Current Research Interests

Thus far, I am focused on two distinct, but interrelated, lines of research, both in the broad area of the neurobiology of learning and memory, and focusing on the contributions of two major brain structures, the cerebellum and the hippocampus, to learning and memory.  The first line of research involves examining the role of the cerebellum and hippocampus in animal models of human clinical conditions, including fetal alcohol spectrum disorder (FASD) and attention deficit/hyperactivity disorder (ADHD). The second line of research involves examining the role in learning of the cerebellum and hippocampus, particularly when both structures are required for learning to take place.

Neurobiology of Learning and Memory: Animal Models of Human Clinical Conditions

Animal Model of FASD. To investigate the precise consequences of exposing the fetal brain to alcohol, researchers have developed a rodent model of fetal alcohol spectrum disorder (FASD). At birth, much of the rat brain is at a stage of development roughly equivalent to a third trimester human fetus, a time that has been referred to as the rat “third trimester equivalent” of humans. A simple motor learning task, eyeblink classical conditioning, is known to require only discrete regions of the brainstem and the cerebellum, making it an ideal behavioral test of cerebellar functionality after early exposure to alcohol. Eyeblink conditioning is a form of classical conditioning in which the organism learns to blink to a previously neutral stimulus, such as a tone, that consistently precedes the delivery of an eyeblink-eliciting stimulus, such as a puff of air to the eye.  If eyeblink conditioning is poor after early exposure to alcohol in the rat, damage to the cerebellum is strongly implicated as the cause.

Since coming to UVM, I have continued a line of research begun as a post-doctoral fellow at Indiana University which uses eyeblink conditioning to examine functionality of the cerebellum after developmental ethanol exposure.  In my laboratory, we have investigated how different doses and patterns of early alcohol exposure affect adult cerebellar-dependent eyeblink conditioning and deep cerebellar nuclei neuron numbers.  The doses and patterns I used were hypothesized to be closer to what “typical” prenatal alcohol exposure would be during the third trimester (i.e., a lower dose for a more extended time period).  We found that a moderate dose of alcohol during the third-trimester equivalent impairs eyeblink conditioning in adult rats.  These rats also have fewer deep cerebellar nuclei neurons.  A manuscript of this work was published in 2006 in Alcohol. We have continued this work by examining how extended and moderate third trimester equivalent ethanol exposure impacts neuron numbers in the neostriatum, which has been shown to be reduced in volume in FASD children.  My graduate student, Alexandra Thanellou, presented a poster of this work at the 2007 Society for Neuroscience meeting in San Diego and the 2007 Vermont Chapter of the Society for Neuroscience meeting.

Thanellou, A.G., & Green, J.T. (2007).  Neuronal loss in the rat caudate-putamen after a moderate dose of ethanol during the third trimester equivalent. Society for Neuroscience Abstracts (published abstract of conference presentation).

Green, J.T., Arenos, J.D., & Dillon, C.J. (2006).  The effects of moderate neonatal ethanol exposure on eyeblink conditioning and deep cerebellar nuclei neuron numbers in the rat.  Alcohol, 39, 135-150.  (Impact Factor = 2.140, 2007 Journal Citation Reports Science Edition: Toxicology)

Green, J.T., Arenos, J.D., & Dillon, C.J. (2005).  Impaired long-delay eyeblink conditioning in rats after moderate doses of ethanol during the third trimester equivalent. Society for Neuroscience Abstracts (published abstract of conference presentation).
Arenos, J.D., & Green, J.T. (2005).  Neuronal loss in the rat lateral and interpositus cerebellar nuclei after moderate doses of ethanol during the third trimester equivalent. Society for Neuroscience Abstracts (published abstract of conference presentation).

Green, J.T. (2004). The effects of ethanol on the developing cerebellum and eyeblink classical conditioning. Cerebellum, 3, 178-187. (Impact Factor = 2.306, 2007 Journal Citation Reports Science Edition: Neurosciences)

