Chem. Senses 27: 769-778,
2002
© Oxford University Press 2002
Identification of Chemosensory Sensilla Activating Antennular Grooming Behavior in the Caribbean Spiny Lobster, Panulirus argus
Department of Biology, Hofstra University, Hempstead, NY 11549-1140, USA
Correspondence to be sent to: Peter C. Daniel, Department of Biology, Hofstra University, Hempstead, NY 11549-1140, USA. e-mail: biopcd{at}hofstra.edu
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Abstract |
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Crustaceans such as crabs and lobsters clean or `groom' their olfactory organ, the antennule, by wiping it through a pair of mouthpart appendages, the third maxillipeds. In the lobster, only a few chemicals found in prey extracts, especially glutamate, elicit grooming. Chemosensory input driving grooming is likely to be mediated via sensilla located on antennules and third maxillipeds. Chemosensory antennular sensilla are innervated by neurons with central projections either to the glomerular olfactory lobe (aesthetasc sensilla) or to non-glomerular antennular neuropils (nonaesthetasc sensilla). By selectively ablating the chemosensory sensilla on the antennules and the third maxillipeds we have determined that the aesthetascs are necessary and sufficient to drive grooming behavior. Chemosensory activation of antennular grooming behavior likely follows a `labeled-line' model in that aesthetasc neurons tuned to glutamate provide adequate input via the olfactory lobe to motor centers in the brain controlling antennular movements.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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An enduring question in chemosensory biology is how olfactory systems encode chemical signals. At one extreme, the labeled-line theory argues that discrete sensory neurons may encode specific qualities of a stimulus. In its simplest derivation, activation of this subset of neurons will drive a behavior. In contrast, the across-neuron pattern theory argues that stimulus quality is encoded by the pattern of activity across a population of neurons. Because many behaviorally relevant chemical signals are complex mixtures requiring activation of many neurons, the latter theory is often favored.
This is quite apparent in aquatic environments in which chemical signals
allowing identification and location of food consist of mixtures that include
amino acids, amines, nucleotides and organic acids
(Carr et al., 1996
).
The chemosensory systems of aquatic predators and scavengers are adapted to
sense and process this complex information. Lobsters possess a diversity of
receptors tuned to specific chemicals in the mixture
(Voigt and Atema, 1992;
Daniel et al., 1994),
including those located on the olfactory organ, the antennules
(Figure 1). The result is that
the peripheral olfactory system both encodes the identity of the mixture as a
unique odor, through the response of the entire population of sensory neurons,
and at the same time retains the identities of the individual elements making
up the mixture, through the responses of subsets of neurons tuned to
particular chemicals (Derby,
2000). Thus it is possible that both labeled-lined and
across-neuron pattern codes may function, although most evidence argues for
the latter.
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Sensory neurons from the antennules appear to follow two independent pathways into the brain of the spiny lobster. The `aesthetasc' pathway consists of afferents from the lateral antennular flagella, most likely innervating aesthetasc sensilla, which terminate on one or a few of the
1100 columnar glomeruli within the paired olfactory lobes (OL) found in
the deutocerebrum. The olfactory lobes also receive some mechanosensory input
(Schmidt and Ache, 1992;
Schmidt et al.,
1992). The `nonaesthetasc' pathway consists of nonaesthetasc
chemosensory and mechanosensory afferents from medial and lateral antennular
flagella, which terminate in the paired nonglomerular lateral antennal
neuropils (LAN) of the deutocerebrum
(Schmidt et al.,
1992; Schmidt and Ache,
1996a). The LAN and associated median antennal neuropil have been
identified as lower motor centers for the antennules
(Roye, 1986;
Roye and Bashor, 1991),
initiating and controlling the antennular movements involved in antennular
withdrawal, flick and grooming (Snow,
1973; Roye and Bashor,
1991; Roye,
1994).
One behavior in lobsters elicited in response to chemical signals is
antennular grooming behavior or AGB
(Barbato and Daniel, 1997;
Daniel et al., 2001).
