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

Medium- and high-intensity rTMS reduces psychomotor agitation with distinct neurobiologic mechanisms

Definitive data are lacking on the mechanism of action and biomarkers of repetitive transcranial magnetic stimulation (rTMS) for the treatment of depression. Low-intensity rTMS (LI-rTMS) has demonstrated utility in preclinical models of rTMS treatments but the effects of LI-rTMS in murine models of depression are unknown. We examined the behavioral and neurobiologic changes in olfactory bulbectomy (OB) mice with medium-intensity rTMS (MI-rTMS) treatment and fluoxetine hydrochloride. We then compared 10-Hz rTMS sessions for 3 min at intensities (measured at the cortical surface) of 4 mT (LI-rTMS), 50 mT (medium-intensity rTMS [MI-rTMS]), or 1 T (high-intensity rTMS [HI-rTMS]) 5 days per week over 4 weeks in an OB model of agitated depression. Behavioral effects were assessed with forced swim test; neurobiologic effects were assessed with brain levels of 5-hydroxytryptamine, brain-derived neurotrophic factor (BDNF), and neurogenesis. Peripheral metabolomic changes induced by OB and rTMS were monitored through enzyme-linked immunosorbent assay and ultrapressure liquid chromatography-driven targeted metabolomics evaluated with ingenuity pathway analysis (IPA). MI-rTMS and HI-rTMS attenuated psychomotor agitation but only MI-rTMS increased BDNF and neurogenesis levels. HI-rTMS normalized the plasma concentration of α-amino-n-butyric acid and 3-methylhistidine. IPA revealed significant changes in glutamine processing and glutamate signaling in the OB model and following MI-rTMS and HI-rTMS treatment. The present findings suggest that MI-rTMS and HI-rTMS induce differential neurobiologic changes in a mouse model of agitated depression. Further, α-amino-n-butyric acid and 3-methylhistidine may have utility as biomarkers to objectively monitor the response to rTMS treatment of depression.

Effects of bilateral olfactory bulbectomy on N-methyl-D-aspartate receptor function: autoradiographic and behavioral studies in the rat

Rat bilateral olfactory bulbectomy (OBX) serves as a useful model in the study of depression and the mechanisms of action of antidepressant treatments. Considering the evidence of NMDA receptors involvement in depression, the present study was undertaken in order to investigate the time-course effects of OBX on the NMDA receptor function. Following bilateral olfactory bulbectomy, rats display an increase in locomotor activity and changes in other types of behavior in a novel environment. Autoradiographic experiments using the noncompetitive NMDA antagonist [(125)I]-iodo-MK-801 as the labeling agent showed that this increase in behavioral activities corresponds to a decrease in [(125)I]-iodo-MK-801 binding in a number of brain regions. In most regions, this reduction reached significance by the third week following OBX. However, in some cortical areas-a nucleus of the thalamus (AV) and one of the amygdala (LA)-this reduction was already significant in the first or second week following OBX and lasted throughout the 4 weeks of the study. We also compared the behavioral modifications induced by a challenge injection of MK-801 (0.2 mg/kg i.p.) in OBX and sham-operated rats. This challenge is known to induce hyperlocomotion and a number of stereotypies in naive rats. These effects were drastically reduced in OBX as compared to sham-operated rats. These data are consistent with the above-mentioned decrease in cerebral binding of MK-801 to NMDA receptors.

Effects of olfactory bulbectomy on NMDA receptor density in the rat brain:

Olfactory bulbectomy (OBX) transects the glutamatergic efferents from the olfactory bulbs, and the changes of glutamatergic N-methyl-D-aspartate (NMDA) receptor-mediated function are though to be involved in the behavioral deficits seen in OBX rats. In the present study, irritability scores in OBX male Wistar rats were correlated with discrete regional effects on NMDA receptor function measured using a [3H] MK-801 binding assay. Irritability scores, measured before and for 2 weeks after OBX, showed a gradual increase in irritability after OBX. A reduction of the NMDA receptor density was observed in the cerebral cortex and amygdala 16 days after OBX, but not in the striatum, olfactory tubercle, entorhinal cortex, and hippocampus. These results demonstrate that OBX causes changes in the NMDA receptor system in certain brain regions and suggest that these changes may be responsible for the behavioral deficits of OBX rats.

