Short intrinsic circuit in the periaqueductal gray matter of the cat

In heavily impregnated Golgi material of cat periaqueductal gray matter (PAG), we were able to follow the course of early axon collaterals to their endings on the soma of neighboring neurons. A short intrinsic circuit is therefore created. Combined Golgi and electron microscope observation give evidence of the inhibitory action of this circuit.     

Intranuclear bodies in neurons of the periaqueductal gray matter in the cat

The nucleoplasm of neurons in the nucleus lateralis of the periaqueductal gray matter in the cat contains fibrillar structures which have no limiting membranes. These intranuclear bodies are associated with neither the nucleolus nor the nuclear membrane and have two characteristic forms. The first, the rodlet, is a compact bundle of fibrils 2 to 8 nm in diameter. It is usually elongated in shape although it appears spherical when sectioned transversely. This rodlike structure appears to correspond to Roncoroni's rodlet or the accessory body of Cajal in light microscopy. The second and more commonly observed form is a long slender bundle of five rows of parallel fibrils. Although similar intranuclear structures have frequently been observed in the highly differentiated neurons of the sympathetic ganglia and the retina, this is the first report of their presence in the undifferentiated neurons of the isodendritic core of the brainstem.     

A golgi study of the periaqueductal gray matter in the cat. Neuronal types and their distribution

Summary In Golgi material, the neurons of the periaqueductal gray matter (PAG) of the cat have been classified into five types, according to the following criteria: number of dendrites per cell, characteristics of secondary arborization, frequency of spines and axon caliber. Type 1 cells, which are multipolar and rich in spines are the most frequent, and are probably intranuclear neurons. Type 4 cells have a short axon which ends in the PAG, but they differ from Type 1 in that their dendritic ramification is of a different type and there are few spines. Type 2 and 3 neurons have a thick axon which runs outside the PAG, and dendrites rich in spines. Type 2 cells have more primary dendrites, while Type 3 neurons have dendrites which may spread outside the PAG. Type 5 cells have dendrites with few spines and no secondary ramification. Their thick and long axon projects outside the PAG. Type 2, 3 and 5 cells have been considered projective neurons. The various neuron types are present in every area of the PAG, although in the ventral region there is a predominance of Type 2 and 5 neurons, in the dorsal regions of Type 2 and 3 cells, and in lateral regions of Type 3 and 5 cells. Local intrinsic circuits have been observed in which both the interneurons and the projective, with early axonic collaterals, are involved. The prevalence of neurons to which an afferent role has been attributed (Type 2 and 3 cells) compared with efferent cells (Type 5), is in agreement with hodological studies which indicate that the PAG receives multiple and numerous afferents in comparison with the relatively scarce efferent fibers. These projections can be intensely and deeply elaborated and modulated by means of local intrinsic circuits.     

Inactivation of the periaqueductal gray attenuates antinociception elicited by stimulation of the rat medial preoptic area

The medial preoptic area (MPOA) is a sexually dimorphic structure that plays key roles in gonado-steroidal regulation and thermoregulation. The MPOA may be involved in sex-based differences in nociceptive processing and steroid hormones effect on pain thresholds. Consistent with this, there is evidence that MPOA can produce antinociception or hyperalgesia. MPOA stimulation inhibits spinal cord or trigeminal neuronal responses to noxious stimuli or produces analgesia, yet most of these studies utilized electrical stimulation which antidromically activates periaqueductal gray (PAG) and rostroventromedial medulla (RVM) neurons involved in descending modulation of nociception. Effects of selective activation of MPOA neurons on behavioral indices of antinociception and the site-specificity of such responses are unknown. To address these questions, we examined the influence of MPOA microinjections of d,l homocysteate (DLH) on hindlimb and tail nocifensive reflexes in lightly anesthetized rats. DLH, but not saline, microinjections into several MPOA subregions markedly increased withdrawal response latencies to noxious thermal stimuli. Antinociceptive effects of MPOA activation were abolished by microinjection of lidocaine into PAG. These results suggest that activation of MPOA neurons produces antinociception that is at least partly mediated by projections to PAG.     

