Background activity of rabbit hippocampal neurons in conditions of functional exclusion of structures which regulate the theta rhythm

Abstract

The functional importance of theta modulation in the activity of hippocampal neurons was further analyzed using a method consisting of controlled sequential short-term (25-30 min) inclusion or exclusion of the theta rhythm by local administration of lidocaine into the median cervical nucleus and medial septal region respectively. Studies were carried out using conscious rabbits with extracellular recording of hippocampal neuron activity in field CA1. Administration of lidocaine into the medial septal nucleus and diagonal tract nucleus (MS-DT) led to complete inhibition of theta modulation in neuronal and total hippocampus activity. The mean frequency of background discharges underwent no change in most neurons, but decreased significantly in a limited group of cells with high-frequency activity (presumptive inhibitory neurons). Administration of lidocaine into the median cervical nucleus (MCN), the source of serotoninergic pathways to the MS-DT and hippocampus, was accompanied by increases in the stability and frequency of theta modulation of neuronal activity, induction of theta modulation in an additional group of neurons, and expression of a continuous theta rhythm in the electrical activity (EA) of the hippocampus. The mean frequency and regularity of discharges increased in most cells. These data support the existence of tonic inhibitory effects on the part of the MCN on the septa-hippocampal system generating the theta rhythm; in this regard, the MCN can be regarded as an antagonist of the activating reticular formation.

Reversible septal inactivitation disrupts hippocampal slow-wave and unit activity and impairs trace conditioning in rabbits (Oryctolagus cuniculus).

“…There is evidence that intraseptal infusion of local anesthetics such as procaine or tetracaine produces disruptions of spontaneous hippocampal theta activity in awake, behaving animals (Golebiewski, Eckersdorf, & Konopacki, 1999; Kichigina, Kudina, Zenchenko, & Vinogradova, 1999; Lawson & Bland, 1993; Mizumori et al, 1990; Oddie, Stefanek, Kirk, & Bland, 1996), as well as disruptions of spontaneous or brainstem stimulation-induced theta in anesthetized animals (Brust-Carmona, Alvarez-Leefmans & Arditti, 1973; Dickson, Kirk, Oddie, & Bland, 1995; Kirk & McNaughton, 1993; Smythe, Christie, Colom, Lawson, & Bland, 1991). However, similarities or differences between the slow wave effects of local anesthetics and intraseptal scopolamine in the context of rabbit classical conditioning are not known.…”

Study of electrical activity of subcortical structures of the brain after destruction of the dorsal amygdalofugal pathway

“…According to the literary data necessary afferent flow(flux) is provided by ascending systems, its transformation into rhythmic activity occurs on a special hippocampal relay that is considered as a pacemaker of hippocampal thetarhythm [1,2,3,4]. Ascending fibres as a part of the medialanterior cerebral bundle enter the medial septal nucleus from the subcortical brain structures which play an important role in the mechanisms of formation of rhythmic hippocampal activity [5,6,7,8,9].…”