Role of the serotoninergic system in inhibiting the jaw-opening reflex induced by stimulation of the central gray matter of the midbrain

Effect exerted by stimulation of the central grey matter (CGM) of the midbrain and nucleus raphe dorsalis (nRD) by a short train of stimuli (20) followed with the rate of 400/s on the high-threshold jaw-opening reflex (HJOR) evoked by electrical irritation of the tooth pulp was studied during serotonin synthesis blockade after parachlorphenylalanine (pCPA) administration (300 mg/kg intraperitoneally) in awake cats. It was found that the inhibitory effect of the CGM and nRD stimulation on HJOR decreased 24 hours after the pCPA injection. Inhibition of HJOR induced by CGM and nRD stimulation became minimal 96 hours after the pCPA administration: reflex magnitude decreased only by 33% and duration of inhibition was 200-250 ms. Conclusion is made about serotonin participation in HJOR depression induced by CGM and nRD stimulation. The possible participation of other neuromodulators in such inhibition is discussed.     

The midbrain central gray substance as a highly sensitive neural structure for the production of ultrasonic vocalization in the rat

To test whether there were any functional differences between the central gray substance (PVG) and other neural structures for the production of ultrasonic vocalization in the rat, an electrical stimulation experiment was systematically undertaken in diencephalic and mesencephalic tegmental regions. The sound production sensitivity to electrical stimulation was the highest in PVG. This suggested that the periventricular fiber system of Schu¨tz's bundle might be a possible neural structure underlying sound emission in the rat.     

A substance P projection from the VMH to the dorsal midbrain central gray: Implication for lordosis

Substance P has been implicated in the modulation of lordosis behavior at the level of the dorsal midbrain central gray (dMCG). Bilateral injections of substance P into the dMCG facilitate estrogen-induced lordosis behavior in ovariectomized female rats. Input from the ventromedial nucleus of the hypothalamus (VMH) to the dMCG is a vital link in the central nervous system control that mediates the expression of lordosis behavior. Substance P-containing cells have been localized in the VMH and substance P binding sites are localized in the dMCG; this suggested to us that substance P neurons originating in the VMH may terminate in the dMCG. The present study examined the projection of substance P-immunoreactive neurons (SP-IR) in the VMH to the dMCG. The present study examined the projection of substance P-immunoreactive neurons (SP-IR) in the VMH to the dMCG. The retrograde tract tracer fluorogold revealed cell bodies throughout the extent of the VMH and sP immunofluorescence labelled a subpopulation of these cells particularly in the ventrolateral part of the VMH. The majority of sP-projection cells was localized in the caudal two-thirds of the VMH. Thirteen percent of the sP-IR cells were observed to project to the dMCG, while approximately 17% of the sP-IR cells of the ventrolateral part of the VMH projected to the dMCG. These results provide morphological evidence for a substance P projection from the VMH to an area where substance P has been demonstrated to facilitate lordosis behavior.(ABSTRACT TRUNCATED AT 250 WORDS)     

The depression of the background neuronal activity of the midbrain central gray substance by stimulation of monoaminergic structures

Three types of neurons which exhibit spontaneous firing were found in the midbrain periaqueductal grey. The stimulation of monoaminergic structures did not change the firing of the first and second types of neurons but suppressed the third one.     

Changes in the neuronal background activity of the central gray substance of the midbrain to motor cortex stimulation in rats

Motor cortex stimulation induced inhibition of the background activity (BA) in 47% periaqueductal gray neurons, excitation--in 20% and had no influence in 32% neurons. Most of the activated neurons (67%) had the 3d type of BA. Neurons with the 1st type of activity did not respond to the cortex stimulation.     

Effect of stimulation of the central gray substance of the midbrain on neuronal responses of the thalamic medial nuclei in the cat

Effect of the central gray matter (CGM) stimulation on neuronal responses in the thalamic nonspecific medial nuclei evoked by stimulation of the teeth pulp, A-alpha and A-sigma fibres of the infraorbital nerve and the caudal nucleus of the spinal trigeminal tract was studied in experiments on cats under thiopental-chloralose anesthesia. Stimulation of the CGM by a train of stimuli evoked excitatory responses with latency from 15 to 40 ms in a part of investigated neurons. Conditioning stimulation of the CGM led to the inhibition of responses evoked by afferent volleys in neurons of the "low-threshold", "high-threshold" and "convergent" groups. Responses evoked by stimulation of the teeth pulp and A-sigma fibres were inhibited in 100% and those evoked by stimulation of A-alpha fibres of the infraorbital nerve only in 86% of neurons. Presence of the inhibitory influence of the CGM on the thalamic neuron responses evoked both by stimulation of peripheral afferents and spinal trigeminal tract nucleus show that the CGM exerts a direct influence on the thalamic neuron activity.     

