Although originally cloned from rat brain, the P2X₇ receptor has only recently been localized in neurones, and functional responses mediated by these neuronal P2X₇ receptors (P2X₇R) are largely unknown. Here we studied the effect of P2X₇R activation on the release of neurotransmitters from superfused rat hippocampal slices. ATP (1–30 mm) and other ATP analogues elicited concentration‐dependent [³H]GABA outflow, with the following rank order of potency: benzoylbenzoylATP (BzATP) > ATP > ADP. PPADS, the non‐selective P2‐receptor antagonist (3–30 µm), Brilliant blue G (1–100 nm) the P2X₇‐selective antagonist and Zn²⁺ (0.1–30 µm) inhibited, whereas lack of Mg²⁺ potentiated the response by ATP. In situ hybridization revealed that P2X₇R mRNA is expressed in the neurones of the cell body layers in the hippocampus. P2X₇R immunoreactivity was found in excitatory synaptic terminals in CA1 and CA3 region targeting the dendrites of pyramidal cells and parvalbumin labelled structures. ATP (3–30 µm) and BzATP (0.6–6 µm) elicited concentration‐dependent [¹⁴C]glutamate efflux, and blockade of the kainate receptor‐mediated transmission by CNQX (10–100 µm) and gadolinium (100 µm), decreased ATP evoked [³H]GABA efflux. The Na⁺ channel blocker TTX (1 µm), low temperature (12°C), and the GABA uptake blocker nipecotic acid (1 mm) prevented ATP‐induced [³H]GABA efflux. Brilliant blue G and PPADS also reduced electrical field stimulation‐induced [³H]GABA efflux. In conclusion, P2X₇Rs are localized to the excitatory terminals in the hippocampus, and their activation regulates the release of glutamate and GABA from themselves and from their target cells.

Despite the profound effect of cannabinoids on motor function, and their therapeutic potential in Parkinson's and Huntington's diseases, the cellular and subcellular distributions of striatal CB₁receptors are not well defined. Here, we show that CB₁receptors are primarily located on GABAergic (vesicular GABA transporter-positive) and glutamatergic [vesicular glutamate transporter-1 (VGLUT-1)- and VGLUT-2-positive] striatal nerve terminals and are present in the presynaptic active zone, in the postsynaptic density, as well as in the extrasynaptic membrane. Both the nonselective agonistWIN55212-2 [(R)-(+)-[2,3-dihydro-5-methyl-3[(4-morpholinyl)methyl] pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate salt] (EC₅₀, 32 nm) and the CB₁-selective agonist ACEA [N-(2-chloroethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide] inhibited [³H]GABA release from rat striatal slices. The effect of these agonists was prevented by the CB₁-selective antagonists SR141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] (1 μm) and AM251 [1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide trifluoroacetate salt] (1 μm), indicating that cannabinoids inhibit the release of GABA via activation of presynaptic CB₁receptors. Cannabinoids modulated glutamate release via both CB₁and non-CB₁mechanisms. Cannabinoid agonists and antagonists inhibited 25 mmK⁺-evoked [³H]glutamate release and sodium-dependent [³H]glutamate uptake. Partial involvement of CB₁receptors is suggested because low concentrations of SR141716A partly and AM251 fully prevented the effect ofWIN55212-2 and CP55940 [5-(1,1-dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]phenol]. However, the effect of CB₁agonists and antagonists persisted in CB₁knock-out mice, indicating the involvement of non-CB₁,CB₁-like receptors. In contrast, cannabinoids did not modulate [³H]dopamine release or [³H]dopamine and [³H]GABA uptake. Our results indicate distinct modulation of striatal GABAergic and glutamatergic transmission by cannabinoids and will facilitate the understanding of the role and importance of the cannabinoid system in normal and pathological motor function.

