The effect of peripherally administered GABA on noradrenaline-induced reflex vagal bradycardia in urethane anaesthetized rats

Organic and Peptidyl Constituents of Snake Venoms: The Picture Is Vastly More Complex Than We Imagined

Small metabolites and peptides in 17 snake venoms (Elapidae, Viperinae, and Crotalinae), were quantified using liquid chromatography-mass spectrometry. Each venom contains >900 metabolites and peptides. Many small organic compounds are present at levels that are probably significant in prey envenomation, given that their known pharmacologies are consistent with snake envenomation strategies. Metabolites included purine nucleosides and their bases, neurotransmitters, neuromodulators, guanidino compounds, carboxylic acids, amines, mono- and disaccharides, and amino acids. Peptides of 2⁻15 amino acids are also present in significant quantities, particularly in crotaline and viperine venoms. Some constituents are specific to individual taxa, while others are broadly distributed. Some of the latter appear to support high anabolic activity in the gland, rather than having toxic functions. Overall, the most abundant organic metabolite was citric acid, owing to its predominance in viperine and crotaline venoms, where it chelates divalent cations to prevent venom degradation by venom metalloproteases and damage to glandular tissue by phospholipases. However, in terms of their concentrations in individual venoms, adenosine, adenine, were most abundant, owing to their high titers in Dendroaspis polylepis venom, although hypoxanthine, guanosine, inosine, and guanine all numbered among the 50 most abundant organic constituents. A purine not previously reported in venoms, ethyl adenosine carboxylate, was discovered in D. polylepis venom, where it probably contributes to the profound hypotension caused by this venom. Acetylcholine was present in significant quantities only in this highly excitotoxic venom, while 4-guanidinobutyric acid and 5-guanidino-2-oxopentanoic acid were present in all venoms.

Functional GABAA receptors on rat vagal afferent neurones

1. In the present study, in vitro electrophysiology and receptor autoradiography were used to determine whether rat vagal afferent neurones possess gamma-aminobutyric acid (GABA)A receptors. 2. GABA (1-100 microM) and isoguvacine (3-100 microM) caused a concentration-dependent depolarization of the rat isolated nodose ganglion preparation at room temperature. When applied to the tissue 20 min before the agonist, SR95531 (3 microM) and bicuculline (3 microM) caused a parallel shift to the right of the GABA and isoguvacine concentration-response curves, yielding shifts of 81 fold and 117 fold for SR95531 and 4 fold and 12 fold for bicuculline, respectively. 3. Baclofen (10 nM-100 microM) was unable to elicit a depolarization of the rat isolated nodose ganglion preparation at either room temperature or at 36 degrees C, whilst 5-aminovaleric acid (10 microM), a GABAB receptor antagonist, was unable to antagonize significantly the GABA-induced depolarization at either room temperature or at 36 degrees C. 4. [3H]-SR95531 (7.2 nM), a GABAA receptor-selective antagonist, bound topographically to sections of rat brainstem. Specific binding was highest in the medial nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus nerve (DMVN). Binding was also observed in certain medullary reticular nuclei, in particular the parvocellular reticular nucleus. 5. Unilateral nodose ganglionectomy caused a reduction in GABAA binding site density in the medial NTS from 93 +/- 7 to 68 +/- 6 d.p.m./mm2. This procedure also caused a reduction in GABAA binding site density in the side of the NTS contralateral to the lesion, from 151 +/- 12 to 93 +/- 7 d.p.m./mm2. Sham surgery had no effect on the binding of [3H]-SR95531 in rat brainstem. 6. The present data provide evidence for the presence of GABAA receptors located on the soma and central terminals of rat vagal afferent neurones. Additionally, a population of GABAA receptors is evidenced postsynaptically in the rat NTS with respect to vagal afferent terminals. These data are discussed in relation to the functional pharmacology of GABA in this region of the NTS.

Differences in cardiovascular responses to peripherally administered GABA as influenced by basal conditions and type of anaesthesia

The cardiovascular (blood pressure, heart rate, cardiac contractility) effects of i.v. gamma-aminobutyric acid (GABA) were investigated in guinea-pigs anaesthetized with barbitone or urethane. GABA (0.1-10 mg kg-1) produced a transient 'depressive' effect on cardiovascular parameters which in barbitone-anaesthetized animals was followed by a transient 'excitatory' effect. Resting cardiovascular parameters were higher in urethane-as compared to barbitone-anaesthetized animals. Picrotoxin pretreatment (2 mg kg-1, i.v.) barely affected the cardiovascular changes produced by GABA in barbitone-anaesthetized animals. In picrotoxin pretreated animals anaesthetized with urethane, GABA produced an initial depression of cardiovascular parameters followed by an excitatory phase. Hexamethonium (20 mg kg-1, i.v.) suppressed or reduced markedly the GABA-induced cardiovascular changes both in barbitone- or urethane- anaesthetized animals. Reserpine pretreatment lowered resting cardiovascular parameters. In these animals, regardless of type of anaesthesia, the effects of i.v. GABA were of the 'excitatory' type only. Reserpine pretreated animals anaesthetized with barbitone were selected for further experiments. Various GABAA receptor agonists (homotaurine, muscimol, THIP, 5-aminovaleric acid) mimicked the 'excitatory' effect of GABA in reserpine pretreated animals anesthetized with barbitone and prevented the effects of subsequent GABA administration. On the other hand (+/-)-baclofen, a selective GABAB receptor agonist, had a slight depressant effect and did not prevent the 'excitatory' cardiovascular effects of GABA. Neither bicuculline nor picrotoxin pretreatment prevented the 'excitatory' cardiovascular effect of i.v. GABA in reserpine pretreated, guinea-pigs anaesthetized with barbitone. In adrenalectomized guinea-pigs or in preparations receiving i.v. phentolamine plus propranolol, GABA produced only a small 'depressant' effect on cardiovascular parameters. These findings demonstrate that GABA exerts a neuromodulatory effect on cardiovascular function via peripheral actions which is influenced by: type of anaesthesia resting values of cardiovascular parameters degree of activity of the sympathetic nervous system and catecholamine release from the adrenal medulla.

Suitability of urethane anesthesia for physiopharmacological investigations in various systems. Part 2: Cardiovascular system

Urethane produces a level of surgical anesthesia characterized by preservation of a number of cardiovascular reflexes. When the proper route of administration is used, and the use of unnecessarily high doses is avoided, urethane anesthesia appears to be suitable for a number of investigations at cardiovascular level. However in certain types of studies involving pharmacological stimulation of peripheral adrenoceptors urethane affects markedly the magnitude of the response under study.