Phenytoin administration reveals a differential role of pontine reticular formation and periaqueductal gray neurons in generation of the convulsive behaviors of audiogenic seizures

The ventrolateral periaqueductal gray (PAG) and pontine reticular formation (PRF) are implicated in the neuronal network for audiogenic seizures (AGS). The AGS of genetically epilepsy-prone rats (GEPR-9s) culminate in tonic hindlimb extension (TE), and elevated acoustically evoked neuronal firing and burst firing, immediately preceding TE, have been observed in PAG and PRF. This study examined changes in PAG and PRF neuronal firing and behavior in GEPR-9s, following phenytoin administration. Recordings involved 16 PAG and nine PRF neurons in GEPR-9s. Phenytoin in doses (mean, 6. 3 mg/kg) that suppressed TE selectively did not consistently alter PAG neuronal firing. However, these doses of phenytoin resulted in significant (51.6% of control) suppression of PRF neuronal firing. Doses of phenytoin (mean, 8.3 mg/kg), which completely blocked AGS, significantly reduced PAG neuronal firing (64.6% of control), and more greatly suppressed PRF firing (25.8% of control). These results are consistent with a critical role for PRF neurons in generation of TE not evident for PAG. The suppression of PAG and PRF neuronal firing induced by phenytoin with complete seizure blockade is consistent with vital roles for both structures in the seizure network. The differential effects of phenytoin on structures requisite to the seizure network indicate that this experimental approach may be able to identify the most sensitive therapeutic target for anticonvulsant drugs, which could be critical to pharmacological suppression of specific seizure behaviors manifest in various types of convulsions, potentially including human epilepsy.     

Membrane and synaptic effects of corticotropin-releasing factor on periaqueductal gray neurons of the rat

Corticotropin-releasing factor (CRF) has been identified as a major component of the hypothalamic-pituitary-adrenal (HPA) axis. By stimulating the release of adrenocorticotropin hormone (ACTH), CRF acts as a key mediator of the stress response. However, CRF receptors and neuronal elements are present in many extrahypothalamic regions of the brain. A region that contains both CRF-ergic neurons and CRF receptors is the midbrain periaqueductal gray (PAG). The physiological effects of CRF in the PAG are unknown. In this study, an in vitro preparation, extracellular and intracellular patch-clamp recordings, were used to examine the effects of CRF, applied through an injecting electrode, on PAG neurons. Recordings were made from 147 neurons in the PAG. CRF injecting electrode concentrations of 0.05 and 1 microM were tested. At the higher concentration, CRF had a predominant excitatory effect on the neurons, and at the lower concentration, CRF produced no significant effect on the neurons. The excitatory effect was dose dependent and was often associated with a depolarization in membrane potential in intracellular recordings. Application of the CRF antagonist, alpha-helical CRF, blocked this excitatory effect. It is concluded that CRF has a predominant excitatory effect on PAG neurons. It is also concluded that CRF is not acting presynaptically. This excitatory effect of CRF on PAG neurons may lead to activation of a descending analgesic pathway.     

Opiate withdrawal increases ProTRH gene expression in the ventrolateral column of the midbrain periaqueductal gray

The midbrain periaqueductal gray matter (PAG) has a critical role in the modulation of behavioral and autonomic manifestations of the opiate withdrawal syndrome. We report a nearly 5-fold increase in proTRH gene expression in neurons of the ventrolateral column of the PAG following naltrexone precipitated morphine withdrawal. The accumulation of immunoreactive proTRH-derived peptides, but not the mature TRH tripeptide was concomitantly observed in these cells. These findings indicate that proTRH-derived peptides synthesized in neurons of the ventrolateral PAG may function as modifiers of opiate withdrawal responses.     

Eyelid apraxia associated with deep brain stimulation of the periaqueductal gray area

We report a patient with eyelid apraxia following deep brain stimulation of the periaqueductal gray area. Based on the position of our electrode, we argue that the phenomenon is linked to inhibition of the nearby central caudal nucleus of the oculomotor nucleus by high frequency stimulation.     

