Dopamine Video

A super-brief tutorial on dopamine function.

Erasing a Memory

Researchers in Sheena Josselyn's lab report that a memory trace for a fearful experience can be selectively deleted in mice. Are we on the verge of localizing specific memories in the brain?

The full article can be found in JH Han et al. (2009) "Selective erasure of a fear memory." 323(5920): 1492-6. Here is the abstract:

Memories are thought to be encoded by sparsely distributed groups of neurons. However, identifying the precise neurons supporting a given memory (the memory trace) has been a long-standing challenge. We have shown previously that lateral amygdala (LA) neurons with increased cyclic adenosine monophosphate response element-binding protein (CREB) are preferentially activated by fear memory expression, which suggests that they are selectively recruited into the memory trace. We used an inducible diptheria-toxin strategy to specifically ablate these neurons. Selectively deleting neurons overexpressing CREB (but not a similar portion of random LA neurons) after learning blocked expression of that fear memory. The resulting memory loss was robust and persistent, which suggests that the memory was permanently erase. These results established a causal link between a specific neuronal subpopulation and memory expression, thereby identifying critical neurons within the memory trace.

Engineering the Next Revolution in Neuroscience

Engineering the Next Revolution in Neuroscience outlines an empirical framework for optimizing discovery in neuroscience. Drawing on illustrations from the molecular and cellular neuroscience of cognition, the framework is rooted in the everyday practice of experimental neuroscience. Toward the optimization of neuroscience, we offer concrete, detailed proposals for studying progress in neuroscience research.

The book is co-authored by Alcino Silva, John Bickle and myself.

Milestones in Neuroscience Research Timeline

There is a nice timeline of research in neuroscience at Eric Chudler's homepage (U. Washington).

Timeline: Molecular and Cellular Neuroscience of Learning and Memory

Here is an evolving time line of the history of the molecular and cellular neuroscience of memory. I say it's evolving, because it is ridiculously incomplete and I intend to update it quite a bit. If there are any inaccuracies or important omissions, let me know. I've included a few important developments in molecular genetics outside of neuroscience because they made possible later critical research in neurogenetics.

1913 Sturtevant discovers linear order of genes
1920 Sturtevant publishes series of articles entitled "Genetic Studies On Drosophila simulans"
1926 Hermann Joseph Muller introduces X-ray mutagenesis
1950 Katz & Halstead hypothesize that memory traces depend on protein synthesis
1957 Scoville & Milner publish on HM
1960 Curtis &Watkins discover glutamate is major brain NT
1963 Flexner shows memory is affected by protein synthesis in mice
1968 Discovery of PKA by Walsh & Krebs
1971 John O'Keefe discovers place cells
1973 Bliss and Lomo discover LTP
1973 Cohen & Boyer introduce a method for creating recombinant plasmids
1974 Jaenisch creates first transgenic mouse using retrovirus
1978 Dunwiddie & Lynch showed LTP depends on extracellular Ca+
1979 Evans and Watkins discover AMPA receptors using quisqualate
1979 Dunwiddie & Lynch show blocking extracellular Ca+ blocks LTP but leaves synaptic transmission, facilitation and PTP intact
1980 Baudry & Lynch first propose receptor unmasking theory of LTP
1982 Morris shows watermaze performance is hippocampal dependent
1982 Turner, Baimbridge and Miller showed transient increase of extracellular Ca+ is sufficient to induce an LTP-like response
1983 Collingridge finds glutamate acts on NMDA receptors in the hippocampus
1983 Lynch using EGTA shows that hippocampal LTP is intracellular Ca+-dependent
1983 Nairn & Greengard discover CaMKII and that synapsin is one of its substrates
1984 Davis & Squire publish influential review "Protein Synthesis and Memory"
1985 Lisman gives theoretical discussion of how an autophosphorylating kinase could serve as a LTM switch
1986 Morris shows blocking NMDA receptor blocks LTP & spatial learning
1986 Montminy showed cAMP regulates somatostatin expression
1987 Montminy introduces CREB as a regulator of somatostatin transcription
1988 Malenka & Nicoll discover second messenger role of Ca+ in triggering LTP
1988 Yamamoto shows that CREB stimulates cAMP transcription
1989 Gonzalez & Montminy show that cAMP stimulates somatostatin transcription via CREB phosphorylation
1989 Malenka & Nicoll showed that LTP depends on CaMKII phosphorylation
1991 Sheng, Thompson & Greenberg suggest that CREB is regulated by CaMKII (turns out false)
1992 Silva shows that null mutation for CaMKII disrupts LTP + spatial learning, first knockout study in neuroscience of learning and memory
1993 Bliss & Collingridge outline their synaptic model of hippocampal-dependent memory, providing roles for both NMDARs & AMPARs
1994 Bourtchuladze shows LTM but not STM affected in CREB mutants
1995 Bartsch shows that CREB can facilitate synaptic growth in Aplysia
1995 Bannerman & Morris upstairs/downstairs experiment
1995 Lledo Malenka & Nicoll show that CaMKII is sufficient to induce LTP
1995 Isaac, Nicoll & Malenka provide evidence for silent synapses AMPARs
1996 Mayford & Kandel introduce CaMKII transgenics
1996 McHugh & Tonegawa show impaired place fields in NMDAR1 knockouts
1996 Rotenberg, Mayford & Kandel show mice expressing activated CaMKII lack low frequency LTP and do not form stable place fields in CA1

Taxonomies of Experiment III: Silva, Bickle and Landreth

A third taxonomy of experiment can be derived from an article by Alcino Silva (UCLA) published in Journal of Physiology - Paris 101 (2007) 203–213 and work that Silva and I are doing along with John Bickle, who is at the University of Cincinnati. (Bickle and Silva have a related article that will soon be published in the Oxford Handbook of Philosophy and Neuroscience. Bickle is the editor of that volume.) This taxonomy is a work in progress.

