Corticosterone, choline acetyltransferase and noradrenaline levels in olfactory bulbectomized rats in relation to changes in passive avoidance acquisition and open field activity

Consequences of olfactory bulbectomy in two behavioural situations, passive avoidance acquisition and activity in a brightly lit open field, were measured in the same animals for which data on four biochemical measures were also obtained. The biochemical measurements were on plasma corticosterone levels, noradrenaline (NA) levels in the midbrain and amygdala + pyriform cortex and the choline acetyltransferase (CAT) levels in the olfactory tubercle. Experimental variation in age groups of rats (7 weeks and 3 months) and in post-bulbectomy periods (1, 2 and 4 weeks) was made. The deficit in passive avoidance as a consequence of olfactory bulbectomy was evident in all groups of young animals and in older animals one and two weeks post-bulbectomy but not in older animals four weeks after bulbectomy. An increase in open field activity was similarly observed in all groups except in the older animals four weeks after bulbectomy. In contrast to reports by other investigators the basal plasma corticosterone levels were not increased in bulbectomized animals nor did we observe any diminution of NA levels in the amygdala (+ pyriform cortex). CAT levels were slightly increased in older animals two weeks after bulbectomy. The absence of a change in the plasma corticosterone level after bulbectomy is discussed in relation to the notion that the olfactory bulbectomized rat is in some way relevant as a test model for predicting efficacy of potential antidepressant drugs.     

Activity-induced manganese-dependent functional MRI of the rat visual cortex following intranasal manganese chloride administration

Manganese-enhanced MRI (MEMRI) of the brain requires delivery of manganese into the target brain regions. It was previously shown that, following intranasal application, ongoing olfactory stimulation facilitates manganese transport along the olfactory nerve into the olfactory bulb, so bypassing the blood–brain barrier (BBB). We report on experiments to evaluate whether visual stimulation can permit manganese transport onwards from the olfactory bulb to the visual cortex. Rats in intact olfactory bulb group were reserved intact olfactory bulb, while those in olfactory bulbectomy group received bilateral bulbectomy. After intranasal MnCl₂ administration, olfactory and visual stimulations were performed on all the animals for a consecutive 20 h. The visual cortex was then examined using MEMRI. Enhanced imaging on T1WI was noted in the visual cortex of the intact olfactory bulb group. Image subtraction revealed that the signal intensity in the visual cortex of the intact olfactory bulb group was significantly higher than that of olfactory bulbectomy group. Volume of interest (VOI) analysis also showed that normalized intensities in the visual cortex of the intact olfactory bulb group were significantly higher as compared with those of the olfactory bulbectomy group. Inductively coupled plasma mass spectrometry (ICP-MS) confirmed that the manganese content in the visual cortex of the intact olfactory bulb group was increased in comparison with that of the olfactory bulbectomy group. These findings indicate that activity-induced manganese-dependent functional MRI (AIM fMRI) of the rat visual cortex can be performed following intranasal administration of manganese and demonstrate that manganese can migrate from the olfactory bulb to the visual cortex.     

Sexual impairment of inexperienced male rats following pre- and postpuberal olfactory bulbectomy.

Olfactory bulbectomy of sexually inexperienced male rats prevented the occurrence of sexual behavior in most of the operated animals. This effect was observed in rats bulbectomized before puberty as well as after puberty. Bulbectomized rats which had heterosexual experience prior to the operation did not deviate from intact rats. Males living in cohabition with intact males prior to the operation showed only minor deficits in their mating performances. It was concluded that the olfactory lobe while of only minor importance for maintenance of mating once sexual behavior has been initiated, plays an important role in initiation of sexual behavior of the male rat. Furthermore, since treatment with testosterone of prepuberally bulbectomized rats did not stimulate the animals to sexual activity it was concluded that the sexual impairment following bulbectomy of isolated males presumably is not due to an impaired production of gonadal secretions.     

