Opiate and muscarinic ligand binding in five limbic areas after bilateral olfactory bulbectomy

An altered spinal serotonergic system contributes to increased thermal nociception in an animal model of depression

The olfactory bulbectomized (OB) rat, an animal model of chronic depression with comorbid anxiety, exhibits a profound dysregulation of the brain serotonergic signalling, a neurotransmission system involved in pain transmission and modulation. We here report an increased nociceptive response of OB rats in the tail flick test which is reverted after chronic, but not acute, administration of fluoxetine. Autoradiographic studies demonstrated down-regulation of 5-HT transporters ([(3)H]citalopram binding) and decreased functionality of 5-HT1A receptors (8-OH-DPAT-stimulated [(35)S]GTPγS binding) in the dorsal horn of the lumbar spinal cord in OB rats. Acute administration of fluoxetine (5-40 mg/kg i.p.) did not modify tail flick latencies in OB rats. However, chronic fluoxetine (10 mg/kg/day s.c., 14 days; osmotic minipumps) progressively attenuated OB-associated thermal hyperalgesia, and a total normalization of the nociceptive response was achieved at the end of the treatment with the antidepressant. In these animals, autoradiographic studies revealed further down-regulation of 5-HT transporters and normalization in the functionality of 5-HT1A receptors on the spinal cord. On the other hand, acute morphine (0.5-10 mg/kg s.c.) produced a similar analgesic effect in OB and sham and OB rats, and no changes were detected in the density ([(3)H]DAMGO binding) and functionality (DAMGO-stimulated [(35)S]GTPγS binding) of spinal μ-opioid receptors in OB rats before and after chronic fluoxetine. Our findings demonstrate the participation of the spinal serotonergic system in the increased thermal nociception exhibited by the OB rat and the antinociceptive effect of chronic fluoxetine in this animal model of depression.

Cholinergic receptor subtypes in the olfactory bulbectomy model of depression

The connection between smoking and depression, the antidepressant actions of nicotine and the targeting of nicotinic acetylcholine receptors (nAChRs) by monoamine re-uptake inhibitors all point to a potential role of nAChRs in the etiology and/or symptomatology of depression. In the current study, we evaluated nAChR subtypes in brain regions of rats subjected to olfactory bulbectomy (OBX), a standard animal model that recapitulates many of the behavioral and neurochemical alterations thought to underlie human depression. Comparisons were made both to sham-operated controls and unoperated animals. OBX led to upregulation of cerebrocortical alpha4beta2 nAChRs and downregulation of striatal alpha7 nAChRs as compared to either the sham-operated or unoperated groups. Striatal alpha4beta2 nAChRs were also downregulated but the sham surgery by itself produced a partial effect, masking the contribution of the OBX lesion. In agreement with earlier studies, we also found downregulation of muscarinic AChRs (both m1 and m2 subtypes) in the striatum when comparing the OBX group to sham-operated controls, but because sham surgery evoked mAChR upregulation, the effect was not apparent when the OBX animals were contrasted to the unoperated group. Accordingly, caution needs to be exercised in interpreting studies of cholinergic function in the OBX model that do not include unoperated animals as an additional comparison group. Our results reinforce a relationship between depression and nAChR expression and point to the need for parallel studies in human depression that might lead to the design of novel therapies targeting specific nAChR subtypes.

Quantitative autoradiographic studies of the effects of bilateral olfactory bulbectomy in the rat brain: central- and peripheral-type benzodiazepine receptors

We investigated the discrete regional effects of bilateral olfactory bulbectomy on central- and peripheral-type benzodiazepine receptors in rat brains at weekly intervals until one month after bulb ablation. Persistent increases in [3H]flunitrazepam binding to central benzodiazepine receptors were observed in the cingulum (27%) and in the frontal (15%) and parietal (14%) cortices. Progressive increases in central benzodiazepine receptors, reaching statistical significance four weeks after olfactory bulbectomy, were observed in the ventromedial thalamic nucleus (35%), the lateral hypothalamic region (22%), the basolateral amygdaloid nucleus (23%) and substantia nigra (25%). Persistent major increases (between four- and six-fold) in [3H]PK-11195 [eH]1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide binding to peripheral-type benzodiazepine receptors were observed in all anterior olfactory nuclei. Similarly, throughout the time period studied, [3H]PK-11195 binding densities were increased two- to three-fold in the piriform cortex and lateral olfactory tract. These observations confirm the usefulness of [3H]PK-11195 binding as a marker of neuronal insult in the brain. Moreover, the persistent regional increases in [3H]flunitrazepam binding to central-type benzodiazepine receptors suggest that GABAergic transmission is altered following olfactory bulb ablation.

