Extracting quantal properties of transmission at central synapses

Predation cues induce predator specific changes in olfactory neurons encoding defensive responses in agile frog tadpoles

Although behavioural defensive responses have been recorded several times in both laboratory and natural habitats, their neural mechanisms have seldom been investigated. To explore how chemical, water-borne cues are conveyed to the forebrain and instruct behavioural responses in anuran larvae, we conditioned newly hatched agile frog tadpoles using predator olfactory cues, specifically either native odonate larvae or alien crayfish kairomones. We expected chronic treatments to influence the basal neuronal activity of the tadpoles' mitral cells and alter their sensory neuronal connections, thereby impacting information processing. Subsequently, these neurons were acutely perfused, and their responses were compared with the defensive behaviour of tadpoles previously conditioned and exposed to the same cues. Tadpoles conditioned with odonate cues differed in both passive and active cell properties compared to those exposed to water (controls) or crayfish cues. The observed upregulation of membrane conductance and increase in both the number of active synapses and receptor density at the postsynaptic site are believed to have enhanced their responsiveness to external stimuli. Odonate cues also affected the resting membrane potential and firing rate of mitral cells during electrophysiological patch-clamp recordings, suggesting a rearrangement of the repertoire of voltage-dependent conductances expressed in cell membranes. These recorded neural changes may modulate the induction of an action potential and transmission of information. Furthermore, the recording of neural activity indicated that the lack of defensive responses towards non-native predators is due to the non-recognition of their olfactory cues.

The functional properties of synapses made by regenerated axons across spinal cord lesion sites in lamprey

While the anatomical properties of regenerated axons across spinal cord lesion sites have been studied extensively, little is known of how the functional properties of regenerated synapses compared to those in unlesioned animals. This study aims to compare the properties of synapses made by regenerated axons with unlesioned axons using the lamprey, a model system for spinal injury research, in which functional locomotor recovery after spinal cord lesions is associated with axonal regeneration across the lesion site. Regenerated synapses below the lesion site did not differ from synapses from unlesioned axons with respect to the amplitude and duration of single excitatory postsynaptic potentials. They also showed the same activity-dependent depression over spike trains. However, regenerated synapses did differ from unlesioned synapses as the estimated number of synaptic vesicles was greater and there was evidence for increased postsynaptic quantal amplitude. For axons above the lesion site, the amplitude and duration of single synaptic inputs also did not differ significantly from unlesioned animals. However, in this case, there was evidence of a reduction in release probability and inputs facilitated rather than depressed over spike trains. Synaptic inputs from single regenerated axons below the lesion site thus do not increase in amplitude to compensate for the reduced number of descending axons after functional recovery. However, the postsynaptic input was maintained at the unlesioned level using different synaptic properties. Conversely, the facilitation from the same initial amplitude above the lesion site made the synaptic input over spike trains functionally stronger. This may help to increase propriospinal activity across the lesion site to compensate for the lesion-induced reduction in supraspinal inputs. The animal experiments were approved by the Animal Ethics Committee of Cambridge University.

Studying Synaptic Connectivity and Strength with Optogenetics and Patch-Clamp Electrophysiology

Over the last two decades the combination of brain slice patch clamp electrophysiology with optogenetic stimulation has proven to be a powerful approach to analyze the architecture of neural circuits and (experience-dependent) synaptic plasticity in such networks. Using this combination of methods, originally termed channelrhodopsin-assisted circuit mapping (CRACM), a multitude of measures of synaptic functioning can be taken. The current review discusses their rationale, current applications in the field, and their associated caveats. Specifically, the review addresses: (1) How to assess the presence of synaptic connections, both in terms of ionotropic versus metabotropic receptor signaling, and in terms of mono- versus polysynaptic connectivity. (2) How to acquire and interpret measures for synaptic strength and function, like AMPAR/NMDAR, AMPAR rectification, paired-pulse ratio (PPR), coefficient of variance and input-specific quantal sizes. We also address how synaptic modulation by G protein-coupled receptors can be studied with pharmacological approaches and advanced technology. (3) Finally, we elaborate on advances on the use of dual color optogenetics in concurrent investigation of multiple synaptic pathways. Overall, with this review we seek to provide practical insights into the methods used to study neural circuits and synapses, by combining optogenetics and patch-clamp electrophysiology.

