Disentangling the nature of the nicotine stimulus

Learning involving interoceptive stimuli likely plays an important role in many diseases and psychopathologies. Within this area, there has been extensive research investigating the interoceptive stimulus effects of abused drugs. In this pursuit, behavioral pharmacologists have taken advantage of what is known about learning processes and adapted the techniques to investigate the behavioral and receptor mechanisms of drug stimuli. Of particular interest is the nicotine stimulus and the use of the two-lever operant drug discrimination task and the Pavlovian drug discriminated goal-tracking task. There is strong concordance between the two methods when using "standard" testing protocols that minimize learning on test days. For example, ABT-418, nornicotine, and varenicline all fully evoked nicotine-appropriate responding. Notably, research from our laboratory with the discriminated goal-tracking task has used an alternative testing protocol. This protocol assesses stimulus substitution based on how well extinction learning using a non-nicotine ligand transfers back to the nicotine stimulus. These findings challenge conclusions based on more "standard" testing procedures (e.g., ABT-418 is not nicotine-like). As a starting point, we propose Thurstone scaling as a quantitative method for more precisely comparing transfer of extinction across doses, experiments, and investigators. We close with a discussion of future research directions and potential implications of the research for understanding interoceptive stimuli.     

Olfactory discrimination ability of CD-1 mice for aliphatic aldehydes as a function of stimulus concentration

Using an operant conditioning paradigm, we tested the ability of CD-1 mice to discriminate between members of a homologous series of aliphatic aldehydes presented at four different concentrations. We found that the mice were clearly capable of discriminating between all odorant pairs when stimuli were presented at concentrations of 1, 0.01, and 0.001 ppm (corresponding to four, two, and one log unit above the highest individual detection threshold) with no significant difference in performance between these concentrations. In contrast, the animals generally failed to discriminate above chance level when stimuli were presented at 0.0001 ppm (corresponding to the highest individual detection threshold) although stimuli were clearly detectable. Further, we found a significant negative correlation between discrimination performance and structural similarity of odorants in terms of differences in carbon chain length. These findings suggest that an increase in stimulus concentration of only one log unit above detection threshold appears to be sufficient for recruitment of additional subpopulations of odorant receptors to allow for qualitative recognition of aliphatic aldehydes.     

Discriminative and reinforcing stimulus properties of music for rats

We trained rats to discriminate music by Bach from that by Stravinsky using operant conditioning. The rats successfully learned the discrimination and transferred their discrimination to novel music by the same artists. Then, we trained rats on concurrent-chain schedule in which the terminal links were associated with different music, Bach or Stravinsky. The rats did not show strong preference for either style of music, although one subject showed a preference for Bach and another subject preferred Stravinsky. Finally, we examined the validity of the concurrent-chain procedure as a method of preference measurement with conspecific vocalization evoked by an aversive experience. Most of the rats preferred white noise to the conspecific vocalization. Therefore, music has a discriminative stimulus property but not a clear reinforcing property for rats.     

Effects of conditioning history on selective stimulus control by elements of compound discriminative stimuli

Effects of prior discrimination training on stimulus control by color and shape dimensions of compound stimuli were studied with college students. In Phase 1, single-stimulus discrimination training was conducted for two values of color and shape. Phase 2 discrimination training employed two 2-dimensional compound stimuli composed of the color and shape stimuli trained in Phase 1. For conflict-compound stimuli, the stimulus-response-consequence contingency was altered between phases for one stimulus dimension (target dimension), but not for the other, non-target, dimension. Level of congruence (100%, 25%, and 0%) of the contingency for the target dimension between phases was manipulated across groups. When each stimulus value was tested in Phase 3, level of Phase-2-consistent responding to the target dimension varied with level of Phase-1-to-Phase-2 congruence. In Experiment 2, training history for the non-target dimension was altered across three conditions: (a) Correlated with reinforcement, as in Experiment 1, (b) No-Training, or (c) Not-Correlated. Phase-2-consistent responding to the target cue in Phase 3 was lower under the latter conditions than under the Correlated condition, indicating that the non-target dimension modulated control by the target dimension, consistent with stimulus competition. The data suggest elemental, rather than configural processing of the compound stimuli during Phase 2.     