Re-emergence of hypothalamically elicited eating following change in food

Rats with lateral hypothalamic electrodes were administered electrical stimulation until the elicited eating of one type of food had emerged to asymptotic levels. Experimental animals later were presented with a different diet, and elicited eating underwent another emergence process. This re-emergence occurred despite the fact that animals had had deprivation experience with the second food immediately prior to stimulation experience with it. These data present interpretative difficulties for the hypothesis that elicited behavior reflects the activation of neural systems underlying deprivation-induced consummatory activity and that the anomalous characteristics of elicited responding can be attributed to an emotionality state which theoretically is also evoked by the stimulation.     

The control exerted by the substantia nigra on the quiet biting attack elicited by hypothalamic stimulation in the cat

The effects of substantia nigra (SN) (pars compacta) stimulation on the quiet biting attack evoked by hypothalamic activation in the cat were studied. The measure of the aggressive behavior was the latency of the biting which did not greatly change from one animal to another when hypothalamic stimulation was performed with the same parameters. Concurrent activation of the SN determined an inhibitory effect on the studied behavior in the form of an increase in the biting latency or loss of the attack pattern. The inhibitory role of the SN on predatory attack behavior is discussed.     

Efferent projections of the periaqueductal gray in the rabbit.

The efferent projections of the periaqueductal gray in the rabbit have been described by anterograde tract-tracing techniques following deposits of tritiated leucine, or horseradish peroxidase, into circumscribed sites within dorsal, lateral or ventral periaqueductal gray. No attempts were made to place labels in the fourth, extremely narrow (medial), region immediately surrounding the aqueduct whose size and disposition did not lend itself to confined placements of label within it. These anatomically distinct regions, defined in Nissl-stained sections, corresponded to the same regions into which deposits of horseradish peroxidase were made in order for us to describe afferent projections to the periaqueductal gray. In this present study distinct ascending and descending fibre projections were found throughout the brain. Terminal labelling was detected in more than 80 sites, depending somewhat upon which of the three regions of the periaqueductal gray received the deposit. Therefore, differential projections with respect to both afferent and efferent connections of these three regions of the periaqueductal gray have now been established. Ventral deposits disclosed a more impressive system of ramifying, efferent fibres than did dorsal or lateral placements of labels. With ventral deposits, ascending fibres were found to follow two major pathways from periaqueductal gray. The periventricular bundle bifurcates at the level of the posterior commissure to form hypothalamic and thalamic components which distribute to the anterior pretectal region, lateral habenulae, and nuclei of the posterior commissure, the majority of the intralaminar and midline thalamic nuclei, and to almost all of the hypothalamus. The other major ascending pathway from the periaqueductal gray takes a ventrolateral course from the deposit site through the reticular formation or, alternatively, through the deep and middle layers of the superior colliculus, to accumulate just medial to the medial geniculate body. This contingent of fibres travels more rostrally above the cerebral peduncle, distributing terminals to the substantia nigra, ventral tegmental area and parabigeminal nucleus before fanning out and turning rostrally to contribute terminals to ventral thalamus, subthalamus and zona incerta, then continuing on to supply amygdala, substantia innominata, lateral preoptic nucleus, the diagonal band of Broca and the lateral septal nucleus. Caudally directed fibres were also observed to follow two major routes. They either leave the periaqueductal gray dorsally and pass through the gray matter in the floor of the fourth ventricle towards the abducens nucleus and ventral medulla, or are directed ventrally after passing through either the inferior colliculus or parabrachial nucleus. These ventrally directed fibres merge just dorsal to the pons on the ventral surface of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)     

Peripheral somatic activation and spontaneous firing patterns of neurons in the periaqueductal gray of the cat

A total of 50 neurons in the periaqueductal gray (PAG) of the cat were studied. Sixty percent of the units were excited by nociceptive pinch stimulation. These units were also excited by non-nociceptive stimulation such as tapping, hair movement and/or joint rotation. Non-nociceptive stimulation excited 20% of the units, whereas nociceptive stimulation did not drive these units. Their receptive fields were large, and often encompassed the entire body. Spontanous discharges of PAG neurons revealed great irregularity.     

