NICOTINE AND RESPONDING MAINTAINED BY CONDITIONED REINFORCERS: EFFECTS OF TWO NICOTINIC ANTAGONISTS

Transcription

1 NICOTINE AND RESPONDING MAINTAINED BY CONDITIONED REINFORCERS: EFFECTS OF TWO NICOTINIC ANTAGONISTS By JEB JONES A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 9 1

2 9 Jeb Jones 2

3 To my parents without whom I never would have made it this far and to Jeffrey for his endless support 3

4 ACKNOWLEDGMENTS I thank my parents for always pushing me and supporting me and encouraging me to work harder and persevere. I thank Jeffrey for his constant support and encouragement. I also thank Jesse Dallery who has been a fantastic and supportive mentor the best advisor I could have asked for. Finally, I would like to thank my colleagues who assisted in conducting the experiment and who reviewed drafts of the current paper: Bethany Raiff, Steve Meredith, Alana Rojewski, Rachel Cassidy, Matthew Weaver, and Matthew Capriotti. 4

5 TABLE OF CONTENTS page ACKNOWLEDGMENTS... 4 LIST OF TABLES... 6 LIST OF FIGURES... 7 ABSTRACT... 8 CHAPTER 1 INTRODUCTION METHOD Subjects Apparatus and Materials Procedure Drug Administration Data Analyses RESULTS Effects of Nicotine Effects of nachr Antagonists Mecamylamine Hexamethonium DISCUSSION LIST OF REFERENCES BIOGRAPHICAL SKETCH

6 LIST OF TABLES Table page 2-1 Dose order by group Rates (± standard deviation) for each dose combination

7 LIST OF FIGURES Figure page 3-1 Effects of nicotine Group graphs of mecamylamine and hexamethonium Individual graphs of mecamylamine: Observing responses Individual graphs of mecamylamine: Food responses Individual graphs of mecamylamine: Extinction responses

8 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science NICOTINE AND RESPONDING MAINTAINED BY CONDITIONED REINFORCERS: EFFECTS OF TWO NICOTINIC ANTAGONISTS Chair: Jesse Dallery Major: Psychology By Jeb Jones December 9 Nicotine has been shown to selectively increase responding maintained by conditioned reinforcers as compared to responding maintained by primary reinforcers. In the current study an observing response procedure was employed in order to test the effects of nicotine and two nicotinic antagonists on responding maintained by conditioned and primary reinforcers and extinction responses. Mecamylamine, a central and peripheral nicotinic antagonist, and hexamethonium, a peripheral nicotinic antagonist, were used in order to localize the effect in the nervous system. Nicotine selectively increased responding maintained by conditioned reinforcers and mecamylamine, but not hexamethonium, attenuated this effect. These results suggest that the reinforcer enhancing effect is mediated exclusively in the central nervous system. 8

9 CHAPTER 1 INTRODUCTION Cigarette smoking provides numerous opportunities for establishing conditioned reinforcers from the olfactory, gustatory, and tactile stimulation directly associated with nicotine administration to the environmental contexts and activities associated with smoking over time. Nicotine, the putative unconditioned stimulus, not only plays a role in establishing these stimuli as conditioned reinforcers, it may also enhance responding maintained by them (e.g., Chaudhri, Caggiula, Donny, Booth et al., 6; Chaudhri, Caggiula, Donny, Palmatier et al., 6; Donny et al., 3). Indeed, enhanced responding for conditioned reinforcers following nicotine administration may be partially responsible for the maintenance of cigarette smoking (Caggiula et al., 1; Rose, 6) Understanding why people smoke requires a systematic and thorough analysis of nicotine s effects on responding maintained by conditioned reinforcers (Clarke, 1991). In the current study we examined the effects of nicotine and two nicotinic acetylcholine receptor (nachr) antagonists on responses maintained by conditioned reinforcers. Experiments with humans have provided some evidence for the role of conditioned reinforcement in the maintenance of smoking. One method of examining the conditioned reinforcers present in smoking is by using de-nicotinized cigarettes, which provide similar stimuli involved in smoking except for nicotine (Rose, 6). Shahan et al. (1999), for example, found similar break points for subjects responding for nicotinized and de-nicotinized cigarettes on a progressive ratio schedule. Indeed, levels of consumption were similar across both types of cigarettes at each unit price, indicating that subjects worked just as hard to consume both types of cigarettes. 9

