Tuesday, April 17, 2007

Got nicotine?

As a pledge-signing member of my elementary school's rigorous D.A.R.E. program, I was regularly exposed to horrific photographs of blackened lungs and deteriorating hearts, and was utterly convinced that cigarettes ravaged and destroyed everything they touched. Thus when my dad started smoking when I was about 5, I (with my mom's encouragement) flushed pack after pack down the toilet until he quit. I still do not smoke, and I don't plan to, but I have grown a bit more discerning with respect to sensationalist demonization of various pleasure-enhancing drugs.

It turns out that of the 4,000 or so compounds in tobacco smoke, including a variety of carcinogens, and toxins such as carbon monoxide, heavy metals, and cyanide, at least one ingredient actually has some beneficial effects: nicotine. There is a large body of research showing that nicotine, the ingredient that drives people to addiction, improves cognitive function in humans and laboratory animals. The most robust effect demonstrated in human smokers is an enhanced ability to sustain attention to a task for a prolonged period of time, an ability inextricably linked to learning and memory. Of course, learning and memory involve a number of processes (acquisition, encoding, storage, and retrieval), but the ability to concentrate on particular stimuli and screen out the rest is critical for the success of this operation.

Nicotine's beneficial effects on these "higher" cognitive functions have prompted efforts to develop nicotinic treatments for diseases associated with cognitive impairment, such as Alzheimer's disease, Parkinson's disease, attention deficit/hyperactivity disorder, and schizophrenia. However, this area of drug development is impeded by the complexity of nicotine's actions, including the observation that cognitive improvements have only been reliably detected in either smokers or the cognitively impaired. In contrast, nicotine tends to have deleterious effects on cognitive performance in "normal" non-smokers. (Another factor hampering the development of nicotine-based therapies is that they offer pharmaceutical companies little potential for financial gain, as nicotine sources are easy to come by.)

So how does nicotine affect cognitive function? First, a bit about how neurons communicate with each other. Some people may be able to skip the next few paragraphs (which I've distinguished with altered formatting), but I've included them anyways because it's important to have at least a rough idea of this process in order to understand how nicotine works in our brains.
Neurons are functionally integrated in expansive neural networks, with each neuron receiving up to thousands of inputs from other neurons. However, the vast majority of neurons* are not actually physically connected to one another; there is a tiny gap that separates neurons, called a synapse.

When a neuron is activated, an electrical pulse (an action potential) travels down its membrane; the neuron is said to "fire" an action potential. When the action potential reaches the end of the neuron, it cannot traverse the synapse, but instead induces the release of chemicals which can. Once liberated from the "pre-synaptic" neuron, these chemicals (called neurotransmitters) navigate across the synapse and bind to specific receptors on the "post-synaptic" neuron. Once bound, the neurotransmitters induce one of many physiological changes: they can make it easier to fire an action potential ("excitatory" neurotrasmitters), more difficult to fire an action potential ("inhibitory" neurotransmitters), or modulate the firing rate or other behavioral properties of the cell.

An overwhelming number of pre-synaptic neurons, all of which are sources of neurotransmitters, impinge on a single post-synaptic neuron, yet the latter responds with a binary decision: fire or don't fire. The cell creates order from this chemical deluge by performing a complex, time-dependent summation of all of its inputs; if it receives a sufficient number of excitatory inputs within a reasonable time window, it will fire an action potential and release its own neurotransmitter, passing the information along the circuit.
Each neurotransmitter can bind to a number of complementary receptors. One of the receptors for a neurotransmitter called acetylcholine (ACh) happens to also bind and respond to nicotine, which is not naturally present in the body. Thus when a post-synaptic neuron containing these particular receptors (called nicotinic ACh receptors, or nAChRs) is exposed to nicotine (as in when someone smokes a cigarette), it behaves as if it has been influenced by ACh; i.e. to an individual nAChR, nicotine and ACh are indistinguishable.
However, there is a crucial difference at the circuit level: ACh is regulated by your body, so it is typically released in small amounts by specific subsets of neurons at any given time. In contrast, nicotine, entering your body from an external source, can potentially act at all nAChR-bearing neurons simultaneously, leading to widespread activation and an assortment of consequences (including the release of other neurotransmitters, such as dopamine, endorphins, and ACh itself).

