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1. The electroresponsive characteristics of neurons in the lateral habenula were studied with intracellular recordings in a brain slice preparation of guinea pig diencephalon maintained in vitro. One hundred and two neurons met the criteria for recording stability, and of these, 18 were analyzed in detail. For these 18 neurons, the mean resting membrane potential was -61.9 mV, the mean input resistance was 124 M omega, and the mean spike amplitude of fast action potentials was 60.3 mV. 2. Lateral habenula neurons were found to have distinct patterns of activity dependent on membrane potential. At membrane potentials more positive than -65 mV, depolarization elicited trains of sodium-dependent fast action potentials. At membrane potentials more negative than -65 mV, slight depolarization elicited a tetrodotoxin-insensitive wave of depolarization, called a low-threshold spike (LTS), from which a burst of fast action potentials were triggered. The principal conductance underlying the LTS is a low-threshold calcium conductance, which is inactivated at membrane potential more positive than -65 mV and deinactivated when the membrane is hyperpolarized to potentials more negative than -65 V. 3. Upon termination of injected hyperpolarizing current, many neurons displayed oscillation in membrane potential at a frequency of 3-10 Hz, thereby generating repetitive bursts of fast spikes. 4. The pattern of neuronal activity in lateral habenula neurons was highly sensitive to slight alterations in membrane potential. The ability of these neurons to fire action potentials in two modes, tonically and in bursts, and the propensity of these neurons to dramatically alter their output in response to transient hyperpolarizing input, indicate that transmission through this relay in the dorsal diencephalic conduction system may be greatly augmented by relatively small hyperpolarizing influences on the individual neurons.

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Rats performed an intermittently reinforced operant response and received periodic presentations of a visual conditioned stimulus (CS) located either very near or far from the operant manipulandum. In three experiments (positive conditioned suppression), the CSs signaled food deliveries, whereas in two other experiments (negative conditioned suppression) the CSs signaled electric shock. When food was signaled, the far CS produced more suppression of operant responding than the near CS. On the other hand, when shock was signaled the near CS produced more suppression. Measurement of approach to CS and approach to the food-delivery site during CS on the positive conditioned-suppression procedure indicated that specific directed skeletal responses develop during CS and play an important role in the reduction of the operant response; presumably by evoking more incompatible approach behavior, the far CS has greater suppressive effects. In the case of negative conditioned suppression, the development of behavior directed away from a CS for shock seems to be involved in the greater suppression produced by the near CS. The findings suggest that there are deficiencies in the general accounts of conditioned suppression that stress the. motivational or emotional effects of Pavlovian CSs and neglect their effects on directed skeletal behavior.Many studies of interactions between clas-by training animals to perform a response sical and instrumental conditioning begin to obtain appetitive reinforcers. When this instrumental response is well established, a ~ ~~~~~ light or tone (CS) is presented indepen-This research was supported by National Ind tf f fa animal ' s behavior and the offstitute of Mental Health Grant MH-

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