Influences of benzodiazepines on hypothalamically elicited emotional responses in the rabbit

Tranquilizers of the benzodiazepine group (oxazepam and diazepam) enhanced the rate of self-stimulation elicited from the lateral hypothalamus. Both tranquilizers induced reversal of escape and avoidance behaviors elicited from the medial hypothalamus to a high rate self-stimulation. The reversal effect persisted for two days and the initial negative emotional responses returned later as the drugs were depleted in the organisms. Thus the effect of minor tranquilizers substantially mimics the ventral hippocampus ablation effect which also produced the reversal of the medial hypothalamus responses in earlier experiments.     

Inhibition of spinal neuronal responses to noxious skin heating by stimulation of mesencephalic periaqueductal gray in the cat.

1. Discharges of lumbar dorsal horn neurons were evoked by noxious radiant skin heating, and inhibition of the heat-evoked responses by stimulation of the mesencephalic periaqueductal gray was investigated in N2O-anesthetized cats. 2. Thirty-seven units selected on the basis of receiving afferent C-fiber input from the posterior tibial and/or superficial peroneal nerves responded vigorously to 50 degrees C heating of the plantar surface of the ipsilateral hindpaw. All discharges were inhibited by periaqueductal gray stimulation (PAGS) at current strengths of 300--900 microA; the mean threshold for inhibition was 167 microamperemeter. The mean frequency of the inhibited discharge was 39% of the control response. 3. Effective PAGS sites were distributed throughout the ventral PAG bilaterally. Stimulus current-distance estimates indicate that small (0.5--1.2 mm diameter) volumes of tissue within the PAG were stimulated. 4. A monotonic relationship between temperature and unitary discharge was found for skin heating from threshold to about 50 degrees C. PAGS resulted in a decrease in the slope of the curve plotting discharge against temperature, without altering the threshold. 5. Inhibition of the heat-evoked discharges rarely outlasted the PAGS. 6. Possible neural substrates for descending inhibition and correlates with neural mechanisms of analgesia are discussed.     

Mapping of jumping, rearing, squealing and switch-off behaviors elicited by periaqueductal gray stimulation in the rat

Rats readily learn to escape from a stimulation applied to most mesencephalic periaqueductal gray (PAG) sites. In the present study, we tried to find out to what extent the differential effects induced by such stimulations actually reflect the existence of intraPAG functional subdivisions. To that end, a row of five electrodes was implanted into the PAG of each of 29 rats. Two kinds of effects were analyzed, the stimulation-elicited overt behaviors and the generalization of switch-off responding from one stimulation site to the others. Further, switch-off latency versus interpulse interval (IPI) relationships were established and both the threshold IPIs and the ceiling switch-off latencies were determined. The most commonly elicited behaviors (jumping, rearing and squealing) as well as the threshold IPIs and the ceiling switch-off latencies were mapped within the PAG. Switch-off behavior was elicited from all the stimulation sites studied. However, in the dorsal PAG the switch-off latency was found to decrease more steeply with decreasing IPI than it did in the ventral PAG. Switch-off generalization was less frequently observed between dorsally located stimulation sites. Jumps were most often elicited from dorsally and rostrally located PAG sites while squeals were more frequently elicited from the caudal part of the PAG and rearings from PAG subareas surrounding the aqueduct.     

