Microelectrode analysis of the cellular mechanisms of conditioned reflex in rabbits.

Spikes and postsynaptic potentials (PSPs) were recorded in the sensorimotor cortex of awake rabbits. (1) A cellular analog of cortico-cortical conditioned reflex (CR) was studied. Direct cortical stimulation of a remote point (conditional stimulus - CS) was paired with the stimulation near the microelectrode insertion (unconditional stimulus - UCS). The most neurons showed response facilitation when short intertrial intervals and strong UCS were used. These changes were short-lasting (up to 30 s) and were explained by posttetanic potentiation. Of 21 units tested with long (7 to 120 s) intertrial intervals, 9 showed significant response modification of longer duration. (2) The analog of cortico-cortical CR was modified using stimulation of homolateral cortex as the CS. The UCS was complimented by lateral hypothalamic reinforcement (LHR) in instrumental or classical paradigm. Of 37 units tested, 5 showed an increase in the probability of short-latency ( less than 25 ms) spike. (3) Extracellular activity of 17 neurons was followed through the entire cycle of elaboration and extinction of a "local conditioned startle response" established by pairing click CS with cortical UCS and LHR. Responses of 7 neurons showed significant modifications. The latency distributions of averaged PSPs were compared for naive (N), conditioned (C) and extinguished (E) states. Latencies in N and E groups were similar but were different from those in C group. Of 41 C group neurons, 26 responded at a latency of less than 17 ms; and 9 responded after less than or equal to 7 ms. A pathway for this simple CR may pass through the cortex. Thus PSP changes were observed in all models. Augmentation of short-latency EPSPs support the idea that an increase in efficacy of excitatory synapses underlies the conditioning. Interneuronal excitatory connections within the sensorimotor cortex are, presumably changed during elaboration of simplest motor CRs. These connections can be analyzed with intracellular recordings triggered by spikes of a neighbouring neuron.