Dorsal Hippocampal Neurons Research Articles

Intrinsic neurons of the septal area are involved in reproductive development of the female rat. Alterations after kainic acid lesioning.

Three experiments were conducted using intracranial infusion of kainic acid (KA) to lesion the medial septal (MS) neurons and dorsal hippocampal (DH) neurons in 25-day-old female rats. Rats of experiment 1 were allowed to develop until vaginal opening (VO) after which vaginal lavages were taken to monitor the vaginal cycle. Rats of experiment 2 received estradiol benzoate (EB) subcutaneously immediately after KA infusion. Blood was obtained at 11.00 and 16.00 h, 2 days after EB and measured for luteinizing hormone (LH) by radioimmunoassay. Rats of experiment 3 received MS KA lesions and 2 days later tested for pituitary responsiveness to LHRH-induced LH release. In experiment 1, MS KA-lesioned rats exhibited a significant delay in VO when compared to the sham-lesioned or the unoperated control groups. Hippocampal KA-lesioned rats in contrast, showed no difference in the age of VO from the control groups. Septal KA-lesioned rats also showed disrupted vaginal cycles after the onset of puberty. In experiment 2, all the EB-treated animals exhibited the expected rise in serum LH on the afternoon 2 days following EB with the exception of the septal KA-lesioned rats. In every case, KA infusion into the medial septum abolished the ability of EB to induce an LH increase at 16.00 h 2 days after EB administration. DH KA lesion on the other hand had no effect on the EB-induced LH increase. The block of LH secretion by MS KA infusion shown in experiment 2 is not a result of decreased pituitary gland sensitivity to LH-releasing hormone (LHRH) as shown in experiment 3. These results indicate that MS neurons play a vital role in the feedback mechanism whereby EB induces an LH increase in the prepuberal female rat, and that they play a role in the timing of VO.

Neuronal conditioned responses in the dorsal hippocampus to combined acoustic stimulation and extracellular polarization

1. Responses of dorsal hippocampal neurons were investigated during combinations of an acoustic stimulus with a direct current (anode), stimulating the cell through the recording microelectrode. Of the 45 cells, 22% formed conditioned responses of activation or inhibitory types which corresponded with the character of the reinforcing agent. Cells not responding to the stimuli did not form temporary connections. Approximately half of the conditionally responding neurons formed extinction and differentiation to an unreinforced stimulus. 2. During periodic presentation of combinations of acoustic and electrical stimulation 30% of the 25 cells formed conditioned responses to time. Significant trace reproduction of conditioned responses was observed during 10 to 12 omissions of the stimuli. Cells forming ordinary conditioned responses could produce conditioned responses to time also. They were formed more rapidly. 3. Prolonged periodic presentation of the acoustic stimulus leads to rhythm binding of the previous stimulation. These responses differ from conditioned responses to time in their short duration, the absence of spontaneous recovery of the responses, and their occurrence within a definite time interval. 4. It is suggested that the characteristics discovered during the formation of simple temporary connections and observed in various model situations in neurons belonging to different brain structures are common to many conditioned-reflex acts.