Bulbar acetylcholine enhances neural and perceptual odor discrimination

Abstract

Experimental and modeling data suggest that the circuitry of the main olfactory bulb (OB) plays a critical role in olfactory discrimination. Processing of such information arises from the interaction between OB output neurons local interneurons, as well as interactions between the OB network and centrifugal inputs. Cholinergic input to the OB in particular has been hypothesized to regulate mitral cell odorants receptive fields (ORFs) and behavioral discrimination of similar odorants. We recorded from individual mitral cells in the OB in anesthetized rats to determine the degree of overlap in ORFs of individual mitral cells after exposure to odorant stimuli. Increasing the efficacy of the cholinergic neurotransmission in the OB by addition of the anticholinesterase drug neostigmine (20 mM) sharpened the ORF responses of mitral cells. Furthermore, coaddition of either the nicotinic antagonist methyllycaconitine citrate hydrate (MLA) (20 mM) or muscarinic antagonist scopolamine (40 mM) together with neostigmine (20 mM) attenuated the neostigmine-dependent sharpening of ORFs. These electrophysiological findings are predictive of accompanying behavioral experiments in which cholinergic modulation was manipulated by direct infusion of neostigmine, MLA, and scopolamine into the OB during olfactory behavioral tasks. Increasing the efficacy of cholinergic action in the OB increased perceptual discrimination of odorants in these experiments, whereas blockade of nicotinic or muscarinic receptors decreased perceptual discrimination. These experiments show that behavioral discrimination is modulated in a manner predicted by the changes in mitral cell ORFs by cholinergic drugs. These results together present a first direct comparison between neural and perceptual effects of a bulbar neuromodulator.

Figure 1.

A, Examples of single-trial responses of a select mitral cell to each of the four odorants under saline conditions. B, Comparison of spontaneous activity between saline and drug conditions. Neo, Neostigmine; Scop, scopolamine.

Figure 2.

Effect of cholinergic modulation in odor responses. Ai, Aii, Examples of mitral cell odor response modulation by application of neostigmine. The graphs show the number of spikes evoked by odor stimulation (difference in the number of spikes recorded during 4 s after odor onset and 4 s before odor onset) in response to the four stimulation odorants (E2–E5). Ai, In this cell, there was no statistical response difference between the four odors (F_(3,16) = 0.34; p > 0.05) in saline. However, in the same cell after infusion of neostigmine, there was sharpening of the ORF after exposure to the same four odors (F_(3,16) = 3.7; p < 0.05). Post hoc analysis shows significant differences between E3/E4 (p < 0.01) and E3/E5 (p < 0.05). A_ii shows a mitral cell for which there was no significant difference between odors in either saline or neostigmine conditions (saline, F_(3,16) = 0.012, p > 0.05; neostigmine, F_(3,16) = 0.049, p > 0.05). B, Number of individual mitral cells exhibiting significantly different responses to odorants differing by one carbon under the control and drug conditions. *p < 0.05, significant difference between saline and drug conditions. C, Proportion of cells responding with significant excitation (Ci) or inhibition (Cii) to the odor chemically most similar to the “best odor” of the cell under saline and neostigmine conditions. *p < 0.05, significant difference between saline and neostigmine conditions. D, Comparison of population responses to pairs of odorants as a function of drug treatment. The graph shows the average correlation between mitral cell responses to odorants differing by one, two, or three carbons. *p < 0.05, significant reduction in correlation between odor pairs differing by one carbon under neostigmine conditions. Neo, Neostigmine; Scop, scopolamine.

Figure 3.

Behavioral experiments. A, Habituation/spontaneous discrimination. Average investigation time during randomized test trials for all drug conditions. *p < 0.05, significant difference between a response to a test odor and response to the last habituation trial (habituated odor). B, Generalization learning, strength, and specificity of association. The graph shows the average digging times in response to the conditioned and test odors for all drug conditions. *p < 0.05, significant difference compared with saline controls.

Figure 4.

Comparison between electrophysiological and behavioral results. A, Average normalized olfactory response functions of recorded mitral cells. All responses are shown as the percentage of the best odor response and arranged as a function of carbon chain length difference to the best response odor. A significant difference between saline and neostigmine response functions increases the discrimination between odor pairs differing by a single carbon (indicated by *p < 0.05). B, Spontaneous discrimination. The graph shows the percentage of habituation to an odorant with respect to the habituated odorant (difference in carbon chain length 0, at 100%). A decrease in habituation signifies an increase in discrimination in this experiment. Note the difference in discrimination between odorants differing by a single carbon between saline and neostigmine conditions (indicated by *p < 0.05). C, Discrimination after associative conditioning. The graph shows the percentage of conditioning (100% being the response to the conditioned odor) as a function of the chain length difference between the conditioned and the test odor. Note the increase in discrimination between odors differing by a single carbon attributable to neostigmine (indicated by *p < 0.05).