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Midbrain dopamine neurons are thought to play key roles in learning by conveying the difference between expected and actual outcomes. While this teaching signal is often considered to be uniform, recent evidence instead supports diversity in dopamine signaling. However, it remains poorly understood how heterogeneous signals might be organized to facilitate the role of downstream circuits mediating distinct aspects of behavior. Here we investigated the organizational logic of dopaminergic signaling by recording and labeling individual midbrain dopamine neurons during associative behavior. We defined combinations of protein expression and cellular localization to sort recorded neurons according to the striatal regions they innervate. Our findings show that reward information and task variables are not only heterogeneously encoded, with multiplexing, but also differentially distributed across populations of dopamine neurons projecting to different regions of striatum. These data, supported by computational modelling, indicate that such distributional coding can maximize dynamic range and tailor dopamine signals to facilitate the specialized roles of different striatal regions.

Dopamine-dependent long-term plasticity is believed to be a cellular mechanism underlying reinforcement learning. In response to reward and reward-predicting cues, phasic dopamine activity potentiates the efficacy of corticostriatal synapses on spiny projection neurons (SPNs). Since phasic dopamine activity also encodes other behavioural variables, it is unclear how postsynaptic neurons identify which dopamine event is to induce long-term plasticity. Additionally, it is unknown how phasic dopamine released from arborised axons can potentiate targeted striatal synapses through volume transmission. To examine these questions we manipulated striatal cholinergic interneurons (ChIs) and dopamine neurons independently in two distinct in vivo paradigms. We report that long-term potentiation (LTP) at corticostriatal synapses with SPNs is dependent on the coincidence of pauses in ChIs and phasic dopamine activation, critically accompanied by SPN depolarisation. Thus, the ChI pause defines the time window for phasic dopamine to induce plasticity, while depolarisation of SPNs constrains the synapses eligible for plasticity.

Introduction. A 4 year retrospective follow up of 996 patients who pre-sented with no DR and 500 with background DR at baseline digital DR screening in 2006. Purpose. To evaluate the safety of increasing screening intervals in patients with no diabetic retinopathy (DR) or with background DR.Methods. A 4 year retrospective follow up of 996 patients who presented with no DR and 500 with background DR at baseline digital DR screening in 2006.results. Background DR Group: Of the 500 subjects that had back-ground DR in 2006, 231 were referred for DR, with an average DR routine referral rate of 12% (46 subjects) per year. nodrgrouP. Of the 996 patients who had no DR at baseline, 51 were referred over the 4 years for sight threatening DR (STDR), of these 45 patients have definite STDR confirmed by ophthalmological examination. 78% of these had type 2 diabetes and mean age at referral was 60 years (25-87). Mean diabetes duration was 10.7 years (3-32), with a mean HbA1c of 7.8% (5.7-11.3%). Eight patients (0.9%) were referred in the first year, 9 (0.9%) in the second year, 19 (1.9%) in the third year and 15 (1.5%) in the fourth year. 86% of referrals were for maculopathy, and all had observable retinopathy and none required ophthalmology clinic assessment or laser treatment.If biannual screening was adopted for patients with no DR at baseline, allowing for patients who subsequently develop background DR and would then revert to annual screening, a total of 7 (0.7%) patients would not have been appropriately referred for STDR and would have waited a further year for identification. None of the 51 referrals across the 4 years required laser treatment apart from just one patient who developed PDR in year 4 (2010) and had background since 2007.conclusIons. It could be recommended that it is safe to screen pa-tients with no DR biannually due to the low risk of developing STDR. However, patients who present with background DR should continue to be screened annually as there is a significant proportion developing STDR and would not be identified at an appropriate screening interval.

