Spontaneous and evoked quantal neurotransmitter release at the neuromuscular junction of the larval housefly, Musca domestica

Physiological evidence that three known mutations in the para-sodium channel gene confer cypermethrin knockdown resistance in Rhipicephalus microplus

BACKGROUND

Acaricide resistance is a central problem for the control of the cattle tick Rhipicephalus microplus. The physiological effects and phenotypes of the mutations that cause acaricide resistance are not always well understood or characterized. Single nucleotide polymorphisms (SNPs) that confer cypermethrin knockdown resistance (kdr) have been reported in R. microplus. These SNPs have been associated and correlated with pyrethroid resistance although there is no direct physiological evidence that their presence does confer kdr in this organism.

METHODS

Resistant and susceptible strain resistance profiles were obtained using the larval packet discriminating dose assay. The relevant genomic regions of the para-sodium channel were amplified using standard PCR; SNPs were detected by sequencing the corresponding amplicons. Ovary response to cypermethrin exposure/treatment was evaluated using videometrical analysis.

RESULTS

We found that the pyrethroid resistance trait is stable in a resistant reference strain after years without selection, suggesting that the resistance conferring mutations are fixed in the population. In this strain, a change in the structure of the pre-synaptic para-sodium channel caused by the G184C, the C190A and the T2134A SNPs appears to confer resistance. These mutations are absent in the susceptible strain used as control. We demonstrate that cypermethrin blocks ovary contraction in cypermethrin-susceptible ticks. We also show that ovaries from organisms that carry the kdr associated SNPs still contract at cypermethrin concentrations that completely block ovary contraction in the susceptible strain. The configuration of the experimental system excludes a xenobiotic detoxification mechanism.

CONCLUSIONS

This is the first report that presents physiological evidence that the presence of the G184C, the C190A, and the T2134A mutations in the para-sodium channel correlates with maintaining muscle contractility in R. microplus exposed to cypermethrin. These SNPs may confer cypermethrin resistance in this organism by avoiding presynaptic blockage, inhibiting the flaccid muscle paralysis characteristic of this acaricide. The videometric assay that we previously validated can be used to detect more rapidly than other assays that involve larval mortality kdr-like cypermethrin resistant tick strains, permitting to directly assay adult pre-engorged females after they are collected on the field without waiting until eggs are laid and larvae eclose.

Homeostasis of synaptic transmission in Drosophila with genetically altered nerve terminal morphology

We present a new test of the hypothesis that synaptic strength is directly related to nerve terminal morphology through analysis of synaptic transmission at Drosophila neuromuscular junctions with a genetically reduced number of nerve terminal varicosities. Synaptic transmission would decrease in target cells with fewer varicosities if there is a relationship between the number of varicosities and the strength of synaptic transmission. Animals that have an extreme hypomorphic allele of the gene for the cell adhesion molecule Fasciclin II possess fewer synapse-bearing nerve terminal varicosities; nevertheless, synaptic strength is maintained at a normal level for the muscle cell as a whole. Fewer failures of neurotransmitter release and larger excitatory junction potentials from individual varicosities, as well as more frequent spontaneous release and larger quantal units, provide evidence for enhancement of transmitter release from varicosities in the mutant. Ultrastructural analysis reveals that mutant nerve terminals have bigger synapses with more active zones per synapse, indicating that synaptic enlargement and an accompanying increase in synaptic complexity provide for more transmitter release at mutant varicosities. These results show that morphological parameters of transmitting nerve terminals can be adjusted to functionally compensate for genetic perturbations, thereby maintaining optimal synaptic transmission.

Improved stability of Drosophila larval neuromuscular preparations in haemolymph-like physiological solutions

Neuromuscular preparations from third instar larvae of Drosophila are not well-maintained in commonly used physiological solutions: vacuoles form in the muscle fibers, and membrane potential declines. These problems may result from the Na:K ratio and total divalent cation content of these physiological solutions being quite different from those of haemolymph. Accordingly haemolymph-like solutions, based upon ion measurements of major cations, were developed and tested. Haemolymph-like solutions maintained the membrane potential at a relatively constant level, and prolonged the physiological life of the preparations. Synaptic transmission was well-maintained in haemolymph-like solutions, but the excitatory synaptic potentials had a slower time course and summated more effectively with repetitive stimulation, than in standard Drosophila solutions. Voltage-clamp experiments suggest that these effects are linked to more pronounced activation of muscle fiber membrane conductances in standard solutions, rather than to differences in passive muscle membrane properties or changes in postsynaptic receptor channel kinetics. Calcium dependence of transmitter release was steep in both standard and haemolymph-like solutions, but higher external calcium concentrations were required for a given level of release in haemolymph-like solutions. Thus, haemolymph-like solutions allow for prolonged, stable recording of synaptic transmission.

Phasic stimulation of the locus coeruleus: effects on activity in the lateral geniculate nucleus

Neurons in the locus coeruleus (LC) encode information related to behavioral state in a tonic pattern of firing and information related to the occurrence of a sensory stimulus in a phasic pattern of firing. The effects of phasic stimulation of the LC (6 pulses at 30 Hz), designed to approximate its physiological activation by sensory stimuli, were studied in the lateral geniculate nucleus (LGN) of anesthetized rats. Phasic stimulation of the LC significantly increased neuronal firing in the LGN with a mean latency 320 ms from onset of stimulation. Receiver operating characteristic analyses on a trial-by-trial basis showed that phasic LC stimulation can result in a highly discriminable signal in the LGN. This increased neuronal firing rate in the LGN was specific for the site of stimulation and was reduced by the norepinephrine synthesis inhibitor alpha-methyl-p-tyrosine and by intravenous WB-4101 (alpha 1-receptor antagonist). Neurons in the LGN have a single-spike firing mode when sensory information is faithfully relayed from retina to cortex and a burst-firing mode when the transfer of this information is degraded. Phasic LC stimulation reduced burst firing (2-5 ms interspike intervals, ISIs) at low frequencies (< or = 4 Hz) in the LGN, and for some neurons there was an absolute decrease in burst-like ISIs after LC stimulation, despite an increase in mean firing rate.