The proposed taxonomy of experiment covers some of the same considerations that Craver and Sweatt considered. But it holds that there are 3 broad classes of experiment that are distinguished by their goals. The goals are: 1) description of phenomena, 2) assessment of causal relations among phenomena, and 3) development of tools to facilitate 1 and 2. Let's call experiments of class 1 Descriptive Experiments, those of class 2 Connective Experiments, and those of class 3 Validation Experiments.

Descriptive experiments focus on the dissection and description of phenomena without regard for the evaluation of causal hypotheses, per se. Causal considerations will of course affect the interpretations of one's measurements in these experiments, e.g. in the use of an imaging technique. But the goal of these experiments is not to assess the causal relations among the phenomena that constitute the subject matter. For example, one can dissect the hippocampus and describe its parts without testing hypotheses about the interactions of those parts.

Connective Experiments attempt to determine whether states of phenomena depend on each other. These assessments are made on the basis of manipulations (intervetions) and measurements of the phenomena of interest. There are 3 forms of connective experiment: 1) positive manipulations, which increase the value of an independent variable; 2) negative manipulations, which decrease the value of an independent variable; and 3) neutral measurements, which measure correlation between an independent and dependent variable under normal test conditions (roughly equivalent to Craver's activation experiments).

Validation Experiments validate the use of a tool, demonstrating that it is a reliable means of manipulating or measuring phenomena of interest. For example, the demonstration that knockout mice can be used to reveal the role a protein (e.g. CamKII) plays in both spatial learning and long-term potentiation validated the use of knockouts in the neuroscience of learning and memory. These experiments did not invent the knockout technique of course, but they did adapt a tool for use in neuroscience and led to a swarm of innovative transgenic approaches.

These forms of experiment are not entirely distinct. Validation experiments draw more attention when they simultaneously introduce a tool and reveal undiscovered phenomena or undiscovered causal dependencies. Descriptive experiments are often performed in such a way as to reveal causal information, e.g. that glutamate receptors can be found in pyramidal cells. The three different goals of experiment are mutually dependent, but any one of them can be performed with little regard for the others.

Taxonomies of Experiment II: Carl Craver

In his book Explaining the Brain (2007), Carl Craver argues that there are 3 basic kinds of experiment in neuroscience: interference experiments, stimulation experiments, and activation experiments. The first two kinds of experiment are bottom-up, involving direct interventions on the components of neural mechanism. The third kind of experiment is top-down. According to Craver, these three forms of experiment are used to help neuroscientists discover neural mechanisms. Neural mechanisms are composed of causal processes whose joint function explains a target phenomenon. For example, the mechanism of the action potential is composed of causal processes with parts including ion gradients and ion channels. Finding explanations for phenomena in neuroscience involves determining which processes comprise the mechanism of the phenomenon. The three kinds of experiment in Craver's taxonomy make possible those deteriminations.

Interference and stimulation experiments are bottom-up in the sense that they are attempts to alter the state of a phenomenon to be explained (explanandum) by interfering with component processes of its mechanism. Lesion-studies are the paradigmatic instance of inference experiments. Other instances might include transcranial magnetic stimulation (TMS), genetic knockout, and receptor blockers.

Stimulation experiments are also bottom-up experiments. As the name suggests, these combine the stimulation of mechanism components with the measurement of a dependent variable. Here, microstimulation studies are the paradigm case.

Activation experiments are top-down experiments. That is to say, they involve interventions on a target phenomenon by going through "the normal causal pathway" that affects that target. To illustrate what he means by an activation experiment, Craver writes:

"There are several common varieties of activation experiment at all levels in neuroscience. In PET and fMRI studies, one activates a cognitive system by engaging the experimental subject in some task while monitoring the brain for markers of activity, such as blood flow or changes in oxygenation... In single- and mutli- unit recording experiments, one engages the subject in a task while recording the electrical activity in neurons. In other studies, researchers monitor the production of proteins, or the activation of immediate early genes such as c-fos and c-jun. The experiments leading up to Hodgkin and Huxley’s model of the action potential involved generating action potentials and monitoring single ionic currents while the neuron spiked..."

(Craver 2007, 151)

Compare these categories of experiment with David Sweatt's system and notice that Craver does not include the "determine" class of experiment that Sweatt offers. Nor does Sweatt offer considerations regarding the top-down or bottom-up nature of experiments. Also notice that Craver's class of activation experiments assumes that some form of manipulation is being performed by the experimenter on the neural system. The manipulation might just be a psychological task that is to be performed while measures of neural activity are taken. Or, the manipulation might be some form of stimulating input, such as in the Hodgkin and Huxley example, so long as that input mimics the normal input to the mechanism (in this case, of the action potential). There are no purely observational forms of experiment in Craver's list.