The behavior consists of deflection of either one or both of the antennules
downward, permitting the lateral flagella of the antennules
(Figure 1A) to be grasped by
the third maxillipeds (Figure
1B), the paired appendages on either side of the mouth. The
antennules are pulled repeatedly through the setal combs of the maxillipeds
(`antennule wiping'). After a number of repetitions the antennules are
returned to their original position and the entire sequence may be repeated.
Usually, at the end of each sequence the maxillipeds are rubbed against each
other repeatedly (`auto-grooming'). Autogrooming is always preceded by a bout
of antennule wiping (Barbato et
al., 1996).
In this way the setae on the lateral flagella are scoured by the serrate
setae of the maxillipeds, which results in the removal of debris accumulating
on the setal surfaces. Prolonged accumulation of debris on the antennular
surface results in breakage and loss of the aesthetascs
(Bauer, 1977).
In lobsters, AGB is elicited almost exclusively by one chemical,
L-glutamate (Glu), found in complex mixtures mimicking food
(Barbato and Daniel, 1997;
Daniel et al., 2001).
In Panulirus argus, for example, the next best stimulus for
activating AGB, glycine, elicits only 30% of the response elicited by an
equimolar concentration of Glu. This suggests that chemosensory activation of
AGB may follow a labeled-line pathway. In contrast, other behaviors associated
with food stimuli, namely search and antennular flick, are elicited in
response to many of the chemicals found in complex mixtures
(Daniel and Derby, 1991;
Fine-Levy and Derby, 1992;
Lynn et al., 1994),
making it likely that across-neuron pattern encoding is predominant. In
effect, the neural processing leading to AGB must deviate considerably from
the processing leading to search and flick behaviors, even though all three
are elicited by the same odorant mixtures encountered in the natural
environment.
A first step towards elucidating the neural processing leading to AGB
requires the identification and characterization of the source of chemosensory
input. Sensilla with known or putative chemosensory function have been
identified on virtually all lobster appendages, including the antennules,
antennae, maxillipeds and periopods, as well as the body surface
(Derby, 1989;
Voigt and Atema, 1992;
Hallberg et al.,
1997; Cate and Derby,
2001). Because AGB involves movements of the antennules and the
third maxillipeds, sensilla on these appendages most likely provide
chemosensory input for the behavior. The most obvious and best-studied
chemosensory sensilla are the aesthetascs found on the distal half of the
lateral flagellum of the antennules (Figure
1). The very dense patches of aesthetascs are the only sensilla
known to have unimodal chemosensory function (Schmiedel-Jakob et al.,
1986). Each of the 1300 aesthetascs found on each antennule is innervated
by 300 olfactory receptor neurons—ORNs
(Cate and Derby, 2001). Closely
associated with the aesthetascs are three other types of setae: guard
(160 per antennule), companion (200 per antennule) and asymmetric
(80 per antennule)— see Figures
1 and
3
(Cate and Derby, 2001). The
four setae together comprise an easily visible `tuft' on the distal half of
the lateral flagellum. Evidence for mechano- or chemo-receptive function in
the guard, companion and asymmetric setae is speculative
(Derby, 1989). In addition to
setae associated only with the tuft, a number of setae are dispersed
throughout the lateral and medial flagella: hooded, plumose, short setuled and
simple (Cate and Derby, 2001).
Hooded setae and two of the three types of simple setae (short and medium) are
innervated by chemoreceptive and mechanoreceptive neurons
(Cate and Derby, 2001). Setae
on the third maxillipeds are largely composed of serrate setae, which are
mechano- and chemoreceptive— see
Figure 1
(Derby, 1989;
Corotto et al.,
1992).
Ablation techniques have been successfully used to examine the behavioral
functions of chemosensory organs in lobsters (Steullet et al.,
2001,
2002). We therefore used
ablation techniques to examine the involvement of the antennules and
maxillipeds in providing chemosensory input driving AGB. Selected appendages
or specific setae on appendages were ablated by either immersion in distilled
water or surgical excision. AGB responses towards Glu were measured before and
after ablation.
