Sensitivity and threshold dynamics of Pinus strobus and Quercus spp. in response to experimental and naturally occurring severe droughts.

Increased drought frequency and severity are a pervasive global threat, yet the capacity of mesic temperate forests to maintain resilience in response to drought remains poorly understood. We deployed a throughfall removal experiment to simulate a once in a century drought in New Hampshire, USA, which coupled with the region-wide 2016 drought, intensified moisture stress beyond that experienced in the lifetimes of our study trees. To assess the sensitivity and threshold dynamics of two dominant northeastern tree genera (Quercus and Pinus), we monitored sap flux density (Js), leaf water potential and gas exchange, growth and intrinsic water-use efficiency (iWUE) for one pretreatment year (2015) and two treatment years (2016-17). Results showed that Js in pine (Pinus strobus L.) declined abruptly at a soil moisture threshold of 0.15 m3 m-3, whereas oak's (Quercus rubra L. and Quercus velutina Lam.) threshold was 0.11 m3 m-3-a finding consistent with pine's more isohydric strategy. Nevertheless, once oaks' moisture threshold was surpassed, Js declined abruptly, suggesting that while oaks are well adapted to moderate drought, they are highly susceptible to extreme drought. The radial growth reduction in response to the 2016 drought was more than twice as great for pine as for oaks (50 vs 18%, respectively). Despite relatively high precipitation in 2017, the oaks' growth continued to decline (low recovery), whereas pine showed neutral (treatment) or improved (control) growth. The iWUE increased in 2016 for both treatment and control pines, but only in treatment oaks. Notably, pines exhibited a significant linear relationship between iWUE and precipitation across years, whereas the oaks only showed a response during the driest conditions, further underscoring the different sensitivity thresholds for these species. Our results provide new insights into how interactions between temperate forest tree species' contrasting physiologies and soil moisture thresholds influence their responses and resilience to extreme drought.

Correcting tree-ring δ13C time series for tree-size effects in eight temperate tree species.

Stable carbon isotope ratios (δ13C) in tree rings have been widely used to study changes in intrinsic water-use efficiency (iWUE), sometimes with limited consideration of how C-isotope discrimination is affected by tree height and canopy position. Our goals were to quantify the relationships between tree size or tree microenvironment and wood δ13C for eight functionally diverse temperate tree species in northern New England and to better understand the physical and physiological mechanisms underlying these differences. We collected short increment cores in closed-canopy stands and analyzed δ13C in the most recent 5 years of growth. We also sampled saplings in both shaded and sun-exposed environments. In closed-canopy stands, we found strong tree-size effects on δ13C, with 3.7-7.2‰ of difference explained by linear regression vs height (0.11-0.28‰ m-1), which in some cases is substantially stronger than the effect reported in previous studies. However, open-grown saplings were often isotopically more similar to large codominant trees than to shade-grown saplings, indicating that light exposure contributes more to the physiological and isotopic differences between small and large trees than does height. We found that in closed-canopy forests, δ13C correlations with diameter at breast height were nonlinear but also strong, allowing a straightforward procedure to correct tree- or stand-scale δ13C-based iWUE chronologies for changing tree size. We demonstrate how to use such data to correct and interpret multi-decadal composite isotope chronologies in both shade-regenerated and open-grown tree cohorts, and we highlight the importance of understanding site history when interpreting δ13C time series.

Representation of spontaneous movement by dopaminergic neurons is cell-type selective and disrupted in parkinsonism.

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 understanding how the firing of different types of dopaminergic neuron relates to movement and how this activity is deciphered in target structures such as 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 or substantia nigra pars lateralis, 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 the 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.

Response of Quercus velutina growth and water use efficiency to climate variability and nitrogen fertilization in a temperate deciduous forest in the northeastern USA.

