Oversized, auditory responsive units of rostral, mid, and posterolateral thalamus

Electrical properties affecting discharge of units of the mid and posterolateral thalamus of conscious cats

Discharge properties in response to intracellularly applied, rectangular currents were measured in units of the mid (lateralis dorsalis and centrolateral nuclei) and posterolateral (lateralis posterior and pulvinar nuclei) thalamus of conscious cats. A separate aim was to determine if neuronal excitability changed in association with changes in stimulus-evoked activity after the animals were trained to discriminate between two acoustic stimuli when performing a conditioned motor response. Low threshold spike (l.t.s.) discharges were observed in three of 272 cells given 1 nA intracellular, hyperpolarizing current pulses of 40 ms duration. This finding supports the view that thalamic neurons of conscious animals operate mainly in the relay as opposed to the oscillatory mode. Application of larger and longer hyperpolarizing currents in the cells produced rebound l.t.s. discharges, supporting the expectation that most thalamic neurons are capable of producing this type of discharge. Decrements of spike afterhyperpolarizations (AHP) and broadening of spike bases upon repeated discharge also were observed in each area of the thalamus studied. After conditioning, changes were found in the posterolateral thalamus (but not in the mid-thalamus) in the proportions of cells with spontaneous, rapid (>/=50 Hz), repetitive, discharges (RRD) and rapid, sustained discharges at rates >/=100 Hz during application of depolarizing current (RSD). In the posterolateral thalamus the percentage of units responding to 1 nA depolarization with RSD fell from 71% before conditioning to 45% after conditioning. The percentage of cells with RRD decreased from 69% to 46%. The changes were accompanied by a 3 mV hyperpolarization of the membrane potentials of the cells and a decrease in baseline activity. After conditioning, increases in excitability were found in cells of the mid thalamus that responded selectively to the click conditioned stimulus (CS) that elicited the conditioned response, and decreases in excitability were found in cells of the posterolateral thalamus that responded to the discriminative acoustic stimulus (DS) to which the animals were trained not to respond. An earlier study showed a potentiation of discharge in response to the CS in units of the midthalamus after similar conditioning and a reduction of the proportion of DS responsive units and peak discharge to the DS in units of the posterolateral thalamus. We conclude that the discharge properties of units of the mid and posterolateral thalamus can change to support discrimination between acoustic stimuli of different functional significance after conditioning.

Acoustic transmission in the dentate nucleus. I. Patterns of activity to click and hiss; II. Changes in activity and excitability after conditioning

Recordings were made from 95 units of the dentate nucleus of naive cats to determine if patterns of response to 70 dB clicks could be distinguished from those to another acoustic stimulus (a hiss) of approximately equal sound pressure level. Further studies of an additional 309 units were conducted to determine if unit excitability and the response to clicks changed after Pavlovian conditioning in which blink responses were elicited by the clicks as conditioned stimuli. Over 50% of units tested before conditioning responded to click or hiss with increased activity, and 8% responded in the first 4-8 ms after the onset of the rapidly rising click. Cross-correlation of the respective 160 ms poststimulus histogram averages of mean activity showed dissimilar patterns of response to clicks and hisses (Pearson product-moment correlation coefficient + 0.02). Thus the averaged population responses distinguished these stimuli. In addition, individual cells were found in each behavioral state that responded selectively to either click or hiss. After conditioning with click as the conditioned stimulus, the number of units responding in the first 4-8 ms to click increased to 23%. The mean magnitude of activity 4-8 ms after presenting the click increased after conditioning but not after sensitization produced by backward pairing of the stimuli used for conditioning. After backward pairing only 6% of the units responded in the first 4-8 ms to click. The changes in activity after conditioning were accompanied by increases in neural excitability to intracellularly applied depolarizing current. In contrast with the changes in activity, the increases in neural excitability were also found after backward pairing. We conclude that short as well as long latency acoustic transmissions to click change in the dentate nucleus after conditioning, that changes in response to click are expressed in 4-8 ms responsive cells, and that many of these cells have different patterns of spike activity in response to click and hiss. The findings support the hypothesis that the dentate nucleus can play a significant role in short as well as long latency, adaptive acoustic transmission that can enhance the response to an acoustic signal used as a Pavlovian conditioned stimulus.