Examination of Age-Related Cholinergic Activity during Nicotine

Exposure and Withdrawal

Luis M Carcoba, M.D, Ph.D.

University of Texas at El PasoDepartment of Psychology

CHARLES R. DREW UNIVERSITY OF MEDICINE AND SCIENCE9th Drug Abuse Research Symposium

September 12, 2014

The University of Texas at El Paso

• Although the prevalence of smoking among the general population has generally decreased over recent years, there has been a steady rise in the rate of smoking behavior among adolescents.

• Adolescents are particularly vulnerable to tobacco abuse, and they are more likely to initiate tobacco use and continue to use tobacco products as adults. As a result, they are at greater risk of smoking-related diseases caused by long-term tobacco use.

• Studies have demonstrated that adolescent rodents display fewer physical symptoms of nicotine withdrawal as compared with adult rats (O’Dell et al. 2006; Shram et al. 2008) and mice (Kota et al. 2007).

• Also, the rewarding effects of nicotine are enhanced during the adolescent period of development (Torres et al., 2008).

Introduction (Adolescents Tobacco Use)

• It is well-accepted that relapse to smoking behavior is due, in large part, to negative reinforcement processes that maintain smoking behavior and relapse behavior in order to avoid the negative consequences of withdrawal.

• Current treatment strategies focus on alleviating withdrawal. However, the role of withdrawal in smoking behavior during adolescence is presently unclear.

Introduction (Nicotine Withdrawal Syndrome)

What We Know ?

There are more than 4,000 chemicals found in the smoke of tobacco products. Of these, nicotine, first identified in the early 1800s, is the

primary reinforcing component of tobacco.

What We Know ?

Koob GF (2005). The neurocircuitry of addiction: implications for treatment. Clin Neurosci Res 5: 89–101.

Role of ACh in NAcc

• The behavioral mechanisms of nicotine are mediated, in large part, in the nucleus accumbens (NAcc), a terminal region of mesolimbic pathway where dopamine levels are increased following nicotine administration and decreased during withdrawal from this drug (Corrigall, 1991, 1992; Mansvelder and McGehee, 2002; Watkins et al., 2000).

Role of ACh in NAcc

• The nucleus accumbens (NAcc) has been implicated in modulating the reinforcing properties of drugs of abuse playing a role in the aversive aspects of drug withdrawal as reported during diazepam and morphine withdrawal.

Role of ACh in NAcc

• Recent work in our laboratory focused on developmental differences to nicotine withdrawal demonstrated that adolescent rats exhibit lesser withdrawal-associated deficits in NAcc dopamine as compared to nicotine-dependent adults (Natividad et al., 2010).

Role of ACh in NAcc

• Our initial studies focused on NAcc dopamine were a logical first step towards understanding the mechanisms of nicotine withdrawal given the importance of dopamine in mediating the behavioral effects of nicotine.

• However, the mechanisms of nicotine withdrawal also appear to involve cholinergic transmission in the NAcc, since nicotine withdrawal produces increased ACh levels in this brain region of adult rats experiencing nicotine withdrawal (Rada et al., 2001).

• Given the potential role of cholinergic systems in mediating age differences to withdrawal, the present study compared ACh levels in the NAcc of adolescent and adult rats during precipitated withdrawal.

Methods

• Rats were divided in three groups:

– Adolescents (PND 28–30)

– Adults (PND 60–75)

– Pre Exposed (PND 95)

Control

Nicotine Treated

Control

Nicotine Treated

Nicotine----Saline

Nicotine----Nicotine

Study Design:•Pump Implant

•Probe Implant

•Assess Nacc ACh during Withdrawal

Nicotine Withdrawal in Rats

To measure ACh activity rats were prepared with osmotic pumps delivering an equivalent nicotine dose in these age groups

Study Design:•Pump Implant•Probe Implant•Assess Nacc ACh during Withdrawal

Dependence Induction:

Administration of nicotine via subcutaneous osmotic pumps for 7-14 days

Withdrawal:

Spontaneous withdrawal: Removal of nicotine pump

Precipitated withdrawal: Administration of the nicotinic antagonist mecamylamine

Nicotine Withdrawal in RatsStudy Design:•Pump Implant•Probe Implant•Assess Nacc ACh during Withdrawal

• Following 13 days of nicotine exposure, rats were implanted with microdialysis probes in the NAcc.

“Bregma 1.70 mm (Paxinos and Watson, 1998).”

Nicotine Withdrawal in RatsStudy Design:•Pump Implant•Probe Implant•Assess Nacc ACh during Withdrawal

• Following 13 days of nicotine exposure, rats were implanted with microdialysis probes in the NAcc.

• Next day, dialysis samples were collected during baseline and following systemic administration of the nicotinic-receptor antagonist mecamylamine. ACh was quantified from 10-μl samples injected into a HPLC system

Study Design:•Pump Implant•Probe Implant•Assess Nacc ACh during Withdrawal

Nicotine Withdrawal in Rats

Baseline levels of ACH are higher in adolescent rats

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Conclusions

• Age Differences– Baseline levels of ACh were higher in adolescent versus adult rats.

– During withdrawal, both groups displayed similar levels of ACh. This suggests that behavioral differences observed in nicotine exposed animals are not ACh-mediated.

• Pre-exposure to Nicotine Effects – Baseline levels of ACh were slightly increased by adolescent nicotine

exposure.

– During withdrawal, the increases in ACh levels observed in naïve adults were blunted by adolescent nicotine exposure.

Acknowledgments

Collaborators:Dr. Laura E O’DellDr. James OrfilaDr. O’Dell Lab

Financial Support from: -Vulnerability Issues in Drug Abuse (VIDA) Program-Research Incentive funds from UTEP-National Institute on Drug Abuse (R01-DA-021274)