Acetylcholine release from rat brain cortical slices evoked by the fraction P4 of the venom of the spider Phoneutria nigriventer has Ca2+ and temperature independent components

c-FOS and n-NOS reactive neurons in response to circulating Phoneutria nigriventer spider venom

Drugs and neurotoxins activate specific neural circuits by increasing or decreasing the formation and release of neurotransmitters, such as nitric oxide (NO), and by inducing immediate early genes, such as FOS. We have previously shown that Phoneutria nigriventer spider venom (PNV) impairs the microtubule-dependent transcellular barrier of the blood-brain interface and causes structural alterations in perivascular astrocytic end-feet without producing morphological changes in central neuronal cells. In the present study, we used FOS and neuronal nitric oxide synthase (n-NOS) immunolabeling to investigate the ability of PNV to activate the central nervous system. Three groups of rats were used: the first group received a sublethal dose of PNV (850 microg/kg, via a tail vein), the second received an equal volume of 0.9% saline (sham group) and the third group received no injection. Envenomed rats showed salivation, lachrymation, tremors and flaccidity followed by spastic paralysis of the hind limbs and convulsions. Cryosections (30 microm thick) were serially collected at 600 microm intervals for free-floating immunohistochemical analysis. FOS-like positive neurons predominated in motor-related areas such as dorsolateral (dlPAG) and ventral periaqueductal gray matter (vPAG), frontal (FCM) and parietal motor cortex (PCM), and periventricular thalamic nucleus (PTN) and in acute stress-related areas (rhinal cortex and lateral septal nuclei). The greatest relative increases in FOS-like positive neurons occurred in the vPAG, PCM and PTN motor-related areas. n-NOS-positive neurons predominated in the periventricular thalamic nuclei, followed by the dorsolateral periaqueductal gray matter and parietal cortex motor area. The marked activation of motor areas and, to a lesser extent, of acute stress-related areas suggested the involvement of neuronal pathways in these regions in the response to envenoming by PNV. In addition, the occurrence of n-NOS immunolabeling in some anatomical regions with FOS-like positive neurons suggests that NO may modulate the response to PNV in these regions.

Effects of the venom of a Mygalomorph spider (Lasiodora sp.) on the isolated rat heart

We studied the effect of the venom of the Brazilian spider, Lasiodora sp. (Mygalomorphae, Theraphosidae), on force generation and electrical activity in the isolated rat heart. Previous work showed that this venom is excitotoxic to excitable cells due to Na(+) channel gating modifier activity [Toxicon 39 (2001) 991]. In the isolated heart, the venom (10-100 microg bolus administration) caused a dose-dependent bradycardia, with transient cardiac arrest and rhythm disturbances. The electrocardiogram showed that the reduction of heart rate was due to sinus bradycardia, sinus arrest and partial or complete A-V block. All of the effects were reversible upon washout of the venom. The effect of the venom was potentiated by the anticholinesterase neostigmine (3.3 microM), suppressed by the muscarinic acetylcholine receptor antagonist atropine (1.4 microM), and inhibited by the vesicular acetylcholine transporter inhibitor (-)-vesamicol (10 microM). Tetrodotoxin (200 nM) did not inhibit the effect of the venom. Together, these data suggest that this Lasiodora venom evokes vesicular release of acetylcholine from parasympathetic nerve terminals by activating tetrodotoxin-resistant Na(+) channels.

Effects of arginine-vasopressin fragment 4-9 on rodent cholinergic systems

Arginine-vasopressin fragment 4-9 (AVP4-9) has been demonstrated in animal studies to facilitate learning and memory. To clarify the mechanisms of this facilitation, we focused on the effects of AVP4-9 on rodent cholinergic systems. AVP4-9 (0.1 microM) enhanced the basal and the high-potassium-evoked acetylcholine (ACh) release from rat hippocampal slices (122.4 and 120.0% of control, respectively) in the presence of 1.3 mM calcium (physiological level) at 60 min after the incubation at 37 degrees C. The AVP4-9-stimulated basal ACh release was inhibited by a V1-selective antagonist ([(beta-mercapto-beta,beta-cyclopentamethylene propionic acid)1, O-methyl-Tyr2, Arg8] vasopressin), but not by a V2-selective antagonist ([adamantaneacetyl1, O-ethyl-D-Tyr2, Val4, aminobutyryl6, Arg8,9]-vasopressin). In addition, AVP4-9 did not affect the basal ACh release under the calcium-free condition at 37 degrees C or in the presence of 1.3 mM calcium at 4 degrees C. However, AVP4-9 facilitated the passive-avoidance response of scopolamine (a cholinergic blocker)-induced memory-deficient mice. These findings demonstrate that AVP4-9 stimulates ACh release via mediation by V1-like vasopressin receptors, and shows dependence on calcium ion and temperature. The results also suggest that the mechanism of the facilitative effects of AVP4-9 on learning and memory consist of the observed stimulation of cholinergic systems and other parallel pathways that would not be inhibited by cholinergic blocking.

Activation by Phoneutria nigriventer spider venom of autonomic nerve fibers in the isolated rat heart

In the isolated rat heart, Phoneutria nigriventer spider venom (10-100 microg) produced a dose-dependent and reversible rise in left ventricular developed pressure. A low dose (10 microg) of venom induced a short-lasting, positive inotropic effect (P < 0.05) with no change in heart rate or coronary flow. At a dose of 50 microg, the venom caused significant positive inotropic and chronotropic responses associated with occasional ventricular arrhythmia, whereas coronary flow was not significantly affected within 10 min after venom administration. The highest dose of venom (100 microg) caused bradycardia, transient cardiac arrest, rhythm disturbances and an increase in end diastolic pressure followed by a reduction in coronary flow. Hearts treated with the non-selective beta-adrenoceptor antagonist propranolol (3 microM) and the selective beta1-adrenoceptor antagonist CGP-20712A (10 microM) were protected against all the cardiac actions of the venom. The selective beta2-adrenoceptor antagonist butoxamine (10 microM) slightly reduced the cardiac response to 50 microg, but not to 100 microg of venom. Butoxamine also prevented the reduction in coronary flow induced by 100 microg of venom. Hearts from reserpine-treated rats (5 mg kg(-1) day(-1), i.p., for 2 days) showed a marked decrease in all venom (< or = 100 microg)-induced cardiac responses. The muscarinic receptor antagonist atropine (1 microM) slightly potentiated the response to 50 microg of venom but had little or no effect on the responses to 100 microg of venom. The cardiac responses to venom (50-100 microg) were unaltered in hearts from rats treated with 8-methyl N-vanillyl-6-nonenamide (capsaicin; 50 mg/kg, s.c.). These findings indicate that P. nigriventer venom releases norepinephrine from cardiac sympathetic nerve endings and this may explain the observed increase in contractile force and heart rate.

Investigation of the effect of PhTx2, from the venom of the spider Phoneutria nigriventer, on the release of [3H]-acetylcholine from rat cerebrocortical synaptosomes

Venoms from spiders are an important source of toxins that can help on the dissection of mechanisms involved in neurotransmission. Among them is the venom of the spider Phoneutria nigriventer that contains several toxic fractions with different targets in mammals and/or insects. We here report that one of these fractions (PhTx2) is able to evoke acetylcholine release from rat cortical synaptosomes and that this effect is dependent on extracellular calcium and is inhibited by the sodium channel blocker tetrodotoxin.