The reactivity of neonatal rabbits to the mammary pheromone as a probe for viability

Newborn rabbits depend on a daily nursing interaction with the mother to gain milk and to survive. During this interaction, they localise and seize the nipples displaying a typical behaviour triggered by maternal odour cues. The mammary pheromone constitutes such a signal in domestic rabbits: it elicits sucking-related movements in more than 90% of the pups. However, some newborns remain unresponsive to the presentation of the pheromone, even pups apparently healthy and highly motivated to suck. The main goal of the present study was therefore to explore the link between the unresponsiveness of rabbit pups to the mammary pheromone and their growth and survival in breeding conditions. To that end, 293 newborns from 30 litters were tested for their head searching-oral grasping responses to the mammary pheromone on days 1 and 3, and their milk intake and mortality were followed up from days 1 to 21. It was hypothesised that unresponsive newborns would have subsequent difficulties in finding nipples, sucking and surviving. Early weight and success in milk intake were further considered as mediating factors in growth and viability. The results showed that pups that were unresponsive to the mammary pheromone on day 1 were less successful in gaining milk and had a higher rate of mortality than the responsive pups. However, this impact was modulated by the weight of pups: it appeared only in the lightest newborns. Moreover, this impact vanished on day 3. On the other hand, the pup weight and sucking success on days 1 to 3 strongly influenced viability and growth during the period extending from days 1 to 21. Taken together, the results show that the day-1 responsiveness of rabbit pups to the mammary pheromone can be considered as an indicator of individual viability in pups having a small weight (<48 g on day 1). The predictive validity of the pups’ pheromonal reactivity seems however time-limited as it works only during the first, but crucial, postnatal days.     

Spinal pathway of the milk-ejection reflex in the rat

The effects of lesions of the spinal cord on the milk-ejection reflex evoked by suckling were studied in urethane-anesthetized lactating rats. All lesions were made between C6 and C7 vertebrae and milk ejection was monitored by recording intramammary pressure. In the first experiment on the rats with bilateral lesions, a 3-h suckling test with 5 pups on each side was performed. Eleven (84.6%) of 13 rats with the section of the dorsal funiculus (Group 2), and 12 (85.7%) of 14 rats with the combined section of the dorsal and ventral funiculi (Group 4) displayed regular milk ejection. The incidence of milk ejection in both groups was not significantly different from 81.8% (9 rats) of the 11 sham-operated rats (Group 1). In contrast, none of the 12 rats with bilateral section of the lateral funiculus (Group 3) displayed milk ejection and the incidence of milk ejection was significantly lower than that in Group 1. In the second experiment on the rats with unilateral section of the lateral funiculus, bilateral suckling with 10 pups (5 pups on each side) and unilateral suckling (both ipsilateral and contralateral to the lesion) with 5 pups were consecutively performed in the 10 rats. Milk ejection was induced in 50% by contralateral suckling and in 100% by bilateral suckling, and the incidence was significantly higher than that (0%) observed during ipsilateral suckling. A significant difference in the incidence of milk ejection was also observed between contralateral and bilateral sucklings.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of partial bulbar section on inhibition of jaw-opening reflexes induced by central gray matter stimulation in the cat

The effects of severing the spinal trigeminal tract and its caudal nucleus on high-threshold jaw-opening reflex elicited by tooth pulp stimulation were investigated during experiments on cats under chloralose-Nembutal anesthesia. Low-threshold jaw-opening reflex produced by stimulating the A--infraorbital nerve at an intensity 2–3 thresholds in relation to the most excitable fibers on this nerve was also observed, as well as suppression of these reflexes induced by central gray matter stimulation. It was found that spinal trigeminal tract section produces a 8–52% increase in high-threshold reflex. The amplitude of low-threshold reflex either remained unchanged or showed a slight tendency to rise or fall. Brief stimulation of the central gray matter produced a 100% decrease in high-threshold reflex in intact animals compared with a 40–60% decrease after section of the trigeminal tract. Protracted stimulation of the central gray brought about an 80% decline in high-threshold reflex in intact animals as against 25–30% after section. The degree to which brief stimulation of the central gray produced depression of low-threshold stimulation remained unchanged by trigeminal tract section. Protracted stimulation of the central gray matter brought about a 25–50% reduction in low-threshold reflex in intact animals and a reduction of 75% in three animals and 15–20% in four animals. This implied that the caudal nucleus of the spinal trigeminal tract exerts a more substantial influence on the process of high- than low-threshold reflex inhibition when the central gray matter is stimulated.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 3, pp. 362–368, May–June, 1987.     