The effect of stimulus discriminability on strategies for learning multiple temporal discriminations

The goal was to identify training conditions under which temporal intervals that are signaled by different stimuli are memorized (i.e., the temporal behavior is readily shown to be under stimulus control). Undergraduate students were trained on three signaled temporal discriminations using a peak procedure. The intervals were trained in either blocks of trials or with trials intermixed within the session, and then they were given a transfer test with intermixed trials. There were two levels of stimulus discriminability, defined by the similarity of the stimuli. Most participants memorized the intervals when the discriminations were intermixed within the session, or were easy, but not when the discriminations occurred in blocks and were difficult. In the transfer tests, those participants trained in the difficult discrimination that occurred in blocks of trials typically continued to perform as they did during the last-trained interval, regardless of the stimulus presented. These results are better explained by a memory retrieval than a memory storage account.     

Effect of US identity on elimination and recovery of autoshaped responding with explicitly unpaired and degraded contingency extinction procedures

The depressive effects of noncontingent and explicitly unpaired food unconditioned stimuli (USs) and the recovery from those effects on autoshaped responding were examined in a series of experiments with pigeons. In Experiments 1 and 2, responding to a keylight conditioned stimulus (CS) previously paired with food was depressed equally by explicitly unpaired presentations of either that same food or a different food. Furthermore, responding recovered equally following removal of the explicitly unpaired foods. In contrast, Experiments 3 and 4 showed that noncontingent presentations of a food US depressed responding more to a keylight CS paired with that same food than to a keylight CS paired with a different food. Moreover, removal of the noncontingent foods led to complete recovery but more rapid extinction of responding to the same keylight relative to the different keylight. The implications of these results for understanding the mechanisms underlying depression and recovery of responding following degraded contingency and explicitly unpaired extinction procedures are discussed.     

Reconciling the influence of predictiveness and uncertainty on stimulus salience: a model of attention in associative learning

Theories of selective attention in associative learning posit that the salience of a cue will be high if the cue is the best available predictor of reinforcement (high predictiveness). In contrast, a different class of attentional theory stipulates that the salience of a cue will be high if the cue is an inaccurate predictor of reinforcement (high uncertainty). Evidence in support of these seemingly contradictory propositions has led to: (i) the development of hybrid attentional models that assume the coexistence of separate, predictiveness-driven and uncertainty-driven mechanisms of changes in cue salience; and (ii) a surge of interest in identifying the neural circuits underpinning these mechanisms. Here, we put forward a formal attentional model of learning that reconciles the roles of predictiveness and uncertainty in salience modification. The issues discussed are relevant to psychologists, behavioural neuroscientists and neuroeconomists investigating the roles of predictiveness and uncertainty in behaviour.     

A comparison of the effects of discriminative and Pavlovian inhibitors and excitors on instrumental responding

In two experiments, the effects of Pavlovian or discriminative conditioned inhibitors on operant responding were investigated in rats. Experiment 1 found that a Pavlovian conditioned inhibitor for food suppressed food-reinforced lever pressing more than a non-differentially trained control stimulus did. Experiment 2 demonstrated that an operant discriminative inhibitor produced greater suppression of lever pressing than a Pavlovian conditioned inhibitor. Experiment 2 also found that compounding an operant discriminative stimulus (SD) for food-reinforced responding with another SD for food-reinforced responding resulted in more additive summation than when an SD was compounded with a Pavlovian conditioned excitor for food. The results of these experiments support two-factor theories that postulate that incentive and response discriminative processes summate algebraically when the processes are inhibitory or excitatory.     

Locomotor activity in a novel environment predicts both responding for a visual stimulus and self-administration of a low dose of methamphetamine in rats

There is evidence that visual stimuli used to signal drug delivery in self-administration procedures have primary reinforcing properties, and that drugs of abuse enhance the reinforcing properties of such stimuli. Here, we explored the relationships between locomotor activity, responding for a visual stimulus, and self-administration of methamphetamine (METH). Rats were classified as high or low responders based on activity levels in a novel locomotor chamber and were subsequently tested for responding to produce a visual stimulus followed by self-administration of a low dose of METH (0.025 mg/kg/infusion) paired with the visual stimulus. High responder rats responded more for the visual stimulus than low responder rats indicating that the visual stimulus was reinforcing and that operant responding for a visual stimulus has commonalities with locomotor activity in a novel environment. Similarly, high responder rats responded more for METH paired with a visual stimulus than low responder rats. Because of the reinforcing properties of the visual stimulus, it was not possible to determine if the rats were responding to produce the visual stimulus, METH or the combination. We speculate that responding to produce sensory reinforcers may be a measure of sensation seeking. These results indicate that visual stimuli have unconditioned reinforcing effects which may have a significant role in acquisition of drug self-administration, a role that is not yet well understood.     