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Materials and methods |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Source and maintenance of spiny lobsters
Caribbean spiny lobsters, P. argus, were obtained from the Florida Keys Marine Laboratory in Long Key, FL and maintained in separate 80 l aquaria (one lobster/aquarium), at 25-27°C with a 12 h: 12 h light:dark cycle. Red light (25 W, ceramic-coated light bulbs) was provided during the dark period. Aquaria were equipped with a gravel bottom filter system, lined with crushed coral and filled with aerated, recirculating artificial seawater (ASW; Instant Ocean®). Spiny lobsters were fed ad libitum scallop and shrimp every other day. Uneaten food was removed after 1 h.
Chemical stimuli
Stock solutions (10 mM, pH 8.1) of Glu were prepared in ASW
(Cavanaugh, 1964) and stored at
-70°C until needed. On the day of an experiment, stock solutions were
thawed and diluted to 0.05 or 0.5 mM with ASW.
Experimental design for ablation procedures
Two ablation techniques—distilled water (DW) ablation and
excision—were used in this experiment
(Table 1). In DW ablation,
antennules are exposed to DW for at least 5 min. The osmotic shock results in
destruction of the dendritic portion of chemosensory cells in contact with DW,
but does not kill the cells (Gleeson
et al., 1997). Excision removes the dendrites of sensory
cells that extend into the setae and leads eventually to the death of ORNs
(Harrison et al.,
2001). Unlike DW ablation, the excision procedure allows removal
of specific setae; however, it is a more laborious procedure and it also
affects mechanosensilla in addition to chemosensilla.
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DW ablation was performed separately on a whole appendage (maxillipeds or medial flagella of antennules) or a region of an appendage (distal halves of lateral flagella of antennules; Table 1). In the excision procedure (Figure 3), specific putative sensilla guard (G) and companion (C) setae (GC excision), and all tuft setae (guard, companion, aesthetasc and asymmetric setae) were excised (Table 1). Behavioral responses to chemical stimuli known to elicit AGB were determined before and after each of the ablation procedures.
DW ablation of whole appendages and region of appendages
Three regions (medial flagella, distal halves of lateral flagella of
antennules, and maxillipeds) of spiny lobster sensory appendages were ablated,
in separate experiments, to identify the source of chemosensory input to AGB.
Six, nine and 12 spiny lobsters were used for ablations of distal halves of
lateral flagella of antennules, medial flagella of antennules and maxillipeds,
respectively. Each lobster was removed from its tank, wrapped in wet paper
towels and restrained in an Instant Ocean® bath (during ablations of
antennules) or hand-held in air (during ablations of maxillipeds). A sham
ablation was performed in which appendages were placed for 15 min in a 20 ml
vial containing ASW. Sham-ablated lobsters were tested with stimuli after a 3
h recovery period. After at least 24 h, the same ablation procedure was
repeated, this time using DW. Lobsters were again allowed a 3 h recovery
period and were tested with stimuli at least 3 h (DW+3-h) and 24-72 h
(DW+24-h) after ablation.
Excision of selected putative sensilla
Different setae on the distal halves of lateral flagella were excised to
identify the specific source of chemosensory input to AGB. Six and nine
lobsters were used for GC excision and excision of all tuft setae,
respectively. Each lobster was removed from its tank, wrapped in wet paper
towels and restrained in an Instant Ocean® bath. Lateral flagella were
then sham-ablated as described above. Lobsters were tested for responses to
chemical stimuli 3 h later. After at least 24 h, each lobster was again
removed from its tank, wrapped in wet paper towels and restrained in an
Instant Ocean® bath. Each lateral flagellum was placed on a dissecting
tray and held in place with a staple pin. Selected setae were identified with
the aid of a dissecting microscope and excised using a scalpel. At least 3 h
later (excise+3-h), spiny lobsters were tested for responses to chemical
stimuli. Finally, after another recovery period of 24-72 h, the lateral
flagella were ablated with DW and the lobsters tested for responses 3 h
(DW+3-h) and 24-72 h (DW+24-h) after excision.
Proper excision of setae was verified using scanning electron microscopy (SEM). Lateral flagella were removed from lobsters and rinsed with DW. The flagella were then air dried, coated with gold—palladium and examined with a Hitachi S-2460 N scanning electron microscope.