Midbrain dopaminergic (mDA) neurons are implicated in cognitive functions, neuropsychiatric disorders, and pathological conditions; hence understanding genes regulating their homeostasis has medical relevance. Transcription factors FOXA1 and FOXA2 (FOXA1/2) are key determinants of mDA neuronal identity during development, but their roles in adult mDA neurons are unknown. We used a conditional knockout strategy to specifically ablate FOXA1/2 in mDA neurons of adult mice. We show that deletion of Foxa1/2 results in down-regulation of tyrosine hydroxylase, the rate-limiting enzyme of dopamine (DA) biosynthesis, specifically in dopaminergic neurons of the substantia nigra pars compacta (SNc). In addition, DA synthesis and striatal DA transmission were reduced after Foxa1/2 deletion. Furthermore, the burst-firing activity characteristic of SNc mDA neurons was drastically reduced in the absence of FOXA1/2. These molecular and functional alterations lead to a severe feeding deficit in adult Foxa1/2 mutant mice, independently of motor control, which could be rescued by L-DOPA treatment. FOXA1/2 therefore control the maintenance of molecular and physiological properties of SNc mDA neurons and impact on feeding behavior in adult mice.

Midbrain dopaminergic neurons are essential for appropriate voluntary movement, as epitomized by the cardinal motor impairments arising in Parkinson’s disease. Understanding the basis of such motor control requires definitions of how the firing of different types of dopaminergic neuron relates to movement, and how this activity is deciphered in target structures like the striatum. By recording and labeling individual neurons in behaving mice, we show that the representation of brief spontaneous movements in the firing of identified midbrain dopaminergic neurons is cell-type selective. Most dopaminergic neurons in the substantia nigra pars compacta (SNc), but not in ventral tegmental area (VTA) or substantia nigra pars lateralis (SNL), consistently represented the onset of spontaneous movements with a pause in their firing. Computational modeling revealed that the movement-related firing of these dopaminergic neurons can manifest as rapid and robust fluctuations in striatal dopamine concentration and receptor activity. The exact nature of the movement-related signaling in striatum depended on the type of dopaminergic neuron providing inputs, the striatal region innervated, and the type of dopamine receptor expressed by striatal neurons. Importantly, in aged mice harboring a genetic burden relevant for human Parkinson’s disease, the precise movement-related firing of SNc dopaminergic neurons and the resultant striatal dopamine signaling were lost. These data show that distinct dopaminergic cell types differentially encode spontaneous movement, and elucidate how dysregulation of their firing in early Parkinsonism can impair their effector circuits.

The membrane potential dynamics of stellate neurons in layer II of the medial entorhinal cortex are important for neural encoding of location. Previous studies suggest that these neurons generate intrinsic theta-frequency membrane potential oscillations, with a period that depends on neuronal location on the dorsal–ventral axis of themedial entorhinal cortex, and which in behaving animals could support generation of grid-like spatial firing fields. To address the nature and organization of this theta-like activity, we adopt the Lombmethod of least-squares spectral analysis. We demonstrate that peaks in frequency spectra that differ significantly from Gaussian noise do not necessarily imply the existence of a periodic oscillator, but can instead arise from filtered stochastic noise or a stochastic random walk. We show that theta-like membrane potential activity recorded fromstellate neurons in mature brain slices is consistentwith stochastic mechanisms, but not with generation by a periodic oscillator. The dorsal–ventral organization of intrinsic theta-likemembrane potential activity, and themodification of this activity during block of HCN channels, both reflect altered frequency distributions of stochastic spectral peaks, rather than tuning of a periodic oscillator. Our results demonstrate the importance of distinguishing periodic oscillations from stochastic processes.We suggest that dorsal–ventral tuning of theta-like membrane potential activity is due to differences in stochastic current fluctuations resulting from organization of ion channels that also control synaptic integration.

SummaryNeurons important for cognitive function are often classified by their morphology and integrative properties. However, it is unclear if within a single class of neuron these properties tune synaptic responses to the salient features of the information that each neuron represents. We demonstrate that for stellate neurons in layer II of the medial entorhinal cortex, the waveform of postsynaptic potentials, the time window for detection of coincident inputs, and responsiveness to gamma frequency inputs follow a dorsal-ventral gradient similar to the topographical organization of grid-like spatial firing fields of neurons in this area. We provide evidence that these differences are due to a membrane conductance gradient mediated by HCN and leak potassium channels. These findings suggest key roles for synaptic integration in computations carried out within the medial entorhinal cortex and imply that tuning of neural information processing by membrane ion channels is important for normal cognitive function.

