Conditioned response evoked by nicotine conditioned stimulus preferentially induces c-Fos expression in medial regions of caudate-putamen

Abstract

Nicotine has both unconditioned and conditioned stimulus properties. Conditioned stimulus properties of nicotine may contribute to the tenacity of nicotine addiction. The purpose of this experiment was to use neurohistochemical analysis of rapidly developing c-Fos protein to elucidate neurobiological loci involved in the processing of nicotine as an interoceptive conditioned stimulus (CS). Rats were injected (SC) in an intermixed fashion with saline or nicotine (16 sessions of each) and placed in conditioning chambers where they were given one of the three conditions depending on group assignment: (a) nicotine paired 100% of the time with intermittent access to sucrose (nicotine-CS condition), (b) nicotine and saline each paired 50% of the time with sucrose (chamber-CS condition), or (c) no sucrose US control (CS-alone condition). Rats in the nicotine-CS condition acquired the discrimination as evidenced by goal-tracking (ie, increased dipper entries before initial sucrose delivery) only on nicotine sessions. The chamber-CS condition showed goal-tracking on all sessions; no goal-tracking was seen in the CS-alone condition. On the test day, rats in each condition were challenged with saline or nicotine and later assessed for c-Fos immunoreactivity. In concordance with previous reports, nicotine induced c-Fos expression in the majority of areas tested; however, learning-dependent expression was specific to dorsomedial and ventromedial regions of caudate-putamen (dmCPu, vmCPu). Only rats in the nicotine-CS condition, when challenged with nicotine, had higher c-Fos expression in the dmCPu and vmCPu. These results suggest that medial areas of CPu involved in excitatory conditioning with an appetitive nicotine CS.

Figure 1

Schematic representation (adapted from Paxinos and Watson, 2007) of areas sampled for c-Fos immunoreactivity assessment. All areas were located within anatomical levels represented on four coronal sections (+3.7, +1.6, −2.80, −5.80). Black squares corresponding to adjacent numbers represent approximate locations where digital images were acquired from: (1) prelimbic cortex (PrL); (2) orbitofrontal cortex (OFC); (3) Cg2 area of anterior cingulate cortex; (4) dorsomedial caudate-putamen (dmCPu); (5) dorsolateral caudate-putamen (dlCPu); (6) ventromedial caudate-putamen (vmCPu); (7) ventrolateral caudate-putamen (vlCPu); (8) nucleus accumbens core (AcbC); (9) nucleus accumbens shell (AcbSh); (10) ventral pallidum (VP); (11) CA1 area of hippocampus; (12) CA2 area of hippocampus; (13) CA3 area of hippocampus; (14) dentate gyrus (DG); (15) centromedial amygdaloid nucleus (CeM); (16) basolateral amygdaloid nucleus (BLA); (17) medial amygdaloid nucleus (MeAD); (18) ventral tegmental area (VTA); (19) substantia nigra (SNR).

Figure 2

Dipper entry rates (±SEM) from rats in (a) nicotine-CS, (b) chamber-CS, and (c) CS-alone conditions of a training phase. ^*Significant from saline session(s) (^*P<0.05, ^***P<0.001).

Figure 3

Dipper entry rates (±SEM) on the final 4-min test. ^**,***Significant from saline treatment. ^aSignificantly different from ^bnicotine-treated CS-alone control (^**P<0.01, ^***P<0.001).

Figure 4

Means (±SEM) of positively labeled c-Fos nuclei. ^*Significant difference indicated by dashed brackets (P<0.05).

Figure 5

Photomicrographs ( × 10 magnification) of c-Fos immunoreactivity in the dmCPu of rats from all groups challenged by nicotine or saline on a final test day. Black arrows are indicating positively labeled nuclei. Scale bar=100 μm.