Participation of neurons of the midbrain reticular formation of the cat in conditioned reflex activity

Spike discharges of mesencephalic reticular formation neurons were investigated during classical and instrumental conditional reflexes in chronic experiments on cats. Neurons were divided into several functional groups according to patterns of their responses. Two groups of neurons produced continuous tonic spike discharges. One of them was activated by positive conditional stimuli and did not respond to the same stimulus after extinction. Another group was involved in reaction only after extinction. Thus the neurons of these two groups developed additional tonic ascending action not only during fulfillment of conditional reflex but under its internal inhibition too. Neurons of the third group revealed double phasic responses: the first--to sound and the second--in connection with initiation of the conditioned movement. They decreased the level of the background and evoked activity during differentiation and extinction. Initial changes of reticular neuron responses were similar under conditioning and under pseudoconditioning. It is concluded that mesencephalic reticular formation takes an active and differential participation in elaboration, fulfillment and internal inhibition of classical and instrumental conditional reflexes.     

The suppression of the postsynaptic responses of the trigeminal nucleus motor neurons evoked by stimulation of the central gray substance of the midbrain

Effects of the midbrain central gray matter (CGM) stimulation on responses of trigeminal motoneurons is investigated. It is shown that the conditioning CGM stimulation suppressed postsynaptic responses of motoneurons, evoked by stimulation of the tooth pulp and high-threshold fibres of the infraorbital nerve. The conditioning CGM stimulation exerts no influence on the antidromic responses of trigeminal motoneurons and jaw opening reflex, evoked by stimulation of the caudal nucleus of the spinal trigeminal tract. It is concluded that suppression of responses of postsynaptic motoneurons in evidently a result of modulation of the afferent flow on the interneuronal level.     

Effect of the stimulation of the central gray substance of the midbrain on the neuronal responses of the trigeminal caudal nucleus during peripheral excitations

Experiment on cats under chloralose-nembutal anaesthesia has shown that 65% responses of caudal trigeminal nucleus neurons to the activation of the tooth pulp, A alpha and/or A delta infraorbital nerve afferents were completely suppressed by conditioning stimulation of the central grey matter (CGM) by a train of stimuli (10-20) that followed with the rate of 200-400/s, if the interval between conditioning and testing stimuli did not exceed 100 ms. Conditioning stimulation of the CGM inhibited responses of the "convergent" neurons to the activation of tooth pulp most efficiently (0.76) and those to the activation of A alpha afferents more weakly (0.6). Effectiveness of "high-threshold" neurons inhibition under the effect of CGM stimulation was 0.71 and that of "low-threshold" neurons--0.48. Ten caudal trigeminal nucleus neurons were activated by the CGM stimulation with the latency of 7.5-20 ms. These neurons did not respond to peripheral nerve stimulation for 200-450 ms after CGM activation. A possible role of caudal trigeminal nucleus neurons in the CGM inhibition of jaw opening reflexes is discussed.     

Role of glutamate intracortical connections in conditioned reflex activity]

The effects of iontophoretic application of glutamate and its blockers on neuronal activity of the sensorimotor cortex were studied in cat during fulfillment of conditioned instrumental placing reflex. 64 neurons were investigated. Application of glutamate caused reliable facilitation of neuronal impulse reactions to the conditioned stimulus. The facilitation appeared some seconds after beginning of the application of glutamate and continued for 5-10 min after cessation of the iontophoretic application. The similar inhibitory effect in neuronal activity was observed with application of APV, kinurenate, ketamine. It is suggested that the NMDA receptors under natural conditions take part in facilitation of the synaptic transmission in glutamatergic intracortical pathways.     

Ascending fiber projections from the midbrain central gray to the ventromedial hypothalamus in the rat

In 18 urethane-anesthetized ovariectomized female rats, 54 neurons in and around the hypothalamic ventromedial nucleus were transynaptically activated by electrical stimulation of the mesencephalic central gray matter. Ventromedial nucleus afferents from either the dorsal longitudinal fascicle (N = 10) or the medial forebrain bundle (N = 8) were disrupted by a small knife cut. In animals which lacked inputs from the dorsal longitudinal fascicle by a cut posterior to the ventromedial nucleus, transynaptic activation followed central gray stimulation in 20 cells; 26 cells were driven from the central gray in animals deprived of afferents through the medial forebrain bundle by a cut placed laterally to the ventromedial nucleus. Two patterns of synaptic activation followed electrical stimulation at 0.5 Hz: one type of response occurred during a latency range of 4 to 20 ms with fluctuation of each response less than 5 ms, the other type was characterized by diffuse activation for 50 ms following central gray stimulation. It was noted that the former time-locked response occurred in 19 among 20 cells in the posterior-cut animals, and the latter diffuse response was seen in 20 of 26 cells in the lateral-cut animals. Electrical stimulation of the ventromedial nucleus in another group of seven animals with the posterior cut caused antidromic activation of 19 central gray cells at latencies of 4.8 to 21.0 ms. Thus, central gray axons in the medial forebrain bundle were shown to have direct access to the ventromedial nucleus and multisynaptic pathways intervened for those in the dorsal longitudinal fascicle.     

