Olfactory receptor neuron responses coding for rapid odour sampling.

Vertebrate olfactory receptor neurons (ORNs) are stimulated in a rhythmic manner in vivo, driven by delivery of odorants to the nasal cavity carried by the inhaled air, making olfaction a sense where animals can control the frequency of stimulus delivery. How ORNs encode repeated stimulation at resting, low breathing frequencies and at increased sniffing frequencies is not known, nor is it known if the olfactory transduction cascade is accurate and fast enough to follow high frequency stimulation. We investigated mouse olfactory responses to stimulus frequencies mimicking odorant exposure during low (2Hz) and high (5Hz) frequency sniffing. ORNs reliably follow low frequency stimulations with high fidelity by generating bursts of action potentials at each stimulation at intermediate odorant concentrations, but fail to do so at high odorant concentrations. Higher stimulus frequencies across all odorant concentrations reduced the likelihood of action potential generation, increased the latency of response, and decreased there liability of encoding the onset of stimulation. Thus an increase in stimulus frequency degrades and at high odorant concentrations entirely prevents action potential generation in individual ORNs, causing reduced signalling to the olfactory bulb. These results demonstrate that ORNs do not simply relay timing and concentration of an odorous stimulus, but also process and modulate the stimulus in a frequency-dependent manner which is controlled by the chosen sniffing rate.

Modifications to the tetracaine scaffold produce cyclic nucleotide-gated channel blockers with widely varying efficacies.

Five new tetracaine analogues were synthesized and evaluated for potency of blockade of cyclic nucleotide-gated channels relative to a multiply charged tetracaine analogue described previously. Increased positive charge at the tertiary amine end of tetracaine results in higher potency and voltage dependence of block. Modifications that reduce the hydrophobic character at the butyl tail are deleterious to block. The tetracaine analogues described here have apparent affinities for CNGA1 channels that vary over nearly 8 orders of magnitude.

Is the olfactory bulb computationally similar to the retina?

The computational role of the olfactory bulb remains a mystery after 60 yr of physiological research. Recently, Fantana and colleagues proposed a new model of bulb function based on sparse inhibitory connections between glomeruli, the functional units of the bulb, rather than the existing lateral inhibition model. I present a summary of their model here and its implications along with comparison to recent work in the very similar Drosophila olfactory system.

Presynaptic muscarinic receptors enhance glutamate release at the mitral/tufted to granule cell dendrodendritic synapse in the rat main olfactory bulb.

The mammalian olfactory bulb receives multiple modulatory inputs, including a cholinergic input from the basal forebrain. Understanding the functional roles played by the cholinergic input requires an understanding of the cellular mechanisms it modulates. In an in vitro olfactory bulb slice preparation we demonstrate cholinergic muscarinic modulation of glutamate release onto granule cells that results in gamma-aminobutyric acid (GABA) release onto mitral/tufted cells. We demonstrate that the broad-spectrum cholinergic agonist carbachol triggers glutamate release from mitral/tufted cells that activates both AMPA and NMDA receptors on granule cells. Activation of the granule cell glutamate receptors leads to calcium influx through voltage-gated calcium channels, resulting in spike-independent, asynchronous GABA release at reciprocal dendrodendritic synapses that granule cells form with mitral/tufted cells. This cholinergic modulation of glutamate release persists through much of postnatal bulbar development, suggesting a functional role for cholinergic inputs from the basal forebrain in bulbar processing of olfactory inputs and possibly in postnatal development of the olfactory bulb.

Store calcium mediates cholinergic effects on mIPSCs in the rat main olfactory bulb.

The significance of endoplasmic reticulum (ER) store calcium in modulating transmitter release is slowly gaining recognition. One transmitter system that might play an important role in store calcium modulation of transmitter release in the CNS is acetylcholine (ACh). The main olfactory bulb (OB) receives rich cholinergic innervation from the horizontal limb of the diagonal band of Broca and blocking cholinergic signaling in the bulb inhibits the ability of animals to discriminate between closely related odors. Here we show that exposing OB slices to carbamylcholine (CCh), a hydrolysis-resistant analog of Ach, increases gamma-aminobutyric acid (GABA) release at dendrodendritic synapses onto the mitral cells. This increase in transmitter release is mediated by the activation of the M1 class of muscarinic receptors and requires the mobilization of calcium from the ER. The site of action of CCh for this effect is developmentally regulated. In animals younger than postnatal day 10, the major action of CCh appears to be on mitral cells, enhancing GABA release by reciprocal signaling resulting from increased glutamate release from mitral cells. In animals older than postnatal day 10, CCh appears to modulate transmitter release from dendrites of the interneurons themselves. Our results point to modulation of inhibition as an important role for cholinergic signaling in the OB. Our data also strengthen the emerging idea of a role for store calcium in modulating transmitter release at CNS synapses.

A multiply charged tetracaine derivative blocks cyclic nucleotide-gated channels at subnanomolar concentrations.

Cyclic nucleotide-gated (CNG) ion channels are central participants in sensory transduction, generating the electrical response to light in retinal photoreceptors and to odorants in olfactory receptors. They are expressed in many other tissues where their specific roles in signaling remain unclear. As is true for many other ion channels, there is a paucity of specific blockers needed to dissect the contributions of these channels to cell signaling. CNG channels are members of the superfamily of voltage-gated ion channels, and the local anesthetic tetracaine is known to block CNG channels in a manner that resembles the block of voltage-gated Na(+) channels. The amine in local anesthetics interacts with the charged selectivity filter of Na(+) channels, while the aromatic ring gets stuck in the inner cavity and has hydrophobic interactions with the residues lining that region. Here we have synthesized a derivative of tetracaine, 3-[(aminopropyl)amino]-N,N-dimethyl-N-(2-[[4-(butylamino)benzoyl]oxy]ethyl)propan-1-aminium acetate (APPA-tetracaine), that contains three positively charged amines at physiological pH instead of one. This compound blocked several different CNG channels in the picomolar to nanomolar concentration range at positive membrane potentials, making it several orders of magnitude more potent than tetracaine. In contrast, significant block of Na(+) channels by APPA-tetracaine required concentrations of hundreds of nanomolar. The results suggest that the highly charged moiety of APPA-tetracaine interacts strongly with the negative charge cluster in the selectivity filter of CNG channels. We propose that a variety of potent and specific ion channel blockers could be generated by expanding on traditional blocker structures to target the selectivity filters of other channels.