Date of Degree

9-2019

Document Type

Dissertation

Degree Name

Ph.D.

Program

Psychology

Advisor

Peter Serrano

Committee Members

Maria E. Figueiredo-Pereira

Thomas Preuss

Wayne Harding

Subject Categories

Biological Psychology | Cognitive Neuroscience | Other Neuroscience and Neurobiology

Keywords

Methamphetamine, Spatial cognition, Hippocampus, Sex differences, Estrogen receptor alpha, Neuroprotection

Abstract

Methamphetamine is an addictive illicit psychostimulant that produces lasting neurochemical and behavioral changes. The mechanisms underlying these deficits have been characterized in animal models using extremely high doses. Currently, better translational models are needed to understand the onset and progression of these deficits that more accurately reflect the gradual and voluntary dosing parameters as chosen by an abuser. To that end, a new model of methamphetamine administration, labeled Voluntary Oral Methamphetamine Administration (VOMA), offers a means to examine the progression of neurotoxicity, behavioral deficits, and the addiction process through a voluntary consumption framework.

Female populations show consistent vulnerabilities to methamphetamine, including greater severity of abuse, and greater shifts in psychological health. Females also show increased neurochemical susceptibilities during abstinence from methamphetamine, exhibiting increased grey-matter loss compared to controls. The mechanisms underlying these female-specific susceptibilities to methamphetamine are unknown.

The overall GOAL of these studies was to establish a voluntary methamphetamine administration model in mice that recapitulates the cognitive and neurochemical deficits shown with previous models to further our understanding of the long-term susceptibilities of at-risk populations, including adolescent and female subjects.

Rodents can exhibit drug-preference and drug-seeking behaviors, and methamphetamine challenges can reproduce the behavioral and neurochemical deficits that human addicts show. However, it is unclear if mice would voluntarily consume methamphetamine, and what effects this administration design would produce in this mice. Thus, the studies presented in this dissertation characterized the utility of the VOMA model to produce 1. naturalistic methamphetamine consumption 2. behavioral deficits and 3. neurochemical changes. This was done in an effort to understand the 4. mechanisms underlying adolescent- and female-specific vulnerabilities to the drug.

To achieve our GOAL, we carried out three (3) specific aims:

Specific Aim 1: Determine the utility of Voluntary Oral Methamphetamine Administration (VOMA) to produce behavioral and neurochemical deficits as seen in previous models [Chapter 2]

In modeling MA abuse with rodents, researchers have developed paradigms that inject neurotoxic or binge doses in mice that are 10 times higher than the lower limits we have characterized in our models 1 (reviewed in2). Previous work in this line of research has found that two (2) 30mg/kg MA doses can acutely increase spatial working-memory performance but also increase spatial working-memory errors in the long-term. These effects were correlated to decreased DA and synaptic plasticity maker expression in the hippocampus 1. These prior studies have contributed significantly to our understanding of the mechanisms underlying MA toxicity 2, but offered very little in understanding the voluntary nature of methamphetamine administration.

In order to characterize the utility of VOMA in producing MA consumption that produced cognitive and neurochemical deficits, we combined a spatial cognition design with a drug administration design as follows: mice were randomly assigned to either a water/control group or MA for 28 consecutive days. To carry out voluntary oral MA administration (VOMA) in mice, MA was mixed into a palatable sweetened oatmeal flake that mice were drawn to and ingested orally. Mice were allowed to consume MA throughout a 3-hour administration period, within 15 min intervals. Following 28-days of VOMA, mice were sacrificed, and tissues were collected.

We found that VOMA:

(1) Decreased working-memory and reference-memory performance of male mice on the radial arm maze. These behavioral deficits were observed after the 28-day drug administration period over a 2-week abstinence period.

(2) Increased neurotoxicity in the hippocampus, as observed through lower dopamine marker expression, increased neuroinflammation, and lower synaptic-plasticity marker expression.

Overall, these results indicate that VOMA can produce the behavioral and the neurochemical deficits observed in previous animal models as well as in human methamphetamine addicts. Additionally, that the molecular deficits were observed in the hippocampus highlights the utility of the model to easily characterize correlative cognitive behaviors and the underlying molecular shifts.