In vitro responses of neurons in the periaqueductal gray to hypoxia and hypercapnia

Hypoxia-sensitive neurons in the caudal hypothalamus (CH) have been shown to project to the periaqueductal gray (PAG) which, in turn, sends descending projections to an area of the ventrolateral medulla (VLM) containing neurons inherently excited by hypoxia. The purpose of this study was to determine if neurons in the PAG are excited by hypoxia or hypercapnia in an in vitro environment. Extracellular responses to hypoxia and hypercapnia of neurons located throughout the PAG were recorded in a rat brain slice (400–500 μm thick) preparation. Hypoxic (10% O₂/5% CO₂/85% N₂) and hypercapnic (7% CO₂/93% O₂) stimuli were delivered to the tissue through gas bubbled into the brain slice chamber. A majority (39 of 53) of the neurons tested responded to hypoxia. Of these neurons, 92% responded to hypoxia with an increase in firing rate. Neurons in the dorsolateral/lateral regions increased firing rates to a greater extent than neurons located in ventrolateral regions. All neurons tested (n=6) also responded to hypoxia after perfusion of the tissue with a low Ca²⁺/high Mg²⁺ solution to block classic synaptic transmission. Only a small proportion (7/33) of neurons tested responded to hypercapnia. These findings indicate that neurons in the periaqueductal gray region of the brain have an inherent responsiveness to hypoxia and, thus, may contribute to the overall coordination of cardiorespiratory responses to systemic hypoxia.     

Opioid receptor subtypes associated with ventral tegmental facilitation and periaqueductal gray inhibition of feeding

Eating was induced in sated animals by lateral hypothalamic electrical stimulation following central microinjections of mu- (morphine), delta-([D-Pen2,D-Pen5]enkephalin) or kappa-(U-50,488H) receptor agonists, or saline. With stimulation intensity fixed at a moderate level, time to eat 3 45-mg food pellets decreased with increases in stimulation frequency, approaching an asymptote near 7 s at ca. 70 Hz. Ventral tegmental injections (8 but not 0.8 nmol) of each of the 3 drugs reduced the minimum frequency required to produce eating of 3 pellets within 20 s and reduced the frequency at which asymptotic performance was produced; the drugs were equally effective at these doses. Naloxone (2 mg/kg) reversed the effects of each drug; naloxone was slightly more effective against morphine than against DPDPE or U-50,488H. These data suggest that all 3 receptor classes may contribute to the ventral tegmental facilitation of feeding. Periaqueductal gray injections (16 but not 1.6 nmol) of morphine had the opposite effect; they increased the stimulation frequency required to cause eating of 3 pellets in 20 s, and decreased the speed of eating across all stimulation frequencies. Periaqueductal gray injections of the delta- and kappa-agonists were each without effect. These data indicate that the periaqueductal gray inhibition of feeding is mediated solely by mu-receptors and their associated periaqueductal gray circuitry.     

Actions of epinephrine on neurons in the rat midbrain periaqueductal gray maintained in vitro

The effect of epinephrine (EPI) on the activity of 150 periaqueductal gray (PAG) neurons was examined using extracellular recordings in an in vitro slice preparation. Drop application of EPI inhibited 45%, excited 35%, and had no effect on 20% of PAG neurons. Both the excitatory and inhibitory effects of EPI were of long duration; excitatory responses averaged 17 min and inhibitory responses averaged 11 min in duration. EPI responses could be blocked by specific alpha-1 and alpha-2 receptor antagonists. In 35% of the neurons tested, blockade of synaptic transmission by perfusion with low calcium-high magnesium physiological saline blocked responses to EPI. The effects of EPI were site specific: 77% of the cells in the caudal ventrolateral region of the PAG were inhibited by EPI; in all other regions of PAG equal numbers of cells were excited and inhibited by EPI. It is concluded that: (a) EPI has potent effects on a majority (80%) of PAG neurons; (b) EPI responses are mediated by presynaptic as well as postsynaptic mechanisms; (c) EPI preferentially inhibits neurons in the ventrolateral subdivision of caudal PAG. As this part of PAG contains many neurons that project to the ventral medulla, it is possible that EPI modulates the PAG-medullary functions such as analgesia, autonomic regulation, defense reactions, and sexual behaviors.     

Inferior colliculus neuronal membrane and synaptic properties in genetically epilepsy-prone rats