Inhibitory effect of dopamine on the transmission of excitation in isolated rat spinal cord

The effects of dopamine on ventral root potential produced by a single supramaximal dorsal root stimulation of the dorsal root was investigated during experiments on isolated superfused spinal cord segments from 10–16-day old rats. A reciprocal dose-dependent inhibition of the mono- and polysynaptic components of reflex response was also observed. Minimum effective concentration was 1×10^–8 M dopamine. Extent of reflex response increased in step with dopamine concentration, so that the amplitude of the monosynaptic component of ventral root potential was decreased by 20% and 87% of baseline level by the action of 10^–4 and 10^–3 M dopamine respectively on the cord. The amplitude of the polysynaptic component was thereby decreased by an average of 18% and 87%. Findings indicate that dopaminergic brainstem-spinal pathways contribute to the governing of impulse transmission in the segmental reflex arcs. Inhibition of dopaminergic synaptic transmission probably underlies the increase in latency already described in the literature, as well as the increase observed in the threshold of reflex motor response to nociceptive action following either stimulation of the dopaminergic brainstem structures or intravenous administration of dopamine agonists.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 5, pp. 616–621, September–October, 1986.     

Effect of rise in plasma osmotic pressure on the milk-ejection reflex in the rat

The effect of an increase in plasma osmolality on the milk-ejection reflex of rats was studied. The lactating rats, at day 8-12 of lactation, were anesthetized with urethane (1.1 g/kg, i.p.) and 8-11 pups which had been separated from their mother 16-18 h were put to the nipples to suckle. In 21 of 47 rats studied an intermittent pattern of milk ejection was recorded with a latency of 15-74 min (group-I rats). The mean interval between recurring milk ejections was 8.3 +/- 0.6 (S.E.M.) min and the mean amount of oxytocin released at each milk ejection estimated in 11 of them was 0.26 +/- 0.04 mU. The remaining 26 rats showed no milk ejection throughout the nursing period of more than 90 min (group-II rats). The intraperitoneal injection of 1.5 M NaCl (1 ml) had no effect on the interval between milk ejections but increased the amount of oxytocin released at each milk ejection by 2.4 times in group-I rats, as the plasma osmolality changed from 297.9 +/- 2.7 mOsm/kg to 310.4 +/- 3.6 mOsm/kg. On the other hand, the 1.5 M NaCl injection induced recurring reflex milk-ejections in all of the group-II rats, while the plasma osmolality increased from 307 +/- 2.8 mOsm/kg to 320 +/- 3.1 mOsm/kg. The mean interval was not significantly different from that observed in the group-I rats. The sensitivity of the mammary gland to oxytocin was not altered by the 1.5 M NaCl injection. The injection of isotonic NaCl had no effect on the milk-ejection reflex.(ABSTRACT TRUNCATED AT 250 WORDS)     

The milk ejection of the rat,as a stimulus and a response to the litter

A finding that the milk ejection of the rat is stimulated effectively only if at least half the pups are suckled is confirmed. This relationship, however, is found if the mother is anaesthetized but not if the mother is conscious. If the mother is conscious, the milk obtained by individual hungry pups is unaffected by the number of other hungry pups suckled. This is true whether the other pups of the litter are removed, or whether they are present but satiated. After the milk ejection, pups tend to leave the mother's nipple. Milk yield decreases with each milk ejection of a series, but getting less milk does not make the pups more likely to leave the nipple in search of another.     

