A multiple species approach to sequential learning: are you a man or a mouse?

We have developed a method for studying list learning in animals and humans, and we use variants of the task to examine list learning in rats, mice, and humans. This method holds several advantages over other methods. It has been found to be easily learned without lengthy pretraining. The data gathered with this procedure provide a measure of correct response rates, of incorrect responses and the locations of these responses, and of response latency on a trial-by-trial basis. We have examined mouse, rat, and human list acquisition of patterns ranging from 12 to 48 items in length. This procedure has also been used to examine many aspects of list learning, such as the effects of the placement of phrasing cues that are either consistent or inconsistent with the structure of the list in rats and mice, the effects of phrasing cues of differing modalities in mice, the sensitivity of subjects to violations of list structure in rats, subjects' abilities to "chunk" from nonadjacent serial positions in structured lists in rats, and subjects' sensitivity to serial patterns with multiple levels of hierarchical organization. The procedure has also been used to examine the effects of drugs on sequential learning.

Suppression of hippocampal slice excitability by 2-, 3-, and 4-methylpyridine.

The present study assessed the effects of 2-, 3-, and 4-methylpyridine on rat hippocampal slice excitability. Tests of excitatory and inhibitory systems in area CA1 of the hippocampal slice were conducted over a period of 3 h postexposure. Following exposures of 100 microM 2-, 3-, or 4-methylpyridine, evoked population excitatory postsynaptic potential and population spike responses recorded in the cell body field of hippocampal area CA1 were slowly suppressed over the course of 3 h, whereas no effects on local inhibitory processes or latency of evoked responses were detected. No significant differences were observed between agents.

Effects of sequential exposure to multiple concentrations of methylmercury in the rat hippocampal slice.

Two experiments explored the effects of sequential exposure to multiple concentrations of methylmercury (MeHg) on rat hippocampal slice synaptic transmission and excitability in area CA1. When hippocampal slices were exposed to 0.1, 1, 10, and 100 microM MeHg chloride in successive 30-min exposures, MeHg produced an increase in excitability over baseline levels throughout the 1 microM exposure and the first 5 min of the 10 microM exposure, followed by profound suppression of excitability at the 100 microM level. When hippocampal slices were exposed to 10, 25, 50, 75, and 100 microM concentrations, MeHg produced an increase in excitability throughout most of the 10 and 25 microM exposures, followed by profound suppression of excitability at the 50 microM level of exposure. In both series of concentrations, MeHg suppressed local inhibitory systems prior to suppressing excitatory systems. In a third experiment, a single exposure of 50 microM MeHg suppressed both presynaptic and postsynaptic responses recorded in stratum radiatum with the same time course, suggesting that the observed suppressive effects of MeHg were not primarily synaptic.

Coding of hierarchical versus linear pattern structure in rats and humans.

In 3 experiments, rats and humans learned serial patterns composed of 24, 30, or 36 items. Patterns had a 2-, 3-, or 4-level hierarchical rule structure. In Experiments 1 and 2, patterns had either perfect hierarchical structure or 2 modified chunks that violated hierarchical structure, thus producing linear structure (i.e., nonhierarchical structure). For both rats and humans, pattern structure predicted pattern learning difficulty and also the nature and relative frequency of errors. Both treated chunks that were inconsistent with hierarchical structure as violation chunks, that is, they made errors that reflected their "tendency to regularize the perception of an irregular pattern" (F. Restle & B. L. Burnside, 1972). The results support the view that rats can abstract and encode a representation of multilevel hierarchical structure in serial patterns in much the same way as humans do in analogous tasks.

Sensitivity to violations of "run" and "trill" structures in rat serial-pattern learning.

Rats learned serial patterns composed of either "run" chunks (e.g., 123 234 ...) or "trill" chunks (e.g., 121 232 ...). For each type of pattern, 1 group of rats encountered an element at the end of the pattern that violated the run or trill structure. In both run and trill patterns, violations were unusually difficult for rats to learn, whereas corresponding elements in "perfect" patterns that did not violate pattern structure were easy. Additionally, rats' errors on violation elements conformed to the structure of the patterns in which they were embedded. Thus, rats were sensitive to the run or trill organization of their patterns and mastered the rules governing the pattern before learning "exceptions to the rule."

The in vitro hippocampal slice preparation as a screen for neurotoxicity.

One of the current goals of neurotoxicology research is to develop methods of assessing the neurotoxicity of chemical agents in the most sensitive, rapid and economical ways possible. Although no single method is likely to fulfil the role of a general screen for toxicity of all organ systems, in vitro brain-slice methods may hold the key to increased sensitivity in screening within the more restricted domain of central nervous system toxicity. The hippocampal brain-slice preparation is particularly well suited for screening purposes because the neurophysiology of the hippocampal slice is relatively well understood and generally matches what is known about the intact hippocampus. Potential practical advantages of a hippocampal slice screen include the fact that multiple tests of a variety of neuronal properties can be carried out using a single slice, many comparable slice 'samples' may be obtained from each animal donor, and testing should be easily automated. In addition, the hippocampal slice assay may have important practical and conceptual advantages over other assay methods because of the putative importance of the hippocampus to a variety of behaviours. Thus the in vitro hippocampal slice preparation as a screen for neurotoxicity offers the advantages of in vitro methods while allowing guarded, but relatively direct extrapolation to dysfunction of learning, memory and other behavioural processes.

