Electrical activity generated in subicular and entorhinal cortices after electrical stimulation of the lateral and basolateral amygdala of the rat

Evoked field potentials and extracellular unitary activity were recorded from entorhinal lateral and subicular ventral cortices under conditions of amygdala stimulation in equithesin-anesthetized rats. The stimulation of the lateral and basolateral nuclei of the amygdaloid complex evoked field potentials consisting of negative-positive waves in layers III-VI and positive-negative deflections in the superficial layers of the ventral subdivision of the entorhinal cortex. The stimulation of the lateral nucleus evoked similar potentials in the dorsal subdivision of this cortex. And the stimulation of the lateral and basolateral nuclei of the amygdala evoked negative-positive field potentials in layer III of the subicular cortex. Cellular activity of the entorhinal and subicular cells evoked by stimulation of the lateral and basolateral nuclei consisted of an excitatory response followed by a prolonged suppression period. This activation coincided with the negative potential recorded in the deeper layers of these cortices. Such observations provide support for amygdaloid projection to the entorhinal and subicular cortices as recent anatomical findings suggested. The functional significance of these observations indicate an amygdaloid influence on entorhinal-hippocampal neurotransmission as well as on the ventral subiculum which provides the major output from the hippocampus.     

Influences of dorsal and ventral hippocampus on hypothalamic self-stimulation

The effects of concurrent stimulation or bilateral ablation of either dorsal or ventral hippocampus on hypothalamic self-stimulation were investigated. Moderate concurrent electrical stimulation of the dorsal hippocampus did not affect the rate of positive self-stimulation elicited from the lateral hypothalamus. Avoidance responses to electrical stimulation of medial or ventromedial hypothalamus were not affected by the dorsal hippocampus stimulation either. Bilateral lesions in the dorsal hippocampus did not change behavioral reactions elicited by the electrical stimulation of any site of the hypothalamus. Concurrent electrical stimulation of the ventral hippocampus did not change negative responses elicited from medial or ventro-medial hypothalamus, but sharply inhibited positive self-stimulation behavior in the lateral hypothalamic animals. Bilateral ablation of the ventral hippocampus had no effect on the positive behavioral responses, but if made in the animals with negative responses, it converted well expressed aversive behavior (escape, thumping etc.) into positive postural self-stimulation responses. Although the rate of the latter was below the lateral hypothalamic self-stimulation rates, nevertheless the acquired responses were perfectly stable. It is suggested that the ventral hippocampus exerts an inhibitory influence upon the hypothalamic motivational centers and the general meaning of this influence must be of protective nature.     

Cortical influence upon cerebellar Purkinje cells responding to natural,peripheral stimulation in the cat

In anesthetized cats cerebellar Purkinje cell activity was recorded. During different sections of a passive wrist movement, electrical stimulation was applied to the corresponding hand region of the sensory-motor cortex, eliciting small dorsal or ventral flexions. The discharge patterns of simple and complex spikes of the Purkinje cell, evoked by passive movements, could be modified by the cortical stimulation either facilitating or diminishing the responses or disrupting the established relationship between complex and simple spikes of the Purkinje cell.     

Climbing fibre inputs to cerebellar Purkinje cells from trigeminal cutaneous afferents and the SI face area of the cerebral cortex in the cat.

1. An investigation was made of climbing fibre (CF) activity evoked in single Purkinje cells in the cerebellum by electrical stimulation of trigeminal cutaneous afferents, the face area of the SI cortex, and the superficial radial nerve, in Nembutal-anaesthetized cats. In addition, both the extent of the cutaneous receptive fields of individual Purkinje cells on the face and the CF responses evoked in these cells by controlled natural stimulation were examined. 2. The pattern of convergence from these inputs on to individual Purkinje cells was found to be complex. DF responses were evoked in 67% of trigeminally-activated cells by electrical stimulation of more than one trigeminal branch. An excitatory CF convergence from the face area of the SI cortex was found on 68% of trigeminally activated cells; 23% also responded to stimulation of the superficial radial nerve. 3. In 81% of the Purkinje cells which were activated by trigeminal nerve stimulation, CF responses were readily elicited by gentle mechanical stimulation of the facial skin. A qualitative relationship was established between the size of the peripheral field of a Purkinje cell, and the probability of convergence on to that cell from the SI cortex. 4. Recordings were made from a limited number of Purkinje cells which were responsive to electrical stimulation of bilateral trigeminal branches and the superficial radial nerve. However, CF responses in these cells often could be elicited only by high intensity stimulation. It is suggested that these cells are analogous to, or perhaps an extension of the population of Purkinje cells described in the anterior lobe which are excited via the ventral funiculus pathway in the spinal cord only by stimulation of 'flexor reflex afferents' in bilateral limb nerves.     