10 In a similar study, Donny, Houtsmuller, and Stitzer (7) demonstrated prolonged maintenance of smoking during eleven days in a residential setting in subjects smoking both de-nicotinized and normal cigarettes. Furthermore, those smoking de-nicotinized cigarettes continued to report suppression of craving throughout the study. This finding is consistent with the notion that smoking generates conditioned reinforcers which contribute to the reinforcing efficacy of cigarettes apart from nicotine s primary reinforcing effects. This stipulation is not without its caveats, however. Eleven days of explicit unpairing of nicotine and stimuli associated with smoking ought to be sufficient time to extinguish the purported conditioned reinforcers. Thus, it is possible that some of the stimuli experienced while smoking de-nicotinized cigarettes were actually serving as primary reinforcers, which may partly explain why extinction of the conditioned reinforcers was not observed in the Donny et al. study. Nevertheless, the results of these two studies, and others (e.g., Perkins et al., 1; Shahan, Bickel, Badger, & Giordano, 1), suggest that conditioned reinforcers play a role in the maintenance of smoking. Evidence of the role of conditioned reinforcers in nicotine self-administration (SA) has also been found in the animal laboratory (Caggiula et al., 1). Presentation of nicotine-paired visual stimuli (e.g., combination of cue lights and house lights) results in increased responding for nicotine (Donny et al., 3). Further, it has been shown that the presentation of nicotine-paired stimuli results in increased SA responding throughout extinction (Donny et al., 1999). Rats first acquired nicotine SA on a progressive ratio schedule, and then the SA response was placed on extinction either by pretreatment with mecamylamine or substituting saline for the nicotine. During this 1

11 exctinction phase subjects responded at higher rates when nicotine-paired stimuli were present versus when they were not present. Nicotine also increases responding for the stimuli with which it has been paired, and this has been shown to be a dose-dependent effect, with greater effects at higher doses (Palmatier et al., 8). Palmatier et al. paired a neutral stimulus with nicotine infusions and subsequently made presentation of the paired stimulus contingent on a novel response. Rats that had received higher doses of nicotine emitted the novel response at higher rates than those that received lower doses. One problem in nicotine SA experiments is separating the effects of nicotine SA from the behavioral effects of nicotine. That is, it is difficult to differentiate these effects because nicotine administration results in direct effects on behavior and, additionally, participates in conditioning novel stimuli as reinforcers. Thus, Raiff and Dallery (6) employed an observing response procedure (Wyckoff, 1952) to assess the effects of presession nicotine on responding for primary and conditioned reinforcers and responding during periods of extinction. In an observing response procedure, a rat can respond on either of two levers. One lever (i.e., the food-extinction lever) provides alternating access to a primary reinforcer or extinction. The other lever (i.e., observing lever) provides access to a stimulus which indicates whether the primary reinforcer is available or whether extinction is in effect. The stimulus displayed contingent on responses on the observing lever has either been explicitly paired with primary reinforcer availability (S+; e.g., solid houselight) or extinction (S-; e.g., blinking houselight). 11

12 The observing response procedure has been used to test for conditioned reinforcement (e.g., Dinsmoor, 1985), and offers advantages over other procedures (e.g., the conditioning-a-new-response procedure) in that it allows testing for conditioned reinforcement over long durations because the conditioned reinforcer does not undergo extinction, as in conditioning a new response procedures. The observing response procedure allowed Raiff & Dallery to demonstrate that nicotine increases responding for conditioned reinforcers but not responding for primary reinforcers or responding during periods of extinction. Note that in the observing response procedure, a period of extinction simply means that food reinforcers are not available, it does not mean that the S+ is undergoing extinction. The S+ is always associated with the availability of food. The current study expanded on the results of Raiff and Dallery in order to determine the effects of mecamylamine and hexamethonium, two nachr antagonists, on responding for conditioned reinforcers. Mecamylamine and hexamethonium have different sites of action in the nervous system. Mecamylamine is a centrally and peripherally active nicotinic antagonist whereas hexamethonium is only active peripherally (Goodman, Gilman, Brunton, Lazo, & Parker, 6). Thus, differential effects of these two antagonists on responding for conditioned reinforcers can help to delineate the neurophysiological locus of nicotine s effects on such responses. Mecamylamine, along with other nachr antagonists (e.g., dihydro-β-erythroidine) has been used to demonstrate that many of nicotine s behavioral effects are centrally mediated. Liu et al. (7) demonstrated that central nicotinic antagonists result in decreased responding for a moderately reinforcing stimulus. Further, nicotine blocks the 12

