Anandamide regulates postnatal development of long-term synaptic plasticity in the rat dorsolateral striatum

Abstract

Long-term changes in synaptic efficacy produced by high-frequency stimulation (HFS) of glutamatergic afferents to the rat dorsolateral striatum exhibit heterogeneity during early stages of postnatal development. Whereas HFS most often induces striatal long-term potentiation (LTP) in rats postnatal day 12 (P12)-P14, the same stimulation tends to induce long-term depression (LTD) at ages P16-P34. Previous studies have shown that striatal LTD induction depends on retrograde endocannabinoid signaling and activation of the CB1 cannabinoid receptor. It is also known that levels of one of the primary endogenous CB1 receptor agonists, anandamide (AEA), increases during development in whole-brain samples. In the present study, we sought to determine whether this developmental increase in AEA also takes place in striatal tissue and whether increased AEA levels contribute to the postnatal switch in the response to HFS. We observed a pronounced increase in striatal levels of AEA, but not the other major endogenous cannabinoid 2-arachidonoylglycerol (2-AG), during the postnatal period characterized by the switch from LTP to LTD. Furthermore, application of synthetic AEA during HFS in field recordings of slices from P12-P14 rats allowed for induction of LTD whereas blocking the CB1 receptor during HFS in animals P16-P34 resulted in expression of LTP. However, blocking 2-AG synthesis with the DAG-lipase inhibitor tetrahydrolipstatin did not alter HFS-induced striatal LTD. In addition, synaptic depression produced by a synthetic CB1 agonist was similar across development. Together, these findings suggest that the robust developmental increase in striatal AEA may be the key factor in the emergence of HFS-induced striatal LTD.

Figure 1.

Synaptic plasticity in the rat DL striatum is age dependent. After obtaining a stable baseline at an intensity that resulted in a field potential recording of half the maximum amplitude for 10 min, the HFS protocol was given and responses were followed for an additional 30 min. Time courses are presented as normalized PS amplitudes. The inset provides representative traces (average of 12 individual responses) from baseline recordings just before giving the HFS protocol (a), response 30 min after giving the HFS protocol (b), and overlay (a+b). A, HFS caused potentiation of the PS amplitude in P13 animals. The average response 30 min after the HFS was 160 ± 10% SEM (n = 11) of the baseline PS amplitude. B, HFS induced a depression of the PS amplitude in slices made from animals P16–P34. The average response 30 min after the HFS was 53 ± 3% SEM (n = 9) of the baseline measure. Error bars indicate SEM.

Figure 2.

AEA but not 2-AG levels increase during postnatal development in striatal tissue. Endocannabinoids were extracted from tissue slices and analyzed using HPLC and mass spectrometry. Endocannabinoid levels are expressed as femtomoles or picomoles per milligram of wet tissue. A, AEA levels increase 1125% between P13 and P17, the two ages where LTP (P13) and LTD (P17) are consistently seen in the DL striatum. B, C, No developmental changes in striatal levels of 2-AG (B) or total AG (2-AG and 1-AG combined; C). *p < 0.01; ***p < 0.001. The n values for each age group are as follows: P12, 10; P13, 14; P14, 22; P15, 14; P16, 9, P17, 11; P22, 4; P30, 6. Error bars indicate SEM.

Figure 3.

Response to a CB1 agonist is similar in P13 and in P16–P34 animals. A, Normalized time course demonstrating depression of the PS amplitude in response to 3 μm WIN 55,212 in slices from P13 rats (○) with an average response 46 ± 7% of the baseline (p < 0.001; n = 7) and P16–P34 rats (•) with an average of 56 ± 6% of baseline (p < 0.001; n = 6) after 20 min of application. B, C, Representative traces (averages of 12 individual responses). Traces represent responses during the baseline period (a), after 20 min application of 3 μm WIN 55,212 (b), and overlay (a+b). Error bars indicate SEM.

Figure 4.

Activation of CB1 receptors during HFS induces LTD in the dorsolateral striatum of P13 rats. A, Time course showing normalized average of PS amplitudes in response to 40 μm AEA and HFS. In P13 rats, the average response 30 min after HFS in the presence of 40 μm AEA was 59 ± 6% of the baseline measure (p < 0.001; n = 5). Inset shows representative traces (average of 12 individual responses) from a typical experiment. Traces represent responses during the baseline period (a), in the presence of 40 μm AEA (b), 30 min after HFS in the presence of 40 μm AEA (c), and overlay (b+c). B, Time course showing normalized average PS amplitudes in response to a coapplication of 40 μm AEA and 3 μm SR 141716 and HFS. The average response 30 min after HFS was 139 + 13% of the baseline measure (p < 0.001; n = 4). Inset shows representative traces (average of 12 individual responses) from a typical experiment. Traces represent responses during the baseline period (a), in the presence of 40 μm AEA and 3 μm SR 141716 (b), 30 min after HFS in the presence of 40 μm AEA and 3 μm SR 141716 (c), and overlay (b+c). Error bars indicate SEM.

Figure 5.

Blocking CB1 receptors during HFS leads to LTP in the dorsolateral striatum of rats P16–P36. A, Representative traces (average of 12 individual responses) from a typical experiment. Traces represent response during the baseline period (a), in the presence of 3 μm SR 141716 (b), 30 min after HFS in the presence of 3 μm SR 141716 (c), and overlay (b+c). B, Time course demonstrating that HFS in the presence of 3 μm SR 141716 induces LTP in P16–P36 rats. The average response 30 min after HFS in the presence of 3 μm SR 141716 was 155 ± 23% of the baseline measure (p < 0.001; n = 6). Error bars indicate SEM.

Figure 6.

Striatal LTD does not depend on 2-AG synthesis. A, Time-course plot for experiments examining THL effects on striatal LTD. Note that inhibition of 2-AG synthesis does not alter HFS-induced LTD. Slices were exposed to 1 μm THL for 60–120 min before and throughout the duration of the field recordings. The average response 30 min after HFS in the presence of 1 μm THL was 60 ± 10% of the baseline measure (n = 4). The inset provides representative traces (average of 12 individual responses from a typical experiment). Traces represent response during the baseline period in the presence of 1 μm THL (a), 30 min after HFS in the presence of 1 μm THL (b), and overlay (a+b). To ensure the efficacy of the drug to inhibit 2-AG synthesis, corticostriatal slices were treated with 1 μm THL or vehicle for 1 h. The striatum was dissected out and endocannabinoids were extracted and analyzed using HPLC and mass spectrometry. B, C, One micromolar THL reduced basal levels of 2-AG to 63% of the vehicle controls (B) (**p < 0.005) but had no effect on basal levels of AEA (C). Error bars indicate SEM.