Effects of subcortical ablations on cortical association responses in the cat

The effects of subcortical lesions on cortical polysensory association area responses to peripheral sensory and direct reticular stimulation were investigated in acute chloralosed cats. Large lesions of the mesencephalic reticular formation were followed by a reduction in the amplitude of polysensory association responses to approximately 0–20% of control levels, with little or no reduction of evoked potentials in the primary sensory cortical areas. Unilateral lesions of posterior medial thalamus or the rostral pole of the thalamus had similar effects of polysensory cortical association responses, and also abolished cortical association area responses to direct electrical stimulation of the mesencephalic reticular formation, but only in the ipsilateral cortex. More ventral lesions, in the subthalamic region, reduced nonspecific evoked responses of orbital cortex to both peripheral and reticular stimulation without impairing responses in dorsal cortical association areas. The results are discussed with regard to the pathways of sensory input to the cortical association responses areas.     

Differentiation of electrical rhythms and functional specificity of the hippocampus of the rat

Electrical activity was recorded from dorsal and ventral hippocampus and from motor cortex of the rat during acquisition and performance of a conditioned discriminated avoidance task. Comparisons of the EEGs taken during the trials indicated that the theta response in the dorsal hippocampus did not habituate, while in the ventral hippocampus, the theta response which was present at the beginning of training was replaced by desynchronization. High voltage bilateral brain stimulation applied during performance of the conditioned avoidance task suppressed performance of the response when stimulation was applied to the dorsal hippocampus or to premotor cortex, but not when applied to the ventral hippocampus. The possible inhibitory motor role of the dorsal hippocampus of the subprimate is discussed, and the analogies between the subprimate ventral hippocampus and the higher primate hippocampal formation are pointed out.     

Functional connections in the human temporal lobe

Summary Connections in the human mesial temporal lobe were investigated using brief, single pulses of electrical stimulation to evoke field potential responses in limbic structures of 74 epileptic patients. Eight specific areas within these structures were stereotactically targeted for study, including amygdala, entorhinal cortex, presubiculum, the anterior, middle and posterior levels of hippocampus and the middle and posterior levels of parahippocampal gyrus. These sites were studied systematically in order to quantitatively assess the response characteristics and reliability of responses evoked during stimulation of pathways connecting the areas. Specific measures included response probability, amplitude, latency and conduction velocities. The results are assumed to be representative of typical human limbic pathways since all recordings were made interictally and response probabilities across sites were not found to differ significantly between non-epileptogenic vs. identified epileptogenic regions. Field potentials ranging in amplitude from less than 0.1 to greater than 6.0 mV were evoked ipsilaterally, with mean onset latencies and conduction velocities ranging from 4.4 ms and 3.64 m/s in the perforant pathway connecting entorhinal cortex to anterior hippocampus to 24.8 ms and 0.88 m/s in the pathway connecting the amygdala and middle hippocampus. Stimulation of presubiculum and entorhinal cortex were most effective in evoking widespread responses in adjacent limbic recording sites, whereas posterior parahippocampal gyrus appeared functionally separated from other limbic sites since its probability of influencing ipsilateral sites was significantly lower than any other area. It was particularly noteworthy that stimulation did not evoke responses in any sites in contralateral hippocampal formation; even though a large number of sites were tested with bilateral implantation of homotopic electrodes. The absence of evidence for a functional contralateral limbic projection in the human brain stands in marked contrast to the anatomical and physiological evidence in lower animals for strong contralateral connections between subfields of the hippocampus via the hippocampal commissure. In addition, it correlates well with anatomical evidence for reduced hippocampal commissural connections in lower primates.     

