Collateral Responses Research Articles

The effect of antibiotic selection on collateral effects and evolvability of uropathogenic Escherichia coli

Trimethoprim is recommended as a first-line treatment of urinary tract infections (UTIs) in the UK. In 2018, 31.4% of Escherichia coli isolated from UTIs in England were trimethoprim-resistant, leading to overreliance on other first and second-line antibiotics. Here, we assessed whether, in principle, prior selection with trimethoprim results in collateral effects to other antibiotics recommended for the treatment of UTIs. As collateral effects, we considered changes in susceptibility, mutation-selection window and population establishment probability. We selected 10 trimethoprim-resistant derivatives from three clinical isolates of uropathogenic Escherichia coli. We found that mutations conferring trimethoprim resistance did not have any collateral effects on fosfomycin. In contrast, resistance to trimethoprim resulted in decreased susceptibility (collateral resistance) to nitrofurantoin, below the clinical breakpoint and narrowed the mutation-selection window, thereby reducing the maximum concentration for selection of nitrofurantoin resistance mutations. Our analyses demonstrate that multiple collateral responses should be accounted for when predicting and optimising antibiotic use, limiting future antimicrobial resistance emergence.

Abstract 1190 Investigating chemoresistance mechanisms and collateral responses in pancreatic ductal adenocarcinoma

Abstract 1190 Investigating chemoresistance mechanisms and collateral responses in pancreatic ductal adenocarcinoma

The ulnar collateral ligament response to valgus stress, repetitive pitching, and elbow muscle contraction in asymptomatic baseball pitchers

In baseball, repetitive pitching leads to medial elbow injuries, particularly to the ulnar collateral ligament (UCL). To prevent pitchers from UCL injuries, it is important to quantify the response to elbow stress. Repetitive elbow external valgus torque and muscular fatigue induced by repetitive pitching could affect markers of the response, that is, humeroulnar joint gap and UCL morphology. The aims of the study were three-folded: to investigate the effect of (1) exerted handgrip force on the humeroulnar joint gap, (2) repetitive pitching on the humeroulnar joint gap and the UCL morphology, and (3) exerted handgrip force on the humeroulnar joint gap for different levels of elbow valgus stress is different after compared to before repetitive pitching in asymptomatic baseball pitchers. Medial elbow ultrasound images were collected in 15 asymptomatic male baseball pitchers. Three levels of static elbow valgus stress (0N, 50N, 100N) were applied with a TELOS device before and after repetitive pitching and with or without handgrip force. These images were used to assess the humeroulnar joint gap size and UCL length and thickness. After 110 fastball pitches or when 80% self-perceived fatigue on a VAS scale was reached, participants were instructed to stop throwing. Repeated measures ANOVAs were used to statistically test significant differences. Handgrip force did not significantly affect the humeroulnar joint gap. The UCL thickness and length and the humeroulnar joint gap were also not different after compared to before repetitive pitching. While higher levels of applied valgus stress significantly increased the humeroulnar joint gap (P < .001), this effect was not significantly different in the interaction with handgrip force and repetitive pitching. The humeroulnar joint gap changes for different levels of elbow valgus stress. However, adult baseball pitchers did not respond to elbow stress after a single pitching session with or without submaximal handgrip force in the humeroulnar joint gap and UCL morphology.

The evolution of antibiotic resistance is associated with collateral drug phenotypes in Mycobacterium tuberculosis

The increasing incidence of drug resistance in Mycobacterium tuberculosis has diminished the efficacy of almost all available antibiotics, complicating efforts to combat the spread of this global health burden. Alongside the development of new drugs, optimised drug combinations are needed to improve treatment success and prevent the further spread of antibiotic resistance. Typically, antibiotic resistance leads to reduced sensitivity, yet in some cases the evolution of drug resistance can lead to enhanced sensitivity to unrelated drugs. This phenomenon of collateral sensitivity is largely unexplored in M. tuberculosis but has the potential to identify alternative therapeutic strategies to combat drug-resistant strains that are unresponsive to current treatments. Here, by using drug susceptibility profiling, genomics and evolutionary studies we provide evidence for the existence of collateral drug sensitivities in an isogenic collection M. tuberculosis drug-resistant strains. Furthermore, in proof-of-concept studies, we demonstrate how collateral drug phenotypes can be exploited to select against and prevent the emergence of drug-resistant strains. This study highlights that the evolution of drug resistance in M. tuberculosis leads to collateral drug responses that can be exploited to design improved drug regimens.