Differential effects of olfactory bulbectomy on GABAA and GABAB receptors in the rat brain

GABAergic mechanisms have been implicated in the bilateral olfactory bulbectomy (OBX) animal model of depression, where GABAB receptor binding sites have been shown to decrease markedly at specific time points after OBX. However, as no detailed time course of events has been determined, the present study investigated the effects of OBX on high-affinity GABAA, GABAB, beta-adrenergic, and benzodiazepine receptor binding parameters in membrane preparations from rat brain regions at weekly intervals (1-4 weeks) after OBX. Persistent significant increases (40-60%) in Bmax values of high affinity GABAA receptors were observed in the frontal cortex throughout the period investigated following OBX. Bmax values in the hippocampus increased significantly after 1 week (53%) but were not statistically significant thereafter. No changes in GABAA binding parameters were observed in the hypothalamus or cerebellum. Conversely, GABAB receptor densities were significantly decreased in the frontal cortex after 1 (-38%) and 2 (-41%) weeks and moderately decreased 3 and 4 weeks (-27 and -23%, respectively) after OBX, while in the cerebellum they were significantly increased after 1 week (96%) and returned to sham-operated levels by 3 weeks. No changes in GABAB receptor binding parameters were observed in the hippocampus or hypothalamus. Binding parameters for benzodiazepine receptor binding sites or beta-adrenoceptors were not modified throughout the time course. GABAergic transmission, reflected by changes in GABAA and GABAB receptor density in the frontal cortex, may be altered in OBX rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of bulbectomy and subsequent antidepressant treatment on brain 5-HT2 and 5-HT1A receptors in mice

The effects of bilateral olfactory bulbectomy on serotonergic 5-HT2 and 5-HT1A receptor binding were studied in the frontal cortex (FC), limbic structures (LS), including the hippocampus, amygdala, olfactory tubercule, and piriform cortex, and hypothalamus (HTH) in mice. Bulbectomy resulted in the increase of Bmax for [3H]spiperone binding with 5-HT2 receptors in FC in C57Bl/6j. The receptors in LS and HTH remained unchanged. Subchronic treatment of the bulbectomized mice with antidepressant trazodone (20 mg/kg/day, IP, 14 days) induced downregulation of 5-HT2 receptors in FC and LS. The other two antidepressants used, amitriptyline (20 mg/kg/day, IP, 14 days) and imipramine (10 mg/kg/day, IP, 14 days), did not alter these receptors. [3H]8-OH-DPAT binding with 5-HT1A receptors was not altered by bulbectomy in any brain area in C57Bl/6j mice. Amitriptyline and trazodone decreased Bmax for these receptors in FC in the bulbectomized mice while imipramine was ineffective. Amitriptyline and imipramine significantly increased Bmax and decreased Kd in HTH, and trazodone displayed the same tendency. Bulbectomy did not alter 5-HT2 receptors in DBA/2j mice. Amitriptyline increased Kd in the all brain areas without changing Bmax in the bulbectomized DBA/2j mice. Trazodone significantly decreased Bmax in FC and increased Kd in FC and LS. Imipramine decreased Bmax while increasing Kd in LS. The possible involvement of the serotonin receptor subtypes in the bulbectomy-induced behavioral deficits and in the restorative action of the antidepressants is discussed.

Olfactory bulbectomy alters alpha-1 acid glycoprotein levels in rat plasma

Olfactory bulbectomy in rats causes neurochemical, behavioral, as well as physiological alterations. These alterations make this surgical procedure a useful animal model for depression. In humans, depression was shown to be accompanied by increases in plasma cortisol, inability to decrease cortisol in the dexamethasone suppression test and increases in plasma alpha-1 acid glycoprotein (AGP), an endogenous modulator for the serotonin uptake site. Utilizing a recently developed radial immunodiffusion assay for rat AGP we were able to confirm the increases in plasma AGP in the rat. However, we did not observe increased corticosterone in the rat. We also observed the aggressive behavior of muricide in olfactory bulbectomized rats. These results seem to indicate that olfactory bulbectomy is a good model for depression in the human condition and that AGP may be a putative marker for this condition.

