Synaptic Plasticity in the Chick Lobus Parolfactorius During Development and Following Memory Formation

Hunter, Alistair (1991). Synaptic Plasticity in the Chick Lobus Parolfactorius During Development and Following Memory Formation. PhD thesis The Open University.



The lobus parolfactorius (LPO) is an avian basal forebrain nucleus. The morphology of synapses within the LPO has been shown to be altered 24 hours following training of chicks (Gallus domesticus), using an aversive taste (methyl anthranilate) as the training stimulus (Stewart et al Brain Res. (1987) 426:69-81). It is not known when synaptogenesis takes place in this nucleus during normal development, and what other factors may influence it. Lesion of the LPO has been shown to cause amnesia for the passively learnt task, only if made post-training (Gilbert et al in press). This suggests that the LPO is involved in storage of memory for the task, and not in its acquisition. The present study aimed to investigate the time course over which changes in synaptic morphology take place in the day-old chick LPO, and to clarify the onset of synaptogenesis during normal development.

Chicks were reared to the ages of either 16 days in ovo, 1-, 9-, or 22-days post-hatch, when they received anaesthesia intra-peritoneally (Sagatai, 6mg/chick). They were then killed by cardiac perfusion, with a solution containing 2% glutaraldehyde and 2% paraformaldehyde in 0.1 M cacodylate buffer. Each brain was extracted from its cranium, and the LPO was subsequently processed for electron microscopy. Unbiased stereological methods were used to make estimates of synaptic density (Nvsyn), on micrographs taken of a systematic random sample of electron microscopic fields. An estimation was also made of the mean projected synaptic height (Hsyn) within the EM section. Results indicated that the 1-day old chick has a lower complement of synapses compared with the 9-day old chick. There is a substantial increase in synaptic density between these ages, although there is a hemispheric asymmetry in this increase, with the left hemisphere exceeding the right by 62%. Synaptic height does not change significantly between 16 days in ovo and 1 day post-hatch, but increases slightly on or before 9 days post-hatch, after which it remains stable.

The time-course study involved the training of day-old chicks, using the aversive taste of a bead coated with methyl anthranilate (M-trained), as the training stimulus. Control chicks were trained using water-coated beads. M-trained chicks avoided pecking a similar, but dry bead, 30 minutes later, whereas control chicks re-pecked. At 1, 6 12, 24 or 48 hours after training, chicks were killed by cardiac perfusion and the LPOs obtained as described above. Using unbiased stereological methods, estimates of NVsyn, Hsyn, synaptic contact surface area (Sasyn), mean dendritic shaft volume (Vvshaft), mean dendritic spine volume (Vvspine) and mean synaptic bouton volume (Vvbouton) were made. A significantly larger mean Nvsyn (approx. 30%) was seen in the left hemisphere of M-trained chicks 24 hours after training, compared with control chicks. A difference of approximately 10% was seen in this hemisphere 48 hours post-training. M-trained chicks also had a greater mean NVsyn (approx. 18%) in the right hemisphere at 48 hours. The estimators of synaptic size showed an increase predominantly in the left hemisphere of M-trained chicks. The analysis of Vvbouton was inconclusive, although no significant differences were found between control and M-trained chicks. Vvspine was significantly increased in the left hemisphere 48 hours following training, but no differences were found in the estimates of Vvshaft between the two groups. These results show that memory formation results in a number of lasting synaptic and dendritic morphological changes in the LPO, and that some of these changes are lateralized to the left hemisphere. The results are consistent with the hypothesis that synaptic plasticity occurs following long-term memory formation.

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