Mechanisms of late-onset cognitive decline after early-life stress

Abstract

Progressive cognitive deficits that emerge with aging are a result of complex interactions of genetic and environmental factors. Whereas much has been learned about the genetic underpinnings of these disorders, the nature of "acquired" contributing factors, and the mechanisms by which they promote progressive learning and memory dysfunction, remain largely unknown. Here, we demonstrate that a period of early-life "psychological" stress causes late-onset, selective deterioration of both complex behavior and synaptic plasticity: two forms of memory involving the hippocampus, were severely but selectively impaired in middle-aged, but not young adult, rats exposed to fragmented maternal care during the early postnatal period. At the cellular level, disturbances to hippocampal long-term potentiation paralleled the behavioral changes and were accompanied by dendritic atrophy and mossy fiber expansion. These findings constitute the first evidence that a short period of stress early in life can lead to delayed, progressive impairments of synaptic and behavioral measures of hippocampal function, with potential implications to the basis of age-related cognitive disorders in humans.

Figure 1.

Early-life stress evokes transitory alterations of neuroendocrine parameters. A, Representative analysis grids of maternal behavior. Left, A dam rearing pups in a normal cage environment; right, a dam rearing pups in a cage with reduced nesting material. Grids depict 25 consecutive 3 min epochs and types of “nurturing” behaviors are color coded as follows: blue, nursing; yellow, carrying pups; green, licking/grooming; red, off pups; and pink, self grooming. B, Dams limited in nesting material spend less time nursing and more time off their pups. C, Parameters indicative of stress immediately after the early-life stress period (P9; left column) and in middle-aged rats (12 months of age; right column). Elevated basal corticosterone levels, higher adrenal gland weights, and modestly lower body weight were found in chronically stressed P9 rats (n = 12 per group), but these changes were no longer present in middle-aged rats (n = 5 controls, 6 early stress). All studies and plasma harvest (by decapitation) were performed from 8:00 A.M. to 10:00 A.M. (^*p < 0.05; Student's t test). Error bars represent SEM.

Figure 2.

Deficient memory is evident in middle-aged (12 months) but not in younger (4- to 5-month-old) rats exposed to chronic early-life stress. A, At 4-5 months of age (left), both early-life stress (n = 8) and control rats (n = 11) require progressively less time to find a hidden platform in the Morris water-maze test. Escape latencies (time to find the hidden platform) on the testing day (day 3) are shown after 2 training days. By 12 months of age (right), early-life stress rats require significantly longer time (p < 0.001; two-way ANOVA; vs controls) to locate a hidden platform. An interaction of early-life stress and age of testing for the last trial (F_(1,24) = 4.23; p = 0.05) suggests that the effects of early-life stress may be progressive. B, A representative probe trial search pattern from a control rat, preferentially exploring the quadrant containing the hidden platform on the previous testing day (left) is compared with that of an early-life stress rat (right) that does not prefer any quadrant. C, Quantitative analysis of the probe trials of middle-aged control and early-life stress rats showing that the latter group spent significantly less time in the quadrant that housed the platform on the previous day (p < 0.05). D, Anxiety level, as determined by elevated plus maze analysis, was not altered in the early-life stressed rats. E, Using the object-recognition paradigm, exploration pattern and duration of two novel objects of early-life stress rats (n = 15) and controls (n = 9) were indistinguishable on day 1 (left). However, 1 d later (day 2), controls discriminated between familiar and novel objects [remembering the familiar object and exploring it for a significantly (asterisk) shorter time], whereas early-life stress rats did not, indicating impairment of recognition memory. As shown on the right, the mean total exploration time on day 2 of the object recognition test did not differ between the groups (p > 0.05). Note that tests were performed and analyzed blindly. Error bars represent SEM.

Figure 3.

Synaptic plasticity defects and physiological abnormalities in field CA3 arise in middle-aged rats subjected to early-life stress. A, LTP instratum radiatum of field CA3 in slices from middle-aged rats. A single HFS train was delivered at the time point indicated by the arrow. Potentiation (illustrated as percentage of average baseline field EPSP amplitude) decayed to near baseline in the stress group but not in age-matched controls (mean ± SEM). B, LTP in field CA3 of slices prepared from young adult rats. There were no evident differences between early stress and control groups 30 min after induction (p > 0.05). C, Representative EPSPs before and after HFS illustrate that LTP was deficient in the early-stress group compared with controls in slices prepared from middle-aged rats (left). There were no detectable differences between potentiated EPSPs in control and stress groups when slices were prepared from young adult rats (right). Calibration: 5 ms, 1 mV. D, Amplitude of field EPSPs in CA3 as a function of stimulus pulse duration (mean ± SEM); the input/output curves were noticeably flatter in slices from early-stress rats. E, Traces are the first 25 responses to the HFS used to induce LTP in area CA3; although initially similar, responses in the early-stress slices exhaust much more quickly than those in control slices. Calibration: 50 ms, 0.5 mV. F, In field CA3, mean amplitude of the fiber volleys (left) and evoked responses (right) were significantly reduced in early stress compared with control slices prepared from middle-aged rats (^*p < 0.05). G, Input/output curves for antidromic responses in field CA3. Antidromic responses to the same stimulation currents were larger in early-stress slices compared with controls, indicating that the stress episode caused CA3 pyramidal cells to be more excitable. Error bars represent SEM.

Figure 4.

