Neuronal basis of age-related working memory decline

Abstract

Many of the cognitive deficits of normal ageing (forgetfulness, distractibility, inflexibility and impaired executive functions) involve prefrontal cortex (PFC) dysfunction. The PFC guides behaviour and thought using working memory, which are essential functions in the information age. Many PFC neurons hold information in working memory through excitatory networks that can maintain persistent neuronal firing in the absence of external stimulation. This fragile process is highly dependent on the neurochemical environment. For example, elevated cyclic-AMP signalling reduces persistent firing by opening HCN and KCNQ potassium channels. It is not known if molecular changes associated with normal ageing alter the physiological properties of PFC neurons during working memory, as there have been no in vivo recordings, to our knowledge, from PFC neurons of aged monkeys. Here we characterize the first recordings of this kind, revealing a marked loss of PFC persistent firing with advancing age that can be rescued by restoring an optimal neurochemical environment. Recordings showed an age-related decline in the firing rate of DELAY neurons, whereas the firing of CUE neurons remained unchanged with age. The memory-related firing of aged DELAY neurons was partially restored to more youthful levels by inhibiting cAMP signalling, or by blocking HCN or KCNQ channels. These findings reveal the cellular basis of age-related cognitive decline in dorsolateral PFC, and demonstrate that physiological integrity can be rescued by addressing the molecular needs of PFC circuits.

Figure 1

Age-related changes in the PFC networks that subserve working memory. a, The region of the DLPFC most needed for spatial working memory and the site of recordings. PS=principal sulcus; AS=arcuate sulcus. b, The oculomotor delayed response (ODR) spatial working memory task. The monkey fixates on the central stimulus and maintains fixation for the duration of the trial. A cue is briefly presented in one of eight locations, followed by a delay period (2.5s) in which no spatial information is present. At the end of the delay period, the fixation spot disappears, and the monkey makes an eye movement (saccade) to the remembered location for juice reward. The cue position randomly changes on subsequent trials. c, A diagram of the recurrent excitatory networks subserving working memory. Pyramidal cells with similar spatial tuning excite each other to maintain persistent firing across the delay period. These networks are concentrated in deep layer III . Spatial tuning is enhanced by GABAergic lateral inhibition (not shown). d, An example of a DLPFC DELAY neuron with spatially-tuned, persistent firing during the delay period. This neuron shows increased firing for the cue, delay and response for the neuron’s preferred direction (highlighted in blue), but not for nonpreferred directions (white backgrounds). The anti-preferred direction opposite to the neuron’s preferred direction is shown in red; note that subsequent figures show only the preferred and anti-preferred directions for the sake of brevity. e, Pyramidal cells synapse on spines where cAMP-PKA signaling regulates the open state of HCN and KCNQ channels, and thus modulates the strength of network connections.. f, Population average activity for the DLPFC DELAY neurons recorded in each age group (102, 101, and 70 neurons for young, middle-aged and old monkeys, respectively). Colors indicate the activity during the trials in which the cue was presented in the neuron’s preferred (blue) and anti-preferred (red) directions; the darker gray background refers to the cue period; the lighter gray background to the delay period.

Figure 2

Age-dependent decline in the spatially-tuned, persistent firing of DLPFC DELAY neurons. a, Marked reduction of DLPFC DELAY activity for the neurons’ preferred direction with advancing age. Activity of individual neurons of each animal was averaged separately for the last 0.5 s during the intertrial interval (pre-fix, gray), the fixation period (fix, black), the cue period (cue, red), and the delay period for the neuron’s preferred direction. Firing during the delay period is represented in a successive series of 0.5-s intervals (color-coded yellow through blue). Lines were obtained using linear regression. b, Firing rates during the delay period for the anti-preferred direction of the same neurons shown in A. There was a significant age-related decline in all epochs, but it was less prominent than the decline in firing for the preferred direction during the delay period. Color-coding for each 0.5s interval as in Fig. A. c, Age-related decline in spatial tuning, whereby the difference between firing for the preferred vs. anti-preferred directions during the delay period declines with advancing age. Color-coding as in Fig. a. d, Age-related decline in d′, i.e. the ability to distinguish preferred from anti-preferred spatial directions based on firing rate patterns during the entire delay period.

Figure 3

Firing rates of DLPFC CUE cells remain stable in aged monkeys. The average firing rates of CUE cells in young monkeys (left graph; 10 neurons from a 7-y old and 2 neurons from a 9-y old monkey), did not differ from the firing rate in the oldest monkey (right graph; 11 neurons from a 21-y old monkey), or from the firing rate averaged for both middle-aged (5 neurons from 13-y old monkey, not shown separately) and old monkeys (t-test, p>0.7). CUE cells may receive direct, “bottom-up” excitation from parietal association cortex, which may be less vulnerable to subtle molecular changes with advancing age. A subset of CUE cells recorded in the aged monkey displayed high firing rates during the Response period.

Figure 4

Iontophoresis of compounds that inhibit cAMP-PKA signaling, or block HCN or KCNQ channel signaling, strengthens delay-related firing in aged PFC DELAY neurons. a–d,. A summary of the results showing a significant increase in population-average firing rate for the neuron’s preferred direction compared to control conditions (paired t-test, p<0.01 for a, c and d, and p<0.05 for b) following iontophoresis of: the α2A adrenergic agonist guanfacine applied at 10nA (GFC; a, significant effects in 7 out of 9 neurons, t-test, p<0.05, indicated by solid lines); the cAMP inhibitor Rp-cAMPS at 50 nA (b, significant in 4 out of 6 neurons); the HCN channel blocker, ZD7288 at 15 nA (c, significant in 4 out of 7 neurons); and the KCNQ channel blocker XE991 at 15 nA (d, significant in 3 out of 6 neurons). In all cases, significant effects were found more frequently than expected by chance (binomial test, p<0.005). The orange lines represent the individual neurons shown in e-h. e–h, Individual examples of neurons under control conditions (top) firing to their preferred (blue trace) or anti-preferred (red trace) directions, compared to their firing patterns following iontophoresis of guanfacine (e), Rp-cAMPS (f), ZD7288 (g) or XE991 (h). The orange lines in a-d indicate the individual neurons shown in e-h. Error bars are s.e.m. * p<0.05, ** p<0.01 significant difference between drug vs. control for the neuron’s preferred direction.