Nitrogen (N) deposition and changing climate patterns in the northeastern USA can influence forest productivity through effects on plant nutrient relations and water use. This study evaluates the combined effects of N fertilization, climate and rising atmospheric CO2on tree growth and ecophysiology in a temperate deciduous forest. Tree ring widths and stable carbon (δ(13)C) and oxygen (δ(18)O) isotopes were used to assess tree growth (basal area increment, BAI) and intrinsic water use efficiency (iWUE) ofQuercus velutinaLamb., the dominant tree species in a 20+ year N fertilization experiment at Harvard Forest (MA, USA). We found that fertilized trees exhibited a pronounced and sustained growth enhancement relative to control trees, with the low- and high-N treatments responding similarly. All treatments exhibited improved iWUE over the study period (1984-2011). Intrinsic water use efficiency trends in the control trees were primarily driven by changes in stomatal conductance, while a stimulation in photosynthesis, supported by an increase in foliar %N, contributed to enhancing iWUE in fertilized trees. All treatments were predominantly influenced by growing season vapor pressure deficit (VPD), with BAI responding most strongly to early season VPD and iWUE responding most strongly to late season VPD. Nitrogen fertilization increasedQ. velutinasensitivity to July temperature and precipitation. Combined, these results suggest that ambient N deposition in N-limited northeastern US forests has enhanced tree growth over the past 30 years, while rising ambient CO2has improved iWUE, with N fertilization and CO2having synergistic effects on iWUE.

The impact of a parkinsonian lesion on dynamic striatal dopamine transmission depends on nicotinic receptor activation.

Dopamine function is disturbed in Parkinson's disease (PD), but whether and how release of dopamine from surviving neurons is altered has long been debated. Nicotinic acetylcholine receptors (nAChRs) on dopamine axons powerfully govern dopamine release and could be critical contributing factors. We revisited whether fundamental properties of dopamine transmission are changed in a parkinsonian brain and tested the potentially profound masking effects of nAChRs. Using real-time detection of dopamine in mouse striatum after a partial 6-hydroxydopamine lesion and under nAChR inhibition, we reveal that dopamine signals show diminished sensitivity to presynaptic activity. This effect manifested as diminished contrast between DA release evoked by the lowest versus highest frequencies. This reduced activity-dependence was underpinned by loss of short-term facilitation of dopamine release, consistent with an increase in release probability (Pr). With nAChRs active, the reduced activity-dependence of dopamine release after a parkinsonian lesion was masked. Consequently, moment-by-moment variation in activity of nAChRs may lead to dynamic co-variation in dopamine signal impairments in PD.

Region-specific deficits in dopamine, but not norepinephrine, signaling in a novel A30P α-synuclein BAC transgenic mouse.

Parkinson's disease (PD) is a neurodegenerative disorder classically characterized by the death of dopamine (DA) neurons in the substantia nigra pars compacta and by intracellular Lewy bodies composed largely of α-synuclein. Approximately 5-10% of PD patients have a familial form of Parkinsonism, including mutations in α-synuclein. To better understand the cell-type specific role of α-synuclein on DA neurotransmission, and the effects of the disease-associated A30P mutation, we generated and studied a novel transgenic model of PD. We expressed the A30P mutant form of human α-synuclein in a spatially-relevant manner from the 111kb SNCA genomic DNA locus on a bacterial artificial chromosome (BAC) insert on a mouse null (Snca-/-) background. The BAC transgenic mice expressed α-synuclein in tyrosine hydroxylase-positive neurons and expression of either A30P α-synuclein or wildtype α-synuclein restored the sensitivity of DA neurons to MPTP in resistant Snca-/- animals. A30P α-synuclein mice showed no Lewy body-like aggregation, and did not lose catecholamine neurons in substantia nigra or locus coeruleus. However, using cyclic voltammetry at carbon-fiber microelectrodes we identified a deficit in evoked DA release in the caudate putamen, but not in the nucleus accumbens, of SNCA-A30P Snca-/- mice but no changes to release of another catecholamine, norepinephrine (NE), in the NE-rich ventral bed nucleus of stria terminalis. SNCA-A30P Snca-/- mice had no overt behavioral impairments but exhibited a mild increase in wheel-running. In summary, this refined PD mouse model shows that A30P α-synuclein preferentially perturbs the dopaminergic system in the dorsal striatum, reflecting the region-specific change seen in PD.

A comparison of the subsecond dynamics of neurotransmission of dopamine and serotonin.

