Ryan Williamson and Dr. Jeff Edwards, Department of Physiology and Developmental Biology
The hippocampus is a region of the brain that mediates learning and memory formation by changing the properties of synapses within its circuitry [2]. The plastic nature of these synapses allows the brain to alter how sensitive one neuron is to stimulation from another neuron. Improper regulation of synaptic plasticity may cause memory loss associated with neurodegenerative disease [4].
Synaptic regulation can be mediated by endogenous signaling molecules called endocannabinoids (eCBs), lipid-based molecules that can decrease presynaptic neurotransmitter release [3]. Electrophysiology data suggests that eCB regulation occurs in synapses between CA3 pyramidal cells and CA1 interneurons [1]. However, there is debate over interneurons’ ability to synthesize eCBs. The purpose of my project was to determine whether stratum oriens interneurons possess the machinery needed to produce eCBs and, if so, whether this machinery is expressed in a subtype-specific manner. The answer to these questions will provide a deeper understanding of mechanisms of memory formation.
I used quantitative polymerase chain reaction (qPCR) to measure relative expression levels of various cDNA products. To do this I used short DNA probes that allowed me to quantify the amount of target cDNA present in a sample through each step of the PCR reaction [6]. After measuring the amount of cDNA during the amplification reaction, I inferred relative quantities of mRNA that occurred in the initial samples. The targets I sought to quantify are categorized in three groupings: interneuron subtype markers, eCB receptors, and eCB synthesizing enzymes (see Table 1).
The results of the qPCR experiments are summarized in Table 2. Our data show expression of eCB enzyme mRNA in several cells (see Figure 1 for an example cell). This supports our hypothesis and confirms that stratum oriens interneurons are capable of synthesizing eCBs. There are especially high levels of DAGLα expression. This agrees with previously published experimental data on hippocampal stratum oriens interneurons [5]. Additionally, expression of 12-LO suggests that interneurons can produce the molecule 12-(S)-Hydroperoxyeicosa-5Z, 8Z, 10E, 14Z-tetraenoic acid (12-HPETE), an agonist of a cation channel called TRPV1, which has been shown to be involved in interneuron long term depression [3].
The data demonstrate that some stratum oriens interneurons possess the machinery necessary to produce eCBs. In the coming months I plan to increase my sample size to correlate expression of eCB enzymes with interneuron subtypes. This work will deepen our understanding of the role stratum oriens interneurons and eCBs play in learning and memory formation.
References
- Edwards, J. G., Gibson, H. E., Jensen, T., Nugent, F., Walther, C., Blickenstaff, J., & Kauer, J. A. (2012). A novel non-CB1/TRPV1 endocannabinoid-mediated mechanism depresses excitatory synapses on hippocampal CA1 interneurons. Hippocampus, 22(2), 209-221. doi: 10.1002/hipo.20884
- Freund, T. F. (1996). Interneurons of the hippocampus. Hippocampus, 6(4), 347-470.
- Mackie, K. (2008). Signaling via CNS cannabinoid receptors. Molecular and Cellular Endocrinology, 286(1-2), S60-S65.
- Malenka, R. C., & Bear, M. F. (2004). LTP and LTD: An embarrassment of riches. Neuron, 44(1), 5-21. doi:10.1016/j.neuron.2004.09.012
- Péterfi, Z., Urbán, G. M., Papp, O. I., Németh, B., Monyer, H., Szabó, G.,…Katona, I. (2012). Endocannabinoid-mediated long-term depression of afferent excitatory synapses in hippocampal pyramidal cells and GABAergic interneurons. The Journal of Neuroscience,32(41), 14448-14463. doi: 10.1523/JNEUROSCI.1676-12.2012
- Vullhorst, D., Neddens, J., Karavanova, I., Tricoire, L., Petralia, R. S., McBain, C. J., & Buonanno, A. (2009). Selective expression of ErbB4 in interneurons, but not pyramidal cells, of the rodent hippocampus. The Journal of Neuroscience, 29(39), 12255-12264. doi:10.1523/JNEUROSCI.2454-09.2009