Appropriate animal models of Attention Deficit/Hyperactivity Disorder (ADHD) and drug reinforcement allow investigation of possible underlying biological bases of ADHD and its co-morbidity with cocaine addiction. Toward this end, Spontaneously Hypertensive Rats (SHR) exhibiting an ADHD phenotype were compared to Wistar-Kyoto (WKY) and Wistar (WIS) rats. Initially, 1.5 mg/kg oral methylphenidate or vehicle was administered between post-natal days 28–55 and acquisition of visual discrimination learning was examined. After discontinuing adolescent treatments, adult rats were evaluated for cocaine self-administration and dopamine transporter (DAT) function in prefrontal cortex and striatum. During adolescence, SHR showed deficits in visual discrimination relative to WKY and WIS when non-medicated. Methylphenidate improved visual discrimination only in SHR. Compared to WKY and WIS, SHR with prior methylphenidate treatment acquired cocaine self-administration fastest, identified cocaine as a highly efficacious reinforcer by displaying an upward shift in the cocaine dose-response function, and showed the greatest motivation to self-administer cocaine by exhibiting the highest progressive ratio breakpoints. In prefrontal cortex, maximal dopamine uptake (Vmax) at DAT was decreased in SHR and increased in WKY and WIS by prior methylphenidate treatment. Affinity (Km) for dopamine at DAT in prefrontal cortex was not different between strains, nor was Vmax or Km altered in striatum by prior methylphenidate treatment in any strain. Methylphenidate-induced decreases in dopamine clearance by DAT in prefrontal cortex may underlie increased cocaine self-administration in SHR. These preclinical findings suggest that caution should be exercised when methylphenidate is prescribed for first-time treatment of ADHD in adolescent patients, as cocaine addiction vulnerability may be augmented.

Abstract(−)-Lobeline (2R,6S,10S; 1a), an antagonist at nicotinic acetylcholine receptors (nAChRs), inhibits the neurochemical and behavioral effects of methamphetamine and inhibits dopamine transporter (DAT) and vesicular monoamine transporter (VMAT2) function. VMAT2 is a target for the development of treatments for methamphetamine abuse. Structural modification of lobeline affords the defunctionalized analogues meso-transdiene (MTD) and lobelane, which have high potency and selectivity for VMAT2. To establish the structure-activity relationships within this novel class of VMAT2 ligands, specific stereochemical forms of MTD, lobelane, and other structurally related analogues have been synthesized. Generally, all of these analogues had lower affinities at α4β2* and α7* nAChRs compared to lobeline, thereby increasing selectivity for VMAT2. The following structural modifications resulted in only modest changes in affinity for VMAT2, affording analogues that were less potent than the lead compound, lobelane: (1) altering the stereochemistry at the C-2 and C-6 positions of the piperidino ring, (2) varying unsaturation in the piperidino C-2 and C-6 substituents, (3) introducing unsaturation into the piperidine ring, (4) ring-opening or eliminating the piperidine ring, and (5) removing the piperidino N-methyl group. Furthermore, incorporating a quaternary ammonium group into defunctionalized lobeline molecules in the cis-series resulted in significant loss of affinity for VMAT2, whereas only a modest change in affinity was obtained in the transseries. The most potent (K i = 630 nM) and VMAT2-selective compound evaluated was the Nmethyl-2,6-cis-bis(naphthaleneethyl)piperidine analogue 28b (1-NAP-lobelane), in which the phenyl groups of lobelane were replaced with 1-naphthyl moieties. Thus, initial structure-activity relationship studies reveal that the most promising structural changes to the lobeline molecule that lead to enhancement of VMAT2 affinity and selectivity are defunctionalization, affording lobelane and MTD, and replacement of the phenyl rings of lobelane with other aromatic moieties that have a π-extended structure.

The current study evaluated a new series of N, analogs with C 6 -C 12 methylene linkers as nicotinic acetylcholine receptor (nAChR) antagonists, for nicotine-evoked [ 3 H]dopamine (DA) overflow, for blood-brain barrier choline transporter affinity, and for attenuation of discriminative stimulus and locomotor stimulant effects of nicotine. bAPi analogs exhibited little affinity for ␣4␤2* (* indicates putative nAChR subtype assignment) and ␣7* high-affinity ligand binding sites and exhibited no inhibition of DA transporter function. With the exception of C 6 , all analogs inhibited nicotine-evoked [ 3 H]DA overflow (IC 50 ϭ 2 nM-6 M; I max ϭ 54 -64%), with N, C 12 ) being most potent. bPiDDB did not inhibit electrically evoked [ 3 H]DA overflow, suggesting specific nAChR inhibitory effects and a lack of toxicity to DA neurons. Schild analysis suggested that bPiDDB interacts in an orthosteric manner at nAChRs mediating nicotine-evoked [ 3 H]DA overflow.To determine whether bPiDDB interacts with ␣-conotoxin MIIsensitive ␣6␤2-containing nAChRs, slices were exposed concomitantly to maximally effective concentrations of bPiDDB (10 nM) and ␣-conotoxin MII (1 nM). Inhibition of nicotine-evoked [ 3 H]DA overflow was not different with the combination compared with either antagonist alone, suggesting that bPiDDB interacts with ␣6␤2-containing nAChRs. C 7 , C 8 , C 10 , and C 12 analogs exhibited high affinity for the blood-brain barrier choline transporter in vivo, suggesting brain bioavailability. Although none of the analogs altered the discriminative stimulus effect of nicotine, C 8 , C 9 , C 10 , and C 12 analogs decreased nicotine-induced hyperactivity in nicotine-sensitized rats, without reducing spontaneous activity. Further development of nAChR antagonists that inhibit nicotine-evoked DA release and penetrate brain to antagonize DA-mediated locomotor stimulant effects of nicotine as novel treatments for nicotine addiction is warranted.Nicotine, the principal tobacco alkaloid, is an agonist at neuronal nicotinic acetylcholine receptor (nAChR) subtypes modulating dopamine (DA) release. Habitual tobacco smoking is maintained via rapid nicotine delivery to brain (Le Foll and Goldberg, 2006) and results from the intrinsic rewarding properties of nicotine, believed to be due to increased DA release. Classical nAChR antagonists mecamylamine and dihydro-␤-erythroidine (DH␤E) inhibit nicotine-evoked DA release and decrease the locomotor stimulant and reinforcing effects of nicotine in rats Watkins et al., 1999;Rahman et al., 2004), suggesting a role for nAChRmediated DA release in these abuse-related behavioral efThis research was supported by National Institutes of Health Grants K02 DA00399, T32 DA007304, and U19 DA017548.Potential royalty payments to L.P.D., P.A.C., and J.T.A. may occur consistent with the University of Kentucky policy.Article, publication date, and citation information can be found at

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