Elicited ponto-geniculo-occipital waves by auditory stimuli are synchronized with hippocampal θ-waves

INTRODUCTION

Ponto-geniculo-occipital (PGO) waves are observed in the pons, the lateral geniculate nucleus (LGN), and the occipital cortex. A PGO wave generator is believed to be located within the pontine caudolateral peribrachial area. Hippocampal θ-waves (hereafter known simply as θ-waves) are observed as rhythmic extracellular field potentials of 3–8 Hz in cats. The medial septum is involved in the generation of θ-waves. A close relationship between PGO waves and θ-waves has been demonstrated electrophysiologically.^1,2 We have also shown previously that PGO waves tend to be phase-locked with θ-waves.³ We hypothesized a possible mechanism for influencing a PGO wave generator by hippocampal θ-waves. The generator is known to receive afferent inputs such as auditory input. Actually, a PGO-like wave is elicited by auditory stimuli, which is called an elicited PGO (PGO_E) wave. To test our hypothesis, we analysed the temporal relationship between hippocampal θ-waves and PGO_E waves.

MATERIALS AND METHOD

Three adult cats (bodyweight range 3.3–5.5 kg; CAT1–CAT3) were used in the study, and standard techniques were used for polygraphic recording.⁴ Twisted bipolar electrodes (stainless, 250 μm φ; gap between tips, 0.5–1.0 mm) were implanted in the LGN (A 5.8–6.9, L 9.0–9.4, D 4.0–4.9) and hippocampus (A 4.0–4.2, L 4.4–6.1, D 4.5–5.0) for recording PGO and θ-waves, respectively.^2–4 The cats could move freely in a sound-attenuated room illuminated by dim lighting. Tone stimuli (2000 Hz, 90 dB, 20 ms duration at intervals of 20 s) were applied at an arbitrary phase in reference to a θ-wave by elaborately equipped small speakers (1 cm distant from ears). The PGO_E waves were defined as those with latencies between 40 ms and 200 ms.⁵ All experimental procedures were performed in accordance with protocols approved by the Institutional Animal Care and Use Committee.

RESULTS AND DISCUSSION

We investigated the time intervals between a negative peak of a PGO_E wave and the preceding and succeeding adjacent positive peaks of a θ-wave; these intervals were denoted by t_ep and t_es, respectively (Fig. 1a). t_ep and t_es were distributed unimodally, as shown in Fig. 1b, whereby the values for t_ep were 0.099 ± 0.054 s (mean ± SD) for CAT1, 0.092 ± 0.051 s for CAT2, and 0.080 ± 0.047 s for CAT3; and the values for t_es were 0.088 ± 0.053 s for CAT1, 0.088 ± 0.049 s for CAT2, and 0.112 ± 0.049 s for CAT3. These results suggest that PGO_E waves tend to be phase-locked with θ-waves rather than phase-resetted because no systematic correlation between t_ep and the period of θ-wave (t_ep + t_es) was observed. In addition, the same analysis was performed for PGO waves, whereby values for t_p were 0.113 ± 0.037 s for CAT1, 0.086 ± 0.047 s for CAT2, and 0.087 ± 0.049 s for CAT3; and values for t_s were 0.078 ± 0.033 s for CAT1, 0.088 ± 0.043 s for CAT2, and 0.116 ± 0.048 s for CAT3. For all cats, there was no significant difference between the corresponding intervals t_p/t_ep and t_s/t_es. In conclusion, PGO_E waves were phase-locked with θ-waves and shared the same phase-locking tendency with spontaneous PGO waves. The results lead us to speculate that the PGO wave generator is driven by θ-waves, which is at least supported anatomically.⁶

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