Blocking the Brake: Fipronil, the Dieldrin Allele, and the Target That Rescued It

1. Introduction: the allele our reversion article left open

Our article on resistance reversion reported a cyclodiene resistance allele persisting at high frequency in German cockroach populations years after the chemistry was withdrawn. This article follows what happened when a new compound arrived at the same target.

1.1 Every clause of that should produce cross-resistance

Same site, widespread mutation, high resistance to the older compound.⁵

1.2 What this article argues

That the prediction fails for a specific and instructive reason, and that the reason undermines using mode-of-action classification as a rotation guide. Sections 11 to 13 are the case.

2. What it does

The mechanism, stated cleanly by one source.

An important mechanism that helps return nerves from excitation to their resting state is the timely opening of chloride channels. Opening of the chloride channels allows negatively charged chloride ions to enter nerves and thereby offset the electrochemical effect of the positively charged sodium ions. Fipronil inhibits the opening of gamma aminobutyric acid (GABA)-mediated chloride channels, and therefore the excitatory effect of sodium is unchecked.²

2.1 It removes an inhibition

The compound does not excite anything. It prevents the system that quiets excitation from operating.²

2.2 Which is why the symptoms differ

An insect losing inhibitory control does not convulse the way one exposed to a sodium channel modifier does. The excitation builds rather than firing immediately, which our articles on knockdown describe as the visible difference between classes.

That connection is ours and it is the practical face of the mechanism in §2.

2.3 And it is classified accordingly

In group 2 of the industry mode of action scheme, that encompasses GABA-gated chloride channel antagonists.⁶

3. Which is the opposite of everything else here

An observation worth making because this journal has now covered most of the classes.

Pyrethroids hold sodium channels open. Organophosphates and carbamates prevent acetylcholine being cleared. Neonicotinoids agonise a receptor. Every one of those drives excitation upward.

3.1 This one removes the brake

Which produces the same endpoint by the reverse route, and is why the two approaches are not cross-resistant to each other despite both killing by uncontrolled nervous activity.

That framing is ours.

3.2 And it has a mammalian analogue

GABA-A receptors are the target of several classes of human sedative, which work by potentiating rather than blocking. The pharmacology is well studied precisely because the mammalian version matters clinically.

4. And the same target as dieldrin

The fact that sets up everything else.

The mode of action involves blocking the GABA-gated chloride channel that is also the target for cyclodiene insecticides such as endosulfan and dieldrin.³

4.1 Which is not a coincidence of classification

The same physical site, in the same channel, in the same receptor.⁵

4.2 Which is why the compound was expected to fail

A new active arriving at a target already defended by a widespread allele is, on the ordinary reasoning, arriving too late. That expectation is what makes §11 worth an article.

4.3 And the cyclodienes are long gone

Our article on the history of pesticide restriction describes their withdrawal, and our reversion article found the resistance to them outliving them by decades.

5. The selectivity

The numbers, which are good.

The binding affinities of fipronil and fipronil sulfone to mammalian GABAA receptors are much less than in arthropods (GABAA receptor binding IC50 human:insect of 135 and 17 respectively) with no binding to other types of mammalian GABA receptor, accounting (in combination with the low systemic bioavailability after dermal administration) for the selectivity of action.²

5.1 A hundred and thirty-five fold

Which is a substantial margin, achieved at the target rather than downstream.²

5.2 Which contrasts with our pro-insecticide article

Where we found a compound whose selectivity sat entirely in a metabolic activation step, because its target was identical across phyla. Here the margin is structural: the insect and mammalian receptors differ.

Two actives, two different bases for the same property, and that comparison is ours.

5.3 And the second clause matters as much

That the selectivity is accounted for in combination with the low systemic bioavailability after dermal administration.²

So the margin is partly a target property and partly an absorption one, which means it applies to a dermal contact and not necessarily to any other route.

5.4 And one factsheet adds a caution

That the lower binding affinity for mammalian receptors enhances selectivity for insects and increases the margin of safety,¹ which §18 then complicates.

6. The second target

And this is the detail that decides the article.

