A [14C]2-deoxyglucose analysis of the functional neural pathways of the limbic forebrain in the rat. III. The hippocampal formation

Memory networks: answering the call of the hippocampus

Without the hippocampus, experiences disappear without a memory trace. A recent study shows that changing synaptic strength within the hippocampus alters circuit function in widely distributed brain networks.

Long-term cosequences of intrahippocampal kainate injection in the Proechimys guyannensis rodent

The Proechimys guyannensis (PG), a spiny rodent specie living in the Amazonian region has been recently studied as an animal model of anti-convulsant mechanisms. The PG was found to be resistant to the administration of the muscarinic cholinergic agonist pilocarpine or the amygdala kindling development. This study examined the susceptibility of this animal species to the intrahippocampal kainic acid (KA) injection. Electrographic, behavioral and neuropathological changes induced by intrahippocampal KA injections were analyzed. PG showed to be extremely sensitive to the acute effects of the KA injection. Although the EEG findings in PG rodents were similar to those typically obtained in Wistar rats the pattern of electrographic activity in PG animals was longer than in Wistar rats. Neuropathological examinations of PG brains that survived KA-induced SE revealed severe cell loss in CA1/CA3 areas of the hippocampus, an extensive cell dispersion in the hilus of DG at the injected site with mossy fiber sprouting in the dentate gyrus supragranular layer. None of PG animals presented spontaneous seizures during the 120 days of observation. These findings confirm our previous observation on the resistance of this animal specie to experimental models of limbic epilepsy.

Electrophysiological analysis of the morphofunctional organization of the limbic control of magnocellular neurosecretory nuclei in the rat hypothalamus

The effects of electrical stimulation of the ventral hippocampus, ventral subiculum, and corticomedial amygdala were used to obtain a general and comparative assessment of the organization of efferent outputs of limbic system structures to the magnocellular neurosecretory nuclei of the hypothalamus, i.e., antidromally identified neurosecretory cells and other groups of identified neurons in these nuclei and in the perinuclear zones. These studies showed that different efferent outputs of the hippocampal formation provide differential control of spike activity of neurosecretory cells in the supraoptic nucleus, with excitatory pathways from the ventral hippocampus and inhibitory pathways from the subiculum. The effects of the amygdala on neurosecretory cells of the paraventricular nucleus were shown to be excitatory, though they were less significant than the excitatory and inhibitory effects of the hippocampus. It was demonstrated that in general, the effect of limbic structures are addressed predominantly to cells which do not project to the posterior lobe of the hypophysis. Projections were mostly to interneurons, which, as convergence sites for excitatory influences both from limbic structures and neurosecretory cells, may thus be responsible for the involvement of the latter in integrative brain functions.

Mapping of limbic seizure progressions utilizing the electrogenic status epilepticus model and the 14C-2-deoxyglucose method

We have previously described a model of limbic status epilepticus in which chronic prolonged seizure states of immobile, exploratory, minor convulsive or clonic convulsive behavior are induced by intracerebral electrical stimulation; these states appear to belong to the same behavioral progression as kindled seizures. We postulated that the underlying seizure substrates, as mapped by the 14C-2-deoxyglucose method, should reflect a corresponding anatomic progression of discharge spread. Status epilepticus was induced in rat by pulsed-train current delivered for up to 90 min to one of several subcortical areas. Autoradiographs revealed that most of the observed patterns of seizure-induced metabolic activation comprised a hierarchical sequence, such that progressively more extensive patterns subsumed anatomic territories activated in less extensive patterns, thus allowing inferences as to the progression of discharge spread. In this sequence, the basolateral amygdala ipsilateral to the induction electrode was among the first structures to be activated. In successively larger activation patterns a small unilateral network related to basolateral amygdala was involved; this evolved through a transitional state to a unilateral extensive limbic pattern; which in turn was succeeded by bilateral extensive limbic activation. This hierarchical sequence culminated in a neocortical activation pattern, in which most of the forebrain was involved in intense seizure-induced activation. Seizure behaviors increased in severity in correspondence with the underlying seizure-activated anatomic substrate. In contrast, patterns of seizure activation were observed which did not fit within the early stages of the above sequence, although analysis indicates that the later stages of spread may be shared. The study of these patterns and those reported in the literature indicates that although limbic seizure networks may be anatomically distinct at their origination, further expansion is characterized by overlap; upon assumption of extensive patterns of activation the number of nuclei participating is so vast that the identity of the limbic originator is lost and common convulsive manifestations occur.

Propagation of hippocampal seizure activity arising from the hippocampus: a local cerebral blood flow study

We studied the propagation of spike discharges and the changes in local cerebral blood flow (LCBF) by means of the electroencephalogram (EEG) and autoradiographs to elucidate the underlying mechanism and the propagation pathway in rats with limbic seizures. Seizures were induced by the unilateral microinjection of kainic acid (KA) into the dorsal hippocampus during which behavioral, EEG and LCBF changes were documented. Overall, the results indicate an initial spreading to the contralateral hippocampus from the ipsilateral hippocampus, followed by activation of the ipsilateral amygdala, other limbic structures, striatum and sensorimotor cortex. Finally, seizure activity spreads to the ipsilateral globus pallidus, substantia nigra, subthalamic nucleus, thalamus, septum and parietal cortex. The activation of the contralateral hippocampus in the KA model studied here occurs earlier than in the acute amygdaloid seizure model. Elucidation of the mechanisms underlying this difference in the propagation pathway will require the application in parallel of techniques examining these and other aspects of seizure activity.

Are differences in dorsal hippocampal kindling related to amygdala-piriform area excitability?

