B Eckersdorf

Theta-like activity in hippocampal formation slices : the effect of strong disinhibition of GABAA and GABAB receptors

The involvement of GABAA and GABAB receptors in neural mechanisms responsible for the production of theta rhythms in hippocampal formation (HPC) slices is addressed in the present study. In a number of papers published in the last decade, we have demonstrated that theta-like activity can be successfully recorded in the limbic cortex maintained in vitro when the cholinergic agonists, acetylcholine, carbachol or muscarine, were added to the bath. Recently, we have also shown a strong GABAA modulation of the cholinergic-induced in vitro theta-like activity. This study presents a report of the first demonstration of in vitro theta-like field responses induced a consequence of simultaneously inhibiting hippocampal GABAA and GABAB receptors. HPC slices (350 microns) were maintained in a gas-liquid interface chamber (35 degrees C). Theta-like activity was induced in the presence of bath perfusion of bicuculline (GABAA antagonist) and 2-hydroxysaclophen (GABAB antagonist). This in vitro induced field response was antagonized both by muscimol (GABAA agonist) and baclophen (GABAB agonist). In addition, the experiments presented here revealed that bicuculline/2-hydroxysaclophen-induced in vitro theta-like activity also had a strong cholinergic M1 involvement: it was abolished by hemicholinium-3 (choline transport blocker) and pirenzepine (specific antagonist of M1 receptor), but not by gallamine (specific antagonist of M2 receptor). The results of the present study provided further evidence for a strong GABAergic/cholinergic interaction in the neural mechanism responsible for production of theta-like activity in the hippocampal formation slices.

Window effect of temperature on carbachol-induced theta-like activity recorded in hippocampal formation in vitro

The effect of different temperatures of ACSF (18-42 degrees C) on carbachol (CCH)-induced field potentials were examined in the present study. Two hundred and thirty one experiments were performed on hippocampal formation slices maintained in a gas-liquid interface chamber. All slices were perfused with 50 microM CCH. A recording electrode was positioned in the region of CA3c pyramidal cells. The experiments gave two main findings. First, in a presence of continuous cholinergic stimulation the temperature of the bathing medium per se determined the rate of synchronization of the field potentials and pattern of EEG activity recorded. Second, within the temperature range from 33 degrees to 37 degrees C a window effect of temperature on CCH-induced theta-like activity (TLA) was noted: in this temperature range all slices tested responded only with one pattern of EEG activity-TLA. The results are discussed in light of temperature effects on hippocampal neuronal networks.

Carbachol-induced theta-like activity in entorhinal cortex slices

The present study was conducted for two purposes: the first was to evaluate whether activation of cholinergic receptors of the entorhinal cortex in vitro (complete deafferentation) with carbachol (100 microM) was capable of producing theta (theta)-like slow activity. The second purpose was to determine whether carbachol-induced slow waveforms were mediated by muscarinic or nicotinic receptors. We demonstrated that carbachol was capable of producing theta-like slow activity. This activity was not altered by nicotinic antagonists, (+)-tubocurarine and hexametonium. Atropine and scopolamine, in contrast, completely blocked in vitro induced slow waves, indicating entorhinal muscarinic receptors to be actively involved in the mechanism generating cholinergic theta rhythm.

Inhibition of locomotor activity during cholinergically-induced emotional-aversive response in the cat

Unilateral injections of carbachol (CCh, 10 micrograms) into the anterior hypothalamic/preoptic area of cats caused an inhibition of locomotor activity measured in two different ways in 4-field and 100-field activity boxes. This inhibition was accompanied by crouched posture of the cats body, general arousal with increased number of searching head movements, autonomic manifestations, and vocalization. Recordings of the growling component of vocalization, serving as a quantitative index of CCh-induced emotional-aversive response, showed that the period of maximal inhibition of locomotor activity coincided with the peak values of vocalization. We conclude that direct injections of CCh into the anterior hypothalamic/preoptic area in cats activate a central mechanism responsible for an inhibition of locomotor activity. This inhibition seems to be one of the natural components of defensive responses, further supporting the suggestion that the CCh-induced emotional-aversive response is equivalent to naturally occurring defensive behavior in cats.

