Highly specific olfactory receptor neurons for types of amino acids in the channel catfish.

Odorant specificity to l-alpha-amino acids was determined electrophysiologically for 93 single catfish olfactory receptor neurons (ORNs) selected for their narrow excitatory molecular response range (EMRR) to only one type of amino acid (i.e., Group I units). These units were excited by either a basic amino acid, a neutral amino acid with a long side chain, or a neutral amino acid with a short side chain when tested at 10(-7) to 10(-5) M. Stimulus-induced inhibition, likely for contrast enhancement, was primarily observed in response to the types of amino acid stimuli different from that which activated a specific ORN. The high specificity of single Group I ORNs to type of amino acid was also previously observed for single Group I neurons in both the olfactory bulb and forebrain of the same species. These results indicate that for Group I neurons olfactory information concerning specific types of amino acids is processed from receptor neurons through mitral cells of the olfactory bulb to higher forebrain neurons without significant alteration in unit odorant specificity.

Responses of olfactory forebrain units to amino acids in the channel catfish.

A paucity of information exists concerning the processing of odorant information by single neurons in any vertebrate above the level of the olfactory bulb (OB). In this report, odorant specificity to four types of L-alpha-amino acids (neutral with long side-chains, neutral with short side-chains, basic and acidic), known biologically relevant odorants for teleosts, was determined for 217 spontaneously active forebrain (FB) neurons in the channel catfish. Group I FB units were identified that were excited by only one of four types of amino acids; no Group I unit was encountered that was excited by an acidic amino acid. The Group I FB units exhibited similar preferences as described previously for OB neurons, suggesting that no major modifications of olfactory information for at least some of these units occurred between the OB and FB. Evidence, however, for the convergence of odor information between the OB and FB was suggested by Group II FB units that exhibited a broader excitatory molecular receptive range (EMRR) than those of previously recorded types of OB units or the Group I FB units. Group II FB units were excited by both neutral and basic amino acids and a few also by acidic amino acids, EMRRs not observed previously in OB units. Stimulus-induced inhibition, likely for contrast enhancement, was also often observed for the many of the FB units encountered. The observed EMRRs of the FB units presently identified and those of the OB units previously studied are consistent with the ability of catfish to behaviorally discriminate these compounds.

Beyond the olfactory bulb: an odotopic map in the forebrain.

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.

Pheromone anosmia in a scarab beetle induced by in vivo inhibition of a pheromone-degrading enzyme.

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.

Odorant specificity of single olfactory bulb neurons to amino acids in the channel catfish.

Odorant specificity to l-alpha-amino acids was determined for 245 olfactory bulb (OB) neurons recorded from 121 channel catfish. The initial tests included 4 amino acids representing acidic [monosodium glutamate (Glu)], basic [arginine (Arg)], and neutral [possessing short: alanine (Ala) and long: methionine (Met) side chains] amino acids that were previously indicated to bind to independent olfactory receptor sites. Ninety-one (37%) units (Group I) tested at 1, 10, and 100 microM showed high selectivity and were excited by only one of the 4 amino acids. Odorant specificity for the vast majority of Group I units did not change over the 3 s of response time analyzed. A total of 154 OB units (63%) (Group II) were excited by a second amino acid, but only at >/=10x odorant concentration. An additional 69 Group I units were tested with related amino acids and derivatives from 10(-9) to 10(-5) M to determine their excitatory odorant thresholds and selectivities. Two groups of units originally selective for Met were evident: those most sensitive to neutral amino acids having branched and linear side chains, respectively. OB units originally selective for Ala responded at low concentration to other similar amino acids. Units originally selective for Arg were excited at low concentration by amino acids possessing in their side chains at least 3 methylene groups and a terminal amide or guanidinium group. The specificities of the OB units determined electrophysiologically are sufficient to account for many of the previous results of behavioral discrimination of amino acids in this and related species.

Peripheral coding of sex pheromone and a behavioral antagonist in the Japanese beetle, Popillia japonica.

Male antennae of the Japanese beetle, Popillia japonica, possess olfactory receptor neurons (ORNs) cocompartmentalized in the same sensilla placodea, one tuned to the sex pheromone, (R)-japonilure, and the other to the detection of a behavioral antagonist, (S)-japonilure. In-depth electrophysiological experiments revealed mutual inhibitory and synergistic effects in ORNs stimulated simultaneously with the two semiochemicals. The olfactory system of P japonica exhibited a remarkable ability to discriminate completely coincident strands of pheromone and behavioral antagonist from strands of the two semiochemicals temporally isolated (by 1.5-3 msec). The mutual inhibition was reflected mainly by the delay of onset or total lack of spikes and by the significant increase in the rise time of potentials generated by blends of (R)- and (S)-japonilure. In contrast, synergist ORNs showed no neural activity (spikes) when stimulated with either the sex pheromone or the behavioral antagonist, but showed clear responses to blends of the two semiochemicals. Evidence for mixture-suppressed responses was observed not only in the Japanese beetle, but also in the Osaka beetle, Anomala osakana, and the Oriental beetle, Exomala orientalis, thus suggesting that it is a common feature in the sensory physiology of scarab beetles.

Odorant-induced olfactory receptor neural oscillations and their modulation of olfactory bulbar responses in the channel catfish.

Peripheral waves (PWs) in the channel catfish are odorant-induced neural oscillations of synchronized populations of olfactory receptor neurons (ORNs) that appear after the initial approximately 500 msec of the response. The mean dominant frequency during the initial 2 sec of PW activity is approximately 28 Hz, declining to approximately 20 Hz in the last sec of a 5 sec stimulus. Recordings of PWs from different regions of a single olfactory lamella and simultaneously from widely separated lamellae within the olfactory organ suggest that PWs are initiated in the sensory epithelium within each olfactory lamella. Simultaneous recordings in vivo from the olfactory organ [electro-olfactogram (EOG) or integrated neural activity], local field potentials (LFPs) from the olfactory bulb (OB), and single and few-unit activity from OB neurons were performed. Cross-correlation analysis of simultaneously recorded odor-induced OB LFPs and either EOG or ORN neural activity showed that oscillations occurring within the OB were lower (<20 Hz) than those of PWs; however, during PW activity, OB LFPs increased both their magnitude and dominant frequencies and became correlated with the PWs. Also during odorant-induced PW activity, the responses of different OB neurons with similar odorant specificity became phase locked to each other and to both the PWs and OB LFPs. PWs are hypothesized to function to strengthen the synaptic transfer of olfactory information at specific glomeruli within the OB.