The behavioural role of glutamate co-release from 5-HT neurons

Thesis

5-hydroxytryptamine (5-HT) is a key neurotransmitter involved in a variety of critical functions including emotional modulation and reward processing. Recent evidence suggests that many 5 HT neurons express the vesicular glutamate transporter 3 (VGLUT3) and release both 5 HT and glutamate. Yet, the role of this co released glutamate is unknown. This thesis aimed to further our understanding of the role of 5 HT glutamate co-release in behaviour using a novel mouse model with conditional VGLUT3 knockout from 5-HT neurons (VGLUT3 cKO5 HT).

A combination of immunohistochemistry and qPCR confirmed the depletion of VGLUT3 expression in 5-HT neurons in VGLUT3 cKO5 HT mice. Initial behavioural analysis showed no alteration in natural behaviour, nor evidence of anxiety-like behaviour in a variety of tests. However, VGLUT3 cKO5 HT mice displayed decreased sucrose preference, which might indicate anhedonia.

C-Fos immunohistochemistry experiments using wildtype mice revealed that VGLUT3 expressing 5-HT neurons in the ventral DRN were activated by exposure to an uncontrollable stressor, acute swim stress. This effect was reversed by the selective serotonin reuptake inhibitor fluoxetine. Additionally, VGLUT3 cKO5 HT mice spent more time climbing during swim stress, suggesting increased active coping.

Reward function was then assessed in VGLUT3 cKO5 HT mice using two operant paradigms. In both paradigms transgenic mice were able to learn over time but showed reduced performance, compared to controls. Conversely, VGLUT3 cKO5 HT mice did not differ from controls in tests probing learning and memory in other domains, suggesting that impairments might be specific to reward-based learning.

Finally, in vivo fibre photometry with a DA biosensor was used to probe DA release in the nucleus accumbens during reward-based tasks. Interestingly, VGLUT3 cKO5 HT mice demonstrated increased DA release in response to a reward-predictive auditory cue but a reduced DA response to the reward as learning progressed. This finding suggested faster transfer of DA neuronal activation from the reward to the reward-predictive cue. This apparent shift toward Pavlovian learning is consistent with a propensity for sign tracking behaviour which may contribute to the observed deficits in reward-based learning.

Altogether, through the use of a novel transgenic mouse model the current thesis provides new insights into the role of 5-HT-glutamate co release in stress coping and reward processing.

Authors: Gullino, LS

• DOI: 10.5287/ora-ko4wn9o4j
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: 5-HT; glutamate; serotonin; co-release; VGLUT3
• Subjects: Neuropsychopharmacology; Neurosciences