Temporal distribution of rapid eye movements and related monophasic potentials in the brain stem following injection of an anticholinesterase.

The temporal distribution of the horizontal rapid eye movements and the related monophasic potentials recorded from the ascending MLF following intravenous injection of o.i mg/kg of anticholinesterase has been investigated in precollicular decerebrate animals. In particular the intervals between individual MLF potentials occurring during successive REM episodes have been evaluated over a total period of 2000 sec on each experiment. 2. There was a bimodal distribution of intervals due to the fact that all the rapid eye movements and the related MLF potentials were grouped in bursts which occurred at quite regular intervals. 3. During the cholinergically induced episodes of REM, there were usually bursts of REM in one direction followed by bursts of REM in the opposite direction. The mean number of individual eye movements within each burst was 4.67 +/- 0.84, S.D., while the average interval between the individual eye movements corresponded to 167 +/- 36 msec, S.D. 4. There was a great regularity in the periodic occurrence of the bursts of REM. In particular the mean interval between the beginning of a burst of REM in one direction (i.e., towards the left side) and that of the next train oriented in the opposite direction (i.e., towards the right side) was 1.97 +/- 0.47 sec, S.D., while the mean interval between the beginning of this last train and that of the successive train oriented in the former direction corresponded to 2.97 +/- 0.48 sec, S.D. Moreover, the duration of the whole period corresponding to the interval between two successive bursts of REM oriented in the same direction (i.e., towards left or towards right) corresponded on average to 4.94 +/- 0.55 sec S.D. and 4.99 +/- 0.52 sec, S.D. respectively. 5. In addition to these "simple bursts" of rapid eye movements oriented in one direction, there were "complex bursts" in which an alternation of the individual eye movements within each burst was observed. In these instances the mean number of spikes was greater (5.35 +/- 1.20, S.D.) and the mean interval shorter 119 +/- 44 msec, S.D.) than those observed in the "simple bursts", About 10-15% of the bursts occurring during the cholinergically induced REM episodes were of the complex type. 6. These findings obtained from an individual experiment were confirmed in all the decerebrate animals treated with the same dose of anticholinesterase; only slight quantitative differences were detected from case to case. 7. Since the bursts of REM induced by the anticholinesterase depend upon the activity of the vestibular nuclei, it is postulated that cholinergic reticular neurons activate structures which show waxing and waning in their activity before acting on the vestibulo-oculomotor system. This system probably contains the inhibitory interneurons which transform the regularly modulated input into a rhythmic vestibular output...     

Syndrome of Rapid Irregular Movements of Eyes and Limbs in Childhood

Electroencephalographic (E.E.G.), polyelectromyographic (poly-E.M.G.), and electro-oculographic (E.O.G.) studies were made of 16 young children and babies referred because of prolonged episodes of rapid irregular movements of eyes and limbs (R.I.M.E.L.) sometimes involving also the trunk and head. This unusual syndrome of unknown aetiology developed over a period of a few days and persisted for months or years. In the E.E.G. there were only minor changes, without discharges of any kind during the episodes of involuntary movements, and during sleep the spindles were unusually fast. In the poly-E.M.G. single or multiple spikes appeared as electrical concomitants of brief myoclonic phenomena at irregular intervals and independently in the various groups of muscles examined. In the E.O.G. the frequent episodes of irregular jerky movements of the eyeballs occurred at various intervals and without rhythmicity. On clinical observation alone this syndrome may not be easily separated from other conditions such as myoclonic epilepsy, gross ataxia, tremors, or choreiform syndromes in young children; the peculiar combination of neurophysiological findings is essential in the identification of this unusual disorder.     

Prostaglandins and sleep. Awaking effect of prostaglandins and sleep pattern of essential fatty acids deficient (EFAD) rats

The experiments were carried out to investigate the effects of prostaglandins (PGs) on the sleep pattern in the cat, and in normal and EFAD rats.The data indicate that the duration of slow wave sleep (SWS) was significantly longer in EFAD rats compared with the normal rats. However, no difference in the REM sleep was observed between the two groups. Intraventricular (i.vc.) administration of PGE₁, PGE₂ and PGF_2α increased wakefulness without a significant alteration of REM sleep.PGE₁ administered i.vc. did not alter the duration of SWS or REM sleep in the chronic cat, but induced ponto-geniculo-occipital (PGO) waves (spikes) which are the phasic phenomenon of REM sleep.The fact that previous administration of 5-hydroxytryptophane abolished the PGE₁-induced PGO spiking, might indicate that this drug triggered the spikes mainly via the functional inhibition of the serotonergic system.     

