Research projects

We are addressing three specific questions:

  • How does chronic exposure to cocaine and ethanol affect the structure and function of neurons and synapses?
  • Is there a difference in synaptic function and morphology between animals that display addictive-like behaviors and animals that do not after they all have been chronically exposed to cocaine or ethanol?
  • Are there differences in the function and morphology of synapses in animals with higher predisposition to become addicted to cocaine?

To answer these questions, we use a combination of techniques: electrophysiology, 2-photon laser scanning microscopy, biochemistry and behavior. The lab employs two models of drug exposure: a passive non-contigent administration (such as intraperitoneal injections of cocaine) and models of ethanol and cocaine self-administration in mice (see models of drug administration ).

Electrophysiological recordings (whole cell voltage clamp) from medium spiny neurons (MSN) of the nucleus accumbens and the striatum are performed in order to determine the functional properties of the synaptic inputs onto D2-positive neurons and D1-positive neurons.

AMPA and NMDA receptor mediated synaptic currents of D2 positive (GFP) medium spiny neurons

We are also studying the morphological changes associated with chronic cocaine and with the development of addictive behaviors by fluorescently labeling MSN using ballistic methods and imaging spine morpholgy with confocal and 2-photon microscopes.

Image of 3 panels with increasing magnification. Panel A shows striatum with stained medium spiny neurons, panel B a single medium spiny neuron and panel C a section of the neurons dendrite with clearly visible spines.

Findings from these studies will provide information about the short-term and long-term effects of drug of abuse, such as cocaine and alcohol, on synapses. In addition, we hope they will lead to the identification of neuronal markers associated with higher vulnerability to addiction and they will aid in the development of new therapies.

Models of drug administration in mice:

Cocaine intraperitoneal injections: Cocaine is a psychostimulant drug that acts by blocking the reuptake of monoamines, such as dopamine, from the synaptic cleft. One of the acute actions of cocaine on mouse behavior is to increase their locomotor activity. In the lab, mice receive single or repetitive cocaine injections and their locomotor activity is measured using infrared beams and detectors located across the sides of the cage (see schematic below). Horizontal locomotion is measured daily for 20—60 min after injection of either saline or cocaine (5—30 mg/kg) in the intraperitoneal cavity. An example of this response is shown in the graph below where an acute response to cocaine is observed on day 1 and subsequent injections increase the psychomotor response furthermore. The enhancement of the locomotor activity with repeated injections is what has been called locomotor sensitization.

Graph 1: Locomotor sensitization to cocaine:

Schema and graph showing the locomotor sensitization to cocaine

Cocaine drug self-administration: The lab uses models of intravenous (IV) cocaine self-administration in mice to investigate the development of addictive-like behaviors in rodents. Mice undergo surgical implantation of a catheter in the jugular vein and, after recovery, are trained to poke their noses in a hole to gain an intravenous infusion of cocaine through this catheter (see a video of a mouse performing the task ). We analyze the number of rewards gained per daily session and track the progression of drug intake during consecutive days and weeks. In one of our models, daily sessions are intermingled with two time-out periods during which the drug is unavailable and nose pokes do not lead to infusions. (Graph below, gray area). We measure the perserverance of nose poking during the time-out periods as one determinant of the development of compulsive drug seeking behavior. The other two parameters are a breakpoint value obtained in sessions of progressive response ratio to test the motivation and one punishment session in which reward is paired to a mild foot shock to determine the perserverance of drug seeking despite aversive consequences (model based on Derroche-Gamonet et al. 2004).
Interestingly, different patterns of drug taking behavior are observed among different mice. The plot below shows an example of a regular (right side) and a more clustered or binging (left side) self-administration pattern.

Graph 2: Different patterns of cocaine self-administration:

Self-administration response pattern to cocaine

Ethanol self-administration: In collaboration with the laboratory of Dr. Kathleen A. Grant ( Oregon National Primate Center and OHSU ) and Dr. Todd Thiele (University of North Carolina at Chapel Hill) we are studying the morphological changes at synapses that occur after chronic ethanol self-administration in non-human primates and rodents, respectively.