Sam A. Deadwyler

Postlesion Axonal Growth Produces Permanent Functional Connections

Lesions of the entorhinal cortex in neonatal rats cause the commissural projections to the dentate gyrus to spread from their normal location in the inner molecular layer into the outer molecular layer, a region normally occupied by afferents from the entorhinal cortex. These lesions also cause the short-latency response to commissural stimulation to spread into the outer molecular layer, a result suggesting that these abnormally located connections are operative.

Physiological studies of the reciprocal connections between the hippocampus and entorhinal cortex.

Abstract Neurophysiological recordings were obtained from the hippocampus, entorhinal cortex, and dentate gyrus under conditions of controlled electrostimulation at interconnecting pathways in order to confirm their bidirectional nature as suggested by recent anatomical findings. The existence of a hippocampal to entorhinal pathway was confirmed physiologically by the presence of evoked field potentials and unitary driving of the entorhinal cortex following stimulation of the CA3 subfields of the ipsilateral and contralateral hippocampus. Activation of the entorhinal cortex by such procedures led to a subsequent excitation of the granule cells of the dentate gyrus through the axonal projections of the perforant pathway. The findings are discussed in the context of known anatomical circuitry which might provide the basis for such bidirectional interactions. The functional significance of the demonstrated physiological connections is indicated by the fact that the entorhinal cortex responds to hippocampal activation in a consistent manner and transmits that information back to the dentate gyrus; thereby completing an important three chain loop between three major components of limbic system circuitry.

Time-dependent changes in commissural field potentials in the dentate gyrus following lesions of the entorhinal cortex in adult rats

Previous neuroanatomical work has shown that lesions of the entorhinal cortex in adult rats cause the commissural projections to spread from their normally restricted locus in the inner molecular layer approximately 40-50 mum into the outer molecular layer (that is, into the zone deafferented by the lesion). In the present study we measured the effects of the entorhinal lesion on the distribution of short-latency potentials elicited by commissural stimulation in the molecular layer. Studies with animals tested at various times after the lesion and with a preparation that permitted recording from the same rat at several post-lesion intervals both indicated that the commissural response spread 100-150 mum towards the deafferented outer molecular layer, while the maximum response spread 50-100 mum. These effects were first detectable by 9 days after the lesion and were fully developed by 15 days post-lesion. These findings suggest that the growth of the commissural system seen after entorhinal lesions results in the rapid formation of functional terminals and are discussed in relationship to the behavioral consequences of brain lesions.

A microdrive for use with glass or metal microelectrodes in recording from freely-moving rats ☆

A new subminiature microdrive assembly is described for electrophysiological recording from behaving rats. This very small, lightweight system allows excellent precision in electrode placement and can maintain stable recordings over extended periods. Since the electrode is nonrotating, tissue damage is minimized. Either metal or glass microelectrodes may be used with the system, offering the possibility of iontophoresis for cell marking or neuropharmacological manipulations.

Synaptically identified hippocampal slow potentials during behavior.

Averaged evoked slow-wave potentials (AEPs) were recorded from various locations within the dentate gyrus and CA1 field of the hippocampus of 16 rats during the performance of an operant tone discrimination task. Consistent time-locked tone averaged evoked potentials (AEPs) were recorded from the perforant path zone in the outer molecular layer of the dentate gyrus. Tone AEPs were never present in this region (1) prior to development of the discrimination, (2) during behavioral extinction, or (3) during the performance of other operant behaviors. Results are discussed in terms of the functions of the perforant path in neural and behavioral plasticity.

Long lasting changes in the spontaneous activity of hippocampal neurons following stimulation of the entorhinal cortex.

The spontaneous activity of hippocampal and denate units was investigated in acute and chronic experiments on adult rats. Spontaneous unitary discharges from both regions were measured before and after brief tetanic stimulation of the entorhinal cortex. An increase in the frequency of spontaneous unit activity was obtained in units recorded from the dentate gyrus and CA3 field of the hippocampus for up to 20 min following stimulation. In chronic recordings of physiologically identified dentate units from freely moving rats similar results were obtained. The findings are discussed with reference to long lasting potentiation of synaptic activity in the perforant path-dentate granule cell connection.

An experimental test of diaschisis

The diaschisis theory of recovery of function was directly tested for the first time. Using the dentate gyrus in adult rats as the model system, extracellular field potentials elicited by commissural stimulation were monitored in both acute and chronic animal preparations before and from 0.5 hr to 11 days following the lesion. The lesion resulted in the loss of the commissurally elicited long-latency potential but did not disrupt the short-latency monosynaptic potential. No changes in latency, amplitude, form, or stimulus threshold in the monosynaptic potential could be detected.

A dual marking technique for microelectrode tracks and localization of recording sites

This paper describes techniques for marking both microelectrode tracks and exact recording loci using a combination of fast green dye and horseradish peroxidase (HRP). The procedure involves coating the exterior of HRP filled microelectrodes with fast green dye in order to identify electrode tracks, and ejecting HRP from the electrode to mark recording loci. Rapid, multiple marks can be made with this technique without harming the recording capabilities of the micropipette.