Mutations in leucine-rich repeat kinase 2 (LRRK2) lead to late-onset, autosomal dominant Parkinson's disease, characterized by the degeneration of dopamine neurons of the substantia nigra pars compacta, a deficit in dopamine neurotransmission and the development of motor and non-motor symptoms. The most prevalent Parkinson's disease LRRK2 mutations are located in the kinase (G2019S) and GTPase (R1441C) encoding domains of LRRK2. To better understand the sequence of events that lead to progressive neurophysiological deficits in vulnerable neurons and circuits in Parkinson's disease, we have generated LRRK2 bacterial artificial chromosome transgenic rats expressing either G2019S or R1441C mutant, or wild-type LRRK2, from the complete human LRRK2 genomic locus, including endogenous promoter and regulatory regions. Aged (18–21 months) G2019S and R1441C mutant transgenic rats exhibit L-DOPA-responsive motor dysfunction, impaired striatal dopamine release as determined by fast-scan cyclic voltammetry, and cognitive deficits. In addition, in vivo recordings of identified substantia nigra pars compacta dopamine neurons in R1441C LRRK2 transgenic rats reveal an age-dependent reduction in burst firing, which likely results in further reductions to striatal dopamine release. These alterations to dopamine circuit function occur in the absence of neurodegeneration or abnormal protein accumulation within the substantia nigra pars compacta, suggesting that nigrostriatal dopamine dysfunction precedes detectable protein aggregation and cell death in the development of Parkinson's disease. In conclusion, our longitudinal deep-phenotyping provides novel insights into how the genetic burden arising from human mutant LRRK2 manifests as early pathophysiological changes to dopamine circuit function and highlights a potential model for testing Parkinson's therapeutics.

Voltage-gated K+ channels activating close to resting membrane potentials are widely expressed and differentially located in axons, presynaptic terminals and cell bodies. There is extensive evidence for localisation of Kv1 subunits at many central synaptic terminals but few clues to their presynaptic function. We have used the calyx of Held to investigate the role of presynaptic Kv1 channels in the rat by selectively blocking Kv1.1 and Kv1.2 containing channels with dendrotoxin-K (DTX-K) and tityustoxin-Kalpha (TsTX-Kalpha) respectively. We show that Kv1.2 homomers are responsible for two-thirds of presynaptic low threshold current, whilst Kv1.1/Kv1.2 heteromers contribute the remaining current. These channels are located in the transition zone between the axon and synaptic terminal, contrasting with the high threshold K+ channel subunit Kv3.1 which is located on the synaptic terminal itself. Kv1 homomers were absent from bushy cell somata (from which the calyx axons arise); instead somatic low threshold channels consisted of heteromers containing Kv1.1, Kv1.2 and Kv1.6 subunits. Current-clamp recording from the calyx showed that each presynaptic action potential (AP) was followed by a depolarising after-potential (DAP) lasting around 50 ms. Kv1.1/Kv1.2 heteromers had little influence on terminal excitability, since DTX-K did not alter AP firing. However TsTX-Kalpha increased DAP amplitude, bringing the terminal closer to threshold for generating an additional AP. Paired pre- and postsynaptic recordings confirmed that this aberrant AP evoked an excitatory postsynaptic current (EPSC). We conclude that Kv1.2 channels have a general presynaptic function in suppressing terminal hyperexcitability during the depolarising after-potential.

The pathological end-state of Parkinson disease is well described from postmortem tissue, but there remains a pressing need to define early functional changes to susceptible neurons and circuits. In particular, mechanisms underlying the vulnerability of the dopamine neurons of the substantia nigra pars compacta (SNc) and the importance of protein aggregation in driving the disease process remain to be determined. To better understand the sequence of events occurring in familial and sporadic Parkinson disease, we generated bacterial artificial chromosome transgenic mice (SNCA-OVX) that express wild-type α-synuclein from the complete human SNCA locus at disease-relevant levels and display a transgene expression profile that recapitulates that of endogenous α-synuclein. SNCA-OVX mice display age-dependent loss of nigrostriatal dopamine neurons and motor impairments characteristic of Parkinson disease. This phenotype is preceded by early deficits in dopamine release from terminals in the dorsal, but not ventral, striatum. Such neurotransmission deficits are not seen at either noradrenergic or serotoninergic terminals. Dopamine release deficits are associated with an altered distribution of vesicles in dopaminergic axons in the dorsal striatum. Aged SNCA-OVX mice exhibit reduced firing of SNc dopamine neurons in vivo measured by juxtacellular recording of neurochemically identified neurons. These progressive changes in vulnerable SNc neurons were observed independently of overt protein aggregation, suggesting neurophysiological changes precede, and are not driven by, aggregate formation. This longitudinal phenotyping strategy in SNCA-OVX mice thus provides insights into the region-specific neuronal disturbances preceding and accompanying Parkinson disease.