Fipronil and its major sulfone metabolite are unique among chloride channel blocking insecticides in that they also potently block GluCls.⁵

6.1 Glutamate-gated chloride channels

A second family of inhibitory channels, gated by a different neurotransmitter, performing a comparable function.⁵

6.2 Unique among the class

Which means the other chloride channel blockers, including the cyclodienes, do not.⁵

6.3 And both forms do it

Parent and sulfone metabolite alike, with an earlier study reporting that both parental and bioactivated compounds were reported to exhibit a neurophysiological effect on the GluCl receptor.⁷

7. Which mammals do not have

The selectivity consequence.

GluCls have been observed only in invertebrates.²

7.1 That is absolute selectivity at that target

Not a hundred and thirty-five fold margin but the absence of the receptor entirely.²

7.2 And it is the basis of a veterinary drug class

Glutamate-gated chloride channels are the target of the avermectins used as animal antiparasitics, which is why those compounds can be given to a mammal to kill the worms inside it.

A target absent from the host is the strongest form of selectivity available to any poison, and that observation is ours.

7.3 Which our RNA-based pesticide article wanted

Where we described the search for targets present in the pest and absent in everything else. This is an existing compound that has one.

8. The mutation

The allele, named.

Mutant receptors carry a naturally occurring amino acid substitution (A302→S) in the putative ion-channel lining region, conferring resistance to dieldrin and picrotoxinin.⁴

8.1 In the channel lining

Which is where a blocker has to sit, so a substitution there changes the shape of the thing being blocked.⁴

8.2 And it is widespread and persistent

In the words of a later review, in insect populations generally.⁵

Which is our reversion article's finding, stated by a second literature.

9. What it does at the receptor

Measured directly.

Mutant receptors were markedly less sensitive to fipronil than the wild-type receptors.⁴

9.1 The experiment is clean

Wild type and mutant channels expressed in the same system, the same compound applied to both.⁴

9.2 And the potency depended on transmitter concentration

With fipronil's effect on the wild type dependent on the GABA concentration but unaffected by membrane potential,⁴ which locates the interaction at the binding site rather than in the electrical field across the membrane.

9.3 And it extends to a structurally different compound

The authors note that the mutation A302→S reduces the potency of both fipronil and picrotoxinin, a compound they describe as structurally distinct.⁴

So the mutation is not defeating a molecular shape but degrading the site itself.

10. So the prediction is cross-resistance

Which anybody would make from §§4, 8 and 9.

Same site, a widespread mutation at that site, demonstrated reduction in sensitivity to the new compound at the receptor level.

10.1 And our own rotation article would predict it

Which concluded that rotating to a product a population is already cross-resistant to achieves nothing, and that mode of action classification is how such rotations are planned.

11. And the prediction fails

At the level that matters.

Fipronil has proven to be effective in controlling many insects, including dieldrin-resistant populations.⁵

11.1 Despite everything in §10

The review's own word is despite, and it frames the situation as requiring explanation.⁵

11.2 And it is not a marginal effect

The compound became a mainstay for ants, termites, cockroaches and fleas on the strength of working, which it would not have done had the widespread allele blunted it.⁷

11.3 So receptor sensitivity and organism survival came apart

Which is unusual. Our resistance articles generally find target-site changes translating directly into field failure.

12. The rescue

The explanation, which follows from §6.

The mutation is in the GABA receptor. Fipronil also blocks glutamate-gated chloride channels, a different protein encoded by a different gene, and the Rdl substitution does not occur there.⁵²

12.1 So a resistant insect still has one target intact

And a compound blocking both inhibitory systems retains half its effect against a population that has protected only one.

That inference is ours, and the review's framing supports it by naming the GluCl activity immediately after posing the puzzle.⁵

12.2 And the argument has a testable form

If the rescue is real, a population carrying a GluCl-site change should lose susceptibility to fipronil in a way an Rdl-only population does not, and the two should be distinguishable by sequencing.

Which is a specific prediction and we found no study testing it, though the patch clamp work in §12.3 examined both receptors in the same animals.⁵

12.3 Which is redundancy working as designed

Our rotation article described mixtures as delivering redundant killing, where resistance to one component leaves the other operating. Here a single molecule does the same thing by hitting two targets.

12.4 And the review set out to study exactly this

Presenting a patch clamp study of the action of fipronil sulfone on native GABA receptors and GluCl receptors from susceptible and dieldrin-resistant German cockroaches, to provide a better understanding of the effect of the Rdl mutation on the function and insecticide sensitivity of these two targets, and its role in resistance.⁵

Which is the right experiment, performed on the species this trade treats.