Turns out that the prefrontal cortex (PFC), a brain structure with a critical role in learning and memory, contains an abundance of nAChRs. This area receives information from all of the senses, and aids the learning process by directing attention to a limited set of input streams at a time. Like many cortical synapses, the excitatory synapses in the PFC are plastic, capable of undergoing systematic changes in synaptic strength/efficacy.

These changes are thought to underlie the processing and storage of information in neural circuits, and for that reason take place in a functionally relevant manner (i.e. one that is dependent on the activity of that particular synapse). Specifically, the robustness and direction (stronger or weaker) of the change in synaptic strength is dependent on the precise timing of pre-synaptic inputs and post-synaptic action potentials (also called "spikes"). This temporal correlation gives the process its name: spike-timing-dependent plasticity (STDP). According to the rules of STDP, a synapse with a high temporal correlation between pre-and post-synaptic activity will strengthen.

Importantly, excitatory synapses in the PFC change during working-memory related tasks, implicating the PFC in these cognitive behaviors. It is likely that nicotine's effects on attention and working memory are effectuated at the nAChR-containing synapses of the PFC, but the mechanistic changes are unknown. Moreover, it is unclear how these synaptic changes affect the functional properties of the circuit underlying these cognitive processes.

A group from Amsterdam, led by Huibert Mansvelder, published a study that explored the cellular and synaptic mechanisms of nicotine's actions in the most recent issue of Neuron, with a focus on how nicotine affects STDP in the PFC.

The scientists cut rat PFCs into slices, and induced STDP by electrically stimulating pre- and corresponding post-synaptic cells simultaneously. After repeating this paired stimulation (50 times), the synapse becomes "potentiated," meaning the pre-synaptic neuron becomes more effective at stimulating the same post-synaptic cell than it was before the procedure; i.e. the synapse is "stronger".

When nicotine was applied to the solution bathing the slice, this potentiation failed to occur. The blockade of STDP could be overcome, however, by increasing the electrical activity of the post-synaptic cell, indicating that the pairing procedure was less effective, but not defunct. The group found that nicotine's net effect was to enhance the release of a major inhibitory neurotransmitter, called GABA. In the context of the synapse, the post-synaptic neuron would thus be bombarded by copious amounts of GABA, which would then dominate the input summation. These actions decrease the likelihood that the post-synaptic neuron will fire, thereby interfering with the concomitant activation of both synaptic partners and interfering with STDP.

So how does impeding STDP, a process thought to provide the cellular foundation for an adaptive nervous system, enhance cognitive function in certain individuals? This question brings us back to attention--the ability to concentrate on relevant stimuli while ignoring that which is irrelevant. During PFC-based cognitive behaviors, the neural activity in the PFC may increase to distracting levels. By enhancing inhibitory neurotransmission, nicotine may enhance the "signal-to-noise" ratio, thereby improving attention selectivity. This may be particularly beneficial to smokers who are accustomed to high nicotinic stimulation, as well as individuals with cognitive impairment, as both these groups are functioning suboptimally in the absence of nicotine.

And what about normal individuals, whose cognitive functioning is often impaired with nicotine? Such "drug-free" individuals are probably already performing at or near their optimal level of performance. As a result, increasing nicotinic stimulation and interfering with STDP will have negative effects in most situations. It is possible, however, that even in "normal" individuals, nicotine may enhance cognitive function under extreme task demands. During such tasks, which necessitate intensified attention over a prolonged period of time, optimal performance may be facilitated by nicotinic stimulation.