Extracellular Ca(2+) modulation of triethyltin neurotoxicity in area CA1 of the rat hippocampal slice.

The present study examined the possible role of extracellular Ca(2+) in triethyltin (TET) neurotoxicity in area CA1 of the rat hippocampal slice. Slices were exposed to 20 mum-TET-Br for 30 min in an environment of normal or no extracellular Ca(2+), and were then monitored for 4 hr post-exposure. In a normal Ca(2+) environment, TET exposure suppressed population excitatory post-synaptic potentials (EPSPs) by 95 min post-exposure, and no recovery was observed following washout of TET. This direct effect of TET on neurotransmission occurred without changes in the conductive properties of the presynaptic Schaffer collaterals; the amplitude of population afferent fibre volley potentials remained stable. In a Ca(2+)-free environment, however, the same TET exposure led to rapid tissue death following the onset of TET exposure. Thus, extracellular Ca(2+) decreased the apparent neurotoxicity of TET observed in a Ca(2+)-free environment. The results suggest that extracellular Ca(2+) modulates TET neurotoxicity, manifested as suppression of synaptic potentials, in area CA1 of the hippocampal slice. However, the mechanisms underlying TET-induced suppression of evoked synaptic potentials in a normal Ca(2+) environment and tissue death in a Ca(2+)-free environment remain to be elucidated.

Carbon monoxide exposure reduces the rewarding quality of brain-stimulation reward in rats.

The effect of carbon monoxide (CO) exposure on hypothalamic brain-stimulation reward (BSR) was examined. Rats were trained in a procedure that daily determined their stimulus duration threshold (SDT), that is, the shortest electrical stimulus to the posterior lateral hypothalamus that would support discrete-trial leverpress responding for BSR. After a stable SDT baseline was established using a single response lever, rats were exposed to 0, 5, 10, 20, and 40 ml/kg pure CO by IP injection. The SDT was significantly elevated by the 40 ml/kg exposure (corresponding to approximately 65% carboxyhemoglobin in the blood) compared to control exposures of an equal volume. No change was observed in response rate at any dose in this 1-lever task. No tolerance was observed when 40 ml/kg CO exposure was repeated on alternating days for 14 exposures, but a small reduction in response rate was observed in this procedure. When rats of a second group were required to alternate responses on two levers some distance apart, SDT was elevated by the highest exposure (40 ml/kg) as before. Additionally, response rate was also significantly suppressed by the highest exposure in this 2-lever task. The results support the view that CO has a direct effect on brain reward systems assessed by the SDT task. Response rate changes due to CO exposure may be due to both direct effects on brain reward systems and other effects such as hypoxia-induced fatigue.

Threshold procedures for assessing the impact of agents on brain reward systems.

Chemical effects on the reinforcing quality of electrical stimulation of the rat brain can be assessed using a variety of methods, most commonly by observing changes in response rates maintained under specific schedules of reinforcement. We present results demonstrating the utility of procedures for assessing the minimum amount of electrical stimulation required to support rat leverpress responding, that is, the brain-stimulation reward (BSR) threshold. In these threshold procedures, each leverpress produced by the rat decreases the duration of the electrical stimulus delivered to the posterior lateral hypothalamus until the rat fails to respond. The stimulus duration is then reset to its initial value and the procedure begins again. The last stimulus duration in a series supporting a response is defined as the stimulus duration (SD) threshold, and the mean SD threshold is determined daily. Stable SD thresholds are achieved within 2 weeks, and this measure is sensitive to agent-induced changes in rats' response to BSR. To illustrate the utility of this approach, data are presented showing that rats' BSR thresholds changed significantly following exposure to triethyltin or carbon monoxide. The results support the view that threshold methods can be used to dissociate agent-induced effects on brain reward systems and BSR quality from changes in performance or effects on other behavioral processes.

Rule abstraction, item memory, and chunking in rat serial-pattern tracking.

Two studies used a stimulus tracking paradigm to test whether rats are sensitive to the rule-based formal properties of structured serial patterns. Hooded rats tracked 16-element patterns of flashing lights by pressing levers under an array of 6 indicator lights. In Experiment 1, rats tracked a pattern similar to one previously used with human subjects and yielded remarkably similar results. More errors and response omissions occurred at boundaries of structural "chunks" than within chunks, and errors often reflected anticipation of the next chunk or extrapolation of the preceding chunk. In Experiment 2, temporal "phrasing" cues encouraged different groups to encode a pattern as a series of either "runs" or "trills." Differential placement of pauses induced rats to encode different rule-based representations of the pattern. Results indicate that under appropriate conditions rats may encode a representation of formal structure when they learn organized response patterns.

