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Time of Day and Nicotine Discrimination
by: Amy DiDio

Welcome

Thank you for visiting my site. The following experiments investigated time-of-day discrimination and nicotine on rats. All training and testing took place in the St. Anselm college animal lab over a period of approximately two months. The following briefly explains the research, testing, results and opinions about the two experiments.

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Introduction

The discriminative effects of nicotine have been well demonstrated in several studies. The drug discrimination paradigm explains and exemplifies how a drug is detected. In this paradigm, drugs are discriminative stimuli. One drug can occasion reinforcement, and one drug can occasion extinction (Pierce & Epling, 1999). On a tandem schedule of food reinforcement, rats rapidly learned to discriminate nicotine from saline (Shoaib & Stolerman, 1996). Another study showed that rats quickly learn to discriminate nicotine from saline and chronic tolerance to nicotine’s discriminative stimulus properties does not readily develop (Shoaib, Thorndike, Schindler, & Goldberg, 1997).

Discriminability of time-of-day (TOD) has also been well investigated through operant procedures. When rats were kept at two meals a day, at 1000 hours and 1400 hours, and running on an exercise wheel to obtain food, the discrimination was clearly evident. Running on the wheel drastically increased prior to the times that predicted when food could be obtained (Bolles & Moot, 1973).

In studying the time-of-day discrimination, the role of the light/dark schedule is an important factor. It has been documented that rats are able to discriminate time-of-day while maintained in a twenty-four hour light, or twenty-four hour dark schedule (Means, Ginn, Arolfo, & Pence, 2000). Rats have also showed discrimination while maintained in a natural light/dark schedule (Means, Arolfo, Ginn, Pence, & Watson, 2000). In both experiments, rats could rapidly discriminate between the times of day.

Nicotine and TOD have been studied together because of the nicotinic receptors in specific areas of the brain. Nicotine has been shown to alter circadian rhythms, controlled by the suprachiasmatic nucleus (SCN). There are an abundant supply of nicotinic receptors in the SCN, playing an important role in the investigation of TOD and the discriminative effects of nicotine (O’Hara, Edgar, Cao, Wiler, Heller, Kilduff, & Miller, 1998).

The present study investigated the relationship between time-of-day and nicotine discrimination. Experiment one hypothesized that subjects would be able to discriminate time-of-day modulated by nicotine discrimination. Experiment two hypothesized that there would be a difference in the ability to discriminate nicotine as a function of time-of-day.

 

 

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Methods

Experiment I:

Subjects:

Eight male rats were used as subjects. They were kept at 80% of their free-feeding weights. The rats were housed in steel cages with limitless access to water and lived in a 12-hour light/dark cycles.

Materials

Each session took place in a steel operant chamber equipped with one lever located to the left of the magazine, which was in the center on the wall of the chamber. Overhead lights were kept dull, and a white noise from a radio was played to drown out other noises.

Experiment II:

Subjects:

Eight different male rats, with no prior experimental history served as subjects. They were kept at 80% of their free-feeding weight. They were housed in individual steel cages with ad lib access to water. Animals were used according to APA guidelines and this institution’s Animal Care and Use Committee.

Materials:

Sessions took place in eight steel/plexiglass operant chambers. Each chamber was equipped with one lever. Low-level overhead light and white noise signaled session time. The experiment was programmed through Med-PC software and by DG interface to 386 PC.

 

 

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Procedure

Experiment I:

During the first week, magazine train took place so that the rats learned that pressing the lever would predict food on a fixed interval one schedule of reinforcement. The rats then learned a variable interval of 30 seconds would predict food. Rats were trained twice a day, Monday through Friday, for the first week. The following week started the discrimination training. The rats were injected twice a day, once in the morning and once in the afternoon with either .2 m/g of nicotine or saline. The drug was randomly chosen for each session and assigned to all eight rats. After the rats were injected, it was ten minutes before put into the operant chambers. For four of the rats, nicotine in the morning predicted food and vice versa for the other four, in the afternoon the reinforcement conditions were flipped. For the four rats that nicotine predicted food for in the morning flipped in the afternoon session. Saline predicted food in the afternoon for those rats and vice versa for the other four. Once the rats were in the operant chambers, they were run on a variable interval schedule of 30 seconds (VI-30) for fifteen minutes.

Experiment II:

Two sessions were run at approximately 0800 hours and 1400 hours. Magazine training took place over the first week. Training gradually increased over the week from a FR-1 to a VI-1 minute schedule. All training sessions were twenty minutes each, and the test session was three minutes. The eight rats were assigned to one of two groups. The first group was made up of four rats. The training and test sessions took place in the morning, 0800 hrs. Within the first group, .02 mg/Kg of nicotine occasioned food reinforcement and saline occasioned non-reinforcement for two of the rats. Saline occasioned food reinforcement and nicotine occasioned non-reinforcement for the other two rats. The second group was trained and tested identical to the first group, but the training and test took place in the afternoon, 1400 hrs.

 

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Discussion

Experiment I:

The purpose of this study was to examine if rats could discriminate the effects of nicotine modulated by the time of day. The study was a follow up study to an experiment that tested the same hypothesis. The present study aimed at the same test but experimented on subjects with no previous experimental history. The previous study examined the effects of nicotine modulated by the time of day, but the subjects had a long history of drug discrimination and time-of-day discrimination. The previous study also trained and tested all subjects initially with .4mg/Kg of nicotine. Because of the long history of the subjects, it was difficult to establish true results. The subjects in the present study had no prior experimental history, and therefore the results relied solely on the training that was implemented for the purpose of this particular experiment.

The results revealed several outcomes. The first test showed that the rats could not significantly distinguish time-of-day under the nicotine condition. Under the saline condition, the subjects significantly distinguished time-of-day. The mean percentages of SD responding under nicotine were marginally different from the percentages SD responding under the saline condition. These results determine that the subjects could not readily discriminate the effects of nicotine as a function of time of day.

In contrast, the results from the second test suggest that the subjects did significantly discriminate time-of-day in the nicotine condition or the saline condition. Again, there was no significant difference in the mean SD responding under the nicotine condition, and only a marginal difference under the saline condition. There was no difference in the mean percentages of SD responses.

Experiment II:

The purpose of experiment two was to examine the discriminative effects of nicotine as a function of time of day. The results suggest that there is a difference in the ability to discriminate nicotine in the morning versus the afternoon. Because of the difference, it is also suggested that the discriminative effect on nicotine in the afternoon is stronger in the afternoon compared to in the morning. Rats that were trained and tested in the afternoon showed a higher average percent of SD responses than the rats that were trained and tested in the morning.

Further investigation of this experiment should test from a larger sample size. Because of the small sample size, no statistical tests were conducted on the data collected. With a larger sample size, a statistical analysis can be run to determine statistical significance. Because the sample size was made up of only four subjects in each condition, no statistical test would show significance. The larger sample size will allow for a more applicable experiment for the research. The larger sample will allow statistical analysis, resulting in determination of either a significant difference or no difference in the discriminative effects. From the results, conclusions can be drawn and applied to previous research and the effects of nicotine on humans.

 

 

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