Changes in blood pressure and heart rate following bilateral olfactory bulbectomy in rats

Changes in blood pressure and heart rate following bilateral olfactory bulb ablation were investigated in unanesthetized and unrestrained rats with chronic arterial cannula implants. After bilateral olfactory bulbectomy, the rat exhibited a marked increase of emotional responses to given stimuli and a high incidence of muricide. Blood pressure and heart rate at 5 and 10 days after olfactory bulbectomy were significantly lower than in intact rats (rats before bulbectomy) and/or sham operated rats. Marked decreases in blood pressure and heart rate were seen especially in the period when the animals showed exploratory behavior after being transferred to the experimental cage from the home cage. These results suggest that olfactory bulbectomy has a great influence not only on emotionality but also on the central regulatory system of blood pressure in the rat.     

Olfactory bulbectomy: influences on maternal behavior in primiparous and multiparous rats

Bilateral olfactory bulb ablation resulted in deficits in several components of maternal behavior in lactating, primiparous female rats. These females frequently cannibalized pups shortly after parturition. Furthermore, bilaterally bulbectomized, primiparous females spent less time nursing their pups and retrieved fewer pups than unilaterally bulbectomized or sham-operated females. In contrast to the situation in primiparous females, in multiparous females bilateral bulbectomy failed to influence any parameter of maternal behavior.     

Brain amine and adrenal enzyme levels in aggressive, bulbectomized rats

Removal of the olfactory bulbs in male Osborne Mendel rats lowers cortical brain norepinephrine without altering brain dopamine or serotonin levels. No difference in brain amines was found between rats which became mouse-killers following surgery and those which remained non-killers. Both shamoperated controls and bulbectomized rats had elevated levels of the adrenal enzyme tyrosine hydroxylase. The bulbectomy group had heavier adrenals than control groups.     

Unilateral but not bilateral olfactory bulbectomy inhibits body weight gain in hamsters.

The relation of the olfactory bulbs and photoperiod to the regulation of body weight was studied in male golden hamsters. Animals underwent sham operation, bilateral olfactory bulbectomy, or unilateral bulbectomy. They were left on long photoperiod for 5 weeks and then were transferred to short photoperiod for 11 weeks. The unilaterally olfactory bulbectomized hamsters gained less weight on long or short photoperiod than the sham operated group, while the bilaterally bulbectomized hamsters gained at least as much weight as the sham group. Thus, we report the novel finding that unilateral but not bilateral olfactory bulbectomy reduces body weight gain in male golden hamsters.     

Olfactory bulbectomy depresses ultrasound production and scent marking by female hamsters

Female hamsters were observed for rates of ultrasound production, vaginal marking and flank marking before and after bilateral olfactory bulbectomy, unilateral bulbectomy or sham operations. Unilaterally bulbectomized and sham-operated subjects maintained preoperative levels of each behavior. In contrast, females with bilateral olfactory bulbectomies showed consistent decreases in rates of vocalization and scent marking. These results emphasize the dependence on an intact olfactory system of several courtship or agonistic behaviors that affect hamster reproduction by functioning as noncontact communications. Together with the results of previous studies suggesting that lordosis by female hamsters is unaffected by olfactory bulbectomy, these data also suggest differences in the brain and sensory mechanisms that control distinct types of reproductive behavior.     

The effect of embryonal amygdala grafts on the impairment of spatial working memory elicited in rats by kainate-induced amygdaloid damage

