Independent effects of seawater pH and high PCO2 on olfactory sensitivity in fish: possible role of carbonic anhydrase

Ocean acidification may alter olfactory-driven behaviour in fish by direct effects on the peripheral olfactory system; olfactory sensitivity is reduced in CO2-acidified seawater. The current study tested whether this is due to elevated PCO2 or the consequent reduction in seawater pH and, if the former, investigate the possible involvement of carbonic anhydrase, the enzyme responsible for the hydration of CO2 and production of carbonic acid. Olfactory sensitivity to amino acids was assessed by extracellular multi-unit recording from the olfactory nerve of the gilthead seabream (Sparus auratus L,) in normal seawater (pH ∼8.2), and after acute exposure to acidified seawater (pH ∼7.7, but normal PCO2; ∼340 µatm) and high PCO2 seawater (∼1400 µatm) at normal pH (∼8.2). Reduced pH in the absence of elevated PCO2 caused reduction in olfactory sensitivity to L-serine, L-leucine, L-arginine and L-glutamine, but not L-glutamic acid. Increased PCO2 in the absence of changes in pH caused reduced olfactory sensitivity to L-serine, L-leucine and L-arginine, including increases in their thresholds of detection, but had no effect on sensitivity to L-glutamine and L-glutamic acid. Inclusion of 1 mM acetazolamide (a membrane-permeant inhibitor of carbonic anhydrase) in the seawater reversed the inhibition of olfactory sensitivity to L-serine caused by high PCO2. Ocean acidification may reduce olfactory sensitivity by reduction in seawater pH and intracellular pH (of olfactory receptor neurones); the former by reducing odorant-receptor affinity, and the latter by reducing the efficiency of olfactory transduction. The physiological role of carbonic anhydrase in the olfactory receptor neurones remains to be explored.

Orthodromically and antidromically evoked local field potentials in the crayfish olfactory lobe

A local field potential, consistent in form and duration, can be recorded from the olfactory lobe of crayfish following electrical stimulation of the outer flagellum of the antennule. The field potential is reversibly blocked by perfusion of the brain with low-[Ca2+] saline or <IMG src="/images/symbols/gamma.gif" WIDTH="9" HEIGHT="12" ALIGN="BOTTOM" NATURALSIZEFLAG="3">-aminobutyric acid and, to a lesser extent, histamine. Paired shocks to the antennule and antidromic electrical stimulation of olfactory lobe output neurones also partially block the field potential. Comparing the field potential with simultaneously recorded intracellular responses of olfactory interneurones reveals a coincidence between excitatory and inhibitory effects in the interneurones and the appearance of identifiable components of the field potential. We interpret the field potential to reflect the response of neural elements in the olfactory lobe to orthodromic activity in the axons of the olfactory receptor neurones on the antennule. We conclude from the blocking experiments that the greater part of the field potential stems from neurones in the olfactory lobe that are postsynaptic to olfactory receptor neurones. As such, it provides a robust indication of olfactory neurone activity.