Neurochemical and behavioral correlates of unilateral striatal acetylcholinesterase inhibition by fasciculin in rats

Neurotoxinas con actividad anticolinesterásica y su posible uso como agentes de guerra

Anatoxin-a(s), onchidal and fasciculins are neurotoxins with anticholinesterase activity. An intoxication by these neurotoxins is characterized by cholinergic syndromes similar to organophosphate insecticide and nerve agent intoxications. Anticholinesterase neurotoxins, as well as other toxins, have some disadvantages if used as weapons of mass destruction. Drawbacks include difficulties to produce them in big quantities and their dissemination in form of aerosols. However, other properties such as high toxicity, improbable identification with common commercial portable detectors for chemical warfare agents and toxic industrial chemicals, as well as the lack of effectiveness of antidotal treatments with oximes may make them attractive in order to be used in military operations or terrorist attacks. For these reasons, it should be necessary to control these neurotoxins through international treaties which have real verification measures such as the Chemical Weapons Convention.

Synthetic peptides derived from the central loop of fasciculin: structural analysis and evaluation as inhibitors of acetylcholinesterase

Fasciculins are peptides present in the venom of green and black mamba snakes, with potent inhibitory activity towards acetylcholinesterase. In order to determine the role of fasciculin loop II in the acetylcholinesterase inhibition, two fasciculin fragments were synthesized by the solid phase procedure using N-alpha-Boc protected amino acids. The two peptides, Fas-A and Fas-B, span the 26-32 and 22-35 sequences of fasciculin and a disulfide bridge links each peptide end, thus ensuring the formation of a looped structure. Both peptides were characterized chemically, structurally and functionally. Circular dichroism indicated the existence of 19.4 and 24.9% of beta-sheet for Fas-A and Fas-B, respectively; SDS-PAGE patterns and mass spectrometry disclosed the intramolecular disulfide formation in both peptides. An inhibitory effect on eel acetylcholinesterase was observed with the longer peptide (Ki = 15.1 microM), without reaching the affinity level of the parent native toxin (Ki = 0.3 nM). This study confirms that fasciculin central loop residues strongly contribute to toxin interaction with acetylcholinesterase.

Acetylcholinesterase inhibitors block acetylcholine-evoked release of dopamine in rat striatum, in vivo

In the rat striatum, acetylcholine (ACh) increases dopamine (DA) release. The role of increased cholinergic activity provoked by acetylcholinesterase inhibitors (AChEi) on DA release is currently under revision after recent papers have shown a blockade of nicotinic transmission by AChEi in vitro. To study the effects of AChEi in vivo, Fasciculin2 (FAS), a peptidergic AChEi, and physostigmine (PHY), a classical carbamate AChEi, were applied through push-pull or microdialysis cannulae respectively, to the striatum of rats, alone or with ACh. Extracellular concentrations of DA were assessed by HPLC with electrochemical detection. Alone, the AChEi studied did not provoke changes in basal extracellular levels of DA, in the different doses studied. ACh (100 microM, 1 and 5 mM) applied through the push-pull cannulae in basal conditions provoked a dose-dependent increase of extracellular DA. This effect was not observed with ACh in concentrations of 100 microM and 1 mM if FAS (0.4 and 4.2 microM) was applied first. Higher concentrations of ACh (5 mM) evoked a partial response after FAS 0.42 microM, an effect still blocked by FAS at 4.2 microM. PHY 50 microM applied through microdialysis completely blocked the increase in DA release provoked by ACh 10, 20 mM, while at ACh 30 mM, PHY 50 microM only partially blocked the evoked increase. A partial blockade was also observed with PHY 20 microM, on the three different concentrations of ACh. On the other hand PHY 10 microM did not block any of the ACh doses perfused. These results showed that AChEi like FAS and PHY interfere with the ACh-evoked DA release in the striatum.

Differential cholinergic and non-cholinergic actions of acetylcholinesterase in the substantia nigra revealed by fasciculin-induced inhibition

The effects of the peptide fasciculin (FAS), a potent inhibitor of acetylcholinesterase (AChE) have been examined, following unilateral microinfusion, on tissue levels of monoamines in the rat substantia nigra and concomitant circling behaviour. Although FAS inhibited 87% of total AChE, the levels of dopamine and its metabolites remained unchanged. Furthermore, the treatment induced modest contraversive rotation which was markedly enhanced in the presence of a systemic challenge with apomorphine. This behavioural effect of FAS was partially reversed by systemically administered atropine. Any possible interaction of FAS with nigral dopamine systems was further investigated by testing the peptide in animals that five days earlier had undergone a 6-hydroxydopamine (6-OHDA) lesion of the SN such that dopamine and AChE were significantly but not completely reduced. In a majority of these animals, FAS treatment caused a reversal of the lesion induced ipsiversive rotation, ie restored contraversive rotation. It is concluded that in the SN, FAS can have biochemical and behavioural actions independent of local dopamine systems and linked to cholinergic transmission. In addition, treatment with FAS in the substantia nigra also reveals the possible existence of at least two distinct pools of AChE with, respectively, non-cholinergic and cholinergic actions.

Catalepsy induced by striatal acetylcholinesterase inhibition with fasciculin in rats

The acetylcholinesterase inhibitor peptide fasciculin (FAS) was bilaterally injected into the striatum of rats. Twenty-four hours after injection, animals showed a cataleptic syndrome that was potentiated by haloperidol (HAL). The catalepsy was significantly decreased by IP atropine. Biochemically, only an increase of the homovanillic acid in the striatum was found 24 h and 7 days after FAS treatment. Seven days after the intrastriatal FAS injection, there was no HAL potentiation of catalepsy, which was even lower than that of rats treated with IP HAL after intrastriatal injection of saline. Results are interpreted as showing the central role of the cholinergic system in the induction of catalepsy in the rat.