The effect of epileptogenic lesion of amygdala on spatial working memory was examined in rats after unilateral (U, n = 9) or bilateral (B, n = 8) injection of 1 or 0.5 mu of 0.2% kainate into the amygdaloid complex. Another group of unilaterally lesioned rats (T, n = 9) received 8 days after kainate injection grafts of embryonal (E 20) amygdala into the damaged region. Twelve days after lesion the three experimental groups and an unoperated control group (C, n = 11) were trained during three weeks in the aversively motivated 8-arm radial water maze (RWM). Training decreased incidence of errors/8 choices from the initial 2.3 to 1.2 in the lesioned rats and to 0.4 in the controls. Transplantation did not enhance RWM acquisition but restored the symmetrical pattern of choices and normalized the emotionality of lesioned animals. Retraining the RWM task 3 months later revealed significant savings in the control group but not in the transplanted group. Histology showed in addition to large lesion at the injection site also distant lesions in the ipsilateral hippocampus, thalamus and neocortex and in the unilaterally injected rats also in the contralateral amygdala. Healthy, well integrated transplants found in 6 out of 9 transplanted rats reduced the incidence of remote lesions (particularly in the contralateral hemisphere). Performance of individual animals reflected the state of the transplant. It is concluded that the impairment of spatial working memory is due not only to destruction of amygdala but also to diffuse damage of the limbic system. The slowly developing seizure-induced distant lesions can be partly prevented by embryonal grafts.     

Tail pinch induces sexual behavior in olfactory bulbectomized male rats

Adult male Long-Evans rats were subjected to bilateral olfactory bulbectomy, sham surgery or no treatment. Of 34 bulbectomized rats, 24 failed to ejaculate on either of 2 tests with a primed ovariectomized female. All control animals exhibited normal sexual behavior, and 10 bulbectomized animals ejaculated at least once during the 2 tests. Later histological examination revealed a relationship between size of lesion and extent of behavioral deficits. After a third test, 16 nonejaculatory animals were subjected to a tail pinch (TP) procedure, immediately followed by a fourth test. The remaining 8 nonejaculatory animals were tested similarly, but without tail pinch. Ten of the 16 tail pinch animals showed complete sexual behavior on the first test, while 2 additional animals began to copulate after a second TP procedure 4 days later. Only 1 of the 8 animals not receiving TP ejaculated on these tests. Thus, TP applied shortly before sexual behavior tests can induce copulation in some males whose behavior had been disrupted by olfactory bulbectomy.     

Effect of bilateral olfactory bulbectomy on discrimination avoidance conditioning in rats

Studies were made with the two-way shuttle box method on the acquisition of discrimination avoidance learning by olfactory bulbectomized rats in relation to changes in emotional behavior. Bulbectomized rats showed a marked increase in locomotor activity, with accompanying augmentation of the reactivity and the appearance of muricidal behavior. Initially, the bulbectomized rats showed elevated conditioned avoidance responses to both the CS+ and the CS-. In later stages, there was a continued slow increase in responses to the CS+ accompanied by a decrease in responses to the CS-, until responses to both stimuli were only slightly elevated above the levels shown by control rats. This result suggests that olfactory bulbectomy does not affect discrimination ability itself, but the impairment of discrimination during the initial stages is resulted from hyperemotionality induced by olfactory bulbectomy.     

Olfactory bulb removal: influences on the aggressive behaviors of male mice

Bilateral removal of the olfactory bulbs of castrated male mice completely prevented the arousal of aggressiveness by exogenous administration of androgen. Unilateralally bulbectomized mice showed fighting comparable to that shown by sham-operated control mice. It was concluded that earlier demonstrations of the abolition of intermale aggressive behavior in mice following olfactory bulb removal could not be attributed to impairment in pituitary-gonadal function. Although in this experiment bulbectomy completely prevented the androgenic arousal of intermale aggression, bulbectomy did not affect the display of aggressive behavior in a competition for food situation.     

Role of central alpha 2-adrenoceptors on the development of muricidal behavior in olfactory bulbectomized rats: effect of alpha 2-adrenoceptor antagonists