Presentation of stimuli
Spiny lobsters were tested with two chemical stimuli (0.5 and 0.05 mM Glu) and ASW. Stimuli were presented in triplicate and in random order. Chemical stimuli (5 ml) were presented using a hand-held pipette, which was placed near the antennules to minimize dilution of stimuli. At least 10 min were allowed to pass before introduction of the next stimulus in order to minimize residual responses to a previous stimulus and avoid desensitization. All trials were videotaped, beginning at least 15 s before each stimulus was presented and continuing for 2 min afterwards.
Data analysis
The magnitude of the AGB response was determined from videotapes. Antennule
wipes were recorded for all ablation experiments. A wipe was defined as a
single pull of either antennule through the setal combs of the third
maxillipeds. In addition, auto-grooms were recorded for the experiment in
which the third maxillipeds were ablated. A groom was defined as the rubbing
back and forth of the third maxillipeds. A preliminary study indicated that
ablation of maxillipeds resulted in a decrease in number of auto-grooms
towards Glu and no change in the number of wipes
(Barbato et al.,
1996). Pre-stimulus responses were determined by counting the
number of auto-grooms (maxilliped ablation experiment only) or number of wipes
that occurred during the 15 s preceding stimulus introduction. Post-stimulus
responses were determined by counting the number of auto-grooms (maxilliped
ablation experiment only) or number of wipes that occurred for the 2 min
following stimulus introduction. Response rates were calculated (wipes/min)
and pre-stimulus rates were subtracted from post-stimulus rates. Responses to
the three presentations of a stimulus were averaged to obtain a mean response
rate.
Differences in response to a given stimulus following ablation procedures were analyzed using either parametric statistical tests (if parametric assumptions were met) or non-parametric statistical tests (if parametric assumptions were not met). With the exception of ablation of medial flagella, one-way repeated measures analysis of variance (ANOVA) or Friedman's repeated measures ANOVA on ranks were used (SigmastatTM; Jandel Scientific) to compare responses to a given stimulus following each ablation or excision procedure in an experiment. For example, in the experiment in which the distal halves of lateral flagella were ablated, responses to 0.05 mM Glu after no ablation (None), sham ablation (Sham), at least 3 h recovery from DW ablation (DW+3-h) and at least 24 h recovery from DW ablation (DW+24-h) were compared. Where statistical differences were found, post hoc pairwise comparisons between responses to a stimulus following ablation procedures were performed using the Student—Newman—Keuls (S—N—K) test. For the experiment involving ablation of medial flagella, we used a paired t-test (parametric assumptions met) or a Wilcoxon signed ranks test (parametric assumptions not met) to compare responses to chemical stimuli after the two ablation procedures (sham and DW ablation).
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Results |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Suppression of wipe response in AGB occurred only following destruction of aesthetascs, either by DW ablation or excision of these setae. Removal of other setae did not affect the wipe rate.
DW ablation of whole appendages and regions of appendages
DW ablation of distal halves of the lateral flagella resulted in a loss of responsiveness to Glu (Figure 2A). This was most evident when lobsters were tested with the higher concentration of Glu. Responses to 0.5 mM Glu were significantly less when tested 3 h after DW ablation, compared to responses before ablation, after sham ablation, or 24-72 h after DW ablation [one-way repeated measures ANOVA, n = 6, F(3,15) = 6.249, P = 0.006, S—N—K tests, P < 0.05]. There were no significant differences between responses before ablation (None), after sham ablation (Sham) and after a 24-72 h recovery period (DW+24-h) for responses to 0.5 mM Glu (S—N—K tests, P > 0.05). Therefore, recovery of wipe responses was evident in spiny lobsters tested at least 24 h after DW ablation. The results were not quite as dramatic when the lobsters were tested with 0.05 mM Glu, although a similar pattern was apparent. At this stimulus concentration, responses measured 3 h after DW ablation were significantly less than those following sham ablation, but not significantly different to responses measured before ablation and 24-72 h after DW ablation [one-way repeated measures ANOVA, n = 6, F(3, 15) = 3.484, P = 0.043, S—N—K tests, P < 0.05]. As expected, very few wipes were produced in response to ASW, which remained unchanged after each ablation technique [one-way repeated measures ANOVA, n = 6, F(3,15) = 0.413, P = 0.746].
