Poikilothermic organisms such as insects have mechanisms to protect neural function under high temperature stress. Natural variation at the foraging (for) locus of the fruit fly, Drosophila melanogaster, encoding a cGMP-dependent protein kinase (PKG), influences neural thermotolerance in Drosophila larvae. The current study re-examines thermotolerance of adult flies to account for inconsistencies in the documented role of for during hyperthermia. We found that adult for (R) (rover) flies with high PKG activity were incapacitated faster under hyperthermic conditions of 39°C compared to their lower PKG activity counterparts for (s) and for (s2) (sitters), but not at higher temperatures. This indicates that lowered PKG activity promotes tolerance to heat stress, and that the for gene influences thermotolerance for a narrow range of temperatures in adult flies.

Key points• The Cl − uptake cotransporter, Na + -K + -2Cl − type 1 (NKCC1), is expressed in the distal synaptic layer of the vertebrate retina, where photoreceptor terminals contact the dendrites of horizontal and bipolar cells.• Light adaptation, dopamine and a D1 receptor agonist increased the expression levels of phosphorylated NKCC1, the active form of the transporter inserted in the cell membrane.• Pharmacological blockage of NKCC1 with bumetanide increased the rod-and cone-mediated excitatory postsynaptic currents in horizontal cells.• Inhibiting NKCC1 increased exocytotic membrane capacitance, intracellular Ca 2+ levels, and voltage-dependent Ca 2+ channel currents in both rod and cone terminals, all of which are associated with increased transmitter release.• This study describes a new function of NKCC1, specifically the suppression of transmitter release at the photoreceptor terminals, a process that serves to prevent the depletion of glutamate and protect retinal neurons from its putative cytotoxic effects.Abstract The Na + -K + -2Cl − co-transporter type 1 (NKCC1) is localized primarily throughout the outer plexiform layer (OPL) of the distal retina, a synaptic lamina that is comprised of the axon terminals of photoreceptors and the dendrites of horizontal and bipolar cells. Although known to play a key role in development, signal transmission and the gating of sensory signals in other regions of the retina and in the CNS, the contribution of NKCC1 to synaptic transmission within the OPL is largely unknown. In the present study, we investigated the function of NKCC1 at the photoreceptor-horizontal cell synapse by recording the electrical responses of photoreceptors and horizontal cells before and after blocking the activity of the transporter with bumetanide (BMN). Because NKCC1 co-transports 1 Na + , 1 K + and 2 Cl − , it is electroneutral and its activation had little effect on membrane conductance. However, recordings from postsynaptic horizontal cells revealed that inhibiting NKCC1 with BMN greatly increased glutamate release from both rod and cone terminals. In addition, we found that NKCC1 directly regulates Ca 2+ -dependent exocytosis at the photoreceptor synapse, raising the possibility that NKCC1 serves to suppress bulk release of glutamate vesicles from photoreceptor terminals in the dark and at light offset. Interestingly, NKCC1 gene and protein expressions were upregulated by light, which we attribute to the light-induced release of dopamine acting on D1-like receptors. In sum, our study reveals a new role for NKCC1 in the regulation of synaptic transmission in photoreceptors.

Key pointsr This study provides experimental evidence that glycinergic interplexiform cells create a centrifugal feedback loop in the vertebrate retina that regulates the transmission of glutamatergic signals between photoreceptors and second-order neurons.r This mechanism serves to reduce glutamate uptake and enhance glutamate release. r Glycine receptors containing the GlyRα3 subunit are expressed on bipolar cell dendrites, and their activation leads to a depolarizing response in a group of rod-dominated ON bipolar cells, and hyperpolarizing responses in OFF bipolar cells.r Using strychnine to block endogenous glycine feedback reduces the amplitudes of light-evoked responses in both ON and OFF bipolar cells, indicating that glycine feedback regulates signal propagation in the distal retina.r Glycinergic feedback provides a neural mechanism that enhances synaptic gain and improves visual sensitivity.Abstract Glycine input originates with interplexiform cells, a group of neurons situated within the inner retina that transmit signals centrifugally to the distal retina. The effect on visual function of this novel mechanism is largely unknown. Using gramicidin-perforated patch whole cell recordings, intracellular recordings and specific antibody labelling techniques, we examined the effects of the synaptic connections between glycinergic interplexiform cells, photoreceptors and bipolar cells. To confirm that interplexiform cells make centrifugal feedback on bipolar cell dendrites, we recorded the postsynaptic glycine currents from axon-detached bipolar cells while stimulating presynaptic interplexiform cells. The results show that glycinergic interplexiform cells activate bipolar cell dendrites that express the α3 subunit of the glycine receptor, as well as a subclass of unidentified receptors on photoreceptors. By virtue of their synaptic contacts, glycine centrifugal feedback increases glutamate release from photoreceptors and suppresses the uptake of glutamate by the type 2A excitatory amino acid transporter on photoreceptors. The net effect is a significant increase in synaptic gain between photoreceptors and their second-order neurons. Abbreviations DHKA, dihydrokainic acid; EAAT2A, type 2A excitatory amino acid transporter; EPSC, excitatory postsynaptic current; GlyRα3, glycine receptor α3 subunit; IPL, inner plexiform layer; LED, light emitting diode; OCT compound, optimal cutting temperature compound; OPL, outer plexiform layer; TEA, tetraethylammonium.

Taurine activates not only Cl−-permeable ionotropic receptors, but also receptors that mediate metabotropic responses. The metabotropic property of taurine was revealed in electrophysiological recordings obtained after fully blocking Cl−-permeable receptors with an inhibitory “cocktail” consisting of picrotoxin, SR95531, and strychnine. We found that taurine’s metabotropic effects regulate voltage-gated channels in retinal neurons. After applying the inhibitory cocktail, taurine enhanced delayed outward rectifier K+ channels preferentially in Off-bipolar cells, and the effect was completely blocked by the specific PKC inhibitor, GF109203X. Additionally, taurine also acted through a metabotropic pathway to suppress both L- and N-type Ca2+ channels in retinal neurons, which were insensitive to the potent GABAB receptor inhibitor, CGP55845. This study reinforces our previous finding that taurine in physiological concentrations produces a multiplicity of metabotropic effects that precisely govern the integration of signals being transmitted from the retina to the brain.