Imidacloprid actions on insect neuronal acetylcholine receptors

The neonicotinoid insecticide Imidacloprid acts at three pharmacologically distinct acetylcholine receptor (AChR) subtypes in the cockroach (Periplaneta americana) nervous system, but is ineffective on muscarinic receptors. Imidacloprid (3-100µmoll-1) induced dose-dependent depolarizations at cockroach cercal afferent/giant interneurone synapses. These responses were insensitive to 20µmoll-1 atropine but were completely blocked by the nicotinic antagonist mecamylamine (50µmoll-1). Similarly, Imidacloprid-induced depolarizations of cultured cockroach dorsal unpaired median (DUM) neurones dissociated from the same (terminal abdominal) ganglion were also completely blocked by 100µmoll-1 mecamylamine. However, two components of the response could be distinguished on the basis of their differential sensitivities to 0.1µmoll-1-bungarotoxin (-BTX), which selectively blocks AChRs with 'mixed' nicotinic/muscarinic pharmacology in this preparation. This indicates that Imidacloprid affects both AChRs sensitive to -BTX and -BTX-insensitive nicotinic acetylcholine receptors (nAChRs). Thus, in the cockroach, Imidacloprid activates -BTX-sensitive synaptic nAChRs in giant interneurones, -BTX-insensitive extrasynaptic nAChRs in DUM neurones, and a recently characterized DUM neurone 'mixed' AChR that is sensitive to both nicotinic and muscarinic ligands. Imidacloprid does not act on muscarinic acetylcholine receptors (mAChRs) present on DUM neurone cell bodies and at the cercal afferent/giant interneurone synapses. This study shows that Imidacloprid can act on pharmacologically diverse nAChR subtypes.

Kappa-bungarotoxin blocks nicotinic transmission at an identified invertebrate central synapse

A comparison was made between the effects of kappa-bungarotoxin and alpha-bungarotoxin upon nicotinic cholinergic transmission at an identified synapse (the cereal afferent, giant interneurone 2 synapse) in the central nervous system of the cockroach (Periplaneta americana). kappa-Bungarotoxin, a snake venom kappa-neurotoxin, completely blocked nicotinic unitary excitatory postsynaptic potentials (EPSPs) and evoked composite EPSPs when applied at a concentration of 1.0 × 10(−7) moll-1. No recovery was observed after a 2h wash in normal saline. kappa-Bungarotoxin produced a decrease in acetylcholine-induced nicotinic responses which paralleled decreases in nicotinic synaptic potentials and currents, indicating that kappa-bungarotoxin blocked postsynaptic nicotinic receptors. This blockade appeared to be specific as resting membrane potential, input resistance and the ability to elicit an action potential in response to direct stimulation of giant interneurone 2 were unchanged following prolonged toxin exposures. Samples of alpha-bungarotoxin which were free from kappa-neurotoxin contamination were also found to be potent antagonists of cockroach neuronal nicotinic receptors. It is concluded that the cockroach receptor is the first reported example of a neuronal nicotinic receptor which is sensitive to blockade by both kappa-neurotoxins and alpha-neurotoxins.

Presynaptic Depolarization Mediates Presynaptic Inhibition at a Synapse Between An Identified Mechanosensory Neurone and Giant Interneurone 3 in the First Instar Cockroach, Periplaneta Americana

Intracellular microelectrodes were used to study presynaptic inhibition at a cholinergic synapse between identified neurones: the lateral filiform hair sensory neurone (LFHSN) and giant interneurone 3 (GI3) in the terminal ganglion of the first instar cockroach Periplaneta americana. The LFHSN-GI3 synapse was shown to fulfil physiological criteria for monosynaptic transmission: the latency of the EPSPs was 1.4 ms and was constant during high-frequency firing of LFHSN; transmission was progressively and reversibly abolished by replacement of Ca2+ with Mg2+. Movement of the lateral filiform hair towards the cereal tip produced a burst of spikes in LFHSN and a burst of EPSPs in GI 3. Movement of the medial filiform hair towards the base of the cercus produced a burst of spikes in the medial filiform hair sensory neurone (MFHSN) and a burst of EPSPs in GI 2. EPSPs evoked in GI 3 by LFHSN spikes were inhibited during bursts of EPSPs in GI 2 which were evoked by MFHSN spikes. LFHSN was depolarized and its spikes were reduced in amplitude during spike bursts in MFHSN. Reduction in LFHSN spike amplitude reduced GI 3 EPSPs. This phenomenon was attributed, therefore, to presynaptic inhibition. The occurrence of presynaptic inhibition was dependent upon the degree of delayed rectification exhibited by the LFHSN axon. Hyperpolarization of LFHSN increased spike height, but did not increase the amplitude of GI 3 EPSPs. The delay between the onset of MFHSN-evoked EPSPs in GI 2 and MFHSNevoked depolarizations in LFHSN suggested that MFHSN does not synapse directly onto LFHSN. Neither depolarization nor hyperpolarization of GI 2 had any effect on MFHSN-mediated presynaptic inhibition of LFHSN-GI 3 transmission, therefore it was considered unlikely that GI 2 synapses onto LFHSN. Prolonged hyperpolarization lowered the LFHSN spike threshold and temporarily abolished presynaptic inhibition. Bursts of spikes in LFHSN mediated presynaptic inhibition of MFHSN-GI2 EPSPs. Mutual presynaptic inhibition by the FHSNs may have a functional significance in sharpening the boundaries of the GIs' directional sensitivities.