This is the first study to estimate the total number of synapses in the human ITG lamina 3 using unbiased stereology coupled with quantitative electron microscopy. All individuals used in these studies were followed longitudinally and initially enrolled as cognitively normal. Compared to age- and PMI-matched individuals with no cognitive impairment, subjects who transitioned to aMCI demonstrated a statistically significant 36% loss of synaptic contacts in the ITG. This decline in cortical connectivity was very similar to the 42% loss observed in the mAD cohort. These results support the idea that synaptic loss in the ITG is an important pathological defect in the early stages of the disease process. The total number of synapses strongly associated not only with the subject’s MMSE but also animal naming, a direct test of semantics access suggesting an underlying mechanism for some verbal fluency dysfunction in early AD. The present findings have both clinical and theoretical implications.
Previous studies assessing synaptic loss [9
] have focused primarily on the hippocampus and the more prominent areas of the frontal, temporal, and parietal lobes. Very few investigations have assessed synaptic change in the early progression of the disease such as aMCI [15
]. Most descriptions of neuropathological alternations have concentrated on the loss of neurons [37
] and the presence of NFT [13
]. Masliah and colleagues reported the loss of synaptophysin staining in the frontal cortex of individuals with a CDR of 0.5 [41
]. Counts et al [17
] reported a decline in the postsynaptic marker drebrin in the temporal lobe in aMCI. Two previous AD studies evaluated changes in synaptophysin as a marker of synaptic connectivity in the ITG. The earliest study [42
] reported a significant AD-related loss of staining, while the second study [43
] failed to find a difference between individuals with AD and a cohort of controls. The excessively long PMI of the tissue used in that study may account for the observed differences. The decline in synaptic connectivity in the aMCI cohort observed in the present study is very striking considering previous investigations that evaluated possible synaptic loss early in the disease process. Ultrastructral data derived from the hippocampus shows an area in transition with many of the aMCI subjects within the lower range of the NCI cohort [15
]. By contrast, only a single aMCI subjects in the present ITG study was within the NCI range with most of the aMCI cohort overlapping with the mAD values, supporting the idea that this cortical structure is involved early in the disease. Several investigators have suggested that the ITG should be included in the routine assessment of cortical involvement in the CERAD determination of AD [44
] along with other heavily involved structures [18
] because of the accumulation of AD-related lesions in this region of the temporal lobe.
Studies in primates have demonstrated interconnections between the inferior and superior temporal gyrus and also between the ITG and perirhinal and parahippocampal cortex [46
]. While it is clear that the ITG projects to entorhinal cortex, whether or not there are reciprocal connections is unclear [47
]. The ITG can be considered as a tertiary visual association cortex as part of the pathway that originates in the primary visual cortex (BA17) and progresses through secondary visual areas BA18, 19 and BA37 and then B20, with some information relayed to the prefrontal cortex [48
]. Ablation studies in primates have identified B20 as being involved in learning tasks that require visual recognition of objects [49
]. Several studies have linked regions of the ITG to the amygdala and nucleus accumbens suggesting the addition of an emotional tone to learning of cognitive tasks [51
]. The temporal lobe is strongly associated with language [53
] with its anterior regions implicated in semantic memory [54
]. Language deficits are important manifestations of cognitive disability associated with AD with verbal fluency (semantic/category) performance a key indicator of the progression of dementia [55
Tests of language dysfunction, in particular those of verbal fluency, are one of the most commonly used diagnostic tools for the assessment of neurologic damage. Both semantic and phonemic fluency are often tested, with semantic fluency more impaired following damage to the temporal lobes [55
]. The semantic fluency test used in the present study required individuals to name as many different animals as they could within 60 seconds. The total number of novel animal names generated consisted of that individual’s score. This particular test appears to be resistant to the influence of practice in MCI and early AD [56
]. Impairment in animal fluency is strongly associated with early stages of AD [1
]. AD patients that develop semantic memory dysfunction also develop significant neuropathology in neocortical association areas linked with semantic language function. Individuals with semantic dementia (SD) differ from AD subjects in that they have preserved episodic memory and perform well on tests such as the BNT. Among the neocortical regions most severely affected in SD is the anterior inferior temporal lobe [58
]. In the present study, there was a greater association between animal naming than the BNT, supporting previous findings that deficits in confrontation naming may be linked to a more advanced stage of AD [55
]. The connectivity of the ITG appears to be strongly associated not only with semantic fluency but also MMSE and word recognition scores.
The current literature suggests that AD-associated decline in cognition may be an inevitable consequence of the age-associated changes not only in neuron number but synaptic loss. While careful unbiased evaluation of cortical neuronal numbers have failed to support this idea [59
], there does appear to be a significant age-related decline in white matter, indicative of a loss of brain connectivity and synapses [61
]. In the present study, we found a significant age-related decline in total synaptic contacts in lamina 3 of the ITG when all subjects were included. This correlation may not necessarily reflect normal age-related associations since both cognitively normal and demented subjects were included in the analysis. How this impacts on the present loss of synapses in the aMCI and AD groups is unclear since there was no significant difference in the mean age of the groups (see ). Immunohistochemical studies have previously reported an age-related loss of synaptic markers in the ITG and other neocortical regions [42
]. This is in contrast to previous ultrastructural studies that demonstrated a preservation of synapses in the superior frontal [67
] and the postcentral gyrus [66
] across a wide age spectrum in cognitively normal individuals. A previous study that included both NCI and AD subjects investigating the superior and middle temporal cortical regions also failed to find an age-related change in synaptic numbers [68
]. The disparity among these studies supports a heterogeneous view of the neocortex and cautions against global statements concerning age-related morphological cortical changes.
Currently, neuroimaging appears to be the easiest way to evaluate possible atrophy in a region of interest and has been limited to estimating gross changes. In the present study, we were able to directly evaluate possible changes in the volume of lamina 3 of the ITG from histological sections coupled with unbiased stereology. We found a significant decline not only in mAD but also in aMCI, although the decline was not as robust as the loss in synapses. This finding also differs from previous hippocampal aMCI studies that evaluated subregion volumetric changes and failed to show a change in either the outer molecular layer of the dentate gyrus [16
] or stratum radiatum of the hippocampal region superior [15
]. The overall significance of this difference may represent subtle differences in cytoarchitecture between the ITG and hippocampus [69
]. As AD progresses, various regions of the neocortex are functionally disconnected from each other due to a loss of corticocortical projections [70
]. Numerous studies suggest that some of the neocortical association areas are the most vulnerable in the progression of AD and it may be the case that the dramatic loss of synapses observed in the present study signals the onset of dementia as suggested by others [18
Finally, it is important to note that the present set of experiments consisted of a cross-sectional picture of a relatively small sample size that may reduce the significance when applied to a larger and more diverse population. The present cohort of subjects was well educated and consisted exclusively of white Caucasians and caution should be maintained when extrapolating the results to a more diverse population. Moreover, the MCI subjects we examined were amnestic and may not reflect the clinical and pathology of other types of MCI individuals.