It has been suggested that several structures associated with the amygdala-piriform (A-P) area are important, if not critical, for convulsive generalization of limbic seizures. In experiment 1, when examining the development of convulsive seizures kindled from the dorsal hippocampus (cornus ammonis; DH), a broad range of kindling rates was observed. Independent of electrode location within the hippocampus, kindling rates were correlated positively with both local and, more dramatically, distant excitability (measured by the duration of the primary and secondary hippocampal afterdischarges, respectively) at all stages of epileptogenesis. Because kindling rates were bimodally distributed, we bisected the distribution into 'faster' and 'slower' kindling rats. Here we examined the magnitude of both electrophysiological and behavioral differences between 'faster' and 'slower' rats and their changes over time. The 'faster' rats had longer primary and secondary afterdischarge (AD) durations than 'slower' rats throughout all stages of kindling. With the appearance of generalized convulsions, the 'faster' rats showed longer latencies to clonus onset, with longer clonus and AD durations than 'slower' rats. Also, the generalized convulsions of 'faster' rats appeared during a much enlarged secondary AD period, while 'slower' rats convulsed during primary AD. In both groups, convulsions were invariably associated with increased discharge in A-P associated structures. We interpreted the differences between 'faster' and 'slower' DH rats to reflect genetic differences in excitability in both local and A-P associated structures. If the DH kindling profile of the 'faster' rats differed from 'slower' rats largely because of naturally greater excitability in A-P associated structures, then experimentally increased excitability in those structures (via amygdala kindling) in a random sample of rats should duplicate much of the 'faster' DH kindling profile. In experiment 2, this outcome was observed. In conclusion, we suggest that either natural or induced differences in the excitability of A-P associated structures affect both the genesis and the profile of convulsive generalization of limbic kindled seizures.

Septotemporal variation of the supragranular projection of the mossy fiber pathway in the dentate gyrus of normal and kindled rats

Previous studies have demonstrated regional variation in the anatomical organization and physiological properties of the hippocampus along its septotemporal (dorsoventral) axis. In this study, regional variation of the supragranular projection of the mossy fiber pathway in the dentate gyrus of normal and kindled rats was characterized with a scoring method for assessment of the distribution of mossy fiber synaptic terminals detected by Timm histochemistry. In normal rats, there was a sparse projection of the mossy fiber pathway into the supragranular region near the tips and crest of the dentate gyrus along the entire septotemporal axis, and a prominent projection into the supragranular region at the temporal pole. Kindling of the perforant path, amygdala, and olfactory bulb induced synaptic reorganization of the mossy fiber pathway into the supragranular region along the entire septotemporal axis of the dentate gyrus. There was regional variation of the seizure-induced synaptic reorganization along this axis, and distinct septotemporal patterns were observed as a function of the site of kindling stimulation. Kindling of the perforant path induced mossy fiber synaptic reorganization that was relatively more prominent in the septal pole than in the temporal pole of the dentate gyrus. In contrast, rats that received kindling stimulation of the amygdala had a more uniform distribution of synaptic reorganization along the septotemporal axis. As there is regional variation of the anatomical and physiological properties of the human epileptic hippocampus, these observations could be pertinent to human epilepsy.

Cerebral hypermetabolism produced by intraventricular endothelin-1 in rats: inhibition by nimodipine

Injection of endothelin-1 (9 pmol) into the lateral cerebral ventricle of rats produced barrel-rotations, convulsions, tonic hindlimb extensions, facial clonus, and kinetic ataxia for up to 45 min. Quantitative metabolic autoradiographs produced from the [14C]deoxyglucose method and analyzed over 60 individual brain structures or subregions revealed intense hypermetabolism in periventricular tissues close to the injection site and in many of their efferent projection sites. Histological examination of these areas proved that this dose of endothelin was without toxic or ischemic effects on neurons or glial cells. Structures metabolically affected ipsilateral to injection were caudate nucleus (+164%), lateral septal nucleus (+270%), and two white matter tracts--corpus callosum (+236%) and hippocampal fimbria (+318%). Distant stimulated structures included cerebellar cortical layers, but not cerebellar nuclei or white matter. Increased rates of glucose metabolism among many other nuclei, particularly distinct subunits of the hippocampal formation and structures in contact with the ventricular system, signify that endothelin induced widespread metabolic stimulation over much of the neuraxis. Furthermore, although the 9 pmol concentration of endothelin produced convulsive movements and diverse metabolic stimulation, it did not evoke detectable electroencephalographic seizure activity assessed by intra- or extracerebral electroencephalography. Both the convulsions and hypermetabolic activation were inhibited by intraventricular pretreatment with the dihydropyridine calcium-channel antagonist, nimodipine. The results identify endothelin-1 as a calcium-mediated 'convulsive' peptide with selective stimulatory effects on cerebral glucose metabolism.

Plasticity in the propagation of hippocampal stimulation-induced activity: a [14C]2-deoxyglucose mapping study

Previous work has suggested that the rewarding effect of hippocampal stimulation develops only as a consequence of neuroplastic change resulting from repeated stimulation experience: naive rats will not initiate self-stimulation for several days, but a prior program of hippocampal kindling greatly facilitates acquisition of self-stimulation. In the current study, the metabolic tracer [14C]2-deoxyglucose was used to measure functional activity during a session of repeated electrical stimulation of the hippocampus to determine whether the resulting activation propagates more widely in experienced hippocampal self-stimulators than in naive rats receiving the same programmed stimulation. A control group of naive unstimulated rats was also included. The results indicated that dorsal CA3 stimulation in naive rats failed to increase metabolic activity in the hippocampus, while in experienced rats, the stimulation produced significant bilateral activation of CA1, CA3, and the ventral subiculum. These results provide support for the hypothesis that development of the rewarding effect of hippocampal stimulation is associated with more widespread propagation of stimulation-induced activity.

Effects of unilateral intrahippocampal injection of MK-801 upon local cerebral glucose utilisation in conscious rats

The effects of unilateral intrahippocampal injections of the non-competitive NMDA receptor antagonist, MK-801, on local cerebral glucose utilisation have been examined in conscious rats using [14C]2-deoxyglucose autoradiography. The intrahippocampal injection of MK-801 (10 nmol) induced significant marked increases in glucose use in the ipsilateral hippocampus molecular layer and dentate gyrus (by 31 and 44%, respectively). Function-related glucose use in brain regions with known neuronal connections with the site of drug administration (e.g. entorhinal cortex, septal nucleus, mamillary body) was minimally altered after intrahippocampal MK-801 administration. Blockade of hippocampal NMDA receptors does not appear to modify activity, as reflected in local glucose utilisation, in hippocampal afferent and efferent circuits in conscious rats.

