Developmental changes in the spatio-temporal pattern of respiratory neuron activity in the medulla of late fetal rat

We investigated how the spatio-temporal pattern of respiratory neuron network activity in the ventral medulla changes during the late fetal period of rat. Brainstem-spinal cord preparations isolated from rat fetuses on embryonic days 17–21 (E17–E21) were stained with a voltage-sensitive dye for optical image analysis of neuronal activity of the ventral medulla. The spatio-temporal pattern of respiratory neuron activity in the preparation from E20 to E21 was basically identical to that of neonatal rat; pre-inspiratory activity in a limited region of the rostral ventrolateral medulla, the para-facial region, preceded by several hundred milliseconds the onset of inspiratory activity in the more caudal ventrolateral medulla, the pre-Bötzinger complex level. In contrast, in E17–E18 specimens, pre-inspiratory activity could not be detected in the rostral medulla at the level of the facial nucleus. Neuronal activity appeared to begin at the pre-Bötzinger complex level shortly before onset of the inspiratory burst. Strong activity then developed in the facial nucleus and peaked in the post-inspiratory phase. The transition of these patterns of respiratory activity occurred at E19. We conclude that the changes in the spatio-temporal pattern of neuronal activity reflect developmental changes in the cellular elements underlying rhythm generation in the fetal respiratory neuron network. We suggest that the pre-inspiratory neuron network of the para-facial region in the rostral ventrolateral medulla functions as the rhythm generator after E19/20.     

Interactions between rostral pontine and ventral medullary respiratory neurons

Lesioning studies have demonstrated that the respiratory rhythm is generated within the brain stem and that connections between the pons and the medulla must be intact for the generation of eupneic breathing in the decerebrate or anesthetized vagotomized cat. However, the nature of proposed functional connections between pontine and medullary respiratory neurons is not well understood. The possibility of interactions between respiratory neurons of the rostral pons (n. parabrachialis medialis, K?lliker-Fuse nucleus) and the ipsilateral ventral respiratory group (VRG; n. retroambigualis, n. ambiguus, retrofacial nucleus) was investigated because of neuroanatomical and electrophysiological evidence for such connections. Phrenic nerve activity and pontine and medullary single-unit respiratory related activities were recorded extracellularly in 44 decerebrate, vagotomized, paralyzed, and artificially ventilated cats. Cross-correlation analysis was employed to detect and evaluate functional associations of pairs of cells. Eighteen (7%) of the 255 pairs of respiratory neurons analyzed showed evidence of short time scale correlations indicative of a functional interaction. The interpretations of the detected correlations suggest that some cell pairs were correlated due to mono- or paucisynaptic connections, while others were correlated due to the influence of an unobserved shared input. The interpretations for 11 of the 15 cell pairs for which a monosynaptic connection may be postulated involve a projection from a tonically active respiratory neuron. Twelve of the 18 positive correlations involved neurons whose maximum rates of discharge occurred during different parts of the respiratory cycle. The results of this study provide the first evidence of functional connections among pontine and medullary respiratory neurons based on the evaluation of simultaneously recorded spike trains and suggest that the role of the rostral pontine respiratory neurons in the control of the respiratory rhythm may be mediated by various types of interactions. When considered with the results of other studies, our data suggest that monosynaptic interactions between VRG and rostral pontine respiratory neurons play a limited role in the control of the respiratory cycle in the decerebrate vagotomized cat. It is likely that the influence of the pons on ventral medullary neurons (and vice-versa) is also exerted via polysynaptic pathways and/or via brain stem neurons not sampled in this study.     

Morphology and electrophysiology of superior laryngeal nerve afferents and postsynaptic neurons in the medulla oblongata of the cat.

