Sexual behavior in peripherally anosmic male hamsters

Sexually experienced and inexperienced male hamsters mated normally after they had been rendered temporarily anosmic by intranasal zinc sulfate treatment (Experiment 1), in contrast to the complete absence of sexual behavior following removal of the olfactory bulbs. The efficacy of our zinc sulfate technique was demonstrated by use of an olfactory discrimination training procedure. Hamsters lost the ability to smell both amyl acetate (Experiment 2) and the female hamster post-estrus vaginal discharge (Experiment 3) following zinc sulfate induced anosmia but at the same time retained normal sexual behavior.     

Differential effects of peripheral and central anosmia producing techniques on spontaneous behavior patterns

The spontaneous behavior patterns of four matched groups of male hooded rats were compared to SHAMs following different anosmia producing techniques. One group was peripherally blocked with lidocaine (XYL), and another with zinc sulphate (Zn). Two operated groups were either centrally deafferented (LB) or sustained olfactory bulb and peduncle destruction (DB). The lesioned and particularly DB animals were hyperactive and reared more than the other groups. Xyls were similar to SHAMs on most measures but many developed unusual locomotor patterns and Zns were hypoactive, reared less and groomed and froze more than all other groups. Food deprivation increased activity and rearing and decreased freezing and grooming in all groups but most intergroup differences were maintained. Some problems associated with anosmia tests were discussed and a technique to distinguish lesion induced anosmia from lesion side effects was investigated. Lesions tended to increase activity, rearing frequency and possibly sniffing frequency somewhat independently of anosmia whereas grooming and possibly freezing changes were more closely related to olfactory cue changes. A second experiment revealed that most of the zinc sulphate effects were due to systemic poisoning. Consequently, the common practice of comparing central and peripheral anosmia producing techniques to demonstrate olfactory deficits aside from lesion effects may be misleading.     

Does intranasal application of zinc sulfate produce anosmia in the rat?

Transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) from olfactory sensory neurons to the olfactory bulb as well as odor detection and discrimination were examined in rats in which each nasal epithelium had been irrigated with 0.1-0.5 ml 5% zinc sulfate. After treatment, rats showed few or no deficits in discriminating among odors and in detecting high (1%-0.01%) concentrations of ethyl acetate, but some had deficits in detecting lower concentrations of the odor. In most cases, HRP reaction product filled more than 30% of olfactory bulb glomeruli 2-4 days after treatment with ZnSO4. The behavioral outcomes are in agreement with recent reports of considerable savings in olfaction even after severe reduction of afferent projections to the olfactory bulb. We conclude that, in the rat, intranasal application of ZnSO4, as generally practiced, does not produce anosmia.     

A study of olfaction and gustatory senses in sheep after olfactory bulbectomy

The importance of taste and smell in discrimination of sodium salts was examined in normal and anosmic sheep. To test for anosmia, faeces of pig, calf and sheep were used as noxious odours. Intact sheep, and sheep with one olfactory bulb removed avoided the aversive stimulus whereas following total bulbectomy, the faecal odour was no longer a deterrent. Olfactory bulbectomy did not affect food intake but changes in fluid intake, urinary loss and electrolyte excretion were apparent. In two-choice preference tests, normal sheep showed a marked aversion for 0.48 M NaHCO3 and 0.51 M NaCl when compared to water. Following either unilateral or bilateral olfactory bulbectomy the aversion for sodium salts persisted but was less extreme. This suggests that in the final discrimination of sodium salts the sense of taste is dominant but olfaction or the olfactory bulb may have a minor role.     

Changes in spontaneous,odor modulated and shock induced behavior patterns following discrete olfactory system lesions

