Gary M. Peterson

Loss of GABAergic neurons in medial septum after fimbria-fornix transection.

Neurons in the medial septum of the rat brain undergo retrograde degeneration after transection of their projection to the hippocampal formation, the fimbria-fornix. This cell death has been characterized for both Nissl-stained neurons and acetylcholinesterase-stained neurons. The major cell type in the medial septum is GABAergic, and many of these GABAergic neurons project to the hippocampal formation. Because the fimbria-fornix transection causes more neuronal death than can be accounted for by the loss of cholinergic neurons, we have sought to determine if the GABAergic neurons undergo a cell death similar to that reported for the cholinergic neurons. We report here that GABAergic neurons are indeed lost after the transection but the time course is considerably slower than that for the cholinergic neurons.

Fate of septohippocampal neurons following fimbria-fornix transection: A time course analysis

Neurons in the medial septum (MS) and vertical limb of the diagonal band (vDB) undergo degenerative changes following transection of their axons. These changes have been well studied by histological techniques such as Nissl stains and immunocytochemistry. A dramatic loss of stained neurons occurs following axotomy and this has been interpreted as indicative of neuronal death. However, since the staining intensity and the size of affected neurons may be reduced by axotomy, it is possible that the apparent neuronal death may actually be due to a decrease in somal size or the ability to detect neurons by routine histological methods. The present study describes the effects of axotomy on MS and vDB neurons which have been labeled by hippocampal injections of the retrograde tracer, Fluoro-Gold (FG), prior to transection of the fimbria-fornix and supracallosal stria. The number of FG-labeled neurons in the MS decreased by 21% at three weeks, 36% at six weeks, and 31% at ten weeks after fimbria-fornix transection. The reduction was statistically significant at 6 and 10 weeks. The number of FG-labeled neurons in the vDB showed no reduction at three weeks but was decreased by 31% and 37% at six and ten weeks, respectively. This was statistically significant only at 10 weeks. By comparison, the number of neurons immunoreactive for choline acetyltransferase (ChAT) was reduced by 75-80% at these time points. The size (area and diameter) of FG-labeled somata decreased in both the MS and vDB within three weeks following fimbria-fornix transection and remained relatively constant at the six- and ten-week time points.(ABSTRACT TRUNCATED AT 250 WORDS)

Anterograde and retrograde axonal transport of Phaseolus vulgaris leucoagglutinin (PHA-L) from the globus pallidus to the striatum of the rat

Iontophoretic administration of PHA-L into the globus pallidus of rats resulted in the labeling of neuronal perikarya in the striatum as well as axons and terminals in the striatum, entopeduncular nucleus, subthalamus and substantia nigra. The labeled striatal perikarya were densely stained in Golgi fashion with virtually complete filling of the dendrites and spines. It is concluded that the striatal cells were filled by the retrograde transport of PHA-L and represent either striatopallidal cells, or striatonigral cells whose axons were interrupted as they passed through the injection site. The anterogradely labeled axon terminals in the striatum were observed in close apposition to the dendrites of the retrogradely labeled neurons suggesting the existence of synaptic contacts between the two groups of cells. This study demonstrates that PHA-L can be transported retrogradely as well as anterogradely following iontophoretic injections.

Direct neurotoxic effects of colchicine on cholinergic neurons in medial septum and striatum

The mitotic/microtubule inhibitor, colchicine, is known to be neurotoxic to certain populations of neurons. We report here that a single intraventricular injection of colchicine exerts specific neurotoxic effects on cholinergic neurons while sparing GABAergic neurons in the medial septal nucleus and striatum. It is hypothesized that the colchicine-induced loss of choline acetyltransferase immunoreactive neurons represents degeneration of the cholinergic neurons as a result of disruption of the retrograde axoplasmic transport of a neurotrophic factor.

