Plasticity of olfactory-guided behaviour and its neurobiological basis: lessons from moths and locusts

The sense of smell plays an important role in guiding the behaviour of many animals including insects. The attractiveness of a volatile is not only dependent on the nature of the chemical, but might change with the physiological status (e.g., age/hormone or mating status) or environmental conditions (e.g., photoperiod or temperature) of the individual. Here we summarize our studies focused on the plasticity of olfactory-guided behaviour and its neurobiological basis linked with the physiological status in Lepidoptera and migratory locusts. In moths and locusts, age and juvenile hormone changed the behavioural responses to pheromones. In moths, mating had an effect on pheromone responses in males and plant odour responses in females. In all cases of behavioural plasticity studied, we found changes in the sensitivity of olfactory interneurons in the antennal lobe, whereas the peripheral system does not seem to show any plasticity in that context. Changes in the central nervous system were slow under the influence of juvenile hormone (days) or fast after mating (minutes). The olfactory system seems thus to adapt to the physiological or environmental situation of an animal to avoid a waste of energy. We discuss possible mechanisms underlying the observed plasticity.     

The most likely voltage path and large deviations approximations for integrate-and-fire neurons

We develop theory and numerical methods for computing the most likely subthreshold voltage path of a noisy integrate-and-fire (IF) neuron, given observations of the neuron’s superthreshold spiking activity. This optimal voltage path satisfies a second-order ordinary differential (Euler-Lagrange) equation which may be solved analytically in a number of special cases, and which may be solved numerically in general via a simple “shooting” algorithm. Our results are applicable for both linear and nonlinear subthreshold dynamics, and in certain cases may be extended to correlated subthreshold noise sources. We also show how this optimal voltage may be used to obtain approximations to (1) the likelihood that an IF cell with a given set of parameters was responsible for the observed spike train; and (2) the instantaneous firing rate and interspike interval distribution of a given noisy IF cell. The latter probability approximations are based on the classical Freidlin-Wentzell theory of large deviations principles for stochastic differential equations. We close by comparing this most likely voltage path to the true observed subthreshold voltage trace in a case when intracellular voltage recordings are available in vitro. Action Editor: Peter Latham     

Protein kinase C enhances glycine-insensitive desensitization of NMDA receptors independently of previously identified protein kinase C sites

Protein kinase C (PKC) phosphorylates the NR1 and NR2A subunits of NMDARs at consensus sites located within their intracellular C-terminal tails. However, the functional consequences of these biochemical events are not well understood. In HEK293 cells expressing NR1/NR2A, activation of endogenous PKC by 4beta-phorbol 12-myristate 13-acetate (PMA) increased NMDAR desensitization as evidenced by a reduced steady-state current without any change in peak. The effects of PMA on NMDAR-mediated responses were prevented by specific PKC inhibitors and were not mimicked by an inactive enantiomer of PMA. The effects of PMA were preserved despite mutagenesis of the major PKC sites on the NR1 subunit (S889A, S890A, S896A and S897A) or removal of the entire NR1 C-terminal tail (NR1(stop838)). When co-expressing NR1(stop838)/NR2A the effects of PMA could only be observed with agonist concentrations sufficient to induce glycine-insensitive desensitization. Moreover, the effects of PMA were observed in receptors composed of NR1/NR2A and NR1/NR2B, but not NR1/NR2C, a subunit combination in which desensitization is absent. The NR2 subunit dependence suggested that the actions of PMA might require specific PKC sites previously identified within NR2A. However, a C-terminal truncated form of NR2A (NR2A(stop905)) remained responsive to PMA. We conclude that activation of PKC increases NMDAR glycine-insensitive desensitization independently of previously identified sites located within the NR1 C-terminus and distal segment of the NR2A C-terminus.     

The M34A mutant of Connexin26 reveals active conductance states in pore-suspending membranes

Connexin26 (Cx26) is a member of the connexin family, the building blocks for gap junction intercellular channels. These dodecameric assemblies are involved in gap junction-mediated cell–cell communication allowing the passage of ions and small molecules between two neighboring cells. Mutations in Cx26 lead to the disruption of gap junction-mediated intercellular communication with consequences such as hearing loss and skin disorders. We show here that a mutant of Cx26, M34A, forms an active hemichannel in lipid bilayer experiments. A comparison with the Cx26 wild-type is presented. Two different techniques using micro/nano-structured substrates for the formation of pore-suspending lipid membranes are used. We reconstituted the Cx26 wild-type and Cx26M34A into artificial lipid bilayers and observed single channel activity for each technique, with conductance levels of around 35, 70 and 165 pS for the wild-type. The conductance levels of Cx26M34A were found at around 45 and 70 pS.     