Three groups of male hooded rats were preoperatively matched and then sham operated (SHAM), bilaterally lateral olfactory tract lesioned or anterior olfactory nucleus/anterior commissure lesioned. In one experiment, spontaneous behavior patterns emitted in an exploration field containing different odors were quantified during satiated and food deprived conditions. In other experiments, flinch and jump thresholds to electric shock and running patterns in an appetitively motivated straight alley experiment were measured. All lesioned animals were hyperactive and typically froze and groomed less than SHAMs while sniffing and rearing patterns differentiated the lesioned and SHAM groups in several ways. All animals had similar flinch and jump thresholds and also emitted similar types of responses to electric shock. In the straight alley, odors from normal and stressed rats had little effect on the running time of the experimental animals while a faint cat odor strongly inhibited SHAM running behavior. Responses to a novel chemical odor (trimethylpentane) and to changes in the alley floor or electric shock applied to the floor were minimal in all animals. The results were discussed against a background of other olfactory system lesions and some evidence separating the effects of olfactory cues from nonolfactory lesion effects was presented. Finally, problems associated with tests for olfactory discrimination were discussed. A need for more rigorous and specific discrimination tests in behavioral studies involving anosmia was emphasized.     

Anosmia and aggression in male owl monkeys (Aotus trivirgatus)

Adult owl monkeys behaved aggressively when paired with unfamiliar members of the same sex. Males showed the greatest aggressive responses. When plugs containing bismuth iodoform paste were inserted into the nasal passages of male owl monkeys they fought less readily and showed less contact aggression. Although intermale aggression was not abolished anosmic males spent longer in proximity and showed an increase in olfactory inspections. Control experiments confirmed that the technique used to induce anosmia had blocked responsiveness to conspecific odours and that decreases in aggression were not due to discomfort or non-specific behavioral effects.     

Sensory influences on homing of stunted rat pups

The effects of 2 methods of restricting food intake—large-litter rearing and rotation between lactating and nonlactating females—on sensory factors involved in homing to the nest by rat pups were examined. Homing was observed in the unaltered home cage, when olfactory cues were altered and when visual cues were altered. Stunted animals homed less in the unaltered cage than did well-nourished controls as a result of a maturational delay. Prior to eye opening, stunted animals showed greater disruption of homing when olfactory cues were altered and after eye opening they showed greater disruption when visual cues were altered. These effects could reflect decreased sensitivity, an inability to use alternate cues, or behavioral disruption by novel stimulation. Nonnutritional factors were also found to affect homing as the 2 well-nourished groups differed in their behaviors. These differences appeared to be due to animals reared in small litters maturing more slowly than animals rotated between females.     

Taste-potentiated odor aversion learning: role of the amygdaloid basolateral complex and central nucleus.

The present study examined the relative contributions of the amygdaloid basolateral complex (ABL) and central nucleus (CN) to taste-potentiated odor aversion (TPOA) learning--an associative learning task that is dependent on information processing in two sensory modalities. In Experiment 1, rats with neurotoxic lesions of these systems were trained on the TPOA task by presenting a compound taste-odor conditioned stimulus, which was followed by LiCl administration. Results showed that ABL damage caused an impairment in potentiated odor aversion learning but no deficit in the conditioned taste aversion. In contrast, rats with CN damage learned both tasks. Experiment 2 examined the effects of ABL damage on TPOA and odor discrimination learning. The odor discrimination procedure used a place preference task to demonstrate normal processing of olfactory information. Results indicated that although ABL-lesioned animals were impaired on TPOA, there was no deficit in odor discrimination learning.     

Mechanisms of olfactory dysfunction in aging and neurodegenerative disorders

Although olfaction is the primal sense in animals, its importance in humans is underappreciated. Extensive literature demonstrates that aging is accompanied by olfactory loss and hyposmia/anosmia which is also a feature of several neurodegenerative disorders. Alzheimer's and Parkinson's diseases are characterized by severe olfactory deficits, while problems of olfactory discrimination are less prominent features in several other disorders. Olfactory loss is accompanied by structural abnormalities of the olfactory epithelium, the olfactory bulb and the central olfactory cortices. This review summarizes our present knowledge about the pathological changes in the olfactory system during aging and in various neurodegenerative diseases.     

The effects of olfactory system lesions and food deprivation on spontaneous behavior patterns of male hooded rats

The behavior of normal male rats was observed in an exploration field in which a number of olfactory cues were present. Locomotor activity, freezing, grooming, sniffing, rearing and boluses dropped were quantified over days and correlations between them were obtained. Following olfactory nerve sections at the bulb surface or total olfactory bulb and peduncle destruction, the measures were made again and lesion induced changes in them and olfactory cue exploration were noted. After a period of food deprivation, the measures were made a third time to assess possible motivational factors on the results. Both lesions and food deprivation altered many variables with the deeply lesioned animals more affected than the shallowly lesioned animals. Possible interactions between partial or complete anosmia and lesion induced changes in reactivity were considered. Some problems associated with the production of and tests for anosmia were also discussed.     