13. Why that matters for rotation

The practical consequence, and it corrects our own earlier advice.

Our rotation article's fourth recommendation was that rotating to a product the population is already cross-resistant to achieves nothing, and that this requires knowing the mechanism rather than the class.

13.1 This case shows the inverse error

A population known to carry a target-site mutation for a class, rotated away from a compound in that class on the assumption of cross-resistance, when the compound would have worked.

13.2 So class membership fails in both directions

It can predict cross-resistance where there is none, as here, and miss cross-resistance that runs across classes, which our metabolic resistance articles describe.

That symmetry is ours and it is the most useful thing in this article.

13.3 And the honest planning input is a bioassay

Which tests the population against the product rather than reasoning from labels. Our resistance monitoring article found diagnostic dose testing effectively absent from structural practice, and this article is a demonstration of what reasoning without it costs.

13.4 Which leaves classification doing less work than it appears to

A mode of action group tells you what a compound binds. Whether a given population survives it depends on how many targets the compound has, which of them the population has altered, and what else the compound does.

14. And within the group it is not uniform either

A second finding that compounds the problem.

A study of the same substitution in a rice pest found that neither of the mutant Rdl subtypes significantly reduced the antagonistic action of fipronil, however there was a significant reduction in response to ethiprole in the two mutated subtypes compared with the wild type.⁶

14.1 Two phenylpyrazoles, one mutation, opposite results

Ethiprole affected, fipronil not, in the same experimental system.⁶

14.2 And the selection history is recorded

That ethiprole selection of two field populations resulted in strong resistance to both ethiprole and fipronil and resulted in fixation of the A301S mutation, alongside a second mutation in one strain.⁶

15. Which compounds the problem

Because the organism-level and receptor-level results diverge again.

Selection produced resistance to both compounds and fixed the mutation, and the mutation alone does not account for the fipronil resistance in the expression system.⁶

15.1 So something else was selected alongside it

Metabolic, penetration, or a further target change, and the study does not identify it.⁶

15.2 Which is the ordinary state of field resistance

Our layered resistance article described populations carrying several mechanisms at once, and this is a case where separating them was attempted and only partly succeeded.

16. The binding is different too

A mechanistic detail that may underlie all of it.

Fipronil exerted only one potent partially reversible inhibitory action on the GABA receptor, whereas dieldrin exhibited an irreversible dual action (potentiating action followed by inhibition).⁷

And: the effect of fipronil was 10-fold faster than that of dieldrin on the cockroach GABA receptor.⁷

16.1 Which is a genuine finding about the site

Two compounds can occupy the same pocket and interact with it entirely differently, which means same target is a coarser statement than it sounds when used to predict anything.⁷

16.2 Same site, different behaviour at it

One reversible and single-acting, the other irreversible and dual-acting, with a tenfold difference in rate.⁷

17. Which may explain the rest

Our hypothesis, offered as such.

If two compounds occupy the same site by different kinetics and with different reversibility, a substitution altering that site need not affect them equally.

17.1 A fast reversible blocker and a slow irreversible one

Have different sensitivities to a change in binding affinity, because one depends on residence time and the other on rate of association.

17.2 We cannot demonstrate this

It is a reading that makes the §14 result unsurprising, and no source we found proposes it. Section 29 says so.

18. The metabolite

Which is where the story turns uncomfortable.

The sulfone is the primary biological metabolite, readily formed via oxidation, and more toxic than the parental compound.¹⁷

18.1 So the insect makes it worse for itself

Which is our pro-insecticide article's structure appearing again, though here the parent is already active and the metabolite adds to it rather than supplying all the effect.

18.2 And both forms hit the second target

Which means the §12 rescue survives metabolism: whatever the sulfone's selectivity, it retains the glutamate-gated activity the Rdl mutation does not touch.⁵⁷

18.3 And the selectivity figure drops

From 135-fold to 17-fold, on the source giving both, which would mean the margin of safety narrows eightfold once the compound is metabolised.²

19. And a conflict we cannot resolve

Between two sources we would ordinarily both trust.

