NMDA receptors in the midbrain periaqueductal gray mediate hypothalamically evoked hissing behavior in the cat

The present study was designed to test the hypothesis that the descending pathway from the medial hypothalamus to the dorsal periaqueductal gray (PAG) is critical for the expression of defensive rage behavior in the cat and utilizes excitatory amino acids as a neurotransmitter. In the first phase of the study, monopolar stimulating electrodes were implanted into the medial hypothalamus from which defensive rage behavior could be elicited by electrical stimulation. For the entire study, the hissing response was used as a measure of defensive rage behavior. Cannula electrodes were implanted into the PAG from which defensive rage sites could be identified and were later used for microinfusion of the NMDA receptor antagonist,dl-2-amino-7-phosphoheptanoic acid (AP-7), into behaviorally identified sites within the PAG. Initially, intracerbral microinjections of the NMDA receptor antagonist, AP-7 (0.2, 2.0 nmol), which were placed directly into sites within the PAG from which defensive rage had been elicited, blocked the occurrence of hypothalamic hissing. Microinjections of similar doses of AP-7 into the PAG also blocked the facilitatory effects of medial hypothalamic stimulation upon hissing behavior elicited from the PAG. However, microinjections of 2 nmol into the PAG had no effect upon hissing that was also elicited from the region of the injection site. This finding indicates that AP-7 selectively blocks hissing elicited from the medial hypothalamus and that the suppressive effects of AP-7 cannot be the result of anesthetic or other nonselective properties of the drug. The next phase of the study, which employed immunohistochemical, receptor autoradiographic techniques, identified NMDA receptors to be present in highest concentrations in the dorsolateral aspect of the PAG where defensive rage is typically elicited. The final phase of the study, which employed a combination of retrograde labeling procedures following microinjections of Fluoro-Gold into defensive rage sites in the dorsal PAG and the immunocytochemical labeling of glutamatergic neurons, identified large numbers of neurons in the medial hypothalamus that were labeled positively for both Fluoro-Gold and glutamate. The overall findings of this study support the hypothesis that descending fibers of the medial hypothalamus that supply the dorsal aspect of the PAG mediate defensive rage behavior and utilize excitatory amino acids that act upon NMDA receptors within the dorsal PAG.     

Locomotor activity and ingestive behavior after damage to ascending serotonergic systems

The effects of electrolytic midbrain raphe lesions on ingestive behavior and locomotor activity of rats were compared to those produced by intracerebral injections of 5,7 dihydroxytryptamine (5,7 DHT) at various points along the ascending serotonergic pathways. Only electrolytic lesions of the median and/or dorsal raphe nuclei produced significant changes in food intake, water intake, body weight gain, and wheel running activity. Intracerebral injections of 5,7 DHT, a selective serotonergic neurotoxin, had no effect on any of these variables. However, 5,7 DHT induced lesions produced decreases in forebrain synaptosomal uptake of serotonin which were equivalent to, or greater than, those resulting from electrolytic lesions of the midbrain raphe nuclei. Failure of 5,7 DHT injections to replicate the behavioral changes resulting from electrolytic lesions of the midbrain raphe nuclei suggests that loss of ascending serotonergic projections was not responsible for the behavioral effects that followed the electrolytic lesions.     

GABA injected into the anterior dorsal tegmentum (ADT) of the midbrain blocks stepping initiated by stimulation of the hypothalamus

Previous work showed that the activity rates of certain neurons in the anterior dorsal tegmentum (ADT) of the midbrain correlated with the onset of stepping elicited by hypothalamic stimulation. This study determined if reversible inactivation of the ADT would block locomotion elicited by hypothalamic stimulation of anesthetized rats (urethane, 800 mg/kg). GABA (concentrations 0.25–1.0 mg/μl in saline) were injected in 52 sites in 21 rats. GABA at volumes of 0.1 or 0.2 μl blocked hindlimb stepping in 18 cases. Locomotor blocks occurred within 5 min of the injection, and typically recovered within 10–20 min. The effective blocking sites were clustered around the interstitial nucleus of the medial longitudinal fasciculus. Sites more dorsal and more anterior were not as effective as sites in and ventral to this nucleus. The data are consistent with a role for the ADT of the midbrain in locomotor initiation.     