Angiotensin III modulates the nociceptive control mediated by the periaqueductal gray matter

Endogenous angiotensin (Ang) II and/or an Ang II-derived peptide, acting on Ang type 1 (AT₁) and Ang type 2 (AT₂) receptors, can carry out part of the nociceptive control modulated by periaqueductal gray matter (PAG). However, neither the identity of this putative Ang-peptide, nor its relationship to Ang II antinociceptive activity was clarified. Therefore, we have used tail-flick and incision allodynia models combined with an HPLC time course of Ang metabolism, to study the Ang III antinociceptive effect in the rat ventrolateral (vl) PAG using peptidase inhibitors and receptor antagonists. Ang III injection into the vlPAG increased tail-flick latency, which was fully blocked by Losartan and CGP 42,112A, but not by divalinal-Ang IV, indicating that Ang III effect was mediated by AT₁ and AT₂ receptors, but not by the AT₄ receptor. Ang III injected into the vlPAG reduced incision allodynia. Incubation of Ang II with punches of vlPAG homogenate formed Ang III, Ang (1–7) and Ang IV. Amastatin (AM) inhibited the formation of Ang III from Ang II by homogenate, and blocked the antinociceptive activity of Ang II injection into vlPAG, suggesting that aminopeptidase A (APA) formed Ang III from Ang II. Ang III can also be formed from Ang I by a vlPAG alternative pathway. Therefore, the present work shows, for the first time, that: (i) Ang III, acting on AT₁ and AT₂ receptors, can elicit vlPAG-mediated antinociception, (ii) the conversion of Ang II to Ang III in the vlPAG is required to elicit antinociception, and (iii) the antinociceptive activity of endogenous Ang II in vlPAG can be ascribed preponderantly to Ang III.     

Functional characteristics of the midbrain periaqueductal gray

The major functions of the midbrain periaqueductal gray (PAG), including pain and analgesia, fear and anxiety, vocalization, lordosis and cardiovascular control are considered in this review article. The PAG is an important site in ascending pain transmission. It receives afferents from nociceptive neurons in the spinal cord and sends projections to thalamic nuclei that process nociception. The PAG is also a major component of a descending pain inhibitory system. Activation of this system inhibits nociceptive neurons in the dorsal horn of the sinal cord. The dorsal PAG is a major site for processing of fear and anxiety. It interacts with the amygdala and its lesion alters fear and anxiety produced by stimulation of amygdala. Stimulation of PAG produces vocalization and its lesion produces mutism. The firing of many cells within the PAG correlates with vocalization. The PAG is a major site for lordosis and this role of PAG is mediated by a pathway connecting the medial preoptic with the PAG. The cardiovascular controlling network within the PAG are organized in columns. The dorsal column is involved in pressor and the ventrolateral column mediates depressor responses. The major intrinsic circuit within the PAG is a tonically-active GABAergic network and inhibition of this network is an important mechanism for activation of outputs of the PAG. The various functions of the PAG are interrelated and there is a significant interaction between different functional components of the PAG. Using the curent information about the anatomy, physiology, and pharmacology of the PAG, a model is proposed to account for the interactions between these different functional components.     

Scopolamine-sensitive and resistant components of increase in cerebral cortical blood flow elicited by periaqueductal gray matter of rats

The present study attempted to evaluate the effects of inhibition of cortical muscarinic receptors on the increase in cortical blood flow (laser-Doppler flowmetry) elicited by chemical stimulation of the periaqueductal gray matter in 28 anesthetized rats with cervical cordotomy. A new device was introduced which allowed focal cortical superfusion with scopolamine, an antagonist for muscarinic receptors, without disturbing the temperature of an exposed cortical area under study. We found that although the flow increase was attenuated by scopolamine (31.6 μmol/l to 1 mmol/l) so applied, about one third of it was resistant to the treatment. Cortical muscarinic and non-muscarinic receptor mechanisms may thus subserve the mediation of the flow increase.     

