The active time model of concurrent choice.

The following paper describes a steady-state model of concurrent choice, termed the active time model (ATM). ATM is derived from maximization principles and is characterized by a semi-Markov process. The model proposes that the controlling stimulus in concurrent variable-interval (VI) VI schedules of reinforcement is the time interval since the most recent response, termed here "the active interresponse time" or simply "active time." In the model after a response is generated, it is categorized by a function that relates active times to switch/stay probabilities. In the paper the output of ATM is compared with predictions made by three other models of operant conditioning: melioration, a version of scalar expectancy theory (SET), and momentary maximization. Data sets considered include preferences in multiple-concurrent VI VI schedules, molecular choice patterns, correlations between switching and perseveration, and molar choice proportions. It is shown that ATM can account for all of these data sets, while the other models produce more limited fits. However, rather than argue that ATM is the singular model for concurrent VI VI choice, a consideration of its concept space leads to the conclusion that operant choice is multiply-determined, and that an adaptive viewpoint-one that considers experimental procedures both as selecting mechanisms for animal choice as well as tests of the controlling variables of that choice-is warranted.

Proprioceptive stimuli and habit formation: Interresponse time mediated behavior in CD-1 mice.

The consolidation of behavioral sequences into relatively ballistic habits is thought to involve the formation of stimulus - response associations. Typically, the stimuli in these associations are assumed to be exteroceptive, i.e., external to the organism. However, responses, themselves, also possess stimulus properties that can mediate behavior. Indeed, these "proprioceptive cues" have long been hypothesized to underlie habit formation (Hull, 1934a, 1934b). One such stimulus involves the time durations between responses - a stimulus termed interresponse time (IRT). We hypothesize that IRTs can come to serve as stimuli that differentially control response elements during habit formation. To examine this hypothesis we report on two experiments that asked whether CD-1 mice utilize IRTs to structure behavior in a two-choice environment. In experiment 1, eight mice were exposed to a free-operant concurrent variable-interval (VI) 30-s VI 60-s reinforcement schedule. We found that switch and stay responses were differentially correlated with IRT durations. In Experiment 2 we directly and differentially reinforced stay/switch responses based on IRT durations in a two-lever procedure. For four of the subjects, the probability of receiving reinforcement after switch responses was proportional to IRT duration. For five of the subjects, these reinforcement probabilities were inversely proportional to IRT duration. Regardless, all of our subjects learned to emit IRT-mediated switching behavior that matched the reinforcement contingencies. Together, Experiments 1 and 2 provide the first evidence of which we are aware that IRTs can come to control sequential choice behavior in mice.

Preference as a function of active interresponse times: a test of the active time model.

In this article, we describe a test of the active time model for concurrent variable interval (VI) choice. The active time model (ATM) suggests that the time since the most recent response is one of the variables controlling choice in concurrent VI VI schedules of reinforcement. In our experiment, pigeons were trained in a multiple concurrent similar to that employed by Belke (1992), with VI 20-s and VI 40-s schedules in one component, and VI 40-s and VI 80-s schedules in the other component. However, rather than use a free-operant design, we used a discrete-trial procedure that restricted interresponse times to a range of 0.5-9.0 s. After 45 sessions of training, unreinforced probe periods were mixed with reinforced training periods. These probes paired the two stimuli associated with the VI 40-s schedules. Further, the probes were defined such that during their occurrence, interresponse times were either "short" (0.5-3.0 s) or "long" (7.5-9.0 s). All pigeons showed a preference for the stimulus associated with the relatively rich VI 40-s schedule--a result mirroring that of Belke. We also observed, though, that this preference was more extreme during long probes than during short probes--a result predicted by ATM.

An effect of inter-trial duration on the gambler's fallacy choice bias.

The gambler's fallacy is defined as the avoidance of a winning outcome in a stochastic environment with a constant probability. We tested the possibility that the gambler's fallacy in humans is responsive to the amount of time between choice allocations. Two groups of subjects were placed in a six-choice betting game in which the choices were clustered into two "patches." Groups were defined by the length of time - 2s or 6s - between trials. On any given trial subjects allocated six points among the alternatives, and retained any points that were bet on the winning alternative. Both groups showed evidence of the gambler's fallacy bias. However, the bias was stronger in the 6-s ITI group than in the 2-s ITI group. This difference was found primarily to be due to differences in the number of subjects showing an opposing bias to the gambler's fallacy, namely a preference for the most recent winning alternative. This choice bias is termed the hot hand fallacy. Our findings contradict predictions derived from a foraging heuristic and from traditional accounts of the gambler's fallacy.

A further application of the active time model to multiple concurrent variable-interval schedules.