Role of the caudate nucleus in the development of evoked synchronized activity

Synchronized activity (spindles, augmentation response) evoked by stimulation of thalamic nonspecific, association, and specific nuclei was investigated in chronic experiments on 11 cats before and after successive destruction of the caudate nuclei. After destruction of the caudate nuclei the duration of spindle activity in the frontal cortex and subcortical formations (thalamic nuclei, globus pallidus, putamen) was reduced to only three or four oscillations. In the subcortical nuclei its amplitude fell significantly (by 50±10%); in the cortex the decrease in amplitude was smaller and in some cases was not significant. Different changes were observed in the amplitude of the augmentation response, depending on where it was recorded. In the subcortical formations it was considerably and persistently reduced (by 50±10%); in the cortex these changes were unstable in character. Unilateral destruction of the caudate nucleus inhibited synchronized activity evoked by stimulation of the thalamic nuclei on the side of the operation only. Destruction of the basal ganglia (caudate nucleus, globus pallidus, entopeduncular nucleus, and putamen) did not prevent the appearance of synchronized activity; just as after isolated destruction of the caudate nucleus, after this operation synchronized activity was simply reduced in duration and amplitude. It is suggested that the caudate nucleus exerts an ipsilateral facilitatory influence on the nonspecific system of the thalamus during the development of evoked synchronized activity.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 9, No. 3, pp. 239–248, May–June, 1977.     

Pulsatile secretion of prolactin and oxytocin during nursing in the lactating rat

The secretory profile of prolactin and oxytocin in response to suckling stimuli by litters was studied in unanesthetized and urethane-anesthetized lactating rats. Serum prolactin levels were determined by radioimmunoassay. Oxytocin released at milk-ejection reflex was monitored by the changes in the intramammary pressure and/or the characteristic pup's reaction associated with the milk ejection. Serum prolactin concentrations began to rise earlier than the first milk ejection in unanesthetized rats, but they were never elevated without the appearance of milk ejections in urethane-anesthetized rats. Pulsatile fluctuation in serum prolactin levels at 6-15 min intervals was observed in the nursing period when 10 pups were suckling continually. The intermittent milk-ejection reflex occurred not always but preponderantly (64-91%) when the serum prolactin levels were at the nadir of the fluctuation. Injection of an estimated dose of oxytocin released at each milk ejection (1 mU) did not change the serum prolactin levels. These results indicate that the mechanism for prolactin release may be more susceptible to the effects of anesthesia than that for oxytocin release in response to the suckling stimuli and that the release of both the hormones is pulsatile in nature and be influenced by a common biological clock during the nursing period.     

Secondary Rhythms of Cardiac Activity in Rat Ontogeny

Endogenous periodic oscillations of the heart beat rate are described in rat pups aged 3–4, 7–8, 10–11, 13–14, 21–22 days and 1.5 month after birth. These oscillations have all characteristic features established earlier for the secondary rhythms of endogenous contractile activity in the wall of various regions of the gastrointestinal tract and for bursts of spontaneous somatomotor excitation in the early postnatal ontogeny of rats: a multi-stage organization, inconstancy, irregularity of components. In frequency spectra of secondary oscillations of the heart rate obtained by means of fast Fourier transform of R–R intervals of the periodogram, age-related changes of the spectral frequency power are demonstrated in 4 ranges, 0.01–0.03, 0.03–0.1, 0.1–1.0, and 1.0–2.5 Hz, which correspond to the about-one-minute, decasecond, and about-one-second waves of the heart rhythm oscillations and to sinus arrhythmia. It is shown that the dominating frequencies of the secondary rhythms in each range do not have regular age-related changes, which is characteristic of all endogenous secondary rhythms.     