Roles of the pontine dorsomedial tegmentum and midbrain central gray in regulating female rat sexual behaviors: Effects of p-chlorophenylalanine

Lesions in the midbrain dorsal central gray (MCGL) markedly suppressed both lordosis and soliciting behavior in estradiol benzoate-progesterone (EB-P)-primed castrated female rats. Similarly, the pontine dorsomedial tegmental lesions (PDMTL) caused a severe loss of lordosis behavior. However, the PDMTL females while demonstrating no lordotic response displayed ear-wiggling. These results suggest that the midbrain dorsal central gray plays an important role in regulation of both lordosis and soliciting behaviors. In contrast, the pontine dorsomedial tegmentum appears to participate in regulating the neural mechanism for lordosis only and not for soliciting behavior. Two weeks later prior to a second test, approximately half of the MCGL and PDMTL females that had shown no lordosis in the first test, were injected with p-chlorophenylalanine (PCPA) and primed with EB-P. PCPA significantly increased the lordotic activity of EB-P-primed MCGL females, whereas it failed to potentiate a lordotic response in the PDMTL females. Since PCPA effectively facilitated the display of lordosis in the absence of the neural substrate of the midbrain central gray, this neural structure may not be the minimally necessary component of the neural circuitry regulating the display of lordosis behavior. On the other hand, the pontine dorsomedial tegmentum may be a neural substrate more closely related with the descending pathway controlling the expression of lordosis.     

Characterization of pretentorial periaqueductal gray matter neurons mediating intraspecific defensive behaviors in the rat by microinjections of kainic acid

Unilateral microinjections of 40 pmol of kainic acid (KA; in 0.2 microliter) within the periaqueductal gray matter (PAG) evoked intraspecific defensive postures (defensive uprights, defensive alterting, defensive sideways, backing) in rats confronted with a conspecific. These reactions, which lasted for up to 30 min, were seemingly identical to the rat's natural defensive reaction to attacks by a conspecific although they were evoked by the investigatory approach, rather than the attack, of another rat. Histological analysis revealed that the strongest defensive reactions were evoked from sites within a restricted part of the pretentorial periaqueductal gray matter. Lower doses of KA induced fewer (20 pmol) or non-significant increases (4 pmol) in defensive reactions. Higher doses (100 and 200 pmol) increased the percentage of defensive behavior and also induced oriented jumps out of the test cage. In tests with a conspecific, defensive reactions were elicited most frequently when investigation by the partner was localized to the side of the body contralateral to the injection site. This was confirmed in a sensory reactivity test in which tactile stimulation by the experimenter elicited most defensive reactions when applied on the side of the body contralateral to the injection side. This test also revealed a somatotopic gradient in the animal's reaction: tactile stimulation of the contralateral head and the forelimb evoked the strongest reactions, whereas no responses were observed upon tactile stimulation of the contralateral flank or hindlimb. Measurement of electroencephalographic activity at the cortical, hippocampal, amygdala and PAG levels indicated that the evoked defensive reactions were not secondary to epileptogenic effects. Finally, quantitative analysis of an autoradiographic study found that [3H]KA diffused within a diameter of 1.0-1.2 mm around the cannula tip. Taken together, these results indicate the existence of a population of neurons within a restricted part of the pretentorial PAG of the rat, the excitation of which produces defensive responses and demonstrate that these defensive reactions have a socially adaptive value.     

Hypothalamic and cortical D-C potential changes induced by stimulation of the midbrain reticular formation

Summary D-C potential changes were evoked in the sensorimotor cortex and the hypothalamus of unanesthetized cats with chronically implanted electrodes by repetitive stimulation of the mesencephalic reticular formation. Stimulation at 30 to 300 p/sec. frequencies induced negative d-c shifts which were quite abrupt in onset, starting after the second or third shock with a peak of negativity reached within the first 50 to 100 msec of the 1 second duration of stimulation. Typically these negative shifts were sustained for the duration of stimulation with a prolonged 1 to 1.5 second return to baseline after termination of stimulation. Such d-c shifts were relatively independent of the site of the reference electrode but highly dependent on the location of the active recording electrodes and the stimulating electrodes. Reasons have been advanced to suggest that these negative d-c shifts reflect summated neuronal discharge in the immediate environs of active recording electrodes in the cerebral cortex and hypothalamus. In contrast to the consistent, stimulus-locked negative d-c shifts, less consistent changes, usually positive in polarity, were observed to begin 2 to 3 seconds after termination of RF stimulation. These slower d-c changes were highly dependent on the site of the reference electrode and the physical state of tissue at the electrode tip and therefore far less directly related to neuronal events in cortical and hypothalamic tissue.This study was supported in part by Public Health Service Grants NB-05638 and NB05199.     