The evolution of antibiotic resistance in an incurable and ultimately fatal infection: A retrospective case study.

The processes by which pathogens evolve within a host dictate the efficacy of treatment strategies designed to slow antibiotic resistance evolution and influence population-wide resistance levels. The aim of this study is to describe the underlying genetic and phenotypic changes leading to antibiotic resistance within a patient who died as resistance evolved to available antibiotics. We assess whether robust patterns of collateral sensitivity and response to combinations existed that might have been leveraged to improve therapy. We used whole-genome sequencing of nine isolates taken from this patient over 279 days of a chronic infection with Enterobacter hormaechei, and systematically measured changes in resistance against five of the most relevant drugs considered for treatment. The entirety of the genetic change is consistent with de novo mutations and plasmid loss events, without acquisition of foreign genetic material via horizontal gene transfer. The nine isolates fall into three genetically distinct lineages, with early evolutionary trajectories being supplanted by previously unobserved multi-step evolutionary trajectories. Importantly, although the population evolved resistance to all the antibiotics used to treat the infection, no single isolate was resistant to all antibiotics. Evidence of collateral sensitivity and response to combinations therapy revealed inconsistent patterns across this diversifying population. Translating antibiotic resistance management strategies from theoretical and laboratory data to clinical situations, such as this, will require managing diverse population with unpredictable resistance trajectories.

Abstract B005: Uncovering polygenic signatures of Ewings Sarcoma drug sensitivity during the evolution of resistance

Abstract Evolution of chemotherapeutic resistance can cause treatment failure in patients with metastatic Ewings sarcoma (EWS). As resistance to the first-line therapy evolves, there is potential for the tumor to develop sensitivities to other chemotherapeutic agents besides the active treatment. This concept is known as collateral sensitivity, and it presents an opportunity for medical providers to plan more effective treatment regimens. However, it can be difficult to predict due to the stochastic nature of evolution. By analyzing gene expression patterns among independent populations of an EWS cell line that have repeatedly evolved collateral responses, we aim to extract a collection of gene signatures, or sets of genes, that will predict states of collateral sensitivity against various existing chemotherapies. To accomplish this, we will treat independently evolving replicates of an EWS cell line (A673) with the standard of care therapy (alternating combinations of vincristine, doxorubicin, cyclophosphamide, and etoposide and cyclophosphamide) to evolve resistance. Throughout this treatment regimen, we will regularly screen for collateral sensitivity against 128 chemotherapeutic agents and perform bulk RNA-sequencing. When collateral sensitivity against a drug is observed in multiple replicates, we will utilize the RNA-sequencing data to extract gene signatures using differential expression analysis. The signatures will then be validated using patient-derived xenografts collected in our lab as well as from collaborators. When completed, the sizable archive of sensitivity signatures we expect to assemble could greatly streamline the process of treatment planning for EWS patients, especially those with advanced disease. When a patient's response to primary therapy begins to decline, RNA- sequencing can be performed on a tumor biopsy, gene expression patterns can be compared with the validated signature library, and the most appropriate drug for the individual patient can be determined and used for subsequent treatment. This proposed method of treatment planning mitigates the need for excessive treatment regimens that are not guaranteed to be effective for every patient, thereby expanding the reach of precision medicine and potentially improving outcomes for EWS patients. Citation Format: Kristi Lin-Rahardja, Jessica Scarborough, Masahiro Hitomi, Jacob Scott. Uncovering polygenic signatures of Ewings Sarcoma drug sensitivity during the evolution of resistance [abstract]. In: Proceedings of the AACR Special Conference on the Evolutionary Dynamics in Carcinogenesis and Response to Therapy; 2022 Mar 14-17. Philadelphia (PA): AACR; Cancer Res 2022;82(10 Suppl):Abstract nr B005.

