Figure 4. Functional linkage of LHb and RMTg. We examined c-Fos in LHb neurons that were labeled by retrograde tracer injections into either the RMTg (A, site also spread to IPN) or VTA (B). Cocaine-induced c-Fos was found in 37 ± 9% of RMTg-projecting LHb neurons (C), but in a much lower percentage of VTA-projecting LHb neurons (D). Black label indicates c-Fos immunoreactivity; brown label indicates retrograde tracer immunoreactivity. Red filled arrows indicate neurons expressing both tracer and c-Fos, whereas open arrows indicate neurons expressing tracer only. Unilateral lesions of the posterior LHb and fasciculus retroflexus (FR, indicated by asterisk) (E), produce a twofold reduction in cocaine-induced c-Fos in the ipsilateral (lesioned) RMTg (G) relative to the contralateral (intact) RMTg (H), suggesting that cocaine-induced RMTg c-Fos is partly dependent on its LHb afferents. G, H Magnified from left and right boxed regions in F, where brown cytoplasmic label indicates immunoreactivity for the neuronal marker NeuN. Red filled arrows indicate neurons immunoreactive for both NeuN and c-Fos, whereas open arrows indicate cells expressing NeuN alone. Graphs show percentages of cocaine-induced double-labeled neurons in the LHb after retrograde tracer injections (I) or LHb/FR lesions (J). K, Animals show conditioned place aversion if placed into conditioning chambers 15 min after intravenous cocaine infusions, whereas placement 0 min postinfusion produces place preference, and placement 30 min postinfusion produces neither preference nor aversion. L, Activation of the RMTg by local AMPA injections produces conditioned place aversion. Scale bars, A, B, E, F, 1 mm; C, D, G, H, 50 μm. **p < 0.01, ***p < 0.001, ap < 0.05, one-tailed.