Effect of 6-hydroxydopamine, 5,6-dihydroxytryptamine and raphe lesions on the antinociceptive actions of morphine in rats

The antinociceptive effect of morphine was studied in rats in which degeneration of catecholamine- or serotonin-containing nerve terminals had been induced. Vocalisation during electric stimulation of the tail was used as a test for nociception. Brain catecholamines were estimated fluorimetrically.Intraventricular injection of 6-hydroxydopamine markedly reduced brain catecholamines without affecting 5-HT levels; this change was accompanied by a significant reduction of the stimulation threshold both before and after application of morphine.Intraventricular injection of 5,6-dihydroxytryptamine significantly depleted central 5-HT content; the stimulation threshold before and after morphine injection was not affected.Lesions of the midbrain raphe system induced a great reduction of forebrain 5-HT; the morphine effect was not significantly changed.The role of catecholamines and 5-HT in the antinociceptive effect of morphine is discussed in respect to these and other results recently published by this laboratory.     

清醒活动家兔杏仁复合体神经元的自发活动及其电刺激中脑中缝的影响

本实验采用慢性微电极技术,观察了233个杏仁复合体神经元的自发活动和电刺激中脑中缝核群的影响。实验见到,杏仁复合体的单位放电频率及型式,以低频单位、单个不均匀占较大比例,并在空间分布上有一定特点。重复电刺激中脑中缝核群,可调制部分杏仁神经元的活动,其中以激活性影响为主,受中缝调制的单位,主要分布于杏仁复合体的基底核。文中对清醒活动家兔杏仁神经元的自发活动特点和中脑中缝核群对杏仁活动调制的生理意义进行了讨论。     

Projections from ventrolateral hypothalamic neurons containing progesterone receptors and somatostatin to the midbrain central gray in the female guinea pig

The goal of this study was to determine, by combining retrograde fluorescent tract tracing with double immunocytochemistry, whether neurons immunoreactive (IR) for both progesterone receptors (PR) and somatostatin (SOM) in the guinea-pig ventrolateral nucleus (VL) send their axons directly to the midbrain. Unilateral microinjections of true blue (TB) fluorescent dye were made into the midbrain central gray (MCG) of ovariectomized animals primed with estradiol to induce PR and injected intracerebroventricularly with colchicine to visualize SOM-IR neurons. The highest number of PR/SOM-IR neurons in the VL that projected to the midbrain was found when the fluorescent retrograde tracer was confined to the lateral region of the MCG at the mid-rostrocaudal level. More specifically, 29% of the progesterone target neurons containing SOM detected in the VL projected directly to the midbrain, accounting for 7% of the PR-IR neurons and for 9% of the SOM-IR neurons. These triple-labeled cells were found throughout the extent of the VL. Double-labeled cells also sent axons to the MCG and were either PR-IR (14%) or SOM-IR (12%). These results provide morphological evidence that a subset of the PR-IR neurons expressing SOM in the VL have long projections to the midbrain and suggest that the SOM system may modulate neural circuits involved in the regulation of steroid-influenced behaviors and neuroendocrine functions.     

Firing rates and patterns of midbrain reticular neurons during steady and transitional states of the sleep-waking cycle

Summary Spontaneous firing of midbrain reticular formation (MRF) neurons was recorded extracellularly in chronically implanted, behaving cats during steady and transitional states of the sleep-waking cycle. Physiological identification of receiver and/or projection MRF neurons was achieved through orthodromically and antidromically elicited discharges.Discharge rates of MRF neurons were more than double in waking (W) and active sleep (D) without phasic motor events, as compared to synchronized sleep (S). During behavioral states associated with EEG activation, the increased firing was essentially due to cells exhibiting high discharge rates, located at relatively ventral levels of the midbrain core. MRF neurons with identified rostrally projecting axons were more active during W and D states; their discharge rates were significantly higher than those of caudally projecting cells. The discharge patterns of MRF neurons increasing their firing rates from S to W and D were of the tonic type. First-order analyses showed a negligible proportion of both very short and long interspike intervals in all states, large interval density around the mode especially in W and D, and the smallest variation coefficients in W. Rhythmic firing with a period near the modal interval was detected during W by autocorrelations.The increase in firing rate of MRF neurons from S to W or D took place before overt EEG desynchronization and behavioral manifestations that define stable W or D states. In our sample a statistically significant increase in discharge rate was found about 15 s before the end of S sleep epochs that developed into awakening.The differences between discharge features of MRF neurons during waking and sleep states and those of neurons in other brainstem reticular fields are emphasized. Taken together, these data support, at a cellular level, Moruzzi and Magoun's concept of a rostral reticular substrate that gives rise to impulses leading to tonic activation of the thalamocortical systems.Supported by grant MT-3689 from the Medical Research Council of CanadaPart of the results was incorporated in the Ph. D. thesis of N. Ropert