In this experiment we show that the active time model (ATM) accurately predicts probe data from multiple concurrent VI VI schedules. Subjects were trained under a concurrent VI 30-s VI 60-s and a concurrent VI 60-s VI 120-s schedule. Two types of unreinforced probes were then conducted. The first paired the two VI 60-s stimuli. These stimuli, while equivalent in their associated absolute rates of reinforcement, differed in their relative rates of reinforcement. The second probe paired the VI 30-s stimulus with the relatively rich VI 60-s stimulus. In contrast with the first probe, these stimuli differed in their absolute rates of reinforcement, while being similar in their relative rates. During the first set of probes, birds preferred the VI 60-s stimulus trained with the VI 120-s schedule. During the second set of probes, birds were indifferent to the two stimuli. These results are less extreme than others reported in the literature. Nonetheless, we found that ATM accurately fit individual subject data in both sets of probes. In contrast a variant of scalar expectancy theory did not fit the data at either the individual or group level.

An application of the active time model to multiple concurrent variable-interval schedules.

The current experiment investigates whether an active time model can account for anomalous results that have emerged from multiple schedule, concurrent variable-interval (VI) VI experiments. The model assumes that (1) during concurrent VI VI training pigeons learn a function that relates time since the most immediate response, i.e., active time, to changeover probabilities and (2) that molar preference is the result of an interaction between inter-response time frequencies and the learned active time changeover functions. Pigeons were trained under a concurrent VI 30-s VI 30-s schedule and a concurrent VI 60-s VI 60-s schedule. Probes were conducted in which VI 30-s and VI 60-s stimuli were paired. During these probes, birds allocated choices equally to the stimuli. The active time model accurately fit individual subject data. In contrast data were not fit by a variant of scalar expectancy theory proposed by Gibbon [Gibbon, J., 1995. Dynamics of time matching: arousal makes better seem worse. Psychon. Bull. Rev. 2, 208-215].

Ontogeny has a phylogeny: background to adjunctive behaviors in pigeons and budgerigars.

Animals coping with operant conditioning tasks often show behaviors that are not recorded by keys, levers and similar response transducers. Nevertheless, these adjunctive behaviors should not be disposed of by classifying them as incidental. Often they are found to be at least partially influenced by the experimentally programmed contingencies, and under certain conditions they can in turn influence conditioned behaviors. Here we describe the occurrence and characteristics of two such behaviors, stimulus grasping in operantly key-pecking pigeons and intra-delay stereotypies in a delayed matching-to-sample task with budgerigars. It is argued that for a proper account of these behaviors it is necessary to refer to a behavioral systems approach that appeals to longer ranging ontogenetic and phylogenetic histories than is usually considered in the psychological literature. The gaping towards on-key stimuli by pigeons is attributed to the hypothesis that operantly conditioned key-pecks probably relate to a grasp-pecking response that is normally executed towards non-edible items covering food. The intra-delay behaviors shown by the budgerigars are assumed to have originated from stress-induced displacement responses that adventitiously came under the influence of differential reinforcement contingencies. Finally, we discuss what kinds of evidence are needed to put these hypothetical explanations on a more certain footing.

A D-amino acid peptide inhibitor of NF-kappa B nuclear localization is efficacious in models of inflammatory disease.

The transcription factor NF-kappa B regulates many genes involved in proinflammatory and immune responses. The transport of NF-kappa B into the nucleus is essential for its biologic activity. We describe a novel, potent, and selective NF-kappa B inhibitor composed of a cell-permeable peptide carrying two nuclear localization sequences (NLS). This peptide blocks NF-kappa B nuclear localization, resulting in inhibition of cell surface protein expression, cytokine production, and T cell proliferation. The peptide is efficacious in vivo in a mouse septic shock model as well as a mouse model of inflammatory bowel disease, demonstrating that NF-kappa B nuclear import plays a role in these acute inflammatory disease models.

Interresponse-time sensitivity during discrete-trial and free-operant concurrent variable-interval schedules.

Two experiments investigated the sensitivity of pigeons' choice to elapsed time since the last response (i.e., to inter-response time [IRT]) during concurrent variable-interval variable-interval schedules. Experiment 1 used a two-key discrete-trial procedure with variable intertrial intervals. Experiment 2 employed a three-key free-operant procedure. In both experiments choice was found to be a function of the active-schedule IRT, defined as the time since the most recent response. Monte Carlo simulations show how this finding permits the joining of several seemingly incompatible data sets held to both support and contradict a kind of choice strategy, termed momentary maximizing, which attempts to maximize momentary reinforcement probabilities. The studies suggest that only two variables are needed to describe the static molecular structure of concurrent variable-interval choice: active-schedule IRTs and "response states" consisting of the last one or two schedule choices.

Identification of a binding site on Hsc70 for the immunosuppressant 15-deoxyspergualin.

