The four-minute neuroscientist

My friend Tim Allsop is currently playing Roger Bannister in an outdoor play which recreates his record-breaking run on the Iffley Road athletics track in 1954. As part of his preparation for the role, Tim met the great man, and found that he was rather bemused by his status as a sporting icon. He would much rather be remembered, apparently, for his research in neurology. His Wikipedia entry contains barely a hint of Dr Bannister's research career - it's all about that glorious 4-minute mile. A quick search on Google scholar reveals that his scientific interest was focussed on diseases of the autonomic nervous system, on which he published widely. From my perspective as a humble PhD student (who is currently training for the Hornsey 10K run!) I can understand his feelings. It seems to me that the commitment and hard work required to get a productive scientific career going probably makes training for an athletics record attempt pale into insignificance by comparison.

Towards an artificial engram

Liu X, Ramirez S, Pang PT, Puryear CB, Govindarajan A, Deisseroth K, Tonegawa S (2012) Optogenetic stimulation of a hippocampal engram activates fear memory recall. Nature.

Tim Bliss and friends recently wrote about the possibilities of using fancy genetic tools to test the hypothesis (quite an old hypothesis too) that memories are encoded in networks of neurons whose synapses have been modified by LTP-like plasticity (Neves, Cooke & Bliss, Nat. Rev. Neurosci. 2008). This week's Nature has a remarkable paper from the Tonegawa group, where they have made a big step forward along this path. They used a clever combination of transgenic mice and virus transfection techniques to produce a mouse where activated neurons in the dentate gyrus (DG) of the hippocampus are not only labelled with YFP but also (and here's the powerful bit) express light activated sodium channels (ChR2). They found that fear conditioning labelled a subset of neurons in the DG, and what's more, that activating this set of neurons a few days later (by shining blue light into the hippocampus), even in a different context to the original conditioning, produced the freezing reaction that you see in fear conditioning.

So, a significant step forward in the quest for the engram. It's interesting to ponder how accurately this result reflects how memory operates in the intact animal. You might expect that more than the DG is involved, for example. Would it be possible to do similar experiments with similar results in CA1 or CA3? And what about other forms of memory, such as episodic memory, would they work in a similar way?

Presynaptic NMDA receptors

Presynaptic NMDARs in the Hippocampus Facilitate Transmitter Release at Theta Frequency
McGuinness et al 2010 (Neuron)

This paper comes from the group of Nigel Emptage in Oxford, who has agreed to take me on for a short placement this summer. So I should probably be careful what I say! However, this does really strike me as a well presented paper with a strong story about the presynaptic role of NMDA receptors.

In rat hippocampus organotypic slices, they patched a CA3 pyramidal neuron and filled it with a calcium-sensitive dye, which enabled them to stimulate action potentials in the cell and observe calcium transients in boutons synapsing onto neurons in CA1. Using fancy stats, they identified less frequent large-amplitude transients which were abolished by the NMDAR-blocker AP5. Perfusing the cell with norketamine, an internal NMDAR blocker also abolished the large transients, confirming that this is definitely a presynaptic phenomenon. Some EM pictures confirm that NMDARs are present at both sides of these CA3-CA1 synapses. They also used glutamate uncaging (my pet technique!) to show that presynaptic NMDARs can produce inward currents visible back at the soma. And in a nice coda, they started to explore the physiological relevance by showing that these large calcium transients are more frequent after induction of LTP by theta-burst stimulation.

So the idea is that a large calcium transient corresponds to the release of a glutamate vesicle at the bouton. This is stochastic - it doesn't happen every time. The kinetics seem to be fast enough to allow for the displacement of magnesium to open up the NMDAR channels within the duration of a single action potential.

Caveat: this work was done in organotypic slices after 7 to 14 days in vitro, so may be more relevant to the physiology of the developing than the adult brain.

Sleep and spine loss

Sleep and waking modulate spine turnover in the adolescent mouse cortex

Maret et al 2011 (Nature Neuroscience)

This is an intriguing little paper just appeared in Nature Neuroscience, looking at spine turnover in sensorimotor (a bit of a vague word this) cortex of adolescent (P23-44) YFP-expressing mice. They imaged the same section of dendrite through the skull of the same mouse on two separate occasions - quite a technical achievement - at the end of a period of wakefulness, and then again after the mouse's next sleeping period. In this situation they found a net loss of spines after the sleeping period, whereas if they reversed the order, imaging after sleep first, then after waking, they found a net gain of spines. These differences disappeared in adults.

