DISCHARGE PATTERNS OF HIPPOCAMPAL THETA-RELATED CELLS IN THE CAUDAL DIENCEPHALON OF THE URETHANE-ANESTHETIZED RAT

1. Single-unit discharge patterns of cells in specific nuclei of the c audal diencephalon were characterized in relation to simultaneously re corded field activity from the stratum moleculare of the dentate gyrus according to the criteria that have been used previously to classify cells in the hippocampal formation (including entorhinal cortex), medi al septum, and cingulate cortex. Theta (theta)-related cells were clas sified as 1) tonic theta-ON, if they discharged nonrhythmically and in creased their discharge rates during hippocampal theta relative to lar ge, irregular hippocampal field activity (LIA); 2) tonic theta-OFF, if they discharged nonrhythmically and decreased their discharge rates d uring theta relative to LIA; or 3) phasic theta-ON, if they discharged rhythmically and in phase with ongoing theta, but nonrhythmically dur ing LIA. Cells not meeting any of the above criteria were classified a s nonrelated. 2. Recordings were obtained in a total of 127 cells from the caudal diencephalon. Recordings were made in 54 cells from the po sterior hypothalamic nucleus (PH), 16 from the supramammillary nucleus (SuM), 20 from the PH/SuM border, and 23 from the medial mammillary n ucleus (MM). Recordings were also made from nine cells from the centra l medial nucleus of the thalamus (CM) and five from the dorsomedial hy pothalamic nucleus (DMH). 3. Of the 54 PH cells, 43 (80%) were classif ied as tonic theta-ON and 11 (20%) as nonrelated. Tonic theta-ON cells in the PH discharged at significantly higher rates during theta, eith er occurring spontaneously (9.6 +/- 1.7 Hz, mean +/- SE) or elicited w ith a tail pinch (TP theta; 10.6 +/- 1.9 Hz), than during LIA (3.6 +/- 1.4 Hz). Of the nine CM cells, seven (78%) were tonic theta-ON and tw o (22%) were nonrelated. Tonic theta-ON cells discharged at significan tly higher rates during theta (17.5 +/- 7.8 Hz) or TP theta (18.0 +/- 7.1 Hz) than during LIA (7.3 +/- 4.8 Hz). All DMH cells were nonrelate d. 4. Of the 20 PH/SuM border cells, 15 (75%) were classified as tonic theta-OFF and discharged at significantly higher rates during LIA (5. 3 +/- 1.5 Hz) than during theta (0.8 +/- 0.4 Hz) or TP theta (0.4 +/- 0.3 Hz). Five (25%) cells in the PH/SuM border were nonrelated. 5. All of the 16 cells (100%) recorded from the body of the SuM were phasic theta-ON. The discharge rates of these cells did not change significan tly across hippocampal field states (LIA = 8.3 +/- 1.6; theta = 7.3 +/ - 1.6; TP theta = 8.6 +/- 1.7 Hz). However, all of these cells changed from a nonrhythmic discharge pattern during LIA to a rhythmic (or pha sic) discharge pattern during theta. Most of the SuM cells (12 of 16, 75%) discharged on the negativegoing phase of the hippocampal theta re corded from the fissure/stratum moleculare of the dentate gyrus (151 /- 14 degrees; 0 degrees = positive peak of theta). 6. Of the 23 cells recorded from the MM, 19 (83%) were also phasic theta-ON. The remaini ng four cells (17%) were nonrelated. The discharge rates of MM phasic theta-ON cells did not change significantly across hippocampal field s tates (LIA = 9.1 +/- 2.0; theta = 9.9 +/- 1.5; TP theta = 9.4 +/- 1.4 Hz). However, all of these cells changed from a nonrhythmic discharge pattern during LIA. to a rhythmic (or phasic) discharge pattern during theta. In contrast to SuM phasic theta-ON cells, most of the MM cells (15 of 19, 75%) discharged on the positive-going phase of theta (300 +/- 12 degrees). 7. On the basis of these and previously published dat a, it is suggested that tonic theta-ON cells in the PH (and possibly C M) act in synergy with phasic theta-ON cells in the SuM during hippoca mpal theta to relay ascending reticular input to the septum. MM cells may, as has previously been suggested, act to relay theta-rhythmic sig nals from the septum or hippocampus to other parts of the limbic syste m. It is suggested that tonic theta-OFF cells in the PH/SuM border inh ibit discharge of theta-ON cells in the PH and SuM during non-theta hi ppocampal states.