Electrophysiological investigation of the hippocampal projections to the neurosecretory cells of the supraoptic nucleus of the rat hypothalamus

The impulse activity of antidromically identified neurosecretory cells of the supraoptic nucleus of the hypothalamus of rats in response to stimulation of the ventral hippocampus was investigated. Short-latency phasic excitation reactions were identified, and inhibition reactions were not found. The presence of excitatory synaptic inputs from the hippocampus to other neurons of the nucleus and of the perinuclear zone, which are predominant by comparison with analogous projections to the neurosecretory cells, was demonstrated. The features of limbic-hypothalamic relationships are discussed in the context of afferent control of the activity of the neurosecretory cells.

Subcortical metabolic alterations in partial epilepsy

The function of subcortical nuclei in partial epilepsy was investigated using positron emission tomography (PET) to measure metabolism in the basal ganglia and thalamus. Sixteen patients undergoing surgical evaluation were studied with 18F-fluorodeoxyglucose (FDG) interictally and had intensive extracranial and intracranial electrophysiologic evaluations. Eight patients had left temporal lobe seizure foci, six had right temporal lobe foci, and two had right posterotemporal or parietal foci. The PET data were analyzed visually and quantitatively, using a multivariate analysis of variance on the quantitative data. Hypometabolism of subcortical nuclei was present ipsilateral to the cortical seizure focus. Cortical hypometabolism was noted focally in the temporal lobe in patients with left temporal lobe seizure foci, whereas patients with right temporal lobe seizure foci had diffuse hemispheric hypometabolism. We postulate that the subcortical hypometabolism is secondary to decreased efferent activity from temporal lobe structures, in particular amygdala and hippocampus, to subcortical nuclei. Diminished subcortical activity may then lead to defective regulation of cortical excitability in the temporal lobe, increasing the likelihood of seizure development and spread.

Relief of seizures from a predominantly posterior temporal tumor with anterior temporal lobectomy

We report the relief of intractable complex partial seizures in a patient with a posteromesial temporal lobe hamartoma after anterior temporal lobectomy, despite minimal tumor removal. We suggest that the key to successful treatment is the mainly medial, or limbic, location of the tumor, which apparently requires anterior limbic structures for full clinical expression of seizures. We conclude that excision anterior to a posterior temporal lesion can result in seizure relief and that a medial tumor location may be important for successful treatment.

Restriction of enhanced [2-14C]deoxyglucose utilization to rhinencephalic structures in immature amygdala-kindled rats

Sixteen-day-old albino rat pups were kindled to varying degrees of seizure severity with amygdala stimulations spaced 15 to 20 min apart. Subsequently, each rat pup was injected (ip) with 10 microCi of [2-14C]-deoxyglucose, and received several additional kindled seizures at regular intervals throughout the following 80 min, at which time it was killed and processed for deoxyglucose autoradiography. Increased seizure severity was associated with correspondingly increased deoxyglucose utilization in many rhinencephalic limbic structures. However, unlike adults, rat pups did not show discernibly increased neocortical, thalamic, or substantia nigra utilization. We postulate that the apparent confinement of seizure activity to limbic structures in pups is related to their relative lack of postictal seizure refractoriness, as well as to other indices of increased seizure susceptibility in immature animals.

Functional [14C]2-deoxyglucose mapping of progressive states of status epilepticus induced by amygdala stimulation in rat

Electrical stimulation of rat amygdala induced self-sustained steady-state seizures (status epilepticus (SE] within 60 min. These SE states varied in behavioral severity from mere alteration of motility to frank clonic convulsions. Four distinct behavioral states were observed: immobility, exploration, mastication and clonus. These SE states were associated with [14C]2-deoxyglucose (2-DG) autoradiography anatomic patterns that were correspondingly more extensive and complex. Four distinct 2-DG activation patterns were observed: a restricted pattern involving several discrete limbic nuclei, including amygdala; more extensive patterns involving numerous limbic areas, first unilaterally, then bilaterally; finally the most extensive pattern involving widespread areas of forebrain. These data imply a systematic progression of seizure activity: originating in the amygdala, then spreading to some direct amygdala projection areas, and from there to a restricted network of interconnected ipsilateral limbic nuclei. This restricted network then recruits most of the remaining limbic structures, first ipsilaterally, then contralaterally. Finally, most of the basal ganglia, thalamus and neocortex are recruited.

Brain output dysregulation induced by olfactory bulbectomy: an approximation in the rat of major depressive disorder in humans?

Mounting evidence indicates that the emotional, cognitive, neurovegetative and behavioral symptoms of patients with major depressive disorder are due to abnormal neurochemical substrates in the brain. Although the specific neurochemical abnormalities responsible have not been identified, the presenting symptoms of major depression are consistent with a disruption of normal neural communications between the limbic system and hypothalamus. Following removal of the olfactory bulbs, rats display a syndrome of behavioral deficits that also reflect a disruption of the limbic-hypothalamic axis. Moreover, the bulbectomy induced deficits are selectively reduced by the chronic administration of the same drugs that alleviate the symptoms of depression when given chronically to the patients. In addition to this pharmacological similarity, there are also numerous behavioral parallels between bulbectomized rats and major depression patients. The bulbectomized rat provides a good model in which to study antidepressant drugs and also may provide neurochemical and neuroanatomical data that are relevant to understanding the biological substrates of emotion and the causes of depression in humans.