13 reinstatement of responding for nicotine-paired stimuli following extinction (Liu et al., 6). Nicotinic antagonists also result in decreased SA. DeNoble & Mele (6) found that presession injections of mecamylamine resulted in decreased nicotine SA in rats, whereas hexamethonium did not have an effect. Pre-cigarette administration of mecamylamine in humans results in self-reported increases in craving and negative affect (McClernon & Rose, 5) and reduced satisfaction of smoking and oral stimulation (Rose et al., 1994). The effect of nicotinic antagonists on nicotine-induced increases in responding maintained by conditioned reinforcers has not been examined using the observing response procedure. In the current study we employed the observing response procedure and administered mecamylamine and hexamethonium in conjunction with.3 mg/kg nicotine. First, we replicated previous studies by demonstrating that nicotine induced a selective increase in responding for conditioned reinforcers, but not for primary reinforcers or extinction responses (Donny et al., 3; Raiff & Dallery, 6, 8). Second, we tested the effects of mecamylamine and hexamethonium on these three responses. If the increase in responding for conditioned reinforcers is centrally mediated then mecamylamine, but not hexamethonium, should result in a decrease in responding for conditioned reinforcers. 13

14 CHAPTER 2 METHOD Subjects Subjects were eight experimentally naive male Long-Evans rats (Harlan; Indianapolis, IN). The rats were approximately 15 days old at the beginning of the experiment. They were individually housed in hanging polycarbonate cages with bedding, in a room that was temperature and humidity controlled. Subjects had free access to water and were maintained at approximately 85% of their 15 day old ad libitum weights, via post-session feeding (Lab Diet Rodent Diet, Formula 51; PMI Nutrition International, LLC; Brentwood, MO). The colony room was on a 12:12 hr light dark cycle (lights on from 8am-8 pm). Apparatus and Materials Four Med Associates extra tall operant conditioning chambers (Model ENV 7; 3.48 cm L x cm W x cm H) were used to conduct experimental sessions. Chambers were contained in large sound attenuating boxes equipped with fans for ventilation and to mask experimental noise. Each intelligence panel contained a food receptacle (5 cm x 5 cm x 3 cm) that was equidistant between two levers (requiring approximately.31 N force), each of which measured 4.5 cm x 2 cm and were located 22 cm from the chamber ceiling. Seven cm above each lever were three light-emitting diodes (LED; red, yellow, green;.8 cm in diameter,.7 cm apart from each other). On the wall parallel to the intelligence panel was a house light (28 volt), centered left to right and 1.5 cm from the ceiling. Purified Rodent Tablets (45 mg sucrose food pellets; TestDiet, Richmand, IN) were located outside of the chamber, but inside the sound 14