*My explanation of synaptic transmission describes, specifically, a chemical synapse. These are the predominant form of synapses in the brain, but neurons can also be connected by channels called "gap junctions." The openings of these channels allow ions to flow from one neuron to the next, enabling electrical signals to pass directly between neurons. This type of connection is called an electrical synapse, and the transmission of information is much more rapid than at chemical synapses.

Thursday, April 12, 2007

If only two rods had gone through Gage's head...

The wonderfully diverse repertoire of animal behaviors is, to a great extent, motivated by a universal desire: to acquire or avoid a particular outcome. Such "goal-directed" behavior is contingent upon learning associations between our actions and the predicted consequences, and, as the name implies, using these associations to direct our behavior. For example, many people drink alcohol because they anticipate enjoying the psychological and physiological results, gamblers place bets based on their expected outcome of the game, and dogs learn tricks because they associate the performance with tasty treats.

A key feature of goal-directed behavior is that it can adapt to new (and unexpected) outcomes caused by changes in the environment. For example, say we learn that a particular route is the fastest and most scenic way to get to work, so we tend to take that route. If construction suddenly begins along that route, that same behavior becomes associated with significant traffic delays and unattractive scenery, which is far less rewarding. Luckily, we have the ability to restructure our mental associations and modify our behavior accordingly, seeking a novel route.

This type of learning, in which our brains are able to respond to altered contingencies, is called "cognitive flexibility." People with deficits in cognitive flexibility, such as those with particular types of brain damage, cannot make such adjustments, and thus the consequences of their actions fail to adequately influence their behavior.

The paradigmatic patient exhibiting such deficits is Phineas Gage, a railroad worker from the mid-1800s. There is an excellent write-up of Phineas's story here; I will offer a brief synopsis: while Phineas was working construction, a 3-foot metal rod (1.25 inches in diameter) blasted through the front of his brain and landed 30 yards away. Phineas survived, but had become, according to his physician, "fitful, irreverent, indulging at times in the grossest profanity (which was not previously his custom), impatient of restraint or advice when it conflicts with his desires, at times pertinaciously obstinate, yet capricious and vacillating, devising many plans of future operation which are no sooner arranged than they are abandoned." In short, he was unable to control his behavior in accordance with the perceived consequences of his actions.

It was concluded, much later, that Phineas's behavior resulted from damage to his prefrontal cortex, which is now associated with a range of high-level cognitive processes, including decision-making, executive function, and working memory. A region of the PFC, the orbitofrontal cortex (OFC), has been implicated particularly in cognitive flexibility. Although it is often regarded as "the" region involved in consequence-motivated behavior, recent research has demonstrated that this role is dependent on its connections with a region called the basolateral amygdala (BLA), an area involved in forming associations between behaviors and outcomes. These two regions are thought to form part of a circuit that underlies goal-directed behavior, but the precise contributions of these regions and the nature of their connections is not well understood.

A group from the University of Maryland, led by Thomas Stalnaker, recently published a paper in Neuron that explored this complicated relationship, using a task called "reversal learning." Basically, rats were trained to poke their noses into ports in response to a particular odor (odor A); this behavior was rewarded with sugar water dispensed into the port. Poking in response to another odor (odor B), however, resulted in a bitter solution (quinine) delivered to the port. Once the rats learn to discriminate the odors and their respective consequences, they will habitually poke in response to odor A, and avoid poking in response to odor B.

At this point, the sly experimenters switch the task: when the rat pokes in response to odor A, expecting a delightfully sweet elixir, it is greeted instead with the sharp, biting taste of quinine. After a bit more poking, the rat will figure out that if it pokes in response to odor B, it can receive the sugar water. Thanks to its cognitive flexibility, it will then rapidly adjust to this novel arrangement of outcomes and poke only in response to B.