Animal Models of ADHD.  Two inbred strains of rats have been proposed as potential animal models of ADHD based on the presence of ADHD-like symptoms, including impulsivity, hyperactivity, and inattention.  Recent studies of children and adolescents with ADHD suggest that ADHD is associated with reduced volume of the cerebellum and impairments in cerebellar-dependent behaviors such as precise timing of motor responses.  Beyond this, very little is known about the role of cerebellar differences in the symptoms of ADHD.  My laboratory conducted the first study of cerebellar-dependent eyeblink conditioning in the most widely-used of these animal models, the spontaneously hypertensive rat (SHR). SHRs were derived out of inbreeding of Wistar-Kyoto rats selected for a hypertension trait but a hyperactivity trait also became fixed.  We tested only male rats in this initial study. Interestingly, SHRs showed a very unusual pattern of eyeblink conditioning whereby they conditioned more quickly than control rats but show very large and mis-timed conditioned eyeblinks, strongly suggesting cerebellar abnormalities. A manuscript of this work was published in 2008 in Behavioral Neuroscience.  We followed up these initial observations in two ways.  First, we began evaluating the number of neurons of different types in the cerebellar cortex of SHRs.  This work is on-going.  Second, we conducted eyeblink conditioning in a second inbred rat strain that exhibits ADHD-like symptoms, the Wistar-Kyoto Hyperactive (WKHA) rat.  This rat strain, created at UVM by Dr. Edith Hendley in the College of Medicine from further inbreeding of SHRs and their progenitor strain, exhibits hyperactivity without hypertension.  SHRs are hypertensive in addition to being hyperactive, leading to potential difficulties in interpreting the underlying causes of their behavior in a particular task.  We found that male WKHA rats showed mis-timed conditioned eyeblink responses very similar in pattern to male SHRs, again suggesting cerebellar abnormalities.  We also tested female WKHA rats in this study.  Somewhat surprisingly, female WKHA rats did not exhibit timing abnormalities in eyeblink conditioning. 

In collaboration with Dr. Betsy Hoza (UVM Department of Psychology), Dr. Alan Smith (Purdue University Department of Health and Kinesiology), and Dr. David Bucci (Dartmouth College Department of Psychological and Brain Sciences), I have begun examining whether aerobic physical activity can normalize SHRs’ behavior. This is a component of a larger collaborative project examining the impact of aerobic physical activity on children with ADHD (overseen by Dr. Hoza and Dr. Smith) and the impact of aerobic physical activity on rats with ADHD-like symptoms (overseen by myself and Dr. Bucci).  Initial results have been very promising, both in terms of the behavior of children with ADHD and in terms of the performance of SHRs on a variety of tasks (attentional orienting, conditioned inhibition, social interaction, eyeblink conditioning) and on measures of brain plasticity (induction of brain-derived neurotrophic factor in the hippocampus).

Thanellou, A., Schachinger, K.M., & Green, J.T. (in preparation). Abnormal timing of conditioned eyeblink responses in male but not female Wistar-Kyoto Hyperactive rats.

Chess, A.C., & Green, J.T. (2008).  Abnormal topography and altered acquisition of conditioned eyeblink responses in a rodent model of Attention-Deficit/Hyperactivity Disorder.  Behavioral Neuroscience, 122, 63-74. (Impact Factor = 2.883, 2007 Journal Citation Reports Science Edition: Neurosciences)

Thanellou, A.G., Chess, A.C., & Green, J.T. (2008).  Abnormal cerebellar-dependent learning in two rodent models of attention-deficit/hyperactivity disorder. Eastern Psychological Association. (paper presented at conference)

Chess, A.C., & Green, J.T. (2007).  Acquisition and timing of conditioned eyeblink responses are differentially affected in a rodent model of attention-deficit/hyperactivity disorder. Society for Neuroscience Abstracts (published abstract of conference presentation).

Neurobiology of Learning and Memory: Roles of the Cerebellum and Hippocampus

In the laboratory, classical (Pavlovian) conditioning procedures have provided some of the best evidence as to exactly how different brain structures are involved in learning and remembering.  The simplest form of classical conditioning (delay conditioning) involves learning that a neutral stimulus, such as a tone (the conditioned stimulus, or CS) consistently precedes and overlaps with a biologically-significant stimulus, such as food or a reflex-eliciting stimulus (the unconditioned stimulus, or US).  Learning is revealed by the emergence of responses to the previously neutral stimulus.  As indicated earlier, one type of classical conditioning, eyeblink conditioning, is known to engage only the brainstem and the cerebellum in its simplest form (i.e., delay conditioning).  Many other, more complex forms of eyeblink conditioning, involving more complicated relations between the CS and US or involving more than one CS, have been shown to engage the hippocampus in addition to the brainstem and cerebellum.  Memory formation in these procedures involves both the hippocampus and the cerebellum, but it is unclear what the relative roles of these structures are and how they communicate during the learning process.  A full understanding of how the brain is involved in learning and memory will require an understanding of how and why these basic conditioning processes engage certain brain structures during learning.