Olfactory bulbectomy alters NMDA receptor levels in the rat prefrontal cortex

Olfactory bulbectomized (OBX) rats show a variety of behavioral and biochemical deficits that parallel human depression. We investigated the expression of glutamate receptor subtypes in cortical and subcortical brain regions following bilateral olfactory bulbectomy in adult rats. Quantitative receptor autoradiography using [(125)I]MK-801 (NMDA receptor), [(3)H]AMPA (AMPA receptor), and [(3)H]kainate (kainate receptor) was performed on brain sections at 1-5 weeks following olfactory bulbectomy. Our results show an elevation of NMDA receptors in the medial prefrontal cortex within 1 week following bulbectomy, which persisted up to at least 5 weeks post-bulbectomy. Neither kainate nor AMPA receptors were altered in any brain region examined. The potential significance of these results is discussed in light of experimental findings supporting a role for NMDA receptors in the mechanism of action of antidepressant drugs and the pathophysiology of major depression.

Effects of olfactory bulbectomy on neuropeptide gene expression in the rat olfactory/limbic system

Bilateral olfactory bulbectomy in the rat produces a well-characterized syndrome that is independent of anosmia. This syndrome is reversed by chronic antidepressant administration, which provides the basis for the olfactory bulbectomy model of depression. The present experiments focused on neuropeptide plasticity in central olfactory/limbic structures following olfactory bulbectomy in rats. Male Sprague-Dawley rats received bilateral surgical ablation of the olfactory bulbs, sham surgery, or no surgery and were killed either three, seven, 14 or 28 days later. Relative levels of messenger RNA encoding neuropeptide Y, somatostatin, thyrotropin-releasing hormone, and corticotropin-releasing factor precursors in the forebrain were measured by quantitative in situ hybridization histochemistry using oligonucleotide probes. Prepro-neuropeptide Y messenger RNA levels in the piriform cortex and dentate gyrus were significantly elevated in bulbectomized rats 14 and 28 days after surgery compared to sham-operated and surgically naive rats. Prepro-somatostatin messenger RNA levels in the piriform cortex were marginally increased in bulbectomized rats at these time-points. Thyrotropin-releasing hormone and corticotropin-releasing factor precursor messenger RNA levels were not altered in the brain regions studied. The results indicate that olfactory bulbectomy causes long-term increases in the expression of the neuropeptide Y gene. These findings suggest that neuropeptide Y plasticity in the olfactory/limbic system may contribute to the olfactory bulbectomy syndrome in rats, and they provide further evidence of a role for neuropeptide Y in the pathophysiology of depression.

The olfactory bulbectomized rat as a model of depression: an update

The olfactory bulbectomized (OB) rat has been proposed as an animal model of depression. The following behavioural changes have been observed following bilateral olfactory bulbectomy: hyperactivity in an enclosed arena, such as the open-field; enhanced nocturnal hyperactivity in a 24-hr home cage activity monitor; deficits in memory, as shown by passive avoidance behaviour and in the Morris maze and the 8-arm radial maze; increased open arm entries in the elevated plus-maze; and changes in food motivated and conditioned taste aversion behaviour. Alterations in the noradrenergic, serotonergic, cholinergic, gamma-aminobutyric acid (GABA)ergic and glutamatergic neurotransmitter systems are also associated with olfactory bulbectomy. The variety of immune changes following olfactory bulbectomy includes reduced neutrophil phagocytosis, lymphocyte mitogenesis, lymphocyte number and negative acute phase proteins, increased leucocyte adhesiveness/aggregation, monocyte phagocytosis, neutrophil number and positive acute phase proteins. An enhanced nocturnal secretion of corticosterone is observed in OB rats, which is normally suppressed by dexamethasone. The most commonly employed behavioural indicator of antidepressant activity is attenuation of the OB-related hyperactivity in the open-field. However, many of the other behavioural, neurotransmitter and immune changes have been shown to be attenuated by chronic (but not acute) antidepressant treatment. Tricyclic antidepressants (amitriptyline, desipramine), atypical agents (mianserin), selective serotonin reuptake inhibitors (paroxetine, sertraline, fluvoxamine), reversible inhibitors of monoamine oxidase A (moclobemide), as well as putative antidepressants such as 5-hydroxytryptamine1A agonists (zalospirone, ipsapirone), noncompetitive N-methyl-D-aspartate antagonists (MK-801) and triazolobenzodiazepines (alprazolam, adinazolam), have demonstrated antidepressant-like activity in this model. As many of the changes exhibited by the OB rat are qualitatively similar to those observed in depressed patients, it may be concluded that the OB rat is a model of depression and not just a means whereby putative antidepressants may be tested.