Upon surgical removal of olfactory bulbs, by the 8th to 10th day after the surgery muricidal behavior has been demonstrated in 75.4% of rats. In the homogenates of forebrain obtained from rats demonstrating muricidal behavior, levels of noradrenaline, 5-hydroxytryptamine and their metabolites (i.e., 3-methoxy-4-hydroxyphenylglycol sulfate and 5-hydroxyindoleacetic acid) were not changed. However, the fractional turnover rate of noradrenaline in the forebrain of the olfactory bulbectomized rats was significantly decreased without alterations in the metabolism of 5-hydroxytryptamine when compared to that of the sham rats. The muricidal behavior and the aggressiveness were suppressed by antagonists of alpha 2-adrenoceptors (yohimbine, idazoxan and rauwolscine), but not by the alpha 1-adrenoceptor antagonist (corynanthine). Furthermore, upon administration of yohimbine or idazoxan to rats demonstrating the muricidal behavior, the level of noradrenaline in the homogenates of forebrain was decreased while that of 3-methoxy-4-hydroxyphenylglycol sulfate was increased. The maximum binding capacity (Bmax) of [3H]yohimbine to the forebrain membranes obtained from the olfactory bulbectomized rats was significantly higher than that from the sham rats without demonstrating any differences in dissociation constants (Kd) between the two brain membranes. Based on these results, it was suggested that olfactory bulbectomy has caused some functional changes in central alpha 2-adrenoceptors.     

Nipple attachment and survival in neonatal olfactory bulbectomized rats

Seven-day-old rat pups were olfactory bulbectomized, cortical lesioned, or sham lesioned, tested for nipple attachment 24 hr after surgery, then housed with their dam and weighed daily for the next 7 days. On the basis of histological findings the bulbectomized animals were divided into two subgroups: Those with removal of all olfactory bulb tissue (n = 9) and those in which some cellular elements remained in the lesioned area (n = 7). There were no differences between sham and cortical lesioned groups for nipple attachment behavior or weight gain. The completely bulbectomized pups did not show nipple attachment, lost weight each day, became moribund and had to be sacrificed by the fifth postoperative day. Deficits in incompletely bulbectomized pups were somewhat less severe and each of these survived and maintained or gained weight in the latter part of the test period. These results suggest that suckling may be critically dependent upon olfaction in 8 day or older pups and that significant savings in this behavior may be mediated by small remnants of olfactory bulb tissue.     

Changes in amygdaloid afterdischarges and kindling effect following olfactory bulbectomy in the rat

Changes in the thresholds for afterdischarges and the formation of kindling effect in the medial amygdala following olfactory bulbectomy were investigated in the rat with chronic electrode implants. The threshold for afterdischarges in the amygdala of the olfactory bulbectomized rat (OB rat) was significantly decreased on day 4 after olfactory bulbectomy, however, no significant difference was found between OB and sham operated rats on days 7, 14 and 21 since the threshold in the sham group was also decreased at these periods after the surgery. The formation of kindling effect was remarkably accelerated in the OB rats. In this case, the number of days required to reach the stage 1 (Racine's classification) was significantly shortened. These results suggest that the activity of the medial amygdaloid nucleus is increased following olfactory bulbectomy.     

Simultaneous and successive olfactory bulb removal: influences on the mating behavior of male mice

Simultaneous olfactory bulb removal abolished mating behavior in sexually experienced male mice. In contrast to simultaneous bulbectomy, removal of both olfactory bulbs in two operations separated by an interval of 30 days had no effect on the mating behavior of male mice. In successively bulbectomized males, the occurrence or nonocurrence of the opportunity for mating behavior in the interval between the removal of each bulb had no influence on the degree to which mating behavior was spared. It was concluded that the abolition of mating behavior produced by simultaneously removing both olfactory bulbs in male mice is not a consequence of a sensory deficit, but is due to the destruction of tissues directly involved in the mediation of sexual behavior.     

Influence of isolation and training on fighting in mice with olfactory bulb lesions

Isolation-induced aggressive mice receiving total bilateral bulbectomy failed to fight after chronic training. Animals receiving sub-total olfactory bulbectomy were capable of being trained to attack but the latency to attack was increased. When mice were bulbectomized before being isolated they were incapable of attack regardless of training or completeness of the lesion. These results indicated that isolation influences aggressive behavior in bilaterally bulbectomized mice.     