Changes in postsynaptic responses in spinal motoneurons during repetitive stimulation of the locus coeruleus

Experiments on cats anesthetized with chloralose showed that repetitive stimulation of the locus coeruleus is accompanied by a decrease in IPSPs evoked by stimulation of flexor reflex afferents in extensor motoneurons. The effect appeared 600 msec after the beginning of stimulation and reached its maximum after 1500–2000 msec. Repetitive stimulation of the locus coeruleus did not change the membrane potential and did not affect EPSPs or IPSPs evoked by stimulation of low-threshold muscle afferents; EPSPs due to activation of high-threshold cutaneous and muscle afferents likewise remained unchanged. Repetitive stimulation of more central regions of the brain stem was accompanied not only by a decrease in IPSPs evoked by stimulation of flexor reflex afferents in extensor motoneurons, but also by a decrease in amplitude of EPSPs arising in response to stimulation of these same afferents in flexor motoneurons. These effects were not connected with activation of monoaminergic structures, for unlike effects arising during stimulation of the locus coeruleus, they were also found in previously reserpinized animals.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 14, No. 1, pp. 51–59, January–February, 1982.     

Inhibition of spinal reflex responses in the last period of embryogenesis

Some characteristics of spinal reflex reaction inhibition were studied in cat fetuses during the last three weeks of antenatal development. The experiments were conducted on fetuses with intact placental circulation. Restoration of the excitability of the spinal reflex arcs was very slow after stimulation of the dorsal root by a single stimulus. In embryos studied 20 days before birth the full inhibition of reflex responses lasted about 500 msec. Even 2–3 sec after a single stimulation of the afferent fibers the amplitude of the reflex response to the second stimulus was only 30–40% of the control value. It was determined that such long postactivation depression is unrelated to refractoriness or antidromic inhibition. The presence of a prolonged intense depolarization of afferent terminal fibers at these stages suggests a presynaptic inhibition as one of the most probable reasons for the prolonged postactivation depression. Another important factor in the appearance of postactivation depression is probably the morphologic and functional immaturity of synaptic structures. A reciprocal inhibition was observed in cat fetuses on the 10–12th antenatal day. On the basis of these data it is suggested that in embryogenesis presynaptic inhibition considerably precedes that of postsynaptic fibers.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 68–75, January–February, 1971.     

Modulation of segmental reflex reactions during actual locomotion in rats

High- and low-threshold reflex segmental reflex reactions produced by stimulating the dorsal root at different stages of the locomotor cycle were investigated during locomotor swimming motions in white rats. Findings show respectively considerable inhibition of and a pronounced increase in extensor and flexor lowthreshold reflex reactions during the absence and presence of activity in the associated muscles. Low-threshold stimulation produced no outstanding effect on the shaping of the muscles' own activity and hence failed to affect time course or amplitude parameters of locomotor movements. Changes in reflex reactions to high-threshold stimulation during the locomotor cycle largely resembled changes in these responses to low-threshold stimulation, although development of highthreshold reactions differed from that of low-threshold response in affecting the parameters of locomotor activity in the associated muscle, while likewise altering frequency parameters of the locomotor rhythm. The physiological significance and mechanisms possibly underlying modulations in the efficacy of afferent peripheral influences during locomotion are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 20, No. 3, pp. 326–333, May–June, 1988.     

Inhibition of jaw opening reflexes by stimulation of the central gray matter and raphe nuclei in cats

The effect of stimulation of the mesencephalic central gray matter and raphe nuclei on jaw opening reflexes evoked by excitation of high-threshold (dental pulp) and low-threshold (A-alpha) fibers of the infraorbital nerve afferents was studied in cats anesthetized with chloralose and pentobarbital. The jaw opening reflex evoked by stimulation of the dental pulp was shown to be effectively suppressed by conditioning stimulation of the central gray matter and raphe nuclei. The reflex evoked by stimulation of low-threshold infraorbital nerve afferents also was depressed (but less deeply and for a shorter period than the reflex evoked by stimulation of the dental pulp) during stimulation of the raphe nuclei and caudal zone of the central gray matter, but was unchanged after stimulation of the points located in the rostral zone of the central gray matter. Application of single stimuli or bursts of five stimuli with a frequency of 100 Hz had no effect on the reflexes studied. Short-term stimulation with a burst of 10–20 stimuli with a following frequency of 200–400 Hz led to inhibition of the reflexes, which lasted 450–1000 msec. Long-term stimulation of the central gray matter and raphe nuclei for 30 sec with a frequency of 50 Hz caused inhibition of jaw opening reflexes evoked by stimulation of both high- and low-threshold afferents for 60 min. Impulses from the central gray matter and raphe nuclei thus have a mainly inhibitory action on the jaw opening reflex evoked by stimulation of high-threshold afferents, but they act less effectively on the reflex evoked by stimulation of low-thres-hold afferents. The duration of inhibition depends essentially on the parameters of stimulation.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 374–387, May–June, 1984.     