Burst control: Synaptic conditions for burst generation in cortical layer 5 pyramidal neurons

The output of neocortical layer 5 pyramidal cells (L5PCs) is expressed by a train of single spikes with intermittent bursts of multiple spikes at high frequencies. The bursts are the result of nonlinear dendritic properties, including Na⁺, Ca²⁺, and NMDA spikes, that interact with the ~10,000 synapses impinging on the neuron’s dendrites. Output spike bursts are thought to implement key dendritic computations, such as coincidence detection of bottom-up inputs (arriving mostly at the basal tree) and top-down inputs (arriving mostly at the apical tree). In this study we used a detailed nonlinear model of L5PC receiving excitatory and inhibitory synaptic inputs to explore the conditions for generating bursts and for modulating their properties. We established the excitatory input conditions on the basal versus the apical tree that favor burst and show that there are two distinct types of bursts. Bursts consisting of 3 or more spikes firing at < 200 Hz, which are generated by stronger excitatory input to the basal versus the apical tree, and bursts of ~2-spikes at ~250 Hz, generated by prominent apical tuft excitation. Localized and well-timed dendritic inhibition on the apical tree differentially modulates Na⁺, Ca²⁺, and NMDA spikes and, consequently, finely controls the burst output. Finally, we explored the implications of different burst classes and respective dendritic inhibition for regulating synaptic plasticity.     

Demonstration of spontaneous and stretch induced urinary bladder EMG in the living rabbit

Spontaneous bladder EMG was recorded in the living rabbit from an isovolumetric bladder without chemical or electrical stimulation. Mechanical intervention, either by lifting the bladder out of the abdomen or by rapid filling, resulted in stretch induced bladder EMG. A self made epoxy resin electrode device that embedded 32 EMG recording electrodes in a matrix like pattern, each electrode Ag/AgCl, d = 0.6 mm with an interdistance of 2.3 mm, was used for registration. The recorder used a common average reference technique and a sample frequency of 400 Hz. A signal bandwidth of 0.05 to 108 Hz was available for analysis. Spontaneous EMG consisted of single spikes and bursts (2-20 spikes), but not of continuous activity. The shape of spikes was triphasic. Single spikes appeared with and without burst activity. Small (2-5 spikes) and large bursts (6-20 spikes) were discerned; small bursts not necessarily propagated across electrodes, large bursts did and were able to organize, suggesting that they were under short neuron system control. Spontaneous EMG was probably related to both contraction and relaxation. Stretch induced EMG was characterised by continuous activity on all electrodes, spikes that followed each other immediately, slowly fading away. The spikes had an elongated third phase when compared to the shape of spontaneous activity. Highest activity and amplitudes were found after lifting the bladder out of the abdomen and placing it on the electrode device. A concept is put forward in which the continuous activity is not unequivocally related to muscle shortening, but where the current stress and strain situation of the bladder tissue can cause a muscle fibre elongation upon the appearance of electrical activity. The EMG activity found was in many aspects similar to results of a previous study using mortalized rabbits. Artifact sources like the heart, respiration, or local movement between electrode and bladder could easily be identified due to the new experimental methodology used.     

Base pair mismatches and carcinogen-modified bases in DNA: an NMR study of G.T and G.O4meT pairing in dodecanucleotide duplexes

High-resolution two-dimensional NMR studies have been completed on the self-complementary d(C-G-C-G-A-G-C-T-T-G-C-G) duplex (designated G.T 12-mer) and the self-complementary d(C-G-C-G-A-G-C-T-O4meT-G-C-G) duplex (designated G.O4meT 12-mer) containing G.T and G.O4meT pairs at identical positions four base pairs in from either end of the duplex. The exchangeable and nonexchangeable proton resonances have been assigned from an analysis of two-dimensional nuclear Overhauser enhancement (NOESY) spectra for the G.T 12-mer and G.O4meT 12-mer duplexes in H2O and D2O solution. The guanosine and thymidine imino protons in the G.T mismatch resonate at 10.57 and 11.98 ppm, respectively, and exhibit a strong NOE between themselves and to imino protons of flanking base pairs in the G.T 12-mer duplex. These results are consistent with wobble pairing at the G.T mismatch site involving two imino proton-carbonyl hydrogen bonds as reported previously [Hare, D. R., Shapiro, L., & Patel, D. J. (1986) Biochemistry 25, 7445-7456]. In contrast, the guanosine imino proton in the G.O4meT pair resonates at 8.67 ppm. The large upfield chemical shift of this proton relative to that of the imino proton resonance of G in the G.T mismatch or in G.C base pairs indicates that hydrogen bonding to O4meT is either very weak or absent. This guanosine imino proton has an NOE to the OCH3 group of O4meT across the pair and NOEs to the imino protons of flanking base pairs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Development of myometrial electrical activity during the first half of pregnancy in the sheep