15 attenuating box. Experimental events and data collection took place on a computer in the same room, using Med-PC software and hardware (MED Associates). Drugs. Nicotine ([-]-Nicotine Hydrogen Tartrate Salt; Sigma, St. Louis, MO) was dissolved in potassium phosphate for all subcutaneous injections. Doses of.3 mg/kg were injected immediately prior to session. Nicotine vehicle injections consisted of potassium phosphate in isolation. Mecamylamine and hexamethonium were dissolved in saline. Doses of 1 and 5 mg/kg of mecamylamine hydrochloride and doses of 1 and 5 mg/kg of hexamethonium bromide (Sigma, St. Louis, MO) were injected subcutaneously 2 min before nicotine injections (Araki et al., 4; Ise, Narita, Nagase, & Suzuki, ; Olausson, Jentsch, & Taylor, 4a; Palmatier, Peterson, Wilkinson, & Bevins, 4). Mecamylamine and hexamethonium vehicle injections consisted of saline in isolation (1mL/kg). Procedure All sessions were conducted on separate days, seven days per week, at approximately the same time each day. Lever training. Research assistants used 45 mg sucrose pellets to reinforce successive approximations of the rats pressing the right and left levers. These initial lever training sessions ended after 3 min or after 2 responses on each lever. One response resulted in a food pellet, with the exception that after three consecutive responses on the same lever food could only be earned by pressing the other lever. Lever training continued for up to three additional sessions, as needed. Once lever pressing was acquired, subjects continued to earn food for pressing levers but were required to strictly alternate between the two levers such that the second consecutive response on one lever did not result in food. These alternation sessions lasted for a 15

16 maximum of 3 min or until 3 responses had been made on each lever, on seven separate days. The houselight was continuously illuminated during all lever training sessions. Discrimination training. After subjects were trained to press both levers, the Discrimination Training condition began. During the last minute of the blackout period, all three LEDs above each lever were illuminated to signal the beginning of the session. The LEDs turned off after 1 min and the houselight was simultaneously illuminated, either blinking or continuous (the first component type was determined randomly at the beginning of each session). This LED signal was in effect for the remainder of the experiment. Components alternated between a continuously illuminated houselight (S+) which signaled periods when food was available for pressing the left, food-extinction, lever (i.e., food components) and a blinking houselight (S-;.2 sec light-dark alternation) which signaled periods when food was not available for pressing the foodextinction lever (i.e., extinction components). Initially, a variable-interval (VI) 15 sec schedule was in effect on the food-extinction lever when the houselight was continuously illuminated. After seven sessions, this value was increased to a VI 2 sec schedule of food delivery. VI distributions were based on the Fleshler-Hoffman distribution (Fleshler & Hoffman, 1962) and were composed of 15 interval values. Components lasted an average of 6 sec (rectangular distribution ranging from 1 to 11 sec); however, if the extinction component was scheduled to change to a food component, it would not change until 5 sec elapsed without a response on either lever (i.e., differential reinforcement of other behavior [DRO] 5 sec). The DRO procedure was implemented to prevent adventitious pairings between responding during extinction 16

17 components and subsequent transitions to food components. Aside from the DRO contingency, responses on the right, observing, lever did not have any additional programmed consequences during Discrimination Training. Performance during the Discrimination Training phase was evaluated using the discrimination index (DI) equal to the rate of responding in the S+ divided by the combined rate of responding in the S+ and S-. Discrimination Training lasted a minimum of 65 days and until all but one subject displayed a DI of.75 or higher. Observing response procedure. The DRO contingency was discontinued when the observing response procedure began. At the beginning of the session, the computer randomly determined whether a VI 2 sec food or extinction component would be in effect; however, the stimulus corresponding to the selected component was only shown contingent on a response to the right, observing lever. Initially, only one response on the observing lever was required to illuminate the schedule correlated stimulus for 1 sec (i.e., fixed-ratio [FR] 1). If a component was scheduled to end during the 1 sec stimulus presentation, the component continued until the stimulus turned off. Immediately after the stimulus turned off the schedule changed. For example, if after 5 sec of S+ stimulus presentation the food component was scheduled to switch to extinction, the S+ and food schedule would remain in effect for the final 5 sec of the stimulus presentation and the component immediately changed to extinction when the stimulus turned off. For the first five sessions of the observing procedure, the first five observing responses resulted in the S+ stimulus being presented. If the extinction component was in effect when one of the first five responses occurred, the component switched to VI 2 sec food (Shahan, 17