Previous studies using single-unit recording techniques have shown that the inferior colliculus is critical for audiogenic seizure initiation in genetically epilepsy-prone rats (GEPR). In order to investigate cellular abnormalities that may be important in causing audiogenic seizure susceptibility, intracellular recording were made from neurons of inferior colliculus dorsal cortex (ICd) in a GEPR variety that exhibits severe audiogenic seizures (GEPR-9). GEPR neuronal membrane and synaptic properties were compared to those of normal Sprague-Dawley rats (SD), the strain from which GEPR were derived. We found six electrophysiological differences between GEPR and normal SD ICd neurons, all of which could promote seizures in GEPR. (1) Input resistances was higher in GEPR than in normal ICd neurons. (2) Threshold for repetitive action potential firing was closer to resting membrane potential in GEPR ICd neurons. (3) GEPR neurons showed faster repetitive spike firing than normal SD neurons. (4) Anode break spikes occured at the offset of a hyperpolarizing pulse more often in GEPR than in normal SD neurons. (5) Stimulation of the commissure of the inferior colliculus caused synaptic paired pulse inhibition in normal ICd neurons, but paired pulse facilitation was always observed in GEPR neurons. (6) In GEPR, a large epileptiform depolarizing event could be elicited by strong electrical stimulation of the commissure of the inferior colliculus. In normal SD rats, similar epileptiform activity was seen only after application of bicuculline or NMDA. Our results suggest that both abnormal neuronal membrane properties and altered synaptic transmission are likely to contribute to seizure predisposition and audiogenic seizure initiation in GEPR.     

Effects of electrical stimulation of thalamic nucleus submedius and periaqueductal gray on the visceral nociceptive responses of spinal dorsal horn neurons in the rat

Electrical stimulation of the nucleus submedius (Sm) has been shown to suppress the viscerosomatic reflex (VSR), which is evoked by colorectal distension (CRD). We have examined the effects of focal electrical stimulation (0.3 ms, 50 Hz, 100 microA, 10 s) of the Sm and the periaqueductal gray (PAG) on the excitatory responses evoked by CRD in spinal dorsal horn neurons within the L6-S1 region in the urethane-anesthetized Wistar rats. Extracellular recordings were made from 32 spinal excitatory CRD responses. All of these neurons were convergent neurons with cutaneous receptive fields. The majority of the neurons (27/32) were wide dynamic range (WDR) neurons (responding to noxious and non-noxious cutaneous stimuli) while the remaining five neurons were nociceptive specific (NS) neurons (responding only to noxious cutaneous stimuli). The effects of electrical stimulation applied to 28 sites within the Sm were assessed for spinal neurons. Electrical stimulation in seven sites within the Sm (25%) inhibited the CRD excitatory response of dorsal horn neurons, while in two sites (7%) the same stimulation yielded facilitation. Electrical stimulation in the majority of the sites in the Sm (19/28, 68%) did not affect spinal excitatory CRD responses. On the other hand, electrical stimulation of the PAG clearly inhibited 20 of 22 (90%) CRD excitatory responses. These results suggest that the majority of Sm neurons may suppress VSR activity at a supraspinal reflex center rather than via a descending inhibition of spinal visceral nociceptive transmission, as is the case for the PAG.     

Opposing roles of the amygdala and dorsolateral periaqueductal gray in fear-potentiated startle

The whole-body acoustic startle response is a short-latency reflex mediated by a relatively simple neural circuit in the lower brainstem and spinal cord. The amplitude of this reflex is markedly enhanced by moderate fear levels, and less effectively increased by higher fear levels. Extensive evidence indicates that the amygdala plays a key role in the potentiation of startle by moderate fear. More recent evidence suggests that the periaqueductal gray is involved in the loss of potentiated startle at higher levels of fear. The influence of both structures may be mediated by anatomical connections with the acoustic startle circuit, perhaps at the level of the nucleus reticularis pontis caudalis. The mediated by anatomical connections with the acoustic startle circuit, perhaps at the level of the nucleus reticularis pontis caudalis. The present chapter reviews these data.     

Differential distribution of parvalbumin- and calbindin-D28K-immunoreactive neurons in the rat periaqueductal gray matter and their colocalization with enzymes producing nitric oxide