Uterine myoelectrical activity was recorded in seven pregnant sheep covering the period between 13 and 75 days post-coitum. Activity in the myometrium was present at day 13 and took the form of intermittent spikes of low amplitude. Bursts of spikes of irregular duration became noticeable between days 25 and 40 but most were not coordinated throughout the myometrium. Coordinated bursts of myoelectrical activity, which could be recorded at several sites simultaneously, first appeared between 40 and 65 days. These bursts had similar characteristics to the myoelectrical activity associated with uterine contractions during the last third of gestation. The myoelectrical activity showed a progressive increase in amplitude during the first half of gestation. There was no relationship between plasma progesterone levels and the increase in amplitude or appearance of coordinated bursts of uterine activity.     

Myocardial Infarction following Surgical Operations

One hundred and forty-one randomly selected surgical patients, aged 35 years or over, were studied preoperatively, followed through their operative procedures, and reassessed during the first post-operative week for evidence of myocardial ischaemia associated with surgical operations under general anaesthesia. Of these patients 38% were found to have preoperative clinical evidence of heart disease, hypertension, or diabetes; 45% had abnormal preoperative E.C.G. patterns.Three patients experienced myocardial infarction during or within 36 hours of operation, all of the occult type; all were in the preoperative abnormal groups. Non-specific postoperative E.C.G. changes were equally common in the groups of patients with normal or abnormal preoperative electrocardiograms.A relationship existed between a rise in serum lactic dehydrogenase (L.D.H.) concentration and the field of the operation, but the diagnosis of infarction was not confused provided serum L.D.H. isoenzyme patterns and a rise in serum aspartate aminotransferase (S.G.O.T.) levels were consistent with the diagnosis.     

BOOK REVIEWS

Book reviewed in this article:
Animal Models in Psychiatry and Neurology. (Editors I. H anin & E. U sdin .)
Receptor Binding Studies in Adrenergic Pharmacology by L. T. W illiams & R. J. L efkowitz .
Kainic Acid as a Tool in Neurobiology (Editors E. G. M c G eer , J. W. O lney & P. L. M c G eer )     

Pattern and correlation of fast and slow spontaneous unit activity in the visual cortex

Spontaneous unit activity in the visual cortex and its changes during stimulation by continuous light or flashes were investigated in waking rabbits. The study of distributions of adjacent intervals showed that the neurons differ in the ratio of burst (fast, with intervals of up to 15–40 msec) and nonburst (slow) activity and in the character of changes from one type of activity to the other. Of the total number of spikes 63% were outside bursts; the ratio of their number to the number of spikes within bursts consisting of two or of three or more spikes was 27:3:1. The relative stability of the burst structure of spontaneous activity and the limited number of spikes in them (on average 2.4) were demonstrated. Bursts of three or more spikes (mean 3.6) were irregular, and in 79% of them a longer interval (18.6±2.4 msec) was observed before the shortest interval (7.9±0.9 msec). Bursts of spikes of most neurons during photic stimulation contain more spikes with shorter intervals; they also began more frequently with the shortest interval, possibly signifying an increase in the steepness and amplitude of the EPSPs lying at their basis. However, in 20% of neurons spontaneous bursts included more spikes and with shorter intervals than bursts evoked by flash stimulation.Research Institute of Psychiatry, Ministry of Health of the RSFSR, Moscow. Translated from Neirofiziologiya, Vol. 11, No. 4, pp. 311–320, July–August, 1979.     

