Joep van den Bercken

Neurotoxicological Effects And The Mode of Action of Pyrethroid Insecticides

Neuroexcitatory symptoms of acute poisoning of vertebrates by pyrethroids are related to the ability of these insecticides to modify electrical activity in various parts of the nervous system. Repetitive nerve activity, particularly in the sensory nervous system, membrane depolarization, and enhanced neurotransmitter release, eventually followed by block of excitation, result from a prolongation of the sodium current during membrane excitation. This effect is caused by a stereoselective and structure-related interaction with voltage-dependent sodium channels, the primary target site of the pyrethroids. Near-lethal doses of pyrethroids cause sparse axonal damage that is reversed in surviving animals. After prolonged exposure to lower doses of pyrethroids axonal damage is not observed. Occupational exposure to pyrethroids frequently leads to paresthesia and respiratory irritation, which are probably due to repetitive firing of sensory nerve endings. Massive exposure may lead to severe human poisoning symptoms, which are generally treated well by symptomatic and supportive measures.

Frequency response of the lateral-line organ of xenopus laevis

The stimulus response relation of the epidermal lateral-line organ of Xenopus laevis was studied by recording activity of single afferent nerve fibres in isolated preparations. Linear frequency response analysis over a frequency range of 0.1–100 Hz was performed under steady-state conditions, using small amplitude, plitude, sinusoidal water displacements produced by a glass sphere at a short distance from the skin.Period histograms of afferent nerve activity were computed, and amplitude, phase and mean activity of the response were determined by means of Fourier analysis. A standardization procedure at the start of each experiment made scaling of the frequency responses of different preparations unnecessary.The results show that for small stimulus amplitudes the response of the lateral-line organ over the whole range of frequencies studied can adequately be described as a modulation of the spontaneous activity. The amplitude of the response is proportional to the stimulus amplitude, and the phase of the response is independent of stimulus amplitude. The lateral-line organ of Xenopus laevis can thus be regarded as a linear system for stimuli which produce modulation of the spontaneous activity.The frequency response demonstrates unequivocally that the lateral-line organ of Xenopus laevis functions as a water velocity detector. For frequencies of stimulation from 0.1–20 Hz the gain increases with a slope of 7.5 dB/oct, and up to 5 Hz the response is almost in phase with the water velocity. The extent to which the different transmission steps between stimulus and response will contribute to the frequency response is discussed.

ACTION OF PYRETHROID INSECTICIDES ON THE VERTEBRATE NERVOUS SYSTEM

Vijverberg H.P.M. & van den Bercken J.1982 Neuropathology and Applied Neurobiology8, 421–440

Frequency-dependent effects of the pyrethroid insecticide decamethrin in frog myelinated nerve fibres

The pyrethroid insecticide decamethrin (10(-6) M) caused a frequency-dependent depression of the action potential in frog myelinated nerve fibres which was associated with a progressive membrane depolarisation brought about by summation of depolarising after-potentials. Voltage clamp experiments with single nodes of Ranvier showed that this afterpotential was most probably due to a long-lasting prolongation of the transient increase in sodium permeability of the membrane. The results indicate that decamethrin, like the other pyrethroids, specifically affects the sodium channels of the nerve membrane.

Temperature- and structure-dependent interaction of pyrethroids with the sodium channels in frog node of Ranvier

(1) The interaction of a series of pyrethroid insecticides with the Na+ channels in myelinated nerve fibres of the clawed frog, Xenopus laevis, was investigated using the voltage clamp technique. (2) Out of 11 pyrethroids 9 insecticidally active compounds induce a slowly decaying Na+ tail current on termination of a step depolarization, whereas the Na+ current during depolarization was hardly affected. These tail currents are most readily explained by a selective reduction of the rate of closing of the activation gate in a fraction of the Na+ channels that have opened during depolarization. (3) The rate of decay of the Na+ tail current varies considerably with pyrethroid structure. After alpha-cyano pyrethroids the decay is at least one order of magnitude slower than after non-cyano pyrethroids. The decay always follows a single-exponential time course and is reversibly slowed when the temperature is lowered from 25 to 0 degrees C. Arrhenius plots in this temperature range are linear. (4) These results indicate that the relaxation of the activation gate in pyrethroid-affected Na+ channels is governed by an apparent first order, unimolecular process and that the rate of relaxation is limited by a single energy barrier. Application of transition state theory shows that after alpha-cyano pyrethroids this energy barrier is 9.6 kJ/mol higher than after non-cyano pyrethroids. (5) Differences in rate of decay of the Na+ tail current account for the reported differences in repetitive nerve activity induced by various pyrethroids. In addition, the effect of temperature on the rate of decay explains the increase in repetitive activity with cooling.

The dopamine response in mouse neuroblastoma cells is mediated by serotonin 5HT3 receptors

Serotonin (5HT) and dopamine (DA) induce, in neuroblastoma N1E-115 cells, a transient membrane depolarization associated with an inward current. The half-maximum response is obtained with 2 microM 5HT or 200 microM DA. The maximum response to 5HT is 2-3 times that to DA. The selective 5HT3 receptor antagonists ICS 205-930 and MDL 72222 at nanomolar concentrations block both the 5HT- and the DA-induced response. High concentrations (10 microM) of 5HT2 receptor antagonists are without effect. It is concluded that, in N1E-115 cells, 5HT and DA activate a single population of 5HT3 receptors.

