Enhanced recognition memory following vagus nerve stimulation in human subjects.

Neuromodulators associated with arousal modulate learning and memory, but most of these substances do not freely enter the brain from the periphery. In rodents, these neuromodulators act in part by initiating neural messages that travel via the vagus nerve to the brain, and electrical stimulation of the vagus enhances memory. We now extend that finding to human verbal learning. We examined word-recognition memory in patients enrolled in a clinical study evaluating the capacity of vagus nerve stimulation to control epilepsy. Stimulation administered after learning significantly enhanced retention. These findings confirm in humans the hypothesis that vagus nerve activation modulates memory formation similarly to arousal.

Cortical edema in moderate fluid percussion brain injury is attenuated by vagus nerve stimulation.

Development of cerebral edema (intracellular and/or extracellular water accumulation) following traumatic brain injury contributes to mortality and morbidity that accompanies brain injury. Chronic intermittent vagus nerve stimulation (VNS) initiated at either 2 h or 24 h (VNS: 30 s train of 0.5 mA, 20 Hz, biphasic pulses every 30 min) following traumatic brain injury enhances recovery of motor and cognitive function in rats in the weeks following brain injury; however, the mechanisms of facilitated recovery are unknown. The present study examines the effects of VNS on development of acute cerebral edema following unilateral fluid percussion brain injury (FPI) in rats, concomitant with assessment of their behavioral recovery. Two hours following FPI, VNS was initiated. Behavioral testing, using both beam walk and locomotor placing tasks, was conducted at 1 and 2 days following FPI. Edema was measured 48 h post-FPI by the customary method of region-specific brain weights before and after complete dehydration. Results of this study replicated that VNS initiated at 2 h after FPI: 1) effectively facilitated the recovery of vestibulomotor function at 2 days after FPI assessed by beam walk performance (P<0.01); and 2) tended to improve locomotor placing performance at the same time point (P=0.18). Most interestingly, results of this study showed that development of edema within the cerebral cortex ipsilateral to FPI was significantly attenuated at 48 h in FPI rats receiving VNS compared with non-VNS FPI rats (P<0.04). Finally, a correlation analysis between beam walk performance and cerebral edema following FPI revealed a significant inverse correlation between behavior performance and cerebral edema. Together, these results suggest that VNS facilitation of motor recovery following experimental brain injury in rats is associated with VNS-mediated attenuation of cerebral edema.

Abnormalities in 5-HT1A and 5-HT1B receptor binding in severe-seizure genetically epilepsy-prone rats (GEPR-9s).

The present study was designed to determine whether abnormalities in serotonin receptor binding co-exist with the presynaptic serotonergic deficits that have previously been identified in the genetically epilepsy-prone rat (GEPR) brain. In vitro binding of [3H]8-OH-DPAT (0.16-10.3 nM) to 5-HT1A receptor sites was found to be decreased in the hippocampus of severe seizure GEPRs (GEPR-9s) when compared to nonepileptic control rats, while no difference in [3H]8-OH-DPAT binding was observed in the GEPR-9 corpora quadrigemina or midbrain tegmentum. The decreased binding of [3H]8-OH-DPAT to hippocampal membranes was due to a decrease in Bmax (P < 0.001), rather than to a change in the Kd. Conversely, in vitro binding of [125I]cyanopindolol (2-400 pM) to 5-HT1B receptor sites was increased in the GEPR-9 hippocampus, corpora quadrigemina and midbrain tegmentum when compared to nonepileptic control rats. The increased binding of [125I]cyanopindolol in all three regions resulted from an increase in the Bmax (P < 0.05), rather than a change in the Kd. These finding suggest that in addition to the innate reduction in 5-HT presynaptic markers, GEPR-9s also exhibit abnormalities in the density of 5-HT1A and 5-HT1B receptors in some regions of the brain. Inasmuch as serotonin acts to attenuate audiogenic seizures in GEPRs, these abnormalities in 5-HT receptor binding may contribute to the seizure susceptibility exhibited by these animals.