Role of amygdala in conditioned and unconditioned fear generated in the periaqueductal gray

The amygdala and ventral portion of the periaqueductal gray (vPAG) are crucial for the expression of the contextual freezing behavior. However, it is still unclear whether the amygdala also plays a role in defensive behaviors induced by electrical stimulation of the dorsal periaqueductal gray (dPAG). In the present study, rats were implanted with electrodes into dPAG for determination of the thresholds for freezing and escape responses before and after sham or electrolytic lesions in the amygdala. Animals were then submitted to a context fear conditioning procedure. Amygdala lesions disrupted contextual freezing but did not affect defensive behaviors induced by dPAG electrical stimulation. These results indicate that contextual and unconditioned freezing behaviors are mediated by independent neural circuits.     

Electrical stimulation of the midbrain central gray facilitates reticulospinal activation of axial muscle EMG

EMG responses were recorded from axial muscles transversospinalis, medial longissimus, and lateral longissimus in urethane-anesthetized rats during combined electrical stimulation of the reticular formation and midbrain central gray. Central gray stimulation facilitated reticular formation-evoked EMG activity in the back muscles of the rat. Electrical stimulation of the central gray lowered the threshold for reticulospinal activation of axial muscles and could maintain firing in these muscles after the end of a reticular formation train. Units were recruited in order of size from small to large. In only one case, central gray stimulation activated axial muscles directly without reticular formation stimulation. The central gray may be important in relaying hypothalamic influences to the reticular formation, which has direct access to the axial muscles responsible for lordosis behavior.     

The neurochemical mechanisms of the inhibitory influences of stimulation of the central gray substance on the high-threshold reflex activity of the reticular formation]

Influences of bulbar microinjections of some neuromodulator antagonists (methysergide, haloperidol, naloxone) on depression dynamics of spino-bulbo-spinal (SBS) reflex from the periaqueductal grey (PAG) stimulation were studied in chloralose anesthetized rats. Microinjections were made into the reticular gigantocellular nucleus which is a main supraspinal centre of SBS reflexes. It is found that methysergide administration (10(-5) mol/l) causes a considerable (two to four times as less) decrease of SBS-reflex depression evoked by PAG stimulation by a short high-frequency series of stimuli. Long-lasting depression evoked usually by a long train of PAG stimuli is less reduced: from 6-10 to 2.5-4 min. A considerable (2 to 5 times) shortening of this depression is revealed after naloxone injections (10(-5)-10(-4) mol/l). The most expressed diminution of all types of inhibitions is shown to occur after haloperidol administration. Data obtained evidence that all the studied neuromodulatory systems (5-HT, catecholamine and opioidergic ones) are involved to inhibitory action of PAG on high-threshold reflex activity of the reticular formation. Some properties of neurochemical mechanisms of the studied PAG-evoked depression are discussed.     

Pituitary adenylate cyclase-activating polypeptide neurons of the ventromedial hypothalamus project to the midbrain central gray

The pituitary adenylate cyclase-activating polypeptide (PACAP) and brain-derived neurotrophic factor (BDNF) neurons have recently been established as markers of the ventromedial hypothalamic nucleus (VMH). However, their neural projections from the VMH remained unknown. We examined whether PACAP and BDNF neurons in the VMH connected to the mesencephalic central gray (MCG), using the combination of in-situ hybridization and immunohistochemical tracing of Fluorogold (FG) injected into the MCG. Approximately 49% of PACAP neurons and 19% of BDNF neurons in the VMH contained the retrograde-transported FG, and 52% of FG-positive cells were PACAP neurons. These results indicate that a large number of PACAP neurons and a small number of BDNF neurons of the VMH project to the MCG.     

Kindling of the midbrain periaqueductal gray in rats

We investigated the role of midbrain periaqueductal gray matter (PAG) in the manifestation of generalized seizures by administering electrical stimulation to this area in rats. Electrical stimulation of 60 Hz biphasic square pulses of a 1-s duration administered to the PAG-induced convulsive responses in the following order: (1) Type I, running (stimulus intensity; range 50--200 microA, mean 73.1 microA) without afterdischarge (AD), (2) Type IIa, running (stimulus intensity; range 50--300 microA, mean 111.8 microA) with AD at the PAG and the amygdala (AMY), and (3) Type IIb, generalized tonic--clonic seizures (GTCS) (stimulus intensity; range 50--250 microA, mean 182.1 microA) with AD at the PAG and AMY. Twenty daily PAG stimulations at the non-GTCS inducing threshold failed to produce kindling. However, 20 daily PAG stimulations at the GTCS threshold produced progressive AD spread involving the motor cortex, and progressive changes in the behavioral seizure pattern. These findings indicate that the PAG can be effectively kindled. However, PAG kindling has no apparent influence on subsequent AMY kindling.     