Allele-specific collateral and fitness effects determine the dynamics of fluoroquinolone resistance evolution

Collateral sensitivity (CS), which arises when resistance to one antibiotic increases sensitivity toward other antibiotics, offers treatment opportunities to constrain or reverse the evolution of antibiotic resistance. The applicability of CS-informed treatments remains uncertain, in part because we lack an understanding of the generality of CS effects for different resistance mutations, singly or in combination. Here, we address this issue in the gram-positive pathogen Streptococcus pneumoniae by measuring collateral and fitness effects of clinically relevant gyrA and parC alleles and their combinations that confer resistance to fluoroquinolones. We integrated these results in a mathematical model that allowed us to evaluate how different in silico combination treatments impact the dynamics of resistance evolution. We identified common and conserved CS effects of different gyrA and parC alleles; however, the spectrum of collateral effects was unique for each allele or allelic pair. This indicated that allelic identity can impact the evolutionary dynamics of resistance evolution during monotreatment and combination treatment. Our model simulations, which included the experimentally derived antibiotic susceptibilities and fitness effects, and antibiotic-specific pharmacodynamics revealed that both collateral and fitness effects impact the population dynamics of resistance evolution. Overall, we provide evidence that allelic identity and interactions can have a pronounced impact on collateral effects to different antibiotics and suggest that these need to be considered in models examining CS-based therapies.

Collateral responses to classical cytotoxic chemotherapies are heterogeneous and sensitivities are sparse

Chemotherapy resistance is a major obstacle to curing cancer patients. Combination drug regimens have shown promise as a method to overcome resistance; however, to date only some cancers have been cured with this method. Collateral sensitivity—the phenomenon whereby resistance to one drug is co-occurrent with sensitivity to a second drug—has been gaining traction as a promising new concept to guide rational design of combination regimens. Here we evolved over 100 subclones of the Eµ-Myc; p19ARF−/− cell line to be resistant to one of four classical chemotherapy agents: doxorubicin, vincristine, paclitaxel, and cisplatin. We then surveyed collateral responses to acquisition of resistance to these agents. Although numerous collateral sensitivities have been documented for antibiotics and targeted cancer therapies, we observed only one collateral sensitivity: half of cell lines that acquired resistance to paclitaxel also acquired a collateral sensitivity to verapamil. However, we found that the mechanism of this collateral sensitivity was unrelated to the mechanism of paclitaxel resistance. Interestingly, we observed heterogeneity in the phenotypic response to acquisition of resistance to most of the drugs we tested, most notably for paclitaxel, suggesting the existence of multiple different states of resistance. Surprisingly, this phenotypic heterogeneity in paclitaxel resistant cell lines was unrelated to transcriptomic heterogeneity among those cell lines. These features of phenotypic and transcriptomic heterogeneity must be taken into account in future studies of treated tumor subclones and in design of chemotherapy combinations.

Leptomeningeal anastomoses: Mechanisms of pial collateral remodeling in ischemic stroke

Arterial collateralization, as determined by leptomeningeal anastomoses or pial collateral vessels, is a well-established vital player in cerebral blood flow restoration and neurological recovery from ischemic stroke. A secondary network of cerebral collateral circulation apart from the Circle of Willis, exist as remnants of arteriole development that connect the distal arteries in the pia mater. Recent interest lies in understanding the cellular and molecular adaptations that control the growth and remodeling, or arteriogenesis, of these pre-existing collateral vessels. New findings from both animal models and human studies of ischemic stroke suggest a multi-factorial and complex, temporospatial interplay of endothelium, immune and vessel-associated cell interactions may work in concert to facilitate or thwart arteriogenesis. These valuable reports may provide critical insight into potential predictors of the pial collateral response in patients with large vessel occlusion and may aid in therapeutics to enhance collateral function and improve recovery from stroke.