Hsc70, the constitutive form of the heat shock protein 70 family of proteins, is involved in a number of biological activities which include protein folding and molecular chaperoning. Previously, we had shown that the immunosuppressant 15-deoxyspergualin (DSG) specifically interacted with Hsc70, as well as the Hsp90 family of proteins. Although the exact binding site on Hsc70 for protein substrates is unknown, a recent study shows that the extreme C-terminal four amino acids 647EEVD650 play a role in regulating AT-Pase activity, substrate binding, and interaction with HDJ-1. These four amino acids are also found at the C-terminus of Hsp90 and may be involved in similar functions. In this study, we show that DSG binds specifically to this EEVD regulatory domain. Binding of DSG to Hsc70 did not affect its ability to bind peptides. These results suggest that in addition to the ATP binding domain, there are two additional substrate binding domains on Hsc70. DSG should provide a tool for understanding the role of the EEVD motif in biological processes.

Differential expression and sequence-specific interaction of karyopherin alpha with nuclear localization sequences.

The process of nuclear protein transport requires the interaction of several different proteins, either directly or indirectly with nuclear localization or targeting sequences (NLS). Recently, a number of karyopherins alpha, or NLS-binding proteins, have been identified. We have found that the karyopherins hSRP1 and hSRP1alpha are differentially expressed in various leukocyte cell lines and could be induced in normal human peripheral blood lymphocytes. We show that the two karyopherins bind with varied specificities in a sequence specific manner to different NLSs and that the sequence specificity is modulated by other cytosolic proteins. There was a correlation between binding of karyopherins alpha to different NLSs and their ability to be imported into the nucleus. Taken together, these data provide evidence for multiple levels of control of the nuclear import process.

Elucidating the mechanism of action of the immunosuppressant 15-deoxyspergualin.

15-Deoxyspergualin (DSG) is an immunosuppressive agent currently in Phase I/II clinical trials. We have previously shown that DSG specifically interacts with Hsc70, a member of the heat shock protein 70 family. In this article we show that DSG appears to bind either kinetically or to a different site on Hsc70 from that of peptides. In addition, we show that DSG inhibits the localization of Hsp70 into the nucleus in response to heat shock. Finally, data are presented showing that there is a correlation between decreases in the transcription factor nuclear factor kappa B and kappa light chain expression in response to varying doses of DSG.

Cytotoxic T lymphocyte-associated molecule-4, a high-avidity receptor for CD80 and CD86, contains an intracellular localization motif in its cytoplasmic tail.

CD28 and CTLA-4, T cell receptors for B7-1 (CD80) and B7-2 (CD86) molecules on antigen-presenting cells, transmit costimulatory signals important for optimal T cell activation. Despite sharing sequence homology and common ligands, these receptors have distinct binding properties and patterns of expression. The function of CTLA-4 during T cell activation is not well understood, although an important role is suggested by complete amino acid sequence conservation of its cytoplasmic tail in all species studied to date. We report here a role of the cytoplasmic tail of CTLA-4 in regulating its subcellular localization and cell surface expression. In activated human peripheral blood T cells, or in several transfected or transduced cell types, CTLA-4 is not primarily a cell surface protein, but rather is localized intracellularly in a region which overlaps the Golgi apparatus. Transfer of 11 cytoplasmic residues, 161TTGVYVKMPPT, from the CTLA-4 cytoplasmic tail to the homologous position in CD28 was sufficient to confer intracellular localization. Mutation of the tyrosine residue (Tyr165) in this motif to phenylalanine resulted in increased surface expression of CTLA-4. Thus, the subcellular localization of CTLA-4 is controlled by a tyrosine-containing motif within its cytoplasmic domain. Contained within this motif is a binding site for SH2 domains of the p85 subunit of phosphatidylinositol 3-kinase.

Effect of interferon-gamma on antigen processing in human monocytes.

The effect of interferon-gamma (IFN-gamma) on the ability of human monocytic cells to process exogenous (major histocompability complex class II) antigens was investigated. The processing (i.e. protein degradation) of antigens that were internalized via Fc gamma receptor (Fc gamma R) was followed for various times after treatment of cells with IFN-gamma. THP-1 cells that had been treated with IFN-gamma for 4 h degraded antigen, internalized as an immune complex, at an enhanced rate. After 24 h of IFN-gamma treatment the rate of processing was similar to untreated cells. Unexpectedly, in cells which had been treated for 48-72 h there was a significant decrease in the rate of processing of the exogenous antigen. These effects were not due to changes in the rate of internalization of immune complex. The inhibition of the rate of processing was independent of the type of antigen, was dependent on the dose of IFN-gamma, and also occurred with normal human peripheral monocytes. Analysis of the degraded peptides by high-pressure liquid chromatography indicated that some of the peptides generated in the IFN-gamma-treated cells were both quantitatively and qualitatively different from the peptides generated in untreated cells. These data suggest that IFN-gamma modulates the way in which antigens, internalized through Fc receptors as immune complexes, are processed. Additionally, the results imply that decreases in the rate of antigen processing may lead to more efficient antigen presentation.