Comparative fos immunoreactivity in the brain after forebrain, brainstem, or combined seizures induced by electroshock, pentylenetetrazol, focally induced and audiogenic seizures in rats.

To help discern sites of focal activation during seizures of different phenotype, the numbers of Fos immunoreactive (FI) neurons in specific brain regions were analyzed following "brainstem-evoked," "forebrain-evoked" and forebrain/brainstem combination seizures induced by a variety of methods. First, pentylenetetrazol (PTZ, 50 mg/kg) induced forebrain-type seizures in some rats, or forebrain seizures that progressed to tonic/clonic brainstem-type seizures in other rats. Second, minimal electroshock induced forebrain seizures whereas maximal electroshock (MES) induced tonic brainstem-type seizures in rats. Third, forebrain seizures were induced in genetically epilepsy-prone rats (GEPRs) by microinfusion of bicuculline into the area tempestas (AT), while brainstem seizures in GEPRs were induced by audiogenic stimulation. A final set was included in which AT bicuculline-induced forebrain seizures in GEPRs were transiently interrupted by audiogenic seizures (AGS) in the same animals. These animals exhibited a sequence combination of forebrain clonic seizure, brainstem tonic seizure and back to forebrain clonic seizures. Irrespective of the methods of induction, clonic forebrain- and tonic/clonic brainstem-type seizures were associated with considerable Fos immunoreactivity in several forebrain structures. Tonic/clonic brainstem seizures, irrespective of the methods of induction, were also associated with FI in consistent brainstem regions. Thus, based on Fos numerical densities (FND, numbers of Fos-stained profiles), forebrain structures appear to be highly activated during both forebrain and brainstem seizures; however, facial and forelimb clonus characteristic of forebrain seizures are not observable during a brainstem seizure. This observation suggests that forebrain-seizure behaviors may be behaviorally masked during the more severe tonic brainstem seizures induced either by MES, PTZ or AGS in GEPRs. This suggestion was corroborated using the sequential seizure paradigm. Similar to findings using MES and PTZ, forebrain regions activated by AT bicuculline were similar to those activated by AGS in the GEPR. However, in the combination seizure group, those areas that showed increased FND in the forebrain showed even greater FND in the combination trial. Likewise, those areas of the brainstem showing FI in the AGS model, showed an even greater effect in the combination paradigm. Finally, the medial amygdala, ventral hypothalamus and cortices of the inferior colliculi showed markedly increased FND that appeared dependent upon activation of both forebrain and brainstem seizure activity in the same animal. These findings suggest these latter areas may be transitional areas between forebrain and brainstem seizure interactions. Collectively, these data illustrate a generally consistent pattern of forebrain Fos staining associated with forebrain-type seizures and a consistent pattern of brainstem Fos staining associated with brainstem-type seizures. Additionally, these data are consistent with a notion that separate seizure circuitries in the forebrain and brainstem mutually interact to facilitate one another, possibly through involvement of specific "transition mediating" nuclei.

GABA in the inferior colliculus plays a critical role in control of audiogenic seizures.

Previous studies have implicated a decreased efficacy of GABA as an important defect subserving the audiogenic seizures of the genetically epilepsy-prone rat (GEPR-9). The inferior colliculus (IC) is a critical site for audiogenic seizure (AGS) initiation, and the pontine reticular formation (PRF) is implicated in the propagation of AGS and in other generalized seizure models. The present study observed that microinjection of baclofen, a GABA-B receptor agonist, into IC protects against AGS, and blockade of the breakdown of endogenous GABA by gabaculine, a GABA transaminase inhibitor, increased GABA levels and blocked AGS susceptibility in the GEPR-9. Microinjection of baclofen or gabaculine into the PRF reduced AGS severity, but the doses required were considerably greater and the degree of anticonvulsant effect was less. Uptake of [3H]GABA into GEPR-9 synaptosomes from the IC is significantly increased as compared to normal, which could contribute to the diminished effectiveness of GABA in the GEPR-9. Previous studies indicate that GABA-A receptor agonists block AGS with IC microinjection, and recent data indicate that blockade of GABA uptake in this nucleus significantly reduced AGS severity. These data taken together strongly support the critical importance of the defect in GABA function in the IC in modulating susceptibility to audiogenic seizure initiation in the GEPR-9.

