Electrophysiological identification of a pathway from the septal area to the medial amygdala: sensitivity to estrogen and luteinizing hormone-releasing hormone

Attenuation of kainic acid-induced status epilepticus by inhibition of endocannabinoid transport and degradation in guinea pigs

Status epilepticus (SE) is a medical emergency associated with a high rate of mortality if not treated promptly. Exogenous and endogenous cannabinoids have been shown to possess anticonvulsant properties both in vivo and in vitro. Here we study the influence of endocannabinoid metabolism on the development of kainic acid-induced SE in guinea pigs. For this purpose, the inhibitors of endocannabinoid transport, AM404, and enzymatic (fatty acid amide hydrolase) degradation, URB597, were applied. Cannabinoid CB1 receptor antagonist, AM251, was also tested. Animal behavior as well as local electric field potentials in four structures: medial septum, hippocampus, entorhinal cortex and amygdala were analyzed when AM404 (120nmol), URB597 (4.8nmol) or AM251 (20nmol) were administrated alone or together with 0.4μg of kainic acid. All substances were injected i.c.v. AM404, URB597 or AM251 administered alone did not alter markedly local field potentials of all four studied structures in the long-term compared with their basal activity. AM404 and URB597 significantly alleviated kainic acid-induced SE, decreasing behavioral manifestations, duration of seizure events and SE in general without changing the amplitude of local field potentials. AM251 did not produce distinct effects on SE in terms of our experimental paradigm. There was no apparent change of the seizure initiation pattern when kainic acid was coadministrated with AM404, URB597 or AM251. The present study provides electrophysiologic and behavioral evidences that inhibition of endocannabinoid metabolism plays a protective role against kainic acid-induced SE and may be employed for therapeutic purposes. Further investigations of the influences of cannabinoid-related compounds on SE genesis and especially epileptogenesis are required.

Exposure to a retrieval cue in rats induces changes in regional brain glucose metabolism in the amygdala and other related brain structures

Pre-test exposure to training-related cues is known to improve subsequent retention performance. To identify brain regions engaged in processes promoted by retrieval cues, a brain imaging approach using the [6-14C]glucose autoradiographic technique was used. Sprague-Dawley rats trained in a brightness discrimination avoidance task were submitted to different cueing conditions after a 1- or a 21-day training-to-test interval (TTI). Animals were either non-cued, cued with a box, or cued with a box and the light that served as a discriminative stimulus. Effects of the different cueing conditions on retention performance or on metabolic activity in 58 different brain regions were investigated. Rats cued with the light exhibited a subsequent improvement of their retention performance relative to controls, when tested at the 1- but not 21-days TTI, confirming our previous results. At the 1-day retention interval, a comparison between rats cued with the box and rats cued with the box and the light showed that the light cue significantly increased glucose uptake in a neuronal network composed of the lateral, basal, and central nuclei of the amygdala, the anterior and suprachiasmatic hypothalamic nuclei, the nucleus accumbens, the medial septum, and the insular cortex. In contrast, at the 21-day retention interval, both groups demonstrated similar cerebral metabolic activity. The present results indicate that exposure to a light cue increased metabolic activity in the previously mentioned brain structures only when the light acted as an effective retrieval cue, suggesting an involvement of this network in the processes triggered by a retrieval cue. Arguments are provided supporting the notion that the amygdala may play a key role in these processes. Whether the amygdala is a part of a neural network involved in retrieval processes or in neuromodulating systems that favour the efficacy of retrieval processes is also discussed.

Role of the septum in the excitatory effect of corticotropin-releasing hormone on the acoustic startle reflex

Intracerebroventricular administration of corticotropin-releasing hormone (CRH) elicits a constellation of behavioral, autonomic, and endocrinological changes typically observed in stress. One of the behavioral changes after intracerebroventricular CRH is a profound increase of startle amplitude (CRH-enhanced startle). The present study examined the role of the septum in CRH-enhanced startle. The septum has direct and indirect connections to the amygdala and inhibits the amygdala. Electrophysiological data show that CRH in the septum is inhibitory. Therefore, it has been hypothesized that intracerebroventricular CRH inhibits the septum, which in turn disinhibits the amygdala, resulting in a constellation of changes via activation of amygdala efferent targets. In testing this hypothesis, it was found that electrolytic lesions of the medial septum, but not the lateral septum, blocked CRH-enhanced startle. However, fiber-sparing chemical lesions of the medial septum did not block CRH-enhanced startle, suggesting that the blockade seen with the electrolytic lesions was caused by damage to fibers of passage. A major fiber bundle passing through the medial septum is the fornix, the primary efferent pathway for the hippocampus. Fimbria transection blocked CRH-enhanced startle almost completely, whereas the large electrolytic lesions of the dorsal hippocampus did not block CRH-enhanced startle. Taken together, these data suggest that perhaps the ventral hippocampus and its efferent target areas, which communicate via the fimbria, may be critically involved in CRH-enhanced startle.

