Do endocannabinoids (eCBs) participate in long-term synaptic plasticity in the brain? Using pharmacological approaches and genetically altered mice, we show that stimulation of prelimbic cortex afferents at naturally occurring frequencies causes a longterm depression of nucleus accumbens glutamatergic synapses mediated by eCB release and presynaptic CB1 receptors. Translation of glutamate synaptic transmission into eCB retrograde signaling involved metabotropic glutamate receptors and postsynaptic intracellular Ca 2؉ stores. These findings unveil the role of the eCB system in activity-dependent long-term synaptic plasticity and identify a mechanism by which marijuana can alter synaptic functions in the endogenous brain reward system. E xogenous cannabinoids, such as the active component of Cannabis sativa L, (Ϫ)-transdelta9-tetrahydrocannabinol, as well as endocannabinoids (eCBs; anandamide and 2-arachidonoyl-glycerol) share the same target in the central nervous system: a Gi͞Go-coupled receptor named CB1 (1, 2). Activation of CB1 receptors inhibits both inhibitory and excitatory synaptic transmission in the hippocampus (3, 4), substantia nigra pars compacta (5), the cerebellum (6), the prefrontal cortex (7), and the nucleus accumbens (NAc) (8, 9). Both eCBs and CB1 have been involved in a short-lasting form of synaptic regulation: the ''depolarization-induced suppression'' of both inhibitory (10-15) and excitatory transmission (16). However, the involvement of eCB in long-lasting activity-dependent synaptic plasticity remained to be documented. The mesocorticolimbic dopaminergic system, and particularly the NAc, is essential to the reinforcing properties of addictive drugs (17, 18). Cannabinoids activate mesolimbic dopamine neurons (19) and increase NAc dopamine levels (20), similarly to other drugs of abuse. Our finding that CB1 are localized at the excitatory afferents to the NAc where exogenous cannabimimetics inhibit glutamatergic synaptic transmission (9) raised two questions: how does synaptic activity lead to the production of eCBs in the NAc, and what are the physiological correlates of eCB release on synaptic transmission? Methods Slice Preparation and Electrophysiology. Whole-cell patch-clamp and extracellular field recordings were made from medium spiny neurons in parasagittal slices of mouse NAc. These methods have been described in detail previously (9). In brief, mice (male C57BL6, 4-6 weeks) were anesthetized with isoflurane and decapitated. The brain was sliced (300-400 m) in the parasagittal plane by using a vibratome and maintained in physiological saline at 4°C. Slices containing the NAc were stored at least 1 h at room temperature before being placed in the recording chamber and superfused (2 ml͞min) with artificial cerebrospinal fluid that contained (in mM): 126 NaCl, 2.5 KCl, 1.2 MgCl 2 , 2.4 CaCl 2, 18 NaHCO 3 , 1.2 NaH 2 PO 4 , and 11 glucose, and was equilibrated with 95% O 2 ͞5% CO 2 . All experiments were done at room temperature. The superfusion medium contained picrotoxin (100 M) to...

The phase of spikes of hippocampal pyramidal cells relative to the local field oscillation shifts forward (''phase precession'') over a full cycle as the animal crosses the cell's receptive field (''place field''). The linear relationship between the phase of the spikes and the travel distance within the place field is independent of the animal's running speed. This invariance of the phase-distance relationship is likely to be important for coordinated activity of hippocampal cells and space coding, yet the mechanism responsible for it is not known. Here we show that at faster running speeds place cells are active for fewer cycles but oscillate at a higher frequency and emit more spikes per cycle. As a result, the phase shift of spikes from cycle to cycle (i.e., temporal precession slope) is faster, yet spatial-phase precession stays unchanged. Interneurons can also show transient-phase precession and contribute to the formation of coherently precessing assemblies. We hypothesize that the speed-correlated acceleration of place cell assembly oscillation is responsible for the phase-distance invariance of hippocampal place cells.cell assembly ͉ interneurons ͉ phase locking ͉ phase precession ͉ oscillations W hile animals navigate in an environment, the hippocampal local field potential (LFP) is characterized by a highly regular oscillation (8-10 Hz). Principal cells in the hippocampus show place-specific firing by two criteria. First, the firing is tuned to a particular location (''place field''), showing a bellshaped tuning curve centered around its preferred location (1). Second, the timing of spikes within subsequent cycles systematically shifts forward (''phase precession''), Ϸ1 full cycle in total, as the rat runs through the place field of the neuron (2, 3) (see also Fig. 1 A and B). Both the firing rate and discharge phase within a cycle are correlated with the rat's position. However, how the rate change and -phase precession of spikes are related is poorly understood. The available experiments support both a rate-phase interdependence (4-6) and independence (7).Several explanations for the place-phase relationship were put forward (4)(5)(6)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). To confront these models, we examined the relationship among running speed, oscillation frequency of place cells and LFP , and timing of spikes within the cycle. We show that principal cells oscillate at a frequency faster than the simultaneously recorded LFP oscillation, and that this oscillation frequency depends on the rat's running speed. Together with the place-and speed-dependent oscillation frequencies of interneurons, the findings support the hypothesis that place coding results from coordinated network activity. We propose that the locomotion speed-dependent oscillation of place cell assemblies may underlie the mechanisms responsible for distance encoding in the hippocampus. ResultsWe recorded the firing patterns of pyramidal cells, interneurons, and the LFP from the CA1 pyramidal layer of rats as they ran on a U-shap...

