Electrophysiological connections of the rat mediobasal hypothalamus with brain areas mediating adrenocortical responses

With the purpose of correlating neuroendocrine and neurophysiological data, the effects of stimulating brain areas which have been shown to be involved in the mediation of adrenocortical responses, i.e. pontine reticular formation (PRF), mammillary peduncle (MP), medial forebrain bundle (MFB), and the suprachiasmatic nucleus (SCN), were studied on unit activity in the mediobasal hypothalamus in the rat. About half of the units recorded responded significantly to one of these modalities by a change in the rate of firing, the MP producing mainly inhibition and SCN facilitation and changes also occurred in the post-stimulus histograms. There was a convergence of different modalities on the same hypothalamic unit. No changes occurred in the time-interval histograms. Stimulating the mediobasal hypothalamus activated synaptically units in the PRF, MP, and MFB but none of them were activated antidromically. The present data demonstrate electrophysiological connections between extrahypothalamic regions participating in adrenocortical regulation and the hypophysiotropic area of the hypothalamus.     

The role of the medial septal nucleus in mediating adrenocortical responses to somatosensory stimulation

In view of the demonstrated involvement of the hippocampus in the mediation of adrenocortical responses following sciatic nerve stimulation, the role of the septum and the preoptic area in the transmission of this response was investigated. Changes in plasma corticosterone following ether stress and photic, acoustic, or sciatic nerve stimulation were studied in intact rats and in animals with lesions in the medial septal nucleus and the preoptic area. The response to ether stress and to photic and acoustic stimulation was normal in these animals. However, the adrenocortical response to sciatic nerve stimulation was partially reduced in the rats with lesions in the superior, but not in the inferior preoptic area, and it was completely blocked in those with medial septal lesions. Our previous and present experiments indicate that the ventral hippocampus, the fimbria, the septum, and anterior hypothalamic afferents mediate the adrenocortical response to somatosensory stimulation.     

AMPA receptor desensitization modulates synaptic responses induced by repetitive afferent stimulation in hippocampal slices

Relationships between the kinetic properties of AMPA receptors and the decay phase of fast excitatory transmission were investigated using modulatory drugs. The benzothiadiazide compound cyclothiazide blocked receptor desensitization in patches excised from hippocampus but had only a weak influence on receptor deactivation, i.e., on the decay of responses produced by a 1-ms pulse of glutamate. The ampakine drug CX516 (BDP-12) produced an opposite pattern of effects: a fourfold slowing of deactivation with little change in desensitization. A structurally related drug (CX554 or BDP-20) had prominent effects on both desensitization and deactivation. The halfwidth of field EPSPs measured in the CA1 region of hippocampal slices increased 50–100% in the presence of CX516 or CX554 but by less than 15% at concentrations of cyclothiazide that fully blocked desensitization in patch experiments. These results indicate that receptor deactivation plays a substantially greater role than receptor desensitization in determining the duration of synaptic responses.     

Reset of hippocampal rhythmical activities by afferent stimulation

Physostigmine induced theta rhythm and unit activity were recorded from the dorsal hippocampus in immobilized locally anesthetized rats. Correlations between theta and rhythmical unit activity and their modifications by hippocampal afferent stimulation were studied. The principal finding was that electrical stimulation of afferents reset theta and rhythmical unit activity in phase. Poststimulus theta displayed a variable frequency which depended upon the structure stimulated. Lower frequencies were evoked by septal, higher frequencies by entorhinal and reticular formation stimulations. When theta rhythms were absent either by spontaneous disappearance or as a consequence of lesions in the fornix superior of septum, the reset was not observed. The reset of the theta rhythms and unit activity by afferent stimulation, suggests that the hippocampus may participate in timing mechanisms.     