Septal lesions block sympathetic activation following frontal cortex stimulation in the rat

The rates of oxygen consumption, colonic and interscapular brown adipose tissue temperature, and discharge of sympathetic nerves innervating the same tissue were recorded before and after orbital frontal cortex stimulation in three groups of rats. These groups consisted of animals with electrolytic lesions of the nucleus medialis septi, with lesions of nucleus lateralis septi or with sham lesions. The values of all the variables considered were similar during the pre-stimulation time in the three groups of rats. There was an increase in all the parameters in sham-lesioned animals after electrical stimulation, while no changes were found in the two groups of injured rats. The results showed that the frontal cortex is involved in the control of thermogenesis through the activation of the sympathetic nervous system. Medial and lateral septal nuclei, in spite of their neurochemical and pharmacological differences, share a common role in the pathway from the frontal cortex to the hypothalamic area and the amygdala, which in turn drive the sympathetic nervous system.

Facilitation of mating behavior in male hamsters by LHRH and AcLHRH5-10: interaction with the vomeronasal system

An intact vomeronasal organ is important for the reproductive physiology and behavior of many species. Vomeronasal sensory input is known to influence hormone levels especially LH, presumably by prior modulation of LHRH. LHRH has been shown independently to facilitate mating behavior in rodents of both sexes. In this study intracerebroventricular LHRH at a dose of 50 ng substantially relieved mating behavior deficits caused by prepubertal removal of vomeronasal organs from inexperienced male hamsters. Intranasal LHRH at a higher dose did not have this effect. Behavioral responses were recorded 30 mins after peptide or saline injection. The LHRH analog, AcLHRH5-10 which we demonstrate does not induce LH release, did facilitate mating behavior in these tests, suggesting that LHRH peptides may facilitate male mating behavior via an extra-pituitary mode of action.

Estrogen-receptor immunoreactivity in hamster brain: preoptic area, hypothalamus and amygdala

The distribution of estrogen-receptor containing cells in the preoptic area, hypothalamus and amygdala of female Syrian hamster brain was studied by immunocytochemical methods. Dense populations of estrogen-receptor immunoreactive (ER-IR) cells were found in the medial preoptic area, the bed nucleus of the stria terminalis, amygdala, ventral and lateral parts of the hypothalamus, and the arcuate nucleus. Injection of estradiol caused a decrease in estrogen-receptor immunoreactivity (ERIR) containing cells within one hour, a decrease that may reflect a change in the ability of the occupied estrogen receptor to bind the particular antibody (H222) used rather than down-regulation of the estrogen receptor. Our findings on the distribution of estrogen-receptor containing cells in these areas using an immunocytochemical technique are consistent with and extend the findings of others using autoradiographic and in vitro binding techniques to study estrogen receptor distribution in hamster brain.

Excitatory and inhibitory synaptic processing in the accessory olfactory system of the female rat

The accessory olfactory system constitutes a sensory system specifically involved in regulating neuroendocrine function and reproductive behavior. The chemosensitive structure of this system, the vomeronasal organ, exclusively innervates the accessory olfactory bulb, which in turn projects via mono- or disynaptic pathways to a limited number of regions implicated in endocrine and sexual function, including the amygdala and hypothalamus. The present study investigated synaptic processing between the different levels of this sensory system, with particular focus on the input from the accessory olfactory bulb to the medial amygdala and the reciprocal connections between the medial amygdala and the ventromedial hypothalamus. Extracellular single-unit recordings were obtained from medial amygdala neurons in anesthetized female rats to study the synaptic responses elicited by stimulation of the olfactory bulb and hypothalamus. Locally applied agonists and antagonists of amino acid neurotransmitters were tested for their ability to mimic and block these synaptic responses in an attempt to identify the neurotransmitters involved in the stimulated pathways. Electrical stimulation of the accessory olfactory bulb induced orthodromic responses in 88% of 288 medial amygdala neurons, with 67% showing orthodromic inhibition and 21% showing excitation as the shortest-latency response. Many cells displayed multiphasic responses with both orthodromic excitation and inhibition. In separate experiments, main olfactory bulb stimulation also produced excitatory (39%) or inhibitory (50%) orthodromic responses in medial amygdala neurons (n = 105), but the latency and distribution of responses were significantly different than with accessory olfactory bulb stimulation. Stimulation of the ventromedial hypothalamus also had prominent excitatory (29%) or inhibitory (59%) orthodromic actions on the majority of amygdala neurons. Out of 288 cells tested with both accessory olfactory bulb and ventromedial hypothalamus stimulation, 79% were orthodromically responsive to both areas, whereas only 3% responded to neither area. Fourteen (6%) medial amygdala neurons were antidromically activated from the ventromedial hypothalamus, and 11 of these 14 cells were simultaneously orthodromically responsive to accessory olfactory bulb stimulation, indicating that the accessory olfactory bulb can influence medial amygdala neurons that project directly to the hypothalamus. Iontophoretic application of drugs to the medial amygdala revealed that glutamate and GABA consistently mimicked the orthodromic excitatory and inhibitory responses, respectively, from both the accessory olfactory bulb and ventromedial hypothalamus. In some cases, ejection of bicuculline could block or reverse excitation of the orthodromic inhibitory responses, whereas kynurenic acid could eliminate the orthodromic excitation, indicating that excitatory and inhibitory amino acid transmitters are at least partially involved in these pathways.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of a biologically active LHRH fragment on CA1 hippocampal neurons