Abstract 166: Loss Of Endothelial Cell Specific EphA4 Improves Acute Leptomeningeal Collateral Growth Following Ischemic Stroke

Leptomeningeal anastomoses, or pial collateral vessels, are a major determinant of patient outcome following ischemic stroke. These vessels form in the pia mater of the brain during development and connect distal arterioles of cerebral arteries. Following vascular obstruction, pial collaterals expand through arteriogenesis, an adaptive process that allows for retrograde perfusion into the obstructed artery and its affected tissue. However, acute arteriogenesis following ischemic stroke has been poorly investigated, therefore our research aims to fill this knowledge gap. Our novel pre-clinical murine findings suggest EphA4, a receptor tyrosine kinase, plays a role in restricting arteriogenesis following permanent middle cerebral artery occlusion (pMCAO). To investigate the role of endothelial cell-specific EphA4 in acute arteriogenesis, we employed 3-month old conditional endothelial cell (EC)-specific EphA4 knockout ( EphA4 fl/fl /Cdh5::Cre ERT2 ; KO) and wild type ( EphA4 fl/fl ; WT) mice. Compared to WT controls, we find KO mice displayed a significant reduction in infarct volume (24.0±1.8mm 3 vs 14.3±2.5mm 3 ; n=9) which correlated with larger ipsilateral pial collateral vessels as early as 4.5-hours (27.31±0.6um vs 32.41±0.8um; n=15) and up to 24-hours post-pMCAO (31.14±0.8um vs 36.7±0.9um; n=15). Additionally, our previous work has indicated that EphA4 may exert its inhibitory function on arteriogenesis by hindering Tie2 signaling. To identify gene expression changes in the collateral vessels and connecting cerebral arteries, the pial surface was carefully isolated from four mice per sample and pooled. Analysis of mRNA expression in the pial surface 24-hours post-pMCAO revealed that WT mice had significantly higher fold change of angiopoietin-2 (1.4±0.2 vs 0.8± 0.09; n=5) and Tie1 (5.3±1.8 vs 0.9±0.2; n=5) compared to KO mice, indicating potential inhibition of Tie2 signaling. These findings demonstrate that EC-specific EphA4 negatively regulates collateral growth and inhibition of this receptor could serve as a novel therapeutic strategy for improving collateral response following ischemic stroke.

Effects of divalent cations on Schaffer collateral axon function.

Previously, we reported that distal Schaffer collaterals undergo biphasic changes in excitability during high-frequency stimulation (HFS), with an early hyper-excitability period followed by an excitability depression period. The extracellular divalent cations calcium and magnesium can regulate membrane excitability in neuronal tissue. Therefore, we hypothesized that altering the concentrations of extracellular calcium and magnesium would alter the biphasic excitability changes. We tested this hypothesis by recording distal Schaffer collateral fiber volleys in stratum radiatum of hippocampal area CA1 during 100Hz HFS in artificial cerebral spinal fluid (ACSF) containing normal and altered concentrations of extracellular divalent cations. Our normal ACSF contained 2.0mM calcium and 2.0mM magnesium. We examined four solutions with altered divalent cation concentrations: (1) high-calcium/low-magnesium (3.8mM/0.2mM), (2) low-calcium/high-magnesium (0.2mM/3.8mM), (3) high-calcium/normal-magnesium (3.8mM/2.0mM), or (4) normal-calcium/high-magnesium (2.0mM/10.0mM), and assessed the effects on Schaffer collateral responses. Increasing or decreasing extracellular calcium enhanced or reduced (respectively) the early hyper-excitable period whereas increasing extracellular magnesium reduced the later excitability depression. Because these results might be explained by altered calcium influx through voltage-gated calcium (CaV) channels, we tested CaV blockers (ω-agatoxin IVA, ω-conotoxin-GVIA, cadmium), but observed no effects on responses during HFS. Some of the effects of altered divalent cation concentration may be explained by altered membrane surface charge. Although this mechanism does not completely explain our findings, calcium influx through CaV channels is not required.

