Optical triggered seizures using a caged 4-Aminopyridine

Photopharmacological modulation of hippocampal local field potential by caged-glutamate with MicroLED probe

AIM

Photopharmacology is a new technique for modulating biological phenomena through the photoconversion of substances in a specific target region at precise times. Caged compounds are thought to be compatible with photopharmacology as uncaged ligands are released and function in a light irradiation-dependent manner. Here, we investigated whether a microscale light-emitting diode (MicroLED) probe is applicable for the photoconversion of caged-glutamate (caged-Glu) in vivo.

METHODS

A needle-shaped MicroLED probe was fabricated and inserted into the mouse hippocampal dentate gyrus (DG) with a cannula for drug injection and a recording electrode for measuring the local field potential (LFP). Artificial cerebrospinal fluid (ACSF) or caged-Glu was infused into the DG and illuminated with light from a MicroLED probe.

RESULTS

In the caged-Glu-injected DG, the LFP changed in the 10-20 Hz frequency ranges after light illumination, whereas there was no change in the ACSF control condition.

CONCLUSION

The MicroLED probe is applicable for photopharmacological experiments to modulate LFP with caged-Glu in vivo.

Ectopic burst induced by blockade of axonal potassium channels on the mouse hippocampal mossy fiber

A potassium channel blocker 4-AP has been shown to exert pronounced convulsive action to generate burst firings when applied to hippocampal slices. However, it remains unclear how the blockade of potassium channels leads to the generation of burst firings. One possibility is ectopic spiking from the sites different from those for physiological spike initiation at the axon initial segment, as suggested for several experimental models of epileptogenesis in vitro. To test for possible ectopic spiking at the distal axon by 4-AP application, direct recordings from large mossy fiber terminals were made with the loose-patch clamp technique in mouse hippocampal slices. To localize the action of 4-AP on the distal axon, focal perfusion, as well as micro-cut to disconnect soma and distal axons, were adopted. Focal application of 4-AP on the distal portion of mossy fibers reliably induced burst discharges of the mossy fiber terminals. Photochemical blockade of potassium channels at distal axons, by the application of RuBi-4-AP, a visible wavelength blue light-sensitive caged compound, and the illumination of blue light caused robust bursting activity originating from distal axons. Computer simulation suggested that local blockade of axonal potassium channels prolongs the duration of action potentials and thereby causes reverberating spiking activities at distal axons and subsequent antidromic propagation toward the soma. Taken together, it was suggested that local blockade of voltage-dependent potassium channels in distal axons by application of 4-AP is sufficient to cause a hyperexcitable state of hippocampal mossy fiber axons.

In Vivo Neuropharmacological Effects of Neophytadiene

Neophytadiene (NPT) is a diterpene found in the methanolic extracts of Crataeva nurvala and Blumea lacera, plants reported with anxiolytic-like activity, sedative properties, and antidepressant-like actions; however, the contribution of neophytadiene to these effects is unknown. This study determined the neuropharmacological (anxiolytic-like, antidepressant-like, anticonvulsant, and sedative) effects of neophytadiene (0.1-10 mg/kg p.o.) and determined the mechanisms of action involved in the neuropharmacological actions using inhibitors such as flumazenil and analyzing the possible interaction of neophytadiene with GABA receptors using a molecular docking study. The behavioral tests were evaluated using the light-dark box, elevated plus-maze, open field, hole-board, convulsion, tail suspension, pentobarbital-induced sleeping, and rotarod. The results showed that neophytadiene exhibited anxiolytic-like activity only to the high dose (10 mg/kg) in the elevated plus-maze and hole-board tests, and anticonvulsant actions in the 4-aminopyridine and pentylenetetrazole-induced seizures test. The anxiolytic-like and anticonvulsant effects of neophytadiene were abolished with the pre-treatment with 2 mg/kg flumazenil. In addition, neophytadiene showed low antidepressant effects (about 3-fold lower) compared to fluoxetine. On other hand, neophytadiene had no sedative or locomotor effects. In conclusion, neophytadiene exerts anxiolytic-like and anticonvulsant activities with the probable participation of the GABAergic system.