The membrane potential dynamics of stellate neurons in layer II of the medial entorhinal cortex are important for neural encoding of location. Previous studies suggest that these neurons generate intrinsic theta-frequency membrane potential oscillations, with a period that depends on neuronal location on the dorsal–ventral axis of themedial entorhinal cortex, and which in behaving animals could support generation of grid-like spatial firing fields. To address the nature and organization of this theta-like activity, we adopt the Lombmethod of least-squares spectral analysis. We demonstrate that peaks in frequency spectra that differ significantly from Gaussian noise do not necessarily imply the existence of a periodic oscillator, but can instead arise from filtered stochastic noise or a stochastic random walk. We show that theta-like membrane potential activity recorded fromstellate neurons in mature brain slices is consistentwith stochastic mechanisms, but not with generation by a periodic oscillator. The dorsal–ventral organization of intrinsic theta-likemembrane potential activity, and themodification of this activity during block of HCN channels, both reflect altered frequency distributions of stochastic spectral peaks, rather than tuning of a periodic oscillator. Our results demonstrate the importance of distinguishing periodic oscillations from stochastic processes.We suggest that dorsal–ventral tuning of theta-like membrane potential activity is due to differences in stochastic current fluctuations resulting from organization of ion channels that also control synaptic integration.

Glutamate is the brain’s main excitatory neurotransmitter. Upon release, its actions are terminated by a class of proteins called glutamate transporters, which are located on the plasma membranes of neurons and glial cells. Glutamate transporters catalyze the movement of glutamate from the extracellular space to the intracellular space and are able to concentrate glutamate to greater than a millionfold under physiological circumstances by extracting energy from the sodium and potassium ionic gradients. This article discusses what is known about the molecular structure and function, the cellular localization, and the synaptic function of these critical components of excitatory synapses.

Whereas recent studies have elucidated principles for representation of information within the entorhinal cortex, less is known about the molecular basis for information processing by entorhinal neurons. TheHCN1gene encodes ion channels that mediate hyperpolarization-activated currents (Ih) that control synaptic integration and influence several forms of learning and memory. We asked whether hyperpolarization-activated, cation nonselective 1 (HCN1) channels control processing of information by stellate cells found within layer II of the entorhinal cortex. Axonal projections from these neurons form a major component of the synaptic input to the dentate gyrus of the hippocampus. To determine whether HCN1 channels control either the resting or the active properties of stellate neurons, we performed whole-cell recordings in horizontal brain slices prepared from adult wild-type and HCN1 knock-out mice. We found that HCN1 channels are required for rapid and full activation of hyperpolarization-activated currents in stellate neurons. HCN1 channels dominate the membrane conductance at rest, are not required for theta frequency (4–12 Hz) membrane potential fluctuations, but suppress low-frequency (

This article summarizes the proceedings of a symposium held at the 2005 Research Society on Alcoholism meeting. The initial presentation by Dr. Wallner provided evidence that selected GABA(A) receptors containing the delta subunit display sensitivity to low intoxicating ethanol concentrations and this sensitivity is further increased by a mutation in the cerebellar alpha6 subunit, found in alcohol-hypersensitive rats. Dr. Mameli reported that ethanol affects gamma-aminobutyric acid (GABA) function by affecting neural circuits that influence GABA release. Dr. Parsons presented data from electrophysiological and microdialysis investigations that ethanol is capable of releasing GABA from presynaptic terminals. Dr. Morrow demonstrated that systemic ethanol increases neuroactive steroids in brain, the absence of which alters various functional responses to ethanol. Dr. Criswell presented evidence that the ability of ethanol to increase GABA was apparent in some, but not all, brain regions indicative of regional specificity. Further, Dr. Criswell demonstrated that neurosteroids alone and when synthesized locally by ethanol act postsynaptically to enhance the effect of GABA released by ethanol in a region specific manner. Collectively, this series of reports support the GABAmimetic profile of acutely administered ethanol being dependent on several specific mechanisms distinct from a direct effect on the major synaptic isoforms of GABA(A) receptors.

