Chemoreceptor

Fig. 1. Schematic for chemoreceptor-modified ISFET biosensor for detection of acetylcholine where the source and drain are both n-ty e siHcon.

The low detection limit, high sensitivity, and fast response times of chemoreceptor-based biosensors result primarily from the extremely high binding constants of the receptor R for the target substrate S. The receptor—substrate binding may be described... 

As of this writing chemoreceptor-based biosensors are not yet on the commercial market. Only a few chemoreceptors have been isolated and their substrates identified. Moreover, those chemoreceptors that have been isolated are fragile and have limited lifetimes. 

The area postrema is a circumventricular brain region positioned on the dorsal surface of the medulla on the floor of the fourth ventricle. The blood-brain barrier and the cerebrospinal fluid-brain barrier are absent in this region and consequently many substances that do not pass across capillaries in other regions of the brain can do so in the area postrema. The chemoreceptor trigger zone (CTZ), located in the lateral area postrema is sensitive to blood-borne emetogens. Nerves from the CTZ connect with the vomiting centre. 

Central nervous system-euphoria, drowsiness, apathy, mental confusion, alterations in mood, reduction in body temperature, feelings of relaxation, dysphoria (depression accompanied by anxiety), nausea, and vomiting are caused by direct stimulation of the emetic chemoreceptors located in the medulla. The degree to which these occur usually depends on the drug and the dose. 

Chemically modified electrodes, 39, 118 Chemometrics, 197 Chemoreceptor, 187 Chip, 194, 195 Chloramphenicol, 70 Chloride electrode, 159 Chlorpromazine, 34 Cholesterol, 182 Cholinesterase, 182 Chromium, 85, 86 Chronoahsorptometry, 42 Chronoamperometry, 21, 60, 130, 135, 132, 177... 

Emesis No distinct pathway DA receptors in chemoreceptor pathway zone Vomiting Anti-emetic (not motion sickness) D2 ... 

ExCDDIs certainly improve the efficacy and duration of action of levodopa so that it can be given in a smaller dose (e.g. 25%) and generally in a 4 1 ratio, levodopa ExCDDI. As might be expected, some DA side-effects such as dyskinesia and psychoses are worse, but hypotension is less (no peripheral effects of DA) and vomiting is actually much reduced or abolished. This is because the chemoreceptor trigger zone of the vomiting centre while in the brain is on the blood side of the blood-brain barrier and will not be stimulated since no DA is formed peripherally (Fig. 15.5). That an... 

With most DA agonists there are the other expected signs of increased DA activity such as hallucinations, psychosis and hypotension which can be worse than with levodopa. Fortunately vomiting can be countered by giving the DA antagonist domperidone. This does not cross the blood-brain barrier and so counteracts only the peripheral (chemoreceptor trigger zone) effect of the DA agonist (Fig. 15.5). 

Opiates activate the chemoreceptor trigger zone in the medulla (by disinhibition) to cause nausea and vomiting, and cough suppression also occurs because of the inhibitory effects of opiates on the brainstem nuclei in the cough reflex pathway. Dextromethorphan is the non-opiate isomer of the opiate levorphanol and is an effective cough suppressant. 

Absence from the first developmental stages as in whales and some Old World monkeys suggests that functional substitution is provided by other nasal chemoreceptors. The ganglia and fibres of the terminal nerve system (N. terminalis, Fig. 2.9) are the principal candidate (Wirsig and Leonard, 1987). The role of the trigeminal input, although a minor sensor, could well be expanded in a limited capacity (Tucker, 1971 Wysocki and Meredith, 1987 Westhofen, 1987). 

The first air-breathing vertebrates have continued not only to differentiate their chemoreceptor cell types, but also to show some increase in morphological complexity. Amphibia show several adaptive features for fluid intake on land, such as the tentacles of the Caecilians, shown in... 

Fig. 2.6 Rostral nasal anatomy of Honey Possum (Tarsipes rostratus) showing (a) section levels and (b) TS at level 4 naso-palatine papilla with taste-buds (TB), gustatory chemoreceptors facing lumen of N-Pd (incisive duct, Id) (from Kratzing, 1987).

Fig. 6.1 Interrelationships of chemoreceptors internal (neurotransmitters) and external chemosignals. Phylogenetic connections for sequences in transmembrane (Fig. 6.2) domains Nos. = bootstrap values from 100 Megaline searches (based on majority consensus tree). Invertebrate — DrOR fruit-fly, CeOR nematode vertebrate — FOR fish, LOR (1 2) lamprey, MOR mouse VR (1 2) vomeronasal (from Dryer and Berghard, 1999).

Fig. 6.2 Topology of GPCRs for the main classes of chemoreceptors. Group A vomeronasal (V1R and V2R) and Group B olfactory (OR) and taste (T1R) (from Tirindelli, 1998 Hoon, 1999 Gilbertson, 2000).

Temporal alterations in peripheral chemoreceptors are rare in vertebrates, but this group provides an example of transient enhancement of signal capture efficiency. The Red-backed salamander (Plethodon cinereus) shows dimorphic and seasonal VNO volume fluctuations. Males always possess a significantly larger vomeronasal area, and both... 

Cooper W.E. Jr. (1997). Correlated evolution of prey chemical discrimination with foraging, lingual morphology and vomeronasal chemoreceptor abundance in lizards. Behav Ecol Sociobiol 41, 257-265. 

Evans C.S. and Schilling A. (1995). The accessory (vomeronasal) chemoreceptor system in some Prosimians. In Creatures of the Dark The Nocturnal Prosimians (Altermann L., Doyle G. and Izard M.K., eds.). Plenum, New York, pp. 393-412. 

Johnson E.W., Eller P. and Jafek B. (1995). Distribution of OMP, PGP 9.5- and CaBP-like immunoreactive chemoreceptor neurons in the developing human olfactory epithelium. Anat Embryol 191, 311-317. 

---