Brain Research, 551 (1991)94-103 © 199l Elsevier Science Publishers B.V. 0006-8993/91/$03.50 A DONIS 000689939116651P
94
BRES 16651
Commissural responses of rat retrohippocampal neurons M a s a k o I s o k a w a ~ a n d D a v i d M. Finch 1'2 t Brain Research Institute and 2Department of Neurology, University of California Los Angeles, Los Angeles, CA 90024-1761 (U.S.A.) (Accepted 8 January 1991) Key words: Dentate gyrus; Subiculum; Entorhinal cortex; Inhibition; In vivo recording
Synaptic responses of commissurally activated rat subicular and entorhinal neurons were studied intracellularly in vivo by stimulating the contralateral dentate gyrus. The most prominent synaptic responses in both subicular and entorhinal neurons were inhibitory postsynaptic potentials (IPSPs). IPSPs were generated in combination with antidromic spikes and/or excitatory postsynaptic potentials (EPSPs) and orthodromic spikes. No dependency between any two response types was found. Commissurally projecting subicular neurons (identified by the presence of antidromic spikes evoked by contralateral stimulation) were found, extending previous anatomical studies. Commissurally projecting entorhinal neurons were found in layer II, confirming previous anatomical studies. Positive correlations between antidromic spike latency and depth of recording sites supported the interpretation that axons projected along the fiber bundles of the hippocampal commissures and angular bundle to distribute to their targets. Possible circuits that could have mediated the excitatory and inhibitory responses of these retrohippocampal neurons are considered.
INTRODUCTION
tures and showed that stimulation of the dorsal hippocampal commissure in the guinea pig evoked large field
There have been a n u m b e r of anatomical studies of the hippocampal commissures of diverse mammalian species. These have shown an extensive commissural system in lower mammals with a more restricted system in primates, and have shown that sources for commissural (and crossed associational) fibers include the hilar region of the dentate gyrus3'11'23'27"28'38'4°; hippocampal fields CA4, CA3 and CA13"11'18'23"33"34'35"38'4°;the subicular
potentials in the subiculum and entorhinal cortex. In the present work, we recorded intracellularly from single neurons within the rat subiculum and entorhinal cortex in
complex3'll'22'3°'31"34'4°;
22,41.
and the entorhinal cortex 3'sAI'
There have been fewer physiological studies of the
hippocampal commissures: electrical stimulation of the dorsal hippocampai commissure evokes field potentials in the hippocampus 32, subicular complex 6 and entorhinal cortex 7. Stimulation of the angular bundle evokes field potentials and unit activity in the contralateral dentate gyrus 9. Stimulation of the hippocampus proper evokes field potentials and unit activity in the contralateral hippocampus 4 and dentate gyrus 1°. A n d stimulation of the dentate gyrus evokes synaptic and antidromic responses in contralateral dentate neurons 19. Most relevant to the present work are physiological studies by Buzs~ki and Eidelberg 9, who provided evidence for a feedforward inhibitory action of the hippocampal commissures on the contralaterai dentate gyrus; and by Bartesaghi and Gessi 6 and Bartesaghi et al. 7, who studied the action of the hippocampai commissures on retrohippocampal struc-
order to provide information about their synaptic responses to stimulation of the contralateral hippocampal formation.
MATERIALS AND METHODS Adult male Sprague-Dawley albino rats (Charles River, MA), weighing 210-370 g at the time of surgery, were used for the present study. The animals were anesthetized with chloral hydrate (400 mg/kg, i.p., supplemented by i.m. injections as necessary), and placed in a stereotaxic frame. Animals were kept warm with a Deltaphase Isothermal pad (Braintree Scientific, Braintree, MA), and some received atropine (0.04 mg, i.m.) to reduce respiratory tract secretions. Skull holes were drilled for acute placement of intracellular recording and stimulating electrodes in locations stereotaxically determined from the atlas of Paxinos and Watsonz6. Coordinates for recordings from the subiculum were 1.2-2.7 mm anterior to the interaural line, 4.5 mm lateral from the midline, and 3-6.5 mm deep from the top of the brain (depending on the anterior-posterior coordinate). The same anterior-posterior and lateral coordinates were used for the entorhinal recording. However for the entorhinal recording, electrodes were further advanced in the horizontal plane to reach the structure, which was 5.0-7.5 mm deep from the top of the brain (again depending on the anteriorposterior coordinate). A glass saline-filled recording micropipette of 8-10 M,Q impedance was lowered slowly to the subiculum or the entorhinal cortex. A twisted bipolar fine wire stimulating electrode (50 /~m in
Correspondence: M. Isokawa, Brain Research Institute, Rm 73-364, CHS. UCLA, Los Angeles, CA 90024-1761, U.S.A.