The contribution of transcortical pathways to long-latency stretch and tactile reflexes in human hand muscles

Long-latency electromyographic (EMG) responses can be evoked in the first dorsal interosseous muscle (FDI) by unexpected slips of an object (skin stretch) held between the index and thumb, or by forcible adduction of the metacarpophalangeal joint (muscle stretch). The former type of response is due to stimulation of tactile afferents in the skin of the digits, whereas the latter also activates muscle receptors. Previous studies have provided good evidence that long-latency reflex responses to stretch of distal muscles involve activity in a transcortical reflex pathway. The present experiments examined whether cutaneous reflexes also utilise a transcortical route. Transcranial magnetic or electrical stimuli were given over the motor cortex to evoke EMG activity during the period of the long-latency reflex response. When evoked by muscle stretch the responses to magnetic stimulation were facilitated more than those to electric stimulation. In contrast, facilitation was equal during the long-latency reflex elicited by cutaneous stimulation. Because of the different ways in which electrical and magnetic stimuli are believed to activate the motor cortex, we interpret these results to mean that the long-latency response to skin stretch is not mediated by a transcortical mechanism in the majority of subjects, whereas that following muscle stretch is. However, these are average data. In a few individual subjects, the opposite results were obtained. We suggest that there may be differences between subjects in the transcortical contribution to long-latency reflex responses. The implication is that, under normal circumstances, several pathways may contribute to these responses. If so, the relative roles of the pathways may change during different tasks, and in pathological states lesions in one system may well be accompanied by compensatory changes in other systems.     

Differential effects of colchicine lesions of dentate granule cells on wet dog shakes and seizures elicited by direct hippocampal stimulation

Direct electrical stimulation of either the dorsal or ventral hippocampal formation elicits wet dog shakes and overt seizures. Destruction of dentate granule cells in the dorsal hippocampal formation does not significantly reduce the number of wet dog shakes elicited by ventral hippocampal stimulation. However, destruction of dentate granule cells in the ventral hippocampus virtually eliminates wet dog shaking elicited by dorsal hippocampal stimulation. Destruction of either dorsal or ventral dentate granule cells lowers the threshold for eliciting forelimb clonus with rearing. These results suggest that dentate granule cells in the ventral hippocampus are essential for wet dog shakes elicited by intrahippocampal stimulation. However, dentate granule cells throughout the hippocampal formation appear to play an important inhibitory role in the spread of seizure activity within the hippocampus.     

"Ventral" area in the rat auditory cortex: a major auditory field connected with the dorsal division of the medial geniculate body

The rat auditory cortex is made up of multiple auditory fields. A precise correlation between anatomical and physiological areal extents of auditory fields, however, is not yet fully established, mainly because non-primary auditory fields remain undetermined. In the present study, based on thalamocortical connection, electrical stimulation and auditory response, we delineated a non-primary auditory field in the cortical region ventral to the primary auditory area and anterior auditory field. We designated it as "ventral" area after its relative location. At first, based on anterograde labeling of thalamocortical projection with biocytin, ventral auditory area was delineated as a main cortical terminal field of thalamic afferents that arise from the dorsal division of the medial geniculate body. Cortical terminal field (ventral auditory area) extended into the ventral margin of temporal cortex area 1 (Te1) and the dorsal part of temporal cortex area 3, ventral (Te3V), from 3.2-4.6 mm posterior to bregma. Electrical stimulation of the dorsal division of the medial geniculate body; evoked epicortical field potentials confined to the comparable cortical region. On the basis of epicortical field potentials evoked by pure tones, best frequencies were further estimated at and around the cortical region where electrical stimulation of the dorsal division of the medial geniculate body evoked field potentials. Ventral auditory area was found to represent frequencies primarily below 15 kHz, which contrasts with our previous finding that the posterodorsal area, the other major recipient of the dorsal division of the medial geniculate body; projection, represents primarily high frequencies (>15 kHz). The posterodorsal area is thought to play a pivotal role in auditory spatial processing [Kimura A, Donishi T, Okamoto K, Tamai Y (2004) Efferent connections of "posterodorsal" auditory area in the rat cortex: implications for auditory spatial processing. Neuroscience 128:399-419]. The ventral auditory area, as the other main cortical region that would relay auditory input from the dorsal division of the medial geniculate body to higher cortical information processing, could serve an important extralemniscal function in tandem with the posterodorsal area. The results provide insight into structural and functional organization of the rat auditory cortex.     