A university extension programme's technical factsheet states that the sulfone is reported to be twenty times more active at mammalian chloride channels than at insect chloride channels.¹

A major publisher's reference compilation gives the sulfone's human to insect binding ratio as 17, meaning seventeenfold selective toward insects.²

19.1 Those are opposite claims

One says the metabolite remains insect-selective by a reduced margin. The other says the selectivity has inverted.¹²

19.2 We cannot tell which is right

They may be measuring different receptor subtypes, different preparations, or different endpoints, and neither gives enough detail in the material available to us to check.

19.3 The factsheet adds a further complication

That the sulfone is reportedly six times more potent in blocking vertebrate GABA-gated chloride channels than fipronil, but demonstrates similar toxicity to the parent compound in mammals.¹

Six times more potent at the receptor and similar in whole-animal toxicity is itself a gap requiring an explanation, presumably in distribution or clearance, and the factsheet does not supply one.

19.4 And it matters practically

If the factsheet is right, the compound's safety margin does not merely narrow on metabolism but reverses, which would make the relevant exposure question about the metabolite rather than the parent.¹

If the reference compilation is right, a seventeenfold margin remains and the picture is unremarkable.² Those imply different cautions.

19.5 Which is why we are reporting the disagreement rather than a number

Our article on damage statistics argued that a figure without a traceable method cannot be assessed. This is the version where two methods exist, both are cited, and they disagree.

20. The photoproduct

The environmental fate, and the most striking finding here.

Whereas fipronil and desulfinylfipronil are equipotent at the insect GABA receptor, the desulfinyl compound is 10-fold more potent than fipronil at the mammalian chloride channel, narrowing the intrinsic selectivity between the insect and mammalian targets.³

20.1 It is the photoproduct

Formed by sunlight, described by its discoverers as a novel photochemical desulfinylation with retention of neurotoxicity, and identified as the primary environmental metabolite.³¹

20.2 The factsheet gives a compatible figure

Nine to ten times more active at the mammalian chloride channel than the parent, reducing the selectivity between insects and humans when exposed to this metabolite.¹

Two sources agreeing, for once.

21. Degradation that reduces selectivity

Stating the consequence plainly.

The compound applied is a hundred and thirty-five-fold selective. The compound sunlight makes of it is equipotent on insects and ten times stronger on mammals.²³

21.1 And the discoverers named the property

Their title records desulfinylation with retention of neurotoxicity, which is the finding stated as a title rather than buried in a result.³

21.2 So weathering does not detoxify it

It retains the insecticidal activity and loses part of the margin, which is a combination our articles on residual chemistry have not previously encountered.

22. Which inverts an assumption

One that runs through this whole subject.

Environmental persistence is treated as the hazard and degradation as the resolution. Our articles on organochlorine restriction, on treated articles and on residue decline all rest on that framing.

22.1 Here degradation produces the less selective compound

So the hazard profile of a treated surface changes over time in a direction nobody would predict from persistence data alone.³

That reading is ours.

22.2 And it is specific to light exposure

Which means an exterior application and an interior one produce different degradation products, and the relevant comparison is not how fast the residue declines but what it declines into.

22.3 And it complicates a regulatory preference

Our article on organochlorine restriction described persistence as the property that drove the withdrawals, and the successor chemistries were selected partly for degrading faster. A compound that degrades into something less selective satisfies the persistence criterion and not the underlying concern.

That is our reading and we are not suggesting the regulator missed it.

22.4 We should be careful about how far to take this

Receptor potency is not whole-animal toxicity, the photoproduct's environmental concentrations are not stated in our sources, and the registered uses presumably account for it. Section 29 repeats that.

23. And the aquatic finding

Recorded briefly.

Fipronil and its metabolites and degradation products are highly toxic to some species of fish.²

23.1 Metabolites and degradation products included

Which is §21 restated for a non-target group.²

24. Selection is happening anyway

Whatever §11 says about current efficacy.

A study reports the detection of the A302S Rdl mutation in fipronil bait-selected strains of the German cockroach.⁷

24.1 Selecting with fipronil bait produced the mutation

Which means the compound is applying pressure at the GABA target even though the GluCl target is rescuing its efficacy.⁷

24.2 And bait is the relevant exposure route

Our horizontal transfer articles describe fipronil baits as depending on delayed action and sharing between individuals, which means a colony receives graded doses rather than a single lethal one.

Graded doses across a population are the classic selection condition, and our reversion article argued that structural settings supply them routinely.

24.3 So the rescue is not permanent

A population accumulating GABA-site resistance is a population in which the compound depends increasingly on the second target alone, and nothing prevents a change there too.