Distinct neuronal coding schemes in memory revealed by selective erasure of fast synchronous synaptic transmission

Neurons encode information by firing spikes in isolation or bursts and propagate information by spike-triggered neurotransmitter release that initiates synaptic transmission. Isolated spikes trigger neurotransmitter release unreliably but with high temporal precision. In contrast, bursts of spikes trigger neurotransmission reliably (i.e., boost transmission fidelity), but the resulting synaptic responses are temporally imprecise. However, the relative physiological importance of different spike-firing modes remains unclear. Here, we show that knockdown of synaptotagmin-1, the major Ca(2+) sensor for neurotransmitter release, abrogated neurotransmission evoked by isolated spikes but only delayed, without abolishing, neurotransmission evoked by bursts of spikes. Nevertheless, knockdown of synaptotagmin-1 in the hippocampal CA1 region did not impede acquisition of recent contextual fear memories, although it did impair the precision of such memories. In contrast, knockdown of synaptotagmin-1 in the prefrontal cortex impaired all remote fear memories. These results indicate that different brain circuits and types of memory employ distinct spike-coding schemes to encode and transmit information.     

A comparison of African buffalo, N''Dama and Boran cattle as reservoirs of Trypanosoma congolense for different Glossina species

Teneral Glossina morsitans centralis Machado were fed on the flanks of the African buffalo (Syncerus caffer Sparrman), N'Dama (Bos taurus L.) or Boran (Bos indicus L.) cattle infected with Trypanosoma congolense Broden. The infected tsetse were maintained on rabbits and on day 30 after the infected feed, the surviving tsetse were dissected to determine the infection rates. The mean infection rates (% +/- SE) in the midgut of tsetse fed on buffalo, N'Damas and Borans were 23.5 +/- 3.3, 31.6 +/- 2.7 and 33.7 +/- 4.6, respectively. The differences were not significant. However, the mean mature infection rate in tsetse fed on the buffalo (13.2 +/- 2.1%) was significantly lower compared to the rates in tsetse fed on the N'Dama (20.4 +/- 1.4) or the Boran cattle (21.4 +/- 1.1). When groups of teneral G.m.centralis, G.pallidipes Austen, G.p.gambiensis Vanderplank, G.f.fuscipes Newstead, G.brevipalpis Newstead and G.longipennis Corti were fed simultaneously on either an infected buffalo, an N'Dama or a Boran steer, the mature infection rates ranged from 0 to 16.1%. Irrespective of the host species used, the T.congolense infection rate was highest in G.m.centralis, lowest in the palpalis and fusca group tsetse, with G.pallidipes being intermediate. Nevertheless, the trypanoresistant African buffalo and N'Dama may serve as reservoirs of T.congolense as can trypanosusceptible Boran cattle.     

Temporal interaction between single spikes and complex spike bursts in hippocampal pyramidal cells

Cortical pyramidal cells fire single spikes and complex spike bursts. However, neither the conditions necessary for triggering complex spikes, nor their computational function are well understood. CA1 pyramidal cell burst activity was examined in behaving rats. The fraction of bursts was not reliably higher in place field centers, but rather in places where discharge frequency was 6-7 Hz. Burst probability was lower and bursts were shorter after recent spiking activity than after prolonged periods of silence (100 ms-1 s). Burst initiation probability and burst length were correlated with extracellular spike amplitude and with intracellular action potential rising slope. We suggest that bursts may function as "conditional synchrony detectors," signaling strong afferent synchrony after neuronal silence, and that single spikes triggered by a weak input may suppress bursts evoked by a subsequent strong input.     

The oscillatory system responsible for the oculomotor activity during the bursts of REM.

