Olfactory discrimination deficits in n-3 fatty acid-deficient rats.

Docosahexaenoic acid (DHA), a long chain n-3 fatty acid, is present in high concentrations in the central nervous system. Although the role that DHA may play in neural function is not well understood, infants fed formulas containing low levels of n-3 fatty acids have decreased visual acuity and neurodevelopmental test scores. The present experiment assessed whether dietary manipulations that decrease the concentration of DHA in the brain interfered with olfactory-based learning. We fed rats a diet that provided adequate n-3 fatty acids or a diet that was deficient in n-3 fatty acids for two generations. The second generation n-3-deficient group had 81% less brain DHA (82% less in olfactory bulb) compared to the n-3-adequate group and made significantly more errors in a series of olfactory-cued, 2-odor discrimination tasks compared to the adequate group. These results suggest that lower levels of central nervous system DHA lead to poorer performance in a series of simple odor discrimination tasks.

Rats with low levels of brain docosahexaenoic acid show impaired performance in olfactory-based and spatial learning tasks.

Studies were carried out to determine if decreased levels of central nervous system docosahexaenoic acid (DHA), a result of consuming an n-3-deficient diet, had an effect on learning- and memory-related behaviors in adult male rats. Females were reared on an n-3-deficient or n-3-adequate diet beginning at 21 d of life. Their male pups, the F2 generation, were weaned to the diet of the dam and tested at 9-12 wk of age. An olfactory-based discrimination and Morris water maze task were used to assess performance. Whole brain was collected after the behavioral experiments and central nervous system fatty acid content was analyzed in olfactory bulb total lipid extracts. F2 generation male rats consuming the n-3-deficient diet had an 82% decrease in DHA compared to rats consuming the n-3-adequate diet. The n-3-deficient animals made significantly more total errors in a 7-problem, 2-odor discrimination task compared to the n-3-adequate group. Furthermore, the escape latency in the Morris water maze task was significantly longer for the n-3-deficient rats compared to the n-3-adequate rats. These results indicate that rats with decreased DHA levels in the central nervous system perform poorer in these tasks compared to rats with higher DHA levels and suggest the presence of learning deficits in these animals.

Olfaction in rats with extensive lesions of the olfactory bulbs: implications for odor coding.

Rats were initially trained on a series of odor detection tasks and then received a unilateral olfactory bulbectomy and removal of different parts of the contralateral bulb. After postoperative recovery they were tested for detection of different concentrations of four odors, on a series of odor discrimination tasks and for their ability to acquire a relatively easy and a more difficult odor mixture discrimination task. Groups were formed based on which region of the bulb was intact (regional savings score) and on amount of bulb intact (bulbar savings score). In general, only rats with bulbar savings scores of less than 21% had deficits in detection or discrimination tasks but most performed as well as controls in most tasks. Correlations between bulbar savings scores and error scores were relatively low across all rats but, within the subgroup with the largest lesions (bulbar savings scores <21%), high correlations between these variables were obtained. There was no evidence for a specific anosmia in any group or individual rat and, except for the more difficult odor mixture discrimination, no one task proved difficult for any subgroup. The present results demonstrate that rats with relatively small remnants of one olfactory bulb can perform a variety of odor detection and discrimination tasks as well or nearly as well as controls. These outcomes provide no support for localization of function within the olfactory bulb but are in accord with recent proposals that odors may be coded by a highly distributed pattern of bulbar input.

Detection and discrimination of propionic acid after removal of its 2-DG identified major focus in the olfactory bulb: a psychophysical analysis.

Prior 2-deoxyglucose and c-fos studies have demonstrated increased metabolic activity in a rostral dorsomedial area of the olfactory bulb in response to the vapor of propionic acid. We used psychophysical tests to assess the effect of removing this area of the bulb on odor sensitivity and discrimination. Normal rats, those with lesions of the rostral dorsomedial bulb or with control lesions of the lateral olfactory bulb were tested for propionic acid absolute detection and intensity difference thresholds and ability to discriminate propionic acid from other odors. There were no differences among groups for absolute or intensity difference threshold or on simple 2-odor discrimination tests but both groups with bulbar lesions made more errors than controls on a relatively difficult odor-mixture task. The results demonstrate that removal of an area of the bulb identified as responsive to propionic acid is essentially without effect on sensitivity to that odor or ability to discriminate it from other odors.