Non-rhythmical hippocampal units, theta rhythm and afferent stimulation

Physostigmine induced theta (θ) rhythm and unit activity were recorded from dorsal hippocampus in immobilized locally anesthetized rats. Two functionally different types of cells were identified, Type 1 which fired in rhythmical bursts phase-locked with γ cycles, and Type 2 which were non-rhythmical. Most of the units (80%) were Type 2 and either had bursting (40%) or non-bursting discharge patterns. Correlations between the θ rhythm and Type 2, Type 1 and Type 2, and between pairs of Type 2 cells were studied. Modifications of correlations by afferent stimulation were also analyzed. The principal findings was that in the presence of θ about half of the Type 2 cells revealed a degree of phase-locking with the rhythm. This finding suggests a caused relationship between both phenomena. Crosscorrelograms between Type 1 and 2 discharges, when positive, showed a symmetrical periodicity suggesting that Type 2 cells might function as an hippocampal output. The activity of Type 2 pairs, positively crossconelated in nearly two thirds of the cases, revealed excitatory interactions. Hippocampal afferent stimulation reset θ activity in phase, modified the temporal relationships between cells and with the θ rhythm, and changed the Type 2 discharge pattern. The above results indicate that θ-related Type 2 cells carry information representing θ rhythmicity. Phase-relationships between Type 2 cells and θ, and their modifications by hippocampal afferent activity may be necessary to establish time-relationships with other brain structures.     

Long-term effects of transcranial magnetic stimulation on hippocampal reactivity to afferent stimulation.

Transcranial magnetic stimulation (TMS) has become a promising treatment of affective disorders in humans, yet the neuronal basis of its long-lasting effects in the brain is still unknown. We studied acute and lasting effects of TMS on reactivity of the rat hippocampus to stimulation of the perforant path. Application of TMS to the brain of the anesthetized rat caused a dose-dependent transient increase in population spike (PS) response of the dentate gyrus to perforant path stimulation. In addition, TMS caused a marked decrease in inhibition and an increase in paired-pulse potentiation of reactivity to stimulation of the perforant path. Also, TMS suppressed the ability of fenfluramine (FFA), a serotonin releaser, to potentiate PS response to perforant path stimulation. Chronic TMS did not affect single population spikes but caused an increase in paired-pulse potentiation, which was still evident 3 weeks after the last of seven daily TMS treatments. After chronic TMS, FFA was ineffective in enhancing reactivity to perforant path stimulation, probably because it lost the ability to release serotonin. In addition, the beta adrenergic receptor agonist isoproterenol, which caused an increase in PS in the control rats, failed to do so in the TMS-treated rats. These results indicate that TMS produces a long-term reduction in efficacy of central modulatory systems.     

Motor responses to afferent stimulation in juvenile myoclonic epilepsy

PURPOSE: To document whether the mechanisms responsible for myoclonic jerks in juvenile myoclonic epilepsy (JME) are similar to those causing other forms of myoclonus. METHODS: We studied somatosensory evoked potentials, the conditioning effect of cutaneous afferents on motor potentials evoked by transcranial magnetic stimulation (TMS), and intracortical inhibition and facilitation in response to paired TMS in a group of nine patients with JME and 20 normal controls. RESULTS: Intracortical inhibition was abnormal, whereas cortical somatosensory evoked potentials and TMS conditioned by cutaneous afferents were unaltered in JME patients. CONCLUSIONS: Abnormal processing of cutaneous afferents would not appear to contribute to myoclonus in JME.     

Hypothalamic paraventricular nucleus lesions decrease pressor responses to subfornical organ stimulation

An asymmetrical LH-RH distribution in rat hypothalamus has been found. In Wistar rats LH-RH content in the right hypothalamus exceeds that in the left one; in albino rats a contrary distribution is observed. LH-RH lateralization changes during a 24-h period. Unilateral castration or cold stress lead to a shift in LH-RH distribution in the hypothalamus.     

A model of status epilepticus based on electrical stimulation of hippocampal afferent pathways

A new model of status epilepticus has been developed in the unanesthetized rat. The model involves repetitive tetanic stimulation of hippocampal afferent pathways. Pulse trains were delivered according to a fixed schedule (0.2 to 0.4-ms monophasic rectangular pulses, 20 Hz, stimulus current adjusted for maximal synaptic response in area CA3 of the hippocampus, 10-s train duration, 30-s intertrain interval) through electrodes chronically implanted in the angular bundle or fimbria. CA3 pyramidal cells responded to each stimulus in the train with little or no decrement. When 10 consecutive trains each produced 30 s of hippocampal afterdischarge, stimulation was terminated and self-sustained electrographic seizure activity was monitored. This procedure was repeated until it yielded at least 15 min of self-sustained seizure activity. Status epilepticus occurred in about 85% of subjects within less than 7 h. Self-sustained electrographic seizures were associated with limbic motor seizures and with brain lesions that resembled Ammon's horn sclerosis. This model holds promise for analyzing the biochemical and physiological bases of seizures, status epilepticus, and neuronal cell death, because the timing of these events during the stimulation protocol is fairly predictable and because seizures are self-sustaining without the need drugs, toxins, or prior kindling.     