rTg(TauP301L)4510 mice exhibit increased VGlut1 in hippocampal presynaptic glutamatergic vesicles and increased extracellular glutamate release

The molecular pathways that contribute to the onset of symptoms in tauopathy models, including Alzheimer’s disease (AD), are difficult to distinguish because multiple changes can happen simultaneously at different stages of disease progression. Understanding early synaptic alterations and their supporting molecular pathways is essential to develop better pharmacological targets to treat AD. Here, we focus on an early onset rTg(TauP301L)4510 tauopathy mouse model that exhibits hyperexcitability in hippocampal neurons of adult mice that is correlated with presynaptic changes and increased extracellular glutamate levels. However, it is not clear if increased extracellular glutamate is caused by presynaptic changes alone, or if presynaptic changes are a contributing factor among other factors. To determine whether pathogenic tau alters presynaptic function and glutamate release, we studied cultured hippocampal neurons at 14–18 days in vitro (DIV) from animals of both sexes to measure presynaptic changes in tauP301L positive mice. We observed that presynaptic vesicles exhibit increased vesicular glutamate transporter 1 (VGlut1) using immunohistochemistry of fixed cells and an established pH-sensitive green fluorescent protein approach. We show that tauP301L positive neurons exhibit a 40% increase in VGlut1 per vesicle compared to tauP301L negative littermates. Further, we use the extracellular glutamate reporter iGluSnFR to show that increased VGlut1 per vesicle directly translates into a 40% increase in extracellular glutamate. Together, these results show that increased extracellular glutamate levels observed in tauP301L mice are not caused by increased vesicle exocytosis probability but rather are directly related to increased VGlut1 transporters per synaptic vesicle.

Overexpression of Neuregulin-1 Type III Has Impact on Visual Function in Mice

Schizophrenia is associated with several brain deficits, including abnormalities in visual processes. Neuregulin-1 (Nrg1) is a family of trophic factors containing an epidermal growth factor (EGF)-like domain. It is thought to play a role in neural development and has been linked to neuropsychiatric disorders. Abnormal Nrg1 expression has been observed in schizophrenia in clinical studies. Moreover, in schizophrenia, there is more and more evidence found about pathological changes of the retina regarding structural, neurochemical and physiological parameters. However, mechanisms of these changes are not well known. To investigate this, we analysed the function of the visual system using electroretinography (ERG) and the measurement of visual evoked potentials (VEP) in transgenic mice overexpressing Nrg1 type III of three different ages (12 weeks, 24 weeks and 55 weeks). ERG amplitudes tended to be higher in transgenic mice than in control mice in 12-week old mice, whereas the amplitudes were almost similar in older mice. VEP amplitudes were larger in transgenic mice at all ages, with significant differences at 12 and 55 weeks (p values between 0.003 and 0.036). Latencies in ERG and VEP measurements did not differ considerably between control mice and transgenic mice at any age. Our data show for the first time that overexpression of Nrg1 type III changed visual function in transgenic mice. Overall, this investigation of visual function in transgenic mice may be helpful to understand corresponding changes that occur in schizophrenia, as they may find use as biomarkers for psychiatric disorders as well as a potential tool for diagnosis in psychiatry.

Model-Based Inference of Synaptic Transmission

Synaptic computation is believed to underlie many forms of animal behavior. A correct identification of synaptic transmission properties is thus crucial for a better understanding of how the brain processes information, stores memories and learns. Recently, a number of new statistical methods for inferring synaptic transmission parameters have been introduced. Here we review and contrast these developments, with a focus on methods aimed at inferring both synaptic release statistics and synaptic dynamics. Furthermore, based on recent proposals we discuss how such methods can be applied to data across different levels of investigation: from intracellular paired experiments to in vivo network-wide recordings. Overall, these developments open the window to reliably estimating synaptic parameters in behaving animals.