Dopamine modulates synaptic plasticity in dendrites of rat and human dentate granule cells

Abstract

The mechanisms underlying memory formation in the hippocampal network remain a major unanswered aspect of neuroscience. Although high-frequency activity appears essential for plasticity, salience for memory formation is also provided by activity in ventral tegmental area (VTA) dopamine projections. Here, we report that activation of dopamine D1 receptors in dentate granule cells (DGCs) can preferentially increase dendritic excitability to both high-frequency afferent activity and high-frequency trains of backpropagating action potentials. Using whole-cell patch clamp recordings, calcium imaging, and neuropeptide Y to inhibit postsynaptic calcium influx, we found that activation of dendritic voltage-dependent calcium channels (VDCCs) is essential for dopamine-induced long-term potentiation (LTP), both in rat and human dentate gyrus (DG). Moreover, we demonstrate previously unreported spike-timing-dependent plasticity in the human hippocampus. These results suggest that when dopamine is released in the dentate gyrus with concurrent high-frequency activity there is an increased probability that synapses will be strengthened and reward-associated spatial memories will be formed.

Fig. 1.

Frequency-dependent dendritic responses observed in DGCs. (A) A neurobiotin-filled DGC with representative whole-cell recording electrode and bipolar stimulation electrode in the MPP. (B) Normalized responses to synaptic trains of four EPSPs from 20–150 Hz (10-Hz increments). The time-voltage integral of the entire synaptic train was divided by the time-voltage integral of the first EPSP in the train to give the EPSP ratio (solid black circles) (n = 10). (Inset) Sample trace at 120 Hz. (C) Synaptic responses to higher frequency stimulus trains are sensitive to NPY action. Responses in PTX (100 μM) + elevated Mg²⁺ (to 5 mM) alone are shown in solid black circles and responses in the additional presence of NPY (1 μM) are shown in solid red squares (***P < 0.001, *P < 0.05; n = 9). (Inset) Sample traces at 120 Hz. (D) Dopamine D1R agonist, SKF 81297 (10 μM) was applied via the bath in control ACSF and trains of EPSPs were evoked as above (***P < 0.001, *P < 0.05; n = 6). (Inset) Sample traces at 150 Hz. (E) Neurobiotin-filled DGC with representative whole-cell recording and rectangular area of Ca²⁺ imaging. (F, Left) Square regions of interest (ROIs) used for Ca²⁺ imaging experiments. X is 44 μM from the cell body. (Right) Trains of somatic APs induced Ca²⁺ influx measured as ΔF/F at five ROIs. Distal ROIs 4 and 5 showed sensitivity to higher frequencies, whereas proximal ROIs 1 and 2 did not. (G) Ca²⁺ influx was normalized to 40-Hz measurements and compared with frequencies up to 150 Hz (difference between ROIs 1 and 5 (***P < 0.001, **P < 0.01, P < 0.05; n = 3). Symbol colors correspond to ROIs in F.

Fig. 2.

4-AP unmasks Ca²⁺-dependent critical frequencies and after-depolarizations. (A) In the presence of 4-AP (100 μM) and synaptic blockers (PTX, 100 μM; APV, 50 μM; kynurenic acid, 100 μM; and Mg²⁺, to 5 mM) distal dendritic Ca²⁺ transients (Above) recorded at 200 μm from the soma become supracritical at the same frequencies as the somatic ADP (120 Hz; Below). (B) Correlation of dendritic and somatic CFs recorded simultaneously (as in A) in individual DGCs (r² = 0.8258; P < 0.0001; n = 12). (C) Cd²⁺ (100 μM) was applied locally from a puffer pipette also containing Alexa 594 (500 nM). Experiments were performed in the presence of the 4-AP solution in DGCs filled with Alexa 594 (1 μM). (D) Plot of ADP time-voltage integral in a single DGC as a function of frequency. The sigmoidal fit was used to determine the CF (shown) for responses in 4-AP (Ctl), and with subsequent local applications of Cd²⁺ to the soma, proximal (Prox) and distal dendrites. In this DGC, distal and somatic application of Cd²⁺ produced a 24-Hz and 4-Hz rightward shift, respectively; however, the shift caused by soma application was not significant across DGCs. (E) Change in CF after local Cd²⁺ application to the soma (−5 ± 2 Hz; n = 9) proximal dendrite (7 ± 5 Hz; n = 5), distal dendrite (19 ± 4 Hz; n = 8), and during washout after distal application (3 ± 4 Hz; n = 7; *P < 0.05).