The patterns of [14C]2-deoxyglucose uptake in female rat brain produced by electrical stimulation of hypothalamic and limbic brain areas

The aim of this study was to investigate the pattern of [14C]2-deoxyglucose uptake in anaesthetized rat brain produced by electrical stimulation of brain areas implicated, by previous electrical stimulation studies, in the neural control of pituitary hormone and especially gonadotrophin secretion. Stimulation of the median eminence led to a significant increase in the relative metabolic activities of the arcuate, ventromedial hypothalamic, supraoptic and paraventricular nuclei and the preoptic area. Stimulation of the suprachiasmatic or paraventricular nuclei or the medial preoptic area, anterior hypothalamic area, the dorsal or ventral hippocampus or amygdala led to an increase in the relative metabolic activity of many brain regions known to have direct connections with these areas, but in addition produced increases in the relative metabolic activity of areas which have secondary connections. Hippocampal stimulation confirmed previous neuroanatomical findings of major intrinsic functional connections between different fields of the ipsilateral and contralateral hippocampus. Stimulation of the amygdala, unexpectedly, did not change the relative metabolic activity of the arcuate nucleus and medial preoptic area which have neuroanatomical connections with the amygdala. Similarly, stimulation of the medial preoptic area did not change significantly the relative metabolic activity of the mamillary body and dorsomedial thalamic area. The effect of preoptic area stimulation on the relative metabolic activity of several brain regions was changed by ovariectomy and by injection of oestradiol benzoate. Stimulation of the preoptic area and suprachiasmatic nuclei, but not the anterior hypothalamic area or other brain regions, increased significantly the plasma concentrations of luteinizing hormone. These results show that (i) electrical stimulation of brain areas concerned with the control of gonadotrophin and other pituitary hormone secretion changes the metabolic activity of nuclei and neural pathways extrinsic as well as intrinsic to the hypothalamic-pituitary system, (ii) the [14C]2-deoxyglucose method can detect changes in antidromic as well as orthodromic activity and in multi-synaptic pathways, (iii) neuroanatomical pathways are not always activated metabolically by electrical stimulation, and (iv) the preoptic-suprachiasmatic nucleus gonadotrophin control system is discrete and is little affected by increased metabolic activity of the hypothalamus produced by stimulation of the anterior hypothalamic area or other brain areas.

The pathways mediating affective defense and quiet biting attack behavior from the midbrain central gray of the cat: an autoradiographic study

The purpose of this study was to describe the pathways which mediate feline affective defense and quiet biting attack behavior elicited from the midbrain central gray. In these experiments, methods of [3H]leucine and 2-deoxy-[14C]glucose (2-DG) radioautography were utilized in concert with the technique of electrical and chemical brain stimulation. Affective defense behavior elicited from the midbrain central gray is characterized by marked vocalization such as hissing and growling, pupillary dilatation, urination and piloerection. In contrast, quiet biting attack elicited from the midbrain central gray lacks overt autonomic signs observed with affective defense response as well as the stalking component which is typically associated with stimulation of the lateral hypothalamus. Nevertheless, central gray-elicited attack resulted in a directed bite of the neck of an anesthetized rat in a manner similar to that observed from the hypothalamus. Affective defense was elicited from the dorsal half of the midbrain central gray, while quiet biting attack was obtained following stimulation of the ventral half of the midbrain central gray, thus indicating a functional differentiation of the central gray with respect to these two forms of aggression. In a separate series of experiments, affective defense or quiet biting attack response was identified by electrical stimulation through a cannula electrode situated in the midbrain central gray. The affective defense responses were subsequently elicited following microinjections of D,L-homocysteic acid through the same cannula electrode in order to demonstrate that these responses were the result of direct stimulation of cell bodies within the central gray. Then, one of the following autoradiographic tracing procedures was utilized: (1) [3H]leucine was injected through a cannula electrode and the animal was sacrificed after a 4- to 14-day survival period; or (2) a 2-DG solution was systemically injected and electrical stimulation was applied through the cannula electrode in order to metabolically activate the pathways associated with each of these responses. In general, the pattern of labelled target regions as indicated by 3H-amino acid radioautography was similar to that obtained from the 2-DG autoradiographic analysis. The principal ascending pathway associated with affective defense was traced to the anteromedial hypothalamus and medial thalamus. Concerning descending projections, label was traced into the central tegmental fields of the midbrain and pons, locus coeruleus and motor and main sensory nuclei of the trigeminal complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Histamine sensitive sites in hippocampus: their probable role on prolactin release in male rats

The effect of histamine (HA), 3-methyl-histamine (3MHA), HA antagonists or drugs interfering with HA synthesis, microinjected into the hippocampus (HPC), on prolactin (PRL) secretion were studied in rats. Three experiments were performed. In Experiment 1, increasing doses of HA or 3-MHA (9-90 nmol) were microinjected stereotaxically into the ventral HPC of adult male rats. In Experiment 2, 135 nmol of pyrilamine (PYR, an H1-HA-antagonist) or ranitidine (RAN, an H2-HA-antagonist) were administered locally into the ventral HPC. Fifteen min later, the rats were microinjected again with 45 nmol of HA. In Experiment 3, rats were microinjected with different doses of HA-antagonists or with 20 nmol of alpha-fluormethyl-histidine (FMH, an inhibitor of the enzyme of HA synthesis) and later subjected to an immobilization stress of 15 min duration. In all cases, the PRL plasma concentrations were measured in blood samples taken at different time intervals (0-120 min) after the last brain injection. Results showed that HA applied locally in ventral HPC induced an increase in PRL levels which was statistically significant from saline-injected rats between 5-30 min after the HPC stimulation. On the contrary, local applications of 3-MHA did not change significantly the PRL blood levels (Experiment 1). Only PYR did block partially the PRL response due to HA in basal conditions. RAN in these later conditions had no effect (Experiment 2). When animals were subjected to stress neither PYR nor RAN, alone or in combination, locally applied were able to block the PRL increase due to stress. Only FMH blunted significantly the hormone response.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium-induced epileptiform activity in area CA3 varies markedly along the septotemporal axis of the rat hippocampus

Hippocampal slices are generally treated as equivalent regardless of their site of origin along the septotemporal axis. In this study, spontaneous epileptiform bursting was induced in area CA3 of rat hippocampal slices by bathing them in 7 mM potassium. The frequency of spontaneous bursting was measured in all viable slices from 12 hippocampi. Burst frequency was found to vary markedly and in a consistent fashion with site of slice origin along the septotemporal axis. Burst frequency was maximal in slices from near the temporal end and declined progressively toward the septal end. This finding was independent of slicing angle. These results demonstrate that site of slice origin along the septotemporal axis is an important confounding variable in in vitro studies of hippocampal neuronal activity. Furthermore, they support the notion that the temporal portion of the hippocampus may be more prone to seizure activity than the septal hippocampus, possibly because of factors intrinsic to the hippocampus.