Ontogeny of odor discrimination: a method to assess novel odor discrimination in neonatal rats.

Recent research using molecular and functional imaging techniques has demonstrated a highly precise spatial representation of odor quality in the rodent olfactory bulb, which is enhanced by extensive lateral inhibitory circuitry. Much of this olfactory bulb circuitry develops postnatally in the rat, leading to the prediction that behavioral discrimination of odor quality may also emerge postnatally. However, currently no behavioral paradigm has been identified to test this prediction. The present report describes the expression and habituation of odor-evoked heart rate-orienting responses in neonatal rats. The results demonstrate that odor-evoked-orienting responses can be observed at least as early as postnatal day 4 (PN 4), and in those animals showing orienting responses, habituation is constant throughout the postnatal period. Furthermore, the results suggest that examination of cross-habituation using this paradigm can be used to explore odor discrimination ability in neonates. These results lay the foundation for future studies of precise mapping of the ontogeny of novel odor discrimination.     

Opposing effects of D1 and D2 receptor activation on odor discrimination learning

Dopaminergic modulation of cortical activity has been implicated in the formation of reward associations. There is abundant evidence for dopaminergic effects on olfactory processing. Using an olfactory discrimination task, the authors show that D1 and D2 dopamine receptors can regulate rats' olfactory discrimination capacities and that the effects of receptor activation functionally oppose one another. Injection of either the D1 agonist SKF 38393 (10 mg/kg) or the D2 antagonist spiperone (0.62 mg/kg) facilitated the discrimination of similar odorants but had no effect on the discrimination of dissimilar odorants, whereas both the D, antagonist SCH 23390 (0.025 mg/kg) and the D2 agonist quinpirole (0.2 mg/kg) significantly impaired rats' ability to discriminate similar and dissimilar odorants.     

The olfactory receptor gene superfamily of the mouse.

Olfactory receptor (OR) genes are the largest gene superfamily in vertebrates. We have identified the mouse OR genes from the nearly complete Celera mouse genome by a comprehensive data mining strategy. We found 1,296 mouse OR genes (including 20% pseudogenes), which can be classified into 228 families. OR genes are distributed in 27 clusters on all mouse chromosomes except 12 and Y. One OR gene cluster matches a known locus mediating a specific anosmia, indicating the anosmia may be due directly to the loss of receptors. A large number of apparently functional 'fish-like' Class I OR genes in the mouse genome may have important roles in mammalian olfaction. Human ORs cover a similar 'receptor space' as the mouse ORs, suggesting that the human olfactory system has retained the ability to recognize a broad spectrum of chemicals even though humans have lost nearly two-thirds of the OR genes as compared to mice.     

Habenular lesions and discrimination responding to olfactory and visual stimuli

Twenty albino rats with either bilateral habenular lesions or sham operations were trained to discriminate on the basis of olfactory cues between two compartments of a Grice apparatus modified for odor presentation. The discriminative stimulus consisted of different relative concentrations of the odor substance Lilial ranging from 0.1% to 10% air-diluted concentrations. The animals were initially trained using a 10% relative odor stimulus and subsequently tested with a descending series of odorant concentrations. All animals except one acquired the initial discrimination, but successive lowering of the odor concentration disrupted the performance of the experimental group while all controls successfully reached criterion at the lowest concentration used. The rat failing to make the initial odor discrimination was able to discriminate between the two compartments when a visual cue was substituted for the olfactory stimulus. We conclude that the habenular area serves as an important link between olfactory sensory input and motor systems in the rat. It appears that the behavioral changes observed are related more to threshold elevation or an impairment in olfactory integration than to generalized disruption of response inhibition mechanisms.     