Intra-axonal recordings were made from 24 afferent fibres of the superior laryngeal nerve in and around the nucleus tractus solitarius, in 26 pentobarbitone-anaesthetized cats. Conduction velocity ranged from 15 to 38 m/s. Four afferents were injected with horseradish peroxidase. They showed dense terminal arborization in the region of the ventral and ventrolateral subnuclei of the nucleus tractus solitarius, both rostral and caudal to the obex. Six other intra-axonal recordings were thought to originate from axons of neurons postsynaptic to superior laryngeal afferents; one of these was injected with horseradish peroxidase and showed a similar arborization pattern to that of the afferent axons. In the same region, intracellular recordings were made from 124 neurons which responded to superior laryngeal nerve stimulation with excitatory postsynaptic potentials (mean latency 2.7 +/- 1.0 ms). Ninety-nine of these neurons were thought to receive a monosynaptic input. The stimulation threshold evoking these responses was similar to that which inhibited phrenic nerve discharge. Eleven of the monosynaptically excited neurons were injected with horseradish peroxidase. They had fusiform or stellate somata and simple dendritic trees, radiating mainly in the transverse plane. In one experiment, in which both a superior laryngeal nerve afferent fibre and a neuron were labelled, afferent terminal varicosities were found in close apposition with the postsynaptic membrane of the injected neuron. Four of 14 (29%) tested neurons could be antidromically activated from the C3 spinal segment. The stimulus thresholds and onset latencies of the responses of superior laryngeal nerve afferents and medullary neurons to stimulation of the superior laryngeal nerve are consistent with their involvement in the reflex inhibition of respiratory neurons evoked by superior laryngeal nerve stimulation.     

Descending axonal projections from the medial parabrachial and K?lliker-Fuse nuclear complex to the nucleus raphe magnus in cats.

In 9 Nembutal-anesthetized and vagotomized cats, a total of 42 units, including 2 respiratory units, recorded from the medial parabrachial (NPBM) and K?lliker-Fuse (KF) nuclear complex were found to be antidromically activated by electrical stimulation of the nucleus raphe magnus (NRM). The latencies ranged from 0.4 to 2.5 ms (mean 1.1 ms). In 5 cats, following injection of WGA-HRP (wheat germ agglutinin-conjugated horseradish peroxidase) into the NRM, a number of retrogradely labelled neurons were observed in the rostral pons, mainly in the NPBM, KF and nearby pontine area. These results demonstrate that mainly non-respiratory neurons in the rostral pons, especially in the NPBM and KF nucleus, send monosynaptic axonal projections to the NRM.     

Facilitation of respiratory rhythm by a mu-opioid agonist in newborn rat pons-medulla-spinal cord preparations

We investigated the effect of a mu-opioid agonist, DAGO, on the respiratory frequency of pons-medulla-spinal cord preparations from newborn rats. Bath application of a low concentration of DAGO (0.2 microM) facilitated respiratory rhythm in pons-medulla-spinal cord preparations, whereas it induced respiratory depression in medulla-spinal cord preparations (without pons). At a higher concentration (1.0 microM), at which the inspiratory burst generation in the medulla was strongly depressed, the respiratory rhythm in half of the pons-medulla-spinal cord preparations increased and then decreased, thus showing a biphasic response. In the other half of these preparations, only the facilitatory effect was observed. The burst rate of pre-inspiratory neurons in the rostral ventrolateral medulla was also facilitated by DAGO application. Such facilitation of the respiratory rhythm is probably due to disinhibition of a pontine inhibitory system. Our findings also suggest the existence of a pontine excitatory system, which is depressed by the pontine inhibitory system under control conditions.     

Activation of Orexin B receptors in the pontine Kölliker-Fuse nucleus modulates pre-inspiratory hypoglossal motor activity in rat

