IJUP08 - Universidade do Porto

Treatment of Myasthenia gravis with cholinesterase inhibitors: on
the role of nicotinic autoreceptors containing α3β2 subunits
Ana Sá-e-Sousa 1 , Tiago Morais 1 , Diogo Trigo 1 , Maria Alexandrina Timóteo 1 ,
Bernardo Matos 1 , Laura Oliveira 1 & P. Correia-de-Sá 1
1 Laboratório de Farmacologia e Neurobiologia, Unidade Multidisciplinar de Investigação
Biomédica (UMIB), Instituto de Ciências Biomédicas de Abel Salazar – Universidade do Porto
(ICBAS-UP), Portugal.
Failure of neuromuscular transmission is rare due to excess of acetylcholine (ACh) release
to levels higher than those required to trigger an action potential at the muscular fiber (i.e.
high safety factor). Transmission safety factor may be limited in several pathological
conditions, namely in Myasthenia gravis. So far, most attempts to improve muscle
weakness that characterize this disorder have been to prevent ACh breakdown by blocking
cholinesterase activity. Recently, we demonstrated that the neuromuscular junction is
equipped with α3β2-containing nicotinic autoreceptors (nAChRs) mediating facilitation of
ACh release, which might also contribute to the therapeutic action of cholinesterase
inhibitors [1]. This prompted us to investigate the effect of neostigmine (Neo, a commonly
used cholinesterase inhibitor) and dihydro-β-erythroidine (DH-β-E, a relatively weak
antagonist at muscle-type α1 nicotinic receptor as compared to the α3β2 subtype) on nerveevoked
contractions and [ 3 H]-ACh release from motor nerve terminals of rats with Toxin-
Induced Myasthenia gravis (TIMG).
Wistar rats (70-100 g) were injected once every 48h with saline (controls) or αbungarotoxin
(α-BTX, an irreversible muscle-type α1 nicotinic receptor antagonist)
(TIMG-model) for a period up to 6 weeks [2]. Dosage of α-BTX was adjusted by
monitoring myasthenic symptoms. [ 3 H]ACh release was evoked by phrenic nerve
stimulation with 50 Hz-bursts (5 bursts of 150 pulses, with a 20s interburst interval).
Fatigue tests were carried out using high frequency (50 Hz) intermittent (17 pulses per sec,
during 3 minutes) nerve stimulation.
Chronic injections of α-BTX decreased the amplitude of nerve-evoked diaphragm
contractions during the fatigue tests. Neo (500 nM) transiently (30s) increased diaphragm
contractile responses in both control and TIMG animals. Following the initial 30-sec
stimulation period, Neo (500 nM) significantly (P