Neuromodulation of Mitral Cells by Serotonin and GLP-1 Neurons in the Olfactory Bulb and the Consequences of Gene Deletion of Kv1.3

• Other Authors: Huang, Zhenbo (author)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Neurosciences
• Online Access: http://purl.flvc.org/fsu/fd/FSU_FALL2017_Huang_fsu_0071E_14226

Neuromodulation plays important roles in adjusting our nervous system to produce behaviors. The same neuromodulator could have different effects on different targets, or the same target could be modulated by multiple neuromodulators. In the first project of my dissertation I investigated differential modulation of mitral cells (MCs) contained in the main (MOB) and accessory (AOB) olfactory bulb by serotonin (5-HT) using an in vitro, brain slice approach in postnatal (P15-30) day mice. In the MOB, 5-HT elicited three types of responses in 94% of 158 cells tested. Cells were either directly excited (73%, n = 115), inhibited (9%, n = 15), or showed a mixed response −first inhibition followed by excitation (12%, n = 19). In the AOB, 83% of 115 cells were inhibited with 17% of cells showing no response. Albeit located in parallel partitions of the olfactory system, 5-HT largely elicited excitation of MOB MCs while it evoked two different kinetic rates of inhibition in MCs of the AOB. Using a combination of pharmacological agents, I found that the excitatory responses in MOB MCs were mediated by 5-HT2A receptors through a direct activation. In comparison, 5-HT-evoked inhibitory responses in the AOB arose due to a polysynaptic, slow-onset inhibition attributed to 5-HT2 receptor activation exciting GABAergic interneurons. The second type of inhibition had a rapid onset as a result of direct inhibition mediated by the 5-HT1 class of receptors. The distinct serotonergic modulation of MCs between the MOB and AOB could provide a molecular basis for differential chemosensory behaviors driven by the brainstem raphe nuclei into these parallel systems. In the second project of my dissertation, I explored the modulation of glucagon-like peptide-1 (GLP-1) neurons in the olfactory bulb (OB). A population of GLP-1 neurons was recently discovered in the OB. The functions of these neurons remain incompletely understood. Herein, I used an in vitro, brain slice approach to investigate the modulations of GLP-1 neurons. Juvenile mice (P20 to P45) of both sexes were used to examine the involvement of centrifugal projections from higher brain areas including serotonergic, cholinergic, and noradrenergic afferents. Bath application of serotonin (40 µM, n = 4) and norepinephrine (100 µM, n = 4) had no effect on the evoked firing frequency. Acetylcholine (ACh; 100 µM), however, led to either inhibition or excitation of GLP-1 neurons. For inhibition, ACh induced a small outward current (5.1 ± 1.8 pA, n = 9) recorded by voltage-clamp when neurons were held at −70 mV. When recorded in current-clamp mode, ACh delayed the latency to first spike (control: 253 ± 30 ms, ACh: 396 ± 4 ms; n = 2). For excitation, bath application of ACh resulted in 1.9 ± 0.6-fold increase in firing frequency (n = 21). Previous evidence showed that GLP-1 neurons in the brainstem could be modulated by metabolic-related hormones such as leptin and cholecystokinin (CCK). I found that GLP-1 neurons could be modulated by CCK, but not by leptin. Bath application of CCK (0.8 µM) led to either cessation of firing (n = 10) or an increase in firing of 1.7 ± 0.4-fold (n = 11). Lastly, mice were injected intraperitoneally with the GLP-1 analogue Exendin-4 (0.4 µM /kg) or control saline and tested 30 minutes post injection in a habituation-dishabituation odor test. Mice receiving Exendin-4 failed to show significant dishabituation, demonstrating impaired ability to discriminate a novel odor from a familiar odor. One primary target of neuromodulation is ion channels. Depending on which group of neurons and in which brain region it is expressed, the same type of ion channel can contribute to multiple functions. In the third project of my dissertation I examined the consequences of loss of function of voltage-gated potassium channel Kv1.3. It has long been recognized that olfaction and emotion are linked. My study aimed to investigate the roles of olfaction in modulating anxiety. Kv1.3 knockout mice (Kv1.3-/-), which have heightened olfaction, and wild-type (WT) mice were examined for anxiety-like behaviors. Because Kv1.3-/- mice have also been observed to show increased locomotor activity, which is one behavior reported in animal models of attention-deficit/hyperactivity disorder (ADHD), inattentive behavior was quantified for both genotypes. Kv1.3-/- mice showed increased anxiety levels compared to their WT counterparts and administration of methylphenidate (MPH) via oral gavage alleviated their increased anxiety. Object-based attention testing indicated Kv1.3-/- mice had attention deficits and treatment with MPH also ameliorated this condition. My data suggest that heightened olfaction does not necessarily lead to decreased anxiety levels, and that Kv1.3-/- mice may be used as a behavioral model of the inattentive subtype of ADHD. === A Dissertation submitted to the Department of Biological Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Fall Semester 2017. === November 16, 2017. === Includes bibliographical references. === Debra Ann Fadool, Professor Directing Dissertation; Timothy M. Logan, University Representative; David M. Gilbert, Committee Member; Lisa C. Lyons, Committee Member; Zuoxin Wang, Committee Member.