Buthus tamulus; BT) venom on cardiac reflexes was examined in urethane anaesthetized adult albino rats of either sex. Intravenous injection of phenyldiguanide (PDG) produced reflex hypotension, bradycardia and apnea lasting for > 60 s. The PDG-induced reflex responses (blood pressure, heart rate and respiration) were augmented greatly (magnitude and time period) after exposure to BT venom (100 microg/kg, i.v., for 30 min). However, there were no great alterations in resting blood pressure, heart rate and respiratory rate. Pretreatment with kallikrein kinin inhibitor (aprotinin; 6000 kallikrein inactivating unit, i.v.) blocked the BT venom-induced augmentation of PDG reflex response. Further, pretreatment with indomethacin (prostaglandin synthetase inhibitor; 10 mg/kg) and heparin (1000 units/kg) also blocked the venom-induced potentiation of the reflex. Captopril (15 mg/kg), an agent known to increase endogenous kinins, also augmented the PDG induced-reflex to the same extent as in BT envenomed rats. The captopril-induced augmentation of the reflex was blocked by aprotinin and heparin, but not by indomethacin. The results indicate that kinins and prostaglandins are involved in the BT venom-induced augmentation of the cardiac reflexes.

Pulmonary oedema produced by scorpion venom augments a phenyldiguanide‐induced reflex response in anaesthetized rats

1 The involvement of pulmonary oedema produced by scorpion venom in augmenting a phenyldiguanide (PDG)‐induced reflex response was evaluated in urethane‐anaesthetized rats. 2 PDG‐induced bradycardiac, hypotensive and apnoeic responses, expressed as time‐response area, exhibited similarities before or after venom treatment. Hence, the time‐response area of bradycardia was taken as a reflex parameter. Pulmonary oedema was determined by physical evaporation and histological methods. 3 Exposure to Indian red scorpion (Buthus tamulus, BT; i.v.) venom for 30 min increased the pulmonary water content (P < 0.05; Students t test) and augmented the PDG‐induced bradycardiac reflex response by more than 2 times (P < 0.001). The increase of pulmonary water content was maximal with 100 μg kg−1 of venom and the augmentation was maximal with 10 μg kg−1. In a separate series of experiments, the venom (100 μg kg−1)‐induced pulmonary oedema was confirmed by histological and physical methods. In this group also, the venom augmented the reflex to the same magnitude. 4 Pulmonary oedema (physical and histological) and augmentation of the bradycardiac reflex response after BT venom (100 μg kg−1; i.v.) were absent in animals pretreated with aprotinin, a kallikrein‐kinin inhibitor (6000 KIU; i.v.). 5 Ondansetron (10 μg kg−1; i.v.), a 5‐HT3 receptor antagonist, failed to block the venom‐induced pulmonary oedema (physical and histological) but blocked the venom‐induced augmentation of the reflex. 6 The results of this study indicate that the venom‐induced augmentation of the PDG reflex is associated with pulmonary oedema involving kinins utilizing 5‐HT3 receptors.

Involvement of nitric oxide in 3-nitropropionic acid-induced striatal toxicity in rats

The roles of nitric oxide (NO) in 3-nitropropionic acid (3-NPA)-induced toxicity were investigated using in vivo and in vitro models. Chronic 3-NPA administration (10 mg/kg) to rats produced selective striatal lesions that were associated with abnormal motor and EMG activities. In these animals, there was loss of glial fibrillary acidic protein (GFAP)-positive cells with extravasation of IgG in the lesion center, although microtubule-associated protein (MAP)-2-positive cells remained, indicating that astrocytes were involved. 3-NPA increased the NO(2)(-)/NO(3)(-) levels in microdialysates obtained from the striatum, thalamus and cerebellum. The basal NO(3)(-) level was much higher in the striatum than in the other areas. The NO(2)(-)/NO(3)(-) levels in the striatum were much higher in animals exhibiting abnormal muscular activity. Expression of endothelial NO synthase (eNOS), but not neuronal NOS (nNOS), was greatly increased in the striatum at 5 h after a second 3-NPA exposure, but not in other areas. In astrocyte cultures, the toxic effects of 3-NPA were associated with corresponding increases in the NO(2)(-) level, and this toxicity was attenuated by hemoglobin (Hb; 20 microM), which quenches NO. The NO(2)(-) generated by 3-NPA, even without cells, was also antagonized by Hb. 3-NPA, S-nitroso-n-acetyl-dl-penicillamine (SNAP) and sodium nitroprusside (SNP) all increased the NO current (detected by NO-sensitive electrodes) in concentration-dependent manners, and Hb significantly attenuated the NO generation induced by 3-NPA, SNAP or SNP. Taken together, these results suggest that 3-NPA generates NO both directly as a donor and indirectly by enhancing NOS expression to produce toxic effects on astrocytes and neuronal toxicity.

Toxicity of scorpion (Buthus tamulus) venom in mammals is influenced by the age and species

The present study was undertaken to determine the toxicity of scorpion (Buthus tamulus) venom in young and adult rats, as well as in different species of adult animals (rats, mice and guinea-pigs). The median lethal dose (LD50; mg/kg s.c.) of scorpion venom in young and adult rats was 2.2 +/- 0.21 and 1.3 +/- 0.14, respectively. The LD50 value for mice (7.2 +/- 1.35) was significantly greater than adult rats or guinea-pigs (1.14 +/- 0.08). The LD50 dose for i.v. route in anaesthetized adult rats was 95 +/- 13.2 micrograms/kg weight, which is 13 times less than that required for s.c. route. The results show that the lethality of scorpion venom in mammals differs with the age and species of the animals.

