Activation of cannabinoid CB1 receptors depresses excitatory neurotransmission in the striatum through a pre-synaptic mechanism believed to involve reduced glutamate release (Huang et al. 2001), although such a reduction has not been demonstrated directly. We have obtained evidence from biochemical experiments that CB1 receptor activation also reduces the uptake of glutamate in the striatum. Here we have investigated the role of glutamate reuptake in the effects of Δ9-tetrahydrocannabinol (THC) on extracellular population spikes in rat striatal slices.
Coronal brain sections, 400 µm thick, were prepared from male Sprague-Dawley rats (7-9 weeks old), humanely killed by cervical dislocation. Hemislices containing the rostral striatum were used for electrophysiological recordings. Stimuli were delivered to the white matter overlying the striatum and extracellular responses recorded from the dorsolateral striatum. Effects of drugs on population spike amplitude (PSamp) are expressed as the mean ± S.E.M. percentage predrug values and analysed by one-way ANOVA followed by Tukey post hoc test.
A persistent depression in synaptic transmission was observed following treatment with 10 µM THC. Thus, PSamp was reduced to 65 ± 5 % predrug values 10-20 min after commencing THC washout (n = 7). THC did not reduce transmission when applied in the presence of AM251, a CB1 receptor antagonist (91 ± 3 % predrug; n = 5; P < 0.05 compared to THC alone). Inhibition of glutamate uptake by L-threo-hydroxyaspartate (THA; 500 µM) also reduced PSamp, but this effect was not prevented by AM251 (65 ± 4 % vs. 62 ± 10% n = 8; P > 0.05). In contrast, the actions of THA were reduced by pre-treatment with 10 µM LY341495, a broad-spectrum metabotropic glutamate receptor (mGluR) antagonist (82 ± 9% n = 7; P < 0.05 compared to THA alone). Agonists of group II and III, but not group I, mGluRs also produced synaptic depressions that could be reversed by application of LY341495.
When LY341495 was applied in the presence of THA, 10 µM THC no longer reduced PSamp in striatal slices (108 ± 18% n = 5; P < 0.05 compared to THC alone), suggesting the electrophysiological effects of this cannabinoid may involve reduced glutamate uptake. Similarly, the actions of 500 µM THA were attenuated when applied 20 min after THC application (83 ± 14% n = 5; P < 0.05 compared to THA alone). Furthermore, pre-treatment with 10 µM LY341495 prevented the THC-induced depression in PSamp (93 ± 4 % predrug; n = 5; P < 0.05 compared to THC alone).
These findings suggest that CB1 receptor-mediated depression of excitatory transmission in the striatum involves an indirect reduction in neurotransmitter release, by reducing glutamate re-uptake and consequently enhancing the activation of presynaptic inhibitory mGluRs.
This work was funded by the BBSRC.