The distribution, colocalization with enzymes producing nitric oxide (NO), and the synaptic organization of neurons containing two calcium-binding proteins (CaBPs) – parvalbumin (Parv) and calbindin-D28K (Calb) – were investigated in the rat periaqueductal gray matter (PAG). Parv-immunopositive (Parv_IP) neurons were detected in the mesencephalic nucleus and rarely in the PAG. Calb_IP neurons were found both in the dorsolateral (PAG-dl) and ventrolateral PAG (PAG-vl); their size ranged from 112.96 μm² (PAG-dl) to 125.13 μm² (PAG-vl). Ultrastructurally Parv and Calb immunoreactivity was mostly found in dendritic profiles. Axon terminals containing each of the two CaBPs formed symmetric synapses. Moreover both Parv and Calb were used to label a subpopulation of NO-producing neurons. Colocalization was investigated using two protocols: (i) a combination of Calb and Parv immunocytochemistry (Icc) with nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry (Hi) and (ii) neuronal NO synthase-Icc (nNOS) (immunofluorescence).Both techniques demonstrated a complete lack of colocalization of Parv and NADPH-d/nNOS in PAG neurons.Double-labeled (DL) neurons (Calb-NADPH-d; Calb-nNOS) were detected in PAG-dl. NADPH-d-Hi/Calb-Icc indicated that 41–47% of NADPH-d-positive neurons contained Calb, whereas 17–23% of Calb_IP cells contained NADPH-d. Two-color immunofluorescence revealed that 53–66% of nNOS_IP cells colocalized with Calb and 24–34% of Calb_IP neurons contained nNOS. DL neuron size was 104.44 μm²; neurons labeled only with NADPH-d or Calb measured 89.793 μm² and 113.48 μm², respectively.Together with previous findings (Barbaresi et al. [2012]) these data suggest that:

• (i) PAG-dl and PAG-vl contain fast CaBPs, (ii) a high degree of heterogeneity exists in PAG-dl, (iii) two subpopulations of NO-producing neurons containing distinct CaBPs are found in PAG-dl.

Therefore the important aspect of the PAG intrinsic organization emerging from this and previous double-labeling studies is the chemical diversity of NO-synthesizing neurons, which is likely related to the different functions in which these neurons are involved.     

Neurons in the deep layers of superior colliculus are a requisite component of the neuronal network for seizures during ethanol withdrawal

Ethanol withdrawal (ETX) in ethanol-dependent animals and humans often results in seizure susceptibility. The deep layers of superior colliculus (DLSC) are proposed to be involved in the neuronal networks of several types of seizures. In rodents, ETX results in susceptibility to audiogenic seizures (AGS), and the DLSC are implicated as a critical component of the seizure network in a genetic form of AGS. Ethanol inhibits NMDA receptors, and the binding at these receptors is increased during ETX in certain brain regions. Therefore, the effect of focal microinjection into DLSC of a competitive NMDA receptor antagonist, -2-amino-7-phosphonoheptanoic acid (AP7) on ETX seizures was examined. AP7 (2 and 5 nmol/side) microinjected bilaterally into DLSC suppressed AGS, supporting a critical role of the DLSC in the AGS network during ETX. DLSC neuronal firing changes in behaving rats were subsequently examined, using chronically implanted microwire electrodes. Acoustically-evoked DLSC firing was significantly suppressed during ethanol intoxication and during ETX. However, DLSC neurons began firing tonically 1–2 s before the onset of the wild running behavior of AGS. Acoustically-evoked DLSC firing was suppressed during post-ictal depression with recovery beginning as the righting reflex returned. These data support a requisite role of the DLSC in AGS during ETX. These neuronal firing changes suggest an important role of DLSC neurons in generation of the wild running phase of AGS during ETX, which may be a general pathophysiological mechanism and a critical event in the initiation of wild running, since a similar pattern was seen previously in a genetic form of AGS.     

Autoradiographic localization of substance P ligand binding sites and distribution of immunoreactive neurons in the periaqueductal gray of the rat

The autoradiographic localization of substance P (SP) binding sites and the distribution of SP immunoreactive (SP-ir) neurons in the periaqueductal gray (PAG) of the rat were studied. The autoradiograms revealed an uneven distribution of specific SP binding sites in the PAG. Throughout the rostrocaudal extent, the densest ligand binding sites were observed in the medial PAG adjacent to the aqueduct, and extended into the dorsal medullary region and to the dorsal raphe nucleus midline region. The distribution of binding sites were denser in the dorsal PAG than the ventral half. In the cuneiform nucleus, a lesser and a denser binding site were observed in the medial and lateral halves respectively. Optical density readings of autoradiograms also supported the differences between these areas. The distribution of SP-ir neurons was also found uneven. In the rostral PAG, SP-ir neurons were found in the entire dorsoventral region. In the caudal PAG, SP-ir neurons were found as 3 clusters: in the dorsomedial, dorsolateral and ventrolateral regions. The present study revealed more SP-ir neurons in the PAG than previously reported.     