1. In precollicular decerebrate cats the electrical activity of single pontine neurons was recorded before, during and after the episodes of postural atonia produced by i.v. injection of 0.03-0.1 mg/kg of eserine sulphate. These episodes were characterized by the regular occurrence of horizontal conjugate eye movements, which were mainly grouped in bursts of REM; moreover, a burst of REM in one direction was generally followed by a burst of REM in the opposite direction. 2. Among the recorded units, 32 showed an increase in their discharge rate during these cataplectic episodes. However, while these units fired at regular frequency when postural rigidity was present, they showed periodic changes in their discharge rate as soon as the bursts of REM appeared in the electrooculogram. In particular a nearly sinusoidal increase in the discharge rate was related to the appearance of an ocular burst in one direction, while a decrease in the unit discharge occurred during an ocular burst in the opposite direction. In some instances neighbouring pontine units located within each side of the brain stem showed reciprocal rate profiles during REM bursts oriented in a given direction, making it likely that the cyclic alternation of their activity depended upon their reciprocal interaction. 3. The alternative hypothesis, i.e., that these periodic changes in unit discharge depend upon the proprioceptive feedback due to the eye movements was excluded by the fact that these changes started before the occurrence of the bursts of REM and began to decline before the end of the burst. Moreover no variation in their firing rate was observed during the positional nystagmus induced by tilting the animal in the control period, i.e., when postural rigidity had reappeared following the end of the cataplectic episode. 4. Most of the neurons showing periodic changes in their discharge frequency during the bursts of REM were located in the pontine reticular formation. Scattered units were also found within the region of the locus coeruleus and the raphe system, close to the surrounding reticular structures. 5. In addition to these neurons, 60 pontine units were recorded, which did not show any changes in their discharge rate during transition from the control period to the cataplectic episode. However, phsiic increases or phasic decreases in their discharge rate appeared synchronously with the individual eye movements. Since in most instances these phasic changes in unit activity coincided with the appearance of the individual monophasic potentials recorded from the ascending MLB, which immediately preceded the rapid eye movements, these units could be attributed either to the premotor neurons responsible for these REM or to the closely related structures which generate their rhythmic discharge. In only a few instances did the discharge of these units not precede but follow the individual eye movements, indicating that they resulted from a proprioceptive feedback originating during the eye movements. 6...     

Upper airway dilating forces during wakefulness and sleep in dogs

We measured the pressure within an isolated segment of the upper airway in three dogs during wakefulness (W), slow-wave sleep (SWS) and rapid-eye-movement (REM) sleep. Measurements were taken from a segment of the upper airway between the nares and midtrachea while the dog breathed through a tracheostoma. These pressure changes represented the sum of respiratory-related forces generated by all muscles of the upper airway. The mean base-line level of upper airway pressure (Pua) was -0.5 +/- 0.03 cmH2O during W, increased by a mean of 2.1 +/- 0.2 cmH2O during SWS, and was variable during REM sleep. The mean inspiratory-related phasic change in Pua was -1.2 +/- 0.1 cmH2O during wakefulness. During SWS, this phasic change in Pua decreased significantly to a mean of -0.9 +/- 0.1 cmH2O (P less than 0.05). During REM sleep, the phasic activity was extremely variable with periods in which there were no fluctuations in Pua and others with high swings in Pua. These data indicate that in dogs the sum of forces which dilate the upper airway during W decreases during SWS and REM sleep. The consistent coupling between inspiratory drive and upper airway dilatation during wakefulness persists in SWS, but is frequently uncoupled during REM sleep.     

Translesion synthesis across O6-alkylguanine DNA adducts by recombinant human DNA polymerases

Previous studies have shown that replicative bacterial and viral DNA polymerases are able to bypass the mutagenic lesions O(6)-methyl and -benzyl (Bz) G. Recombinant human polymerase (pol) delta also copied past these two lesions but was totally blocked by O(6)-[4-oxo-4-(3-pyridyl)butyl] (Pob)G, an important mutagenic lesion formed following metabolic activation of the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. The human translesion pols iota and kappa produced mainly only 1-base incorporation opposite O(6)-MeG and O(6)-BzG and had very low activity in copying O(6)-PobG. Human pol eta copied past all three adducts. Steady-state kinetic analysis showed similar efficiencies of insertion opposite the O(6)-alkylG adducts for dCTP and dTTP with pol eta and kappa; pol iota showed a strong preference for dTTP. pol eta, iota, and kappa showed pre-steady-state kinetic bursts for dCTP incorporation opposite G and O(6)-MeG but little, if any, for O(6)-BzG or O(6)-PobG. Analysis of the pol eta O(6)-PobG products indicated that the insertion of G was opposite the base (C) 5' of the adduct, but this product was not extended. Mass spectrometry analysis of all of the pol eta primer extension products indicated multiple components, mainly with C or T inserted opposite O(6)-alkylG but with no deletions in the cases of O(6)-MeG and O(6)-PobG. With pol eta and O(6)-BzG, products were also obtained with -1 and -2 deletions and also with A inserted (opposite O(6)-BzG). The results with pol eta may be relevant to some mutations previously reported with O(6)-alkylG adducts in mammalian cells.     

