An analysis of cholinoceptive neurons in the hippocampal formation by direct microinfusion.

Microinfusions of cholinergic agents were made in various sites in the dorsal hippocampal formation of urethane anaesthetized rats. Infusions of eserine or carbachol elicited hippocampal theta activity when made in areas containing high levels of cholinergic markers: the stratum oriens and radiatum of the CA1 and CA3, the stratum moleculare and stratum granulosum of the dentate gyrus and the infragranular region of the hilus. Subsequent infusions of atropine sulfate antagonized the theta activity. Control infusions of equal volumes of saline in active sites were without effect. Infusions of eserine or carbachol in the vicinity of the hippocampal fissure, the stratum lacunosum/moleculare of the CA1 or CA3, in the deep regions of the hilus, and in the lateral ventricle and overlying neocortex, were also without effect. Furthermore, in active sites, the latency to onset of theta and subsequent theta frequency, were both directly related to the total amount of carbachol infused. Thus, areas in which theta could be elicited with a cholinergic agonist (carbachol), or an anticholinesterase (eserine) and antagonized with atropine, were found to correspond well to areas previously found to contain a high density of cholinoceptive neurons, using autoradiographic and immunohistochemical techniques. These results provide further support for the involvement of acetylcholine as a neurotransmitter in the generation of type 2 theta in the hippocampal formation.

The relation of multiple hippocampal theta cell discharge rates to slow wave theta frequency.

Multiple theta cells in the dentate granule cell area of the hippocampal formation of the freely moving rabbit were analyzed during three behavioral conditions: (1) voluntary motor patterns, termed Type 1 theta behaviors; (2) automatic motor patterns, termed Type 2 LIA behaviors; (3) alert immobility with presentation of sensory stimuli, termed Type 2 theta behavior. Multiple theta cells showed a linear increase in discharge rates, with increasing frequency of slow wave theta, in both the Type 1 and Type 2 theta conditions. Although there was no mean overlap in discharged rates (from one frequency to the next highest frequency of slow wave theta), individual data points overlapped considerably in the middle ranges. Multiple theta cell discharge rates were significantly lower in the Type 2 theta condition compared to the Type 1 theta condition, even at identical slow wave theta frequencies. Multiple theta cells discharged in an irregular fashion during the Type 2 LIA behavior condition. The discharge pattern was characterized as on or off, the on pattern most commonly observed being asynchronous bursting.