Mossy fiber anatomy and physiology in slices from middle-aged rats subjected to early-life stress compared with age-matched controls. A, Sections of CA3 pyramidal cell fields from control and early-life stress (killed at 12 months; n = 6 in each group) rats, subjected to Timm's stain for visualizing the high zinc content of mossy fiber terminals (axons of the CA3-innervating granule cells). In early-life stressed rats, these terminals were abnormally abundant with in CA3 stratum oriens (so; arrow). Scale bar, 50 μm. sp, Stratum pyramidale. B, Quantification of the Timm's stained sections confirmed that mossy fiber sprouting was significantly increased in early-life stress animals (^*p < 0.0001; Student's t test). C, The amplitudes (in millivolts) of mossy fiber synaptic responses, recorded from stratum lucidum, in control (white; n = 10) and stress (gray; n = 11) groups were not significantly different in CA3a or CA3b (n = 10 in each group). D, The degree of paired-pulse facilitation (i.e., the ratio of the size of the second pulse over the size of the first pulse in a pair of pulses separated by 50 ms) observed at the mossy fiber synapse in early-stress slices (gray; n = 11) was not different from control values (white; n = 10). Typical mossy fiber paired-pulse responses from CA3b are illustrated for control (left) and early-stress (right) slices from middle-aged rats (inset). Calibration: 20 ms, 1 mV. E, The metabotropic glutamate agonist DCG-IV (1 μm) depressed mossy fiber responses in control and early-stress slices (n = 4 in each group). Response amplitudes were normalized to predrug baseline values. Error bars represent SEM.

Figure 5.

Deficits in LTP are not accompanied by disturbances in basic physiology in field CA1 of middle-aged rats subjected to early-life stress. A, Percentage of LTP at the Schaffer collateral/commissural synapse in stratum radiatum of field CA1 of middle-aged rats. Potentiation was equivalent in control and early-stress slices immediately after HFS but decayed much more quickly in the latter group and was significantly lower than control values at 30 min after HFS (p < 0.05; data illustrated as mean ± SEM). Typical EPSPs from early-stress (left) and control (right) slices before HFS illustrate that response size and waveform were comparable between groups. Calibration: 5 ms, 1 mV. B, The average amplitude of Schaffer collateral responses was not different between control (cont; white) and early-stress (gray) groups. C, Input/output curves for EPSPs in CA1 stratum radiatum indicate that an identical magnitude of stimulation current elicited responses in early-stress slices (n = 5) that were not measurably different from those recorded in control slices (n = 5). D, Input/output curves for antidromic responses in field CA1. Changes in response amplitude as a function of stimulation pulse duration were not significantly different in slices from early-life stress rats compared with age-matched controls. E, Current-clamp recordings from CA1 pyramidal cells in the presence of picrotoxin (50 μm). EPSPs were similar in control (left) and stressed (right) slices across a range (0.06-0.11 ms) of stimulus pulse durations. Calibration: 60 ms, 4 mV. F, Grouped current-clamp data for the effect depicted in E. Responses in CA1 pyramidal cells to the same level of stimulation current were not detectably different in control (white circles) and stress (gray circles) slices. G, In voltage-clamp experiments, the voltage-sensitive NMDA current was measured in the presence of 50 μm picrotoxin at the time point indicated by the dotted line. Small NMDA currents begin to emerge by -40 mV, whereas more prominent NMDA currents are apparent at -20 and 0 mV. No obvious abnormalities in NMDA currents were seen in the early-stress slices (right) when compared with controls (left). Calibration: 20 ms, 100 pA. H, The NMDA contribution to EPSCs in CA1 pyramidal cells is expressed as a percentage of the overall response across a range of holding potentials (left) and is not detectably different in recordings from control (white; n = 6) and early-stress (gray; n = 10) slices. The size of the AMPA- and NMDA-mediated currents are plotted for control (white) and early-stress (gray) groups at a holding potential of 0 mV (right). These quantifications reveal no differences in the NMDA-mediated component of EPSCs in CA1 pyramidal cells between the early-stress and control groups. I, A train of theta bursts (indicated by an arrow) was delivered to current-clamped CA1 pyramidal neurons to elicit an AHP; the top trace illustrates an example recorded in control tissue. Calibration bars for top trace: 0.3 s, 10 mV. Magnification of the AHP in representative traces from control and early-stress slices shows that AHPs were similar in both groups. The depolarizing responses to the stimulation train (indicated with an arrow on the control trace) were cutoff for illustration purposes in the magnified traces. The similarity between groups was quantified (right). Neither the amplitude nor the area of AHPs in CA1 neurons was significantly affected by early-life exposure to stress. Calibration for magnified, bottom traces: 0.3 s, 3 mV. Error bars represent SEM.

Figure 6.

Dendritic atrophy of CA1 pyramidal cells from early-life stressed middle-aged rats. Photomicrographs of biocytin-labeled pyramidal cells in field CA1 (left) illustrate the reductions in total dendritic length and in dendritic arborization in the early-stress group. Scale bar, 80 μm. These observations were quantified in the graphs on the right. The number of dendritic intersections, a measure of arborization, was lower in CA1 apical dendrites (right, top panel) of middle-aged early-stress rats. This effect diminished with increasing distance from the soma, indicating that the more distally situated dendrites in stratum lacunosum/moleculare, the region in which the perforant path axons from the entorhinal cortex terminate, were not affected. The number of intersections in the basal dendritic field of CA1 was also reduced (right, middle panel) in the early-stress group (gray circles) compared with age-matched controls (white circles). The total length of dendritic processes in CA1 (bottom right) was also lower in the early-stressed compared with the control group. ^*p = 0.018; n = 8-10 per group. Error bars represent SEM.