Embryonic Methamphetamine Exposure Inhibits Methamphetamine Cue Conditioning and Reduces Dopamine Concentrations in Adult N2 <b><i>Caenorhabditis elegans</i></b>

Abstract

Methamphetamine (MAP) addiction is substantially prevalent in today's society, resulting in thousands of deaths and costing billions of dollars annually. Despite the potential deleterious consequences, few studies have examined the long-term effects of embryonic MAP exposure. Using the invertebrate nematode Caenorhabditis elegans allows for a controlled analysis of behavioral and neurochemical changes due to early developmental drug exposure. The objective of the current study was to determine the long-term behavioral and neurochemical effects of embryonic exposure to MAP in C. elegans. In addition, we sought to improve our conditioning and testing procedures by utilizing liquid filtration, as opposed to agar, and smaller, 6-well testing plates to increase throughput. Wild-type N2 C. elegans were embryonically exposed to 50 μ<smlcap>M</smlcap> MAP. Using classical conditioning, adult-stage C. elegans were conditioned to MAP (17 and 500 μ<smlcap>M</smlcap>) in the presence of either sodium ions (Na⁺) or chloride ions (Cl^-) as conditioned stimuli (CS+/CS-). Following conditioning, a preference test was performed by placing worms in 6-well test plates spotted with the CS+ and CS- at opposite ends of each well. A preference index was determined by counting the number of worms in the CS+ target zone divided by the total number of worms in the CS+ and CS- target zones. A food conditioning experiment was also performed in order to determine whether embryonic MAP exposure affected food conditioning behavior. For the neurochemical experiments, adult worms that were embryonically exposed to MAP were analyzed for dopamine (DA) content using high-performance liquid chromatography. The liquid filtration conditioning procedure employed here in combination with the use of 6-well test plates significantly decreased the time required to perform these experiments and ultimately increased throughput. The MAP conditioning data found that pairing an ion with MAP at 17 or 500 μ<smlcap>M</smlcap> significantly increased the preference for that ion (CS+) in worms that were not pre-exposed to MAP. However, worms embryonically exposed to MAP did not exhibit significant drug cue conditioning. The inability of MAP-exposed worms to condition to MAP was not associated with deficits in food conditioning, as MAP-exposed worms exhibited a significant cue preference associated with food. Furthermore, our results found that embryonic MAP exposure reduced DA levels in adult C. elegans, which could be a key mechanism contributing to the long-term effects of embryonic MAP exposure. It is possible that embryonic MAP exposure may be impairing the ability of C. elegans to learn associations between MAP and the CS+ or inhibiting the reinforcing properties of MAP. However, our food conditioning data suggest that MAP-exposed animals can form associations between cues and food. The depletion of DA levels during embryonic exposure to MAP could be responsible for driving either of these processes during adulthood.

Caenorhabditis elegans Show Preference for Stimulants and Potential as a Model Organism for Medications Screening

“…C. elegans have conserved neurobiological systems with established mapping of all neurons and synapses in the entire animal. We have shown that, C. elegans develops a conditioned preference for cues that had previously been paired with either cocaine or methamphetamine exposure that is similar to findings using place conditioning with rats and mice ( Musselman et al, 2012 ; Katner et al, 2016 ). Moreover, conditioning required functional dopamine neurotransmission ( Musselman et al, 2012 ).…”

“…Cholesterol was not included in the salt-free agar in order to obtain clearer images of worms during testing. Although the lack of salts and cholesterol in the agar may have long-term effects on worms, our previous work indicating intact cue-conditioned learning ( Musselman et al, 2012 ; Katner et al, 2016 ) and the differential responses with the SOAs vs. controls (food or benzaldehyde) show that the agar preparation as used in this paradigm does not prevent normal chemotaxic responses.…”

A novel functional cross-interaction between opioid and pheromone signaling may be involved in stress avoidance in Caenorhabditis elegans

“…3b,c), they could be useful as an animal model for the in vivo screening of potential antagonists for opioid expression. In fact, C. elegans has already been demonstrated to study chemical antagonists against cocaine and methamphetamine exposure, and these experiments yielded similar findings to those obtained using place conditioning with rats and mice 55,56 . Furthermore, C. elegans and vertebrates share a conserved opioid system 4 , the effects of pheromone on the expression of nlp-24, which encodes a β-endorphin-homolog, suggest possible approaches for modulating opioid drug function via pheromone mimetics and preference via manipulation of genetic or biochemical conditions.…”

Caenorhabditis elegans as a model system to identify therapeutics for alcohol use disorders

“…In addition, a relatively short life cycle and a 3-day generation time from egg to adult can lead to a dramatic increase in the pace of discovery at a fraction of the cost inherent when using higher level organisms. We have discovered that, like mammals and other invertebrates, C. elegans also develops a conditioned preference for cues after previous pairings with methamphetamine or cocaine that is dependent on dopamine neurotransmission [9,10]. Together, these data indicate that invertebrates, specifically C. elegans, show behavioral responses to addictive drugs that are consistent with those of higher level organisms.…”