Effect of intracisternal 6-hydroxydopamine on seizure susceptibility in rats.

Rats with 60--70% deficits in brain catecholamines produced by the intracisternal injection of 6-hydroxydopamine were examined by 3 tests of seizure susceptibility. In all procedures (minimal and maximal electroshock and flurothyl convulsions) evidence for increased sensitivity to seizures was obtained. All the animals displayed normal rectal temperature at the times of seizure testing and normal blood chemistry at approximately the time of the third seizure test. These observations strengthen the widely held hypothesis that brain catecholaminergic neurons play a fundamental role in governing sensitivity to convulsions.

In vivo rate of serotonin synthesis in brain and spinal cord of young, spontaneously hypertensive rats.

The accumulation of 5-hydroxytryptophan (5-HTP) following decarboxylase inhibition was measured in hypothalamus (HYP), pons-medulla (PM) and spinal cord (SC) of 4- and 8-week-old spontaneously hypertensive (SH) and normotensive (WKY) rats. Using this method, a significant increase in the in vivo activity of tryptophan hydroxylase was observed in the PM and SC of 4-week-old, but not 8-week-old SH rats. These findings show a transient elevation in rate of 5-HT synthesis prior to the onset of hypertension, which does not appear to continue during the maintenance phase.

Enhancement of the anticonvulsant effect of fluoxetine following blockade of 5-HT1A receptors.

Serotonin reuptake inhibitors, such as fluoxetine, have been shown to exert anticonvulsant effects in several animal models of epilepsy. In view of recent studies showing that 5-HT1A receptor antagonists (somatodendritic autoreceptor antagonists) enhance the increase in extracellular 5-hydroxytryptamine (5-HT, serotonin) produced by serotonin reuptake inhibitors, it was of interest to determine if these antagonists also enhance the anticonvulsant effect of fluoxetine in Genetically Epilepsy-Prone Rats (GEPRs). The 5-HT1A receptor antagonists (-)-pindolol and LY 206130 (1-[1-H-indol-4-yloxy]-3-[cyclohexylamino]-2-propanol maleate) were examined in the present study and both enhanced the anticonvulsant action of fluoxetine in severe seizure GEPRs (GEPR-9s). The latter effect of LY 206130 was found to be dose- and 5-HT-dependent. These findings provide further evidence that the increase in extracellular serotonin observed after administering fluoxetine in combination with a 5-HT1A receptor antagonist is physiologically important and that the anticonvulsant effect of fluoxetine in the GEPR is mediated through an increase in extracellular 5-HT.

Fos in locus coeruleus neurons following audiogenic seizure in the genetically epilepsy-prone rat: comparison to electroshock and pentylenetetrazol seizure models.

Seizures in genetically epilepsy-prone rats (GEPRs) may result from hypoactivity of locus coeruleus (LC) neurons during seizures. This study examined Fos-like-immunoreactivity (FLI) in the LC following audiogenic seizures in two strains of GEPRs (GEPR-9s and -3s), and following pentylenetetrazol (PTZ) or maximal electroshock seizures (MES) in normal rats. After tonic seizure, GEPR-9s showed an identical LC-FLI response to that of normal rats following tonic seizures induced by either PTZ or MES. GEPR-3s, having clonic seizures, had less FLI in the LC. Therefore, stimulus-transcription coupling in the GEPR LC is apparently normo-typic in its FLI response to seizure and thus is not likely the root cause of NE abnormalities in this seizure model.

In vivo increase in hypothalamic cyclic AMP following 5-hydroxytryptophan administration in rats.