Bulbectomy and sensitivity to estrogen: Anatomical and functional specificity

In order to clarify the influence of the olfactory system on female sexual behavior, ovariectomized rats were given sham operations (SHAM), total bilateral olfactory bulbectomy (TBULBX), partial bulbectomy (PBULBX), anterior olfactory nucleus lesions (AON) or accessory olfactory bulb lesions (AOB), and tested for lordosis behavior. Only TBULBX resulted in increased sensitivity to estradiol benzoate (EB) in that lordosis quotients (LQ) were increased and rejection behavior decreased following administration of 2, 4 or 8 μg EB/kg/day for 3 days. Only TBULBX group rats were anosmic on 2 postoperative tests. TBULBX group rats showed very mild hyperresponsiveness on an emotionality test. Effects of TBULBX on LQ are not due to general sensory hyperresponsiveness or EB-induced hyperresponsiveness since no differences in the quality of lordosis occurred, and no differences occurred in latency to paw-lift on hot plate tests with or without EB. Heightened EB sensitivity in the TBULBX group is not due to adrenal steroids since following adrenalectomy and 8 μg EB/kg treatment, TBULBX group LQ scores were still elevated relative to those of SHAM controls. The LQ scores of PBULBX group rats were intermediate to those of SHAM and TBULBX group rats. Bulbectomy-induced alterations in sensitivity to EB as measured by the LQ do not appear to be due to alterations in “arousal” mechanisms in general. While deficits in olfactory perception might exacerbate the effect, it is unlikely that anosmia per se is sufficient to induce major alterations in the degree of sexual receptivity following EB. The magnitude of behavioral effects of bulbectomy on EB sensitivity may be related, to some extent, to the amount of bulb tissue removed. It is possible that bulbectomy may enhance behavioral sensitivity to EB by disrupting biochemical responses to EB in limbic system structures which normally exert an inhibitory influence over sexual receptivity.     

Behavioral and molecular changes in the mouse in response to prenatal exposure to the anti-epileptic drug valproic acid

Experiments in rodents have indicated that maternal valproic acid (VPA) exposure has permanent adverse effects upon neurological and behavioral development. In humans, prenatal exposure to VPA can induce fetal valproate syndrome, which has been associated with autism. The present study examined mouse pups exposed in utero to VPA, measuring physical development, olfactory discrimination, and social behavior as well as expression of plasticity-related genes, brain derived neurotrophic factor (BDNF) and NMDA receptor subunits NR2A and NR2B. VPA-exposed mice showed delayed physical development, impaired olfactory discrimination, and dysfunctional pre-weaning social behavior. In situ hybridization experiments revealed lower cortical expression of BDNF mRNA in VPA animals. These results support the validity of the VPA mouse model for human autism and suggest that alterations in plasticity-related genes may contribute to the behavioral phenotype.     

Electrophysiological and behavioral phenotype of insulin receptor defective mice

The olfactory bulb expresses one of the highest levels of insulin found in the brain. A high level of expression of the concomitant insulin receptor (IR) kinase is also retained in this brain region, even in the adult. We have previously demonstrated in a heterologous system that insulin modulates the voltage-dependent potassium channel, Kv1.3, through tyrosine phosphorylation of three key residues in the amino and carboxyl terminus of the channel protein. Phosphorylation also induces current suppression of the Kv1.3-contributed current in cultured olfactory bulb neurons (OBNs) of rodents. In order to explore the behavioral importance of this kinase-induced modulation of the channel for the olfactory ability of the animal, mice with a targeted-gene deletion of the insulin receptor were electrophysiologically and behaviorally characterized. Mice heterozygous for the insulin receptor kinase (IR+/-) gene performed the same as wild-type (+/+) mice when challenged with a traditional, non-learning-based task to test gross anosmia. There was also no significant difference across the two genotypes in tests designed to measure exploratory behavior or in a battery of systems physiology experiments designed to assess metabolic energy usage (locomotion, ingestive behaviors, weight, oxygen consumption, and respiratory quotient). Object memory recognition tests suggest that IR+/- mice have an impairment in recognition of familiarized objects; IR+/- mice demonstrate poor performance for both short-term (1 h) and long-term (24 h) memory tests in comparison to that of wild-type mice. Electrophysiological experiments indicate that mitral cell neurons cultured from both heterozygous and homozygous-null mice (IR+/- and IR-/-) have an decreased peak current amplitude compared with that recorded for wild-type (+/+) animals matched for days in vitro (DIV). These data indicate that the loss of one allele of the IR kinase gene modifies the electrical phenotype of the mitral cell neurons in the olfactory bulb without a change in gross olfactory ability. Given our findings that there are no significant changes in metabolic balance of the IR (+/-) mice but some impairment in memory retention, future experiments testing for specific olfactory behaviors or functional deficits in IR-/+ mice models of diabetes will need to either be tasks that do not require learning or will require a different model (such as diet-induced diabetes) that may evoke a stronger phenotype.     