Polysynaptic activation of the dentate gyrus of the hippocampal formation: An olfactory input via the lateral entorhinal cortex

Summary The possibility that olfactory input is transmitted to specific subregions of the hippocampal formation via the entorhinal cortex was investigated electrophysiologically by analyzing the laminar profiles of potentials evoked in the hippocampal formation by stimulation of the lateral olfactory tract (LOT). LOT stimulation resulted in long latency (14–20 ms) evoked responses in the dentate gyrus of the hippocampal formation ipsilateral to the stimulation. The variable long latency of these responses and their inability to follow stimulus rates of 40/s suggested that these potentials reflected polysynaptic activation. Analysis of the laminar profiles of the evoked potentials indicated that the responses originated from a synaptic field localized in the outer portion of the stratum moleculare of the dentate gyrus, a terminal distribution which overlaps that of the lateral entorhinal cortical (LEC) projection to the dentate gyrus. Lesions of the LEC eliminated the long latency responses in the dentate gyrus evoked by LOT stimulation. In addition, a conditioning pulse delivered either to the LOT or to the LEC produced paired pulse potentiation of the response elicited by subsequent stimulation of the other structure. No evidence was found to indicate that responses were generated in regio superior of the hippocampus proper following LOT stimulation. Taken together, these results suggest that stimulation of the LOT activates the dentate gyrus of the hippocampal formation by multisynaptic pathways which relay through the lateral portion of the entorhinal area. This finding is discussed with regard to entorhinal cortical organization and the known olfactory projections to the LEC.Some of this material was presented in abstract form at the 7th Annual Meeting of the Society for Neuroscience, 1977     

The functional anatomy of middle-latency auditory evoked potentials: thalamocortical connections.

1. An 8 x 8-channel microelectrode array was used to map epicortical field potentials from a 4.375 x 4.375-mm2 area in the right parietotemporal neocortex of four rats. Potentials were evoked with bilaterally presented click stimuli and with electrical stimulation of the ventral and dorsal divisions of the medial geniculate body. 2. Epicortical responses to click stimuli replicated earlier findings. The responses consisted of a positive-negative biphasic waveform (P1a and N1) in the region of primary auditory cortex (area 41) and a positive monophasic waveform (P1b) in the region of secondary auditory cortex (area 36). Two potential patterns, one at the latency of the N1 and the other at the latency of the P1b, were used to represent activation of cells within areas 41 and 36. A linear combination of these patterns was sufficient to explain from 90 to 94% of the variance of the evoked potential complex at all latencies. 3. In the same animals, epicortical responses to electrical stimulation of the ventral and dorsal divisions of the medial geniculate body were also localized to areas 41 and 36, respectively. A linear combination of potential patterns from these separate stimulation conditions was sufficient to explain from 80 to 93% of the variance of the original click-evoked potential complex at all latencies. 4. These data provide functional evidence for anatomically defined topographical thalamocortical projections to primary and secondary auditory cortex. They suggest that short-latency cortical evoked potentials (10-60 ms poststimulus) are dominated by parallel thalamocortical activation of areas 41 and 36.     