Involvement of phospholipase A2 pathway for the Indian red scorpion venom-induced augmentation of cardiopulmonary reflexes elicited by phenyldiguanide.

The present study was conducted to examine the role of phospholipase A(2) and prostaglandins in Indian red scorpion (Mesobuthus tamulus; MBT) venom-induced augmentation of cardiopulmonary reflexes elicited by phenyldiguanide (PDG). Trachea, femoral artery and jugular vein were cannulated in urethane anesthetized adult albino rats. The effect of jugular venous injection of PDG on ECG, BP and respiratory activity were recorded. Injection of PDG (10 microg/kg) evoked tachypnea/apnea, bradycardia and hypotension lasting for 60s. After injecting MBT venom (100 microg/kg) for 30 min, the PDG evoked reflex responses were augmented by two times and increased the pulmonary water content in envenomed animals, significantly. The venom-induced augmentation of PDG reflex and the increase in pulmonary water content were blocked in animals pretreated with B(2) kinin receptor antagonist (Hoe 140; 2.32 microg/kg). These responses induced by venom were also blocked by a phospholipase A(2) antagonist (PACOCF(3); 1 mg/kg) and a prostaglandin synthase inhibitor (indomethacin; 10 mg/kg). The observations indicate that the venom-induced responses (augmentation of PDG reflex response and increased pulmonary water content) involve PLA(2)-prostaglandin pathway that is triggered by B(2) kinin receptors to sensitize the receptors located on the vagal C-fibres.

Cardio-respiratory reflexes evoked by phenylbiguanide in rats involve vagal afferents which are not sensitive to capsaicin

Aim:  Stimulation of pulmonary C fibre receptors by phenylbiguanide (PBG, 5‐HT3 agonist) produces hypotension, bradycardia and tachypnoea or apnoea. However, tachypnoeic or apnoeic responses are not consistent. Therefore, this study was undertaken to delineate the actions of PBG on respiration and compared with those evoked by capsaicin (TRPV1 agonist).

B2 kinin receptors mediate the Indian red scorpion venom‐induced augmentation of visceral reflexes via the nitric oxide cyclic guanosine monophosphate pathway

Aim:  This study was performed to delineate the kinin (receptor)‐dependent pathways in the Indian red scorpion (Mesobuthus tamulus; MBT) venom‐induced pulmonary oedema as well as the augmentation of cardio‐pulmonary reflexes evoked by phenyldiguanide (PDG).

Indian red scorpion venom-induced augmentation of cardio-respiratory reflexes and pulmonary edema involve the release of histamine.

Pulmonary edema is a consistent feature of Mesobuthus tamulus (MBT) envenomation. Kinins, prostaglandins and other inflammatory mediators are implicated in it. Since, histamine also increases capillary permeability, this study was undertaken to evaluate whether MBT venom utilizes histamine to produce pulmonary edema and augmentation of cardio-respiratory reflexes evoked by phenylbiguanide (PBG). Blood pressure, respiratory excursions and ECG were recorded in urethane anaesthetized adult rats. Injection of PBG (10 μg/kg) produced apnoea, hypotension and bradycardia and the responses were augmented after exposure to venom (100 μg/kg). There was increased pulmonary water content in these animals. Pretreatment with pheniramine maleate (H₁ antagonist, 3 mg/kg) blocked both venom-induced augmentation of PBG response and pulmonary edema. In another series, compound 48/80 (mast cell depletor) was treated for 4 days then the PBG responses were elicited as before. At the end of the experiments, mast cells were counted from the peritoneal fluid. The venom-induced pulmonary edema and the augmentation of PBG reflex were not observed in compound 48/80 treated animals. Further, mast cells in the peritoneal fluid were absent in this group as compared to vehicle treated group (29 ± 7.9 cells/mm³). These observations indicate that venom-induced pulmonary edema and augmentation of PBG reflexe are mediated through mast cells by involving H₁ receptors.

Bisphenol A depresses compound action potential of frog sciatic nerve in vitro involving Ca2+-dependent mechanisms

Bisphenol-A (BPA), a toxic chemical from polycarbonate plastics, is known for behavioural and neural abnormalities. These neuro-behavioural changes reflect the changes in neural activity. However the effect of BPA on nerve action potential is not available. Therefore, present investigation was undertaken to study the effect of BPA on compound action potential (CAP) of frog sciatic nerve. Bundle containing small group of nerve fibres in a sciatic nerve was dissected and placed in a Perspex chamber perfused with Ringer solution. Suction electrodes were applied to the cut ends of the nerve for stimulating and recording purposes. The stimulation of one end (with supramaximal strength) produced CAP in the recording electrode. BPA (1-100 μM) decreased the amplitude and repolarization time of CAP in a concentration-dependent manner, without any alteration in latency, rise time and threshold. The decrease in amplitude was directly correlated with decrease in repolarization time (r=0.76). The BPA-induced decreases were absent in Ca(2+)-free medium or in presence of L-type Ca(2+)-channel antagonist (nifedipine/deltiazem). T and P type Ca(2+) channel antagonist (Ni(2+)) failed to block the BPA-induced responses. Pre-treatment with an Erα antagonist (tamoxifen) blocked the BPA-induced decrease in CAP parameters. These observations indicate that the BPA decreased the amplitude and repolarization time of CAP involving L-type Ca(2+)-channel dependent mechanisms. Further involvement of Erα in the modulation of Ca(2+) channels is a possibility.