An estradiol-induced protein synthesized in the ventral medial hypothalamus and transported to the midbrain central gray

Estradiol (E2) facilitates the lordosis reflex that occurs in response to flank stimulation in female rats. Lordosis appears to be regulated in part by the synthesis of proteins in the ventral medial hypothalamus (VMH) that are transported to the midbrain central gray (MCG). We developed a strategy involving microinfusion of radioactive amino acids, followed by 2-dimensional gel electrophoresis, to identify proteins that may be regulated by E2 in the VMH and transported to the MCG. A mixture of 35S-methionine and 35S-cysteine (2:1, total 500-1000 microCi), suspended in 1 microliter PBS, was infused bilaterally into the VMH over a period of 2 hr into matched pairs of ovariectomized female rats, one of which was given a Silastic implant containing E2 at the beginning of infusion or 1 week earlier. The rats were sacrificed 12 hr after the end of infusion, and several brain regions were obtained by microdissection. Samples were analyzed by 2-dimensional gel electrophoresis, entailing isoelectric focusing in the first dimension and SDS-PAGE (molecular-weight separation) in the second dimension, followed by fluorography. We could routinely separate at least 250 spots. We consistently found a protein spot with an apparent molecular weight of 70 kDa, pI of about 5.9, which almost always appeared in the VMH and MCG of rats given E2 replacement but very rarely in samples from ovariectomized rats given no E2 replacement. A spot immediately acidic to this protein (70 kDa, pl about 5.8) appeared to vary inversely with this E2-induced protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Activation of protein kinase C in the hypothalamic ventromedial nucleus or the midbrain central gray facilitates lordosis

Many neurotransmitters and neuropeptides can act through the hypothalamic ventromedial nucleus (VMN) or midbrain central gray (MCG) to facilitate lordosis. Since these lordosis-facilitating agents can also stimulate the phosphoinositide (PI) second-messenger pathway, it was hypothesized that direct activation of this pathway can also potentiate the behavior. To evaluate this possibility, a phorbol ester, TPA (12-O-tetradecanoyl phorbol 13-acetate), was used to activate a key enzyme, protein kinase C (PKC), of the PI pathway in ovariectomized (OVX) rats either primed or not primed with estrogen. These female rats were paired with males for mating tests before and after an intracerebral infusion of TPA, and both the lordosis quotient (LQ) and the lordosis strength (LS) were measured. Bilateral infusion of TPA (5 μg/0.5 μl or 0.2 μg/0.2 μl, but not 0.1 μg/0.2 μl/side) into the VMN or MCG of estrogen-primed subjects facilitated both LQ and LS in 30 min, peaked at 60–90 min, and the facilitation lasted for more than 180 min. This facilitatory effect of TPA was: (1) not observed in OVX rats not primed with estrogen; (2) not observed if the infused TPA did not reach both sides of the VMN or MCG; (3) not mimicked by 4-phorbol 12,13-didecanoate, which does not activate PKC; (4) blocked by PKC inhibitors (H7 10 mM or staurosporine 1 μM, 0.2 μl/ side), which by themeselves did not facilitate lordosis; and (5) was not affected by pretreatment of the progestin antagonist RU486. These observations indicate that TPA facilitates lordosis in a dose-dependent fashion by activating, and not by depleting, PKC in the VMN or MCG, and that the TPA effect requires estrogen priming but not the activation of progestin receptors. Thus, the PI pathway or the activation of PKC may be a common mediator for lordosis facilitation in these two brain regions; and the requirement of estrogen priming further raises the possibility that this second-messenger system or its substrates in the VMN and MCG are modulated by estrogen.     

Electrical stimulation of the midbrain central gray facilitates lateral vestibulospinal activation of back muscle EMG in the rat

Electrical stimulation in the midbrain central gray in urethane-anesthetized female rats increased responses of the deep back muscles lateral longissimus and medial longissimus to stimulation of the lateral vestibular nucleus (LVN). During central gray stimulation, LVN stimuli led to larger muscle responses, recruitment of new motor units, and decreased latency of muscle response. Effective central gray sites are hypothesized to act through axons descending to medullary reticular formation. Results are consistent with participation of these neuronal groups in the activation of lordosis behavior, a vertebral dorsiflexion that requires deep back muscle contraction, but these electromyographic results could also be relevant for other behaviors that require vertebral postural adjustments.