The olfactory conditioning in the early postnatal period stimulated neural stem/progenitor cells in the subventricular zone and increased neurogenesis in the olfactory bulb of rats

The olfactory memory acquired during the early postnatal period is known to be maintained for a long period, however, its neural mechanism remains to be clarified. In the present study, we examined the effect of olfactory conditioning during the early postnatal period on neurogenesis in the olfactory bulb of rats. Using the bromodeoxyuridine-pulse chase method, we found that the olfactory conditioning, which was a paired presentation of citral odor (conditioned stimulus) and foot shock (unconditioned stimulus) in rat pups on postnatal day 11, stimulated the proliferation of neural stem/progenitor cells in the anterior subventricular zone (aSVZ), but not in the olfactory bulb, at 24 h after the conditioning. However, the number of newborn cells in the olfactory bulb was increased at 2 weeks, but not 8 weeks, after such conditioning. Neither the exposure of a citral odor alone nor foot shock alone affected the proliferation of neural stem/progenitor cells in the aSVZ at 24 h after and the number of newborn cells in the olfactory bulb at 2 weeks after. The majority of newborn cells in the olfactory bulb of either the conditioned rats or the unconditioned rats expressed the neural marker NeuN, thus indicating that the olfactory conditioning stimulated neurogenesis in the olfactory bulb. These results suggest that olfactory conditioning during the early postnatal period temporally stimulates neurogenesis in the olfactory bulb of rats.     

Effects of ovarian hormones on sexual receptivity, proceptivity, and motivation in olfactory bulbectomized female rats.

The purpose of the present experiment was to assess the effect of olfactory bulbectomy and ovarian hormones on female sexual motivation. Ovariectomized female rats underwent either bilateral bulbectomy or sham surgery. Females received one of four subthreshold hormone treatments: 0% estradiol (E2) plus 500 micrograms progesterone (P), 100% E2 alone, 10% E2 plus 500 micrograms P, or 100% E2 plus 500 micrograms P. Sexual motivation (as indicated by a female's preference for a sexually active male over a castrated male) and proceptivity (dart and ear wiggling sequences) were measured in a three compartment partner preference apparatus. Sexual receptivity (lordosis) was measured separately in a glass arena with a sexually active male. Results showed that olfactory bulb removal facilitates sexual receptivity and proceptivity in females exposed to 10% or 100% E2 in combination with 500 micrograms P. In contrast, sexual motivation was only demonstrated by olfactory bulbectomized females which received 100% E2 in combination with 500 micrograms P. These findings support the hypothesis that olfactory bulbectomy induces a behavioral hypersensitivity to estrogen, and suggest that sexual motivation is an estrogen-mediated response which requires a higher level of estrogen stimulation than sexual receptivity and proceptivity.     

Enhancement of 5-hydroxytryptamine-induced head-twitch response after olfactory bulbectomy

5-Hydroxytryptamine(2A) receptor agonists evoke the head-twitch response in mice. The head-twitch response in olfactory bulbectomized mice elicited by the administration of 5-hydroxytryptamine (40 microgram/mouse, i.c.v.) was increased about threefold as compared with controls on the 14th day after the operation. The injection of ketanserin (1 mg/kg, i.p.), a 5-hydroxytryptamine(2A) receptor antagonist, inhibited this enhancement of 5-hydroxytryptamine-induced head-twitch response after olfactory bulbectomized. On the 14th day, the number of head-twitch response induced by 5-hydroxytryptophan (40, 80 and 160 mg/kg, i.p.), a precursor of 5-hydroxytryptamine, did not differ between olfactory bulbectomized and control mice. Monoamine oxidase-B activity in the forebrain of olfactory bulbectomized mice was higher than that in controls while monoamine oxidase-A activities were unchanged. The 5-hydroxytryptamine uptake into synaptosomes in the forebrain homogenates of olfactory bulbectomized mice was lower than that in controls. These findings indicate that olfactory bulbectomized causes the enhancement of head-twitch response by a supersensitivity of 5-hydroxytryptamine(2A) receptors in cerebral cortex derived from degeneration of neurons projecting from the olfactory bulb.