Occlusion of activity dependent synaptic plasticity by late hypoxic long term potentiation after neonatal intermittent hypoxia

To elucidate the mechanisms of memory impairment after chronic neonatal intermittent hypoxia (IH), we employed a mice model of severe IH administered at postnatal days 3 to 7. Since prior studies in this model did not demonstrate increased cell death, our primary hypothesis was that IH causes a functional disruption of synaptic plasticity in hippocampal neurons.In vivo recordings of Schaffer collateral stimulation-induced synaptic responses during and after IH in the CA1 region of the hippocampus revealed pathological late phase hypoxic long term potentiation (hLTP) (154%) that lasted more than four hours and could be reversed by depotentiation with low frequency stimulation (LFS), or abolished by NMDA and PKA inhibitors (MK-801 and CMIQ). Furthermore, late phase hLTP partially occluded normal physiological LTP (pLTP) four hours after IH. Early and late hLTP phases were induced by neuronal depolarization and Ca2+ influx, determined with manganese enhanced fMRI, and had increased both AMPA and NMDA – mediated currents. This was consistent with mechanisms of pLTP in neonates and also consistent with mechanisms of ischemic LTP described in vitro with OGD in adults. A decrease of pLTP was also recorded on hippocampal slices obtained 2 days after IH. This decrease was ameliorated by MK-801 injections prior to each IH session and restored by LFS depotentiation. Occlusion of pLTP and the observed decreased proportion of NMDA-only silent synapses after neonatal hLTP may explain long term memory, behavioral deficits and abnormal synaptogenesis and pruning following neonatal IH.

Systematic Investigation of Resistance Evolution to Common Antibiotics Reveals Conserved Collateral Responses across Common Human Pathogens.

As drug resistance continues to grow, treatment strategies that turn resistance into a disadvantage for the organism will be increasingly relied upon to treat infections and to lower the rate of multidrug resistance. The majority of work in this area has investigated how resistance evolution toward a single antibiotic effects a specific organism's collateral response to a wide variety of antibiotics. The results of these studies have been used to identify networks of drugs which can be used to drive resistance in a particular direction. However, little is known about the extent of evolutionary conservation of these responses across species. We sought to address this knowledge gap by performing a systematic resistance evolution study of the ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter cloacae) under uniform growth conditions using five clinically relevant antibiotics with diverse modes of action. Evolved lineages were analyzed for collateral effects and the molecular mechanisms behind the observed phenotypes. Fourteen universal cross-resistance and two global collateral sensitivity relationships were found among the lineages. Genomic analyses revealed drug-dependent divergent and conserved evolutionary trajectories among the pathogens. Our findings suggest that collateral responses may be preserved across species. These findings may help extend the contribution of previous collateral network studies in the development of treatment strategies to address the problem of antibiotic resistance.

Collateral adaptive responses induced by desiccation stress in Salmonella enterica

Some Salmonella enterica serovars adapt effectively to low-water activity (low-aw) environments but this desiccation stress also induces collateral responses that have not been fully explored. Additionally, it is not known how long these responses are retained during refrigerated storage of the dehydrated pathogen. To investigate these issues, two S. enterica serovars known to acquire desiccation resistance (Tennessee and Eimsbuettel) were studied. Incubating Salmonella in broth under dehydration stress for 72 h increased the biofilm-forming ability of serovar Tennessee but not Eimsbuettel. Both serovars entered a viable but nonculturable (VBNC) state in response to desiccation for 24 or 48 h. When desiccation-adapted cells were stored at 4 °C for 2 days, four desiccation-related genes, proV, STM1494, kdpA, and otsB, were significantly upregulated (>2-fold increase) and the universal stress response regulator, rpoS, was highly upregulated (>50-fold increase). When storage at 4 °C was extended to 14 days, the two serovars increased the expression of proV and rpoS genes. In contrast, the expression of two virulence regulatory genes, hilA and invA, was downregulated (>2-fold decrease) at both storage periods. In conclusion, desiccation-adaptation of Salmonella invoked multiple stress responses which could serve as targets for developing pathogen mitigation strategies in low-aw foods.