Orexins (splice variants A and B) are hypothalamic neuropeptides that have essential functions in control of arousal and nutrition. Lack of Orexins is strongly associated with narcolepsy and sleep disordered breathing. However, the role of Orexins and particularly that of Orexin-B (OXB), in respiratory centres controlling upper-airway patency are less defined. In the present study we performed microinjections of OXB into the pontine K?lliker-Fuse nucleus (KF) of the dorsolateral pons, since this nucleus is particularly involved in the pre-motor control of upper airway muscles. The OXB mediated effects on heart, phrenic (PNA) and hypoglossal (XII-A) nerve activities were analysed in an in situ perfused brainstem preparation. Injection of OXB into the KF evoked significant augmentation of the respiratory frequency. Importantly, OXB provoked particularly prolonged pre-inspiratory discharge of the XII nerve, while no cardiovascular response was observed after KF microinjections. In summary, OXB in the KF exerts an excitatory effect on XII pre-motoneurones. Since pre-inspiratory activity of the XII is important for the decrease in upper airway resistance during inspiration, we conclude that OXB release in the KF has strong implications in the state-dependent control of upper airway patency under physiological and pathophysiological conditions.     

Whole cell recordings from respiratory neurons in the medulla of brainstem-spinal cord preparations isolated from newborn rats

In brainstem-spinal cord preparations isolated from newborn rats, a whole cell recording technique was applied to record membrane potentials of inspiratory (Insp) and pre-inspiratory (Pre-I) neurons in the ventrolateral medulla. Labelling of these respiratory neurons with Lucifer Yellow allowed analysis of their locations and morphology. Intracellular membrane potentials from 25 Insp neurons were recorded. Average resting membrane potential was –49 mV (n=25) and input resistance was 306 M. Insp neurons were classified into three types from the patterns of synaptic potentials. Type I neurons (n=11) had a high probability of excitatory postsynaptic potentials (EPSPs) in the pre- and post-inspiratory phases. Type II neurons (n=7) showed abrupt transition to the burst phase from the resting potential level without increased EPSPs in the preinspiratory phase. Type III neurons (n=7) were hyperpolarized by inhibitory postsynaptic potentials (IPSPs) in the pre- and post-inspiratory phases. These Insp neurons, located in the ventrolateral medulla 80–490m from the ventral surface, were 10–30 m in diameter, and had various soma shapes (pyramidal, spherical or fusiform). Intracellular membrane potentials from 24 Pre-I neurons were recorded. The average resting membrane potential was –45 mV (n=24), and the input resistance was 320 M. Typical Pre-I neurons showed fairly great depolarization accompanied by action potentials during their burst phase and repolarization during the inspiratory phase. Most Pre-I neurons appeared to have a high level of synaptic activity. These cells were located in the ventrolateral medulla 50–440 m below the ventral surface and had pyramidal or fusiform somas of 10–25 m in diameter. Stimulation of the ipsilateral IXth, Xth roots or the spinal cord (C3 level) induced orthodromic responses in most Insp or Pre-I neurons. An antidromic action potential was induced in only one Pre-I neuron by stimulation at the ipsilateral C3 level. Many Insp or Pre-I neurons had dendrites that terminated close to the ventral surface of the medulla. The present study revealed postsynaptic activity of respiratory neurons in the rostral ventrolateral medulla, which is consistent with the excitatory and inhibitory synaptic connections from Pre-I neurons to Insp neurons, and inhibitory synaptic connections for Insp neurons to Pre-I neurons.     

Pre-inspiratory burst in the caudal medulla of rabbits

The activity of 48 respiratory units in the paraolivary region from the middle to the rostral end of the hypoglossal cranial nerve root, and the effect of electrical stimulation and L-glutamate applied to the region on phrenic nerve activity was investigated in 14 rabbits. Electrical stimulation (50 Hz, 0.2 ms current pulses at intensities 5-20 microA) and L-glutamate (30-100 ng) shortened the expiratory time and increased the respiratory rhythm with no change in tidal phrenic nerve activity. Rhythmic activity preceding the phrenic nerve activity (pre-inspiratory burst) was recorded in the paraolivary region. The temporal relationship between the pre-inspiratory (pre-I) burst and the phrenic activity remained constant even when the respiratory frequency was altered by passive lung inflation. These results suggest that structures in the paraolivary region may influence the respiratory rhythm in rabbits and that pre-I burst neurons may play a role in triggering periodic phrenic activity.     