Effect of olfactory bulbectomy and chronic amitryptiline treatment in rats. 3H-imipramine binding and behavioral analysis by swimming and open field tests

An 'animal model' of depression, based on bulbectomy, followed by chronic treatment with amitryptiline was used in rats. In the synaptosomal membranes of the cerebral cortex plus hippocampus, the number of binding sites for 3H-imipramine increased significantly when bulbectomy was associated with the antidepressant. In the bulbectomized rats the tendency was toward a decrease in binding. The treatment with 0.2% Triton X-100 of the membranes revealed a large increase in postsynaptic sites in the bulbectomized treated rats. The behavioral parameters analyzed by the swimming with a water wheel and the open field test revealed a series of differences in the various groups of rats, with respect to handling, bulbectomy and antidepressant treatment. Handling resulted in an increase in swimming time in controls, while bulbectomy reduced this parameter. In both the swimming and open fields tests, chronic bulbectomy reduces the motility of the rat. In control rats chronic amitryptiline increases locomotion and exploratory activity, a behavioral effect that is even more prominent in bulbectomized treated rats.

Brain output dysregulation induced by olfactory bulbectomy: an approximation in the rat of major depressive disorder in humans?

Mounting evidence indicates that the emotional, cognitive, neurovegetative and behavioral symptoms of patients with major depressive disorder are due to abnormal neurochemical substrates in the brain. Although the specific neurochemical abnormalities responsible have not been identified, the presenting symptoms of major depression are consistent with a disruption of normal neural communications between the limbic system and hypothalamus. Following removal of the olfactory bulbs, rats display a syndrome of behavioral deficits that also reflect a disruption of the limbic-hypothalamic axis. Moreover, the bulbectomy induced deficits are selectively reduced by the chronic administration of the same drugs that alleviate the symptoms of depression when given chronically to the patients. In addition to this pharmacological similarity, there are also numerous behavioral parallels between bulbectomized rats and major depression patients. The bulbectomized rat provides a good model in which to study antidepressant drugs and also may provide neurochemical and neuroanatomical data that are relevant to understanding the biological substrates of emotion and the causes of depression in humans.

A new immunization procedure for obtention of anti-leucine-enkephalin antibodies. Part II. Effects of olfactory bulb removal on pro-enkephalin related peptides in rat brain

Bilateral Olfactory Bulb Removal (OBR) induced both complex behavioral alterations and a decrease of many neurotransmitter levels. We studied brain levels of the pro-enkephalin related peptides 45 days after OBR. Opioid levels were studied using three different highly specific antisera exhibiting very high affinities in radioimmunoassays in striatum, hypothalamus, hypophysis, brain stem and cortex. Methionine enkephalin levels increase significantly in striatum (42%), hypophysis (94%) and hypothalamus (25%) and non-significantly in the other areas. Leucine-enkephalin levels tended to increase in all dissected structures but a significant increase only occurred in striatum (42%). Octapeptide levels (Methionine-enkephalin-Arg-Gly-Leu) significantly increase in striatum (22%) and decrease in hypophysis (97%) and in brain stem (76%). All these results are partially consistent with the decrease of opiate binding described previously after OBR and suggest a complex imbalance in neurotransmitters after such a sensorial deprivation. It is suggested that the modifications of enkephalinergic neurotransmission might be related to the stressful state induced by OBR.

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.

Differential effects of antidepressant drugs on [3H]dihydroalprenolol and [3H]imipramine ligand recognition sites in olfactory bulbectomized and sham-lesioned rats

In the olfactory bulbectomy rat model of major depression, the binding of [3H]imipramine is increased by 60% in the midbrain, reduced by 30% in the pons and by 20% in the hippocampus, and unchanged in the hypothalamus 6 weeks after the bulbectomy. Binding of [3H]dihydroalprenolol is unchanged in the midbrain but is increased by 30% in the pons and 15% in the hippocampus. The i.p. administration of the antidepressants amitriptyline, mianserin, tranylcypromine (all at a dose of 10 mg/kg) or iprindole (25 mg/kg) for 28 days followed by a 5-day drug washout period, alters brain part [3H]imipramine and [3H]dihydroalprenolol binding in a manner that is a function of the particular drug, brain part and lesion effect. Only in the hippocampus did the lesion increase beta-binding that was reduced by all four antidepressant drugs.