Effectiveness of Parent Education in Pivotal Response Treatment on Pivotal and Collateral Responses

In two studies, we investigated the effectiveness of parent education in Pivotal Response Treatment (PRT) on parent-created opportunities and spontaneous child initiations in two community-based treatment facilities for children with autism spectrum disorder (ASD). Changes in parental stress and self-efficacy were explored. Participants were 26 parents and their children who participated in group (Study 1) or individual (Study 2) parent education in PRT. Results indicated that group-based parent education resulted in moderate increases in opportunities, functional initiations, and empathic social initiations. Furthermore, parental stress reduced and self-efficacy increased. Individual parent education resulted in large increases in opportunities and functional initiations, but parental stress and self-efficacy did not change. Implications for clinical practice and directions for future research are discussed.

Conserved collateral antibiotic susceptibility networks in diverse clinical strains of Escherichia coli

There is urgent need to develop novel treatment strategies to reduce antimicrobial resistance. Collateral sensitivity (CS), where resistance to one antimicrobial increases susceptibility to other drugs, might enable selection against resistance during treatment. However, the success of this approach would depend on the conservation of CS networks across genetically diverse bacterial strains. Here, we examine CS conservation across diverse Escherichia coli strains isolated from urinary tract infections. We determine collateral susceptibilities of mutants resistant to relevant antimicrobials against 16 antibiotics. Multivariate statistical analyses show that resistance mechanisms, in particular efflux-related mutations, as well as the relative fitness of resistant strains, are principal contributors to collateral responses. Moreover, collateral responses shift the mutant selection window, suggesting that CS-informed therapies may affect evolutionary trajectories of antimicrobial resistance. Our data allow optimism for CS-informed therapy and further suggest that rapid detection of resistance mechanisms is important to accurately predict collateral responses.

Cholinergic responses of acoustically-characterized cochlear nucleus neurons: An in vivo iontophoretic study in Guinea pig

The responses of guinea pig cochlear nucleus neurons to in vivo iontophoretic application of various neurotransmitter agonists were recorded with extracellular multi-barrelled electrodes. Where possible, neurons were physiologically identified using strict criteria. Emphasis was placed on the action of cholinergic agonists in relation to the possible action of olivocochlear collateral innervation. Excitatory responses (increase in action potential firing) to glutamate were confirmed in a number of neuronal response types. Application of acetylcholine (ACh) or the broad spectrum cholinergic agonist carbachol produced reliable excitatory responses in about 47% of neurons (n = 29 out of 61 neurons). The remaining neurons were unresponsive to cholinergic agonists and no inhibitory responses were observed. Cholinergic responses were more common in dorsal cochlear nucleus (DCN) (73% of 30 neurons tested) than in ventral cochlear nucleus (VCN) (23% of 31 neurons). Of the total neuron sample in which cholinergic responses were investigated, 41 neurons were able to be categorized according to established acoustic response features. Excitatory responses to cholinergic agonists were seen in “Pauser-buildup” (Pb) and “Transient chopper” (Ct) response types. Primary-like neurons (PL and Pn) as well as “Onset chopper” (Oc) neurons (n = 6) were unresponsive to either ACh or carbachol. Oc neurons also did not show any effect on their acoustic responses. Robust cholinergic responses were also seen in several VCN and DCN neurons that were either unresponsive to sound, or had acoustic response properties that did not fit standard classification. The results suggest a relatively more robust cholinergic innervation of DCN compared to VCN. The excitatory cholinergic responses of some Ct neurons and the lack of effect on Oc neurons are consistent with previous results in mouse brain slice studies, but are in conflict with reports of medial olivocochlear collateral excitatory responses in onset-type neurons in vivo. The results also indicate that a number of neurons of unknown identity may also receive cholinergic input.