Activity-related extracellular potassium transients in the neonatal rat spinal cord: an in vitro study

Transient increases and decreases in extracellular potassium (delta[K+]o) were recorded from the gray matter of hemisected, neonatal rat spinal cords isolated from 3, 4, 9- and 10-day-old pups. delta[K+]o were evoked in both the ventral and dorsal regions of the gray matter by electrical stimulation. In the ventral horn, repetitive stimulation of the ventral root was required to elicit detectable delta[K+]o. By contrast, single dorsal root stimuli evoked clear delta[K+]o. In the dorsal horn, single orthodromic stimuli elicited delta[K+]o as large as 4-5 mM from a baseline of 4.5 mM. With repetitive stimulation the [K+]o reached, but never exceeded, a ceiling of 10-11 mM. Undershoots were seen only after repetitive stimulation. Spontaneous delta[K+]o were observed in the ventral horn and were well correlated with ventral root activity. Spontaneous delta[K+]o were rare in the dorsal cord, but were recorded after bath application of apamin or tetraethylammonium. The magnitude and distribution of evoked K+ transients and postsynaptic components of the evoked field potential were well correlated in both the dorsal and the ventral gray matter. delta[K+]o were reversibly blocked by 1 mM CdCl2 in the bath and diminished by 1 mM BaCl2. Bath application of mephenesin, apamin or tetraethylammonium diminished evoked delta[K+]o in a stimulus-dependent manner. In apamin and tetraethylammonium, decreases from control responses were largest with high intensity stimulation, the opposite was the case with mephenesin. These results are interpreted in terms of the spinal circuits activated by high- and low-intensity electrical stimulation. We conclude that activity-related delta[K+]o in neonatal spinal cord are large enough to modulate neuronal electrical activity and the [K+]o is well regulated compared to other immature CNS regions studied. Thus, local increases in [K+]o may, by modulating neuronal activity, play a role in neonatal spinal cord developmental processes.     

The effect of stimulation of the reticulo-hypothalamic-hippocampal systems on the cerebral blood flow and neocortical and hippocampal electrical activity in cats

Summary The effect of stimulation of the medial and lateral reticulo-hypothalamic-hippocampal (RHH) systems on cerebral blood flow (CBF) and electrical activity of the hippocampus and neocortex was examined in 19 encéphale isolé cats. ECoG was recorded from posterior sigmoid gyri and marginal gyri and hippocampal activity from dorsal hippocampus. Changes in hippocampal activity were evoked by electrical stimulation of RHH systems. CBF was measured by external monitoring of the clearance of ¹³³Xe given as a single bolus in the carotid artery. Stimulation of the lateral system resulted in desynchronisation of ECoG and hippocampal activity without changes in CBF. Stimulation of the medial system elicited desynchronisation in ECoG modulated by theta-like synchrony, theta activity in the hippocampus and a 45% CBF increase. After atropine administration, low frequency, high voltage waves appeared in both ECoG and hippocampal activity, but no change in CBF was observed. During stimulation of the medial system there were no changes in the type of electrical activity but the CBF response was still preserved (increase by 50%). Stimulation of the lateral system did not change either the type of electrical activity or the CBF. The results indicate that the two systems of neuronal pathways, which mediate two different patterns of electrical response in the dorsal hippocampus but similar ECoG activity in the neocortex, elicit different CBF responses. It is argued that the alterations of electrical activity of the neocortex and hippocampus mediated by these two pathways depend on the cholinergic system, whereas the CBF changes depend on a different mechanism.     

Involvement of the trisynaptic hippocampal pathway in generating neural representations of object–place associations (an analytical review)

The possible mechanisms by which neural representations of object–place associations are generated in different parts of the network consisting of the hippocampus and the parahippocampal complex are analyzed. Spatial and non-spatial information arrives in the hippocampus via two streams from the parahippocampal complex, which consists of the perirhinal, postrhinal, and entorhinal areas of the cortex. It can be suggested that because there are no connections between the lateral and medial areas of the entorhinal cortex, these representations, as particular patterns of connected and discharging neurons, are generated mainly in the hippocampus, though they may also be generated in the entorhinal cortex because of the input from the postrhinal cortex. As both information streams converge on neurons in the dentate gyrus and field CA3, the trisynaptic pathway through the hippocampus may play a key role in generating these representations. As spatial information arrives in the neocortex and passes from there via the parahippocampal complex to the hippocampus about 20 msec earlier than non-spatial information, spatial information is processed first in the dentate gyrus and field CA3. Later, because of the return of excitation from field CA3c to the dentate gyrus, neural representations of object–place associations start to be generated in the dentate gyrus. Signals are transferred from the dentate gyrus to field CA3, where information arriving from the entorhinal cortex is superimposed on the neuronal patterns activated by these signals. As a result, more complex neural representations are generated in field CA3 and signals are sent to field CA1. In the dorsal (ventral) part of field CA1, non-spatial (spatial) information arriving from the lateral (medial) part of the entorhinal cortex is superimposed on the activated neuronal pattern. The result is that higher-order representations are generated in field CA1. In the parahippocampal cortex, the generation of neuronal representations of object–place associations can result from the transfer of activity from the dorsal part of hippocampal field CA1.     