Olfaction and Alzheimer's disease

Disturbances in the sense of smell may be important both clinically and theoretically in Alzheimer's disease. Initial evidence of poor olfactory recognition performance in Alzheimer's and parkinsonian dementias was followed by two reports which corroborated olfactory dysfunction in Alzheimer's disease. The neuroanatomical and neurochemical bases for this disturbance are discussed. Despite an abundance of preclinical and clinical data linking olfaction with acetylcholine, a preliminary study failed to show any effect of scopolamine on olfactory thresholds. Two of the olfaction-Alzheimer's studies found significantly better performance in other demented groups (alcoholics and patients with vascular dementia), suggesting possible utility in the differential diagnostic workup. The effect of aging per se on olfactory performance cannot be assessed without rigorous control for cognitive dysfunction in sampled populations.

Animal models of depression: parallels and correlates to severe depression in humans

Drugs with antidepressant properties in patients with severe depression also have various behavioral and neurochemical effects in animals. This has given rise to numerous animal models that have been suggested to be valid for research into the neurobiology of depression and the neurochemical mechanisms of the antidepressant drugs. However, considerable evidence from many avenues of research indicates that severe depression is a biochemical disorder that develops in those individuals with some predisposing neurochemical vulnerability. Although the predisposing biochemical abnormality has not been identified, it may be related to the neurochemical mechanisms that regulate impulse traffic in various neural systems and maintain the homeostatic balance of neural activity within the brain. Therefore, the appropriate animal model for severe depression should have some disruption of neural functioning that is returned to normal by the chronic administration of antidepressant drugs. Of the numerous animal models of depression that have been presented in the literature, only the rat with olfactory bulb lesions meets this requirement. The behavioral and endocrine abnormalities induced by the olfactory bulb lesions are reversed by chronic (but not acute) treatment with antidepressants of various classes. Of the existing animal models of severe depression, the olfactory bulbectomy model holds the most promise for elucidating the neurobiology of depression and the neurochemistry of antidepressant drugs.

Autoradiographic analysis of [3H]dopamine and [3H]dopa uptake in the turtle olfactory bulb

Uptake and retention of exogenous tritiated dopamine and L-dopa was observed within turtle olfactory bulb slices. In the more superficial layers, periglomerular and superficial tufted cells, as well as their processes, and intraglomerular dendrites were recognized as labeled. Within the deeper part of the bulb, some labeled cells between the tanycytes, as well as nerve fibers and terminals within the granule cell layer, are reported. The results confirm the presence of specific intrinsic dopaminergic cells within the reptilian olfactory bulb.

Distribution of enkephalin-like immunoreactivity in the rat main olfactory bulb

Enkephalin-like immunoreactivity was localized within the main olfactory bulb of the rat using immunohistochemical techniques. These studies utilized well characterized antisera directed to either leu5- or met5-enkephalin. Specificity was established by absorption of the antisera with either 10 microM synthetic leu5- or met5-enkephalin. Specific enkephalin-like immunoreactivity was observed within several different cell populations including (1) periglomerular cells, (2) granule cells and their processes within the external plexiform layer and (3) occasional short-axon (horizontal) cells within the granule and external plexiform layers. The granule cell layer contained the greatest number of immunoreactive cells. Only a limited number of immunoreactive cells were found in both the periglomerular and granule cell layers, suggesting the enkephalin-containing neurons represent a sub-population within each layer. The absence of immunoreactive processes in the periventribular white matter, as well as the morphologies of immunoreactive bulbar neurons, indicates that enkephalin is found exclusively within intrinsic olfactory bulb neurons.

Regional changes in brain cholinergic enzyme activities after bilateral olfactory bulbectomy in relation to mouse-killing behavior by rats

Bilateral olfactory bulbectomy produced the increased tendency of mouse-killing behavior in nonkiller rats (60% on the 14th day after surgery). Scopolamine hydrobromide (4 and 8 mg/kg, IP) significantly suppressed the killing response in a dose-dependent manner, whereas methylscopolamine nitrate was ineffective. In order to investigate a possible neural mechanisms, choline acetyltransferase (CAT) and acetylcholinesterase (ACh-E) activities were measured in 7 discrete brain areas: cortex, amygdala, hypothalamus, thalamus, tegmentum, hippocampus, and pons plus medulla oblongata. Although the central anticholinergic drug suppressed mouse-killing, no significant difference in either CAT and ACh-E activities was found between the killer and nonkiller rats in any of the brain areas determined in this study. The evidence suggests that the neurochemical findings may not fit the pharmacological findings for supporting a unified cholinergic hypothesis for mouse-killing behavior.