Distribution and medullary projection of respiratory neurons in the dorsolateral pons of the rat

The dorsolateral pons around the parabrachial nucleus including the Kölliker-Fuse nucleus is closely linked with the medullary respiratory center and plays an important role in respiratory control. We aimed to elucidate the firing properties, detailed distributions, and medullary projections of pontine respiratory neurons in pentobarbitone-anesthetized, paralyzed, and artificially ventilated rats with intact vagi.     

Membrane potentials of individual cells of isolated gastric glands of rabbit

In urethane-anaesthetized, paralyzed and artificially ventilated rabbits, medullary respiration-related neurons (RRU) were classified according to the phase relation of their burst discharge to phrenic nerve activity. Phase-bound inspiratory (I) or expiratory (E) neurons were discriminated from phase-spanning expiratory-inspiratory (EI) or inspiratory-expiratory (IE) units. Mechanisms of termination of inspiration by electrical stimulation of rostral pontine nuclei (Nc. parabrachialis medialis; Lc. coeruleus) were examined firstly to demonstrate whether RRU receive descending excitatory and inhibitory afferents as well as ascending efferents and secondly to analyse the time course of the neuronal pathways involved. Of 120 RRU, 38 neurons were demonstrated to receive pontine afferents. About 33% of all E neurons became orthodromically excited during rostral pons stimulation whereas 18.2% of all I cells became orthodromically inhibited. Some RRU were shown to project up to the rostral pons. 50% of these were of the phase-spanning IE type. The onset of inspiratory inhibition induced by rostral pons stimulation occurred 3.4 ms after the onset of single electrical pulse stimulation. Based on these results a neuronal model for a pontine mechanism terminating inspiration is proposed.     

Respiration-related rhythmic activity in the rostral medulla of newborn rats

There are at least two respiration-related rhythm generators in the medulla: the pre-B?tzinger complex, which produces inspiratory (Insp) neuron bursts, and the parafacial respiratory group (pFRG), which produces predominantly preinspiratory (Pre-I) neuron bursts. The pFRG Pre-I neuron activity has not been correlated with motor neuron activity in slice or block preparations of rostral medulla. In this study, we attempted to detect pFRG Pre-I activity as motor output in the rostral medulla. We recorded respiratory activity of the facial nerve in the brain stem-spinal cord preparation of 0- to 2-day-old rats. Facial nerve activity consisted of preinspiratory, Insp, and postinspiratory activity. Pre- and postinspiratory activity corresponded well with membrane potential trajectories of Pre-I neurons in the rostral ventrolateral medulla. In response to perfusion of 1 microM DAMGO (a mu-opiate agonist), fourth cervical ventral root (C4) Insp activity was depressed and facial nerve activity continued to synchronize with Pre-I neuron bursts. After transverse sectioning between the levels of the pre-B?tzinger complex and the pFRG, C4 Insp activity recovered within 15 min, but facial nerve activity was inhibited. When DAMGO was applied, C4 Insp activity was inhibited, and rhythmic facial nerve activity recovered. Subsequent elevation of K+ concentration reinduced C4 activity, but facial nerve activity was inhibited. Whole cell recordings in the rostral block revealed the presence of putative Pre-I neurons, the activity of which was synchronized with facial nerve activity. These results show that the rostral medulla, not including the pre-B?tzinger complex, produces Pre-I-like rhythmic activity that can be monitored as facial nerve motor output in newborn rat in vitro preparations.     

