Objective: Systemic insulin increases muscle sympathetic nervous system activity (MSNA) in healthy adults by augmenting firing frequency of medium-sized sympathetic action potentials (AP) and recruiting previously latent, larger axons. Whether these neural coding patterns are due to central or peripheral effects of insulin is unclear. We examined the impact of elevated central insulin on AP firing patterns in healthy adults. We hypothesized intranasal insulin administration, which increases central insulin levels, would increase MSNA via elevated firing frequency of APs and recruitment of previously latent, larger axons. Methods: Ten participants were assigned to time control [TC, n=6 (3M/3F)] or 160 IU of intranasal insulin [n=4 (3M/1F)] administered over 5 min. MSNA (fibular microneurography) was assessed at baseline and for 30 min following insulin administration. Sympathetic APs were identified using a matched mother wavelet and continuous wavelet transform. APs were divided into the proportion of those firing in small, medium, or large amplitude clusters. AP firing frequency, percent of APs firing within an MSNA burst (synchronous), and the probability of clusters firing more than once within an MSNA burst were identified. Data are reported as median (interquartile range). Results: MSNA burst frequency increased 15 min after intranasal insulin administration [24(20) to 31(23) bursts/min, p=0.03]. At this time, the percent of synchronous APs [72(27) to 90(24)%, p=0.02] and the probability of medium-sized AP clusters firing more than once within an MSNA burst [13(14) to 19(17)%, p=0.03] increased. No changes in MSNA burst frequency [23(14) to 25(13) bursts/min, p=0.27], percent of synchronous APs [74(15) to 75(34), p=0.48], nor firing probability of medium-sized AP clusters [6(12) to 8(7)%, p=0.97] occurred in TC. The total number of detected clusters did not change in either group (p>0.05). Conclusions: Intranasal insulin increases MSNA burst frequency due to a higher percent of synchronous APs, particularly medium-sized AP clusters; however, we did not observe recruitment of latent, larger axons. These exploratory data highlight potential differences in the sympathetic response to central versus peripheral insulin exposure in healthy adults. CAFNR Joy of Discovery (JKL, JP). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

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