A quantitative analysis of the crossed septohippocampal projection in the rat brain

SummaryThe projection from the medial septal-diagonal band complex (MSDB) to the hippocampal formation is generally considered to be primarily an uncrossed projection. The present study examined the number of MSDB neurons which project to the contralateral hippocampus, the position of these cells, their identity as either cholinergic or GABAergic, and the question of bilateral collateral projections. The fluorescent dyes Fluoro-Gold or diamidino yellow were injected into 5 equally-spaced sites along the septotemporal extent of the right hippocampus of adult female Sprague-Dawley rats. In some animals True Blue (5%) was also injected in a similar fashion into the left hippocampus. Five days later the brains were processed for fluorescence microscopic examination. The number of retrogradely labeled cells on each side was counted in every fifth section (200 μm interval) through the MSDB and the percentage of contralaterally projecting neurons was calculated. Averaging across all sections and all brains, it was found that approximately 15% of the medial septal neurons and 17% of the diagonal band neurons project to the contralateral hippocampus. Most of the labeled neurons, both ipsilateral and contralateral, were present in the caudal half of the MSDB. Within the medial septum contralaterally placed cells were distributed uniformly throughout the mediolateral extent and some of these stained immunocytochemically for choline acetyltransferase or glutamic acid decarboxylase. In brains in which diamidino yellow was injected into the right hippocampus and True Blue into the left numerous neurons were observed to contain both dyes, thus indicating projections to both hippocampi.

Evidence for the Corelease of Dynorphin and Glutamate from Rat Hippocampal Mossy Fiber Terminals

Abstract: Hippocampal mossy fiber (MF) nerve endings may be isolated in a subcellular fraction (P3) that releases both prodynorphin‐derived peptides and glutamate (Glu) in a calcium‐dependent manner when depolarized. However, this isolation procedure does not yield a pure preparation of MF synaptosomes. The present study evaluates the proportion of dynorphin (Dyn) and Glu that is released from synaptosomes in the P3 fraction that are of MF origin. We have addressed this issue by determining the degree to which a selective lesion of the dentate granule cell/MF system in vivo concomitantly reduces the exocytosis of Dyn and Glu from the P3 subcellular fraction. Unilateral injections of colchicine into the dentate gyrus resulted in a substantial and selective degeneration of the granule cell/ MF pathway in the rat hippocampal formation. The overall integrated density of the Timm‐stained band, which corresponds to the position of the MF terminal field, was estimated to be reduced by 75%. After this extensive loss of MF boutons, the K+‐evoked release of Dyn and Glu from the P3 fraction was reduced by 95 and 51 %, respectively. The loss of Timm staining and evoked Dyn release indicate that colchicine effectively eliminated MF synaptosomes from the P3 fraction. Those subcellular entities that were not destroyed by colchicine comprised ∼50% of the protein and evoked Glu release measured by using the P3fraction. In addition, the present results demonstrate that the inhibitory potency of the K opioid agonist U‐50.488H was not altered by the elimination of MF boutons from this synaptosomal preparation. This finding indicates that U‐50,488H is capable of suppressing Glu exocytosis from both MF and non‐MF synaptosomes. These results are consistent with the hypothesis that Dyn peptides and Glu are coreleased from hippocampal MF terminals.

Morphological evidence for a substance P projection from medial septum to hippocampus

The medial septal nucleus provides one of the major afferents to the hippocampal formation. The two major types of neurons present in the medial septum are cholinergic and GABAergic, but other types of neurons are also present. A small population of substance P-containing neurons is present along the border between the medial and lateral septum, but it is unclear whether these project to the hippocampus. The present study, by employing both anterograde and retrograde tracing techniques, combined with immunocytochemistry for substance P, provides direct morphological evidence for a substance P projection from the lateral region of medial septum to a portion of CA2/3a, which is restricted to the mid-septotemporal portion of the hippocampus.

Studies related to the use of colchicine as a neurotoxin in the septohippocampal cholinergic system.

