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This article discusses the logic underlying the use of invertebrate model systems for investigating the neurobiological basis of learning and memory, the kinds of questions which can be asked of these systems as well as their limitations. A model system selected to answer specific questions about learning and memory is most useful if its selection is based on strategy rather than chance.

A one-session T-maze training procedure for cockroaches in which animals were trained to turn right or left to avoid a shock was utilized to investigate the effects of dieldrin and chlordimeform (two neurotoxic pesticides) on learning and memory. These animals were trained and then tested for retention 5 h later. Three behavioral measures were recorded: choice behavior or direction turned, the time taken to proceed down the runway (runway time), and the time taken to proceed from the runway to either choice arm (choice point time). In control animals the number of correct choices, the runway time, and the choice point time increased with succeeding trials during training. Furthermore, control animals showed retention of correct choice behavior from training to testing. A nontoxic dose (no overt behavioral signs) of dieldrin was injected 2 h before training or 15 min after training. Pretraining injections of dieldrin eliminated correct-choice learning but did not alter the increase with training in runway or choice point times. Posttraining dieldrin administration did not interfere with retention of correct-choice behavior upon testing. These findings indicate that dieldrin induces behavioral alterations in a low acute dose and that detrimental effects are more likely with more complex behavior. Furthermore, behavioral disruption is more likely the closer exposure is to the initial learning of a task. Nontoxic doses of chlordimeform injected 1 h before training eliminated correct-choice learning and facilitated an increase in runway times during training.

The interest in antibiotics as amnestic agents stemmed from their effects on protein synthesis. Initially they were viewed as an avenue for exploring various hypotheses concerning the role of protein synthesis in memory formation. Three classes of protein synthesis inhibitors have been widely used in memory-disruption experiments: puromycin (Puro), the glutarimides such as cycloheximide (CXM) and acetoxycycloheximide, and, more recently, anisomycin (ANI). Each of these agents has been found effective in producing experimental amnesia within dose ranges that are not significantly toxic to the animal. Although a factor common to each is the ability to block protein synthesis, the biochemical mechanisms for protein synthesis inhibition differ for these drugs in a consequential manner. Each of these agents may have other biochemical effects as well. In fact, it appears that protein synthesis inhibition is not a crucial factor underlying their behavioral effects. For example, data from experiments with CXM and ANI suggest that protein synthesis may not be essential for the storage of longer-term memory in mammals, birds, or insects. Even when retention deficits are produced by protein synthesis inhibition, the effects appear to result from deficits in retrieval or access processes rather than in the storage of memory. Puro, on the other hand, appears to exert its effects on memory in a unique manner. Of all the antibiotics, it alone is effective in producing amnesia when injections are delayed until 24 h after training.

A new one-session T-maze training procedure for cockroaches, in which animals were trained to turn right or left to avoid shock, is described. This paradigm was utilized to investigate effects of protein synthesis inhibiting drugs on learning and retention. Cycloheximide (CXM), which inhibited protein synthesis by over 90% during the training period, did not impair acquisition and did not produce retention deficits an any interval up to 1 day after training. Puromycin (PURO), which inhibited protein synthesis by about 70% during the training period, produced amnesia 5 hr after training, while acquisition was not affected. Thus invertebrates, as well as vertebrates, are susceptible to amnesic effects of puromycin. Although PURO-injected animals showed retention deficits as measured by the number of correct turns, no retention deficit occurred for the behavioral modification consisting of an increase in runway time during the training period. Therefore, PURO appears to show specificity for the different types of longer-term memories that are formed in a training situation.

The muscarinic cholinergic blocker, scopolamine, has no effect on acquisition or retention of the choice behavior of the cockroach, P. americana trained to turn right or left to avoid shock in a T-maze. Scopolamine does, however, prevent runway habituation during training, suggesting that such habituation may be dependent on central cholinergic synapses as there are no known peripheral cholinergic receptors in cockroaches. Previous work demonstrating that puromycin interferes with correct choice retention five hours after training taken together with our results demonstrating no effect of scopolamine on such retention, suggests that puromycin does not produce its amnesic effect on choice behavior by interferring with central cholinergic synaptic processes.

There are probably few terms in evolutionary studies regarding neuroscience issues that are used more frequently than ‘behavior', ‘learning', ‘memory', and ‘mind'. Yet there are probably as many different meanings of these terms as there are users of them. Further, investigators in such studies, while recognizing the full phylogenetic spectrum of life and the evolution of these phenomena, rarely go beyond mammals and other vertebrates in their investigations; invertebrates are sometimes included. What is rarely taken into consideration, though, is that to fully understand the evolution and significance for survival of these phenomena across phylogeny, it is essential that they be measured and compared in the same units of measurement across the full phylogenetic spectrum from aneural bacteria and protozoa to humans. This paper explores how these terms are generally used as well as how they might be operationally defined and measured to facilitate uniform examination and comparisons across the full phylogenetic spectrum of life. This paper has 2 goals: (1) to provide models for measuring the evolution of ‘behavior'and its changes across the full phylogenetic spectrum, and (2) to explain why ‘mind phenomena'cannot be measured scientifically at the present time. [ABSTRACT FROM PUBLISHER]