Cholinergic mechanisms related to REM sleep. III. Tonic and phasic inhibition of monosynaptic reflexes induced by an anticholinesterase in the decerebrate cat

The depression of the postural activity induced by intravenous injection of eserine sulphate (0.1 mg/kg), an anticholinesterase, has been studied in precollicular decerebrate cats. The extensor and flexor monosynaptic reflexes elicited by single shock stimulation of the GS, P1-FDHL and DP nerves are tonically depressed during the episodes of postural atonia induced by the anticholinesterase. A further phasic depression of the monosynaptic reflexes occurs during the bursts of rapid eye movements (REM) typical of these episodes. These changes in spinal reflex activity closely resemble the tonic depression of the spinal reflexes described in the unrestrained cats during the desynchronized sleep as well as the phasic depression of the spinal reflexes characteristic of the hypnic bursts of REM. Results obtained after spinal cord section indicate that both the tonic and the phasic depression of the spinal reflexes induced by eserine are due to active inhibitory influences originating from supraspinal structures. A complete bilateral destruction of the vestibular nuclei or limited to the medial and descending vestibular nuclei abolishes not only the cholinergically induced bursts of REM, as reported in a previous paper, but also the related phasic depression of the monosynaptic reflexes. These findings can be related with previous observations showing that a bilateral lesion of the vestibular nuclei abolishes the REM bursts of desynchronized sleep, as well as the related phasic inhibition of the spinal reflexes. The tonic depression of the monosynaptic reflexes induced by the anticholinesterase, on the other hand, remains unmodified by this vestibular lesion. This depression, therefore, can be attributed to supraspinal descending inhibitory volleys originating from extravestibular structures.     

Pulse flows of neuronal populations of the cerebral cortex exposed to low intensity microwaves

Pulse flows of populations of cortical neurons in rabbit were investigated prior to, during, and after 1-min microwave irradiation (wavelength 37.5 cm, power density 0.5-1 mW/cm2). It was found that irradiation caused rearrangements in burst activity, which manifested themselves predominantly as changes in the number of spikes forming the bursts. The direction of shifts depended on the time threshold levels of revelation of bursts. These effects took place during microwave irradiation and after its cessation.     

Clofibrate,Serum Enzymes,and Muscle Pain

Serum creatine kinas (C.K.), aspartate amino-transferase (G.O.T.), and alkaline phosphatase (A.P.) activities were measured in 211 men with serum cholesterol concentrations in the upper one-third of the normal distribution. Of these, 110 were receiving clofibrate and 101 were given identical capsules containing olive oil. These investigations were also carried out on 85 healthy men with low serum cholesterol levels not receiving clofibrate.No differences were observed in C.K. and G.O.T. activities between any of these groups; A.P. was significantly lower in the clofibrate-treated group. No significant alterations in C.K. occurred during serial observations made in 15 patients with ischaemic heart disease over a period of five months. No instance of myalgia or muscle stiffness was recorded in 452 men who had received clofibrate for one year.It is concluded that raised C.K. and G.O.T. concentrations and the occurrence of myalgia are uncommon accompaniments of clofibrate treatment.     

Patterns of spontaneous activity in single rat olfactory receptor neurons are different in normally breathing and tracheotomized animals

Spontaneous firing of olfactory receptor neurons (ORNs) was recently shown to be required for the survival of ORNs and the maintenance of their appropriate synaptic connections with mitral cells in the olfactory bulb. ORN spontaneous activity has never been described or characterized quantitatively in mammals. To do so we have made extracellular single unit recordings from ORNs of freely breathing (FB) and tracheotomized (TT) rats. We show that the firing behavior of TT neurons was relatively simple: they tended to fire spikes at the same average frequency according to purely random (Poisson) or simple (Gamma or Weibull) statistical laws. A minority of them were bursting with relatively infrequent and short bursts. The activity of FB neurons was less simple: their firing rates were more diverse, some of them showed trends or were driven by breathing. Although more of them were regular, only a minority could be described by simple laws; the majority displayed random bursts with more spikes than the bursts of TT neurons. In both categories bursts and isolated spikes (outside bursts) occurred completely at random. The spontaneous activity of ORNs in rats resembles that of frogs, but is higher, which may be due to a difference in body temperature. These results suggest that, in addition to the intrinsic thermal noise, spontaneous activity is provoked in part by mechanical, thermal, or chemical (odorant molecules) effects of air movements due to respiration, this extrinsic part being naturally larger in FB neurons. It is suggested that spontaneous activity may be modulated by respiration. Because natural sampling of odors is synchronized with breathing, such modulation may prepare and keep olfactory bulb circuits tuned to process odor stimuli.