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No other DW ablation procedure attenuated wipe rates towards Glu. DW
ablation of the medial flagella did not produce a statistically significant
change in response to 0.5 mM Glu, 0.05 mM Glu or ASW, compared to responses
following sham ablation (Figure
2B; Wilcoxon signed rank test, n = 9; 0.5 mM Glu,
W = -15.000, P = 0.426; ASW, W = 0.0000, P
= 1.000; 0.05 mM Glu, paired t-test, n = 9, t =
0.727, P = 0.488). Changes in responses (wipe rate and auto-groom
rate) to chemical stimuli following DW ablation of the third maxillipeds were
also not significant compared to responses following sham ablation
(Figure 2C). Results for wipe
rate were: one-way repeated measures ANOVA, N = 12, 0.5 mM Glu,
F(2,22) = 2.459, P = 0.109; ASW, F(2,22) = 1.446,
P = 0.257; 0.05 mM Glu, Friedman repeated measures ANOVA, n
= 12, 
2 = 1.319, P = 0.517. Results for auto-groom
rate were: one-way repeated measures ANOVA, n = 12, 0.5 mM Glu,
F(2,22) = 2.084, P = 0.148; 0.05 mM Glu, F(2,22) =
0.799, P = 0.463; ASW, Friedman repeated measures ANOVA, n =
12 2 = 2.3389, P = 0.303.
Excision of selected putative sensilla
Excision of guard and companion setae (GC excision) had no effect on AGB responses. However, when all tuft setae were excised, AGB responses were extinguished.
While there were no decreases in wipe rates toward 0.05 and 0.5 mM Glu 3 h after GC excision compared to wipe rates following sham ablation, there were significant decreases in responses toward both 0.5 and 0.05 mM Glu 3 h after DW ablation compared to responses to 0.5 and 0.05 mM Glu following GC excision [Figure 3A, one-way repeated measures ANOVA, 0.5 mM Glu, n = 6, F(2,10) = 12.044, P = 0.002; 0.05 mM Glu, n = 6, F(2,10) = 3.484, P = 0.043, S—N—K tests for either 0.5 or 0.5 mM, P < 0.05]. Recovery of behavior (DW+24-h) was tested on only four out of six lobsters and therefore was not included in the statistical tests. However, AGB appeared to return to previous magnitudes following at least a 24 h recovery period.
Excision of all tuft setae produced a significant decrease in wipe rate in response to 0.5 mM Glu compared to responses following sham ablation [Figure 3B, one-way repeated measures ANOVA, n = 9, F(2,16) = 41.374, P < 0.001; S—N—K multiple comparison tests, P < 0.05]. No AGB recovery was observed even after 24 h following excision of setae (S—N—K tests, P > 0.05). Decreases in wipe rates toward 0.05 mM Glu were not evident, probably due to the overall lower sensitivity toward less concentrated stimuli [one-way repeated measures ANOVA, n = 9, F(2,16) = 1.542, P = 0.244].
Excision techniques were effective in removing specific setae as determined by visual inspection of scanning electron micrographs. GC excision removed guard and companion setae as expected, leaving intact aesthetascs as well as asymmetric setae and simple setae (Figure 3A). Excision of all tuft setae removed all setae on the ventral side of the distal half of the lateral flagellum, leaving intact the simple setae found dorsally (Figure 3B).
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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These results provide further evidence that AGB is initiated via a labeled-line pathway. Of the four chemosensory sensilla that have been identified on the antennules—aesthetascs, hooded, and short and medium simple setae—only the aesthetascs appear to provide the necessary and sufficient chemosensory input driving AGB (Table 2). Functional removal of aesthetascs and other tuft setae by DW ablation or surgical excision resulted in complete attenuation of AGB towards Glu. DW ablation of the medial flagella, which reduced the number of non-tuft setae available for chemosensory detection, had no effect on the ability to respond to Glu. In excision experiments, attenuation of AGB occurred only with removal of all tuft setae but not guard and companion setae alone. It should be noted that removal of aesthetascs along with guard and companion setae also resulted in removal of asymmetric setae. While it is not possible unequivocally to rule out the contribution of asymmetric setae to chemosensory input, there is no evidence at present that asymmetric setae are chemosensory. Furthermore, aesthetascs outnumber asymmetric setae (16:1) and are most likely more heavily innervated:
300 per aesthetasc versus <15 per asymmetric seta if same
as other nonaesthetasc sensilla (Cate and
Derby, 2001). Finally, DW ablation of maxillipeds, which bear
chemosensory serrate setae, had no effect on either the wiping or
auto-grooming components of Glu-induced AGB.