The administration of 5-hydroxytryptophan (5-HTP, 100 mg/kg, i.p.) consistently increased hypothalamic cyclic AMP levels in rats treated 10 days earlier with the serotonin neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), to produce 5-HT receptor supersensitivity. However 5-HTP (100 mg/kg), failed to cause an increase in hypothalamic cyclic AMP in rats not pretreated with 5,7-DHT. The 5-HTP-induced increase in cyclic AMP was blocked by the decarboxylase inhibitor, benserazide (RO 4-4602, 800 mg/kg) and by the 5-HT antagonist metergoline (5 mg/kg). Other treatments that caused a significant elevation of hypothalamic cyclic AMP included: (a) L-Tryptophan plus the monoamine oxidase inhibitor, tranylcypromine, and (b) the serotonin agonist, 1-(m-trifluromethylphenyl)-1-piperazine. The 5-HT antagonist, methysergide, blocked the serotonin receptor mediated behavioral syndrome, but failed to prevent the increase in hypothalamic cyclic AMP. Moreover, the 5-HT agonist, 5-methoxy-N, N-dimethyltryptamine, (5-Me-DMT), induced a strong behavioral syndrome but failed to significantly increase hypothalamic cyclic AMP. These findings suggest that activation of 5-HT receptors somewhere in the brain causes an increase in hypothalamic cyclic AMP, but further studies will be needed to determine whether this is a direct result of activation of the 5-HT receptors in the hypothalamus.

Assessment of the effects of a synthetic gonadotropin-releasing hormone associated peptide on hormone release from the in situ and ectopic pituitaries in adult male rats.

The in vivo effects of synthetic human gonadotropin-releasing hormone associated peptide (GAP) were evaluated in adult male rats. In normal rats, intracerebroventricular (III ventricle) injection of 5 ng GAP significantly increased plasma LH levels after 60 minutes. Intracerebroventricular administration of 5, 25 or 125 ng of GAP elevated circulating LH levels also at 120 minutes of injection but did not alter plasma FSH, prolactin or testosterone concentrations. In hypophysectomized-pituitary-grafted rats injection of 125 ng GAP directly into the ectopic pituitary induced no changes in peripheral hormone levels. However, intrapituitary graft injection of 25 ng of GnRH significantly elevated circulating levels of LH and testosterone. These results indicate that the ectopic pituitary graft can respond to acute exogenous GnRH stimulation and that the commercially available synthetic GAP fails to inhibit prolactin release in adult male rats.

Expression of Fos in the superior lateral subdivision of the lateral parabrachial (LPBsl) area after generalized tonic seizures in rats.

Generalized tonic-clonic seizures of brain stem origin in rats are associated with acute induction of neuronal Fos in several discrete regions of the brain. One particular site in the dorsal pons shows remarkable Fos induction following generalized tonic seizures induced by maximal electroshock in normal rats or by audiogenic stimulation in genetically epilepsy-prone rats (GEPRs). Although this area shows the most intense Fos induction of any brain area following generalized tonic seizures, its identity has been uncertain. Based on its general location, we hypothesized that this nucleus was either 1) a component of the pedunculopontine tegmentum nucleus-pars compacta (PPTn-pc) or 2) the superior lateral subnucleus of lateral parabrachial area (LPBsl). The present study used Fos-protein immunocytochemistry in combination with the reduced form of nicotinamide-adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry, cholecystokinin (CCK) immunocytochemistry, and neuronal tract-tracing to determine the identity of this cluster of Fos-immunoreactive neurons in the dorsal pons. Following maximal electroshock seizure (MES), Fos labeling was compared to NADPH diaphorase staining (a marker for cholinergic neurons of the PPTn-pc); retrograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injected into the ventromedial nucleus of the hypothalamus (VMH; to identify the LPBsl) or CCK immunoreactivity (also a marker for LPBsl neurons). Results showed this cluster of Fos immunoreactive (FI) neurons to be closely associated, but not overlapping, with the lateral and most caudal aspect of the PPTn-pc. Alternatively, WGA-HRP retrograde-labeled neurons corresponded precisely with the seizure-induced FI neurons. Additionally, the location of CCK immunoreactive neurons directly overlapped with the FI neurons, although they were not nearly as prevalent. These results demonstrate that the seizure-induced FI neurons in this area are neurons of the LPBsl and not cholinergic neurons of the PPTn-pc. This is the first report of seizure-induced Fos expression specifically localized to the superior lateral subnucleus of the lateral parabrachial area.

