Effects of stimulus spatial frequency, size, and luminance contrast on orientation tuning of neurons in the dorsal lateral geniculate nucleus of cat.

It is generally thought that orientation selectivity first appears in the primary visual cortex (V1), whereas neurons in the lateral geniculate nucleus (LGN), an input source for V1, are thought to be insensitive to stimulus orientation. Here we show that increasing both the spatial frequency and size of the grating stimuli beyond their respective optimal values strongly enhance the orientation tuning of LGN neurons. The resulting orientation tuning was clearly contrast-invariant. Furthermore, blocking intrathalamic inhibition by iontophoretically administering γ-aminobutyric acid (GABA)A receptor antagonists, such as bicuculline and GABAzine, slightly but significantly weakened the contrast invariance. Our results suggest that orientation tuning in the LGN is caused by an elliptical classical receptive field and orientation-tuned surround suppression, and that its contrast invariance is ensured by local GABAA inhibition. This contrast-invariant orientation tuning in LGN neurons may contribute to the contrast-invariant orientation tuning seen in V1 neurons.

Role of GABAergic inhibition in shaping the spatial frequency tuning of neurons and its contrast dependency in the dorsal lateral geniculate nucleus of cat.

To understand how GABAergic inhibition contributes to the elaboration of spatial frequency (SF) tuning of neurons in the lateral geniculate nucleus, we examined the effects of the microiontophoretic administration of bicuculline methiodide (BIC) and gabazine, both antagonists of GABAA receptors, on visual responses to grating stimuli with high or low contrasts. BIC administration changed the shape of the SF tuning curve of the spike response from band-pass to low-pass. We took the tuning curve obtained under the BIC condition as an estimated excitatory contribution to the control tuning curve and then estimated the difference between tuning curves recorded with and without BIC as the tuning curve of the estimated GABAergic inhibitory contribution. The SF tuning profile of estimated inhibition (Estimated-Inh) varied widely from cell to cell, as did estimated excitation (Estimated-Ex). Nonetheless, the relationship that Estimated-Inh exhibited more low-pass tuning than did Estimated-Ex was well conserved in the majority of cells, and the relationship refined the SF tuning of Estimated-Ex toward the band-pass tuning of the geniculate output. Lowering the stimulus contrast decreased the response magnitude, but did not change the degree of band-pass tuning. The GABAergic refinement of the SF tuning was also observed at low stimulus contrast, but was weaker than at high contrast, suggesting that GABAergic inhibition is regulated in coordination with excitatory inputs to keep the degree of the band-pass tuning constant. We therefore concluded that the degree of band-pass tuning is conserved contrast invariantly in the lateral geniculate nucleus on the basis of the dynamic regulatory action of GABAergic inhibition.

Enriched expression of serotonin 1B and 2A receptor genes in macaque visual cortex and their bidirectional modulatory effects on neuronal responses.

To study the molecular mechanism how cortical areas are specialized in adult primates, we searched for area-specific genes in macaque monkeys and found striking enrichment of serotonin (5-hydroxytryptamine, 5-HT) 1B receptor mRNA, and to a lesser extent, of 5-HT2A receptor mRNA, in the primary visual area (V1). In situ hybridization analyses revealed that both mRNA species were highly concentrated in the geniculorecipient layers IVA and IVC, where they were coexpressed in the same neurons. Monocular inactivation by tetrodotoxin injection resulted in a strong and rapid (<3 h) downregulation of these mRNAs, suggesting the retinal activity dependency of their expression. Consistent with the high expression level in V1, clear modulatory effects of 5-HT1B and 5-HT2A receptor agonists on the responses of V1 neurons were observed in in vivo electrophysiological experiments. The modulatory effect of the 5-HT1B agonist was dependent on the firing rate of the recorded neurons: The effect tended to be facilitative for neurons with a high firing rate, and suppressive for those with a low firing rate. The 5-HT2A agonist showed opposite effects. These results suggest that this serotonergic system controls the visual response in V1 for optimization of information processing toward the incoming visual inputs.

RAR1 and HSP90 form a complex with Rac/Rop GTPase and function in innate-immune responses in rice.

A rice (Oryza sativa) Rac/Rop GTPase, Os Rac1, is involved in innate immunity, but its molecular function is largely unknown. RAR1 (for required for Mla12 resistance) and HSP90 (a heat shock protein 90 kD) are important components of R gene-mediated disease resistance, and their function is conserved in several plant species. HSP90 has also recently been shown to be important in mammalian innate immunity. However, their functions at the molecular level are not well understood. In this study, we examined the functional relationships between Os Rac1, RAR1, and HSP90. Os RAR1-RNA interference (RNAi) rice plants had impaired basal resistance to a compatible race of the blast fungus Magnaporthe grisea and the virulent bacterial blight pathogen Xanthomonas oryzae. Constitutively active Os Rac1 complemented the loss of resistance, suggesting that Os Rac1 and RAR1 are functionally linked. Coimmunoprecipitation experiments with rice cell culture extracts indicate that Rac1 forms a complex with RAR1, HSP90, and HSP70 in vivo. Studies with Os RAR1-RNAi and treatment with geldanamycin, an HSP90-specific inhibitor, showed that RAR1 and HSP90 are essential for the Rac1-mediated enhancement of pathogen-associated molecular pattern-triggered immune responses in rice cell cultures. Furthermore, the function of HSP90, but not RAR1, may be essential for their association with the Rac1 complex. Os Rac1 also regulates RAR1 expression at both the mRNA and protein levels. Together, our results indicate that Rac1, RAR1, HSP90, and HSP70 form one or more protein complexes in rice cells and suggest that these proteins play important roles in innate immunity in rice.

Biosynthesis of castor oil: effect of polyamines on the acylation of lysophosphatidic acid at the sn-2 position with ricinoleic acid.

Polyamines with diamine structures of chain length longer than 3C were essential for the synthesis of phosphatidic acid (PA) from ricinoleoyl-CoA and lysophosphatidic acid (LPA) by the castor LPA acyltransferase reaction, suggesting that polyamines modulate enzyme affinity for the acyl-CoA substrate in vivo.