Histamine modulates reactivity of hippocampal CA3 neurons to afferent stimulation in vitro

Intracellular activity was recorded from hippocampal CA3 pyramidal cells maintained in vitro. Histamine (HA) produced a slow depolarization associated with minimal conductance changes. In addition, there was an increase in action potential discharge rates and the emergence of bursting firing patterns. EPSP size increased by about 50% and spontaneous dendritic spikes were observed. These effects were markedly reduced by retrodotoxin. Extracellular recording of population spikes revealed a marked difference between CA1 and CA3 regions; in the former HA produced an increase in population spike size whereas in the latter this increase was larger and was associated with the appearance of secondary and tertiary population spikes. It is suggested that HA produces its effects by enhancing release of neurotransmitters from excitatory synapses on the recorded neurons.     

Presynaptic inhibition of Schaffer collateral synapses by stimulation of hippocampal cholinergic afferent fibres

It has been known for decades that muscarinic agonists presynaptically inhibit Schaffer collateral synapses contacting hippocampal CA1 pyramidal neurons. However, a demonstration of the inhibition of Schaffer collateral synapses induced by acetylcholine released by cholinergic hippocampal afferents is lacking. We present original results showing that electrical stimulation at the stratum oriens/alveus with brief stimulus trains inhibited excitatory postsynaptic currents evoked by stimulation of Schaffer collaterals in CA1 pyramidal neurons of rat hippocampal slices. The increased paired-pulse facilitation and the changes in the variance of excitatory postsynaptic current amplitude that paralleled the inhibition suggest that it was mediated presynaptically. The effects of oriens/alveus stimulation were inhibited by atropine, and blocking nicotinic receptors with methyllycaconitine was ineffective, suggesting that the inhibition was mediated via the activation of presynaptic muscarinic receptors. The results provide a novel demonstration of the presynaptic inhibition of glutamatergic neurotransmission by cholinergic fibres in the hippocampus, implying that afferent cholinergic fibres regulate the strength of excitatory synaptic transmission.     

Hypothalamic pituitary adrenocortical and sympathetic nervous system responses to the cold pressor test in Alzheimer's disease.

BACKGROUND: Increased basal activity of the hypothalamic-pituitary-adrenocortical (HPA) axis has been repeatedly demonstrated in Alzheimer's disease (AD), and some studies suggest increased basal activity of the sympathetic nervous system (SNS) in this disorder; however, the effects of AD on HPA axis or SNS responses to a standardized aversive stressor have not been examined. The neuroendocrine response to aversive stress may be relevant to the pathophysiology of AD. METHODS: Plasma adrenocorticotropic hormone (ACTH), cortisol, norepinephrine (NE), and epinephrine responses to a 1-min cold pressor test (CPT) were measured in nine medically healthy AD outpatients (age 76 +/- 2 years) and nine age- and gender-matched medically healthy cognitively normal older subjects (age 76 +/- 1 year). RESULTS: The cortisol response to CPT was increased in the AD group but the ACTH response did not differ between groups. Basal NE concentrations were higher in the AD group. Although NE responses to CPT did not differ between groups, the blood pressure response to CPT was higher in the AD subjects. CONCLUSIONS: These results suggest increased HPA axis responsiveness to CPT at the level of the adrenal cortex in AD. The results also suggest increased basal sympathoneural activity and increased cardiovascular responsiveness to sympathoneural stimulation in AD under the conditions of this experimental protocol. Increased SNS stimulatory modulation of the adrenal cortex is a possible mechanism contributing to the observed enhanced cortisol response to CPT in these AD subjects.     

alpha-Adrenoceptor subtypes mediating regional kidney blood flow responses to renal nerve stimulation