GABA-immunoreactive neurons and terminals in the cat periaqueductal gray matter: a light and electron microscopic study

Immunocytochemical and electron microscopic methods were used to study the GABAergic innervation in adult cat periaqueductal gray matter (PAG). A mouse monoclonal antibody against γ -aminobutyric acid (GABA) was used to visualize the inhibitory neuronal system of PAG. At light microscopy, GABA-immunopositive (GABA_IP) neurons formed two longitudinally oriented columns in the dorsolateral and ventrolateral PAG that accounted for 36% of the neuronal population of both PAG columns; their perikaryal cross-sectional area was smaller than that of unlabeled (UNL) neurons found in the same PAG subdivisions. At electron microscopic level, patches of GABA immunoreactivity were readily detected in neuronal cell bodies, proximal and distal dendrites, axons and axon terminals. Approximately 35–36% of all terminals were GABA_IP; they established symmetric synapses with dendrites (84.72% of the sample in the dorsolateral PAG and 86.09% of the sample in the ventrolateral PAG) or with cell bodies (7–10% of the sample). Moreover, 49.15% of GABA_IP axon terminals in the dorsolateral and 52.16% in the ventrolateral PAG established symmetric synapses with GABA_IP dendrites. Immunopositive axon terminals and unlabeled terminals were also involved in the formation of a complex synaptic arrangment, i.e. clusters of synaptic terminals in close contact between them that were often observed in the PAG neuropil. Moreover, a fair number of axo-axonic synapses between GABA_IP and/or UNL axon terminals were present in both PAG subdivisions. Several dendro-dendritic synapses between labeled and unlabeled dendrites were also observed in both PAG subdivisions. These results suggest that in the cat PAG there exist at least two classes of GABArgic neurons. The first class could exert a tonic control on PAG projecting neurons, the second could act on those GABAergic neurons that in turn keep PAG projecting neurons under tonic inhibition. The functional implications of this type of GABAergic synapse organization are discussed in relation to the dishinibitory processes that take place in the PAG.     

Effects of hypothalamically elicited emotional behaviors on the plasma levels of estradiol and IGF-1.

We examined changes in the plasma levels of estradiol (E2), insulin-like growth factor-1 (IGF-1), ACTH, cortisol and catecholamines accompanying various kinds of hypothalamically elicited emotional behaviors in female cats. The emotional behaviors consisting of restlessness, threat and searching-biting (S-B) were elicited intermittently for 6 h by electrical stimulation of the anterior hypothalamus (AH), ventromedial hypothalamus (VMH) and lateral hypothalamus (LH), respectively, in awake and free-moving conditions. The blood was sampled three times immediately before, 1 h after and 6 h after the start of stimulation. The plasma levels of ACTH, cortisol and catecholamines significantly increased in both restlessness and threat behaviors, whereas in the S-B behavior, the ACTH level significantly increased, while the cortisol level showed a slight nonsignificant increase. No changes were observed in the plasma catecholamine levels in the S-B behavior. The plasma E2 level significantly increased in threat behavior after 1 and 6 h of stimulation compared to the prestimulation levels, and the level also increased in comparison to the control group after 1 h. In contrast, the restlessness and S-B behaviors had little or no effect on the E2 level. No significant changes were observed in the plasma levels of IGF-1 in all behavior groups. These findings suggest that various hypothalamically elicited emotional behaviors have differential effects on the plasma E2, but not on the IGF-1 levels. Therefore, E2 and IGF-1 are regulated independently of each other.     

Shock-induced hyperalgesia: II. Role of the dorsolateral periaqueductal gray.

