Stereotypy, but not locomotion, requires CB1 receptor signaling in the NAc core
We performed a series of microinjection experiments to determine if psychomotor activation requires NAc CB1 receptor signaling. Rats received bilateral, intracranial microinfusions of vehicle, rimonabant (1 μg) or AM251 (1 μg) into either the NAc core or shell, followed by intravenous methamphetamine (3 mg/kg). Shell treatment had no effect on methamphetamine-induced locomotion (data not shown; unpaired t test, t(10) = 0.01, p > 0.05) or stereotypy (data not shown; unpaired t test, t(10) = 0.52, p > 0.05). However, drug application in the core attenuated methamphetamine-induced stereotypy (; 1 way ANOVA, F(3,23) = 12.52, p < 0.001; rimonabant vs. vehicle, Tukey’s multiple comparisons test, p < 0.05; AM251 vs. vehicle, Tukey’s multiple comparisons test, p < 0.05), but not locomotion (; 1 way ANOVA, F(3,23) = 0.14, p > 0.05). Anatomic specificity of this effect was confirmed by an additional group of rats that received rimonabant infusions into the dorsal striatum followed by methamphetamine (, core rimonabant vs. dorsal striatum rimonabant, Tukey’s multiple comparisons test, p < 0.01).
Methamphetamine heterogeneously affects NAc firing rates
Local pharmacological manipulation of the NAc was sufficient to modulate stereotypy; therefore, we investigated methamphetamine’s electrophysiological effects in the NAc. We recorded NAc multiple single-unit activity in rats given intravenous vehicle or rimonabant (0.3 mg/kg), followed by saline, followed thereafter by methamphetamine (0.01, 0.1, 1, 3 mg/kg; cumulative dosing). A subpopulation of neurons were responsive to methamphetamine (86/139, 61.9%). Units responsive to saline or expressing biphasic methamphetamine responses were excluded from further analysis. The remaining units followed a bimodal distribution, and were characterized as either methamphetamine-excited (E neurons; Gaussian fit, R2 = 0.46; normality of residuals, p < 0.001) or methamphetamine-inhibited (I neurons; Gaussian fit, R2 = 0.49; normality of residuals, p < 0.001) neurons (). No core/shell differences in E/I neuron activity were observed (data not shown; E neurons, 1 way ANOVA, F(1,25) = 0.86, p > 0.05; I neurons, 1 way ANOVA, F(1,23) = 0.20, p > 0.05). Therefore, E and I populations were pooled across brain regions for further analysis. Methamphetamine increased E neuron firing rates (1 way ANOVA, F(74,1850) = 18.00, p < 0.001) and decreased I neuron firing rates (1 way ANOVA, F(74,1850) = 21.50, p < 0.001).
Tonic NAc firing rates do not correlate with psychomotor activation
Rimonabant alone had no effect on E (1 way ANOVA, F(1,27) = 1.49, p > 0.05) or I (1 way ANOVA, F(1,23) = 3.46, p > 0.05) neuron activity. However, rimonabant attenuated methamphetamine-induced changes in I (2 way ANOVA, F(74,1702) = 1.99, p < 0.001), but not E (2 way ANOVA, F(74,1998) = 1.12, p > 0.05) tonic firing (, top). Additionally, rimonabant attenuated methamphetamine-induced stereotypy (2 way ANOVA, F(74,444) = 7.26, p < 0.001), but not locomotion (2 way ANOVA, F(74,444) = 1.06, p > 0.05) (, bottom). We therefore hypothesized that functional E neuron/locomotion and I neuron/stereotypy relationships exist. No core/shell differences in activity/behavior correlations were observed (data not shown; E neuron/locomotion, linear regression, F(3,172) = 0.30, p > 0.05; I neuron/stereotypy, linear regression, F(3,717) = 2.02, p > 0.05). Therefore, neurons were pooled across anatomic regions for further analysis. Neither E, nor I, tonic activity correlated with methamphetamine-induced locomotion (; vehicle, R2 = 0.10, p > 0.05; rimonabant, R2 = 0.03, p > 0.05), or stereotypy (; vehicle, R2 = 0.01, p > 0.05; rimonabant, R2 = 0.02, p > 0.05), respectively.
Rimonabant attenuates phasic correlates of stereotypy, but not locomotion
In contrast with tonic activity, phasic firing patterns were time-locked to the onset of locomotion and stereotypy (). Locomotion encoding was expressed uniformly across the NAc core and shell (data not shown; unpaired t test, t(23) = 0.30, p > 0.05). Responses from both brain regions were therefore pooled for further analysis. Conversely, although individual neurons in both brain regions expressed phasic stereotypy correlates, mean population activity in the core (), but not the shell (), expressed stereotypy encoding. Rimonabant attenuated phasic correlates of methamphetamine-induced stereotypy (; unpaired t test, t(15) = 2.20, p < 0.05), but not locomotion (; unpaired t test, t(23) = 0.37, p > 0.05).
FSIs preferentially encode psychomotor activation
To determine if a sub-population of neurons preferentially encode psychomotor activation, we classified units based on waveform shape and firing rate (). A disproportionate percentage of FSIs (locomotion, 41.2 %; stereotypy, 70.6 %) expressed psychomotor activation encoding, as compared to MSNs (locomotion 18.6%; stereotypy 18.6 %) (). Rimonabant did not differentially affect MSN or FSI encoding strength (data not shown; 2 way ANOVA, F(1,10) = 0.03, p > 0.05). Additionally, the majority of FSIs were found in the NAc core (58.8 %).