It is known that animals with damaged OFCs can learn the initial discrimination task, but take much longer than normal animals to relearn the task once the contingencies are reversed. It is also known that neurons in both the OFC and the BLA are known to fire selectively in response to cues associated with particular rewards; in this example, some neurons (group 1) would fire in response to odor A, while others (group 2) would fire in response to odor B. After the rat learns the reversal, this relationship will switch, and group 1 will fire after odor B, and group 2 after odor A.

Until a few years ago, researchers believed that the OFC was responsible for acquiring and signaling these reversed associations to the BLA. The BLA, encoding the behavior/consequence association, would then modify its association to be congruent with the information from the OFC. More recently, however, this group discovered a key difference in firing between these two areas: neurons in the BLA reverse their firing behavior more rapidly than those in the OFC, indicating that the BLA encodes the reversal before the OFC. This pattern is inconsistent with the original hypothesis.

This group developed a new hypothesis: the activity of neurons in the OFC might encode the expected outcome of a behavior, which can then be compared to an actual outcome encoded by neurons in a different area (or areas). If there is a mismatch between the two, as in the reversal learning task, the OFC communicates this conflict with the BLA, which can then modify its association (by reversing the neuronal firing behavior). Once the animal's behavior changes in response to this new association, the expected outcome changes, thus resulting in a change to the firing behavior of the OFC neurons. This communication allows the animal to rapidly reverse previously acquired discriminations.

If the OFC is damaged, the mismatch is undetected. This allows the association in the BLA to persist and continue to guide behavior, even though it is no longer relevant. It's important to keep in mind that animals with damaged OFCs do eventually learn the reversed contingency, it just takes much longer. This group believed that it is this persistence and inflexibility of the BLA associations which impedes learning the novel discrimination, but other brain areas are eventually able to compensate.

This belief prompts a rather counterintuitive prediction: after damaging the OFC, if you remove the persistent association in the BLA, you will remove the impediment to relearning; i.e. cure the problem caused by OFC damage by damaging the BLA too!

So the group tested four groups of animals: those with OFC damage, BLA damage, OFC and BLA damage, and no area damaged (they damaged the areas by injecting an excitatory neurotoxin specifically into the area of interest). All four groups were able to learn the initial discrimination task, but, as had been shown before, animals with damaged OFCs took twice as long to learn the reversed discrimination. Strikingly in accordance with their hypothesis, animals with damage to both the OFC and BLA could learn the reversed association as well as animals which had not been damaged at all (and as well animals with BLA damage, who show no deficits in reversal learning).

They concluded that the OFC is not directly involved in learning the association, but rather generates representations of expected outcomes, which can be communicated with other regions. The OFC/BLA circuit may be the default pathway recruited for directing behavior, but in their absence the brain can call upon other areas that are capable of supporting reversal learning. Thus, damaging the BLA removes the persisting association, allowing mercenary regions to compensate for the loss.

The critical point is that the connections between the OFC and areas like the BLA are important for cognitive flexibilty, not just one brain area or the other. Brain areas do not work in isolation, but in the context of a network (which includes other regions as well). It is this network, and the cooperation between regions, that encodes behavior-outcome associations, and uses them to motivate and guide behavior.

Saturday, April 7, 2007

Color me fantastic

For some background on vision, particularly color vision, read my previous post.

Millions of years ago, before the dinosaurs roamed, the remote ancestors of mammals probably had magnificent color vision. When the dinosaurs came to ecological power, these ancestors (mammal-like reptiles), were banished from the daylight hours and became creatures of the night. During this period, their visual systems evolved be maximally sensitive to light, allowing their proficiency at color discrimination to deteriorate in exchange. Although many mammals have returned to reclaim their diurnal dominion, their color vision still pales (get it?) in comparison to that of other vertebrates, such as reptiles, fish, and birds, whose ancestors did not suffer this period of exile.

