Dopamine Modulates the Processing of Food Odour in the Ventral Striatum.

Food odour is a potent stimulus of food intake. Odour coding in the brain occurs in synergy or competition with other sensory information and internal signals. For eliciting feeding behaviour, food odour coding has to gain signification through enrichment with additional labelling in the brain. Since the ventral striatum, at the crossroads of olfactory and reward pathways, receives a rich dopaminergic innervation, we hypothesized that dopamine plays a role in food odour information processing in the ventral striatum. Using single neurones recordings in anesthetised rats, we show that some ventral striatum neurones respond to food odour. This neuronal network displays a variety of responses (excitation, inhibition, rhythmic activity in phase with respiration). The localization of recorded neurones in a 3-dimensional brain model suggests the spatial segregation of this food-odour responsive population. Using local field potentials recordings, we found that the neural population response to food odour was characterized by an increase of power in the beta-band frequency. This response was modulated by dopamine, as evidenced by its depression following administration of the dopaminergic D1 and D2 antagonists SCH23390 and raclopride. Our results suggest that dopamine improves food odour processing in the ventral striatum.

Diet Protein Content and Individual Phenotype Affect Food Intake and Protein Appetence in Rats.

BACKGROUND: A low-protein diet can induce compensatory intake of excess energy. This must be better evaluated to anticipate the obesogenic risk that may result from the dietary recommendations for reducing animal protein consumption. OBJECTIVES: We aimed to further characterize the behavioral and physiological responses to a reduction in dietary protein and to identify the determinants of protein appetite. METHODS: Thirty-two male Wistar rats [4 wk old, (mean ± SEM) 135 ± 32 g body weight] were fed a low-protein (LP; 6% energy value) or normal-protein (NP; 20%) diet for 8 wk. Food intake and body mass were measured during the entire intervention. During self-selection sessions after 4 wk of experimental diets, we evaluated rat food preference between LP, NP, or high-protein (HP; 55%) pellets. At the end of the experiment, we assessed their hedonic response [ultrasonic vocalizations (USVs)] and c-Fos neuronal activation in the olfactory tubercle and nucleus accumbens (NAcc) associated with an LP or HP meal. RESULTS: Rats fed an LP diet had greater food intake (24%), body weight (5%), and visceral adiposity (30%) than NP rats. All LP rats and half of the NP rats showed a nearly exclusive preference for HP pellets during self-selection sessions, whereas the other half of the NP rats showed no preference. This suggests that the appetite for proteins is driven not only by a low protein status but also by individual traits in NP rats. LP or HP meal induced similar USV emission and similar neuronal activation in the NAcc in feed-deprived LP and NP rats, showing no specific response linked to protein appetite. CONCLUSIONS: Protein appetite in rats is driven by low protein status or individual preferences in rats receiving adequate protein amounts. This must be considered and further analyzed, in the context of current recommendations for protein intake reduction.

Odour conditioning of positive affective states: Rats can learn to associate an odour with being tickled.

Most associative learning tests in rodents use negative stimuli, such as electric shocks. We investigated if young rats can learn to associate the presence of an odour with the experience of being tickled (i.e. using an experimenter's hand to mimic rough-and-tumble play), shown to elicit 50 kHz ultrasonic vocalisations (USVs), which are indicative of positive affect. Male, pair-housed Wistar rats (N = 24) were all exposed to two neutral odours (A and B) presented in a perforated container on alternate days in a test arena. Following 60s of exposure, the rats were either tickled on days when odour A (n = 8) or odour B (n = 8) was present, or never tickled (n = 8). When tickled, rats produced significantly more 50 kHz USVs compared to the days when not being tickled, and compared to control rats. The level of anticipatory 50 kHz USVs in the 60s prior to tickling did not differ significantly between the tickled and control rats. As a retrieval test following the odour conditioning, rats were exposed successively in the same arena to three odours: an unknown neutral odour, extract of fox faeces, and either odours A or B. Compared to controls, 50 kHz USVs of tickled rats increased when exposed to the odour they had previously experienced when tickled, indicating that these rats had learned to associate the odour with the positive experience of being tickled. In a test with free access for 5 min to both arms of a T-maze, each containing one of the odours, rats tickled with odour A spent more time in the arm with this odour. This work is the first to test in a fully balanced design whether rats can learn to associate an odour with tickling, and indicates that positive odour conditioning has potential to be used as an alternative to negative conditioning tests.

Sexual responses of male rats to odours from female rats in oestrus are not affected by female germ-free status.