Fos expression and 2-deoxyglucose uptake following seizures in developing genetically epilepsy-prone rats.

Juvenile genetically epilepsy-prone rats (GEPR)-3s display one of three types of seizures in response to sound: a typical class 3 seizure consisting of an explosive running/bouncing episode followed by a clonic seizure (audiogenic response score, ARS-3); an ARS-3 seizure followed by a forebrain seizure that includes facial and forelimb (F&F) clonus with rearing (ARS-3f); or, a running/bouncing episode followed by a severe tonic seizure with complete hindlimb extension (ARS-9) not accompanied with subsequent F&F clonus. The adult seizure phenotype, manifest in all GEPR-3s by age 45 days of age, consists of an ARS-3 not followed by F&F clonus or tonic extension. The present studies sought to determine the neuronal networks activated during these various developmental convulsive patterns by examining anatomical patterns of [(14)C]2-deoxyglucose (2-DG) uptake or immediate-early-gene (Fos) expression subsequent to seizures. Many, but not all, brain areas of control rats showed age-related increases in Fos expression in response to the acoustic stimulation. An age effect was not observed in 2-DG uptake. In GEPRs, the profiles of Fos expression and 2-DG uptake following seizures were often parallel; however, there were notable exceptions. For example, increased 2-DG uptake in the cochlear nuclei, central region of the inferior colliculi, and the substantia nigra were not accompanied by increased Fos expression in these areas regardless of the seizure phenotypes. Reciprocally, other regions, particularly in the amygdala, ventromedial hypothalamus and parabrachial areas, displayed intense seizure related Fos labeling without detectable increases in 2-DG uptake. Fos and 2-DG uptake patterns in response to acoustic stimulation varied according to brain region, seizure phenotype and severity. In general, the degree of 2-DG uptake correlated with seizure severity. For example, the ARS-9 seizures, being the most intense, resulted in significant increases in 2-DG uptake in almost all brain regions examined. 2-DG uptake following the ARS-3f and ARS-3 seizures, although increased, did not reach statistical significance in most brain areas. In contrast to the 2-DG findings, a seizure-severity dependent effect was not seen with Fos. Rather, the induction of Fos associated with acoustic stimulation and seizure was more associated with age and seizure-phenotype. Thus, the developmental profiles of Fos expression and 2-DG uptake in response to seizures are distinctly different and concurrent examination of both markers is useful in the identification of brain circuitry involved in seizure development.

Noradrenergic abnormalities in the genetically epilepsy-prone rat.

The genetically epilepsy-prone rat (GEPR) has central nervous system noradrenergic deficits as compared to normal rats. It is possible that these deficits contribute to seizure predisposition because they are exhibited by seizure-naive as well as by seizure-experienced GEPRs. On the basis of pharmacological studies, it is hypothesized that there is an inverse relation between seizure predisposition and levels of noradrenergic activity in brain. Neurochemical studies indicate that deficits exist in areas innervated by both the locus ceruleus and the lateral tegmental noradrenergic systems. These deficits exist in GEPRs without seizure experience and are more pronounced in the severe seizure strain as compared to the moderate seizure strain. We review eight experimental steps undertaken to identify more precisely the anatomical location of noradrenergic determinants of seizure predisposition. These steps illustrate the theoretical bases for the studies and describe the specific experiments completed. Evidence supports the hypothesis that noradrenergic deficits in the superior colliculus and/or ventrally adjacent regions are determinants of seizure predisposition.

Electroshock- and pentylenetetrazol-induced seizures in genetically epilepsy-prone rats (GEPRs): differences in threshold and pattern.

