[Intracellular recordings and multi-parameter analysis of long-term potentiation of synaptic responses in chick brain slices.].

Day-old chick is unique animal model in brain development and behavior study. The intermediate medial mesopallium (IMM), a region of the chick forebrain, is intimately involved in the early learning processes, which offers the ideal opportunity to study the neural changes that underlie behavioral plasticity. In this paper, the intracellular recordings were conducted from IMM neurons in chick forebrain slices, in which electrophysiological properties, synaptic responses and long-term potentiation (LTP) were observed. Coronal sections of left forebrains (500 mum thick), containing IMM, were prepared from domestic chicks, aged 2-10 days. In 69 IMM neurons, the resting membrane potential was measured to be (-59.4+/-5.3) mV, slope membrane resistance (70.8+/-27.2) MΩ, and time constant (10.2+/-4.3) ms. The amplitude, threshold, overshoot, half-width, max rise slope and max decay slope of action potential evoked by intracellular current injection were (85.2+/-9.4) mV, (-38.7+/-7.6) mV, (25.6+/-8.9) mV, (2.1+/-0.5) ms, (150.5+/-41.2) mV/ms and (-64.3+/-14.0) mV/ms, respectively. Spike-firing frequency was increased with depolarizing current intensity in 32 of 69 tested cells [linear regression slope was (21.5+/-10.9) Hz/nA, P<0.05 in all cells]. The depolarizing synaptic responses (i.e. EPSPs), with stimulus intensity- and membrane potential-dependent properties, were elicited by dorsal (n=25) or ventral (n=62) focal electrical stimuli at 0.1 Hz in all tested IMM neurons and could be nullified reversibly by perfusion with 100 mumol/L AP5 (NMDA receptor antagonist) and 3 mumol/L DNQX (non-NMDA receptor antagonist), but enlarged by 6 mumol/L bicuculline (GABA(A) receptor antagonist). The EPSPs evoked by ventral stimulation were persistently increased after tetanic stimulation (5 Hz, 300 pulses/train, 2 trains, train interval 10 min) in 6 of 12 tested IMM neurons. The amplitude of EPSPs was potentiated to more than 120% of control level (when analyzed at 45 min of enhancement, P<0.05, n=5), which lasted at least 30 min and then could be referred to as LTP. Moreover, area under curve, duration and max rise slope of EPSPs were also enhanced (P<0.05), while no significant changes were observed in the electrophysiological parameters of IMM neurons following induction of LTP (P>0.05). These results suggest that the intracellular recording techniques in the chick brain slices can be used to perform multi-parameter analysis of synaptic responses and their LTP.