Response properties of lobster chemoreceptors: tuning of olfactory neurons sensitive to hydroxy-proline

1. A prominent population of olfactory receptor cells from the lobsterHomarus americanus is narrowly tuned to trans-4-hydroxyl-L-proline (Hyp, Fig. 1) suggesting that Hyp may be an important chemical signal for this animal (Johnson and Atema 1983). However, Hyp is usually bound in connective tissue proteins of lobster prey and thus may be unavailable in sufficient quantities as a free amino acid to stimulate chemoreceptors. To determine other possible adequate stimuli for Hyp sensitive cells we further examined their tuning using a variety of substances including other amino acids also found in collagens, Hyp isomers (Fig. 1), synaptic receptor agonists, ecdysones, purified natural collagens and their gelatins (Fig. 2), and different molecular weight fractions of a commercial gelatin solution (Table 1). 2. Of a first group of Hyp sensitive cells (N=29) tested with the compounds in Table 1 A, 21 responded best to Hyp. Only a commercial gelatin solution (SG1) and its one-tenth dilution consistently elicited responses from these cells (Fig. 3). The remaining 8 cells responded best to the SG1 solutions (5 cells) or to one of the other test substances (Table 2). 3. Of a second group of Hyp sensitive cells (N=27) tested with the collagen and gelatin solutions (Table 1b), 19 responded best to Hyp. Again, the Hyp best cells rarely responded to any test substance other than Hyp and a commercial gelatin solution, SG2, and its greater than 12 kD fraction, SG2-12 (Fig. 4). SG2 and SG2-12 were equally effective for the Hyp best cells. The remaining 8 cells responded best to either SG2 (2 cells), SG2-12 (2 cells) or one of the purified gelatin or collagen solutions (Table 3). 4. A third group of Hyp sensitive cells (N=21) was tested with SG2, SG2-12, and a greater than 1 kD fraction of SG2 (SG2-1; Table 1c). Based on the mean response, the most effective stimulus for these cells was SG2-12, followed by SG2-1, SG2 and Hyp. The high mean response for the SG2 solutions was mainly due to a few cells giving large responses to these stimuli (Fig. 6). Ten of these 21 cells responded best to Hyp; all but 2 responded to one or more of the SG2 solutions; the other 11 cells responded best to either SG2, SG2-12 or SG2-1, which were all best stimuli for different cells (Fig. 5). 5. Overall, the 77 Hyp sensitive cells tested here can be divided into two main types; 65% Hyp best cells and 31% gelatin best cells. The Hyp best cells seem to be a distinct population of receptors: they have no spontaneous activity and give low responses (15 spikes in 5 s) even to their ‘best’ stimulus, Hyp. In contrast, the gelatin best cells are not infrequently spontaneously active and can give high responses to their best stimulus (up to 150 spikes in 5 s). In addition, when tested specifically in the third group, the Hyp cells appear to have a tuning spectrum distinct from the gelatin best cells (Fig. 7). 6. Stimulation of Hyp cells by gelatin solutions may be due to Hyp-containing peptides derived from the gelatin. Enzymatic tissue breakdown from the lobster's prey could produce chemical mixtures that stimulate prominent receptor populations which respond to both high (gelatin best cells) and low (Hyp best cells) molecular weight substances. This could create a central representation of food based on parallel receptor lines of somewhat overlapping sensitivity. Together, Hyp best and gelatin best receptor cell populations may give important information on the presence and state of decay of the lobster's food.