Parabrachial–lateral pontine neurons link nociception and breathing

We investigated the role of the parabrachial complex in cutaneous nociceptor-induced respiratory stimulation in chloralose-urethane anesthetized, vagotomized rats. Noxious stimulation (mustard oil, MO) applied topically to a forelimb or hindlimb enhanced the peak amplitude of the integrated phrenic nerve discharge and, with forelimb application, increased phrenic nerve burst frequency. Bilateral inactivation of neural activity in the parabrachial complex with injection of the GABA agonist muscimol (3 nl) markedly attenuated the response to MO application. Injection of the retrograde tracer FluoroGold within the medullary ventral respiratory column labeled neurons in dorsolateral pontine regions known to receive nociceptive inputs (i.e., Kölliker-Fuse, lateral crescent, and superior lateral subnuclei of the parabrachial complex). Extracellular recordings of 65 dorsolateral parabrachial neurons revealed about 15% responded to a noxious cutaneous pinch with either an increase or a decrease in discharge and 40% of these exhibited a phasic respiratory-related component to their discharge. In conclusion, parabrachial pontine neurons contribute to cutaneous nociceptor-induced increases in breathing.     

Pontine respiratory group neuron discharge is altered during fictive cough in the decerebrate cat

A network of neurons in the rostral dorsal lateral pons and pons/mescencephalic junction constitute the pontine respiratory group (PRG) and is essential for reflex cough. As a next step in understanding the role of the PRG in the expression of the cough reflex, we examined neuron firing rates during fictive cough in cats. Decerebrated, thoracotomized, paralyzed, cycle-triggered ventilated adult cats were used. Extracellular activity of many single neurons and phrenic and lumbar neurograms were monitored during fictive cough produced by mechanical stimulation of the intrathoracic trachea. Neurons were tested during control periods for respiratory modulation of firing rate by cycle-triggered histograms and statistical tests. Most respiratory modulated cells were continuously active with various superimposed respiratory patterns; major categories included inspiratory decrementing (I-Dec), expiratory decrementing (E-Dec) and expiratory augmenting (E-Aug). There were alterations in the discharge patterns of respiratory, as well as, non-respiratory modulated neurons during cough. The results suggest an involvement of the PRG in the configuration of the cough motor pattern.     

Characteristics of late vasoconstrictor A-response in cats after decerebration at different levels of the brain stem

Single stimulation of A-fibers of the tibial nerve in cats decerebrated at the rostral border of the mesencephalon (mesencephalic animals) at various levels of the pons, including the region of the pontobulbar junction, and the most rostral levels of the medulla (pontine animals), or rather more caudally to this region (bulbar animals), evoked a late response in the renal nerve, consisting of excitatory and inhibitory components. In 53% of experiments on pontine animals, 42% of experiments on mesencephalic animals, but only 18% of experiments on bulbar animals the excitatory component of the response was small or even absent. The system generating the inhibitory component of the response was most active and most excitable in the pontine cats. However, features indicating relative potentiation of the inhibitory component of action of impulses in A-afferents on vasoconstrictor neurons in the pontine animals were not sufficiently constant to account for the switch from hypertensive reflexes to impulses from somatic A-afferents into hypotensive, taking place after disconnection of the structures of the pontobulbar junction and rostral levels of the medulla from the mesencephalon.Laboratory of Biophysics and Pathophysiology of the Circulation, Institute of General Pathology and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR A. M. Chernukh.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 87, No. 2, pp. 114–117, February, 1979.     

Inspiration-Promoting Vagal Reflex in Anaesthetized Rabbits after Rostral Dorsolateral Pons Lesions

The centrally generated respiratory rhythm is under strong modulation by peripheral information, such as that from the slowly adapting pulmonary stretch receptors (SA-PSRs) conveyed via the vagus nerve. We have already demonstrated that vagal afferent stimulation at a low frequency (5–40 Hz), or holding the lung volume at the end-expiratory level (no-inflation test) prevents spontaneous termination of the inspiratory (I) phase or initiates I activity in anaesthetized rabbits in which the NMDA receptors (NMDA-Rs) are pharmacologically blocked. Here we show that this I-promoting vagal reflex also becomes manifest in animals where the pontine respiratory groups are ablated. Following lesions of the rostral dorsolateral pons, including the nucleus parabrachialis medialis and Kölliker-Fuse nucleus, with radio-frequency current or local injection of kainic acid, low-frequency stimulation of the vagus nerve and the no-inflation test significantly prolonged the I phase in a manner highly similar to that observed in rabbits with NMDA-R block. Brief stimuli at low frequency during the mid-expiratory (E) phase evoked I discharge with a latency significantly smaller and less variable than that before the lesions. It is concluded that low-frequency input from the SA-PSR suppresses I-to-E phase transition and promotes central I activity when the medullary respiratory network is released from pontine influence, which involves NMDA-R-mediated signalling.     