Structure-related effects of pyrethroid insecticides on the lateral-line sense organ and on peripheral nerves of the clawed frog, Xenopus laevis

The effects of seven different pyrethroid insecticides on the lateral-line sense organ and on peripheral nerves of the clawed frog, Xenopus laevis, were investigated by means of electrophysiological methods. The results show that two classes of pyrethroid can be clearly distinguished. (i) Pyrethroids without an α-cyano group (permethrin, cismethrin, and bioresmethrin). These noncyano pyrethroids induce short trains of nerve impulses in the lateral-line sense organ. In peripheral nerve branches they induce a depolarizing afterpotential and repetitive firing. These effects are very similar to those previously reported for allethrin. (ii) Pyrethroids with an α-cyano-3-phenoxybenzyl alcohol (cypermethrin, fenpropathrin, deltamethrin, and fenvalerate). In the lateral-line sense organ these α-cyano pyrethroids induce very long trains of nerve impulses which may last for seconds and may contain hundreds or even thousands of impulses. The α-cyano compounds do not cause repetitive activity in peripheral nerves. Instead they induce a quickly reversible, stimulus frequency-dependent suppression of the action potential. Since the chemical structure of cypermethrin differs from that of permethrin only in the α-cyano group and because all four α-cyano compounds act in a very similar way, it is concluded that the α-cyano substituent is responsible for the large differences in neurotoxic effects. In the lateral-line sense organ the duration of nerve impulse trains induced by the noncyano as well as the α-cyano pyrethroids increases dramatically when the temperature is lowered. Further, in sensory fibers the effects of both classes of pyrethroid on the nerve action potential are more pronounced compared to their effects on motor fibers. It is argued that the different neurotoxic effects reported here originate from a common mechanism of action of pyrethroids, which is a prolongation of the transient increase of sodium permeability of the nerve membrane associated with excitation. It is concluded that the sodium channel in the nerve membrane is the major target site of noncyano and α-cyano pyrethroids.

Presynaptic action of the pyrethroid insecticide allethrin in the frog motor end plate

Abstract • The action of pyrethroid insecticide allethrin on synaptic transmission in the frog motor end plate was studied by means of intracellular microelectrodes. Allethrin in concentrations as low as 10−7 M produced pronounced repetitive activity in the motor end plate, and a single nerve stimulus could evoke a train of up to 13 end plate potentials. This repetitive activity was presynaptic in origin and was due to repetitive firing of the motor nerve Terminal. The repetitive activity in the nerve terminal was, however, not accompanied by repetitive firing in the more proximal part of the motor fibres. By recording nervous activity along the whole length of the nerve innervating the muscle, from the spinal roots down to the smallest nerve branches in the muscle, it was demonstrated that repetitive firing did not start till the motor nerve fibres reached the vicinity of the muscle. From that point on their tendency to fire repetitively after treatment with allethrin increased in the distal direction till ultimately all motor nerve terminals showed pronounced repetitive activity. This difference in response to allethrin between proximal and distal parts of the motor nerve fibres is probably not due to a different pharmacological action of allethrin, but may be explained by a shift in membrane conductance parameters, specially the rate of recovery from sodium inactivation, as the nerve fibre approaches the nerve terminal. • The allethrin-induced repetitive activity in the nerve terminals was highly dependent on temperature and became much more pronounced as the temperature was lowered, thus showing a truly negative temperature coefficient. Apart from repetitive activity of the presynaptic nerve membrane, no genuine synaptic effect of allethrin was observed, either pre- or post-synaptic, nor was there any significant effect of allethrin on the muscle fibre membrane.

Effects of ototoxic antibiotics on sensory hair cell functioning

The isolated lateral-line organ of Xenopus laevis was used to study reversible effects of low concentrations of aminoglycoside antibiotics on sensory hair cell functioning. Extracellular receptor potentials and single fibre afferent nerve activity were recorded during stimulation of the lateral-line organ with sinusoidal water movements. The results show that dihydrostreptomycin in concentrations between 2.5 and 17.5 micrograms/ml caused an increase in spontaneous afferent nerve activity, probably by increasing the spontaneous transmitter release at the afferent synapse. In addition, the same concentrations of dihydrostreptomycin markedly affected the mechano-electric transduction process, resulting in a phase lag in th receptor potential, the magnitude of which increased with increasing concentration up to a value of 0.4 pi radians. The effect of dihydrostreptomycin on the amplitude of the receptor potentials depended on the stimulus frequency. Other aminoglycoside antibiotics, namely streptomycin, gentamycin and kanamycin, produced similar effects but at different concentrations. It is concluded that low concentrations of aminoglycoside antibiotics have a dual action upon the sensory hair cells. They increase the spontaneous activity by affecting the hair cell membrane and at the same time they impair the mechano-electric transduction process resulting in a large phase lag in the receptor potentials. The latter effect, which is antagonized by Ca2+, may be due to interference by the antibiotics with the mode of motion of the sensory hairs.

Excitatory and depressant effects of dieldrin and aldrin-transdiol in the spinal cord of the toad (Xenopus laevis)

An investigation was made into the action of the insecticide dieldrin and one of its metabolites, aldrin-transdio, on the isolated spinal cord of the toad, Xenopus laevis. Conventional electrophysiological techniques were used for stimulating and recording of dorsal and ventral spinal roots. An augmentation of polysynaptic reflex activity along with a marked reduction of orthodromic postsynaptic inhibition could be demonstrated in preparations isolated from dieldrin-poisoned animals. However, application of dieldrin to the isolated spinal cord failed to produce any significant effect. Application of aldrin-transdiol, on the other hand, caused a potentiation of spinal reflex activity and an increase in spontaneous activity of ventral and dorsal roots. Aldrin-transdiol also produced a marked reduction of spinal inhibitory mechanisms. The excitatory effects of aldrin-transdiol were followed by a strong depressant action on spinal excitability.