That is our inference and it is the reason §11 should not be read as reassurance.

25. What a contractor should take from this

Four points.

A mode of action group is not a cross-resistance prediction. Sections 11 to 14.

This compound is unusual in having two targets. Which is why it survived an existing widespread resistance allele, and is not a general property of anything else.⁵

Its selectivity changes as it is metabolised and as it weathers. Sections 18 and 20.

And the aquatic caution is real. Section 23, which matters for any exterior application near water.²

25.1 The second point is the one to hold

Because it is a property of this molecule rather than a general lesson. Nothing licenses assuming that another compound will survive an existing resistance allele, and the reason fipronil did is specific enough that it should not be generalised.⁵

25.2 And it is a reason to value the compound

An active that works against a population carrying a widespread and persistent resistance allele is a scarce thing, and our bed bug history article described what happens when the supply of such compounds stops.

26. The successor chemistry

Worth a mention because it addresses the same target.

A study of an isoxazoline compound found it several orders of magnitude more potent than picrotoxinin and dieldrin, performing 5-236 fold better than fipronil on the arthropod RDLs, while a rat GABACl remained unaffected.⁵

26.1 A rat receptor unaffected

Which is a stronger selectivity claim than any figure in §5.⁵

26.2 And the same study found the target hierarchy

That one tick species' RDL is 52-fold more sensitive than its GluCl to fluralaner inhibition, confirming that the GABA-gated chloride channel is the primary target.⁵

26.3 And the 5 to 236 fold range is worth noticing

A forty-seven-fold spread in how much better the newer compound performs, across different arthropod receptors.⁵

Which means potency at this target is strongly species-dependent, and a figure obtained in one organism should not be carried to another. That is the same caution our articles on bioassay transfer make.

26.4 Which would make it vulnerable where fipronil was not

If a compound depends primarily on the GABA target, the §12 rescue is not available to it. That prediction is ours and we found nobody stating it.

27. Our own position

The disclosure.

This is an active we use. Section 13 says our own earlier rotation advice was incomplete, and sections 20 to 22 report a property of it that no product literature we have read mentions.

27.1 None of which is an argument against the compound

The selectivity is genuine, the second target is a real advantage, and the efficacy against resistant populations is documented.⁵

27.2 It is an argument for knowing what it does

Which our label article established is not what a label tells you.

28. The Manitoba position

Two notes.

The German cockroach work is directly applicable. Both the mutation detection and the patch clamp study used that species, which is the one our cockroach articles describe.⁷⁵

And the aquatic caution matters here. A province of rivers, lakes and a high water table, with exterior applications draining to them.

28.1 And the pollinator question sits alongside

This active has been the subject of substantial international argument over non-target insect effects, which our article on pollinator exposure from structural application touches. Nothing in this article addresses that literature and §29 says so.

28.2 What we could not find

Any Canadian resistance monitoring for this allele in structural pests, any local fipronil susceptibility data, and any Manitoba surface water monitoring for the compound or its metabolites.

29. Limitations and open questions

Most sources were read as abstracts. The electrophysiology papers and the review reach us through published abstracts and extracts rather than full texts.⁴⁵

Two sources disagree and we could not resolve it. Section 19, and it concerns the main metabolite's selectivity.

The rescue explanation is our reading. The review names the GluCl activity immediately after posing the puzzle, which we take as the intended connection, and does not state it as the answer in the material we have.⁵

The kinetics hypothesis in §17 is unsupported. It follows from §16 and no source proposes it.

Receptor potency is not toxicity. Sections 20 to 22 concern binding and channel blocking, and the whole-animal consequence depends on exposure, absorption and clearance we have not examined.

And we did not examine the regulatory assessment. The photoproduct is well known and presumably addressed in registration, which our registration article describes as a substantial exercise, and we have not read what was concluded.

Nor the non-target literature. Section 28.1. This article is about mechanism and resistance, and the environmental argument around this compound is substantial and untouched here.

And several findings are single studies. The receptor-level cross-resistance, the planthopper divergence and the binding kinetics each rest on one paper, and none has been checked against a replication.⁴⁶⁷

Sections 1.2, 3.1, 5.2, 7.2, 10, 12.1, 12.2, 13, 15.2, 17, 18.1, 21, 22, 24.2, 25 and 26.3 are our reasoning. The comparison of selectivity bases, the rescue interpretation, the argument that class membership fails in both directions, the kinetics hypothesis, the inversion of the degradation assumption and the prediction about the successor chemistry are ours rather than sourced positions.