Transport of the immunosuppressant 15-deoxyspergualin in human peripheral blood lymphocytes.

15-deoxyspergualin (DSG) is a potent immunosuppressive compound currently in clinical trials. In this study, we have characterized the uptake and intracellular localization of DSG in human peripheral blood lymphocytes (PBL's). DSG is transported into human PBL's and reaches an estimated maximum concentration of approximately 500 microM in 6 hours. The majority of the [3H]-DSG remains in the cytoplasm of cells and that which is associated with the nucleus is only loosely associated. DSG was transported by HeLa cells, as well, suggesting uptake is not specific for hematopoietic cells. Positively charged amino acids and polyamines, which are structurally similar to DSG, were unable to compete for DSG transport suggesting that DSG is transported into cells via a pathway distinct from amino acids or polyamines.

Determination of immunoreactivity of doxorubicin antibody immunoconjugates by a Le(y) competitive RIA.

Many monoclonal antibody-drug immunoconjugates have been evaluated for their ability to deliver cytotoxic drugs to tumors. It is essential to establish that the ability of the conjugates to bind antigen, i.e. their immunoreactivity, is not adversely affected by the drug conjugation procedure. We have described herein a measurement of the immunoreactivity of BR96-DOX, a conjugate comprised of BR96, a chimeric monoclonal antibody specific for the Le(y) tetrasaccharide commonly expressed on human carcinomas, and doxorubicin, an anticancer agent in widespread clinical use. We have employed a competitive RIA, in which microtiter wells were coated with synthetic Le(y) conjugated to human serum albumin and then incubated with 125I-labeled antibody BR96 in the presence of test conjugate or intact BR96 mAb. The test conjugates were found to compete as effectively as unconjugated BR96. This assay is highly applicable to QC processes with the intra-assay CV = 2.0% and the interassay CV = 4.3%.

A microtiter-based assay for the detection of protein tyrosine kinase activity.

A 96-well microtiter enzyme-linked immunosorbent assay (ELISA) for protein tyrosine kinases has been developed. This assay uses one of several substrates that are phosphorylated by tyrosine kinase, an antibody to phosphotyrosine, and a peroxidase-linked second antibody. Color development is monitored by a change in absorbance at 450 nm and is dependent upon time, enzyme, ATP, and substrate concentrations. Specificity of the ELISA for phosphotyrosine was shown by inhibition of binding of the anti-phosphotyrosine antibody with phenyl phosphate. Results obtained in the ELISA compared favorably with those obtained by direct phosphorylation of the substrate with [32P]ATP. Staurosporine and K252a, protein kinase inhibitors, showed titratable inhibition of tyrosine kinase activity. This assay is a rapid, nonradioactive alternative to traditional methodology and is also amenable to automation.

Changes in nucleoside transport of HL-60 human promyelocytic cells during N,N-dimethylformamide induced differentiation.

The rate of nucleoside transport decreased profoundly in human promyelocytic leukemia HL-60 cells after myeloid differentiation was induced by 5-6 days of exposure to 0.8% N,N-dimethylformamide (DMF). The facilitated diffusion of 100 microM radiolabeled adenosine and 2'-deoxyadenosine, measured by rapid transport assays, decreased 10- to 20-fold. The transport of 2 microM coformycin or 2'-deoxycoformycin, which is mediated by the same mechanism and was monitored by the adenosine deaminase titration assay, decreased 29-fold. The reduction in nucleoside transport capacity after DMF treatment was confirmed by a 19-fold decrease in the number of specific binding sites per cell (from 24-30 X 10(4) to 1.2-1.7 X 10(4)) for [3H]-6-p-nitrobenzylthioinosine, a nucleoside transport inhibitor. The binding affinity of 6-p-nitrobenzylthioinosine was not altered significantly and nucleoside transport remained sensitive to the transport inhibitors, 6-p-nitrobenzylthioinosine, dipyridamole, and dilazep after DMF-induced maturation. Time-dependence studies showed that the rate of 100 microM deoxyadenosine transport was unchanged for the first 24 h of exposure to DMF but fell to about 36% of control rates at 24-26 h and then gradually decreased further to about 4-5% of control rates after 5-6 days. In contrast, transport rates of the purine bases were reduced only 2- to 3-fold in HL-60 cells after 5 days of DMF treatment. The rates of adenosine and deoxyadenosine transport were unchanged or reduced by no more than 2-fold after 5-6 days of exposure to 0.8% DMF in the following human tumor cell lines that are not inducible with DMF: ARH-77 (multiple myeloma), KG-1 (acute myelogenous), and K-562 (chronic myelogenous). Thus, changes in nucleoside transport may serve as an early, membrane-associated marker of differentiation of the HL-60 cell line.