Extended Data Fig. 1: Recording of optogenetically identified oxytocin neurons, local field potential, phase locking, synchronization and spike distribution.

a, The cell type specificity of rAAV-OTp-ChR2-mCherry expression in OT cells. OT-immunosignal (green) colocalizes with ChR2-mCherry (red) signal; the overlay appears in yellow. Scale bar = 100 µm. Bottom, right: quantification of the colocalization mCherry/ChR2 (red bar, 90.8 ± 3.9 %) and mCherry/OT immnoreactive cells (green bar, 82.6 ± 7.7 %). Bar plots show mean ± SEM. b, Post-hoc verification of implanted optoprobe location in the PVN in a representative animal (one of five rats). Scale bar = 100 µm. c, Sorted extracellular spike waveforms (n = 175 action potentials) of a representative single unit optically-identified as OT neurons. d, Silicon probe (NeuroNexus) with 32-channel single shank were used in acute (anesthetized) single units recording. Sorted units and their location in the channels map were visualized with Phy-GUI (klusta, Python). Spike sorting was manually done in Plexon Offline Sorter 4.0 (Plexon, Inc.). e, PSTHs illustrating two optogenetically-identified oxytocin neurons by their response to blue light pulses (1 Hz, 5 Hz and 10 Hz laser stimulation, 10 ms, λ=473 nm, 10 mW/mm²). In both neurons, low frequency stimulation evoked spikes with a relatively constant short latency of 2-10 ms. f, Power spectrogram of the local field potential (LFP) in the PVN recorded before (open field, OF), during and after (OF) a free social interaction (FSI) session. g, Average theta (5-10 Hz) power recorded during and FSI session is significantly higher than before FSI session (p = 0.03, n = 5 rats, paired two-sided t test). All data represented as mean + SEM. h, P value distribution of phase-locking between theta (5-10 Hz) oscillations and OT cells spikes during exploratory (OF) or social (FSI) behavior (** p = 0.0089, n = 15 cells from 5 rats, paired two-sided t test; box plot shows median 10^th, 25^th, 75^th, and 90^th percentiles; min/max: OF, 0.07/0.99; SI, 0.002/0.08). i, j, Example traces (black) of LFP in the PVN and band-pass (5-10 Hz) filtered theta oscillations (red) during exploratory (OF) or social (FSI) behavior. Examples of distribution of four OT neurons firing in relation to LFP theta oscillations; OT neurons spikes are phase-locked with theta oscillations (** P = 0.0014, n = 15 cells) during social interaction (FSI, j), but not during exploratory behavior (i). Circular representation of OT neurons firing in relation to theta oscillation phase shows phase locking during social behavior (l) exclusively. No significant difference of spike-phase coupling between social behavior subtypes (P = 0.28). Significance of phase locking are determined by Rayleigh test for circular uniformity. k, l, Cross-correlation of pairs of oxytocin neurons recorded simultaneously. During open field (k) test there is no detectable correlation between oxytocin neurons spiking activity, but during social interaction (l) there is a significant increase (P = 0.0038, n = 12 cell pairs) of temporally correlated spikes within a time window of τ = 1.2 ± 0.5 s (mean correlation half-time). m, n, Examples of interspike interval (ISI, time bins = 10 ms) histograms of four OT neurons recorded during exploratory behavior (OF, m) and during social interaction (FSI, n). During FSI there is a prominent increase of spikes with short intervals due to increased spike clustering.