18 2). Otherwise, components alternated every 6 sec on average as described for Discrimination Training. The presence or absence of schedule-correlated stimuli did not affect the probability of food deliveries during VI components. In other words, if the VI 2 food timer elapsed, the next response on the food-extinction lever during the VI component resulted in a food delivery, regardless of whether the schedule-correlated stimuli were on or off. Furthermore, the presence of schedule-correlated stimuli during the VI component did not ensure that the next response on the food-extinction lever would result in food this only occurred if the VI timer had elapsed. If the VI timer elapsed, but a food-extinction response did not occur during the VI component, the arranged food delivery was saved for the next VI component. After 11 sessions under the conditions described above, the VI 2 sec food schedule was increased to a VI 3 sec food schedule. Ten sessions later the observing response requirement was increased from FR1 to VI 5 sec. Thus, the terminal parameters of the observing response procedure consisted of a VI 3 sec food schedule alternating with extinction approximately every 6 sec, and stimuli were presented for 1 sec on a VI 5 sec schedule. All sessions from this point forward were arranged according to these terminal parameters and were 3 min in duration. Drug Administration Establishing increases with nicotine. Drug administration began after a minimum of 65 sessions on the observing response procedure. The first 14 drug sessions were the antagonist vehicle (i.e., saline) 2 minutes presession and.3 mg/kg nicotine immediately presession. These doses were administered in order to demonstrate that nicotine reliably and selectively increased rates of the observing 18

19 response. Injections were administered every other day and occasionally every fourth day. Sessions were always conducted during the intervening days. Antagonist administration. Drug administration continued to occur every other day. As stated above injections of antagonist occurred 2 minutes prior to session and injections of nicotine occurred immediately prior to the start of the session. The subjects were randomly divided into two groups and received a counterbalanced order of antagonist drug. Group 1 received mecamylamine first and Group 2 received hexamethonium first. The order of doses is shown in Table 2-1. The antagonists were given in descending order of dose (i.e., 5. then 1. mg/kg). The complete series of probe injections was administered two times. Data Analyses Paired samples t-tests were conducted to evaluate the effects of nicotine on observing, food, and extinction responses as compared to vehicle. Repeated measures analysis of variance, with Huynh-Feldt correction, were performed to evaluate whether there were significant differences across the different dose combinations for each type of response (i.e., observing, food, and extinction), as well as to test for effects based on order of administration of the antagonists. Bonferroni post-hoc analyses were performed to make direct comparisons of the effects of each dose on each response type. Alpha was set to.5 for all analyses. 19

20 Table 2-1. Dose order by group Group 1 Group 2 2 min presession Immediately presession 2 min presession Immediately presession Saline.3 Nicotine Saline.3 Nicotine Saline KPO 4 Saline KPO 4 Mecamylamine 5..3 Nicotine Hexamethonium 5..3 Nicotine Mecamylamine 1..3 Nicotine Hexamethonium 1..3 Nicotine Hexamethonium 5..3 Nicotine Mecamylamine 5..3 Nicotine Hexamethonium 1..3 Nicotine Mecamylamine 1..3 Nicotine Mecamylamine 5. KPO 4 Hexamethonium 5. KPO 4 Mecamylamine 1. KPO 4 Hexamethonium 1. KPO 4 Hexamethonium 5. KPO 4 Mecamylamine 5. KPO 4 Hexamethonium 1. KPO 4 Mecamylamine 1. KPO 4 Order of probe doses presented to both groups. Probes were administered every other day. This order of doses was presented twice. 2

21 CHAPTER 3 RESULTS Rates of responding for each of the three response types (observing, food, and extinction) are presented in Table 3-1 for each dose combination. All statistical analyses below were calculated using the percentage of vehicle responding for the relevant conditions. Effects of Nicotine Prior to administering the nachr antagonists, we examined the effects of nicotine alone on observing, food, and extinction responses. Figure 3-1 shows observing, food, and extinction responses plotted as percent of vehicle following vehicle and nicotine injections. Paired samples t-tests showed that nicotine selectively increased observing responses (t(15) = , p <.1), but not food or extinction responses compared to vehicle. Effects of nachr Antagonists There were no main effects of order of antagonist administration on observing or food responses. There was, however, a main effect of order on extinction responses, F(1, 14) = , p <.1. Extinction responding was consistently lower in the group that received mecamylamine first. Mecamylamine Figure 3-2 (left panels) shows the aggregated effects of both doses of mecamylamine on observing, food, and extinction responses when administered with vehicle and with nicotine. Mecamylamine resulted in a statistically significant reduction of observing responses when administered with vehicle, F(2, 3) = 23.61, p <.1, and nicotine, F(2, 3) = 36.5, p <.1. Mecamylamine also resulted in statistically 21