Stimulus magnitude effects on the extinction of conditioned consummatory responses to flavored sucrose solution in mice

Paradoxical extinction effects in the conditioned consummatory behavior of rodents have remained largely elusive. Here, appetitive flavor conditioning was studied to determine if a paradoxical magnitude of reinforcement extinction effect (MREE) can occur in the consummatory behavior of mice. During acquisition training of two experiments with factorial design, animals received daily access to either 32% or 4% sucrose solution, and goal tracking time was measured in one-minute bins. In Experiment 1 the solutions were flavored with either 5% or 0.5% almond essence and in Experiment 2 with 2% almond essence, but combined with continuous or partial schedules of reinforcement. During extinction tests of Experiment 1 and 2, water flavored with 0.5% or 2% almond was presented, respectively. Consummatory performance decreased more abruptly during the initial portion of the extinction sessions after training with 32% as compared to 4% sucrose solution. Furthermore, when given a choice test after extinction training (Experiment 2), animals trained with 32% sucrose, preferred the flavored solution, but animals trained with 4% preferred the unflavored solution. These results are interpreted as indicative of the occurrence of a paradoxical MREE in conditioned consummatory behaviors.     

The effect of elevated blood cortisol levels on the extinction of a conditioned stress response in rainbow trout

Following previously published observations that a conditioned response (CR) was lost more quickly by rainbow trout (Oncorhynchus mykiss) exhibiting a high responsiveness to stressors than by low responding individuals this study was designed to investigate the effects of exogenous cortisol on the retention of a CR in unselected rainbow trout. Fish held in isolation were conditioned over a 10-day period by pairing an innocuous signal (conditioned stimulus, CS: a water jet played on the surface of the tank water) with a mild stressor (unconditioned stimulus, US: 30 min of confinement). This resulted in a brief elevation of plasma cortisol levels (the CR) when the fish was exposed to the CS only. The effect of exogenous cortisol on the retention of the CR was evaluated by comparing the performance of fish that received cortisol-containing slow-release intraperitoneal implants, with fish receiving vehicle-only implants. Retention of the CR was assessed at intervals up to 35 days after conditioning ceased. The CR was considered to be evident when 30 min following presentation of the CS, mean plasma cortisol levels were significantly higher in conditioned than untrained fish. On day 1 both cortisol-implanted and vehicle-implanted conditioned fish exhibited a CR. However, from day 5 onwards the CR was observed only in the vehicle-implanted and conditioned group. This finding indicates that administration of cortisol accelerated the extinction of the CR in the cortisol-implanted fish, suggesting that elevated plasma cortisol levels can impair memory processes in rainbow trout.     

Characterizing psychophysical measures of discrimination thresholds and the effects of concentration on discrimination learning in the moth Manduca sexta

What is the spatial and temporal nature of odor representations within primary olfactory networks at the threshold of an animal's ability to discriminate? Although this question is of central importance to olfactory neuroscience, it can only be answered in model systems where neural representations can be measured and discrimination thresholds between odors can be characterized. Here, we establish these thresholds for a panel of odors using a Pavlovian paradigm in the moth Manduca sexta. Moths were differentially conditioned to respond to one odor (CS+) but not another (CS-) using undiluted odorants to minimize salience-dependent learning effects. At 24 and 48 h postconditioning, moths were tested for the presence of a conditioned response (CR) with a blank, then the CS+ and CS- (pseudorandomly) across a 5-log step series of increasing concentration. Results identified discrimination thresholds and established that differential CRs to the CS+ and CS- increased with stimulus concentration. Next, 3 separate groups of moths were differentially conditioned at either one-log step below, at, or one log step above the identified discrimination threshold. At 24 and 48 h postconditioning, moths were tested sequentially with a blank, the concentration used for conditioning, and then undiluted odor. Conditioning at one log step below the discrimination threshold established a CR, indicating both stimulus detection and learning, but was insufficient to establish evidence of discrimination. Moths conditioned at the discrimination threshold were able to discriminate but only when stimulated with undiluted odors, indicating learning, but discrimination measures were hampered. When conditioned above the discrimination threshold, moths had no difficulty in discriminating. These results establish methods for psychophysical characterization of discrimination and indicate that differential conditioning at lowered concentrations biases threshold measures.     