Short-term odor memory: effects of posterior transection of the lateral olfactory tract in the rat.

Rats were trained on a series of novel 2-odor discrimination problems before and after combined unilateral bulbectomy and posterior transection of the contralateral lateral olfactory tract. In postoperative tests, experimental rats performed as well as controls when a short intertrial interval (30 seconds) was used but, in contrast to controls, failed to learn a 2-odor discrimination when the intertrial interval was 10 minutes. When tested on a reversal task, controls showed memory for original learning by making many errors while experimental rats quickly acquired the task. The results suggest that lateral olfactory tract afferents to posterior olfactory cortex may play a significant role in short-term memory for odors.

Social organization and aggression in a group of olfactory bulbectomized male mice.

Olfactory bulbectomized and control male CF-1 strain mice were housed in separate, large, seminatural environments over a 30-day observation period. Control mice engaged in vigorous fighting behavior before a dominant animal emerged. The alpha mouse established the floor area as its territory and vigorously attacked all other mice that ventured from the second tier of the enclosure to the floor. Lower ranking mice had numerous body scars, were much less aggressive, and engaged in a variety of nonaggressive social interactions. In contrast, bulbectomized mice did not fight and roamed freely through all areas of the environment. These mice largely ignored one another, engaged in little or no social behavior, and had no body scars. When two anosmic mice came into physical contact, they appeared startled and moved away from one another. These results, together with those of prior studies, indicate that olfaction plays a critical role in virtually all aspects of social behavior in male mice.

Flavor and the frontal cortex.

Rats were trained to discriminate an aqueous compound of an odor and taste (amyl acetate and NaCl) from the components of the compound before removal of one olfactory bulb and the contralateral ventrolateral frontal cortex. In postoperative tests, experimental rats performed much more poorly than nonlesioned controls or controls which had all lesions made in the same hemisphere. However, there were no significant differences among groups on tests for detection of amyl acetate and NaCl. These results provide evidence that integration of taste and smell in the production of flavor occurs in the ventrolateral frontal cortex.

The horizontal vertical illusion: evidence for strategic factors in feedback-induced illusion decrement.

This study assessed whether feedback would improve performance on a horizontal vertical illusion task and whether such improvement would transfer to different forms of the illusion. Subjects shortened the extended vertical line of small, medium, and large inverted-T figures to be equal in length to the horizontal line before and after being shown a correctly adjusted medium-sized figure (visual feedback). Next, they were tested on three alternate forms of the illusion; a production task (drawing 1-in. lines in the horizontal and vertical planes), an adjustment task using L figures, and a task requiring them to choose which of several sailboat drawings had mast height equal to hull length. Prior to feedback, vertical lines were adjusted shorter than the horizontal lines on each size inverted-T figure. After feedback on the medium-size figure, experimental subjects were more accurate than the controls on each size inverted-T figure. The results suggest that transfer of illusion decrement was also obtained for the boat-selection and, to a lesser extent, the L-figure adjustment tasks but not to the line-production task. These findings are consistent with the notion that improvement after feedback is not due to structural changes in visual processing or to simple feedback-induced compensation in performance but involves some strategic or cognitive change in judging line length.

Visual inspection alone produces a decrement in the horizontal-vertical illusion.

Subjects inspected either inverted-T or L figures for 5 min. and then shortened the extended vertical lines of inverted-T figures in an attempt to make the vertical line equal in length to the horizontal line. Those who inspected inverted-T figures were more accurate on initial trials than controls (who did not inspect figures), those who inspected L figures, or those who inspected inverted-T figures and made adjustments on the inspected figures. The results indicate that visual inspection alone can produce a decrease in the Horizontal-Vertical illusion and may account, in part, for changes in strategic factors which have been hypothesized to underlie illusion decrement.