The actions of a behaviorally active luteinizing hormone-releasing hormone fragment, Ac-LHRH(5-10), on CA1 pyramidal cells were studied utilizing conventional intracellular recordings from the in vitro rat hippocampal slice preparation. The behaviorally active fragment (10(-7) M) and the natural decapeptide [LHRH(1-10), 10(-8) M] had similar actions on CA1 neurons: a long-duration depolarization associated with increased input resistance, a reduction in the slow afterhyperpolarization (AHP), and a decrease in accommodation. In contrast, a biologically inactive LHRH fragment [LHRH(1-6) at 10(-7) M] had no effect on electrical properties of CA1 neurons. These data suggest that Ac-LHRH(5-10), like LHRH(1-10), may have a modulatory action on hippocampal neurons.

Modulation of single-unit activity in the rat medial amygdala by neurotransmitters, estrogen priming, and synaptic inputs from the hypothalamus and midbrain

The medial amygdala (m-AMG) appears to act as an integrative center for sensory, synaptic, and endocrine signals important in the regulation of reproductive function. Extracellular single-unit recordings from anesthetized, ovariectomized female rats were used to investigate neuropharmacological, hormonal, and synaptic modulation of neurons in the m-AMG. Electrical stimulation of the ventromedial hypothalamus (VMH) elicited excitatory or inhibitory orthodromic responses in 72% and antidromic responses in 7% of m-AMG neurons, whereas stimulation of the midbrain central gray (MCG) induced orthodromic responses in 43% of m-AMG neurons. Interestingly, most cells that were influenced by MCG stimulation were also orthodromically driven by the VMH, as 40% of all m-AMG cells responded orthodromically to both the VMH and MCG. Furthermore, the majority of these cells tended to be modulated by both areas in the same direction. Iontophoretic application of glutamate, GABA, ACh, and LHRH could modulate the spontaneous firing rate of m-AMG neurons. In particular, ACh had a predominantly excitatory action, which was more effective on m-AMG neurons that were orthodromically driven by the VMH and that were from estrogen-primed animals. In addition to increasing chemical responsiveness to ACh, estrogen priming of ovariectomized animals also increased the spontaneous firing rate of m-AMG neurons and decreased the number of silent cells. These modulatory actions on m-AMG neurons may be important in the medial amygdala's regulation of the behavioral and endocrine aspects of reproductive function in the female rat.

Lesions of the central nucleus of the amygdala block the excitatory effects of septal ablation on the acoustic startle reflex

Many studies have investigated the role of the septum and the amygdala in emotional behavior. While the literature is somewhat inconsistent, most studies suggest a role for the septal nuclei in the inhibition of fear and stress responses (at the behavioral, autonomic and hormonal levels) while the central nucleus of the amygdala is involved in the production of such responses. The present study examined the ability of lesions of the central nucleus of the amygdala to block the excitatory effects of complete septal ablation on the acoustic startle reflex. Septal ablation produced a significant increase in startle amplitude which was blocked by concomitant lesions of the central nucleus of the amygdala. These results suggest that the increase in startle amplitude resulting from septal damage might be due to a disinhibition of neuronal activity in the central nucleus of the amygdala, a structure known to mediate the increase in startle associated with conditioned and unconditioned fear, or from antagonistic interactions at other target sites which themselves modulate startle.