Of the five excitatory amino acid transporters (EAATs) identified, two genes are expressed by neurons (EAAT3 and EAAT4) and give rise to transporters confined to neuronal cell bodies and dendrites. At an ultrastructural level, EAAT3 and EAAT4 proteins are clustered at the edges of postsynaptic densities of excitatory synapses. This pattern of localization suggests that postsynaptic EAATs may help to limit spillover of glutamate from excitatory synapses. In an effort to study transporter localization in living neurons and ultimately to manipulate uptake at intact synapses, we have developed viral reagents encoding neuronal EAATs tagged with GFP. We demonstrate that these fusion proteins are capable of Na(+)-dependent glutamate uptake, that they generate ionic conductances indistinguishable from their wild-type counterparts, and that GFP does not alter their glutamate dose-dependence. Two-photon microscopy was used to examine fusion protein expression in Purkinje neurons in acute cerebellar slices. Both EAAT3-GFP and EAAT4-GFP were observed at high levels in the dendritic spines of transfected Purkinje neurons. These findings indicate that functional EAAT fusion proteins can be synthesized and appropriately trafficked to postsynaptic compartments. Furthermore, they validate a powerful system for looking at EAAT function in situ.

Potassium channels are crucial regulators of neuronal excitability, setting resting membrane potentials and firing thresholds, repolarizing action potentials and limiting excitability. Although most of our understanding of K+ channels is based on somatic recordings, there is good evidence that these channels are present in synaptic terminals. In recent years the improved access to presynaptic compartments afforded by direct recording techniques has indicated diverse roles for native K+ channels, from suppression of aberrant firing to action potential repolarization and activity-dependent modulation of synaptic activity. This article reviews the growing evidence for multiple roles and discrete localization of distinct K+ channels at presynaptic terminals.

Low-threshold voltage-gated potassium currents (I(LT)) activating close to resting membrane potentials play an important role in shaping action potential (AP) firing patterns. In the medial nucleus of the trapezoid body (MNTB), I(LT) ensures generation of single APs during each EPSP, so that the timing and pattern of AP firing is preserved on transmission across this relay synapse (calyx of Held). This temporal information is critical for computation of sound location using interaural timing and level differences. I(LT) currents are generated by dendrotoxin-I-sensitive, Shaker-related K+ channels; our immunohistochemistry confirms that MNTB neurons express Kv1.1, Kv1.2, and Kv1.6 subunits. We used subunit-specific toxins to separate I(LT) into two components, each contributing approximately one-half of the total low-threshold current: (1) I(LTS), a tityustoxin-Kalpha-sensitive current (TsTX) (known to block Kv1.2 containing channels), and (2) I(LTR), an TsTX-resistant current. Both components were sensitive to the Kv1.1-specific toxin dendrotoxin-K and were insensitive to tetraethylammonium (1 mm). This pharmacological profile excludes homomeric Kv1.1 or Kv1.2 channels and is consistent with I(LTS) channels being Kv1.1/Kv1.2 heteromers, whereas I(LTR) channels are probably Kv1.1/Kv1.6 heteromers. Although they have similar kinetic properties, I(LTS) is critical for generating the phenotypic single AP response of MNTB neurons. Immunohistochemistry confirms that Kv1.1 and Kv1.2 (I(LTS) channels), but not Kv1.6, are concentrated in the first 20 microm of MNTB axons. Our results show that heteromeric channels containing Kv1.2 subunits govern AP firing and suggest that their localization at the initial segment of MNTB axons can explain their dominance of AP firing behavior.