Unit responses in the anterior zone of the suprasylvian gyrus to peripheral stimuli of different modalities

Unit responses in the anterior zone of the suprasylvian gyrus to visual, electrodermal, and acoustic stimulation were investigated in experiments on unanesthetized cats immobilized with tubocurarine. Electrical activity was recorded from 131 units, 121 of which were spontaneously active. In 65.5% of cells responses consisted of a short or long increase or a decrease in intensity of spike activity. Most cells (58.2%) were monosensory. Responses to visual stimulation were given by 72% of neurons, to electrodermal by 61.6%, and to acoustic by 9.3%. The corresponding latent periods were 20–40, 20–30, and 15–20 msec. Responses of the same neurons to different peripheral stimuli were uniform or they differed in their dynamics. Intracellular recording gave responses in the form of EPSPs (amplitude 4–5 mV, duration 60–80 msec) or, rarely, IPSPs (amplitude 2–3 mV, duration 160–200 msec). The functional organization of the associative cortex and mechanisms of analysis of incoming afferent information are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 4, pp. 368–374, July–August, 1972.     

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

The effects were studied in waking cats of brief stimulation (20 stimuli at a rate of 400 Hz) of the central gray matter (CGM) and dorsal raphe nucleus (DRN) on high-threshold jaw-opening reflex (HJOR) evoked by tooth pulp stimulation during blockade of serotonin synthesis produced by application of 300 mg/kg parachlorophenylalanine (PCPA) i.p. Inhibitory effects of CGM and DRN stimulation had already declined in comparison with post-stimulation (but pre-PCPA) level within 24 h after PCPA application; 96 h afterwards, inhibition of HJOR induced by CGM and DRN stimulation had become only minimal: amplitude of the reflex had declined to 30–35% and duration of inhibitory effects ws 200–250 msec. It is therefore deduced that serotonin contributes to the HJOR depression induced by CGM and DRN stimulation and the possible involvement of other neuromodulators in this effect is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 1, pp. 45–52, January–February, 1989.     

Sensory stimuli from pups involved in inhibition of milk secretion in rats during late lactation

Lactating rats of two strains, Holtzman and Sprague-Dawley-Rolfsmeyer, were removed from the animal room on postpartum Day 20 and were placed in a testing room where there were no other rats. Milk secretion was stimulated in mothers of each strain 24 hr later in response to 30 min of exposure to a rack of lactators and their pups, placed 3 ft in front of the test mothers; this stimulation to milk secretion was prevented, provided their own pups were placed under, but not when they were placed alongside, the mothers 3–4 min beforehand. The milk secretion responses on the whole were quantitatively less in the Holtzman rats. The sensory signals emanating from the pups which were responsible for the inhibitory effect upon milk secretion were analyzed in Holtzman rats. Sound and odor each proved to be inhibitory; sight, however, was not. The function of the exteroceptive inhibition of milk secretion in the control of milk secretion in late lactation is discussed.     

Segmental inhibitory response in the frog spinal cord during orthodromic motoneuron activation

Different types of reflex discharges were produced in various preparations by stimulating the dorsal root of isolated frog spinal cord. These ranged from multiphasic low-amplitude waves to distinctly synchronized monosynaptic response. The discharges were followed by facilitation in the former and deep, protracted inhibition of response to test dorsal root stimulation in the latter. When interstimulus intervals measured 40–50 msec, inhibitory action was less pronounced than at shorter (15–30 msec) or longer (60–100 msec) intervals, thus indicating that at least two types of inhibition were at work, one at an earlier and the other at a later stage. Strychnine at a concentration of 10^–5 M effectively reinforced the former and blocked the latter, while 10^–4 M d-tubocurarine attenuated both types of inhibition substantially. It is concluded that inhibition of response occurs mainly as a result of recurrent activation of inhibitory systems via recurrent motoneuron axon collaterals when frog spinal cord afferents are excited. Intensity of the later (presynaptic) and earlier (postsynaptic) inhibition of reflex transmission is determined by the degree of synchrony in motoneuronal discharge in response to orthodromic stimulation.Institute of Medical Radiology, Academy of Medical Sciences of the USSR, Obninsk, Kaluga Oblast. Translated from Neirofiziologiya, Vol. 19, No. 3, pp. 343–350, May–June, 1987.     

