Interictal spikes precede ictal discharges in an organotypic hippocampal slice culture model of epileptogenesis.

In organotypic hippocampal slice cultures, principal neurons form aberrant excitatory connections with other principal cells in response to slicing induced deafferentation, similar to mechanisms underlying epileptogenesis in posttraumatic epilepsy. To investigate the consequences of this synaptogenesis, the authors recorded field-potential activity from area CA3 during perfusion with the complete growth medium used during incubation. At 7 days in vitro, slice cultures only displayed multiunit activity. At 14 days in vitro, the majority displayed population bursts reminiscent of interictal-like spikes, but sustained synchronous activity was rare. Band-pass filtering of interictal discharges revealed fast ripple-like complexes, similar to in vivo recordings. Spontaneous ictal-like activity became progressively more prevalent with age: at 21 days in vitro, 50% of organotypic hippocampal slice cultures displayed long-lasting, ictal-like discharges that could be suppressed by phenytoin, whereas interictal activity was not suppressed. The fraction of cultures displaying ictal events continually increased with incubation time. Quantification of population spike activity throughout epileptogenesis using automatic detection and clustering algorithms confirmed the appearance of interictal-like activity before ictal-like discharges and also revealed high-frequency pathologic multiunit activity in slice cultures at 14 to 17 days in vitro. These experiments indicate that interictal-like spikes precede the appearance of ictal-like activity in a reduced in vitro preparation. Epileptiform activity in cultures resembled in vivo epilepsy, including sensitivity to anticonvulsants and steadily increasing seizure incidence over time, although seizure frequency and rate of epileptogenesis were higher in vitro. Organotypic hippocampal slice cultures comprise a useful model system for investigating mechanisms of epileptogenesis as well as developing antiepileptic and antiepileptogenic drugs.

Timing of administration mediates the memory effects of intraseptal carbachol infusion.

Medial septal neurons innervate the entire hippocampal formation. This input provides a potent regulation of hippocampal formation physiology (e.g. theta) and memory function. Medial septal neurons are rich in cholinergic receptors and thus are potential targets for the development of cognitive enhancers. Direct intraseptal infusion of cholinomimetics alters hippocampal physiology and can produce either promnestic or amnestic effects. Several variables (e.g. age of animal, integrity of septohippocampal circuits, task difficulty) may influence treatment outcome. We have previously demonstrated that intraseptal carbachol (12.5-125 ng) infusion immediately after the sample session of a delayed-non-match-to-sample radial maze paradigm produces a dose-dependent amnesia. The present study examined whether manipulating the timing of intraseptal carbachol infusion with respect to the sample session would alter the amnestic effect. A within-subjects design was used to examine the effect of intraseptal carbachol (125 ng/0.5 microl) in a delayed-non-match to sample radial maze task. During a sample session, rats retrieved rewards from six of 12 maze arms. At the test session (3 h later), only the alternate set contained reward and entries into the sample set arms constituted errors. Intraseptal carbachol was administered: 1) 30 min prior; 2) immediately prior; 3) immediately after and 4) 90 min after the sample session. Intraseptal carbachol prior to the sample had no effect on any index of accuracy. Infusion immediately after the sample, or delayed 90 min into the retention interval, produced an acute amnesia. These findings demonstrate that the timing of treatment is a critical variable in determining the memory effects of septohippocampal manipulations and that dynamic changes in cholinergic tone are important for memory.

Intraseptal infusion of oxotremorine impairs memory in a delayed-non-match-to-sample radial maze task.

The medial septal nucleus is part of the forebrain circuitry that supports memory. This nucleus is rich in cholinergic receptors and is a putative target for the development of cholinomimetic cognitive-enhancing drugs. Septal neurons, primarily cholinergic and GABAergic, innervate the entire hippocampal formation and regulate hippocampal formation physiology and emergent function. Direct intraseptal drug infusions can produce amnestic or promnestic effects depending upon the type of drug administered. However, intraseptal infusion of the cholinomimetic oxotremorine has been reported to produce both promnestic and amnestic effects when administered prior to task performance. The present study examined whether post-acquisition intraseptal infusion of oxotremorine would be promnestic or amnestic in a delayed-non-match-to-sample radial maze task. In this task rats must remember information about spatial locations visited during a daily sample session and maintain that information over extended retention intervals (hours) in order to perform accurately on the daily test session. Treatments may then be administered during the retention interval. Alterations in maze performance during the test session an hour or more after treatment evidences effects on memory. In the present study, intraseptal infusion of oxotremorine (1.0-10.0 microg) produced a linear dose-related impairment of memory performance. Importantly, we also observed disrupted performance on the day after treatment. This persistent deficit was related only to memory over the retention interval and did not affect indices of short-term memory (ability to avoid repetitive or proactive errors during both the pre- and post-delay sessions). The persistent deficit contrasts with the acute amnestic effects of other intraseptally administered drugs including the cholinomimetics carbachol and tacrine. Thus, intraseptal oxotremorine produced a preferential disruption of memory consolidation as well as a persistent alteration of medial septal circuits. These findings are discussed with regards to multi-stage models of hippocampal-dependent memory formation and the further development of therapeutic strategies in the treatment of mild cognitive impairment as well as age-related decline and Alzheimer's dementia.