22 significant reductions in food responses when administered with vehicle, F(2, 3) = 2.51, p <.1, and nicotine, F(2, 3) = 19.59, p <.1. Mecamylamine did not have a statistically significant effect on extinction responses. Graphs of the individual subject data are presented for observing, food, and extinction responses in Figures 3-3, 3-4, and 3-5, respectively. Individual data were generally representative of the observed group effects. Post hoc analyses revealed that only the 5. mg/kg dose of mecamylamine, and not the 1. mg/kg dose, reduced observing and food responses when administered with vehicle, suggesting a general rate reducing effect of this higher dose. When administered with nicotine, 5. mg/kg mecamylamine reduced both observing and food responses. However, only the 1. mg/kg dose resulted in a reduction in nicotineinduced increases of observing responses. This lower dose had no effect on observing or food responses when administered with vehicle, nor did it have an effect on food responses when paired with nicotine. Effects of mecamylamine on both observing and food responses were dose dependent. Hexamethonium Figure 3-2 (right panels) shows the effects of both doses of hexamethonium on observing, food, and extinction responses when administered with vehicle and with nicotine. Hexamethonium did not result in statistically significant changes in any of the three different response types. Individual data were generally representative of the observed group effects. 22

23 Table 3-1. Rates (± standard deviation) for each dose combination Nicotine - Immediately presession Vehicle (KPO 4 ).3 Nicotine Vehicle (KPO 4 ).3 Nicotine Vehicle (KPO 4 ).3 Nicotine Vehicle (Saline) Mecamylamine 5. Nicotinic Antagonist 2 min presession Mecamylamine 1. Observing Responses Hexamethonium 5. Hexamethonium Food Responses Extinction Responses

24 Observing Responses Food Responses EXT Responses Percent of Vehicle Saline-KPO4 Saline-.3mg/kg Nicotine Saline-KPO4 Saline-.3mg/kg Nicotine Saline-KPO4 Saline-.3mg/kg Nicotine Figure 3-1. Effects of nicotine versus vehicle on observing (first panel), food (second panel), and extinction (third panel) responses. Error bars represent standard deviations. 24

25 Observing Responses Observing Responses Percent of Vehicle Responding 4 3 Food Responses Extinction Responses 4 3 Food Responses Extinction Responses Saline - KPO4 5. Mec - KPO4 1. Mec - KPO4 Saline -.3 Nic 5. Mec -.3 Nic 1. Mec -.3 Nic Saline - KPO4 5. Hex - KPO4 1. Hex - KPO4 Saline -.3 Nic 5. Hex -.3 Nic 1. Hex -.3 Nic Figure 3-2. Effects of mecamylamine (left panels) and hexamethonium (right panels) on observing (top panels), food (middle panels), and extinction responses (bottom panels) as percent of vehicle responding. Bars represent group means and closed circles are individual data points. 25

26 Observing Responses Percent of Vehicle Responding Sal - KPO4 5. Mec - KPO4 1. Mec - KPO4 Sal - Nic 5. Mec - Nic 1. Mec - Nic Sal - KPO4 5. Mec - KPO4 1. Mec - KPO4 Sal - Nic 5. Mec - Nic 1. Mec - Nic Figure 3-3. Effects of both doses of mecamylamine on observing responses for each subject plotted as percent of vehicle responding. Error bars represent the range. 26

27 Food Responses Percent of Vehicle Responding Sal - KPO4 5. Mec - KPO4 1. Mec - KPO4 Sal - Nic 5. Mec - Nic 1. Mec - Nic Sal - KPO4 5. Mec - KPO4 1. Mec - KPO4 Sal - Nic 5. Mec - Nic 1. Mec - Nic Figure 3-4. Effects of both doses of mecamylamine on food responses for each subject plotted as percent of vehicle responding. Error bars represent the range. 27