Investigation of pallidofugal influences on hypothalamic activity

In chronic experiments on nine cats electrical stimulation of the posterior zones of the lateral hypothalamus led to reproduction of a conditioned food-getting reflex produced previously in response to clicks. During stimulation of the globus pallidus a food-getting reflex also appeared in 3 of 10 zones studied, but it did so only irregularly. In response to combined stimulation of the hypothalamus and these three zones of the globus pallidus facilitation of this reflex was observed: its intensity was increased and its latent period shortened. Stimulation of other areas of the globus pallidus inhibited the food getting reflex of hypothalamic origin on account of the intensification of purposeless motor responses. Stimulation of the globus pallidus evoked desynchronization of spontaneous hypothalamic electrical activity. After bilateral electrolytic destruction of the globus pallidus and its principal efferent pathway (the ansa lenticularis) no conditioned food-getting reflex was reproduced during hypothalamic stimulation. After pallidectomy the amplitude of the spontaneous waves and evoked potentials was reduced by a statistically significant degree in the hypothalamic zones tested. The results are evidence of the existence of regulatory functional influences of the globus pallidus on the lateral hypothalamus.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 6, No. 6, pp. 592–601, November–December, 1974.     

Formation of "caudate spindles" in the rabbit sensomotor cortex during ontogeny

Electrical activity of the sensomotor and visual areas of the neocortex during stimulation of the caudate nucleus was recorded in young rabbits aged 3–60 days and in adults. Single stimulation of the caudate nucleus was found to cause the appearance of characteristic bursts of spindle-like rhythmic activity ("caudate spindles"), described previously in cats and monkeys, in the adult rabbit cortex. The latent period of the caudate spindle was about 200 msec, its duration 1–3 sec, and the frequency of its oscillations of the order of 12 Hz. Caudate spindles were most marked in the sensomotor cortex of the ipsilateral hemisphere. In rabbits under 10 days old caudate spindles were not found even if the intensity of stimulation was increased many times. Starting from the age of 15 days bursts of rhythmic activity resembling caudate spindles, but with lower frequency (about 8 Hz), longer latent period (up to 350 msec), and also with a higher threshold, appeared in the sensomotor cortex. The definitive type of caudate spindles was established toward the end of the first month of postnatal life, corresponding to the time of formation and complication of conditioned-reflex activity in developing animals.Brain Institute, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 17, No. 1, pp. 11–15, March, 1985.     

Effect of microelectrophoretically applied acetylcholine, noradrenaline, dopamine and serotonin on the discharge of paraventricular oxytocinergic neurones in the rat

The effects of microelectrophoretic applications of neurotransmitter substances and their antagonists on the activity of paraventricular oxytocinergic neurones were studied in urethane anesthetized lactating rats. Oxytocinergic neurones were identified by their antidromic response to the stimulation of the neurohypophysis and by their characteristic high frequency discharge of action potentials approximately 15-20s before reflex milk ejection. Acetylcholine (ACh) excited the majority (75%) of paraventricular oxytocinergic neurones, and none of the cells was inhibited in its activity by ACh. In about half of the oxytocinergic cells, atropine and hexamethonium reduced the number of action potentials during the burst discharge preceding reflex milk ejection. Noradrenaline (NE), dopamine (DA) and serotonin (5-HT) reduced the activity of most (75-100%) of oxytocinergic neurones, and none of the cells was excited by these catecholamines. These results suggest that paraventricular oxytocinergic neurones receive excitatory cholinergic inputs and inhibitory noradrenergic, dopaminergic and serotonergic inputs.