30. Conclusion

Fipronil blocks the GABA-gated chloride channel, which is the nerve's brake: chloride entering the cell offsets the excitatory effect of sodium, and preventing it leaves excitation unchecked.² That is the same site the cyclodienes blocked, and a substitution in the channel lining conferring high dieldrin resistance is widespread and persistent in insect populations decades after that chemistry went, and makes mutant receptors markedly less sensitive to fipronil in direct measurement.⁴⁵ Every element of a cross-resistance prediction is present, and fipronil controls dieldrin-resistant populations anyway.

The reason is a second target. Fipronil and its sulfone metabolite are described as unique among chloride channel blockers in also potently blocking glutamate-gated chloride channels, which have been observed only in invertebrates and which the Rdl substitution does not affect.⁵² A compound hitting two inhibitory systems retains half its effect against a population that has protected one. Which means mode of action classification fails in both directions: it predicted cross-resistance that did not appear here, and our metabolic resistance articles describe cross-resistance appearing across classes it says are unrelated. And within the group it is not even uniform, since the same substitution reduced ethiprole's action and not fipronil's in one expression system.⁶

Two things about this compound do not appear in any product literature we have read. Its selectivity is reported at a hundred and thirty-five-fold for the parent and seventeen for the sulfone by one source, while another states the sulfone is twenty times more active at mammalian channels than insect ones, and we cannot reconcile them.²¹ And the photoproduct sunlight makes of it is equipotent with the parent at the insect receptor and ten times more potent at the mammalian channel.³ Degradation is treated everywhere in this subject as the process that resolves a hazard. Here it retains the insecticidal activity and removes part of the margin, which makes the useful question about a residue not how fast it declines but what it declines into.