So sleep seems to be an important period of synaptic pruning in adolescent mice. I wonder if this applies to humans as well. Teenagers seem to go through these periodic bouts of excessive sleeping. Perhaps they're not being lazy after all - they're engaging in important synaptic pruning, remodelling their brain circuits in preparation for adult life?

Seaside fun!

I'm writing this on the train from Devon back to London.

I've just spent a fortnight at the Marine Biological Association in Plymouth, attending the 28th annual Microelectrode Techniques Workshop. Two weeks of very intensive work - lectures, demonstrations, practical sessions in the lab, and an excellent lunch every day in the common room overlooking Plymouth Sound and the Americas Cup.

I have to say this course has been a real eye-opener for me - I first learned whole-cell patch clamping 3 years ago, but this is the first time I've really been taken through the physics and electronics of what the technique involves. It's one thing to know to press this button and twiddle that knob until the spiky bits go away, it's quite another to understand where that capacitative transient comes from, and how the compensation circuits work that can remove it from your signal. (This is no reflection on my teachers by the way - it reflects rather my own laziness and lack of curiosity.)

The very first day in the lab exemplified the approach - a solid lecture on electronics, followed by a happy day soldering up various op-amp circuits in various configurations: current to voltage converter, voltage follower, differentiator.

A huge amount of learning was packed into these two weeks. Lots of time in the lab, which was kitted out with many rigs of various kinds, gave me opportunities to try out several techniques, including iontophoretic cell injection with sharp electrodes, and single channel recording. The demonstrators were friendly, dedicated, and in many cases pretty distinguished. There was also a generous schedule of lectures, with highlights for me from Boris Barbour on amplifier electronics, and David Ogden on photolysis.

Another highlight was a fabulous morning spent on the MBA research vessel trawling for crabs and shrimps and miscellaneous denizens of the deep.

We finished about 7:30pm each day, and went in 6 days a week, which has left me tired but exhilarated - my head is buzzing with ideas for things I want to try out when I get back to the lab at UCL. I would recommend this course without hesitation to anyone who wants to be a better electrophysiologist. One clear lesson I have come away with is this - a good scientist does the homework to really understand the techniques they use. It's not good enough to just copy the protocol from a paper you read once, blindly following someone else's recipe.

Reading Week

This week is Reading Week, where the whole lab decamps down to my supervisor's lovely seaside home, and we spend the week reading neuroscience papers, and discussing them over huge meals and abundant bottles of wine. Alongside shorts, sunhat and various neuroscience texts, I also - with a vague sense of shame - packed GRR Martin's "Game of Thrones" to read in my spare time.

Which highlights a point of tension for me: should I even be reading for fun at all? Shouldn't all my reading hours be given over to papers, reviews and books about neuroscience? If I'm serious about this PhD shouldn't I be willing to sacrifice reading fiction for fun for a few years?

And it's not just a reading issue - what about my hobbies? When I was a wage-slave in the IT industry all those years, frustrated and bored at work much of the time, I lived for my free time, and hobbies were a big part of that. Paragliding, boardgaming, wargaming, mountain walking, cycling, drawing, even dancing for a couple of years. Now that I'm in the lab doing something interesting and exciting with my working day, it's not so urgently important for my happiness to be doing all these hobbies. Plus there's less time and energy left for them these days. But I am still pretty interested in some of them, especially walking and boardgaming and books. Should I still be making time for this stuff? My supervisor on my masters told me "science should be your hobby too". Even at this early stage, I can feel the totalizing pressure of science on my life, and I can see that for some of the really successful scientists it becomes an all-consuming passion. Should I let that happen to me? Or is there still room for a hobby or two in my life?

Terence Smith: imaging complex neuronal behavior in the enteric nervous system

I went to a fascinating seminar today at UCL given by Terence Smith of the University of Nevada. He was talking about his work on the enteric nervous system. Turnout was low - most of us are not that excited by anything without "hippocampus" in the title! But I was glad I went. Prof Smith described this fine mesh of ganglia and interconnecting fibres sitting between the smooth muscle layers of the gut. Did you know that the ENS has as many neurons as the spinal cord?! And what a fascinating array of neuron types - sensory, excitatory motor, inhibitory motor, pacemaker and many varieties of interneuron. All talking to each other by means of strange and unfamiliar (at least to me) neurotransmitters - 5HT, acetyl choline, ATP, NO and suchlike. Prof Smith and his lab have been using calcium sensitive dyes to track patterns of neuronal activation. The nice thing about the ENS is that it is a mammalian system that is also a tractable problem - at least compared with brain areas like the hippocampus. A relatively simple circuit producing a relatively small repertoire of "behaviours" - it should be possible to get to the bottom of this in the next few decades. Which is more than we could hope for with the hippocampus for example....