Alpha-conotoxin MII-sensitive nicotinic acetylcholine receptors in the nucleus accumbens shell regulate progressive ratio responding maintained by nicotine

Abstract

Beta2 subunit containing nicotinic acetylcholine receptors (beta2(*)nAChRs; asterisk ((*)) denotes assembly with other subunits) are critical for nicotine self-administration and nicotine-associated dopamine (DA) release that supports nicotine reinforcement. The alpha6 subunit assembles with beta2 on DA neurons where alpha6beta2(*)nAChRs regulate nicotine-stimulated DA release at neuron terminals. Using local infusion of alpha-conotoxin MII (alpha-CTX MII), an antagonist with selectivity for alpha6beta2(*)nAChRs, the purpose of these experiments was to determine if alpha6beta2(*)nAChRs in the nucleus accumbens (NAc) shell are required for motivation to self-administer nicotine. Long-Evans rats lever-pressed for 0.03 mg/kg, i.v., nicotine accompanied by light+tone cues (NIC) or for light+tone cues unaccompanied by nicotine (CUEonly). Following extensive training, animals were tested under a progressive ratio (PR) schedule that required an increasing number of lever presses for each nicotine infusion and/or cue delivery. Immediately before each PR session, rats received microinfusions of alpha-CTX MII (0, 1, 5, or 10 pmol per side) into the NAc shell or the overlying anterior cingulate cortex. alpha-CTX MII dose dependently decreased break points and number of infusions earned by NIC rats following infusion into the NAc shell but not the anterior cingulate cortex. Concentrations of alpha-CTX MII that were capable of attenuating nicotine self-administration did not disrupt locomotor activity. There was no effect of infusion on lever pressing in CUEonly animals and NAc infusion alpha-CTX MII did not affect locomotor activity in an open field. These data suggest that alpha6beta2(*)nAChRs in the NAc shell regulate motivational aspects of nicotine reinforcement but not nicotine-associated locomotor activation.

Figure 1

α-Conotoxin MII (α-CTX MII)-sensitive nicotinic acetylcholine receptors (nAChRs) in the nucleus accumbens (NAc) shell regulate motivation for nicotine and/or nicotine-associated cues during a progressive ratio schedule of reinforcement. (a) Nicotine-reinforced animals (NIC, n=12) showed a concentration-dependent decrease in the number of responses they gave for a single infusion of nicotine and light+tone cues (break point) as well as (b) a decrease in number of nicotine infusions/cue deliveries following infusion of α-CTX MII into the NAc shell. NAc shell infusion of α-CTX MII had no effect on either break points or number of cue deliveries for animals that received light+tone reinforcement unaccompanied by nicotine (CUEonly, n=7). Data are expressed as mean±SEM; ^*p<0.05 compared to vehicle.

Figure 2

α-Conotoxin MII (α-CTX MII)-sensitive nicotinic acetylcholine receptors (nAChRs) in the anterior cingulate cortex do not appear to regulate motivation for nicotine or associated cues. (a) Infusion of α-CTX MII into the anterior cingulate cortex had no effect on break point, the number of depressions that rats gave for a single delivery of nicotine and cues (NIC, n=7), or cues alone (CUEonly, n=4). (b) Anterior cingulate cortex infusion of α-CTX MII also had no effect on the total number of infusions and/or cues earned by NIC and CUEonly rats.

Figure 3

I¹²⁵-α-CTX MII (α-conotoxin MII) infusion into the nucleus accumbens (NAc) shell. (a, c) Representative infusions are shown by overlay of X-ray film with coronal sections from an animal that received a 1 μl infusion of 10 pmol I¹²⁵-α-CTX MII (shown in black) into the NAc shell. (b, d) Nissl-stained coronal sections showing the NAc shell and core of the samples depicted above.

Figure 4

α-Conotoxin MII (α-CTX MII) infusion into the nucleus accumbens (NAc) shell did not depress locomotor activity in an open field. Animals received s.c. injection of 0.175 mg/kg nicotine (by weight of freebase) in 0.9% saline (NIC) or saline vehicle (VEH). Sessions were preceded by intra-accumbens shell infusion of saline vehicle (VEH), 5 pmol, or 10 pmol of the α6β2^*nAChR antagonist, α-CTX MII. (a) Planned comparisons between VEH–VEH and VEH–NIC showed a significant increase in distance traveled following nicotine injection (n=8, p=0.002), indicating that the dose of nicotine used in these studies resulted in locomotor activation. Locomotor-activating effects of nicotine were not affected by antagonism of α6β2^*nAChRs in the NAc shell. (b) Intra-accumbens shell infusion of α-CTX MII also had no effect on basal locomotor activity. Data are expressed as mean±SEM; ^*p<0.05 compared with activity following vehicle infusion and vehicle injection.