Cannabinoids impair hippocampus-dependent memory in both humans and animals, but the network mechanisms responsible for this effect are unknown. Here we show that the cannabinoids Delta(9)-tetrahydrocannabinol and CP55940 decreased the power of theta, gamma and ripple oscillations in the hippocampus of head-restrained and freely moving rats. These effects were blocked by a CB1 antagonist. The decrease in theta power correlated with memory impairment in a hippocampus-dependent task. By simultaneously recording from large populations of single units, we found that CP55940 severely disrupted the temporal coordination of cell assemblies in short time windows (<100 ms) yet only marginally affected population firing rates of pyramidal cells and interneurons. The decreased power of local field potential oscillations correlated with reduced temporal synchrony but not with firing rate changes. We hypothesize that reduced spike timing coordination and the associated impairment of physiological oscillations are responsible for cannabinoid-induced memory deficits.

Despite the role of excitatory transmission to the nucleus accumbens (NAc) in the actions of most drugs of abuse, the presence and functions of cannabinoid receptors (CB1) on the glutamatergic cortical afferents to the NAc have never been explored. Here, immunohistochemistry has been used to show the localization of CB1 receptors on axonal terminals making contacts with the NAc GABAergic neurons. Electrophysiological techniques in the NAc slice preparation revealed that cannabimimetics [WIN 55,212,2 (WIN-2) and CP55940] strongly inhibit stimulus-evoked glutamate-mediated transmission. The inhibitory actions of WIN-2 were dose-dependent (EC(50) of 293 +/- 13 nm) and reversed by the selective CB1 antagonist SR 141716A. In agreement with a presynaptic localization of CB1 receptors, WIN-2 increased paired-pulse facilitation, decreased miniature EPSC (mEPSC) frequency, and had no effect on the mEPSCs amplitude. Perfusion with the adenylate cyclase activator forskolin enhanced glutamatergic transmission but did not alter presynaptic CB1 actions, suggesting that cannabinoids inhibit glutamate release independently from the cAMP-PKA cascade. CB1 did not reduce evoked transmitter release by inhibiting presynaptic voltage-dependent Ca(2+) currents through N-, L-, or P/Q-type Ca(2+) channels, because CB1 inhibition persisted in the presence of omega-Conotoxin-GVIA, nimodipine, or omega-Agatoxin-IVA. The K(+) channel blockers 4-aminopyridine (100 micrometer) and BaCl(2) (300 micrometer) each reduced by 40-50% the inhibitory actions of WIN-2, and their effects were additive. These data suggest that CB1 receptors are located on the cortical afferents to the nucleus and can reduce glutamate synaptic transmission within the NAc by modulating K(+) channels activity.

The striatum is required for the acquisition of procedural memories but its contribution to motor control once learning has occurred is unclear. Here we created a task in which rats learned a difficult motor sequence characterized by fine-tuned changes in running speed adjusted to spatial and temporal constraints. After training and extensive practice, we found that the behavior was habitual yet tetrode recordings in the dorsolateral striatum (DLS) revealed continuous integrative representations of running speed, position and time. These representations were weak in naive rats hand-guided to perform the same sequence and developed slowly after learning. Finally, DLS inactivation in well-trained animals preserved the structure of the sequence while increasing its trial-by-trial variability. We conclude that after learning the DLS continuously integrates task-relevant information to constrain the execution of motor habits. Our work provides a straightforward mechanism by which the basal ganglia may contribute to habit formation and motor control.

Endogenous cannabinoids (eCB) mediate synaptic plasticity in brain regions involved in learning and reward. Here we show that in mice, a single in-vivo exposure to Delta 9-tetrahydrocannabinol (THC) abolishes the retrograde signaling that underlies eCB-mediated synaptic plasticity in both nucleus accumbens (NAc) and hippocampus in vitro. This effect is reversible within 3 days and is associated with a transient modification in the functional properties of cannabinoid receptors.

No abstract

The integrity of the hippocampus is critical for both spatial navigation and episodic memory, but how its neuronal firing patterns underlie those functions is not well understood. In particular, the modality by which hippocampal place cells contribute to spatial memory is debated. We found that administration of the cannabinoid receptor agonist CP55940 (2-[(1S,2R,5S)-5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]-5-(2-methyloctan-2-yl)phenol) induced a profound and reversible behavioral deficit in the hippocampus-dependent delayed spatial alternation task. On the one hand, despite severe memory impairment, the location-dependent firing of CA1 hippocampal place cells remained mostly intact. On the other hand, both spike-timing coordination between place cells at the theta timescale and theta phase precession of spikes were reversibly reduced. These results raise the possibility that cannabinoids impair memory primarily by altering short-term temporal dynamics of hippocampal neurons. We hypothesize that precise temporal coordination of hippocampal neurons is necessary for guiding behavior in spatial memory tasks.