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Let us now go back to the example in Chapter 3. In the last section of that chapter, we showed that the integral of a function, : ℝ → ℝ, over a surface parameterized by : ⊂ ℝ → ℝ is

Let us now go back to the example in Chapter 3. In the last section of that chapter, we showed that the integral of a function, : ℝ → ℝ, over a surface parameterized by : ⊂ ℝ → ℝ is

Histological analyses of presynaptic neurons and physiological recordings from postsynaptic cells suggest that photoreceptor, bipolar, and ganglion cells release glutamate as their neurotransmitter. Multiple glutamate receptor types are present and differentially distributed in the retina. iGluRs, found on neurons within the OFF pathway, directly gate ion channels and mediate rapid synaptic transmission. IGluRs are also present on ON-type amacrine and ganglion cells in proximal retina, while ON-bipolar cells express the metabotropic APB receptor. Signaling through the APB receptor is important during development as bipolar cell input to ganglion cells mediate the formation of ON and OFF processes in the IPL. Glutamate transporters are also present at retinal glutamatergic synapses. Transporters remove excess glutamate from the synaptic cleft to prevent neurotoxicity. Thus, postsynaptic responses to glutamate are determined by the distribution of receptors and transporters at glutamatergic synapses which, in retina, determine the conductance mechanisms underlying visual information processing and development within the ON and OFF pathways.

Much research remains to be done. The state trait nature of the marker needs to be elucidated. It would be illuminating to know if the marker settled with improvement in clinical condition. Treatment specificity with regard to the marker would also be interesting. It would be illuminating to know if the marker had any predictive value in terms of treatment. More research in a wider range of psychiatric illnesses would be interesting. It is also necessary to look at studies to correlate the peripheral marker with central markers to validate the use of peripheral markers.

It is hoped that platelet intracellular second messenger responses to glutamate will reflect the pathogenesis of the disease processes. Further development of these accessible and practical markers may allow for a better understanding of these disorders, and may guide in the rational development of treatments of these conditions.

Glutamate plays a double role in the physiology of TRCs. As a free-occurring component of some foodstuff, glutamate is detected by TRCs and conveys information about the presence of protein-rich source. As a substance released by TRC and/or nerve endings, it is involved in cell-to-cell communication at chemical synapses in taste organs. In both cases, glutamate is sensed by TRC membrane through specific receptors, some of which (iGluRs) are similar to those found in the CNS, whereas others (taste-mGluR4 and T1R1/T1R3) are specifically expressed by TRCs. One interesting aspect of the biology of glutamate receptors in TRCs is that their expression seems to depend on the specific papillary localization of TRCs. For example, T1R1/T1R3 receptors are found predominantly in fungiform papillae but not in the foliate/vallate papillae of the mouse. Evidence for taste-mGluR4 has been obtained, on the other hand, by analyzing foliate/vallate papillae in rat. It is then reasonable to conceive that food glutamate may give rise to different patterns of TRC activation depending on their localization on the tongue surface. This may have a profound impact on the sensory coding for glutamate compared to other taste stimuli, including the central representation of this substance as “umami” taste.