Collateral response modulates the time-penumbra relationship in proximal arterial occlusions.

While clinical benefit from thrombolysis decreases with increase in time from stroke onset, the relationship of acute physiologic tissue compartments and collateral response to stroke onset time remains unclear. We studied consecutive patients with proximal arterial occlusions (n = 355) with whole-brain perfusion CT with CT angiography within 6 hours of stroke onset. Penumbra and core were defined using voxel-based thresholds. Tissue mismatch was defined as the ratio of penumbra to core. Collateral scores were assessed using a previously validated visual score. Mean (SD) age was 72.1 (12.4) years, median (interquartile range) NIH Stroke Scale score 16 (4), mean (SD) time to imaging 152.5 (69.7) minutes. Penumbra volume (Spearman ρ = 0.119, p = 0.026) and mismatch increased (Spearman ρ = 0.115, p = 0.030) with time from onset. Core volume decreased (Spearman ρ = -0.112, p = 0.035) while collateral scores increased with time (Spearman ρ = 0.117, p = 0.028). On multivariable regression, good collateral scores predicted longer time since onset (β = 0.101, p = 0.039) while mismatch was not a predictor (β = 0.001, p = 0.351). Good collateral score was the strongest independent predictor of final infarct volume and improvement in clinical deficit. In our large patient cohort study of proximal arterial occlusions, we found an incremental collateral response and preserved penumbral volume with time. Thus, tissue viability can be maintained in this time window (0-6 hours) after stroke if leptomeningeal collaterals are able to sustain the penumbra. Our findings suggest that a longer therapeutic window may exist for intra-arterial intervention and that multimodal imaging may have a role in strokes of unknown onset time.

Can Collateral Behavior Account for Transitions in the Stimulus Control of Speech?

The task of extending Skinner's (1957) interpretation of verbal behavior includes accounting for the moment-to-moment changes in stimulus control as one speaks. A consideration of the behavior of the reader reminds us of the continuous evocative effect of verbal stimuli on readers, listeners, and speakers. Collateral discriminative responses to verbal stimuli, beyond mere echoic or textual behavior, are potential sources of control and must be included in any complete account of both verbal and nonverbal behavior.

Nonspecific block of voltage-gated potassium channels has greater effect on distal schaffer collaterals than proximal schaffer collaterals during periods of high activity

Previous studies established different responses between proximal and distal portions of Schaffer collateral axons during high‐frequency and burst stimulation, with distal axons demonstrating biphasic changes in excitability (hyperexcitability followed by depression), but proximal axons showing only monophasic depression. Voltage‐dependent potassium (KV) channels are important determinants of axonal excitability, and block of KV channels can promote axon hyperexcitability. We therefore hypothesized that block of KV channels should lead to biphasic response changes in proximal Schaffer collaterals, like those seen in distal Schaffer collaterals. To test this hypothesis, we made extracellular recordings of distal Schaffer collateral responses in stratum radiatum of hippocampal area CA1 and proximal Schaffer collateral responses in stratum pyramidale of area CA3 during high‐frequency stimulation (HFS) at 100 Hz and burst stimulation at 200 msec intervals (5 Hz or theta frequency). We then applied a nonselective KV channel blocker, tetraethlylammonium (TEA, 10 mmol/L) or 4‐aminopyridine (4‐AP, 100 μmol/L), and assessed effects on Schaffer collateral responses. Surprisingly, block of KV channels had little or no effect on proximal Schaffer collateral responses during high‐frequency or burst stimulation. In contrast, KV channel blockade caused more rapid depression of distal Schaffer collateral responses during both high‐frequency and burst stimulation. These findings indicate that KV channels are important for maintaining distal, but not proximal, Schaffer collateral excitability during period of sustained high activity. Differential sensitivity of distal versus proximal Schaffer collaterals to KV channel block may reflect differences in channel density, diversity, or subcellular localization.