Electrophysiology Recordings

Mice were anesthetized using isoflurane and decapitated with a guillotine. The brain was then rapidly removed and 400 μm thick coronal slices of hippocampus were made with a vibrating microtome (Campden) using standard methods (Li et al., 2017). Slices from ventral hippocampus were collected and CA3 was removed from each slice. Dissection solution was kept ice cold (1–3°C) and contained the following (in mM): 120 NaCl, 3.5 KCl, 0.7 CaCl₂, 4.0 MgCl₂, 1.25 NaH₂PO₄, 26 NaHCO₃, and 10 glucose, bubbled with 95% O₂/5% CO₂, pH 7.35–7.45. Slices were allowed to recover at room temperature in the dissection solution for >1 hour prior to recording. Recordings were conducted in a submersion recording chamber perfused with external recording solution, a custom artificial cerebrospinal fluid, containing (in mM): 120 NaCl, 3.5 KCl, 2.5 CaCl₂, 1.3 MgCl₂, 1.25 NaH₂PO₄, 26 NaHCO₃ and 10 glucose, bubbled with 95% O2, pH 7.35–7.45. Recordings were conducted at 22–24° C.

Field postsynaptic potential (fPSP) were measured in the temporoammonic (TA) pathway in stratum lacunosum-moleculare (SLM). A recording electrode (glass micropipette filled with external recording solution; 2–5 MΩ) and a bipolar tungsten stimulating electrode (FHC, Bowdoinham, ME) were placed in SLM. The synaptic response was measured as the initial slope of the fPSP. Paired-pulse stimulation was applied using 50, 100, 200, 500, and 1000 ms intervals. A 15–20 minute stable baseline was obtained before the onset of each experiment by setting the stimulation at the intensity that generated 50–75% of the maximum synaptic response (the largest fPSP before population spikes are generated). Paired-pulse ratios were calculated as the slope of response 2/slope of response 1. The input/output curve was determined as the slope of the fPSP plotted against the amplitude of the fiber volley. The neuropeptide Y dose response curve was plotted with slopes at each dose normalized to the baseline.

All electrophysiology experiments were conducted in the presence of the NMDA antagonist D-2-amino-5-phosphonovalerate (D-APV, 50 μM) to prevent long-term potentiation. The NPY dose response curve was also measured in the presence of the GABA_A receptor antagonist picrotoxin (100 μM) to block inhibition so that the effect of NPY on excitation could be directly measured. Neuropeptide Y (Tocris, 1176/200U) was freshly resuspended in external recording solution before each experiment.