Memory for brief, widely spaced odor presentations in the rat.

Rats trained on a series of 16 novel 2-odor discrimination tasks using a 10-s intertrial interval (ITI) rapidly improved in performance and made only 0-3 errors by the end of the test series. They were then tested on other novel pairs of odors, but with a 10- and a 30-min interval between trials. There was no decrement in performance accuracy in the longer ITI tests and, in most cases, criterion performance was achieved after making zero or 1 error after the first (information) trial. These results demonstrate that rats have the capacity to remember for at least 30 min whether a single brief presentation of a novel odor was followed by a reward.

Odor-cued taste avoidance: a simple and efficient method for assessing olfactory detection, discrimination and memory in the rat.

Rats quickly learn to avoid licking at a drinking tube containing an odorant and quinine hydrochloride. After training, the rat responds readily to water but sniffs at but does not lick a tube containing a solution of isoamyl acetate. Learning occurs after only one or two exposures to the combination of odor and tastant. This method can be used to demonstrate odor discrimination, odor memory, and to assess detection of low concentrations of odors.

Olfactory bulbectomy decreases social behavior in male prairie voles (M. ochrogaster).

Prairie voles (Microtus ochrogaster) are social rodents that show characteristics of monogamy including high levels of social behavior and male parental care. Behavioral studies of prairie voles have implicated chemosignals in the control of various components of social behavior and reproduction. In the present study, the role of the olfactory system in male behavior was examined following surgical removal of the olfactory bulbs. Decreases in measures of sexual, paternal, and other social behaviors were observed following bilateral olfactory bulbectomy, but not unilateral or sham bulbectomy. Some bilaterally bulbectomized males also showed a stereotypy, but this probably did not account for the changes in social behaviors.

Recognition of propionic acid vapor after removal of the olfactory bulb area associated with high 2-DG uptake.

Rats were trained in an olfactometer to recognize the odor of propionic acid using a go, no-go operant discrimination tasks in which propionic acid vapor served as the negative stimulus and a series of novel odors served as positive stimuli. After training, rats readily responded to all novel odors but not to propionic acid. Removal of the olfactory bulb region previously identified with 2-deoxyglucose (2-DG) as being responsive to propionic acid had no effect on recognition of propionic acid, ability to discriminate novel odors from propionic acid or odor mixtures containing propionic acid or to discriminate propionic acid from a very similar odor (acetic acid). These results demonstrate that the 2-DG-identified propionic acid focal area is not essential for detection, discrimination or recognition of the odor. The outcomes are in accord with and extend those of other behavioral studies that have failed to demonstrate a functional correlate for the regional increases in metabolic activity produced by exposure to specific odors.

The enigma of olfactory learning revisited.

If, as indicated in olfactory conditioning studies, rats are able to demonstrate primate-like behavior by their acquisition of response strategies, rodent olfactory learning may provide a particularly useful model for neurobiological analyses of learning and memory. Reid and Morris [Proc. R. Soc. Lond., Ser. B 247, 137-143 (1992); Trends Neurosci. 16, 17-20 (1993)] have recently challenged these views. They suggested that the evidence for acquisition of an olfactory learning set in rodents is insufficient and that rat olfactory learning may provide no special advantage for the neurobiologist. This essay reviews the relevant studies and concludes that there are serious shortcomings in the Reid and Morris argument, that the available data base provides clear evidence for the claim that rats acquire abstract rules for responding and that olfactory learning may be qualitatively different from other forms of instrumental learning in rodents.

The horizontal-vertical illusion: transfer of illusion decrement.