Hippocampal- and Cerebellar-Dependent Learning: Trace Eyeblink Conditioning.  Trace conditioning is the simplest procedure that requires the hippocampus for learning and memory formation.  In contrast to delay eyeblink conditioning, in trace eyeblink conditioning the CS and the US do not overlap but are separated by a brief, stimulus-free interval of about half a second.  This simple change in procedure requires brain structures, including the hippocampus, in addition to the brainstem and cerebellum for conditioning to occur at a normal rate.  Therefore, it is believed that both the cerebellum and the hippocampus are necessary for learning the CS-US association in trace eyeblink conditioning.  However, we have an incomplete understanding of the relative roles that these structures play in the learning process.  These questions are important to answer since the hippocampus is one of the key brain structures involved in many types of learning and remembering, including the formation of memories for personal events in our lives and the ability to learn relationships between many of the seemingly arbitrary stimuli we encounter on a daily basis.  A complete understanding of trace conditioning would provide a window on fundamental aspects of hippocampal involvement in learning and memory.

I recently completed a study in which we compared the activity of neurons in the hippocampus and the cerebellum during delay and trace eyeblink conditioning.  This study was conducted as a first step towards understanding whether a procedure that engages both the hippocampus and the cerebellum (trace conditioning) shows a different neural “signature” from a procedure which engages only the cerebellum (delay conditioning). This work was published in 2007 in Neurobiology of Learning and Memory. In a related project, I was involved in equipment setup and data analysis for a study conducted at Temple University that examined delay and trace eyeblink conditioning in mutant mice with abnormalities in cerebellar cortex.  In this study, we found impairments in delay conditioning but not in trace conditioning. 

Green, J.T., & Arenos, J.D. (2007).  Hippocampal and cerebellar single-unit activity during delay and trace eyeblink conditioning in the rat.  Neurobiology of Learning and Memory, 87, 269-284. (Impact Factor = 3.443, 2007 Journal Citation Reports Science Edition: Neurosciences)

Green, J.T., & Arenos, J.D. (2006).  Hippocampal versus cerebellar single-unit activity during delay versus trace eyeblink classical conditioning in the rat. Society for Neuroscience Abstracts (published abstract of conference presentation).

Woodruff-Pak, D.S., Green, J.T., Levin, S.I., & Meisler, M.H. (2006).  Inactivation of sodium channel Scn8A (Na_v1.6) in Purkinje neurons impairs learning in Morris water maze and delay but not trace eyeblink classical conditioning.  Behavioral Neuroscience, 120, 229-240.  (Impact Factor = 2.883, 2007 Journal Citation Reports Science Edition: Neurosciences)

Retention of Cerebellar-Dependent Learning. After conditioning has occurred, the learned response to the CS will cease if the CS is now presented without the US (extinction).  However, many studies have shown that this is not an unlearning process, and that traces of the original conditioning remain intact but suppressed. For example, in many types of classical conditioning, simply presenting the US again (by itself) can restore the learned response to the CS (reinstatement).  Furthermore, reinstatement has been shown to require the hippocampus.  For her Master’s Thesis research, my graduate student, Alexandra Thanellou, examined whether reinstatement of the extinguished eyeblink response can be produced.  We reasoned that this would tell us about the retention of learned responses in eyeblink conditioning and would allow us to examine the roles of the cerebellum and the hippocampus in this retention.

Thanellou, A., & Green, J.T. (2006).  Reinstatement of the extinguished eyeblink conditioned response in the rat. Society for Neuroscience Abstracts (published abstract of conference presentation).

Neural Substrates of Response Timing in Cerebellar-Dependent Learning. The cerebellum is involved in the timing of discrete movements that require rapid execution in addition to the learning of movements to previously neutral stimuli.  Eyeblink conditioning can be used to model these processes in the laboratory.  Different regions of the cerebellum (deep nuclei, cortex) appear to play different roles in eyeblink conditioning (CS-US association formation, and timing of learned responses to the CS, respectively).  In terms of cerebellar cortex (a very large area), it has remained unclear which regions play a role in the timing of learned eyeblink responses.  While at UVM, I analyzed data collected while I was a postdoctoral fellow at Indiana University in which I recorded from cerebellar cortical neurons in the anterior lobe of the cerebellar cortex during eyeblink conditioning and found neuronal activity that matched the timing of learned eyeblink responses. This study was featured on the cover of the May/June 2005 issue of Learning and Memory.  

Green, J.T., & Steinmetz, J.E. (2005).  Purkinje cell activity in the cerebellar anterior lobe after rabbit eyeblink conditioning. Learning and Memory, 12, 260-269.  (Impact Factor = 4.037, 2007 Journal Citation Reports Science Edition: Neurosciences)  (featured on cover)