Regional cerebral blood flow during enkephalin-induced seizures in the rat

Blood flow, determined by the radioactive microsphere technique during epileptiform seizures induced by [D-Ser2,Leu5]enkephalyl-Thr (DSLET), a specific delta-opioid receptor agonist, was examined in different areas of the brain of the rat at various time intervals. An increase in blood flow to the hippocampus and brain stem was observed 2.5 min after administration of DSLET into the left lateral ventricle. An additional increase in flow occurred in the striatum and cerebellum 2.5 min later (5 min after the injection), at which time both the neural and vascular effects of the drug were most marked. Ten minutes after the administration of the drug, cerebral blood flow in all regions except the hippocampus, returned to the respective baseline values. Since the time-course and the magnitude of functional activity and blood flow in the hippocampus showed a good correlation, it is suggested that this region of the brain may play an essential role in triggering and maintaining the seizure phenomena induced by enkephalin.

Kindling of the hippocampus induces spatial memory deficits in the rat

Since kindling produces electrophysiological and morphological changes in the brain area stimulated, it may well affect behavioural functions dependent on the kindled area. Using an 8-arm maze, it was found that hippocampal kindling can induce specific memory deficits in spatial tasks. Reference (long-term) memory as well as working (short-term) memory were impaired. The largest impairment was observed during the period in which generalized convulsions occurred. Working memory but not reference memory impairment was reversible. Hippocampal kindling may be a useful experimental model for investigating behavioural deficits correlated with epileptogenesis.

Functional mapping of limbic seizures originating in the hippocampus: a combined 2-deoxyglucose and electrophysiologic study

The pathways by which seizures spread from the hippocampus were studied both with multiple electroencephalographic recordings and 2-deoxyglucose autoradiography. The rapid kindling model described in the previous report was employed to compare mild versus severe limbic seizures. Seizures were accompanied by an increased glucose utilization in localized brain areas. The transition from mild to severe limbic seizures involved a greater spatial extent of paroxysmal electroencephalographic activity and metabolic signals. However, electrical recordings proved more sensitive in mapping seizures, as regions shown to be involved in mild or severe limbic seizures with electrical recordings did not necessarily show an increased glucose metabolism. Three types of circuits are important in dissemination of these seizures: interhippocampal connections, pathways leading out of the hippocampus to other limbic regions, and connections to certain extralimbic areas. The nucleus accumbens, amygdala, and substantia nigra emerge as important relay points in the spread of hippocampal-based seizures.

A [14C]2-deoxyglucose analysis of the functional neural pathways of the limbic forebrain in the rat. V. The septal area

The [14C]2-deoxyglucose (2-DG) metabolic mapping technique has been used to identify the regions responding with an augmented rate of metabolism following focal electrical stimulation of various sites within the lateral septal nucleus and medial septal nucleus/diagonal band (MSN/DB) complex in the rat. Since 2-DG uptake has been correlated with rates of functional activity, it was the intention of this study to suggest the anatomical substrates underlying various physiological and behavioral responses elicited by stimulation of the septal area. The results show that stimulation of any region within the lateral septal nucleus produced a profound bilateral activation of both the lateral septal nucleus, as well as the hippocampal formation. While stimulation of a number of different fiber systems associated with the lateral septum could contribute to the observed pattern of labeling, the data suggest that, functionally, a major consequence of such stimulation is the antidromic activation of CA3----lateral septum fibers to axonal branch points, beyond which, orthodromic propagation of the impulse produces activation in CA3 target regions, including subfields CA1 and CA3, as well as the lateral septal nucleus, bilaterally. In addition, regions typically manifesting metabolic activation following stimulation of the lateral septal nucleus included the ipsilateral diagonal band of Broca, nucleus accumbens, lateral preoptic area and lateral hypothalamus, posteriorly, and the prelimbic cortex, anteriorly. Occasionally, target regions of the postcommissural fornix, including the medial mammillary nucleus and anterior thalamic nuclei were also activated following stimulation of the lateral septal nucleus. In contrast to the widespread pattern of activation resulting from stimulation of the lateral septal nucleus, stimulation of the MSN/DB complex produced activation which was largely confined to the medial forebrain bundle. In a final phase of the experiment, afterdischarge activity was elicited by sodium penicillin injection into the lateral septal nucleus. Such treatment produced more widespread 2-DG uptake, including more extensive activation within the lateral septal nucleus, hippocampal formation, amygdala, and thalamus. Additionally, the prefrontal cortex and temporal neocortex were activated.

Positron emission tomography: human brain function and biochemistry

Positron emission tomography (PET) is an analytical imaging technique that provides a way of making in vivo measurements of the anatomical distribution and rates of specific biochemical reactions. This ability of PET to measure and image dynamic biochemistry builds a bridge between the basic and clinical neurosciences founded on the commonality of the types of measurements made. Clinical findings with PET in humans are suggesting hypotheses that can be tested rigorously in the basic science laboratory.