Stimulation of ATPase activity by auxin is dependent on ATP concentration

The effect of auxin on membrane-bound ATPase activity was studied in a plasma membrane-enriched fraction from zucchini hypocotyls. The apparent K_M of ATPase activity for ATP was decreased in the presence of 10^-6 M auxin so that at very low ATP concentrations the stimulation of ATPase activity was most obvious. The weak auxin analogue, 2,3-dichlorophenoxyacetic acid, stimulated much less than the active auxin 2,4-dichlorophenoxyacetic acid.     

A Salience Theory of Learning and Behavior: With Perspectives on Neurobiology and Cognition

Traditional behaviorists have described behaviors fundamentally as responses to stimuli or, perhaps more liberally, as behaviors under the control of discriminative stimuli or contexts. They have held responses or behaviors to be established, strengthened, sustained, and inhibited or extinguished by contingent events: notably reinforcers, punishers, or the absence of either. In addition, they believed reinforcement acts on the response, the behavior, not on the organism. Here, and in support of Hebb’s view, we advance a contrarian view. A key principle of our framework is that species’ brains are uniquely designed to perceive and to relate stimulus events that are contiguous, salient, and relevant to adaptation. In accordance with what we here view as the constructive biases of species’ brains, stimuli are differentially organized into amalgams that reflect an exchange of salience and response-eliciting properties of component units, which are then integrated to form a basis of knowledge about the organism and its ecological niche. One can then base adaptation on overarching principles and rules, not just on simple associations. Species may create emergent behaviors with no history of specific training, and even new capacities, to service adaptation to both familiar and novel challenges.     

Inhibition of tobacco mosaic virus replication in lateral roots is dependent on an activated meristem-derived signal

Summary. Viral invasion of the root system of Nicotiana benthamiana was studied noninvasively with a tobacco mosaic virus (TMV) vector expressing the green-fluorescent protein (GFP). Lateral root primordia, which developed from the pericycle of primary roots, became heavily infected as they emerged from the root cortex. However, following emergence, a progressive wave of viral inhibition occurred that originated in the lateral-root meristem and progressed towards its base. Excision of source and sink tissues suggested that the inhibition of virus replication was brought about by the basipetal movement of a root meristem signal. When infected plants were inoculated with tobacco rattle virus (TRV) expressing the red-fluorescent protein, DsRed, TRV entered the lateral roots and suppressed the host response, leading to a reestablishment of TMV infection in lateral roots. By infecting GFP-expressing transgenic plants with TMV carrying the complementary GFP sequence it was possible to silence the host GFP, leading to the complete loss of fluorescence in lateral roots. The data suggest that viral inhibition in lateral roots occurs by a gene-silencing-like mechanism that is dependent on the activation of a lateral-root meristem. Received July 23, 2001 Accepted October 11, 2001     

The effect of 3-bromopyruvate on human colorectal cancer cells is dependent on glucose concentration but not hexokinase II expression

Cancer cells heavily rely on the glycolytic pathway regardless of oxygen tension. Hexokinase II (HKII) catalyses the first irreversible step of glycolysis and is often overexpressed in cancer cells. 3-Bromopyruvate (3BP) has been shown to primarily target HKII, and is a promising anti-cancer compound capable of altering critical metabolic pathways in cancer cells. Abnormal vasculature within tumours leads to heterogeneous microenvironments, including glucose availability, which may affect drug sensitivity. The aim of the present study was to elucidate the mechanisms by which 3BP acts on colorectal cancer (CRC) cells with focus on the HKII/Akt signalling axis. High HKII-expressing cell lines were more sensitive to 3BP than low HKII-expressing cells. 3BP-induced rapid Akt phosphorylation at site Thr-308 and cell death via both apoptotic and necrotic mechanisms. Cells grown under lower glucose concentrations showed greater resistance towards 3BP. Cells with HKII knockdown showed no changes in 3BP sensitivity, suggesting the effects of 3BP are independent of HKII expression. These results emphasize the importance of the tumour microenvironment and glucose availability when considering therapeutic approaches involving metabolic modulation.     