Most mammals (some primates, including humans, are exceptions) rely on a dichromatic visual system, with cones that respond optimally to either short (S) or medium (M) wavelengths. (For more on this, see the primer) Our remote ancestors, along with the aforementioned other vertebrates, had three, four, or possibly more types of cones. A broader range of cones enables an exponentially enhanced ability to distinguish colors, but was unnecessary during our ancestors' tenure as nocturnal animals and was subsequently lost.

Our more proximal ancestors, apes and Old World (African) monkeys, regained a third cone, the long (L) wavelength-responsive cone, an adaptation that was instrumental to our evolutionary success. This cone allowed primates to distinguish red from green, which would have been advantageous for purposes of foraging. There are two major questions that arise from this trichromatic revival: How did it happen? and How did our higher visual systems adapt to interpret/perceive this new dimension of sensory experience?

The first is a question of genetics, which I'll answer briefly before delving into the neuroscience. Outside of the lab, new genes aren't added to the genome de novo; they arise from duplications of extant genes. Over time (on an evolutionary timescale), mutations change one or both copies of the gene such that they end up coding for different proteins. The genes that code for the M and L cones are very similar, and are on the same chromosome; it is thus likely that when Old World monkeys regained trichromacy, the gene for the M cone duplicated and mutated to form a gene that coded for the L cone (the divergence of color sensitivities would have been favored by natural selection). It is further believed that the M cone originated from a duplication/mutation/diversion combo from the S cone (which is on a different chromosome).

So that's how it happened on a genetic/peripheral level, but what about the mind? What good is it to be able to have this increased diversity in the retina if the brain is only capable of processing a dichromatic visual world? How much time had to pass before we could not only see this new dimension of light, but could interpret it as well?

A very exciting paper came out recently in Science, in which a group led by Gerald Jacobs at UCSB (the king of color vision, I'm told) used genetically engineered mice to model this crucial adaptation to trichromacy. Mice are, like most mammals and our ancestors, dichromatic, with only S and M cones. The group furnished the mice with the gene for the human L cone, resulting in mice with all the retinal components of trichromatic vision. They then sought to determine whether the mouse nervous system would be able to capitalize on this new information, thus endowing the mouse with an enhanced sensory experience. Alternatively, the visual system may be genetically wired such that it could only handle inputs from the two existing classes of cones, implying that the homologous primate adaptation would have required generations for adaptive rewiring.

To address this question, the group wanted to see if the "trichromatic" mice were able to distinguish between two colors which their dichromatic siblings (and ancestors) would have considered the same. To do this, they put them in front of three colored panels, on which two different wavelengths or intensities were displayed (they should be able to distinguish, for example, a dim green light from a bright red light...for background see primer). The mice were trained to identify which of three panels was illuminated with a different color than the other two; a correct choice was rewarded with drops of soymilk. After about 17,000 tests (literally), most of the mice with the S, M, and human L cone were able to learn the task, and chose correctly 80% of the time! (Mice with only the S and M cones still performed no better than chance after a similar number of trials.)

This demonstrates, quite wonderfully, that "the mammalian brain is sufficiently plastic that it can extract and compare a new dimension of sensory input." It follows, they speculate, that the primate that first inherited an L cone would have immediately reaped the benefits, and would likely have enjoyed a selective advantage.

I think this is quite a marvelous find, and the paper made me happy and excited, but I am also not entirely surprised by the result. It is merely a reminder of the essence of evolution, in that it tends to be guided by everyday biological mechanisms; things that seem special or mysterious in our natural history and our living world are made possible by these same mechanisms. I've previously discussed this idea that the parsimony of evolution would allow novel adaptations in the periphery to exploit previously evolved central circuitry. For this study, I think it's helpful to supplement this idea with a brief discussion of the striking plasticity of the developing nervous system.