Synaptic effects of intercostal tendon organs on membrane potentials of medullary respiratory neurons

1. Stimulation of intercostal muscle tendon organs or their afferent fibers reduces medullary inspiratory neuron activity, decreases motor output to inspiratory muscles, and increases the activity of expiratory laryngeal motoneurons. The present study examines the synaptic mechanisms underlying these changes to obtain information about medullary neurons that participate in the afferent limb of this reflex pathway. 2. Membrane potentials of medullary respiratory neurons were recorded in decerebrate paralyzed cats. Postsynaptic potentials (PSPs) elicited in these neurons by intercostal nerve stimulation (INS) were compared before and after intracellular iontophoresis of chloride ions. After chloride injection, the normal hyperpolarization caused by inhibitory (I) PSPs is "reversed" to depolarization. 3. In inspiratory neurons, reversal of IPSPs by chloride injection also reversed hyperpolarization produced by INS when applied during any portion of the respiratory cycle. This observation suggests that increased chloride conductance of the postsynaptic membrane mediated the inhibition. Further, it is very likely that the last-order interneuron in the afferent pathway must be excited by INS and alter inspiratory neuron activity via an inhibitory synapse. The linear relationship between the amplitude of the INS induced PSP and membrane potential of inspiratory neurons provided evidence that neurons in the afferent pathway are not respiratory modulated. 4. The membranes of expiratory vagal motoneurons and post-inspiratory neurons were depolarized by INS during all portions of the respiratory cycle before IPSP reversal. Reversal of IPSPs affected neither this depolarization of expiratory vagal motoneurons during stage I and II expiration nor that of post-inspiratory neurons during stage I expiration. Thus this depolarization probably resulted from synaptic excitation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Responses of ventral respiratory neurones in the rat to vagus stimulation and the functional division of expiration.

In anaesthetized rats, extracellular and intracellular recordings were taken from 106 respiratory neurones in the intermediate region of the nucleus ambiguus. We observed unprovoked shortening of expiratory time accompanied, in all classes of respiratory neurone, by the elimination of the changes in membrane potential that were characteristic of stage II expiration. The demonstration of the elimination of stage II expiration in both the rat and cat strongly supports the functional division of expiration into stage I expiration (post-inspiration) and stage II expiration. In order to identify the neurones in the rat that receive inputs from vagal afferents and modulate the central respiratory rhythm, we examined whether any respiratory neurones responded to stimulation of the vagus nerve. Some post-inspiratory and stage II expiratory neurones responded. The short latency (< 2 ms) of four of the responses indicates that some vagal afferents act on post-inspiratory neurones via a disynaptic pathway. While repetitive stimulation of the vagus nerve could inhibit the phrenic rhythm, it appears that most inspiratory neurones in the intermediate region of the nucleus ambiguous complex are not directly involved in integrating the information from vagal afferents with the central respiratory rhythm.     

Presynaptic GABAA receptors facilitate spontaneous glutamate release from presynaptic terminals on mechanically dissociated rat CA3 pyramidal neurons