Somatic afferent input to posterior thalamic neurones and their axon projection to the cerebral cortex in the cat

1. A technique of reversible block of synaptic transmission through the dorsal column nuclei and the trigeminal nucleus caudalis (n. caudalis) has been employed to assess the somatic afferent input to individual posterior thalamic neurones in the cat. The axon projection to the cerebral cortex of these neurones has been identified by antidromic activation following cortical stimulation.2. Unitary responses in the nucleus ventralis posterolateralis (VPL) evoked by cutaneous stimulation were abolished or depressed following block of transmission in the dorsal column nuclei. Block in n. caudalis, however, depressed unitary responses in nucleus ventralis posteromedialis (VPM) evoked by facial skin stimulation in less than 10% of cells.3. A more complex source of the somatic input to the posterior nuclear region of the thalamus (PO) was found. It was most commonly noted that PO unitary activity evoked by cutaneous stimulation of the face was unaffected by block of synaptic transmission in n. caudalis. No uniform effect was observed on unitary responses in PO evoked by limb stimulation when transmission in the dorsal column nuclei was blocked.4. Antidromic activation from the cerebral cortex was seen in 69% of ventrobasal neurones. Most cells (66%) had ;antidromic cortical fields' restricted to a region consisting of a third to a half of the specific somatic projection areas. In 10% of cells evidence was obtained for discontinuous cortical ;antidromic fields' suggesting subcortical bifurcation of the projecting axon.5. An axon projection to the cortex was found in 35% of PO cells, about half of which projected only to specific somatic projection areas. Evidence for subcortical branching of the axon was obtained for seven PO cells.     

Tonic effects of the dopaminergic ventral midbrain on the auditory cortex of awake macaque monkeys

This study shows that ongoing electrical stimulation of the dopaminergic ventral midbrain can modify neuronal activity in the auditory cortex of awake primates for several seconds. This was reflected in a decrease of the spontaneous firing and in a bidirectional modification of the power of auditory evoked potentials. We consider that both effects are due to an increase in the dopamine tone in auditory cortex induced by the electrical stimulation. Thus, the dopaminergic ventral midbrain may contribute to the tonic activity in auditory cortex that has been proposed to be involved in associating events of auditory tasks (Brosch et al. Hear Res 271:66–73, 2011) and may modulate the signal-to-noise ratio of the responses to auditory stimuli.     

Neuropeptide-mediated facilitation and inhibition of sensory inputs and spinal cord reflexes in the lamprey