When an animal is developing, the pathways of the visual system are highly sensitive to certain visual stimuli, and grow and refine under the guidance of the animal's experience. Some of you may have heard of experiments in which kittens were raised in boxes in which their only visual stimuli were black and white vertical stripes. Their visual pathways developed such that neurons in the visual processing areas responded primarily to vertical lines, and failed to respond to horizontal lines when presented later in life. In a sense, their brains were not able to see horizontal planes.

It seems to follow that if an animal is endowed with a "new" element of visual stimulation, rather than deprived, its developing visual pathways would accommodate this visual world by wiring up accordingly. As the brain of a dichromat was already capable of comparing information from two different classes of cones to compute a color representation in the brain, it may not have been too demanding a request to factor a third into the equation. Importantly, this adaptation would take place during the development of the first animal that possessed the new cone, as opposed to an evolutionary timescale.

This brings up an intriguing thought: what would happen if a mutation arose that endowed humans with a fourth cone? Our resulting percept of the world is a bit difficult to fathom...would we be able to distinguish light mixtures that normal people cannot? What would these colors look like? What if this new cone was able to detect light outside our visible spectrum--ultraviolet light, like bees and some birds, or infrared light, like rattlesnakes?

Friday, April 6, 2007

Vision primer

My trichromatic mouse post was way too long, so I decided to take all the background information on the visual system, which some of you may not need, and put it in a different post, thus breaking it into more digestible bits. This intro focuses on topics relevant to the article (photoreceptors and the initial processing stages of color vision), so although I would love to discuss physical nature of light, the complexity of the vertebrate eye, and higher visual processing, I'm leaving all that for another day!

Our visual system exploits light to generate a useful representation of our environment. This process involves a highly specialized division of labor, in which our eyes function as the light "detectors." They then convey this data in an organized fashion to the visual areas of the brain, which process this deluge of information such that we can make sense of our visual world.

This sounds basic, but I want to make sure I preclude the common misconception that our eyes are like tiny cameras, which project tiny inverted images of the world for the brain to "watch" like a movie on a screen. Instead, when light (with wavelengths between 700 billionths of a meter to 420 billionths of a meter) hits our eyes, it activates a certain pattern of sensory neurons in the retina (photoreceptors), and after some preliminary organization at the level of the retina, a modified pattern is conveyed to the brain, where it is decoded in the brain by a mechanism which is far from understood. So not only do our photoreceptors not relay "picture of a tree," they do not even convey information such as "green."

Color Vision
How, then, do we see "green?" Our eyes rely on a three-color system ("trichromacy"); that is, they contain three distinct classes of color photoreceptors (called cones), each of which respond differently to a given wavelength and intensity of light. The classes are generally called short-wave (S), medium-wave (M), and long-wave (L), in reference to their optimal responsiveness to either short, medium or longer waves of light. They are more commonly called Blue, Green, and Red cones, colors which correspond to their respective wavelength (although Red is somewhat a misnomer because the neurons actually respond optimally to Yellow).

However, these cones are pretty broadly tuned, meaning they are actually capable of responding to wide ranges of light; the functional difference between an optimal wavelength and a suboptimal wavelength for a given class is a difference in the firing rate of the cone. To add further confusion, the firing rates are dependent not only on the wavelength, but also to the amount (intensity) of light.

This explains why the magnitude of response of a single class of cones cannot tell us anything about the color of a light: a "green" cone will respond to a dim green light with the same firing rate as it will to a bright red light. In order to deduce the color of an object, it is thus necessary for the brain to compare the responses of representatives from each class of cones. With trichromatic vision, our brains can compare the firing rate of three classes of cells, all with different properties, giving us the ability to perceive and differentiate among a dazzling array of hues.

Most mammals have dichromatic vision, with only two cones (short and medium); seeing through their eyes would be akin to missing a cone, which is the case with the 8% of men and 0.5% of women who are colorblind. Many fish and reptiles, on the other hand, are tetrachromatic, and the retinas of birds and turtles may exhibit even more diversity.

To see what happens when you give a dichromatic animal trichromatic vision, read on...