Employing ICA and SOM for spike sorting of multielectrode recordings from CNS

For classification of action potential shapes in multineuron recordings, we present a spike sorting system employing independent component analysis (ICA) and an unsupervised artificial neural network (Kohonen's self-organizing map, SOM). We focus on how ICA in the first stage of the spike sorting system can be used to address specific problems arising in recordings using multielectrode arrays in the CNS. Using real data recorded from the pontine nuclei in rats and simulated data, we evaluate the performance of several ICA algorithms to remove cross-talk between electrodes using data from continuous recording (or simulation). When using cut-out data, the standard format of extracellular spike recordings, new problems emerge and robust algorithms are needed. We demonstrate that several ICA algorithms show a good performance on cut-out data from multielectrode array recordings (simulated and real data). In tetrode recordings the same neuron is purposely recorded by several electrodes simultaneously and we show, how independent component analysis can be used in this case to identify redundant information and hence to compress relevant information, improving subsequent clustering of a SOM.     

Dynamic control of deactivation gating by a soluble amino-terminal domain in HERG K(+) channels

K(+) channels encoded by the human ether-à-go-go-related gene (HERG) are distinguished from most other voltage-gated K(+) channels by an unusually slow deactivation process that enables cardiac I(Kr), the corresponding current in ventricular cells, to contribute to the repolarization of the action potential. When the first 16 amino acids are deleted from the amino terminus of HERG, the deactivation rate is much faster (Wang, J., M.C. Trudeau, A.M. Zappia, and G.A. Robertson. 1998. J. Gen. Physiol. 112:637-647). In this study, we determined whether the first 16 amino acids comprise a functional domain capable of slowing deactivation. We also tested whether this "deactivation subdomain" slows deactivation directly by affecting channel open times or indirectly by a blocking mechanism. Using inside-out macropatches excised from Xenopus oocytes, we found that a peptide corresponding to the first 16 amino acids of HERG is sufficient to reconstitute slow deactivation to channels lacking the amino terminus. The peptide acts as a soluble domain in a rapid and readily reversible manner, reflecting a more dynamic regulation of deactivation than the slow modification observed in a previous study with a larger amino-terminal peptide fragment (Morais Cabral, J.H., A. Lee, S.L. Cohen, B.T. Chait, M. Li, and R. Mackinnon. 1998. Cell. 95:649-655). The slowing of deactivation by the peptide occurs in a dose-dependent manner, with a Hill coefficient that implies the cooperative action of at least three peptides per channel. Unlike internal TEA, which slows deactivation indirectly by blocking the channels, the peptide does not reduce current amplitude. Nor does the amino terminus interfere with the blocking effect of TEA, indicating that the amino terminus binding site is spatially distinct from the TEA binding site. Analysis of the single channel activity in cell-attached patches shows that the amino terminus significantly increases channel mean open time with no alteration of the mean closed time or the addition of nonconducting states expected from a pore block mechanism.We propose that the four amino-terminal deactivation subdomains of the tetrameric channel interact with binding sites uncovered by channel opening to specifically stabilize the open state and thus slow channel closing.     

Binocular visual integration in the crustacean nervous system

Although the behavioral repertoire of crustaceans is largely guided by visual information their visual nervous system has been little explored. In search for central mechanisms of visual integration, this study was aimed at identifying and characterizing brain neurons in the crab involved in binocular visual processing. The study was performed in the intact animal, by recording intracellularly the response to visual stimuli of neurons from one of the two optic lobes. Identified neurons recorded from the medulla (second optic neuropil), which include sustaining neurons, dimming neurons, depolarizing and hyperpolarizing tonic neurons and on-off neurons, all presented exclusively monocular (ipsilateral) responses. In contrast, all wide field movement detector neurons recorded from the lobula (third optic neuropil) responded to moving stimuli presented to the ipsilateral and to the contralateral eye. In these cells, the responses evoked by ipsilateral or contralateral stimulation were almost identical, as revealed by analysing the number and amplitude of the elicited postsynaptic potentials and spikes, and the ability to habituate upon repeated visual stimulation. The results demonstrate that in crustaceans important binocular processing takes place at the level of the lobula.     

Volatile organic compounds as signals in a plant-herbivore system: electrophysiological responses in olfactory sensilla of the moth Cactoblastis cactorum

The morphological sensillum types on the antennae of male and female Cactoblastis cactorum were visualized by scanning electron microscopy. Electrophysiological recordings were performed for the first time on single olfactory sensilla of C. cactorum. The male sensilla trichodea house a receptor cell responding to the putative pheromone component (9Z,12E)-tetradecadienyl acetate. The sensilla trichodea of the females were much shorter than those of the males and contained specialized receptor cells responding to certain terpenoids, the most frequent being the nerolidol-sensitive cell. The sensilla auricillica and sensilla basiconica of both sexes contained cells responding less specifically to terpenoid compounds as well as to green leaf volatiles. Cells of the sensilla coeloconica responded to aliphatic aldehydes and acids. Eight volatile organic compounds emitted by Opuntia stricta, a host plant of C. cactorum, were identified using gas chromatography-mass spectrometry, beta-caryophyllene being the major compound. Five compounds identified by gas chromatography in the headspace of O. stricta elicited responses in olfactory receptor cells of C. cactorum, nonanal being the most active compound and therefore a candidate attractant of C. cactorum.