28 Extinction Responses Percent of Vehicle Responding Sal - KPO4 5. Mec - KPO4 1. Mec - KPO4 Sal - Nic 5. Mec - Nic 1. Mec - Nic Sal - KPO4 5. Mec - KPO4 1. Mec - KPO4 Sal - Nic 5. Mec - Nic 1. Mec - Nic Figure 3-5. Effects of both doses of mecamylamine on extinction responses for each subject plotted as percent of vehicle responding. Error bars represent the range. 28

29 CHAPTER 4 DISCUSSION The results of the current study add to the growing corpus of data suggesting that nicotine has a selective, enhancing effect on responding maintained by conditioned reinforcers. We demonstrated 1) that nicotine selectively increased responding maintained by conditioned reinforcers, and 2) that mecamylamine, but not hexamethonium, antagonized this effect. Because mecamylamine readily crosses the blood-brain barrier and hexamethonium does not, these results suggest that nicotine s effect on responding maintained by conditioned reinforcers is mediated via nachrs in the central nervous system. Further, by using the observing response procedure we were able to conduct a rigorous, extended test of conditioned reinforcement. This procedure also allowed us to compare responding maintained by conditioned reinforcers to responding maintained by primary reinforcers and responding in extinction. It has previously been demonstrated that the S+ is responsible for maintaining responding for the observing stimulus (Dinsmoor, Browne, & Lawrence, 1972). When presented in isolation, Dinsmoor et al. found that only the S+, and not the S-, maintained levels of responding similar to when the two stimuli were both available. Thus, it is not information per se that maintains responding on the observing lever, rather it is the conditioned reinforcer aspects of the stimulus that has in the past been paired with primary reinforcer delivery. Indeed responding is higher when only an S+ is available as compared to when an S+ and S- are available, as in the current study. It has been suggested that the increases in responding observed following nicotine administration are due to a general behavioral activation as opposed to a 29

30 reinforcer-enhancing effect (Frenk & Dar, 4). However, it should be noted that the behavioral activation explanation does not account for the selectivity of response increases in the current study and others (e.g., Raiff & Dallery, 8). That is, a general behavioral activation should increase responding across the board; however we demonstrated that nicotine selectively increases responding maintained by conditioned reinforcers, but not responding maintained by primary reinforcers or responses under extinction. The effects engendered by nicotine might be characterized as rate-dependent, particularly the food responses. In a comparison of rates following administration of vehicle versus nicotine, Pearson s r for observing, food, and extinction responses was -.65, -.84, and -.69, respectively. Thus, particularly for food responses the effects appear to be correlated with the rate of vehicle responding. It should be noted, however, that rate-dependency is merely descriptive of the relationship between vehicle and drug responding and does not account for why the relationship exists (Poling & Byrne, ). Specifically, it does not account for the disparity in increases across response types nor for the effects of the nachr anagonists. Nicotinic interaction with nachrs results in wide ranging effects on nicotine SA and other responses influenced by nicotine across species. Mecamylamine attenuates nicotine SA (DeNoble & Mele, 6; Donny et al., 1999) or increases preference for higher doses of nicotine (Glick, Visker, & Maisonnneuve, 1996) in rats. Mecamylamine also blocks reinstatement of nicotine SA brought on by nicotine-paired stimuli (Liu et al., 6; Liu, Caggiula et al., 7). It has also been noted that the β2 subunit of nachrs plays a vital role in the reinforcing properties of nicotine (Wannacott, Sidhpura, & 3