95
RESULTS
36 entorhinal recordings were classified as quasi-intracellular (20-39 mV, with clear synaptic potentials). The remaining 12 out of 34 subicular recordings and 12 out of 36 entorhinal recordings showed smaller a m p l i t u d e action potentials (
94
BRES 16651
Commissural responses of rat retrohippocampal neurons M a s a k o I s o k a w a ~ a n d D a v i d M. Finch 1'2 t Brain Research Institute and 2Department of Neurology, University of California Los Angeles, Los Angeles, CA 90024-1761 (U.S.A.) (Accepted 8 January 1991) Key words: Dentate gyrus; Subiculum; Entorhinal cortex; Inhibition; In vivo recording
Synaptic responses of commissurally activated rat subicular and entorhinal neurons were studied intracellularly in vivo by stimulating the contralateral dentate gyrus. The most prominent synaptic responses in both subicular and entorhinal neurons were inhibitory postsynaptic potentials (IPSPs). IPSPs were generated in combination with antidromic spikes and/or excitatory postsynaptic potentials (EPSPs) and orthodromic spikes. No dependency between any two response types was found. Commissurally projecting subicular neurons (identified by the presence of antidromic spikes evoked by contralateral stimulation) were found, extending previous anatomical studies. Commissurally projecting entorhinal neurons were found in layer II, confirming previous anatomical studies. Positive correlations between antidromic spike latency and depth of recording sites supported the interpretation that axons projected along the fiber bundles of the hippocampal commissures and angular bundle to distribute to their targets. Possible circuits that could have mediated the excitatory and inhibitory responses of these retrohippocampal neurons are considered.
INTRODUCTION
tures and showed that stimulation of the dorsal hippocampal commissure in the guinea pig evoked large field
There have been a n u m b e r of anatomical studies of the hippocampal commissures of diverse mammalian species. These have shown an extensive commissural system in lower mammals with a more restricted system in primates, and have shown that sources for commissural (and crossed associational) fibers include the hilar region of the dentate gyrus3'11'23'27"28'38'4°; hippocampal fields CA4, CA3 and CA13"11'18'23"33"34'35"38'4°;the subicular
potentials in the subiculum and entorhinal cortex. In the present work, we recorded intracellularly from single neurons within the rat subiculum and entorhinal cortex in
complex3'll'22'3°'31"34'4°;
22,41.
and the entorhinal cortex 3'sAI'
There have been fewer physiological studies of the
hippocampal commissures: electrical stimulation of the dorsal hippocampai commissure evokes field potentials in the hippocampus 32, subicular complex 6 and entorhinal cortex 7. Stimulation of the angular bundle evokes field potentials and unit activity in the contralateral dentate gyrus 9. Stimulation of the hippocampus proper evokes field potentials and unit activity in the contralateral hippocampus 4 and dentate gyrus 1°. A n d stimulation of the dentate gyrus evokes synaptic and antidromic responses in contralateral dentate neurons 19. Most relevant to the present work are physiological studies by Buzs~ki and Eidelberg 9, who provided evidence for a feedforward inhibitory action of the hippocampal commissures on the contralaterai dentate gyrus; and by Bartesaghi and Gessi 6 and Bartesaghi et al. 7, who studied the action of the hippocampai commissures on retrohippocampal struc-
order to provide information about their synaptic responses to stimulation of the contralateral hippocampal formation.
MATERIALS AND METHODS Adult male Sprague-Dawley albino rats (Charles River, MA), weighing 210-370 g at the time of surgery, were used for the present study. The animals were anesthetized with chloral hydrate (400 mg/kg, i.p., supplemented by i.m. injections as necessary), and placed in a stereotaxic frame. Animals were kept warm with a Deltaphase Isothermal pad (Braintree Scientific, Braintree, MA), and some received atropine (0.04 mg, i.m.) to reduce respiratory tract secretions. Skull holes were drilled for acute placement of intracellular recording and stimulating electrodes in locations stereotaxically determined from the atlas of Paxinos and Watsonz6. Coordinates for recordings from the subiculum were 1.2-2.7 mm anterior to the interaural line, 4.5 mm lateral from the midline, and 3-6.5 mm deep from the top of the brain (depending on the anterior-posterior coordinate). The same anterior-posterior and lateral coordinates were used for the entorhinal recording. However for the entorhinal recording, electrodes were further advanced in the horizontal plane to reach the structure, which was 5.0-7.5 mm deep from the top of the brain (again depending on the anteriorposterior coordinate). A glass saline-filled recording micropipette of 8-10 M,Q impedance was lowered slowly to the subiculum or the entorhinal cortex. A twisted bipolar fine wire stimulating electrode (50 /~m in
Correspondence: M. Isokawa, Brain Research Institute, Rm 73-364, CHS. UCLA, Los Angeles, CA 90024-1761, U.S.A.
95
RESULTS
36 entorhinal recordings were classified as quasi-intracellular (20-39 mV, with clear synaptic potentials). The remaining 12 out of 34 subicular recordings and 12 out of 36 entorhinal recordings showed smaller a m p l i t u d e action potentials (