Fig. 3.

D1 agonist increases excitability of DGC dendrites. Data in A–C were recorded in the absence of 4-AP. (A) Sample traces at 60, 110, and 120 Hz shown superimposed in control, in the presence of the D1R agonist SKF 81297 (10 μM) and with Cd²⁺ and Ni²⁺ also added (each 50 μM). SKF application produced two distinct effects: (B) Unmasking of a clear CF in some DGCs (mean CF: 110 ± 13 Hz; **P < 0.01, ***P < 0.001; 50 Hz vs. all frequencies; n = 5). (C) In other DGCs, a significant increase in ADP was seen with SKF across all frequencies (compared with control, *P < 0.05), but with no clear CF emerging (no significant differences across all frequencies; n = 9). (D) In 4-AP (100 μM), SKF increased the ADP and shifted the CF leftward (4-AP vs. SKF, **P < 0.01; n = 6). (E) In 4-AP (100 μM), SKF reversibly decreased the CF by 23 ± 4 Hz (n = 10). After a washout of SKF, subsequent NPY application increased the CF by 32 ± 6 Hz (n = 3). In other DGCs, the membrane-permeant cAMP analog, dibutyrl-cAMP (10 μM) decreased the CF (23 ± 6 Hz; n = 6). Coapplication of SKF and NPY resulted an increase of the CF (mean: 28 ± 7 Hz; n = 4) in yet other DGCs. (F) In DGCs with no CF in low 4-AP (10 μM), SKF unmasked a CF and increased the ADP integral. Still in the presence of SKF and low 4-AP, NPY inhibited the ADP at and above the CF (n = 8; *between 4-AP and SKF; ^†between SKF and SKF + NPY, ** and ^††P < 0.01, ***P < 0.001).

Fig. 4.

D1 agonist alters the threshold for LTP induction in rat and human DGCs. (A) A neurobiotin-filled human DGC with representative whole-cell recording and extracellular MPP stimulation. (B) Sample trace recorded during the theta-burst pairing protocol (TBP₁). The ADP is increased slightly after a subsequent theta-burst pairing (TBP₂) in the presence of bath applied D1 agonist (SKF 81297; 10 μM) in the same DGC (superimposed gray trace). (C) The ADP integral was calculated from the onset of the fifth action potential until return to resting potential. Each pair represents a single DGC that exhibited LTP after TBP₂ + SKF (*P < 0.05; n = 7). (D) Sample EPSP traces before (control), after TBP₁, and after TBP₂ + SKF from the same DGC. (E) TBP at 100 Hz did not evoke LTP under control conditions (TBP₁). However, with the application of SKF (10 μM) a second TBP (TBP₂) resulted in a long-lasting potentiation (*P < 0.05; n = 7). (F) NPY (3 μM) applied via a focal pipette containing Alexa 594 (300 nM) to the distal dendrites together with bath application of SKF (10 μM) for 4 min before TBP₁ did not change EPSP amplitude 15 min after TBP₁. After washout, SKF (10 μM) was applied alone. Under these conditions, TBP₂ induced LTP (*P < 0.05; n = 4). (G) SKF significantly induces LTP in human DGCs from 350-μm thick hippocampal slices with otherwise the same protocol as in E (*P < 0.05, **P < 0.01; n = 5).