The effects of neuromodulators present in the dorsal horn [tachykinins, neuropeptide Y (NPY), bombesin, and GABAB agonists] were studied on reflex responses evoked by cutaneous stimulation in the lamprey. Reflex responses were elicited in an isolated spinal cord preparation by electrical stimulation of the attached tail fin. To be able to separate modulator-induced effects at the sensory level from that at the motor or premotor level, the spinal cord was separated into three pools with Vaseline barriers. The caudal pool contained the tail fin. Neuromodulators were added to this pool to modulate sensory inputs evoked by tail fin stimulation. The middle pool contained high divalent cation or low calcium Ringer to block polysynaptic transmission and thus limit the input to the rostral pool to that from ascending axons that project through the middle pool. Ascending inputs and reflex responses were monitored by making intracellular recordings from motor neurons and extracellular recordings from ventral roots in the rostral pool. The tachykinin neuropeptide substance P, which has previously been shown to potentiate sensory input at the cellular and synaptic levels, facilitated tail fin-evoked synaptic inputs to neurons in the rostral pool and concentration dependently facilitated rostral ventral root activity. Substance P also facilitated the modulatory effects of tail fin stimulation on ongoing locomotor activity in the rostral pool. In contrast, NPY and the GABAB receptor agonist baclofen, both of which have presynaptic inhibitory effects on sensory afferents, reduced the strength of ascending inputs and rostral ventral root responses. We also examined the effects of the neuropeptide bombesin, which is present in sensory axons, at the cellular, synaptic, and reflex levels. As with substance P, bombesin increased tail fin stimulation-evoked inputs and ventral root responses in the rostral pool. These effects were associated with the increased excitability of slowly adapting mechanosensory neurons and the potentiation of glutamatergic synaptic inputs to spinobulbar neurons. These results show the possible behavioral relevance of neuropeptide-mediated modulation of sensory inputs at the cellular and synaptic levels. Given that the types and locations of neuropeptides in the dorsal spinal cord of the lamprey show strong homologies to that of higher vertebrates, these results are presumably relevant to other vertebrate systems.     

Differential effects of medial forebrain bundle lesions on adrenocortical responses following limbic stimulation

Adult male rats had electrolytic lesions placed bilaterally in the medial forebrain bundle and were subsequently implanted with stimulating electrodes in one of the following limbic regions: (1) dorsal hippocampus; (2) ventral hippocampus; (3) medial septal nucleus; (4) basolateral amygdala; (5) mesencephalic reticular formation. Following electrical stimulation, blood was drawn by acute venesection, under either, for plasma corticosterone determinations. In non-lesioned animals, electrical stimulation in all of the limbic regions led to elevated plasma corticosterone levels. In rats with lesions in the medial forebrain bundles, the adrenocortical response to stimulation in the dorsal hippocampus, the basolateral amygdala or the reticular formation was markedly attenuated. On the contrary, the same lesions were without effect upon the corticosterone secretory response to medial septal stimulation, and had only a slight inhibitory effect upon the response to electrical stimulation in the ventral hippocampus. The results demonstrate that the medial forebrain bundle plays a major role in the transmission of impulses to the mediobasal hypothalamus, originating in the dorsal hippocampus, basolateral amygdala or mesencephalic reticular formation, which activate adrenocortical secretion; its role in the transmission of cues arising in the ventral hippocampus or medial septum is, however, minor.     

Stimulation of the parasubiculum modulates entorhinal cortex responses to piriform cortex inputs in vivo

Although a major output of the hippocampal formation is from the subiculum to the deep layers of the entorhinal cortex, the parasubiculum projects to the superficial layers of the entorhinal cortex and may therefore modulate how the entorhinal cortex responds to sensory inputs from other cortical regions. Recordings at multiple depths in the entorhinal cortex were first used to characterize field potentials evoked by stimulation of the parasubiculum in urethan-anesthetized rats. Current source density analysis showed that a prominent surface-negative field potential component is generated by synaptic activation in layer II. The surface-negative field potential was also observed in rats with chronically implanted electrodes. The response was maintained during short stimulation trains of < or =125 Hz, suggesting that it is generated by activation of monosynaptic inputs to the entorhinal cortex. The piriform cortex also projects to layer II of the entorhinal cortex, and interactions between parasubicular and piriform cortex inputs were investigated using double-site stimulation tests. Simultaneous activation of parasubicular and piriform cortex inputs with high-intensity pulses resulted in smaller synaptic potentials than were expected on the basis of summing the individual responses, consistent with the termination of both pathways onto a common population of neurons. Paired-pulse tests were then used to assess the effect of parasubicular stimulation on responses to piriform cortex stimulation. Responses of the entorhinal cortex to piriform cortex inputs were inhibited when the parasubiculum was stimulated 5 ms earlier and were enhanced when the parasubiculum was stimulated 20-150 ms earlier. These results indicate that excitatory inputs to the entorhinal cortex from the parasubiculum may enhance the propagation of neuronal activation patterns into the hippocampal circuit by increasing the responsiveness of the entorhinal cortex to appropriately timed inputs.