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The Role of ORNs in chemosensory mediation of AGB
Because of the stimulus specificity of chemosensory activation of AGB and the localization of that input to one specific type of sensilla, it is likely that chemosensory activation of AGB follows a labeled-line model. In effect, ORNs tuned to Glu and located in the aesthetascs provide adequate input to motor centers in the brain controlling antennule and 3rd maxilliped movements.
While ORNs express multiple excitatory receptor types
(Cromarty and Derby 1997), one
receptor type appears to predominate, either due to a higher density or
affinity. In electrophysiological studies, cells best for a particular
chemical, such as Glu or taurine, have been identified. Equivalent responses
to the best and next-best chemicals can be activated by concentrations that
are several orders of magnitude apart
(Derby et al., 1991;
Daniel et al., 1994;
Cromarty and Derby, 1997).
Biochemical ligand binding assays of cell membranes isolated from aesthetascs
have identified independent olfactory receptor sites for
adenosine-5'-monophosphate, taurine, D- and L-alanine and Glu
(Michel et al., 1993;
Olson and Derby, 1995;
Burgess and Derby, 1997). The
narrow tuning of AGB clearly reflects these characteristics of ORNs.
The aesthetasc pathway mediates odorant activation of AGB
In an earlier paper we proposed that chemosensory processing leading to AGB
most likely followed the nonaesthetasc pathway because this provided the most
direct route leading to activation of AGB
(Barbato and Daniel, 1997). In
this hypothesis, chemosensory input from neurons tuned narrowly to Glu and
innervating nonaesthetasc sensilla project to the LAN. However, the results of
the present study argue strongly against this hypothesis and suggest instead
activation through the aesthetasc pathway.
Further processing of chemosensory input by the OL leading to AGB must
fulfill several criteria. First, the integrity of the response properties of
ORNs tuned to Glu must be retained within the OL. This could occur if specific
functional ORN types terminate on specific glomeruli. In effect, ORNs most
responsive to Glu might converge on a specific glomerulus or glomeruli. The
olfactory system of vertebrates appears to behave in this manner. Considerable
evidence now exists to conclude that the individual glomeruli in the olfactory
bulb receive input only from ORNs expressing a specific olfactory receptor
type (Hildebrand and Shepherd,
1997; Mombaerts,
1999). In spiny lobsters, neuroanatomical and physiological
studies indicate that most of the chemosensory neurons terminate on single
glomeruli, while at least 10% project to multiple glomeruli
(Schmidt and Ache, 1992). It
has not been determined whether glomeruli receive convergent input from ORNs
with specific response properties.
Secondly, output from the OL mediating AGB activation must go through a
narrow band filter allowing mostly Glu responses to pass. This could be
accomplished by projection neurons innervating only glomeruli receiving input
from Glu-best ORNs. While projection neurons are multi-glomerular, dense
innervation is observed in only three to four glomeruli
(Wachowiak and Ache, 1994;
Schmidt and Ache, 1996b). In
one study of the crayfish, identified OL projection neurons were each
responsive to a number of the single chemicals tested
(Arbas et al., 1988).
In spiny lobsters, extracellular recordings from interneurons further
down-stream from the OL, namely interneurons in the optic tract ascending from
the brain and interneurons in the circumesophageal connectives descending from
the brain, were found to be generally more responsive to a broader spectrum of
chemicals than chemosensory afferents
(Derby and Ache, 1984;
Derby et al., 1984).
In both crayfish and spiny lobsters, a small number of interneurons were
characterized by narrow response spectra.