Phasic but not tonic REM-selective discharge of periaqueductal gray neurons in freely behaving animals: relevance to postulates of GABAergic inhibition of monoaminergic neurons

To determine if ventrolateral periaqueductal gray contains neurons that selectively increase their discharge activity before and during rapid eye movement (REM) sleep, and hence might furnish GABAergic inhibition of monoaminergic neurons, we recorded the extracellular activity of 33 neurons across sleep-wakefulness in freely behaving cats. Several types of state-specific neuronal populations were found in the periaqueductal gray, although we did not find any neurons that had a tonic discharge increase before and during REM. Thus, these data suggest that, although periaqueductal gray neurons may regulate phasic components of REM sleep, they do not have the requisite tonic pre-REM and REM activity to be a source of GABAergic inhibition of monoaminergic neurons.     

Diazepam dissociates the analgesic and aversive effects of periaqueductal gray stimulation in the rat

Electrical stimulation of the periaqueductal gray matter (PAG) of the rat can produce both analgesia and aversive reactions. To determine if these two effects can be dissociated, diazepam, a benzodiazepine, was administered to rats chronically implanted with electrodes in the PAG. The threshold for stimulation-produced analgesia or aversion, whichever was lowest, was determined before and after drug administration. Diazepam (1 mg/kg) attenuated stimulation-produced aversive reactions at 12 of 20 stimulation sites, allowing analgesia to be measured at the same threshold. Diazepam did not alter baseline pain sensitivity or thresholds for stimulation-produced analgesia. These results indicate that aversive reactions and analgesia from PAG stimulation can be pharmacologically dissociated.     

Arginine vasopressin enhances periaqueductal gray synthesis and secretion of enkephalin and endorphin in the rat

Previous study has proven that arginine vasopressin (AVP) enhances periaqueductal gray (PAG) secreting enkephalin and endorphin in vivo in the rat. Present work investigated that AVP effect on PAG secretion and synthesis of enkephalin and endorphin in vitro. Radioimmunoassy results showed that AVP increased leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek), beta-endorphin (beta-Ep) rather than dynorphin A(1-13) (DynA(1-13)) concentrations in PAG slice culture liquid, and V(2) receptor antagonist: d(CH(2))(5)[D-Ile(2), Ile(4), Ala(9)-NH(2)]AVP decreased L-Ek, M-Ek and beta-Ep, not DynA(1-13) concentrations in PAG slice culture liquid in a dose-dependent manner, but V(1) receptor antagonist: d(CH(2))(5)Tyr(Me)AVP did not change these peptide concentrations in PAG slice culture liquid. RT-PCR data displayed that AVP enhanced proenkephalin and proopiomelanocortin (Pro-beta-Ep) rather than prodynorphin mRNA expressions in culture PAG tissues, and V(2) receptor antagonist weakened proenkephalin and proopiomelanocortin (Pro-beta-Ep), not prodynorphin mRNA expressions in culture PAG tissues, but V(1) receptor antagonist did not influence these mRNA expressions in culture PAG tissues. The data suggest that AVP enhances PAG synthesizing and secreting enkephalin and endorphin rather than dynorphin through V(2) receptors.     

Dorsolateral and ventral regions of the periaqueductal gray matter are involved in distinct types of fear

Stepwise increases in the electrical stimulation of the dorsolateral periaqueductal gray (dlPAG) produces alertness, then freezing and finally escape. This paper examines whether this freezing is (i) caused by Pavlovian fear conditioning to the contextual cues present during stimulation and (ii) the result of the stimulation of neurons located inside the dlPAG or elsewhere. To this end, freezing behavior was assessed in rats exposed either to the same or a different environment (context shift test) following the application of either footshocks or stimulation of the dlPAG at the freezing threshold. Rats submitted to footshocks presented freezing to the context 24 h later whereas rats submitted to the dlPAG stimulation showed freezing only immediately after the stimulation, regardless of the context. In the second experiment, aversive states generated by activation of the dlPAG were assessed either by measuring the thresholds for freezing and escape responses or the duration of these responses following microinjections of semicarbazide inside the dlPAG. The duration of freezing behavior was also measured in rats submitted to a contextual fear-conditioning paradigm using footshocks as unconditioned stimulus. Lesions of the ventral periaqueductal gray (vPAG) disrupted conditioned freezing to contextual cues associated to footshocks but vPAG lesions did not change the threshold of either freezing or escape responses elicited by electrical stimulation of the dlPAG. Lesions of the vPAG did not change the amount of freezing or escape responses produced by microinjections of semicarbazide into the dlPAG. These results indicate that stimulation of dlPAG neurons produce freezing behavior independent of any contextual fear conditioning and add to previously reported evidence showing that the vPAG is a critical structure for the expression of conditioned fear. In contrast, the neural substrate of unconditioned dlPAG stimulation-induced freezing is likely to elaborate unconditioned fear responses to impending danger, which have been implicated in panic disorder.     