Retrograde messengers adjust the precise timing of neurotransmitter release from the presynapse, thus modulating synaptic efficacy and neuronal activity. 2-Arachidonoyl glycerol, an endocannabinoid, is one such messenger produced in the postsynapse that inhibits neurotransmitter release upon activating presynaptic CB₁ cannabinoid receptors. Cognitive decline in Alzheimer’s disease is due to synaptic failure in hippocampal neuronal networks. We hypothesized that errant retrograde 2-arachidonoyl glycerol signalling impairs synaptic neurotransmission in Alzheimer’s disease. Comparative protein profiling and quantitative morphometry showed that overall CB₁ cannabinoid receptor protein levels in the hippocampi of patients with Alzheimer’s disease remain unchanged relative to age-matched controls, and CB₁ cannabinoid receptor-positive presynapses engulf amyloid-β-containing senile plaques. Hippocampal protein concentrations for the sn-1-diacylglycerol lipase α and β isoforms, synthesizing 2-arachidonoyl glycerol, significantly increased in definite Alzheimer’s (Braak stage VI), with ectopic sn-1-diacylglycerol lipase β expression found in microglia accumulating near senile plaques and apposing CB₁ cannabinoid receptor-positive presynapses. We found that microglia, expressing two 2-arachidonoyl glycerol-degrading enzymes, serine hydrolase α/β-hydrolase domain-containing 6 and monoacylglycerol lipase, begin to surround senile plaques in probable Alzheimer’s disease (Braak stage III). However, Alzheimer’s pathology differentially impacts serine hydrolase α/β-hydrolase domain-containing 6 and monoacylglycerol lipase in hippocampal neurons: serine hydrolase α/β-hydrolase domain-containing 6 expression ceases in neurofibrillary tangle-bearing pyramidal cells. In contrast, pyramidal cells containing hyperphosphorylated tau retain monoacylglycerol lipase expression, although at levels significantly lower than in neurons lacking neurofibrillary pathology. Here, monoacylglycerol lipase accumulates in CB₁ cannabinoid receptor-positive presynapses. Subcellular fractionation revealed impaired monoacylglycerol lipase recruitment to biological membranes in post-mortem Alzheimer’s tissues, suggesting that disease progression slows the termination of 2-arachidonoyl glycerol signalling. We have experimentally confirmed that altered 2-arachidonoyl glycerol signalling could contribute to synapse silencing in Alzheimer’s disease by demonstrating significantly prolonged depolarization-induced suppression of inhibition when superfusing mouse hippocampi with amyloid-β. We propose that the temporal dynamics and cellular specificity of molecular rearrangements impairing 2-arachidonoyl glycerol availability and actions may differ from those of anandamide. Thus, enhanced endocannabinoid signalling, particularly around senile plaques, can exacerbate synaptic failure in Alzheimer’s disease.

There is considerable evidence showing that the neurodegenerative processes that lead to sporadic Parkinson's disease (PD) begin many years before the appearance of the characteristic motor symptoms. Neuropsychiatric, sensorial and cognitive deficits are recognized as early non-motor manifestations of PD, and are not attenuated by the current anti-parkinsonian therapy. Although loss-of-function mutations in the parkin gene cause early-onset familial PD, Parkin-deficient mice do not display spontaneous degeneration of the nigrostriatal pathway or enhanced vulnerability to dopaminergic neurotoxins such as 6-OHDA and MPTP. Here, we employed adult homozygous C57BL/6 mice with parkin gene deletion on exon 3 (parkin^−/−) to further investigate the relevance of Parkin in the regulation of non-motor features, namely olfactory, emotional, cognitive and hippocampal synaptic plasticity. Parkin^−/− mice displayed normal performance on behavioral tests evaluating olfaction (olfactory discrimination), anxiety (elevated plus-maze), depressive-like behavior (forced swimming and tail suspension) and motor function (rotarod, grasping strength and pole). However, parkin^−/− mice displayed a poor performance in the open field habituation, object location and modified Y-maze tasks suggestive of procedural and short-term spatial memory deficits. These behavioral impairments were accompanied by impaired hippocampal long-term potentiation (LTP). These findings indicate that the genetic deletion of parkin causes deficiencies in hippocampal synaptic plasticity, resulting in memory deficits with no major olfactory, emotional or motor impairments. Therefore, parkin^−/− mice may represent a promising animal model to study the early stages of PD and for testing new therapeutic strategies to restore learning and memory and synaptic plasticity impairments in PD.