References

1. Fipronil technical fact sheet, university pesticide information programme. Used for the statement that the lower binding affinity for mammalian receptors enhances selectivity for insects and increases the margin of safety for people and animals; for the report that the sulfone, the primary biological metabolite, is reported to be twenty times more active at mammalian chloride channels than at insect chloride channels; for the report that the sulfone is reportedly six times more potent in blocking vertebrate GABA-gated chloride channels than the parent while demonstrating similar toxicity to the parent compound in mammals; and for the report that the desulfinyl photoproduct, the primary environmental metabolite, is nine to ten times more active at the mammalian chloride channel than the parent compound, reducing the selectivity between insects and humans when exposure is to this metabolite. https://npic.orst.edu/factsheets/archive/fiptech.html
2. Fipronil overview, reference compilation entry assembled by a major academic publisher from its own published works. Used for the mechanistic account that an important mechanism returning nerves from excitation to their resting state is the timely opening of chloride channels, that chloride ions entering offset the electrochemical effect of sodium ions, and that fipronil inhibits the opening of GABA-mediated chloride channels so the excitatory effect of sodium is unchecked; for the statement that glutamate-gated chloride channels have been observed only in invertebrates; for the binding data that affinities of fipronil and its sulfone to mammalian GABA-A receptors are much less than in arthropods, with human to insect binding concentration ratios of 135 and 17 respectively and no binding to other types of mammalian GABA receptor, which together with low systemic bioavailability after dermal administration accounts for the selectivity of action; and for the statement that fipronil and its metabolites and degradation products are highly toxic to some species of fish. https://www.sciencedirect.com/topics/neuroscience/fipronil
3. Fipronil insecticide: novel photochemical desulfinylation with retention of neurotoxicity. Journal article in a national academy proceedings. Used for the description of fipronil as the first phenylpyrazole introduced for pest control, providing long-term protection at low dosage against named pest groups; for the statement that the mode of action involves blocking the GABA-gated chloride channel that is also the target for cyclodiene insecticides including two named compounds; for the finding that while fipronil and the desulfinyl compound are equipotent at the insect GABA receptor, the desulfinyl compound is tenfold more potent than fipronil at the mammalian chloride channel, narrowing the intrinsic selectivity between insect and mammalian targets; and for the authors' observation that the compound's activity may be due in part to the combined action of the parent and the sulfone derivative, which is similar in potency to fipronil and is its principal metabolite. https://www.pnas.org/doi/10.1073/pnas.93.23.12764
4. Actions of the insecticide fipronil on dieldrin-sensitive and resistant GABA receptors of a named fruit fly. Journal article in a pharmacology title, read as abstract. Used for the study design examining the blocking actions of fipronil on GABA responses recorded from oocytes expressing either wild type dieldrin-sensitive or mutant dieldrin-resistant forms of the GABA-gated chloride channel homo-oligomer, the product of the resistance to dieldrin locus; for the finding that in the wild type receptor fipronil blocked GABA-induced currents, shifting the dose-response curve and depressing maximum response amplitude, with potency dependent on GABA concentration and unaffected by membrane potential; for the finding that mutant receptors carrying a naturally occurring amino acid substitution in the putative ion-channel lining region, conferring resistance to dieldrin and to a named convulsant, were markedly less sensitive to fipronil than wild-type receptors; and for the observation that the same substitution reduces the potency of both fipronil and the structurally distinct convulsant. https://bpspubs.onlinelibrary.wiley.com/doi/10.1111/j.1476-5381.1995.tb15896.x
5. The role of GABA and glutamate receptors in susceptibility and resistance to chloride channel blocker insecticides. Review and research article in a pesticide biochemistry title, read as abstract and extracts. Used for the central statement that despite a point mutation in the pore-forming segment of the Rdl GABA receptor subunit that is widespread and persistent in insect populations and confers high levels of resistance to dieldrin and other related insecticides, the phenylpyrazole fipronil, which binds at the same site, has proven effective in controlling many insects including dieldrin-resistant populations; for the statement that fipronil and its major sulfone metabolite are unique among chloride channel blocking insecticides in that they also potently block glutamate-gated chloride channels; for the described patch clamp study of fipronil sulfone on native GABA and glutamate-gated chloride receptors from susceptible and dieldrin-resistant German cockroaches, undertaken to understand the effect of the mutation on both targets and its role in resistance; for the note that the resistant strain was isolated from a strain shown to carry the substitution in the relevant membrane segment; and for the comparative findings that an isoxazoline compound was several orders of magnitude more potent than two named older compounds and performed 5 to 236 fold better than fipronil on arthropod RDL receptors while a rat receptor remained unaffected, with one tick species' RDL being 52-fold more sensitive than its glutamate-gated chloride channel to that compound. https://www.sciencedirect.com/science/article/abs/pii/S0048357509001436
6. Influence of a named substitution in the RDL GABA-gated chloride channel of a rice pest on the activity of phenylpyrazole insecticides. Open-access journal article. Used for the description of phenylpyrazoles as non-competitive blockers of the GABA-gated chloride channel, a member of the pentameric transmembrane ligand-gated ion channel family mediating synapse inhibition in the insect central nervous system, and their placement in group 2 of the industry mode of action classification scheme encompassing GABA-gated chloride channel antagonists; for the note that these compounds were introduced for control of the species after resistance to a named neonicotinoid became commonplace; for the discovery of the substitution in resistant field populations by sequencing; for the finding that selection of two field populations with one phenylpyrazole resulted in strong resistance to both that compound and fipronil and fixation of the substitution, alongside emergence of a second mutation in one selected strain; and for the expression-system result that neither mutant receptor subtype significantly reduced the antagonistic action of fipronil while both showed significant reduction in response to the other phenylpyrazole compared with wild type. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5672059/
7. Detection of the A302S Rdl mutation in fipronil bait-selected strains of the German cockroach. Journal article in an economic entomology title, read as abstract and reference material. Used for the study's subject, namely detection of the named substitution in strains of that species selected with fipronil bait; for the description of fipronil as a phenylpyrazole widely used against German cockroaches; for the comparison that fipronil exerted only one potent partially reversible inhibitory action on the GABA receptor whereas dieldrin exhibited an irreversible dual action of potentiation followed by inhibition; for the finding that the effect of fipronil was tenfold faster than that of dieldrin on the cockroach GABA receptor; for the statement that fipronil is readily metabolised via oxidation to form a sulfone byproduct that is more toxic than the parent compound; and for the note that both parental and bioactivated compounds were reported to exhibit a neurophysiological effect on the glutamate-gated chloride receptor. https://academic.oup.com/jee/article/106/5/2167/879688