Cannabinoids enhance subsecond dopamine release in the nucleus accumbens of awake rats

Abstract

Dopaminergic neurotransmission has been highly implicated in the reinforcing properties of many substances of abuse, including marijuana. Cannabinoids activate ventral tegmental area dopaminergic neurons, the main ascending projections of the mesocorticolimbic dopamine system, and change their spiking pattern by increasing the number of impulses in a burst and elevating the frequency of bursts. Although they also increase time-averaged striatal dopamine levels for extended periods of time, little is known about the temporal structure of this change. To elucidate this, fast-scan cyclic voltammetry was used to monitor extracellular dopamine in the nucleus accumbens of freely moving rats with subsecond timescale resolution. Intravenous administration of the central cannabinoid (CB1) receptor agonist, R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-(1-naphthalenyl) methanone mesylate, dose-dependently produced catalepsy, decreased locomotion, and reduced the amplitude of electrically evoked dopamine release while markedly increasing the frequency of detected (nonstimulated) dopamine concentration transients. The CB1 receptor antagonist [N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide] reversed and prevented all agonist-induced effects but did not show effects on dopamine release when injected alone. These data demonstrate that doses of a cannabinoid agonist known to increase burst firing produce ongoing fluctuations in extracellular dopamine on a previously unrecognized temporal scale in the nucleus accumbens.

Figure 1.

Spontaneous and electrically evoked dopamine release in the NAc monitored by cyclic voltammetry. A, The voltammetric current (encoded in color) is plotted against the applied potential (ordinate) and the acquisition time (abscissa). During electrical stimulation of the MFB (biphasic pulses, 0.4 sec duration, 60 Hz, ±120 μA; red bar), dopamine rapidly increased (green spot shows oxidative current). The dark blue area after the green spot is caused by a nondopaminergic (ionic) change. The time courses of the signals observed at different applied potentials (indicated by the dashed white lines) are shown to the right of the color plot: x, current at the peak oxidation potential (0.62 V) for dopamine; y, current at 0.21 V, a potential insufficient to oxidize dopamine but centered on the ionic change. Subtraction of y from x yields a signal that is attributable to dopamine changes; red boxes are times at which electrical stimulation began and ended; asterisk identifies the dopamine transient. B, Cyclic voltammograms extracted from this data set: Evoked DA, the cyclic voltammogram recorded at the end of the stimulation; DA transient, the cyclic voltammogram recorded at the time indicated by the asterisk on the color plot; r², the correlation coefficient for the comparison of the two voltammograms. The cyclic voltammogram characteristic of the ionic change (at the time of the second vertical dashed bar on the color plot) is shown to the right.

Figure 2.

WIN55,212–2 increases the number of dopamine concentration transients in the NAc. Cyclic voltammograms recorded in the same animal and location as in Figure 1 after administration of WIN (125 μg/kg). The trace above the color plot is the current at the potential where dopamine is oxidized, corrected for ionic changes. Asterisks indicate times at which cyclic voltammograms reveal an increase in dopamine concentration. Electrical stimulation of the MFB (biphasic pulses, 0.4 sec duration, 60 Hz, ±120 μA) occurred at the time indicated by the red bar. The cyclic voltammogram corresponding to stimulated release (red bar) in the bottom panel (solid line) exhibits a correlation coefficient of 0.93 when superimposed on the WIN-induced transient (dashed line) occurring immediately before it.

Figure 3.

Time course of change in dopamine transients after WIN55,212–2. Dopamine concentrations extracted from cyclic voltammograms as in Figure 2 but recorded over a 6 min interval. The rat was administered WIN (125 μg/kg), which induced catalepsy at the time indicated. Dopamine release was evoked with one MFB stimulation per minute (biphasic pulses, 0.4 sec duration, 60 Hz, ±120 μA; black bars). Asterisks between the evoked responses indicate peaks that were identified as dopamine by cyclic voltammetry.

Figure 4.

Lack of effect of WIN55,212–2 on dopamine uptake in freely moving rats. The descending portions of the evoked dopamine signals before and after WIN administration (125 μg/kg) are overlaid. Five evoked responses were averaged for each treatment. Data were time-shifted so that the initial dopamine concentration was 0.73 μm in each case (n = 3).

Figure 5.

WIN55,212–2 increases the amplitude of dopamine concentration transients detected in the NAc. Histogram distributions of the amplitude of dopamine transients (WIN1: 125 μg/kg; n = 9). Bin size is 50 nm.

Figure 6.

Changes in dopamine neurotransmission after WIN55,212–2 are reversed by SR141716A. Left panel, Spontaneous dopamine transients (asterisk) and dopamine release evoked by electrical stimulation of the MFB (biphasic pulses, 0.4 sec duration, 60 Hz, ±120 μA; black bar). Center panel, Dopamine changes monitored in the 1 min interval after administration of WIN (125 μg/kg). Note that electrically evoked release is reduced by ∼50%. The right trace shows the reversal of these effects by SR (300 μg/kg).

Figure 7.

Mean responses of dopamine to a cannabinoid agonist and antagonist. A, The maximal dopamine concentration ([DA]_max) elicited by electrical stimulation of the MFB is decreased after the first injection of WIN (125 μg/kg; WIN1) but not further decreased by a larger dose (250 μg/kg; WIN2). This effect is reversed and prevented by SR (300 μg/kg; SR; ^&p < 0.05 compared with WIN2), and a subsequent injection of WIN (250 μg/kg; WIN2) does not affect this (**p < 0.005 compared with vehicle; n = 9 rats). B, The frequency of NAc dopamine concentration transients (n = 9 rats) induced by an initial injection of WIN (125 μg/kg; WIN1) and a subsequent one (250 μg/kg; WIN2). SR (300 μg/kg; SR) reversed the effects of WIN and blocked further effects of WIN (250 μg/kg; WIN2). (***p < 0.001 compared with pre-drug). C, Administration of SR (300 μg/kg; SR) does not affect the frequency of dopamine transients (n = 4 rats) but prevents changes by WIN (250 μg/kg; WIN2). In each panel, vehicle consists of a 1:1:18 ethanol/emulphor/saline (0.9%) ratio. Bolus doses were each spaced by 15 min.

Figure 8.

Dose-dependent effects of WIN55,212–2 on dopamine release and behavior. For all panels vehicle = [1:1:18, ethanol/emulphor/saline (0.9%) ratio], 25 = 25 μg/kg, 50 = 50 μg/kg, and 75 = 75 μg/kg (n = 4 rats). Intravenous bolus doses were spaced by 15 min. A, Frequency of dopamine concentration transients in the NAc during a cumulative dosing regimen (*p < 0.05). B, The maximal dopamine concentration ([DA]_max) elicited by electrical stimulation of the MFB. The response tends to decrease with the cumulative dosing regimen but was not significant at any dose. C, Locomotor activity after treatment with WIN. Black bars represent WIN-treated animals, whereas time-matched, saline controls are depicted with white bars. (***p < 0.001 compared with vehicle;^&&&p < 0.001 compared with time-matched saline control;^&&p < 0.005 compared with time-matched saline control). D, Administration of WIN dose-dependently increases catalepsy as measured using the bar test (*p < 0.05, ***p < 0.001 compared with vehicle;^&&&p < 0.001 compared with time-matched saline control).