Brain's defence against cocaine

Abstract

Warm Low density Cool High density Warm Platinum film Spin current a b c Electric field Electric field Chemical potential m m ↓ m ↑ Ferromagnet Magnetic field ('spin-up') Ferromagnet Metal bar Cool Warm Cool process. In the rest frame of the electron, the charged impurity rushing towards it constitutes a current filament, so the electron 'sees' a weak magnetic field circling the filament. This non-uniform magnetic field imparts a force on the electron along a direction that depends on its spin orientation 3,4. The net result is that spin-up electrons are pushed to the right of the impurity whereas spin-down electrons are pushed to its left. In effect, each impurity acts like a spin filter that selectively kicks electrons to one side or the other, depending on their spin. As shown in Figure 1c, the excess spin-up population in the incident beam results in more charge accumulating on the far face than on the near face of the platinum film. The voltage difference between the two faces is observable as a Hall signal. The asymmetric scattering of electrons is especially large in materials with a high atomic number, such as platinum. Following its prediction 3,4 , the spin Hall effect was first observed by applying purely optical techniques to semiconductors 5,6 , and was later detected electrically in metals 7,8. In a series of tests, Uchida et al. 2 convincingly show that the Hall voltage in the platinum film arises from the spin voltage. The Hall signal in the platinum film tracks both the magnitude and the direction of the magnetization in the nickel-iron film. Moreover, by moving the platinum film along the length of the nickel-iron film, they show that the spin voltage varies linearly over the 6-mm length of the sample. In demonstrating that the spin Seebeck effect can produce a large, calibrated spin-voltage source that can be 'tapped' anywhere along the length of the ferromagnet, Uchida and colleagues have added an important tool to the spintronics toolbox. ■ N. P. Ong is in the Figure 1 | The spin Seebeck effect. a, In the ordinary Seebeck effect, a temperature gradient in a metal bar causes more electrons to accumulate at the cool end, producing a tilt in the chemical potential (μ), which is observable as an electric field. b, Uchida et al. 2 extend the Seebeck effect to spins. In a ferromagnet, the temperature gradient results in an excess of spin-up electrons at the cool end, and an excess of spin-down electrons at the warm end. Their respective spin-chemical potentials, μ and μ , have tilt profiles of opposite signs (solid lines), the average (dashed line) giving the electric field. c, The spin Hall effect. The excess of spin-up electrons (red arrows) at the cool end of the ferromagnet drives a spin current that flows vertically into the platinum film (yellow arrow). Here, spin-up means that the direction of the spin is parallel to the magnetic field, and thus points to the right. By spin-orbit coupling, electrons 'see' a weak magnetic field circulating around a charged impurity (circles around blue dot). Scattering from the charged impurity causes spin-up electrons to accumulate preferentially on the far face of the platinum film, whereas spin-down electrons (blue arrows) end up on the near face. The imbalance is observed as a Hall voltage difference between the two faces. Long-term exposure to cocaine changes the organization of synaptic connections within the addiction circuitry of the brain. This process might protect against the development and persistence of addiction. Neurons modify their structure and communication with other neurons in response to experiences. Such experience-dependent neuro plasticity is crucial for survival because it allows learning from, and responses to, changes in the environment. But the cellular mechanisms that mediate this process can also be co-opted by drugs of abuse. Reporting in Neuron, Pulipparacharuvil et al. 1 describe how some of the chemical, structural and behav-ioural changes in neurons that are induced by repeated exposure to cocaine are regulated at a molecular level. Drug addiction is characterized by compulsive drug seeking. It resembles a chronic relapsing disorder in which the addict resumes taking drugs after a period or periods of abstinence. Human and animal studies indicate that the recalcitrant nature of addiction results from drug-induced stimulation of reward-related learning processes in the brain. The pleasure-producing effects of the drug trigger cellular and molecular processes that are normally activated by natural rewards such as food and sex. Repeated exposure to an addictive drug leads to a long-lasting associative memory of its rewarding properties through experience-dependent neuroplasticity. In effect, drug-seeking behaviour becomes hard-wired in the addict's brain, and the persistent memory trace is easily reactivated by drug-associated environmental stimuli, such as the sight of drug paraphernalia. Along the dendritic processes of a neuron, morphologically specialized structures called dendritic spines receive most of the excitatory signals from other neurons through synap-tic junctions. These spines are considered to be a primary cellular site for mediating the synaptic plasticity that is thought to underpin memory formation 2. One regulator of the density of excitatory signals on dendritic spines is the gene transcription factor MEF2 (ref. 3). When active (dephosphorylated), MEF2 favours elimination of dendritic spines, and when inactive (phosphorylated) it allows spine formation 3,4. Repeated exposure to cocaine and other psychostimulants increases the number of dendritic spines on medium spiny neurons drives a spin current into the platinum film. The decay of this spin current leads to the appearance of an electrical signal through the spin Hall effect 3,4 , which enables spin currents to be detected using a sensitive voltmeter. To understand the spin Hall effect, one can track an electron as it enters the platinum film and scatters off a charged impurity (Fig. 1c). The spin-orbit interaction-the interaction between an electron's spin and its motion-imparts a left-right asymmetry to the scattering 743 NATURE|Vol 455|9 October 2008 NEWS & VIEWS