Locus coeruleus degeneration exacerbates olfactory deficits in APP/PS1 transgenic mice

Neuronal loss in the locus coeruleus (LC) is 1 of the early pathological events in Alzheimer's disease (AD). Projections of noradrenergic neurons of the LC innervate the olfactory bulb (OB). Because olfactory deficits have been reported in early AD, we investigated the effect of induced LC degeneration on olfactory memory and discrimination in an AD mouse model. LC degeneration was induced by treating APP/PS1 mice with N-(2-chloroethyl)-N-ethyl-bromo-benzylamine (DSP4) repeatedly between 3 and 12 months of age. Short term odor retention, ability for spontaneous habituation to an odor, and spontaneous odor discrimination were assessed by behavioral tests. DSP4 treatment in APP/PS1 mice resulted in an exacerbation of short term olfactory memory deficits and more discrete weakening of olfactory discrimination abilities, suggesting that LC degeneration contributes to olfactory deficits observed in AD. Importantly, DSP4 treatment also increased amyloid β (Aβ) deposition in the olfactory bulb of APP/PS1 mice, which correlated with olfactory memory, not with discrimination deficits.     

Early recognition of newborn goat kids by their mother: I. Nonolfactory discrimination

Mother sheep and goats develop an early bond with their neonate on the basis of olfactory recognition. We investigated whether goats were also able to show early (<24 hr postpartum) nonolfactory discrimination of their kids, as already reported in sheep. In a first experiment, we found that goats are not able to recognize their kid at 1 m away on the basis of olfactory cues alone. By contrast, they showed a significant preference for their own kid in a two-choice test as early as 4 hr postpartum, and prepartum maternal anosmia did not impede the ability of mothers to show discrimination. We conclude that goats, like sheep, are fully able to discriminate their neonate without the help of olfactory cues very early after parturition. The difference in the early spatial mother-young relationship between the two species due to the different behavior of the young (kids = hiders, lambs = followers), is not associated with marked differences in the dynamics or mechanisms controlling the development of recognition of the neonate by its mother.     

The perirhinal-entorhinal cortex, but not the hippocampus, is critical for expression of individual recognition in the context of the Coolidge effect

The Coolidge effect is a phenomenon in which males show renewed sexual interest in a novel female following copulation to satiety with another female. In golden hamsters, this phenomenon depends on the ability to recognize conspecifics using chemosensory cues processed through the main olfactory system. Here we tested whether olfactory targets in the hippocampal system support this natural form of recognition memory. Male hamsters received ibotenic acid lesions of the perirhinal-entorhinal cortex (PR-ENT) or hippocampus (H) and were allowed to copulate to satiety with a female conspecific, then were presented with two anesthetized females, the familiar mate and an unfamiliar female that copulated with another male. Sham-operated and H-lesioned subjects preferentially investigated the novel female, indicating intact recognition of individual identity. By contrast, PR-ENT-lesioned males failed to discriminate familiar and novel females, and this deficit could not be attributed to abnormal copulatory behavior during mating. All subjects were able to detect and discriminate between female odors when presented in isolation during a habituation-discrimination test, indicating that behavioral deficits shown by PR-ENT males were not due to anosmia or a general investigatory deficit. Thus, the perirhinal-entorhinal cortex, but not the hippocampus, is critical for the recognition of familiar conspecifics in this naturalistic situation. This study reveals an essential role for the perirhinal-entorhinal cortex, but not the hippocampus, in a natural form of recognition memory within the social behavior of hamsters. The findings show a strikingly similar pattern to the effects of selective damage to the same brain regions on performance in standard recognition memory tasks by rats and monkeys. Therefore, the present data extend our understanding of the differential role of structures of the hippocampal memory system, showing continuity across species and between formal laboratory tests and the function of memory in natural social behavior.