Gabrb3 is required for the functional integration of pyramidal neuron subtypes in the somatosensory cortex

Dysfunction of gamma-aminobutyric acid (GABA)ergic circuits is strongly associated with neurodevelopmental disorders. However, it is unclear how genetic predispositions impact circuit assembly. Using in vivo two-photon and widefield calcium imaging in developing mice, we show that Gabrb3, a gene strongly associated with autism spectrum disorder (ASD) and Angelman syndrome (AS), is enriched in contralaterally projecting pyramidal neurons and is required for inhibitory function. We report that Gabrb3 ablation leads to a developmental decrease in GABAergic synapses, increased local network synchrony, and long-lasting enhancement in functional connectivity of contralateral-but not ipsilateral-pyramidal neuron subtypes. In addition, Gabrb3 deletion leads to increased cortical response to tactile stimulation at neonatal stages. Using human transcriptomics and neuroimaging datasets from ASD subjects, we show that the spatial distribution of GABRB3 expression correlates with atypical connectivity in these subjects. Our studies reveal a requirement for Gabrb3 during the emergence of interhemispheric circuits for sensory processing.

Precise spatiotemporal control of voltage-gated sodium channels by photocaged saxitoxin

Here we report the pharmacologic blockade of voltage-gated sodium ion channels (Na_Vs) by a synthetic saxitoxin derivative affixed to a photocleavable protecting group. We demonstrate that a functionalized saxitoxin (STX-eac) enables exquisite spatiotemporal control of Na_Vs to interrupt action potentials in dissociated neurons and nerve fiber bundles. The photo-uncaged inhibitor (STX-ea) is a nanomolar potent, reversible binder of Na_Vs. We use STX-eac to reveal differential susceptibility of myelinated and unmyelinated axons in the corpus callosum to Na_V-dependent alterations in action potential propagation, with unmyelinated axons preferentially showing reduced action potential fidelity under conditions of partial Na_V block. These results validate STX-eac as a high precision tool for robust photocontrol of neuronal excitability and action potential generation.

Theoretical Design, Synthesis, and In Vitro Neurobiological Applications of a Highly Efficient Two-Photon Caged GABA Validated on an Epileptic Case

In this paper, we present an additional, new cage-GABA compound, called 4-amino-1-(4'-dimethylaminoisopropoxy-5',7'-dinitro-2',3'-dihydro-indol-1-yl)-1-oxobutane-γ-aminobutyric acid (iDMPO-DNI-GABA), and currently, this compound is the only photoreagent, which can be applied for GABA uncaging without experimental compromises. By a systematic theoretical design and successful synthesis of several compounds, the best reagent exhibits a high two-photon efficiency within the 700-760 nm range with excellent pharmacological behavior, which proved to be suitable for a complex epileptic study. Quantum chemical design showed that the optimal length of the cationic side chain enhances the two-photon absorption by 1 order of magnitude due to the cooperating internal hydrogen bonding to the extra nitro group on the core. This feature increased solubility while suppressing membrane permeability. The efficiency was demonstrated in a systematic, wide range of in vitro single-cell neurophysiological experiments by electrophysiological as well as calcium imaging techniques. Scalable inhibitory ion currents were elicited by iDMPO-DNI-GABA with appropriate spatial-temporal precision, blocking both spontaneous and evoked cell activity with excellent efficiency. Additionally, to demonstrate its applicability in a real neurobiological study, we could smoothly and selectively modulate neuronal activities during artificial epileptic rhythms first time in a neural network of GCaMP6f transgenic mouse brain slices.

Role of inhibitory control in modulating focal seizure spread

Focal seizure propagation is classically thought to be spatially contiguous. However, distribution of seizures through a large-scale epileptic network has been theorized. Here, we used a multielectrode array, wide field calcium imaging, and two-photon calcium imaging to study focal seizure propagation pathways in an acute rodent neocortical 4-aminopyridine model. Although ictal neuronal bursts did not propagate beyond a 2-3-mm region, they were associated with hemisphere-wide field potential fluctuations and parvalbumin-positive interneuron activity outside the seizure focus. While bicuculline surface application enhanced contiguous seizure propagation, focal bicuculline microinjection at sites distant to the 4-aminopyridine focus resulted in epileptic network formation with maximal activity at the two foci. Our study suggests that both classical and epileptic network propagation can arise from localized inhibition defects, and that the network appearance can arise in the context of normal brain structure without requirement for pathological connectivity changes between sites.

Light-triggered release of photocaged therapeutics - Where are we now?