Using facial and forelimb (F&F) clonus (a proposed forebrain marker) and running-bouncing (R/B) clonus and tonus (proposed brain-stem markers), the responsiveness of forebrain and brain-stem to electroshock or pentylenetetrazol seizures was assessed in GEPRs. The most striking finding was the failure of GEPR-9s to display F&F clonus in response to transcorneal electroshock at any stimulus intensity. Indeed, GEPR-9s displayed only R/B clonus or tonus indicative of brain-stem seizure discharge. GEPR-3s and normal rats, on the other hand, displayed F&F clonus in response to the least effective electroshock stimulus, and R/B clonus and tonus at higher stimulus intensities. After treatment with phenytoin (50 mg/kg) to inhibit the tonic seizure, the least effective electroshock stimulus also produced F&F clonus in GEPR-9s. These findings suggest that the threshold for triggering brain-stem seizure discharge by electroshock is lower than that for triggering forebrain seizure discharge in GEPR-9s, whereas the reverse relationship is true in normal rats and GEPR-3s. The rank ordering of the electroshock thresholds was: normals greater than GEPR-3s greater than GEPR-9s. Both GEPR-3s and GEPR-9s were found to be hyper-responsive to pentylenetetrazol as evidenced by shorter latency for the tonic seizure and a greater seizure severity than normal rats. The rank ordering of seizure severity in response to pentylenetetrazol was: GEPR-9 greater than GEPR-3 greater than normal rats.

Injections of noradrenergic and GABAergic agonists into the inferior colliculus: effects on audiogenic seizures in genetically epilepsy-prone rats.

Genetically epilepsy-prone rats (GEPRs) which display tonic seizures (GEPR-9s) in response to acoustic stimulation were used in these studies. Other laboratories have shown that GEPR-9s have a reduced concentration of brain norepinephrine (NE). Previous reports have also indicated that audiogenic seizures (AGS) in these animals are inhibited by treatments that enhance noradrenergic (NA) neurotransmission. AGS in GEPRs are believed to be initiated in the inferior colliculus (IC) where GABA has been shown to exert inhibitory influences in GEPRs that display submaximal AGS. The present study examined whether the IC is a crucial site for NA suppression of tonic seizures by examining the effect of microinfusing NA agonists into the IC. The intracollicular effect of a GABA agonist, muscimol, on sound-induced tonic convulsions in GEPR-9s was also examined. Bilateral microinfusion of NE, phenylephrine, clonidine or isoproterenol failed to alter the AGS. In contrast, muscimol (30 or 60 ng/side) infused into the IC abolished the tonic and clonic components of the AGS in GEPR-9s. These findings suggest that enhancement of GABAergic neurotransmission in the IC markedly attenuates AGS in the GEPR, while augmentation of NA neurotransmission has little effect in this brain region.

Noradrenergic terminal fields as determinants of seizure predisposition in GEPR-3s: a neuroanatomic assessment with intracerebral microinjections of 6-hydroxydopamine.

The genetically epilepsy-prone rat (GEPR) and other mammals with genetically based epilepsy are characterized by an innate predisposition to seizures evoked by a wide variety of stimuli (including those of endogenous origin). The present investigation was undertaken to identify the anatomical location of the noradrenergic terminal fields responsible for regulation of seizure predisposition. In this study, audiogenic seizure severity was used as the index of seizure predisposition. The effect of widespread destruction of noradrenergic terminal fields was compared with the effect of destroying regionally distinct terminal fields. These lesions were produced by microinfusion of 6-hydroxydopamine (6-OHDA) into the locus ceruleus, the A1 noradrenergic area, the noradrenergic dorsal bundle, the cerebellar peduncles and spinal intrathecal space. Selective depletion of norepinephrine in the forebrain, the cerebellum, or the spinal cord failed to alter audiogenic seizure severity. An increase in seizure severity was always associated with marked depletion of norepinephrine in the midbrain excluding the inferior colliculus. Also a significant correlation existed between the seizure intensification and reduction of norepinephrine in this structure in all instances where a seizure intensification was observed. An association of seizure intensification also existed in all cases except one with depletion in the pons/medulla. The present findings support the hypothesis that the noradrenergic terminal fields of the midbrain excluding the inferior colliculus are determinants of seizure predisposition. Inasmuch as audiogenic seizures are a type of brainstem seizure, the present findings do not a priori pertain to the noradrenergic regulation of forebrain seizures.

