In this supplementary section, we briefly discuss evidence for the role of the neurotransmitter acetylcholine (ACh) in learning and memory. Some of this evidence involves selective breeding of rats for ability to learn mazes. Rats were tested for their ability to learn mazes. Good maze learners were bred with other good maze learners and poor maze learners were bred with other poor maze learners, generation after generation. After a number of generations of this type of selective breeding, two separate strains of rats were established, one consisting of rats that were good maze learners and a second strain of poor maze learners. Researchers found more whole brain concentrations of ACh, and more ACh metabolism in the cerebral cortex, in rats selectively bred to be good maze learners compared to the rats bred to be poor maze learners. These relationships were interpreted as evidence of an involvement of ACh in learning and memory. Another line of research involved "enriched environments" and their effects on the brain in a variety of species. Small lab animals such as rats and mice are often kept in small wire cages, two animals per cage. In studies using "enriched environments" animals may be kept in large group cages housing together many animals with "toys" such as running wheels, blocks, and other novel or interesting objects. These "enriched environments" not only provide the animals with many things to do, to experience, and to learn about, but these group environments allow much more social interaction than can occur in small cages which house only two animals together. Sometimes, animals are raised in "impoverished environments" in which animals kept by themselves, one animal per cage. Animals raised in "enriched environments" compared to those raised in "impoverished environments" developed thicker and heavier cerebral cortex, greater dendritic branching, more dendritic spines, and other anatomical changes, as well as changes in brain chemistry. Recent research clarifies the mechanisms involved in the effects of ACh on learning and memory. ACh increases the excitability of dendrites of post-synaptic neurons in the CA3 region of the hippocampus involved in memory.
Differences in brain ACh metabolism induced by selective breeding for learning ability
The amnesia and dementia associated with Alzheimer's Disease involve damage to the frontal lobes, the medial temporal lobes, and involve depletion of acetylcholine (ACh) neurotransmitter due to damage in the basal forebrain, located just above the hypothalamus. The basal forebrain is the main source of ACh in the brain (Pinel and Barnes, 2021). Research in animals had long suggested that ACh might be involved in memory.
One way to test for an involvement of ACh in memory is to use selective breeding in animals. If differences in learning ability could be associated with specific differences in brain function, such as ACh activity, these differences might give clues about the physical bases of learning and memory and might contribute to our understanding of some forms of amnesia and dementia.
Rosenzweig and colleagues found chemical differences in the brains of rats selectively bred for maze learning ability (Rosenzweig, 2007). Early experiments focused on the neurotransmitter acetylcholine (ACh). Activity of acetylcholinesterase (AChE), an enzyme which inactivates ACh following its stimulation of cholinergic receptors (post-synaptic receptor sites that receive ACh), was used an indicator of acetylcholine (ACh) metabolism. Comparisons between AChE activity in cerebral cortex in maze-bright and maze-dull rats (rats selectively bred over multiple generations either for superior or inferior maze learning) showed higher cortical AChE activity indicating higher ACh metabolism in the maze-bright strain compared to the maze-dull strain, and within each strain there were significant correlations between behavioral performance and AChE activity in cerebral cortex. In addition, whole brain ACh concentrations were higher in the maze-bright strain than in the maze-dull, suggesting a possible role of ACh synapses in learning and memory. In addition, Rosenzweig and his research group found that AChE activity increases with age in rats up to about 100 days and then declines.
Anatomical and biochemical changes in the brain induced by enriched experience
Additional experiments by Rozensweig and his research group tested the effects of experience on the brain. Rats raised from an early age in an enriched environment, with stimulus objects in a group cage, not only showed differences in AChE activity and increased RNA and protein, but also showed increased weight of cortex and about a 5% increase in thickness of cerebral cortex compared to rats raised by themselves in individual cages in an impoverished environment. This unexpected finding was early evidence that experience (resulting in learning and memory) can cause anatomical changes in the brain, as well as changes in brain chemistry.
Similar results were found even when older rats were exposed to an enriched environment for as little as 30 days, and several additional studies showed that exposure to enriched environments for as little as 4 days was sufficient to induce cortical weight changes and increased dendritic branching. Later studies in occipital cortex found a 14% increase in glial cells, increased numbers of pyramidal cell bodies, and increased sizes of synaptic junctions, all induced by enriched experience, suggesting possible effects of learning and memory in the brain. These studies also showed increased numbers of dendritic spines in the rats exposed to enriched experience compared to rats in the impoverished environmental condition. As you have already learned, these small spiny structures on dendrites, associated with synapses, undergo transient structural change followed by sustained change in spine volume lasting about 30 minutes or more after stimulation. Renner and Rozensweig (1987) reported that cerebral effects of experience occur in a wide range of species tested, including rats, mice, squirrels, cats, monkeys, fish, and birds. Rosenzweig (2007) reports that Kozorovitskiy, et al. (2005) found that exposure of adult marmosets to an enriched environment in group cages, for just 30 days, "resulted in increases in dendritic spine density, dendritic length, and dendritic complexity of neurons in the hippocampus and the prefrontal cortex, and it raised the expression levels of several synaptic proteins in the same regions."
Acetylcholine Increases Dendritic Excitability in Hippocampus
More recent research is helping to clarify the role of acetylcholine in learning and memory. Humphries, et al. (2022) reported that by inhibiting potassium (K+) ion channels, acetylcholine enhances the excitability of dendrites in the CA3 regions of the hippocampus reducing the number of NMDA receptors that needed to be excited in order to trigger spike activity at NMDA synapses, known to be involved in long-term potentiation (LTP) and long-term memory (see Section 10.4). Thus, these authors propose that "acetylcholine facilitates dendritic integration and NMDA spike generation in selected CA3 dendrites which could strengthen connections between specific CA3 neurons to form memory ensembles" (Humphries, et al., 2022, p. 69).
Humphries, R., Mellor, J. R., & O'Donnell, C. (2022). Acetylcholine boosts dendritic NMDA spikes in a CA3 pyramidal neuron model. Neuroscience, 489, 69-83.
Kozorovitskiy, Y., Gross, C. G., Kopil, C., Battaglia, L., McBreen, M., Stranahan, A. M., & Gould, E. (2005). Experience induces structural and biochemical changes in the adult primate brain. Proceedings of the National Academy of Sciences, 102(48), 17478-17482.
Renner MJ, & Rosenzweig MR. Enriched and Impoverished Environments: Effects on Brain and Behavior. Springer Verlag; New York: 1987.
Rosenzweig, M. R. (2007). Modification of brain circuits through experience. In Neural Plasticity and Memory: From Genes to Brain Imaging. CRC Press/Taylor & Francis, Boca Raton (FL); 2007. PMID: 21204433.
Section 10.9, "Supplement 2: Acetylcholine, Enriched Experience, and Memory," written by Kenneth A. Koenigshofer, PhD., Chaffey College, licensed under CC BY 4.0