Triethyltin exposure suppresses synaptic transmission in area CA1 of the rat hippocampal slice.

To examine the effects of TET on the electrophysiology of area CA1 of hippocampus, hippocampal slices were obtained from adult hooded rats and were maintained in vitro using standard techniques. Stimulating and recording electrodes were placed in the Schaffer collaterals and CA1 pyramidal cell body layer, respectively. Following baseline measurements, slices were exposed to either 0, 1, 3, 6, or 10 microM TET in the incubating medium. Both pyramidal cell excitability and recurrent/feedforward inhibition were suppressed in a dose-dependent manner within 3 hr postexposure. The evoked population spike and population excitatory postsynaptic potential (EPSP) were suppressed significantly by 2 hr postexposure for 1 and 3 microM TET exposures, and by 45 min postexposure for 6 and 10 microM exposures. A similar dose-dependency was observed for the suppression of recurrent/feedforward inhibition in hippocampal CA1. A second procedure tested the specificity of TET effects to axonal conduction of Schaffer collaterals. Both the stimulating and recording electrode were placed in the Schaffer collaterals so that both the Schaffer collateral population fiber volley and the CA1 pyramidal cell population EPSP could be recorded. TET exposure suppressed pyramidal cell EPSPs without significantly affecting the amplitude of Schaffer collateral fiber volleys. The results support the view that acute TET exposure suppresses synaptic transmission in area CA1 of hippocampus.

Aspartame exposure and in vitro hippocampal slice excitability and plasticity.

Aspartame (APM) is a low-calorie sweetener recently approved and released for widespread use in the United States. However, concerns still exist that APM consumption may be responsible for adverse neurological and psychological effects in some people. In addition, recent reports indicate that APM exposure may alter regional brain neurotransmitter levels. The present study assessed the effects of APM and its amino acid moieties on rat hippocampal slice excitability and plasticity. Specifically, tests of excitatory systems, inhibitory systems, and synaptic plasticity (induction of long-term potentiation--LTP) were administered postexposure. Exposures of 0.01, 0.1, 1, and 10 mM APM potentiated the response of hippocampal CA1 pyramidal cells, but had no apparent effect on local inhibitory systems. APM exposure did not block the establishment of LTP at any dose despite the potentiation of pyramidal cell response observed postexposure. In addition, 0.1 mM phenylalanine (PHE) produced a greater increase in excitability than that produced by an equivalent dose of APM, 0.1 mM aspartic acid (ASP) and 0.1 mM phenylalanine methyl ester (PM) produced effects comparable to those produced a smaller, but reliable, change in hippocampal CA1 excitability relative to baseline. Like APM, none of the amino acids produced detectable changes in inhibitory systems or neuronal plasticity.

Neuronal plasticity in the mammalian brain: relevance to behavioral learning and memory.

Much recent activity in the neurosciences relates to the search for the brain mechanisms underlying learning and memory. In recent years a brain circuit in cerebellum and brainstem has been discovered that is responsible for the learning of a simple motor response (nictitating membrane movement). This has provided a model for neuroscientists to use in understanding the brain circuits involved in this simple form of learning and, by extension, to more complex forms ultimately, and a means of exploring the changes in neural function underlying the learning. An enduring change in neural function is represented by long-term potentiation (LTP), an alteration in synaptic efficacy seen in hippocampus and other areas. LTP can be induced experimentally and occurs as a concomitant of learning. We review data suggesting that different brain circuits may underlie different forms of learning and memory. Several current theories of learning and memory with respect to hippocampal and other brain circuit involvement are considered. We conclude with the behavioral and physiological effects of exposure to teratogens or toxins and the CNS alterations associated with dementia.

Behavioral consequences of intraperitoneal carbon monoxide administration in rats.

Blood carboxyhemoglobin (HbCO) was determined 15, 30, 45, 60, 90, and 120 min following ip injection of 2.5, 5, 10, 20, and 40 ml pure carbon monoxide (CO)/kg body wt in rats. These CO doses produced HbCO concentrations of 12, 24, 35, 45, and 60%, respectively, at 30 min postinjection. Once these normative data were obtained, a group of eight naive rats were trained to produce a rapid sequence of responses in a stimulus-tracking task, then they were exposed to each of the doses of CO. The 10-ml/kg dose produced a slight decrement in performance, the 20-ml/kg dose reduced correct responses by nearly half, and the 40-ml/kg dose resulted in virtually complete cessation of responding. CO exposure resulted in longer pauses in responding with increasing dose, but the distribution of errors produced in the stimulus-tracking task remained relatively parallel across the range of CO exposures. Thus CO exposure impaired tracking performance but had relatively little effect on the pattern of errors rats produced during the stimulus-tracking test. Taken together these results contradict previous reports purporting to show that CO by ip administration has no behavioral effects; instead, the results indicate that CO administration via the ip route has very similar effects to inhaled CO on behavior.