Specific Aim 2: Characterize the role of abstinence in perpetuating methamphetamine-induced deficits using VOMA [Chapter 3]

MA abuse can produce long-term cognitive impairments in abstinent individuals3-5 but other studies have shown that long periods of abstinence can improve the recovery of decision making-skills and emotional symptoms6. This may help individuals to avert relapse and yet, other reports indicate that relapse can occur after abstinence7, as a result of exposure to specific environmental cues8. The neurochemical pathways affected by abstinence from MA are also unclear. An understanding of the progression of behavioral and neurochemical changes produced by MA abuse and by abstinence would provide pharmacological targets to aid in recovery from addiction.

In order to characterize the role of abstinence in producing MA-induced behavioral and molecular deficits, we used an acute VOMA design to do the following: put adolescent mice through cognitive assessments directly after 14 days of MA exposure and after a prolonged abstinence period. Tissue was collected directly after VOMA and after a 4-week abstinence period.

We found that VOMA:

(1) Produced a transient sensitization to MA as seen through behavior analyses in the drug-context

(2) Produced an acute spatial working-memory deficit on the RAM directly after VOMA

We found that abstinence:

(1) Did not produce long-term working-memory, reference learning, nor retention deficits on the RAM

(2) Increased expression of neuroinflammatory markers in the hippocampus

(3) Modulated expression of monoamine proteins in the hippocampus

Overall, these results indicate that abstinence from VOMA acutely affects behavioral performance that is ameliorated by abstinence. Further, abstinence produces neurochemical changes not observed directly following VOMA. This demonstrates that even acute MA exposure is sufficient to produce molecular changes in the brain despite not manifesting in behavior. This highlights the susceptibility of the adolescent brain to acute MA exposure and provides new avenues for pharmacological studies aimed at preventing MA-induced deficits.

Specific Aim 3: Characterize the sex-differences in behavior and neurochemistry following escalating dose of methamphetamine using VOMA [Chapter 4]

Previous work has shown that females exhibit susceptibilities to psychostimulant-addiction 9-12. It has been reported that methamphetamine use in female populations can be attributed to factors including weight loss, desire for increased energy and work output, and decreased exhaustion 13-14. Data on drug-rehabilitation admissions show that young female participants are admitted at higher rates compared to males 15. One hypothesis for sex differences in psychostimulant addiction highlights estrogen’s facilitation of neuronal and behavioral development of drug addiction processes in the brain. 16-18. However, this idea remains untested.

Additionally, previous work has shown that escalating doses of methamphetamine have intrinsic neuroprotective qualities that protect the brain and behavior from a chronic administration of the drug. The nature and source of this neuroprotection is unclear, but previous work has shown that this phenomenon occurs in binge injection models, 19-20, associated with decreases in methamphetamine binding to DAT and decreases in the resulting DA availability in the synapse. However, the underlying mechanisms that produce this change in DA sensitivity remain unknown.

To test how female subjects respond differentially to VOMA, we modified our administration design to include a 10-day escalation period, wherein mice would receive gradually escalating doses of and access to methamphetamine prior to receiving the maximum access and dose. We hypothesized that that this administration design would produce the maximal voluntary consumption over a shorter (18-day) period and produce the maximal behavioral and molecular deficits that could be observed.

We found that Escalation-VOMA:

(1) Decreased working-memory of female mice on the radial arm maze shortly after the conclusion of VOMA. This deficit was not observed in male mice that were placed into VOMA.

(2) Discrete shifts in neuroprotective signaling (D1, ERα and Akt/GSK3β pathway) in the hippocampus and nucleus accumbens of female mice after a 2-week abstinence period. Male mice that underwent VOMA did not exhibit any molecular changes.

Overall, this study showed that through VOMA, and with the aid of an escalating-dose design, we are able to observe sex-differences in the behavioral and neurochemical changes that methamphetamine can produce. Interestingly, we also observed a previously reported neuroprotective effect of escalating doses in male subjects, but not in females. Our molecular investigations will provide future direction in basic research and clinical endeavors to target methamphetamine addiction.

Conclusion: The novel VOMA design has provided a more naturalistic approach to investigating the behavioral and molecular deficits produce by methamphetamine and abstinence during adolescence. With this model, we have successfully characterized longer-term susceptibilities in mice than previous studies have shown. Furthermore, it has provided new insights into the sex-differences surrounding meth-abuse, including the role of neuroprotective pathway signaling as markers for susceptibilities/resilience to methamphetamine abuse

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