Rats detect and use odorant molecules as a source of information about their environment. Some of these molecules come from conspecifics, and many arise as by-products from microbial activity. Thus, compared to conventionally housed rats, germ-free rats are raised in an environment with fewer odorants, but this reduction is rarely quantified. Using gas chromatography-mass spectrometry, we found that germ-free rat faeces samples contained half as many volatile molecules than conventional rat faeces (52 vs 109 (±2.4) molecules; P < 0.001) and overall these were only 12% as abundant. We then investigated if odours from female germ-free rats in oestrus would have pro-erectile effects in conventional male rats. For this aim, conventionally housed Brown Norway (BN) rats (n = 16) with sexual experience with either Fischer or BN females, were exposed to four different odour types: faeces from germ-free Fischer rat in oestrus, faeces from conventional rats in oestrus and di-oestrus (either from Fischer or BN), and a control (either 1-hexanol or male rat faeces). The number of penile erections per test as well as the duration of freezing behaviour was significantly higher with the oestrous odours (germ-free and conventional) compared to the control, with intermediate responses to the di-oestrous faeces. The findings indicate that, despite a significantly reduced composition in terms of volatiles compared to conventionally housed rats, the faeces of germ-free rats contain sufficient odorants to evoke sexual responses in conventional male rats. Oestrous odours of rats thus appear not to be of microbial origin.

Where is the TMT? GC-MS analyses of fox feces and behavioral responses of rats to fear-inducing odors.

TMT (2,5-dihydro-2,4,5-trimethylthiazoline) is known as a component of fox feces inducing fear in rodents. However, no recent chemical analyses of fox feces are available, and few studies make direct comparisons between TMT and fox feces. Fox feces from 3 individuals were used to prepare 24 samples to be analyzed for the presence of TMT using gas chromatography-mass spectrometry (GC-MS). When TMT was added in low amounts (50-2000 nmol/g), TMT was detected in 10 out of 11 samples. When no TMT was added, TMT was detected in only 1 out of 13 samples. In a second experiment, we tested the behavioral response of male Brown Norway (BN) and Wistar rats to either fox feces, a low amount of TMT (0.6 nmol) or 1-hexanol. TMT induced freezing in the rats, but fox feces induced significantly more freezing episodes and longer total duration of freezing in both rat strains. In experiment 3, male BN rats were exposed over several days to fox feces, rat feces, 1-hexanol, cadaverine, 2-phenylethylamine, and TMT, one odor at a time. Fox feces induced significantly more freezing episodes of a longer total duration than any of the other odors, with rat feces and 1-hexanol giving rise to the lowest amount of freezing. This finding, together with our inability to verify the presence of TMT in fox feces, indicates that the concentration of TMT in our fox feces samples was below 50 nmol/g. It may also be that other compounds in fox feces play a role in its fear-inducing properties.

Behavioural response of sexually naïve and experienced male rats to the smell of 6-methyl-5-hepten-2-one and female rat faeces.

Sexually experienced male rats display penile erections when exposed to faeces from mammalian females in oestrus (Rampin et al., Behav Brain Res, 172:169, 2006), suggesting that specific odours indicate female receptiveness across species. However, it is unknown to what extent the sexual response observed results from an odorous conditioning acquired during sexual experience. We tested the behavioural response of male Brown Norway rats both when sexually naïve and experienced to four odours, including oestrous rat faeces and 6-methyl-5-hepten-2-one (methylheptenone; a molecule found in higher concentrations during oestrus in female rats, foxes and horses). Odour had a significant effect on the sexual response of the naïve rats, with oestrus faeces provoking significantly more erections than herb odour, and with methylheptenone and di-oestrus faeces being intermediate. This indicates that sexually naïve male rats have an unconditioned ability to detect oestrous mediated via odour. After gaining sexual experience, the response to methylheptenone, di- and oestrus faeces was significantly higher than that observed with herb odour. These results strongly suggest that methylheptenone is part of the odorous bouquet of oestrus and contributes to the olfactory determination of female receptiveness.

Brain processing of biologically relevant odors in the awake rat, as revealed by manganese-enhanced MRI.