Firing cell repertoire during carbachol‐induced theta rhythm in rat hippocampal formation slices

One hundred and seven cells were recorded extracellularly in hippocampal formation (HPC) slices during carbachol‐induced theta. The data obtained provided evidence of a population of HPC neurons which, when activated cholinergically, participate in the generation of in vitro theta. The activity patterns of in vitro recorded theta‐related cells were shown to be similar to those of theta‐related cells recorded in vivo and cells recorded in vitro during cholinergically induced theta, and non‐theta intervals were successfully classified according to previously developed criteria for in vivo recorded theta‐related cells. The current in vitro experiments showed that, in addition to theta‐on and theta‐off cells, the HPC contained cells that were probably involved in programming the appearance and duration of theta epochs and the intervals between theta epochs. These novel types of cells were termed ’gating cells’.

Electrical coupling underlies theta rhythm in freely moving cats.

The role of gap junction coupling in the generation of theta rhythms in freely moving cats was investigated in a present study. Two gap junction blockers, carbenoxolone and quinine, were administered intraperitoneally and intrahippocampally; both gap junction blockers abolished or diminished (respectively) hippocampal formation theta. The inhibitory effect developed approximately 30 min after drug administration. This effect was found to be reversible. Our results provide the first direct in vivo evidence for the contribution of gap junction communication in mechanisms of neural synchrony, underlying the production of theta in in vivo conditions.

The effect of intraseptal procaine injection of hippocampal theta in freely moving cat

Numerous studies have demonstrated that in the rodent, the septum is a critical site for the generation of hippocampla rhythmical field activity (theta, RSA). The aim of the present study was to clarify the role of the medial septal area in theta generation in the cat. Results indicate that (1) the microinfusion of the local anesthetic procaine into the medial septal region temporarily suppressed spontaneous as well as sensory and electrically induced hippocampal theta; (2) the suppression lasted 15 min and then amplitude and power of the hippocampal theta showed a progressive increase reaching preprocaine levels 60 min from the infusion onset. Theta frequency, in contrast, did not differ from control levels. The results provide evidence for the medial septal mediation of the hippocampal theta in the cat. This region is responsible for controlling theta amplitude but not its frequency. The present data supported the earlier results obtained in the rat and determined that the medial septal region plays a critical role in the generation of the hippocampal theta activity in cat.

Muscarinic (M1) mediation of hippocampal spontaneous theta rhythm in freely moving cats

The effects of intrahippocampally applied different doses of muscarinic (atropine sulphate, pirenzepine, gallamine) and nicotinic (hexamethonium, mecamylamine) antagonists on the spontaneous theta rhythm in the cat hippocampal formation were investigated. The injections of atropine and pirenzepine abolished spontaneous theta but administration of gallamine did not affect the EEG pattern. The intrahippocampally administered nicotinic blockers, hexamethonium and mecamylamine, were completely ineffective in antagonizing theta waves. The data suggest that the spontaneous theta rhythm in the cat is mediated by the M1 receptor subtype.

Septal cholinergic mediation of hippocampal theta in the cat.

The effects of intraseptally microinjected muscarinic (atropine sulfate, pirenzepine and gallamine) and nicotinic (hexamethonium) antagonists on spontaneous, sensory and electrically-induced hippocampal (HPC) theta EEG activity were investigated in the freely behaving cat. Administration of hexamethonium failed to elicit a detectable effect on HPC theta. Injections of atropine and pirenzepine abolished, whereas the injection of gallamine only reduced hippocampal theta. Moreover, a gradual recovery of theta amplitude and power was observed, while frequency recovered rapidly. Our data provide further evidence that the septal M1 and M2 muscarinic receptor subtypes mediate the HPC theta in this species. Intraseptal microinjection of cholinergic agonist (carbachol) produced almost a continuous HPC theta with increased amplitude and power. The contribution of the medial septal cholinergic projections to HPC theta frequency and amplitude was also discussed.

Muscarinic (M1) mediation of carbachol-induced theta in the cat entorhinal cortex in vitro.

Entorhinal cortex slice preparations obtained from the cat exhibited theta rhythm during perfusion with 50 microM carbachol. The effect of carbachol was antagonized by the muscarinic blocker atropine sulphate, but not by hexamethonium and mecamylamine, which are antagonists of the nicotinic receptor. Further analysis of the pharmacological profile of these carbachol-induced theta oscillations showed that M1 receptor subtype to be involved in mediation of this EEG activity: the theta rhythm was antagonized by the M1 receptor blocker pirenzepine, but was unaffected by gallamine, an antagonist of the M2 receptor subtype.