31 Balfour, 5). Because it is known to be active on these nachr subtypes (Papke, Sanberg, & Shytle, 1), some of mecamylamine s behavioral effects might likely be mediated through β2 receptor subtypes. In humans, contrary to its effect on rats, mecamylamine results in increased nicotine SA, whether via smoking (Rose & Corrigall, 1997) or intravenous injection (Rose, Behm, Westman, & Bates, 3). Rose and Corrigall suggest that the disparate effects between rats and humans might be due to the fact that human study participants are nicotine dependent but rats are not. Thus humans respond more for nicotine following mecamylamine administration in order to overcome the nachr antagonizing effect of the drug. Even still, mecamylamine does result in reduced smoking satisfaction (Rose et al., 1994). Nicotine also effects behavior in ways which may be indirectly related to SA. It has been shown to have a reinforcer-enhancing effect in rats (Chaudhri, Caggiula, Donny, Palmatier et al., 6; Olausson et al., 4a; Olausson, Jentsch, & Taylor, 4b; Raiff & Dallery, 8). Olausson et al. demonstrated that nicotine selectively increases responding for conditioned reinforcers and that mecamylamine, a nachr antagonist, offsets this effect. Nicotine did not, however, increase responding for other stimuli suggesting that the increase in responding was dependent on the conditioned reinforcing aspects of the stimulus and was not due to a general, undifferentiated increase in responding. Additionally, this effect is present whether nicotine delivery is response-dependent or response-independent (Chaudhri, Caggiula, Donny, Booth et al., 6). 31

32 Activation of nachrs in the central nervous system results in widespread neural effects involving many neurotransmitter systems including dopamine, glutamate, and γ- aminobutyric acid. Indeed, complex interactions over time result in different effects on dopaminergic systems in acute versus chronic dosing (Laviolette & van der Kooy, 4). For this reason it would be beneficial to replicate the current study under a chronic dosing regimen to determine if the results are disparate with the current data. However, it is important to note that Laviolette and van der Kooy report that mesolimbic dopamine signaling might serve to signal stimuli which predict reward and that this effect might be similar in both acute and chronic dosing. Activation of dopaminergic tracts in the nucleus accumbens is also widely implicated in as one factor influencing reward, and thus abuse liability, of drugs of abuse (Di Chiara et al., 4). Indeed, nicotine elicits a similar increase in dopamine release in the nucleus accumbens as cocaine and amphetamine, albeit to a lesser extent. Furthermore, it has been noted that the two subunits of the nucleus accumbens, the core and the shell, have different behavioral effects (Balfour, 4). It is beyond the scope of the current study to tease apart the effects of these antagonists on accumbal dopamine expression, particularly due to mecamylamine s lack of specificity. However, it would be beneficial for future studies to examine the role of more specific nicotinic antagonists on sensitivity to conditioned reinforcers, as well as to nicotine in acquisition and maintenance of nicotine self-administration. Balfour (4) has also argued for a specific role of extracellular dopamine in the medial shell of the nucleus accumbens. His hypothesis is that behavior that is accompanied by an increase in extracellular dopamine is reinforced. Thus, any and all 32

33 behavior occurring in conjunction with nicotine-induced increases in extracellular nicotine should increase in rate. However, as the current study indicates, it is not all behavior that is increased, but rather only behavior maintained by specific types of consequences, namely conditioned reinforcers. The current study suffered from a few limitations. In order to make comparisons across subjects, each subject received the same dose of nicotine (.3 mg/kg). Although this dose has frequently been the most behaviorally active in previous experiments in our lab, testing for the dose of nicotine that resulted in the greatest increases in responding maintained by conditioned reinforcers might have resulted in clearer effects of the two different antagonists. Further, there were only two tests of each antagonist dose. More replications would have been desirable. Future studies should conduct more extend investigations of these drugs as well as more specific nachr antagonists (e.g., dihydro-β-erythroidine [DHBE] or varenicline). Even still, the results of the current study strongly implicate central, but not peripheral, nachrs in the mediation of nicotine s effects on responding maintained by conditioned reinforcers. 33

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38 BIOGRAPHICAL SKETCH Jeb Jones received his Bachelor of Science with High Honor in applied psychology from the Georgia Institute of Technology in May of 5. Subsequently he worked at the Marcus Institute, a research and treatment facility for children and adolescents with developmental disabilities and severe behavior problems, eventually being promoted to serve as a primary therapist and assist in the development of treatment and research protocols. He was accepted for admission to the behavior analysis program in the Department of Psychology at the University of Florida for the fall semester of 7 to study under the leadership of Dr. Jesse Dallery. There he conducted the current experiment as well as assisted in numerous other experiments with animals and humans. He earned his Master of Science degree from the University of Florida in the fall of 9. 38