BACKGROUND: So far, an overall view of olfactory structures activated by natural biologically relevant odors in the awake rat is not available. Manganese-enhanced MRI (MEMRI) is appropriate for this purpose. While MEMRI has been used for anatomical labeling of olfactory pathways, functional imaging analyses have not yet been performed beyond the olfactory bulb. Here, we have used MEMRI for functional imaging of rat central olfactory structures and for comparing activation maps obtained with odors conveying different biological messages. METHODOLOGY/PRINCIPAL FINDINGS: Odors of male fox feces and of chocolate flavored cereals were used to stimulate conscious rats previously treated by intranasal instillation of manganese (Mn). MEMRI activation maps showed Mn enhancement all along the primary olfactory cortex. Mn enhancement elicited by male fox feces odor and to a lesser extent that elicited by chocolate odor, differed from that elicited by deodorized air. This result was partly confirmed by c-Fos immunohistochemistry in the piriform cortex. CONCLUSION/SIGNIFICANCE: By providing an overall image of brain structures activated in awake rats by odorous stimulation, and by showing that Mn enhancement is differently sensitive to different stimulating odors, the present results demonstrate the interest of MEMRI for functional studies of olfaction in the primary olfactory cortex of laboratory small animals, under conditions close to natural perception. Finally, the factors that may cause the variability of the MEMRI signal in response to different odor are discussed.

A mixture of odorant molecules potentially indicating oestrus in mammals elicits penile erections in male rats.

A common set of odorous molecules may indicate female receptiveness across species, as male rats display sexual arousal when exposed to the odour of oestrous faeces from rats, vixens and mares. More than 900 different compounds were identified by GC-MS analyses performed on faeces samples from di-oestrous and oestrous females and from males of the three species. Five carboxylic acids were found in lower concentrations in faeces from all oestrous females. We subjected 12 sexually trained male rats to a 30 min exposure to different dilutions of a mixture of these five molecules in the same proportions as found in female oestrous faeces. The behavioural responses of the rats were compared to those displayed when exposed to water (negative control) and faeces from oestrous female rats (positive control). Frequency of penile erections were found to be significantly dependent on mixture dilution, with two intermediate dilutions eliciting frequencies of penile erections that did not differ from those obtained during exposure to oestrous female rat faeces. Higher and lower dilutions did not elicit more penile erections than observed with water. These results support our hypothesis that a small set of odorous molecules may indicate sexual receptiveness in mammalian females.

Are oestrus odours species specific?

Adult male rats were exposed to faeces odours of three animal species (rat, fox and horse). They displayed erections in the presence of faeces from oestrous females (whatever the species). In addition, fox faeces (whatever the gender or hormonal status) elicited an expected freezing reaction. It is suggested that oestrous female faeces of these three species share common odorants which depend on the hormonal status and characterize female receptivity.

Spinal control of erection by glutamate in rats.

The lumbosacral spinal network controlling penile erection is activated by information from peripheral and supraspinal origins. We tested the hypothesis that glutamate, released by sensory afferents from the genitals, activates this proerectile network. In anesthetized intact and T8 spinalized (i.e., freed from supraspinal inhibition) male rats, the parameters of electrical stimulation of the dorsal penile nerve (DPN) that elicited intracavernous pressure (ICP) rises were determined. In T8 spinalized rats, DPN stimulations were applied in the presence of d(-)-2-amino-5-phosphonopentanoic acid (d-AP5), a competitive NMDA receptor antagonist, or of 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulphonamide (NBQX), an AMPA-kainate receptor antagonist, injected intrathecally at the lumbosacral level. Both antagonists, alone or in combination, dose dependently decreased the ICP rise and increased its latency. In conscious rats, reflexive erections were depressed by d-AP5 and NBQX, as revealed by an increased latency of the first erection and by decreases of the number of rats displaying erections, of the number of erection clusters and of the number of erections per cluster. In anesthetized ats, the combined administration of the glutamatergic agonists NMDA and AMPA elicited ICP rises in the absence of DPN stimulation. In contrast, both agonists moderately decreased the ICP rise elicited by DPN stimulation but did not affect its latency. These results support our hypothesis that glutamate, released on stimulation of the genitals and acting at AMPA and NMDA receptors, is a potent reactivator of the spinal proerectile network.

Proerectile effects of apomorphine in mice.

Dopaminergic pathways play a key role in the central control of sexual behavior. Stimulation of central dopaminergic receptors elicits penile erection in a variety of species and has been proposed as a treatment option for erectile dysfunction in humans. The present study investigated the proerectile effects of apomorphine in mice. In this species, subcutaneous injection of apomorphine (range: 0.11-110 microg/kg sc) elicited three different behavioral responses: erection, erection-like responses and genital grooming. Proerectile effects of apomorphine were dose-dependent. More than 50% of mice displayed erections after administration of 1.1-11 microg/kg of apomorphine sc. Proerectile effects of apomorphine were blocked by haloperidol, a central D2 antagonist, but not by domperidone, a peripherally active dopaminergic antagonist. We conclude that apomorphine elicits erection in mice. This effect is dose-dependent and due to activation of central D2 dopaminergic receptors. The mouse model may be useful for pharmacological approaches designed to provide a better understanding of the central mechanisms of penile erection and sexual behavior.