2 TYPES OF INTRINSIC OSCILLATIONS IN NEURONS OF THE LATERAL AND BASOLATERAL NUCLEI OF THE AMYGDALA

Intracellular recordings in the guinea pig and cat basolateral amygdal oid (BL) complex maintained as slices in vitro revealed that a subpopu lation of neurons (79%) in the lateral (AL) and basolateral (ABI) nucl ei generated two types of slow oscillations of the membrane potential upon steady depolarization from resting potential. The cells were of a stellate or pyramidal-like shape and possessed spiny dendrites and an axon leaving the local synaptic environment, and thus presumably repr esented projection neurons. Similar oscillatory activity was observed in projection neurons of the cat AL nucleus recorded in vivo. Oscillat ory activity with a low threshold of activation (low-threshold oscilla tion, LTO) appeared as rhythmic deflections (amplitudes, 2-6 mV) of th e membrane potential positive to -60 mV. Fast Fourier transformation ( FFT) demonstrated a range of frequencies of LTOs between 0.5 and 9 Hz, with >80% occurring at 1-3.5 Hz and an average at 2.3 +/- 1.1 Hz. LTO s were more regular after pharmacological blockade of synaptic transmi ssion and were blocked by tetrodotoxin (TTX). Blockade of LTOs and Na spikes revealed a second type of oscillatory activity (high-threshold oscillation, HTO) at depolarizations beyond -40 mV, which was capable of triggering high-threshold spikes. HTOs ranged between 1 and 7.5 Hz , with >80% occurring at 2-6 Hz and an average at 5.8 ir 1.1 Hz. HTOs vanished at a steady membrane polarization positive to -20 mV. Current versus voltage relations obtained under voltage-clamp conditions reve aled two regions of negative slope conductance at -55 to -40 mV and at around -30 mV, which largely overlapped with the voltage ranges of LT Os and HTOs. TTX abolished the first region of negative slope conducta nce (-55 to -40 mV) and did not significantly influence the second reg ion of negative slope conductance. Neuronal responses to maintained de polarizing current pulses consisted of an initial high-frequency disch arge (up to 100 Hz), the frequency of which depended on the amplitude of the depolarizing current pulse, followed by a progressive decline ( ''adaptation'') toward a slow-rhythmic firing pattern. The decay in fi ring frequency followed a double-exponential function, with time const ants averaging 57 +/- 28 ms and 3.29 +/- 1.85 s, and approached steady -state frequencies at 6.3 +/- 2.9 Hz (n = 17). Slow-rhythmic firing re mained at this frequency over a wide range of membrane polarization be tween approximately -50 and -20 mV, although individual electrogenic e vents changed from Na+ spikes and underlying LTOs to high-threshold sp ikes and underlying HTOs. Rhythmic regular firing was only interrupted at an intermediate range of membrane polarization by the occurrence o f spike doublets. In conclusion, the integrative behavior of a class o f neurons in the BL complex appears to be largely shaped by the slow-o scillatory properties of the membrane. While LTOs are likely to synchr onize synaptic signals near firing threshold, HTOs are a major determi nant for the slow steady-state firing patterns during maintained depol arizing influence. These intrinsic oscillatory mechanisms, in turn, ca n be assumed to promote population activity at this particular frequen cy, which ranges well within that of the limbic theta (theta) rhythm a nd the delta (delta) waves in the electroencephalogram during slow-wav e sleep.