Subjects first adjusted 20 vertical lines to be equal in length to the horizontal line in small or large inverted-T figures and were then given 10 additional trials on the other size figure. Vertical lines were made significantly shorter than the horizontal line but accuracy gradually improved and performance was not disrupted by the switch in figure size. By Trial 30, adjusted vertical lines did not differ from the length of the horizontal line. Other subjects were first tested on the small inverted-T figure and then asked to produce 1-in. lines in the vertical plane and in the horizontal plane. These subjects also improved on the adjustment task but, on the production task, were no more accurate than control subjects who were not given practice on the adjustment task. Thus, the magnitude of the Horizontal-Vertical Illusion decreases with practice alone and this practice effect readily transfers to figures of other sizes but not to an alternate form of the illusion. The potential factors responsible for the improvement and transfer are discussed.

Olfactory sensitivity of rats with transection of the lateral olfactory tract.

Rats with discrete transection of the lateral olfactory tract (LOT) were tested using operant conditioning and psychophysical methods for their amyl acetate intensity difference threshold and absolute detection threshold. Experimental rats performed as well as controls on the easiest problems of both threshold series but their intensity difference threshold was approximately 2.5 times as high as controls and their absolute detection threshold was approximately 2.25 orders of magnitude higher than controls. The deficit in sensitivity in both tests was related to the frontal level at which the tract was cut; rats with the most rostral transection had the greatest increase in threshold. The absolute detection threshold of rats with transection of the LOT was increased to that of normal human subjects tested with the same apparatus.

Odor matching and odor memory in the rat.

To assess olfactory matching-to-sample learning, rats were exposed to two odors separated by a 1-s presentation of clean air. If, and only if, the odors were identical, a response produced a water reinforcer. High levels of performance were maintained over a series of 10 novel three-odor matching-to-sample problems on this conditional go/no-go discrimination procedure. In general, performance accuracy improved over problems and errorless or near errorless performance on many stimulus combinations, particularly near the end of training, indicated acquisition of a learning set. There was little decrement in performance when the interstimulus interval was increased gradually from 1 to 10 s and matching-to-sample was not disrupted when a novel odor was presented during the interstimulus interval. These results demonstrate that rats readily learn an olfactory matching-to-sample task, maintain high levels of performance even with delays of 10-s between stimuli, and can acquire a matching-to-sample learning set. The outcomes are in agreement with prior studies demonstrating exceptional learning of instrumental tasks by rats when they are provided with odor cues.

Olfactory bulb removal affects partner preference development and estrus induction in female prairie voles.

In female prairie voles (Microtus ochrogaster) bilateral olfactory bulbectomy reduced affiliative behavior, as measured by social contact, and prevented the formation of partner preferences. Unilateral olfactory bulb removal did not significantly influence affiliative behavior, but did inhibit partner preferences. Bilateral, but not unilateral, bulbectomy significantly reduced the proportion of females exhibiting behavioral estrus following male exposure. In contrast to affiliative and sexual behavior, parental behavior was not significantly affected by either bilateral or unilateral olfactory bulbectomy. These results suggest that divergent sensory-neural pathways underlie social, sexual, and parental behaviors in this species.

Decreased NaCl sensitivity in zinc-deprived rats.

NaCl thresholds and ability to discriminate between NaCl and sucrose were assessed in rats using an operant discrimination conditioning procedure before and during moderate and severe zinc deprivation and during zinc supplementation. NaCl thresholds were approximately 1 mM before dietary zinc manipulation. They increased in all zinc-deprived rats tested 10 and 17 days after initiation of deprivation but did not change in pair-fed controls maintained on supplemental zinc. Threshold changes were greater for those rats severely zinc deprived than for those only moderately deprived and were greater as the period of deprivation lengthened. Plasma zinc concentrations decreased significantly in deprived rats from values obtained at baseline, values in severely deprived rats being significantly lower than in those only moderately deprived. Although zinc-deprived rats discriminated NaCl from sucrose, they made more discrimination errors than controls. Following 24 days of zinc supplementation, previously deprived rats exhibited no significant improvement in gustatory performance, although their body weight increased and plasma zinc concentrations increased; but these later changes were not significant. These results demonstrate that zinc deprivation induces decreased gustatory sensitivity and confirm a role for zinc in taste.