Comparison of neuronal response patterns in the external and central nuclei of inferior colliculus during ethanol administration and ethanol withdrawal

Convulsive seizures during ethanol withdrawal (ETX) in rodents can be precipitated by acoustic stimulation. The inferior colliculus (IC) is strongly implicated in the neuronal network for these audiogenic seizures (AGS) in animals undergoing ETX. Previous evidence indicates that the central nucleus of IC (ICc) is important in AGS initiation in ETX, but the ICc does not project directly to motor pathways. The external nucleus of IC (ICx) receives convergent output from a broad range of ICc neurons, which is not tonotopically organized, and projects to several nuclei with major motor connections. Lesion, neuroanatomical, and stimulation experiments suggest the involvement of the ICx in the AGS network in several forms of AGS, including ETX. The present study examined ICx neuronal firing patterns in awake behaving rats during ethanol administration and during ETX to examine the role of this structure directly. ICx neuronal responses during both ethanol intoxication and ETX were significantly suppressed as compared to pre-ethanol responses. ICx neuronal responsiveness was reduced (habituated) at faster (>0.25 Hz) rates of stimulus presentation. However, immediately prior to the onset of AGS, there was an increase in ICx neuronal responses that continued into the wild running phase of AGS. This increase in neuronal responses temporally corresponded to the sustained ICc neuronal responses during ETX just prior to AGS. The enhanced ICx neuronal responsiveness may be mediated, in part, by changes in GABA and glutamate receptor regulation previously observed during ETX. The net result of these changes involves a functional reversal of response habituation normally observed in ICx neurons. These data illuminate the nature of the changes in ICx neuronal function that serves to transmit the sensory input that originates in the ICc and propagates seizure to the brainstem AGS network nuclei responsible for the convulsive motor behaviors of ETX seizures.     

Decreased GABA and increased glutamate receptor-mediated activity on inferior colliculus neurons in vitro are associated with susceptibility to ethanol withdrawal seizures

Cessation of ethanol administration in ethanol-dependent rats results in an ethanol withdrawal (ETX) syndrome, including audiogenic seizures (AGS). The inferior colliculus (IC) is the initiation site for AGS, and membrane properties of IC neurons exhibit hyperexcitability during ETX. Previous studies observed that ETX alters GABA and glutamate neurotransmission in certain brain sites. The present study evaluated synaptic properties and actions of GABA or glutamate antagonists during ETX in IC dorsal cortex (ICd) neurons in brain slices from rats treated with ethanol intragastrically 3 times daily for 4 days. A significant increase of spontaneous action potentials (APs) was observed during ETX. The width, area and rise time of excitatory postsynaptic potentials (EPSPs) evoked by stimulation in the commissure of IC were significantly elevated during ETX. A fast EPSP was sensitive to block by the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and a slow EPSP was sensitive to the NMDA receptor antagonist, 2-amino-5-phosphonovalerate (AP5). However, during ETX the concentration of CNQX or AP5 needed to block these EPSPs was elevated significantly. Inhibitory postsynaptic potentials (IPSPs) in ICd neurons evoked in both normal and ETX rats were blocked by the GABA(A) antagonist, bicuculline. However, IPSPs during ETX displayed a significantly greater sensitivity to bicuculline. These data indicate that decreased GABA(A)-mediated inhibition and increased glutamate-mediated excitability in IC may both be critical mechanisms of AGS initiation during ETX, which is similar to observations in a genetic form of AGS. The common changes in IC neurotransmission in these AGS forms may be general mechanisms subserving AGS and other forms of auditory system pathophysiology in which the IC is implicated.     

Structure-antinociceptive activity of neurotensin and some novel analogues in the periaqueductal gray region of the brainstem

Neurotensin, an endogenous tridecapeptide, produces a potent, naloxone-insensitive antinociceptive response when it is microinjected into the periaqueductal gray region of the rat brainstem. In the present study, the ED50 for neurotensin in inducing antinociception was 1.5 nmol, two times more potent than morphine. We sought to find whether neurotensin's antinociceptive effects were mediated by the same receptor that mediates its other functions. We found that the structure-activity relationship of neurotensin-induced antinociception was different from that required for the stimulation of intracellular cyclic GMP production in neuroblastoma clone N1E-115 and the binding to N1E-115 cells, human brain tissue, or rat periaqueductal gray. These data suggest there exists a subtype of neurotensin receptors in neural tissue that mediates its antinociceptive actions.