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Long-Lasting Intrinsic Optical Changes Observed in the Neurointermediate Lobe of the Mouse Pituitary Reflect Volume Changes in Cells of the Pars Intermedia

Background/Aims: Complex intrinsic optical changes (light scattering) are readily observed in the neurointermediate lobe of the mouse pituitary gland following electrical stimulation of the infundibular stalk. Our laboratory has previously identified three distinct phases within the light scattering signal: two rapid responses to action potential stimulation and a long duration recovery. The rapid light scattering signals, restricted to the neurohypophysial portion (posterior pituitary) of the neurointermediate lobe, consist of an E-wave and an S-wave that reflect excitation and secretion, respectively. The E-wave has the approximate shape of the action potential and includes voltage- and current-related components and is independent of Ca2+ entry. The S-wave is related to Ca2+ entry and exocytosis. The slow recovery phase of the light scattering signal, which we designated the R-wave, is less well characterized. Methods: Using high temporal resolution light scattering measurements, we monitored intrinsic optical changes in the neurointermediate lobe of the mouse pituitary gland. Pharmacological interventions during the measurements were employed. Results: The data presented here provide optical and pharmacological evidence suggesting that the R-wave, which comprises signals from the posterior pituitary as well as from the pars intermedia, mirrors volume changes in pars intermedia cells following a train of stimuli applied to the infundibular stalk. These volume changes were blocked by the GABA-receptor antagonists bicuculline and picrotoxin, and were mimicked by direct application of GABA in the absence of electrical stimulation. Conclusions: These results emphasize the importance of central GABAergic projections into the neurointermediate lobe, and the potential role of GABA in effecting hormone release from the pars intermedia.