The neuromodulators dopamine (DA) and serotonin (5-hydroxytryptamine; 5-HT) are similar in a number of ways. Both monoamines can act by volume transmission at metabotropic receptors to modulate synaptic transmission in brain circuits. Presynaptic regulation of 5-HT and DA is governed by parallel processes, and behaviorally, both exert control over emotional processing. However, differences are also apparent: more than twice as many 5-HT receptor subtypes mediate postsynaptic effects than DA receptors and different presynaptic regulation is also emerging. Monoamines are amenable to real-time electrochemical detection using fast scan cyclic voltammetry (FSCV), which allows resolution of the subsecond dynamics of release and reuptake in response to a single action potential. This approach has greatly enriched understanding of DA transmission and has facilitated an integrated view of how DA mediates behavioral control. However, technical challenges are associated with FSCV measurement of 5-HT and understanding of 5-HT transmission at subsecond resolution has not advanced at the same rate. As a result, how the actions of 5-HT at the level of the synapse translate into behavior is poorly understood. Recent technical advances may aid the study of 5-HT in real-time. It is timely, therefore, to compare and contrast what is currently understood of the subsecond characteristics of transmission for DA and 5-HT. In doing so, a number of areas are highlighted as being worthy of exploration for 5-HT.

Striatal dopamine release is triggered by synchronized activity in cholinergic interneurons.

Striatal dopamine plays key roles in our normal and pathological goal-directed actions. To understand dopamine function, much attention has focused on how midbrain dopamine neurons modulate their firing patterns. However, we identify a presynaptic mechanism that triggers dopamine release directly, bypassing activity in dopamine neurons. We paired electrophysiological recordings of striatal channelrhodopsin2-expressing cholinergic interneurons with simultaneous detection of dopamine release at carbon-fiber microelectrodes in striatal slices. We reveal that activation of cholinergic interneurons by light flashes that cause only single action potentials in neurons from a small population triggers dopamine release via activation of nicotinic receptors on dopamine axons. This event overrides ascending activity from dopamine neurons and, furthermore, is reproduced by activating ChR2-expressing thalamostriatal inputs, which synchronize cholinergic interneurons in vivo. These findings indicate that synchronized activity in cholinergic interneurons directly generates striatal dopamine signals whose functions will extend beyond those encoded by dopamine neuron activity.

Regulation of ß-adrenergic control of heart rate by GTP-cyclohydrolase 1 (GCH1) and tetrahydrobiopterin.

AIMS: Clinical markers of cardiac autonomic function, such as heart rate and response to exercise, are important predictors of cardiovascular risk. Tetrahydrobiopterin (BH4) is a required cofactor for enzymes with roles in cardiac autonomic function, including tyrosine hydroxylase and nitric oxide synthase. Synthesis of BH4 is regulated by GTP cyclohydrolase I (GTPCH), encoded by GCH1. Recent clinical studies report associations between GCH1 variants and increased heart rate, but the mechanistic importance of GCH1 and BH4 in autonomic function remains unclear. We investigate the effect of BH4 deficiency on the autonomic regulation of heart rate in the hph-1 mouse model of BH4 deficiency. METHODS AND RESULTS: In the hph-1 mouse, reduced cardiac GCH1 expression, GTPCH enzymatic activity, and BH4 were associated with increased resting heart rate; blood pressure was not different. Exercise training decreased resting heart rate, but hph-1 mice retained a relative tachycardia. Vagal nerve stimulation in vitro induced bradycardia equally in hph-1 and wild-type mice both before and after exercise training. Direct atrial responses to carbamylcholine were equal. In contrast, propranolol treatment normalized the resting tachycardia in vivo. Stellate ganglion stimulation and isoproterenol but not forskolin application in vitro induced a greater tachycardic response in hph-1 mice. ß1-adrenoceptor protein was increased as was the cAMP response to isoproterenol stimulation. CONCLUSION: Reduced GCH1 expression and BH4 deficiency cause tachycardia through enhanced ß-adrenergic sensitivity, with no effect on vagal function. GCH1 expression and BH4 are novel determinants of cardiac autonomic regulation that may have important roles in cardiovascular pathophysiology.

Opposing alterations in anxiety and species-typical behaviours in serotonin transporter overexpressor and knockout mice.

