Using drugs acting on nicotinic acetylcholine receptors (nAChRs), we examined temporal-parietal and frontal cortex, hippocampus, and cerebellum to identify sites of cognition enhancement in 4-and 27-month rabbits. First, we compared radioligand receptor binding for neuronal ab heteromeric nAChRs ([ 3 H]epibatidine) and a7 homomeric nAChRs ([ 3 H]methyllycaconitine) in rabbits and rats. In cerebellum, nAChR levels of both species are low, about at the detection limit of the radioligand binding assays. Next, we compared nAChRs in 4-and 27-month vehicle-treated rabbits trained in delay eyeblink conditioning. Older rabbits conditioned more poorly and had lower ab heteromeric nAChR binding in hippocampus than young rabbits. For cognition enhancement, galantamine (mild cholinesterase inhibitor and allosteric modulator of nAChRs) or MEM-3389 (a7nAChR agonist formerly identified as AR-R 17779) was injected before conditioning. Drugs improved learning in both age groups. In 27-month rabbits, drugs increased expression of frontal and temporal-parietal ab heteromeric nAChRs and hippocampal ab and a7nAChRs. In 4-month rabbits, drugs increased expression of a7 homomeric nAChRs in frontal and temporal-parietal cortex and hippocampus, but increased expression of ab heteromeric nAChRs only occurred in temporal-parietal cortex. Increased expression of ab nAChRs was more extensive in older drug-treated rabbits, whereas increased expression of a7nAChRs was more prevalent in younger drug-treated rabbits, suggesting different substrates for amelioration (27-month rabbits) vs facilitation (4-month rabbits) of learning. Results provide evidence for cortical as well as hippocampal nAChR modulation of delay eyeblink conditioning and demonstrate that more sensitive binding assays are required to assess nAChR effects in cerebellum.

The aim of this study was to examine parameters affecting age differences in eyeblink classical conditioning in a large sample of young and middle-aged rabbits. A total of 122 rabbits of mean ages of 4 or 26 mo were tested at inter-stimulus intervals (ISIs) of 600 or 750 msec in the delay or trace paradigms. Paradigm affected both age groups dramatically, with superior performance in the delay paradigm. ISI was salient as middle-aged rabbits were significantly impaired in 750-msec compared with 600-msec delays, and young rabbits were significantly less impaired in 600-msec than in 750-msec trace. Young rabbits performed equally well at both delay ISIs, and consequently, there were significant age differences in 750-msec but not in 600-msec delays. Middle-aged rabbits performed poorly at both 600- and 750-msec trace, resulting in significant age differences in 600-msec but not in 750-msec trace. Timing of the conditioned response has been associated with cerebellar cortical function. Normal aging of the cerebellar cortex likely contributed to the magnitude of the effect of ISI in delay conditioning in middle-aged rabbits. Results demonstrate that the magnitude of age differences in eyeblink conditioning can be enlarged or eliminated by ISI and paradigm.

Neuronal αβ heteromeric and α7 homomeric nicotinic acetylcholine receptors (nAChRs) were compared in 4-and 27-month rabbits selected for learning proficiency. Sixty 4-and 60 27-month rabbits received the α7 nAChR agonist (MEM-3389), galantamine, or vehicle during training in trace eyeblink classical conditioning. Brain tissue from the best and worst young and older learners was analyzed with radioligand binding. Vehicle-treated 4-and 27-month good learners had higher αβ heteromeric nAChR binding in hippocampus and temporal-parietal cortex than poor learners, and this result was replicated in both age groups of rabbits treated with galantamine. Results indicate that anatomically more numerous nAChRs or functional activation of a greater number of nAChRs may characterize animals demonstrating optimal learning. During normal aging the expression of high-affinity binding sites declines. Age-related changes in the expression of hippocampal αβ heteromeric nAChRs may account for some of the documented age-related impairment in learning. However, individual differences in αβ heteromeric nAChRs also exist early in life, as better learning in 4-month rabbits was associated with significantly higher binding.