Increased expression of peripheral benzodiazepine receptors in the facial nucleus following motor neuron axotomy

Peripheral benzodiazepine receptors (PBRs) are expressed in a variety of tissues but are normally found at low levels in the brain. Following various types of nerve injury, a reactive gliosis results that exhibits a high expression of this receptor. To further characterize the expression of PBRs following neuronal injury, we evaluated PBR expression in the facial nucleus following facial nerve axotomy (FNA). Injury to a peripheral nerve results in a complex series of metabolic and morphological changes around the injured neuron. Transections of the facial nerve results in a rapid activation of both astrocytes and microglia around axotomized motor neurons. FNA resulted in an increase in the staining for both astrocytes (glial fibrillary acidic protein) and activated microglia (OX42). There was also a reduction in synaptic contacts with the motor nucleus as evidenced by reduced staining for the synaptic marker, synaptophysin. In sections labeled with [3H]-PK11195, the subsequent autoradiograms displayed marked increases in the labeling for PBRs. This increase was observed at 5, 7 and 10 days after nerve transection. The increase was primarily in the level of expression (Bmax), with no change in the affinity of the ligand (Kd). The increase in PBR expression after FNA supports the hypothesis that PBRs can be used as a sensitive marker for CNS injury.

The autoradiographic perspective of central benzodiazepine receptors; a short review

1. We reviewed studies performed to characterize central benzodiazepine binding sites. 2. An overview of the different radioligands used to characterize BZ1 and BZ2 binding sites and a mapping of these central benzodiazepine sites are described. 3. Saturation studies carried out by autoradiogram quantification also are reviewed. 4. The specific use of the autoradiographic technique to carry out studies on ontogeny, development, and phylogeny is discussed, as well as studies performed using this technique on some diseases and experimental conditions, such as drug treatments or chemical and mechanical lesions.

Antidepressants reverse the olfactory bulbectomy-induced decreases in splenic peripheral-type benzodiazepine receptors in rats

The present study investigated the effects of 21-day administration of clorgyline (1 mg/kg/day), desipramine (10 mg/kg/day) or paroxetine (10 mg/kg/day) on peripheral-type benzodiazepine receptors in rat peripheral tissues following bilateral olfactory bulbectomy. Thymus and spleen weights decreased as a result of bulbectomy. Subsequent antidepressant drug administration had no further effects on the weights of thymus glands but increased those of spleens. In thymus glands, higher densities of peripheral-type benzodiazepine receptors were observed in medulla than in cortex; no significant variations were observed following bulbectomy or antidepressant drug administration. In spleen, higher densities were observed in white pulp than in red pulp. The bulbectomy-induced decreases in binding densities observed in both regions were reversed following administration of antidepressants. Adrenal peripheral-type benzodiazepine receptors were not altered by bulbectomy or subsequent treatment with clorgyline or desipramine while paroxetine upregulated these receptors. No changes in kidney peripheral-type benzodiazepine receptors were observed. The present study confirms that cell lines of the rat immune system possess high densities of peripheral-type benzodiazepine receptor binding sites and further support the contention that, following olfactory bulbectomy, rats may present an antidepressant-reversible immunitary dysfunction.

Antidepressant-induced modulation of GABAA receptors and beta-adrenoceptors but not GABAB receptors in the frontal cortex of olfactory bulbectomised rats

The effects of prolonged administration of antidepressant drugs, belonging to three different classes, on high-affinity GABAA receptor, GABAB receptor and beta-adrenoceptor binding parameters were determined in the frontal cortex of olfactory bulbectomised rats. Clorgyline (1 mg/kg/day), paroxetine (10 mg/kg/day) or desipramine (10 mg/kg/day) were administered for 21 days via subcutaneous osmotic minipumps implanted in the scapular region 7 days after bulbectomy. Cortical GABAA receptor densities, defined with [3H]gamma-aminobutyric acid ([3H]GABA), were significantly increased following bulbectomy. This effect on Bmax values was reversed by all three antidepressant drugs. GABAB receptor densities decreased slightly after bulbectomy. Chronic antidepressant administration had no effect on GABAB receptor binding parameters. Olfactory bulbectomy did not induce any changes in cortical beta-adrenoceptor binding parameters determined with [3H]CGP-12177 ((-)-4-(3-t- butylamino-2-hydroxypropxy)- [5,7-3H]benzimidazol-2-one). However, prolonged administration of all three antidepressant drugs induced a downregulation of beta-adrenoceptors. The results of the present study confirm the involvement of cortical GABAA rather than GABAB receptors in the olfactory bulbectomy animal model of human depression. Moreover, the data further support the hypothesis that a decrease in function of the GABAA receptor complex could play a role in the therapeutic effects of antidepressant treatments.