Human gene association studies have produced conflicting findings regarding the relationship between the 5-HT transporter (5-HTT) and anxiety. In the present study genetically modified mice were utilised to examine the effects of changes in 5-HTT expression on anxiety. In addition, the influence of 5-HTT expression on two innate "species-typical" behaviours (burrowing and marble burying) and body weight was explored. Across a range of models, 5-HTT overexpressing mice displayed reduced anxiety-like behaviour whilst 5-HTT knockout mice showed increased anxiety-like behaviour, compared to wildtype controls. In tests of species-typical behaviour 5-HTT overexpressing mice showed some facilitation whilst 5-HTT knockout mice were impaired. Reciprocal effects were also seen on body weight, as 5-HTT overexpressors were lighter and 5-HTT knockouts were heavier than wildtype controls. These findings show that variation in 5-HTT gene expression produces robust changes in anxiety and species-typical behaviour. Furthermore, the data add further support to findings that variation of 5-HTT expression in the human population is linked to changes in anxiety-related personality traits.

Comparing the role of the anterior cingulate cortex and 6-hydroxydopamine nucleus accumbens lesions on operant effort-based decision making.

Both the anterior cingulate cortex (ACC) and mesolimbic dopamine, particularly in the nucleus accumbens (NAc), have been implicated in allowing an animal to overcome effort constraints to obtain greater benefits. However, their exact contribution to such decisions has, to date, never been directly compared. To investigate this issue we tested rats on an operant effort-related cost-benefit decision-making task where animals selected between two response alternatives, one of which involved investing effort by lever pressing on a high fixed-ratio (FR) schedule to gain high reward [four food pellets (HR)], whereas the other led to a small amount of food on an FR schedule entailing less energetic cost [two food pellets, low reward (LR)]. All animals initially preferred to put in work to gain the HR. Systemic administration of a D2 antagonist caused a significant switch in choices towards the LR option. Similarly, post-operatively, excitotoxic ACC lesions caused a significant bias away from HR choices compared with sham-lesioned animals. There was no slowing in the speed of lever pressing and no correlation between time to complete the FR requirement and choice performance. Unexpectedly, no such alteration in choice allocation was observed in animals following 6-hydroxydopamine NAc lesions. However, these rats were consistently slower to initiate responding when cued to commence each trial and also showed a reduction in food hoarding on a species-typical foraging task. Taken together, this implies that only ACC lesions, and not 6-hydroxydopamine NAc lesions as performed here, cause a bias away from investing effort for greater reward when choosing between competing options

Increased expression of the 5-HT transporter confers a low-anxiety phenotype linked to decreased 5-HT transmission.

A commonly occurring polymorphic variant of the human 5-hydroxytryptamine (5-HT) transporter (5-HTT) gene that increases 5-HTT expression has been associated with reduced anxiety levels in human volunteer and patient populations. However, it is not known whether this linkage between genotype and anxiety relates to variation in 5-HTT expression and consequent changes in 5-HT transmission. Here we test this hypothesis by measuring the neurochemical and behavioral characteristics of a mouse genetically engineered to overexpress the 5-HTT. Transgenic mice overexpressing the human 5-HTT (h5-HTT) were produced from a 500 kb yeast artificial chromosome construct. These transgenic mice showed the presence of h5-HTT mRNA in the midbrain raphe nuclei, as well as a twofold to threefold increase in 5-HTT binding sites in the raphe nuclei and a range of forebrain regions. The transgenic mice had reduced regional brain whole-tissue levels of 5-HT and, in microdialysis experiments, decreased brain extracellular 5-HT, which reversed on administration of the 5-HTT inhibitor paroxetine. Compared with wild-type mice, the transgenic mice exhibited a low-anxiety phenotype in plus maze and hyponeophagia tests. Furthermore, in the plus maze test, the low-anxiety phenotype of the transgenic mice was reversed by acute administration of paroxetine, suggesting a direct link between the behavior, 5-HTT overexpression, and low extracellular 5-HT. In toto, these findings demonstrate that associations between increased 5-HTT expression and anxiety can be modeled in mice and may be specifically mediated by decreases in 5-HT transmission.