Colchicine has been shown to be neurotoxic to cholinergic neurons in the medial septum 1 week following intracerebroventricular injections. The experiments described here were designed to examine the selectivity of this effect over a longer time course, and to examine the role of axoplasmic transport in the neurotoxic effect. As previously reported, 1 week after intracerebroventricular injections of colchicine, the numbers of choline acetyltransferase (ChAT)-immunoreactive neurons in the medial septum-diagonal band complex (MSDB) were reduced to 38% of control; this reduction was stable 2 and 3 weeks post injection. Injections of colchicine placed into the body of the fornix produced similar results. GAD-immunoreactive somata, the other major population of neurons in the MSDB, were unaffected 3 weeks following colchicine, as previously reported 1 week following similar injections. The normal AChE staining pattern in the hippocampus, particularly the dentate gyrus, was depleted following either ICV or intrafornical injections of colchicine. This depletion was more severe with longer survival times. Injections of lumicolchicine, an isomer of colchicine which does not bind tubulin, had no effect on ChAT-immunoreactive neurons in the MSDB or on AChE staining in the hippocampus. Injections of colchicine, but not of lumicolchicine, partially blocked the retrograde transport of the fluorescent dye Fluoro-Gold from the hippocampus to the MSDB. In addition, the content of NGF in the hippocampus rose 84% above control values 2 weeks following colchicine and remained elevated at three weeks. Together these results indicate that colchicine is selectively toxic for cholinergic neurons in the septohippocampal system, and suggest that the alkaloids neurotoxic effects work via the blockade of axoplasmic transport.

Delayed development of spontaneous seizures and prolonged convulsive state in rats after massed stimulation of the anterior piriform cortex

We studied the short- and long-term epileptogenic effects of massed stimulation (MS) of the piriform cortex. Sprague-Dawley rats with electrodes implanted bilaterally in the anterior piriform cortex and the dorsal and ventral hippocampi underwent MS: electrical stimulation of the left piriform cortex every 5 min for 6 h (afterdischarge threshold, 60 Hz, 1 ms, 1 s). Animals were retested (5 stimulations) 3-4 times later at different time points to check for the kindled state. Our data showed that MS resulted in delayed development of severe epilepsy. The interval between MS and the first appearance of convulsive response (2 weeks) was characterized by deep refractoriness to seizure (silent period). Unexpectedly, dramatic seizure activity occurred 4-7 weeks after MS. This was manifested by (1) generalized tonic-clonic convulsions with multiple failings, which were elicited repeatedly during retest; (2) frequent progression of elicited generalized convulsions into a prolonged (> 8 min) postictal convulsive state expressed mainly by continuous partial seizures and even new bouts of generalized seizures, and (3) development of mild spontaneous seizures. We found that epileptiform activity predominated in the ventral hippocampus. Mossy fiber sprouting was also most pronounced in this area. We propose that the MS resulted in formation of pathological circuits which involve both piriform cortex and ventral hippocampus and lead to severe epilepsy.

Differential projections to the hippocampus by neurons of the medial septum and vertical limb of the diagonal band

Neurons of the medial septum and vertical limb of the diagonal band of Broca project topographically to the hippocampus through the fornix and fimbria, the supracallosal stria and via a ventral route through the amygdala. Injection of the fluorescent dyes Fluoro-Gold or Diamidino yellow into the fimbria-fornix retrogradely labeled neurons in the medial septum and in the ventromedial portion of the vertical limb of the diagonal band. Injection of True blue into the vicinity of the supracallosal stria labeled only neurons in the dorsolateral portion of the diagonal band. No double labeled neurons were observed. These results indicate that (1) neurons of the medial septum and the ventromedial diagonal band project to (or towards) the hippocampus via the fornix and fimbria, (2) neurons in the dorsolateral portion of diagonal band project via the supracallosal stria, and (3) neurons of the medial septum and diagonal band do not send collaterals via both routes. The differential projection of the two groups of neurons may explain the differences in degenerative changes in the two nuclei after damage to their axons.