The purpose of this study was to investigate the role of response latency in discrimination of chemical stimuli by geniculate ganglion neurons in the rat. Accordingly, we recorded single-cell 5-s responses from geniculate ganglion neurons (n = 47) simultaneously with stimulus-evoked summated potentials (electrogustogram; EGG) from the anterior tongue to signal when the stimulus contacted the lingual epithelium. Artificial saliva served as the rinse solution and solvent for all stimuli [(0.5 M sucrose, 0.03-0.5 M NaCl, 0.01 M citric acid, and 0.02 M quinine hydrochloride (QHCl)], 0.1 M KCl as well as for 0.1 M NaCl +1 μM benzamil. Cluster analysis separated neurons into four groups (sucrose specialists, NaCl specialists, NaCl/QHCl generalists and acid generalists). Artificial saliva elevated spontaneous firing rate and response frequency of all neurons. As a rule, geniculate ganglion neurons responded with the highest frequency and shortest latency to their best stimulus with acid generalist the only exception. For specialist neurons and NaCl/QHCl generalists, the average response latency to the best stimulus was two to four times shorter than the latency to secondary stimuli. For NaCl-specialist neurons, response frequency increased and response latency decreased systematically with increasing NaCl concentration; benzamil significantly decreased NaCl response frequency and increased response latency. Acid-generalist neurons had the highest spontaneous firing rate and were the only group that responded consistently to citric acid and KCl. For many acid generalists, a citric-acid-evoked inhibition preceded robust excitation. We conclude that response latency may be an informative coding signal for peripheral chemosensory neurons.

Previous biochemical evidence suggests that a cytochrome P450 specific to male antennae of the pale-brown chafer, Phyllopertha diversa, has evolved as a pheromone-degrading enzyme. By using a bioinformatics approach, we have now cloned three P450 cDNAs: CYP4AW1, CYP4AW2, and CYP6AT1. RT-PCR indicated that CYP4AW2 is expressed in all tissues examined, that CYP6AT1 is antennae-rich, and that CYP4AW1 is antennae-specific. Both tissue specificity and electrophysiological studies strongly support that CYP4AW1 in P. diversa is a pheromone-degrading enzyme involved in pheromone inactivation. Highly sensitive, pheromone-specific olfactory receptor neurons in male antennae were completely desensitized by direct application of metyrapone into the sensillar lymph. When tested in the same or different individuals, the metyrapone treatment had no effect on olfactory receptor neurons tuned to the plant volatile (Z)-3-hexenyl acetate, which might be inactivated by an esterase. Metyrapone treatment did not affect pheromone reception in the Japanese beetle, Popillia japonica, in the scarab beetle, Anomala octiescostata, or in the Oriental beetle, Exomala orientalis. Metyrapone-induced anosmia was restricted to the pheromone detectors in P. diversa, which became insensitive to physiological concentrations of pheromones for a few minutes. As opposed to previous trials, the specificity of the inhibitor and pheromone system led to unambiguous evidence for the role of pheromone-degrading enzymes in the fast inactivation of pheromones. Insects rely heavily on their chemical communication skills to perform fundamental behaviors. Females, for example, advertise their readiness to mate and recruit males for reproduction by releasing sex pheromones. To avoid ''chemical conspicuousness,'' they release minute amounts of the species-specific chemical signals. To detect low levels of such unique signals in a noisy environment, males have evolved sensory systems with remarkable sensitivity that are highly tuned to these species-specific pheromones. Females of the pale-brown chaffer Phyllopertha diversa (Coleoptera or Scarabaeidae), for example, produce an alkaloid pheromone, 1,3-dimethyl-2,4-(1H,3H)-quinazolinedione (DMQ) (Fig. 1) (1), which is specifically detected by highly sensitive olfactory receptor neurons in male antennae (2, 3).While taking an odorant-oriented flight toward a pheromoneemitting female, a male encounters intermittent chemical signals comprising short bursts of high flux separated by periods during which the flux is zero (4, 5). Sustainable flight and orientation requires that the sensory system be reset on a millisecond timescale while navigating through the ''clean'' space between filaments. Two dichotomous hypotheses have been suggested for the fast inactivation of chemical signals. Based on the estimation that in the presence of an antennae-specific pheromonedegrading enzyme (PDE) (6) the pheromone has a half-life of 15 ms, it has been suggested that pheromone signals are deactivated by an enzymatic process (7). On th...

