SYNAPTIC POTENTIATION AND DEPRESSION IN SLICES OF MEDIOROSTRAL NEOSTRIATUM-HYPERSTRIATUM COMPLEX, AN AUDITORY IMPRINTING-RELEVANT AREA IN CHICK FOREBRAIN

Long-term potentiation, a tetanic stimulation-evoked, persistent incre ase in synaptic efficiency, is the most extensively studied form of sy naptic plasticity. Intracellular correlates of long-term potentiation have been analysed in mammalian hippocampus and cortex, but not in bir d cortical analogues. We present here studies on long-term potentiatio n in slices of the chick forebrain area mediorostral neostriatum-hyper striatum complex which receives thalamic afferents and is relevant for auditory filial imprinting. Following afferent tetanic stimulation, p opulation spike potentiation was extracellularly recorded in 25% of th e tested neurons for longer than 40 min. Using intracellular recording s, the membrane potential, the amplitude of excitatory postsynaptic po tentials, the latency between the test stimulus and the evoked action potentials, and the cellular excitability (excitatory postsynaptic pot ential-spike relationship) were found to change after the tetanus. A l ong-term depression following the tetanus was also seen in some units in this area. Furthermore, the mechanisms underlying long-term potenti ation were investigated. A large depolarization of resting membrane po tential (approx. 36 mV) was characteristic after the tetanic stimulati on. N-methyl-D-aspartate receptor channels are necessary for induction of this depolarization, as well as for long-term potentiation, as dem onstrated by the effect of DL-2-amino-5-phosphonovaleric acid. After i ntracellular recordings, the cells were injected with Lucifer Yellow. The combination of electrophysiological characterization and morpholog ical identification suggested that the potentiation came chiefly from type I neurons, which have the largest soma among the neuron types in this area and up to eight dendrites. The results demonstrate that the recognized major phenomena of long-term potentiation are found in an a uditory imprinting-relevant area of the chick forebrain, and that this potentiation is dependent on N-methyl-D-aspartate receptor channels. It is noteworthy that behavioural imprinting was previously shown to i nduce a reduction of up to 47% of the spine frequency of type I neuron s and a growth of the remaining spine synapses, all resembling a synap tic selection process. Therefore, the intriguing possibility emerges t hat mechanisms underlying long-term potentiation are instrumental for this selection process, which involves regressive and proliferative mo rphological changes.