Long-Term Shaping of Corticostriatal Synaptic Activity by Acute Fasting

Abstract

Food restriction is a robust nongenic, nonsurgical and nonpharmacologic intervention known to improve health and extend lifespan in various species. Food is considered the most essential and frequently consumed natural reward, and current observations have demonstrated homeostatic responses and neuroadaptations to sustained intermittent or chronic deprivation. Results obtained to date indicate that food deprivation affects glutamatergic synapses, favoring the insertion of GluA2-lacking α-Ammino-3-idrossi-5-Metil-4-idrossazol-Propionic Acid receptors (AMPARs) in postsynaptic membranes. Despite an increasing number of studies pointing towards specific changes in response to dietary restrictions in brain regions, such as the nucleus accumbens and hippocampus, none have investigated the long-term effects of such practice in the dorsal striatum. This basal ganglia nucleus is involved in habit formation and in eating behavior, especially that based on dopaminergic control of motivation for food in both humans and animals. Here, we explored whether we could retrieve long-term signs of changes in AMPARs subunit composition in dorsal striatal neurons of mice acutely deprived for 12 hours/day for two consecutive days by analyzing glutamatergic neurotransmission and the principal forms of dopamine and glutamate-dependent synaptic plasticity. Overall, our data show that a moderate food deprivation in experimental animals is a salient event mirrored by a series of neuroadaptations and suggest that dietary restriction may be determinant in shaping striatal synaptic plasticity in the physiological state.

Keywords: GluA1; calcium-permeable AMPA; dietary restriction; dorsolateral striatum; food deprivation; naphthyl-acetyl spermine.

Figure 1

Acute food restriction is associated with changes in glutamatergic transmission in spiny projection neurons (SPNs). (A) Experimental plan of Naïve and food-restricted condition. (B) Representative firing traces and (C) current-voltage (I/V) graphs of Naïve (n = 17) and FR mice (n = 15), obtained after applying hyperpolarizing and depolarizing steps of current to SPNs recorded in dorsolateral striatum. (D) The aligned dot plot shows the mean number of spikes triggered by a step that generates a maximum response. (E) Current/voltage curve and bar graph show the rectification pattern and the rectification index in SPNs of Naïve and FR mice. The α-Ammino-3-idrossi-5-Metil-4-idrossazol-Propionic Acid receptor (AMPAR)- excitatory postsynaptic currents (EPSCs) were pharmacologically isolated by application of the N-Methyl-d-aspartate (NMDAR) antagonist D-(-)-2-Amino-5-phosphonopentanoic acid (D-APV, 50 µM) (current/voltage curve, two-way ANOVA time x group interaction, Naïve n = 19 vs. FR n = 11, F_(2,56) = 5.36, *** p < 0.001; bar graph, unpaired t-test, Naïve n = 13 vs. FR n = 15, t = 7.942, df = 28, *** p < 0.001). Example traces of evoked AMPAR-EPSCs recorded at -70, 0, and+40 mV. Scale bar: 100 ms, 100 pA. (F) Group mean AMPA:NMDA ratio calculated in Naïve and FR SPNs in the presence of D-APV (unpaired t-test, Naïve n = 10, vs. FR n = 8, t = 2.911, df = 16, * p < 0.05); example traces of evoked AMPA- and NMDA-EPSCs at +40 mV (Dual: AMPA + NMDA EPSCs; NMDA EPSCs: obtained by subtraction of the EPSCs measured before and after the application of 50 µM D-APV; AMPA EPSCs: isolated by application of 50 µM D-APV). Scale bar: 100 ms, 100 pA.

Figure 2

Enhanced activity of GluA2-lacking AMPARs in food-restricted (FR) mice is associated with changes in the direction of corticostriatal synaptic plasticity in SPNs. (A) Frequency and amplitude of sEPSCs glutamatergic transmission in Naïve and FR mice. In the upper part, comparison traces of spontaneous activity recorded from groups are shown. The frequency of sEPSC is increased in FR mice compared to Naïve mice (unpaired t-test, Naïve n = 10, vs. FR n = 8, t = 3.088, df = 19, ** p < 0.01). (B) Left panel: time course of excitatory postsynaptic potential (EPSP) amplitude of SPNs from Naïve and FR mice after induction of long-term depression (LTD) protocol (high-frequency stimulation, HFS) (paired t-test pre vs. 20 min post-HFS, Naïve n = 7, t = 12.10, df = 12, *** p < 0.001, FR n = 9, t = 7.299, df = 17, *** p < 0.001). Grouped analysis shows significant group effect (two-way ANOVA: time x group interaction F_(24,336) = 18.85, Bonferroni’s post hoc ^### p < 0.001). The scale factor is 50 ms/5 mV for all traces. Right panel, representative traces of single SPNs recorded from Naïve and FR mice before (solid lines) and after HFS (dotted lines). (C) Left panel: time course of SPNs EPSP amplitude, recorded from Naïve and FR mice in the presence of 30 μM 1-naphthylacetyl spermine (NASPM) bath application for the whole duration of the experiment (paired t-test pre vs. 20 min post-HFS, Naïve n = 8, t = 8.017, df = 15; FR n = 9, t = 10.55, df = 17, *** p < 0.001 for both groups). The scale factor is 50 ms/5 mV for all traces. Right panel, representative traces of single SPNs recorded from Naïve and FR mice before (solid lines) and after HFS (dotted lines).

Figure 3

AMPAR subunit composition is critical for the maintenance of long-term potentiation (LTP). Left panel: time course of EPSP amplitude of SPNs from Naïve and FR mice after induction of LTP protocol. Grouped analysis shows significant group effect (two-way ANOVA: time x group interaction F_(24,264) = 4.23, 11–20 min, Bonferroni’s post hoc ^# p < 0.05). The scale factor is 50 ms/5 mV for all traces. Right panel: representative traces of single SPNs recorded from Naïve and FR mice before (solid lines) and after HFS (dotted lines).