Thirdly, output from the OL mediating AGB activation must project
monosynaptically or polysynaptically to the antennular neuropils. In crayfish,
two classes of interneurons have been identified with dendritic branches in
both the OL and the LAN (Arbas et
al., 1988; Mellon and
Alones, 1994). While these may provide a monosynaptic connection
between the two regions, they do not display characteristics consistent with a
labeled-line pathway. Arborization in the OL is extensive, impinging on many
glomeruli, and a broad range of chemical stimuli elicit activity (Mellon and
Allones, 1994). Alternatively, a polysynaptic pathway from the OL to the LAN
might exist via the protocerebrum. Projection neurons associated with the OL
ascend into the protocerebrum via the olfactory globular tract
(Schmidt and Ache, 1996b).
Chemosensitive projection neurons ascending and descending the protocerebral
tract to the protocerebrum have been identified
(Derby and Blaustein, 1988;
Schmidt and Ache, 1996b).
Other, as yet undefined pathways are also possible.
Aesthetasc and nonaesthetasc pathways mediate search behavior
There is considerable evidence that search behavior, in which spiny
lobsters orient and move towards sources of food odorants, requires
chemosensory input from a number of types of sensilla. Early studies indicated
that the lateral flagella played a major role in search behavior, although the
medial flagella appeared to provide a minor role
(McLeese, 1973; Reeder and
Ache, 1980; Devine and Atema,
1982). Recent ablation studies suggest that aesthetasc and
nonaesthetasc sensilla provide sensory input mediating search behavior.
Ablation of aesthetascs must be accompanied by ablation of at least one other
group of nonaesthetasc sensilla (i.e. ablation of all tuft sensilla, ablation
of aesthetascs and medial flagella, ablation of all tuft sensilla and medial
flagella) in order to observe attenuation of search behavior
(Steullet et al.,
2001). The ability to orient towards and locate successfully an
upstream odor source and to discriminate between different odorants following
aversive conditioning procedures is diminished only slightly in spiny lobsters
in which either aesthetascs or nonaesthetasc antennular sensilla have been
ablated (Derby et al.,
2001; Steullet et
al., 2002). Thus, processing leading to search behavior
likely utilizes both aesthetasc (via the olfactory lobe) and nonaesthetasc
pathways (via the LAN). These results, coupled with evidence that a large
number of chemicals in odorant mixtures can activate search behavior, provide
further argument for an across-neuron pattern of encoding for this behavior.
It is becoming apparent that both models of stimulus encoding exist
concurrently in lobster olfactory systems.
Recovery of AGB from osmotic shock
Within 24-72 h following DW ablation of antennules, spiny lobsters
recovered behavioral sensitivity to Glu. This cannot be a result of
replacement of damaged ORNs within this time. Normal turnover of aesthetasc
sensilla and associated ORNs occurs over a period of three to six molts
(Steullet, 2000). New
aesthetascs proliferate at the proximal part of the flagellum and are not
functionally active for several weeks after emergence. Damage to specific
regions of the antennule via excision of aesthetascs can also cause death of
ORNs and their replacement by new ORNs
(Harrison et al.,
2001). Over a period of several weeks, damaged cells degenerate
and are replaced by new cells which must reconnect to the olfactory lobe.
Thus, quick replacement of damaged ORNs with functional ORNs is unlikely. This
is consistent with the lack of recovery observed after excising all tuft setae
(Figure 3B).
It is more likely that the damage caused by osmotic shock is reversible.
Studies on blue crabs reveal that osmotic shock vesiculates the outer
dendritic segments of ORNs (Gleeson et
al., 1997). Physiological recordings from the antennular
nerve of freshwater-acclimated blue crabs showed greatly diminished olfactory
responses. Olfactory responses begin to show recovery within 24-48 h after
transferring these crabs to saltwater. Recovery of olfactory function is
correlated with lengthening of the outer dendritic segment, the location of
receptors. Thus it is likely that the recovery of chemosensory-mediated AGB in
spiny lobster is due to regeneration of the outer dendritic segments following
DW ablation.
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Acknowledgments |
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We wish to thank Lonny Anderson and the staff of the Florida Keys Marine Laboratory in Long Key for supplying spiny lobsters, Drs Holly Cate, Charles Derby and Paul Harrison for enlightening discussions on neuroanatomy, Dr Charles Derby, Dr Barry Ache and the two anonymous reviewers for invaluable comments on the manuscript. We thank Dr Gary Grimes posthumously for his assistance with SEM.
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References |
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Accepted July 30, 2002
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