Neurite extension of developing noradrenergic neurons is impaired in genetically epilepsy-prone rats (GEPR-3s): an in vitro study on the locus coeruleus.

A primary determinant of seizure susceptibility and severity in genetically epilepsy-prone rats (GEPRs), is a generalized deficiency in the central noradrenergic system of these animals. In particular, this deficiency includes reduced numbers of norepinephrine (NE) synaptic terminals in several brain areas and distinctly fewer NE axons within the auditory tectum. Two strains of GEPRs have been developed: GEPR-3s that have moderately severe clonic seizures and GEPR-9s that have severe tonic seizures culminating in complete hindlimb extension. Seizures in animals of each substrain are preceded by a brief episode of wild running. The developmental profile of NE axonal growth in GEPRs compared to control rats is not known, but may be causally related to NE deficiencies in this seizure model. The present study compared developmental neurite extension of fetal NE neurons in vitro between GEPR-3s and Sprague-Dawley control rats, the strain from which GEPR-3s were originally derived. Neurite arborization of individual NE neurons was assessed by quantitative morphometry following immunocytochemical identification of tyrosine hydroxylase (TH). Preliminary studies using explant and dispersed-cell cultures of control-rat tissues showed that optimal culture parameters to support neuritogenesis of LC neurons included the use of dispersed-cell cultures, Pronectin-F substrate, day-14 gestation donor-tissue, no use of cytosine-arabinofuranoside (ARA-c, a glial mitotic inhibitor) and the presence of co-cultured tectal tissue. Compared to fetal control-rat NE neurons co-cultured with fetal control-rat tectum, NE neurons derived from fetal GEPR-3 LC in co-culture with GEPR-3 tectum exhibited only 30% of the neurite extension of control-rat LC neurons and GEPR-3 LC neurons had a similarly deficient amount of branching. This study suggests, but does not prove, that deficiency in tectal NE in GEPR-3s involves a developmental deficiency in neurite extension from GEPR-3 LC neurons. Hypothetically, this deficiency may also contribute to the well described NE deficiency in other regions of the adult GEPR brain.

Neuroanatomical localization of structures responsible for seizures in the GEPR: lesion studies.

Identification of the neural substrates subserving audiogenic convulsions in the GEPR is an important task and while it is not yet complete, many laboratories employing various techniques have contributed importantly to our current understanding. The present review focuses on the use of lesions to identify the neural substrates of audiogenic convulsions. Lesions in brain stem nuclei appear to have a much greater ability to attenuate audiogenic convulsions than do forebrain lesions. In fact, some forebrain lesions (dorsal hippocampus, caudate, intralaminar thalamic nuclei) appear to enhance the severity of audiogenic seizures. On the other hand, bilateral lesions in the inferior colliculus (IC) have been shown to completely abolish audiogenic convulsions, while lesions in the pontine reticular formation (PRF nucleus) abolish all aspects except the running episode suggesting that these two brain stem structures are important neural substrates involved in the expression of audiogenic convulsions. Large bilateral lesions of the substantia nigra also appear to attenuate audiogenic convulsions. The effect of lesions on audiogenic convulsions is basically similar to their effect on other generalized seizure models and the data appear to support the hypothesis that there are two anatomical systems involved in the expression of all generalized convulsions: a forebrain system responsible for the expression of face and forelimb clonus; and a brain stem system responsible in the expression of running-bouncing clonus and tonus.

Variation in threshold and pattern of electroshock-induced seizures in rats depending on site of stimulation.

Although most laboratories employ transcorneal stimulation as a means of producing electroshock seizures, transauricular stimulation is also used by many investigators. The present study shows that seizures produced with transcorneal electroshock differ from those produced by transauricular electroshock in several ways: transauricular stimulation is more effective at eliciting tonic convulsions; the threshold for clonus is lower when transcorneal electrodes are used; and the face and forelimb clonus produced by transcorneal stimulation cannot be produced with transauricular stimulation at any current. The present findings are consistent with the hypothesis that tonic seizures are more easily triggered with transauricular stimulation because they originate in the brainstem and because this brain region is preferentially activated when ear-clip electrodes are used.