Mossy fiber-derived giant spontaneous miniature excitatory postsynaptic currents have been suggested to be large enough to generate action potentials in postsynaptic CA3 pyramidal neurons. Here we report on the functional roles of presynaptic GABA(A) receptors on excitatory terminals in contributing to spontaneous glutamatergic transmission to CA3 neurons. In mechanically dissociated rat hippocampal CA3 neurons with adherent presynaptic nerve terminals, spontaneous excitatory postsynaptic currents were recorded using conventional whole-cell patch clamp recordings. In most recordings, unusually large spontaneous excitatory postsynaptic currents up to 500 pA were observed. These large spontaneous excitatory postsynaptic currents were highly sensitive to group II metabotropic glutamate receptor activation, and were still observed even after the blockade of voltage-dependent Na(+) or Ca(2+) channels. Exogenously applied muscimol (0.1-3 microM) significantly increased the frequency of spontaneous excitatory postsynaptic currents including the large ones. This facilitatory effect of muscimol was completely inhibited in the presence of 10 microM 6-imino-3-(4-methoxyphenyl)-1(6H)-pyridazinebutanoic acid HBr, a specific GABA(A) receptor antagonist. Pharmacological data suggest that activation of presynaptic GABA(A) receptors directly depolarizes glutamatergic terminals resulting in the facilitation of spontaneous glutamate release. In the current-clamp condition, a subset of large spontaneous excitatory postsynaptic potentials triggered action potentials, and muscimol greatly increased the frequency of spontaneous excitatory postsynaptic potential-triggered action potentials in postsynaptic CA3 pyramidal neurons. The results suggest that presynaptic GABA(A) receptors on glutamatergic terminals play an important role in the excitability of CA3 neurons as well as in the presynaptic modulation of glutamatergic transmission onto hippocampal CA3 neurons.     

Potentiation of GABAergic synaptic transmission in the rostral nucleus of the solitary tract

Whole-cell recordings were made from neurons in the rostral nucleus of the solitary tract in horizontal brainstem slices. Monosynaptic GABAA receptor-mediated inhibitory postsynaptic potentials were evoked by single stimulus shocks or by high-frequency tetanic stimulation in the presence of glutamate receptor blockers. While single stimulus-evoked inhibitory postsynaptic potentials had variable amplitudes, tetanic stimulation-induced, hyperpolarizing postsynaptic potentials were of a more constant amplitude. Furthermore, tetanic stimulation resulted in potentiation of the amplitude of single stimulus shock-evoked inhibitory postsynaptic potentials. Of 55 neurons that were tested, potentiation lasted over 30 min for 11, 10-30 min for 13, less than 10 min for 23 and no potentiation occurred in eight. Tetanic stimulation did not result in potentiation of the tetanic stimulus-evoked hyperpolarizing postsynaptic potentials. Both the single stimulus shock- and tetanic stimulus-evoked potentials had similar inhibition concentration-response curves to the GABAA antagonist, bicuculline methiodide (EC50 = 0.75 and 0.83, respectively), indicating that they were mediated by the same postsynaptic receptors. By comparing the effect of bicuculline methiodide on the amplitude of the single stimulus shock-evoked inhibitory postsynaptic potentials and the tetanic stimulus-evoked hyperpolarizing potentials, we concluded that a single stimulus shock does not activate all postsynaptic GABAA receptors. However, tetanic stimulation results in activation of all postsynaptic GABAA receptors and induces long-lasting changes in the presynaptic GABAergic neuron. These long-lasting changes of the presynaptic neuron facilitate the release of GABA during single stimulus shock and, as a consequence, more postsynaptic receptors are activated during single stimulus shock-evoked synaptic transmission. This conclusion is supported by the results of experiments in which the extracellular Ca2+ concentration was manipulated to change the amount of neurotransmitter released from the presynaptic GABAergic terminals. The single stimulus shock-evoked inhibitory postsynaptic potentials were sensitive to the extracellular Ca2+ concentration, whereas tetanic stimulus-evoked inhibitory post-synaptic potentials were essentially insensitive to extracellular Ca2+ concentration. The relationship between the single stimulus shock-evoked inhibitory postsynaptic potential amplitude and extracellular Ca2+ concentration indicates that, in control physiological saline containing 2.5 mM Ca2+, a single stimulus shock activates less than half the postsynaptic GABA receptors. The phenomenon of long-lasting potentiation of inhibitory transmission within the rostral nucleus of the solitary tract may be important in the processing of gustatory information and play a role in taste-guided behaviors.