The role of the anterior hypothalamus in affective defense behavior elicited from the ventromedial hypothalamus of the cat

In the preceding paper a hypothalamic circuit subserving feline affective defense behavior was described. This circuit included an ascending component from the ventromedial nucleus to the anterior hypothalamus and a descending component from the anterior hypothalamus to the midbrain central gray substance. The present study was undertaken to test the hypothesis that the anterior hypothalamus plays a central role in the organization of this functional pathway. In the first part of this study, dual stimulation methods were utilized to demonstrate that concurrent stimulation of the ventromedial hypothalamus facilitates the occurrence of affective defense responses elicited from the anterior hypothalamus. In the second part of the study, lesions placed in the anterior hypothalamus significantly increased the latency and threshold current for affective defense responses elicited from the ventromedial hypothalamus. [14C]2-deoxyglucose autoradiography confirmed the fact that anterior hypothalamic lesions effective in blocking affective defense were placed in regions where the vast majority of ventromedial hypothalamic fibers terminate. In contrast, lesions which had little or no effect upon the latency or threshold for affective defense elicited from the ventromedial hypothalamus appeared to leave intact the connections from the ventromedial to the anterior hypothalamus. These findings are consistent with the proposed intrahypothalamic anatomical substrate subserving affective defense behavior described in the preceding paper.

The organization of the hypothalamic pathways mediating affective defense behavior in the cat

The purpose of this study was to describe the hypothalamic pathways which mediate affective defense in the cat utilizing the methods of [14C]2-deoxyglucose (2-DG) and [3H]leucine radioautography in concert with the technique of electrical brain stimulation. The feline affective defense response, characterized by pupillary dilatation, piloerection, ear retraction, hissing, growling and striking with the forepaws, was elicited consistently by stimulation of sites within the ventromedial hypothalamus and anterior aspect of the medial hypothalamus. In one series of experiments, 2-DG autoradiography was employed to describe the brain regions activated following stimulation of sites in the region of the ventromedial hypothalamus from which affective defense had been elicited. Ventromedial hypothalamic stimulation produced activation primarily in forebrain regions situated rostral to the level of the stimulating electrode. These structures included principally the anteromedial hypothalamus and medial preoptic area, as well as the bed nuclei of the stria terminalis and anterior commissure, diagonal band and lateral septal area. The caudal extent of activation included only the dorsal and perifornical hypothalamus at the level of the stimulation site. In a second series of experiments, affective defense sites in the anteromedial hypothalamus were stimulated and the regional distribution of 2-DG label was identified. In contrast to the results obtained from ventromedial hypothalamic stimulation, these experiments revealed a marked descending distribution of label within the posterior hypothalamus, midbrain central gray and ventral tegmental area. Results obtained from studies in which tritiated amino acids were injected into affective defense sites in both the ventromedial nucleus and anteromedial hypothalamus confirmed the general findings observed with 2-DG autoradiography. From these observations, we have concluded that the organization of the pathway mediating affective defense behavior from the ventromedial hypothalamus to the midbrain involves an initial synapse within the region of the anteromedial hypothalamus and a second synapse in the midbrain central gray substance. The significance of the anteromedial hypothalamus for the expression of affective defense behavior was considered in the Discussion.

Seizures produced by pilocarpine in mice: a behavioral, electroencephalographic and morphological analysis

Increasing doses of pilocarpine, 100-400 mg/kg, were given intraperitoneally to mice and the resulting behavioral, electroencephalographic and neuropathological alterations were studied. No behavioral phenomena were observed in mice treated with the lowest dose of pilocarpine. Occasional tremor and myoclonus of hindlimbs were found in animals which received pilocarpine in a dose of 200 mg/kg. At doses of 300, 325 and 350 mg/kg, pilocarpine produced a sequence of behavioral alterations including staring spells, limbic gustatory automatisms and motor limbic seizures that developed over 15-30 min and built up progressively into a limbic status epilepticus lasting for several hours. The highest dose of pilocarpine, 400 mg/kg, was generally lethal to mice. Pilocarpine produced both interictal and ictal epileptiform activity in the electroencephalogram (EEG). The earliest EEG alterations appeared in the hippocampus and then spread to cortical areas. EEG seizures started 10-15 min after injection of large doses of pilocarpine, 300-350 mg/kg. Ictal periods lasted for 1-2 min, recurred every 5-10 min and were followed by periods of depression of the EEG activity. By 30-45 min paroxysmal activity resulted in a status epilepticus. Examination of frontal forebrain sections with light microscopy revealed a widespread damage to several brain regions including the hippocampus, amygdala, thalamus, olfactory cortex, neocortex and substantia nigra. Scopolamine, 10 mg/kg, and diazepam, 10 mg/kg, prevented the development of convulsive activity and brain damage produced by pilocarpine. The results emphasize that excessive and sustained stimulation of cholinergic receptors can lead to seizures and seizure-related brain damage in mice. It is proposed that systemic pilocarpine in mice provides a useful animal model for studying mechanisms of and therapeutic approaches to temporal lobe epilepsy.

Phagocytic and metabolic reactions to intracerebral electrical stimulation of rat brain

Electrical stimulation of the brain has been used with a variety of different electrode types and different stimulating intensities. Few studies have explored neural damage resulting from electrical stimulation that may render the experiment unreliable. The present study used conventional intracerebral bipolar metal stimulating electrodes, delivered a range of defined charge levels, and systematically measured "acute" metabolic effects on glucose (2-deoxy[14C]glucose) metabolism as well as "chronic" (96 h poststimulation) neuronal death and phagocytosis at the electrode surfaces. Neither charge density per phase (QD) nor total charge density per phase (TQD) were independent predictors of "safe" or damaging stimulation; however, together they provided ranges of clearly safe or damaging stimulating conditions that lasted 1 to 2 h. The lowest QD (11.11 microC/cm2/ph) was safe even at 9.1 TQD but not when the pulses totaled 9.6 TQD in 4 h. The middle QD (22.22) was safe even at 9.3 TQD but not when the 2-h pulsing totaled 11.0 TQD. The highest QD (44.44) was safe at 0.4 and 0.8 TQD but irreversibly damaging at 1.6 and 9.6 TQD. These comparisons demonstrated conditions suitable for 1 to 2 h of safe stimulation, but did not provide evidence on the cumulative effects of longer periods or higher frequencies.