Inhibition of THP-1 cell adhesion to endothelial cells by alpha-tocopherol and alpha-tocotrienol is dependent on intracellular concentration of the antioxidants

Vitamin E analogs such as alpha-tocopherol and alpha-tocotrienol have been shown to reduce endothelial expression of adhesion molecules. The reactivity of alpha-tocopherol and alpha-tocotrienol in inhibiting lipid peroxidation in vitro was essentially identical but the inhibition of adhesion of THP-1 cells, a monocytic-"like" cell line, to endothelial cells differs substantially. To determine the mechanism underlying this response, human umbilical vein endothelial cells (HUVECs) were assessed for their ability to accumulate vitamin E analogs. alpha-Tocotrienol accumulated in HUVECs to levels approximately 10-fold greater than that of alpha-tocopherol. The decrease in expression of vascular cell adhesion molecule-1 (VCAM-1) and the adhesion of THP-1 cells to HUVECs by alpha-tocopherol and alpha-tocotrienol was also determined. Both alpha-tocopherol and alpha-tocotrienol suppressed VCAM-1 expression and adhesion of THP-1 cells to HUVECs in a concentration-dependent manner. The efficacy of tocotrienol for reduction of VCAM-1 expression and adhesion of THP-1 cells to HUVECs was also 10-fold higher than that of tocopherol. The inhibitory effects of vitamin E analogs on the adhesiveness of endothelial cells clearly correlated with their intracellular concentrations. The data demonstrated that, in assessing the biological responses of antioxidants, intracellular accumulation and metabolism were additional important factors that must be considered.     

Voltage dependence of the Ca2+-activated K+ conductance of human red cell membranes is strongly dependent on the extracellular K+ concentration

Summary The conductance of the Ca²⁺-activated K⁺ channel (g _K(Ca)) of the human red cell membrane was studied as a function of membrane potential (V _m ) and extracellular K⁺ concentration ([K⁺]_ex). ATP-depleted cells, with fixed values of cellular K⁺ (145mm) and pH (7.1), and preloaded with 27 m ionized Ca were transferred, with open K⁺ channels, to buffer-free salt solutions with given K⁺ concentrations. Outward-current conductances were calculated from initial net effluxes of K⁺, correspondingV _m , monitored by CCCP-mediated electrochemical equilibration of protons between a buffer-free extracellular and the heavily buffered cellular phases, and Nernst equilibrium potentials of K ions (E _K) determined at the peak of hyperpolarization. Zero-current conductances were calculated from unidirectional effluxes of⁴²K at (V _m –E _K)0, using a single-file flux ratio exponent of 2.7. Within a [K⁺]_ex range of 5.5 to 60mm and at (V _m –E _K) 20 mV a basic conductance, which was independent of [K⁺]_ex, was found. It had a small voltage dependence, varying linearly from 45 to 70 S/cm² between 0 and –100 mV. As (V _m –E _K) decreased from 20 towards zero mVg _K(Ca) increased hyperbolically from the basic value towards a zero-current value of 165 S/cm². The zero-current conductance was not significantly dependent on [K⁺]_ex (30 to 156mm) corresponding toV _m (–50 mV to 0). A further increase ing _K(Ca) symmetrically aroundE _K is suggested as (V _m –E _K) becomes positive. Increasing the extracellular K⁺ concentration from zero and up to 3mm resulted in an increase ing _K(Ca) from 50 to 70 S/cm². Since the driving force (V _m –E _K) was larger than 20 mV within this range of [K⁺]_ex this was probably a specific K⁺ activation ofg _K(Ca). In conclusion: The Ca²⁺-activated K⁺ channel of the human red cell membrane is an inward rectifier showing the characteristic voltage dependence of this type of channel.