The mechanisms underlying the relative insensitivity of the renal medullary circulation to renal sympathetic nerve stimulation (RNS) remain unknown. Therefore, we tested the effects of systemic alpha(1)- and alpha(2)-adrenoceptor blockade on responses to electrical RNS in pentobarbitone anaesthetized rabbits. Renal blood flow (RBF), cortical laser Doppler flux (CLDF), and to a lesser extent medullary LDF (MLDF) were reduced by RNS in a frequency-dependent manner. Prazosin decreased responses of RBF and CLDF, but not MLDF, to RNS. For example, during the control period 4 Hz stimulation reduced RBF, CLDF and MLDF by 85+/-3%, 89+/-2%, and 20+/-12%, respectively, but after prazosin, corresponding responses were 39+/-3%, 42+/-5% and 28+/-7%, respectively. Prazosin markedly blunted pressor and renal vasoconstrictor responses to intravenous phenylephrine, without altering pressor responses to intravenous xylazine. Rauwolscine enhanced renal vasoconstrictor responses to RNS, although this was statistically significant for RBF and CLDF but not MLDF. For example, during the control period 2 Hz stimulation reduced RBF, CLDF and MLDF by 63+/-7%, 58+/-7%, and 29+/-17%, respectively, and after rauwolscine, corresponding responses were 83+/-4%, 87+/-1%, and 53+/-12%, respectively. Rauwolscine markedly blunted renal vasoconstrictor responses to renal arterial guanabenz, but not phenylephrine. These data suggest that alpha(1)-adrenoceptors contribute to RNS-induced vasoconstriction in the renal cortex, but contribute less in vascular elements controlling medullary perfusion. Activation of alpha(2)-adrenoceptors appears to blunt RNS-induced renal vasoconstriction, but this mechanism does not underlie the relative insensitivity of medullary perfusion to RNS.     

Electrophysiological responses to muscarinic receptor stimulation in cultured hippocampal neurons.

Electrophysiological recordings of responses to muscarinic receptor stimulation in cultured embryonic hippocampal neurons have been largely unsuccessful to date. In this study muscarinic receptor binding was demonstrated in 2-week-old embryonic rat hippocampal cultures. Using whole-cell patch-clamp recording we found that 1-5 microM carbachol produced multiple effects including depolarization, increased action potential firing rate, increased synaptic activity and a reduction in the amplitude of medium-duration afterhyperpolarizations. Voltage-clamp analysis revealed a time-dependent current relaxation with hyperpolarizing steps from a holding potential of about -40 mV which was inhibited by 10 microM muscarine or 50 microM carbachol and had characteristics similar to those of the m-current. Both atropine and pirenzepine inhibited all of these effects indicating that these cholinergic actions were mediated by muscarinic receptors. This study shows that muscarinic responses obtained classically in hippocampal brain slices can also be produced in cultured hippocampus.     

A correlation between hippocampal responses to interhemispheric stimulation, hippocampal slow rhythmic activity and behaviour.

The spontaneous electrical activity of the hippocampus as well as its responses to commissural stimulation were recorded in waking rats. Changes in magnitude of the early and late components of the response to commissural stimulation were associated with the behavioural state of the rat. The magnitude of the initial potential was higher in non-theta 'states' than in theta 'states', whereas the opposite was true for the late potential.     

Spatial distribution of hippocampal responses to dental pulp stimulation and acoustic clicks

Evoked potentials and unitary discharges in responses to tooth pulp and acoustic click stimuli were recorded from the hippocampus of freely moving rats. The spatial distribution of evoked field responses to tooth pulp stimulation and acoustic clicks were identical. Averaged evoked potentials consisted of a large negative deflection (N1) preceded by a small positive potential (P1). The shortest latency N1 was recorded from the middle third of the dentate molecular layer and the outer portion of apical dendrites of Ca3 (27 ms). The peak latency of N1 was significantly longer (34 ms) in the stratum radiatum of CA1. Laminar profiles of N1 in the dentate gyrus and CA3 were similar to that evoked by electrical stimulation of the perforant path and in CA1 similar to the response profile evoked by the Schaffer collaterals. The largest amplitude P1 was observed above the pyramidal layer of CA1 and the hilus. Both sensory modalities were able to modify the discharge rate of neurons in all hippocampal regions. A conclusion is made that information about sensory stimuli can reach the hippocampus by two distinctive pathways: a short-latency inhibitory input via the fimbrial fornix and a longer-latency path via the entorhinal cortex.