Anterior thalamic afferents from the mamillary body and the limbic cortex in the rat

Anterior thalamic afferents from the mamillary body and the limbic cortex were studied by using single and double retrograde transport methods in the rat. The medial mamillary nucleus was divided on the basis of the cytoarchitecture into four subnuclei: the pars medialis centralis, pars medialis dorsalis, pars lateralis, and pars basalis. Extensive connections were seen between each of these subdivisions of the mamillary body and the anterior thalamic nuclei, topographically organized so that the anteromedial thalamic nucleus receives projections exclusively from the pars medialis centralis, while the anteroventral thalamic nucleus receives projections from the pars medialis dorsalis and pars lateralis. Nuclei in the dorsal half of these two mamillary subdivisions project predominantly to the medial half of the anteroventral thalamic nucleus, and those in the ventral half to the lateral half of the nucleus. The pars basalis was found to have numerous projections to the magnocellular part of the anteroventral nucleus. All limbic cortical areas send projections bilaterally to all regions of the anteromedial nucleus as well as to the parvicellular parts of the anteroventral thalamic nucleus, while the anterodorsal nucleus receives ipsilateral projections originating exclusively from the preagranular, anterior limbic, and cingular regions. The magnocellular part of the anteroventral nucleus, however, receives only ipsilateral projections from all of the limbic cortex. Some neurons in the infralimbic region also project bilaterally to all of the anterior thalamic nuclei except the anterodorsal nucleus. All of these cortical projections to the anterior thalamus originate in layers V and VI of the limbic cortex.

A [14C]2-deoxyglucose analysis of the functional neural pathways of the limbic forebrain in the rat. IV. A pathway from the prefrontal cortical-medial thalamic system to the hypothalamus

The present study utilized the [14C]2-deoxyglucose (2-DG) cell labeling procedure to characterize a functional pathway from the prefrontal cortex (Pfc) and mediodorsal thalamic nucleus (MD) to the hypothalamus. Rats were injected with 2-DG prior to a 45 min experimental paradigm consisting of alternating 30 s on-off periods of electrical brain stimulation. Standard procedures were utilized for the removal and processing of brain tissue for X-ray autoradiography. In the first phase of this study, stimulation applied to the prefrontal cortex generally yielded a pattern of 2-DG distribution consistent with the findings of classical anatomical studies. Stimulation of the dorsomedial and ventromedial prefrontal cortex or the infralimbic cortex produced the most effective activation of the diencephalon. This activation was primarily limited to MD, with no involvement of any region of the hypothalamus. In the second phase of this study, brain regions activated following stimulation of sites along the rostro-caudal axis of MD were examined. Stimulation of MD resulted in the activation of the nucleus reuniens and other midline and non-specific thalamic nuclei. Stimulation of this nucleus also activated the ventromedial thalamic nucleus, medial aspects of the nucleus accumbens and the medial and sulcal prefrontal cortices. Again, in each of these cases, labeling within any region of the hypothalamus could not be detected. Since MD stimulation activated the midline thalamus, and the nucleus reuniens in particular, the last phase of this experiment involved stimulation of the nucleus reuniens in order to determine the source of medial thalamic inputs to the hypothalamus. Stimulation of the nucleus reuniens activated fibers which were distributed to both the medial and lateral hypothalamus. In addition, stimulation also activated the descending periventricular system, which could be followed to the level of the midbrain central gray and such limbic structures as the hippocampal formation, septal area, amygdala and prefrontal cortex. These findings indicate that Pfc-MD activation of the hypothalamus is achieved indirectly via interneurons within the nucleus reuniens.

An analysis of the mechanisms underlying septal area control of hypothalamically elicited aggression in the cat

This experiment was performed in order to examine several of the underlying mechanisms by which the septal area and adjacent regions regulate quiet biting attack behavior elicited from electrical stimulation of the hypothalamus in the cat. The results clearly indicate that stimulation of the septal area and anterior cingulate gyrus increased the latency for the occurrence of quiet biting attack behavior. Those sites within the septal area from which inhibition of attack can be produced are linked to sensory mechanisms associated with trigeminal reflexes activated during hypothalamic stimulation. Stimulation of these septal area sites decreased the lateral extent of the 'effective sensory fields' of the lipline established during hypothalamic stimulation, but did not appear to have any affect upon the latency of the hypothalamically elicited jaw-opening response. Deoxyglucose autoradiography revealed that the inhibition resulting from stimulation of the lateral septal area may be due to either the monosynaptic activation of the lateral hypothalamus or the disynaptic activation of this area utilizing a circuit involving the nuclei of the diagonal band of Broca.

Neural pathways mediating hypothalamically elicited flight behavior in the cat

This study has sought to identify hypothalamic pathways mediating flight behavior in the cat. Flight behavior, characterized by an initial pupillary dilatation and followed by vigorous attempts to leap out of the observation chamber, was elicited primarily by electrical stimulation of the medial preoptic region and dorsomedial hypothalamus, and to a lesser extent from the perifornical region. A [14C]-2-deoxyglucose analysis was utilized to examine brain regions functionally activated by stimulation of hypothalamic sites which elicited flight behavior. In a second series of experiments, [3H]leucine injected into regions surrounding electrode tips from which flight had previously been elicited, permitted identification of pathways arising from such functionally characterized sites. We describe for the first time pathways arising from the hypothalamus which mediate flight behavior. In spite of individual variation in placement of electrodes eliciting flight, a consistent pattern of labeling was observed following injection of either [14C]-2-deoxyglucose systemically or [3H]amino acids into the hypothalamus. The primary rostral target structures receiving inputs from flight electrode sites included the nuclei of the diagonal band, bed nucleus of the stria terminalis, medial amygdaloid nucleus, lateral septal nucleus, and anterior medial preoptico-hypothalamus. Caudal to the level of stimulation, the principal target nuclei involved the centrum medianum-parafascicular complex and the midbrain central gray substance. Possible roles of these nuclear regions in organization and regulation of flight behavior is discussed.