Exposure to 3 moderately intense (1-mA) tailshocks has been shown to lower vocalization thresholds to both heat and shock. Previous shock exposure also facilitates the acquisition of conditioned fear as measured by freezing. These observations suggest that shock induces hyperalgesia (enhanced pain). This study explored whether shock-induced hyperalgesia depends on neurons within rostral or caudal portions of the dorsolateral periaqueductal gray (dlPAG). Experiment 1 examined the impact of dlPAG lesions on the acquisition of conditioned fear. Sham-operated rats demonstrated enhanced acquisition after shock exposure; both rostral and caudal lesions eliminated this effect. Experiment 2 showed that tailshock lowered vocalization thresholds to heat in sham-operated but not lesioned subjects. These results suggest that the dlPAG plays a critical role in the production of shock-induced hyperalgesia.     

Afferent projections to the periaqueductal gray in the rabbit

The afferents to the periaqueductal gray in the rabbit have been described following hydraulic pressure injection of horseradish peroxidase at various sites throughout this structure. Every third section was reacted with tetramethylbenzidine, for the localization of afferent neurons. At the site of the deposit alternate sections were reacted with tetramethylbenzidine, Hanker-Yates reagent, or diaminobenzidine, for comparative assessment of the injection site. A large number of retrogradely labelled cells, assessed by bright- and dark-field microscopy, were observed in a wide range of areas throughout the brain. Major labelled areas within the telencephalon were cortical areas 5, 20, 21, 32 and 40. Within the diencephalon, the hypothalamus contained quantitatively by far the largest number of labelled cells. Of these nuclei, the dorsal pre-mammillary nucleus contained the largest number of labelled cells. Considerable labelling was also found within medial and lateral preoptic nuclei, anterior hypothalamic area, and ventromedial hypothalamic nucleus. Another diencephalic region containing a significant number of retrogradely labelled neurons was the zona incerta. At midbrain, pontine and medullary levels, additional labelled regions were: the substantia nigra, cuneiform nucleus, parabigeminal nucleus, raphe magnus, and reticular areas. Heavy labelling was seen within the periaqueductal gray itself, rostral and caudal to deposits placed within each subdivision. In addition, a large number of other areas labelled throughout the brain (Tables 2A-D). Not only were some differences noted in the pattern of labelled cells with deposits placed rostrally or caudally within periaqueductal gray, but certain topographical differences with respect to the degree of labelling within nuclei were also seen with injection sites ventral, lateral or dorsal to the aqueduct. In addition, a further difference was noted, in that over one third of the areas labelled with deposits in just one or other of the "divisions" within periaqueductal gray. The results therefore suggest that the periaqueductal gray might be divisible to some extent on the basis of connectivity with intrinsic subdivisions of the complex. It is hoped that, with time, it might prove possible to resolve any such differential input in functional terms. The wide variety of afferent input to the periaqueductal gray, and its strategic location, would seem to place it in a unique position for integrating and modifying a diversity of motor, autonomic, hormonal, sensory and limbic influences.(ABSTRACT TRUNCATED AT 400 WORDS)     

Connections of midbrain periaqueductal gray in the monkey. I. Ascending efferent projections

1. To obtain a comprehensive map of the ascending projection of the midbrain periaqueductal gray, small amounts of [3H]leucine were injected into discrete regions of the periaqueductal gray (PAG) of the monkey. 2. Despite the fact that different regions of the PAG were injected in separate animals, the majority of the PAG's efferent projections to the forebrain remained constant. 3. The diencephalic projection areas include the nucleus reticularis thalami, the nucleus medialis dorsalis, the midline thalamic nuclei, the intralaminar thalamic nuclei, the preoptic area, and the anterior, dorsal, periventricular, ventromedial, periarcurate, lateral, and posterior hypothalamic nuclei. 4. More caudal areas receiving a projection from the PAG include the zona incerta and the mesencephalic reticular formation. 5. The main route for PAG-diencephalic projections is through the periventricular bundle. This implies that if a stimulating electrode was placed in this region, both the diencephalic leads to PAG and PAG leads to diencephalic pathways would be stimulated. This observation may explain the wide variety of side effects often encountered in stimulation-produced analgesia in humans. 6. These results support our hypothesis that the PAG functions as a visceral, nociceptive, and cognitive integrator.