The current available therapeutics face several challenges such as the development of ideal drug delivery systems towards the goal of personalized treatments for patients benefit. The application of light as an exogenous activation mechanism has shown promising outcomes, owning to the spatiotemporal confinement of the treatment in the vicinity of the diseased tissue, which offers many intriguing possibilities. Engineering therapeutics with light responsive moieties have been explored to enhance the bioavailability, and drug efficacy either in vitro or in vivo. The tailor-made character turns the so-called photocaged compounds highly desirable to reduce the side effects of drugs and, therefore, have received wide research attention. Herein, we seek to highlight the potential of photocaged compounds to obtain a clear understanding of the mechanisms behind its use in therapeutic delivery. A deep overview on the progress achieved in the design, fabrication as well as current and possible future applications in therapeutics of photocaged compounds is provided, so that novel formulations for biomedical field can be designed.

High efficiency two-photon uncaging coupled by the correction of spontaneous hydrolysis

Two-photon (TP) uncaging of neurotransmitter molecules is the method of choice to mimic and study the subtleties of neuronal communication either in the intact brain or in slice preparations. However, the currently available caged materials are just at the limit of their usability and have several drawbacks. The local and focal nature of their use may for example be jeopardized by a high spontaneous hydrolysis rate of the commercially available compounds with increased photochemical release rate. Here, using quantum chemical modelling we show the mechanisms of hydrolysis and two-photon activation, and synthesized more effective caged compounds. Furthermore, we have developed a new enzymatic elimination method removing neurotransmitters inadvertently escaping from their compound during experiment. This method, usable both in one and two-photon experiments, allows for the use of materials with an increased rate of photochemical release. The efficiency of the new compound and the enzymatic method and of the new compound are demonstrated in neurophysiological experiments.

Sulforhodamine 101, a widely used astrocyte marker, can induce cortical seizure-like activity at concentrations commonly used

Sulforhodamine 101 (SR101) is a preferential astrocyte marker widely used in 2-photon microscopy experiments. Here we show, that topical loading of two commonly used SR101 concentrations, 100 μM and 250 μM when incubated for 10 min, can induce seizure-like local field potential (LFP) activity in both anaesthetized and awake mouse sensori-motor cortex. This cortical seizure-like activity develops in less than ten minutes following topical loading, and when applied longer, these neuronal discharges reliably evoke contra-lateral hindlimb muscle contractions. Short duration (<1 min) incubation of 100 μM and 250 μM SR101 or application of lower concentrations 25 μM and 50 μM of SR101, incubated for 30 and 20 min, respectively, did not induce abnormal LFP activity in sensori-motor cortex, but did label astrocytes, and may thus be considered more appropriate concentrations for in vivo astrocyte labeling. In addition to label astrocytes SR101 may, at 100 μM and 250 μM, induce abnormal neuronal activity and interfere with cortical circuit activity. SR101 concentration of 50 μM or lower did not induce abnormal neuronal activity. We advocate that, to label astrocytes with SR101, concentrations no higher than 50 μM should be used for in vivo experiments.

Reduction in focal ictal activity following transplantation of MGE interneurons requires expression of the GABAA receptor α4 subunit

Despite numerous advances, treatment-resistant seizures remain an important problem. Loss of neuronal inhibition is present in a variety of epilepsy models and is suggested as a mechanism for increased excitability, leading to the proposal that grafting inhibitory interneurons into seizure foci might relieve refractory seizures. Indeed, transplanted medial ganglionic eminence interneuron progenitors (MGE-IPs) mature into GABAergic interneurons that increase GABA release onto cortical pyramidal neurons, and this inhibition is associated with reduced seizure activity. An obvious conclusion is that inhibitory coupling between the new interneurons and pyramidal cells underlies this effect. We hypothesized that the primary mechanism for the seizure-limiting effects following MGE-IP transplantation is the tonic conductance that results from activation of extrasynaptic GABA_A receptors (GABA_A-Rs) expressed on cortical pyramidal cells. Using in vitro and in vivo recording techniques, we demonstrate that GABA_A-R α4 subunit deletion abolishes tonic currents (I_tonic) in cortical pyramidal cells and leads to a failure of MGE-IP transplantation to attenuate cortical seizure propagation. These observations should influence how the field proceeds with respect to the further development of therapeutic neuronal transplants (and possibly pharmacological treatments).