Metabolic rate in different rat brain areas during seizures induced by a specific delta opiate receptor agonist

The glucose utilization during specific delta opiate agonist-induced epileptiform phenomena, determined by the [14C]2-deoxyglucose technique (2-DG), was examined in various rat brain areas at different time intervals. The peak in EEG spiking response and the most intensive 2-DG uptake occurred 5 min after intraventricular (i.v.t.) administration of the delta opiate receptor agonist. The most pronounced 2-DG uptake at this time interval can be observed in the subiculum, including the CA1 hippocampal area, frontal cortex and central amygdala. A general decrease of glucose consumption, compared to control values, is observed after 10 min, in all regions, with exception of the subiculum. Since functional activity and 2-DG uptake are correlated, we suggest that the subiculum and/or CA1 area, are probably the brain regions most involved in the enkephalin-induced epileptic phenomena.

The topological anatomy of the hippocampus: a clue to its function

The anatomy of the hippocampus and dentate gyrus is considered with respect to the topology of its afferents and efferents. The topological attributes of the hippocampal formation represent a four-dimensional array with the demensions being: the afferent input to laminated dendritic zones, the intrinsic tri-synaptic hippocampal system, the longitudinal and commissural association systems, and time. Within this four-dinmensional array, active foci can vary dynamically in space and time, depending upon the pattern of afferent activity. The features of hippocampal topology may relate to the role of the hippocampal formation in information storage and processing.

Opioid-induced epileptogenic phenomena: anatomical, behavioral, and electroencephalographic features

Recent animal studies have indicated a possible role of opioids in epilepsy. Intraventricular opioid administration induces a prolonged nonconvulsive stuporous state characterized by epileptiform electroencephalographic patterns, and reversed by naloxone. In high doses, naloxone itself causes generalized clonic convulsions. We compared opioid-induced and naloxone-induced epileptogenic phenomena using quantitative 2-deoxyglucose autoradiography in order to define the anatomical structures involved in these two different seizure types. When opioid-induced seizures occurred, limbic structures were preferentially activated, but when naloxone-induced clonic convulsions occurred, pyramidal and extrapyramidal motor areas and some limbic structures were activated. Based on the present experiments and currently available evidence, we speculate that opioid-mediated epileptogenic phenomena are similar to those occurring during the postictal state of a fully kindled seizure, whereas naloxone-induced epileptogenic phenomena are similar to the ictal state. Therefore, simple pharmacological manipulation of endogenous opioid systems may allow selective study of ictal and postictal phenomena.

Metabolic influence of the hippocampus on hypothalamus, preoptic and basal forebrain is exerted through amygdalofugal pathways

The spread of ventral hippocampal afterdischarge activity to nuclei of the basal diencephalon was studied in rats with fornix lesions and combined lesions of the fornix and stria terminalis. The results indicate that a major hippocampal influence on specific hypothalamic, preoptic and basal forebrain structures is conveyed by way of the amygdala's stria terminalis. An additional hippocampal influence on certain basal forebrain nuclei is consistent with projections via the ventral amygdalofugal pathway.

The use of positron emission tomographic scanning in epilepsy

Positron emission tomography (PET) with fluorine-18-labeled fluorodeoxyglucose (18FDG) has demonstrated the epileptogenic lesion in partial epilepsy to be hypometabolic interictally . This finding is useful for localizing the area of resection when surgical therapy is contemplated. 18FDG scans during partial seizures show increased metabolism in areas of ictal onset and spread and in other regions of decreased metabolism that could reflect postictal effects. In the generalized epilepsies, petit mal absences and generalized convulsions induced by electroconvulsive shock therapy (ECT) are associated with global hypermetabolism, while global hypometabolism is seen in the postictal period following ECT. More information about the factors that influence the interictal hypometabolic zone in partial epilepsy should improve the diagnostic value of this finding for presurgical localization and perhaps also for the evaluation of other therapeutic regimens. New techniques for more dynamic PET studies with improved resolution, combined with computerized electroencephalographic analysis, should allow more accurate interpretation of ictal, as well as interictal, phenomena. Application of PET technology to other paroxysmal disorders may provide a basis for new diagnostic classifications that have therapeutic and prognostic value and may allow clearer differentiation among epileptic phenomena, myoclonus, and movement disorders. More clinical and animal research is needed, however, before we can delineate fundamental mechanisms of human epilepsy from PET data. To this end, it is now possible to use combined multidisciplinary parallel approaches in patients and animals to define specific aspects of epileptic disorders clinically, to intensively investigate them with experimental models in the animal laboratory, and to verify the relevance of these experimental results by returning to clinical studies.

Hippocampal kindling in the rat: intrastructural differences

Kindling patterns were compared in different regions of the hippocampus of the rat. The duration, morphology, and frequency of the electrical responses were different after stimulation of the dorsal and the ventral hippocampus, but these parameters showed the same evolution as kindling progressed. Comparison of the threshold intensity and of the kindling rate revealed some differences not only between dorsal and ventral hippocampus, but also within the dorsal hippocampus: The central layers had higher thresholds and longer kindling periods than the outer layers and the fimbria hippocampi. We suggest that stimulation of hippocampal efferents is responsible for the greater ease of kindling in the outer layers than in the central regions of this structure. The degree of connection of the different parts of the hippocampus with the amygdala and the influence of the intrinsic inhibitory GABAergic circuitry are discussed.

Role of the amygdala in development of hippocampal kindling in the rat

The influence that the amygdala may exert on the development of hippocampal kindling was investigated using three different approaches: (i) after uni- or bilateral amygdalectomy by thermocoagulation, (ii) after prestimulation of the amygdala until the appearance of masticatory movements, and (iii) after increasing the GABA concentration in the amygdala with gamma-vinyl GABA. Hippocampal kindling was not significantly modified in amygdalectomized animals. On the contrary, prestimulation of the ipsilateral amygdala facilitated the subsequent hippocampal kindling. Finally, microinjection of gamma-vinyl GABA in both amygdalae either reduced the seizures to a prekindled level or strongly delayed the appearance of the motor signs of kindling. It is suggested that hippocampal kindling may develop preferentially through the amygdala.