Extracellular electrophysiological recordings from single olfactory bulb (OB) neurons in the channel catfish, Ictalurus punctatus, indicated that the OB is divided into different functional zones, each processing a specific class of biologically relevant odor. Different OB regions responded preferentially at slightly above threshold to either a mixture of 1) bile salts (10(-7) to 10(-5) M Na(+) salts of taurocholic, lithocholic, and taurolithocholic acids), 2) nucleotides [10(-6) to 10(-4) M adenosine-5'-triphosphate (ATP), inosine-5'-monophosphate (IMP), and inosine-5'-triphosphate (ITP)], or 3) amino acids (10(-6) to 10(-4)M L-alanine, L-methionine, L-arginine, and L-glutamate). Excitatory responses to bile salts were observed primarily in a thin, medial strip in both the dorsal (100-450 microm) and ventral (900-1,200 microm) OB. Excitatory responses to nucleotides were obtained primarily from dorsal, caudolateral OB, whereas excitatory responses to amino acids occurred more rostrally in the dorsolateral OB, but continued more medially in the ventral OB. The chemotopy within the channel catfish OB is more comparable to that previously described by optical imaging studies in zebrafish than by field potential studies in salmonids. The present results are consistent with recent studies, suggesting that the specific spatial organization of output neurons in the OB is necessary for the quality coding/decoding of olfactory information.

We report electrophysiological evidence that a simple odotopy, the spatial mapping of different odorants, is maintained above the level of the olfactory bulb (OB). Three classes of biologically relevant odorants for fish are processed in distinct regions of the forebrain (FB) in the channel catfish. Feeding cues, mainly amino acids and nucleotides, are represented in lateral, pallial portions of the FB, equivalent to the olfactory cortex of amniote vertebrates, whereas social signals mediated by bile salts are represented in medial FB centers, possibly homologous to portions of the amygdala. As in the OB, the different odorant classes map onto different territories; however, the response properties of units of the olfactory areas of the FB do not simply mirror those of the OB. For some units, distinctive response properties emerged, because the FB is the first center where odors subserving a common behavioral function (i.e., food function) converge.fish ͉ odotopy ͉ amygdala ͉ piriform cortex I n the primary olfactory centers of both vertebrates (the olfactory bulb, OB) and invertebrates (e.g., the antennal lobe, AL), odor quality is represented in a spatial map within the structure (1). Both the OB and AL are organized into an array of glomeruli, which are round areas of neuropil in which the receptor cell axons terminate. In all systems studied to date, each glomerulus serves as a target for receptor cells expressing a common odorant receptor. Thus, the chemospecificity of a glomerular network is related to the chemospecificity of a particular odorant receptor molecule. The proposed function of an odotopic map across the glomerular array is to enhance both the detection and discrimination of odorants by means of lateral inhibitory interactions that sharpen the response specificity of the glomerular output neurons (2, 3).Currently, a major question is whether odotopic maps occur in olfactory brain centers superior to the OB͞AL and, if so, how odors represented there compare with the OB͞AL. That is, is the odotopic map of the OB͞AL maintained intact, altered, or eliminated? Recent anatomical studies in rodents indicate that the output neurons of single glomeruli project widely to downstream forebrain (FB) targets and evidence considerable overlap (4), a logic clearly different from the odotopic organization within the OB (5, 6). This organizational pattern suggests that, in vertebrates, third-order neurons in the olfactory pathway integrate odor information arriving from multiple OB glomeruli, possibly encoding features of odorant quality more directly related to the odor's behavioral significance (e.g., food or social signal) (4,7,8). A recent report relying on expression of the transcription factor c-FOS as a marker of neuronal activity showed that, in mice, single odorants evoke repeatable, complex patterns of activation spread across the olfactory cortex (9). Such complex patterns are quite different from the simple odotopy of the OB. Further, the odorants selected for study were not of particular biological ...