Neuronal input contributes to sequence of Aplysia egg laying behaviors

Egg laying in Aplysia is controlled by the bag cell neuroendocrine system, which releases multiple peptides during a long-lasting electrical discharge. Following the discharge, a fixed sequence of head and neck movements is performed in which two phases can be distinguished: an appetitive or preparatory phase, in which the substrate is prepared, and a consummatory phase, when the egg string is deposited. During egg laying, feeding responses are suppressed. In this study, Aplysia fasciata was used. When the movement of the egg string through the genital groove was prevented by ligation, lesions of the nerve innervating the genital pore completely abolished the consummatory egg-laying behaviors. This shows that a nervous connection between the genital pore area and the central nervous system is important for the consummatory egg-laying behaviors.We found that suppression of feeding responses to seaweed occurred only during the consummatory phase of egg laying in controls, but animals with ligated genital grooves continued to show normal responses to food. It is hypothesized that a neuronal feedback, possibly together with the bag cell peptides, is critical for the temporal organization of egg-laying behavior in Aplysia.Abbreviations CNS central nervous system - ELH egg laying hormone     

Egg laying inAplysia

1. Central pathways for bag cell activation were identified by examining the frequency of spontaneous egg laying episodes in animals with central connective lesions. Bilateral lesions of the cerebropleural (but not the cerebropedal) connectives abolished spontaneous egg laying. In contrast, bilateral lesions of all cerebral ganglion peripheral nerves did not abolish spontaneous egg laying, suggesting that sensory input to the cerebral ganglion is not necessary for activating the bag cells. 2. Backfilling either pleuroabdominal connective labelled cell bodies in the cerebral ganglia (via the ipsilateral cerebropleural connective) that could project to the bag cells. Focal extracellular stimulation of these stained clusters activated the bag cells in isolated brains. 3. Central pathways for initiating egg laying behaviors were identified by selectively eliciting bag cell discharges in animals with central connective lesions. Bilateral lesions of the cerebropedal (but not the cerebropleural) connectives completely abolished elicited egg laying behaviors. 4. Pathways for motor output during rhythmic head and neck movements were identified by eliciting bag cell discharges in animals with peripheral nerve lesions. Bilateral lesions of the four tegumentary nerves in combination with the anterior pedal nerve completely abolished elicited egg laying behaviors, indicating that these nerves are necessary for normal motor output. A normal pattern of egg laying behaviors occurred when the four tegumentary and the anterior pedal nerves were left intact and all other pedal ganglion nerves were lesioned bilaterally, indicating that these nerves are also sufficient for normal motor output.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of bag cells in egg deposition ofAplysia brasiliana

Summary Egg deposition behaviors are analyzed from time-lapse recordings during which spontaneous discharges of the neuroendocrine bag cells are recorded with chronically implanted cuff electrodes. In the laboratory,Aplysia brasiliana normally deposit long egg cordons on the substrate in a characteristic figure 8 pattern similar to the configuration of egg masses observed in the natural environment. The overt behaviors associated with egg deposition are rhythmic head movements consisting of three components that overlap with characteristic relative latencies: up-and-down undulations, side-to-side weaves and in-and-out tamps. The characteristics of the three behaviors and their time courses relative to the appearance of eggs on the substrate suggest that undulations prepare the substrate, weaves distribute the egg cordon and tamps attach the cordon to the substrate. The same rhythmic head movements are also elicited by injections of homogenized abdominal ganglia (HAG) containing bag cell clusters, with comparable relative latencies and maximum frequencies but for shorter total durations. The overt behaviors begin earlier for normal than for triggered egg laying, often before the spontaneous release of bag cell hormones. This suggests that the head oscillations in intact animals are not normally initiated by bag cell activity. The mean latency to the appearance of the egg cordon on the substrate is the same (about 34 min) following either HAG injections or spontaneous bag cell discharges, confirming previous suggestions that the bag cell discharge triggers ovulation. Furthermore, the head movements appear to terminate at the same time following release or injection of hormone. The accompanying paper demonstrates that the full expression of the behavioral effects of bag cell injections depend upon normal movement of eggs in the reproductive tract.Abbreviations CPG central pattern generator - ELH egg laying hormone - HAG homogenized abdominal ganglia     

Pheromones linked to sexual behaviors excite the appetitive phase of feeding behavior of Aplysia fasciata. I. Modulation and excitation of appetitive behaviors

Pheromones presumably secreted by mating conspecifics – as well as homogenates containing tissue that is homologous with the atrial gland – increase the time that Aplysia fasciata spend feeding. This effect is caused by increasing the number of feeding episodes initiated in response to food, whereas the duration of a feeding bout remains unchanged. The increase in the number of feeding episodes is related to increases in head waving and crawling, i.e., appetitive movements that bring the animal into contact with food, as well as an increase in the responsiveness to food after it is contacted. Releasing a homogenate containing atrial gland tissue, or egg laying hormone, in the water near the animal elicited head lifting similar to that seen when animals are food aroused. The data indicate that the facilitation of Aplysia feeding caused by pheromones arises in part by an excitation of appetitive behaviors. These findings suggest that neurons generating appetitive behaviors will be affected by pheromones. Accepted: 28 November 1997     

Studies on the Neurosecretory Control of Egg Laying in Aplysia

Studies are reviewed on the electrophysiological and endocrinologicalfeatures of a group of neurosecretory cells (the big cells)in the abdominal ganglion of Aplysia. Electrophysiological studiessuggest that the bag cells are involved in the regulation ofa phasic behavioral function such as egg laying. Egg layingoccurs approximately one hour following the injection into thehemocele of a crude extract of the bag cells or of the perfusateof an abdominal ganglion in which a pleural abdominal connectiveis electrically stimulated. The bag cells appear to act as aunit, releasing a measured dose of egg laying hormone when theyare triggered into activity.     

The Muscle Cells of the Follicle of the Ovotestis in Aplysia as the Probable Target Organ for Bag Cell Extract

It has recently been demonstrated that activity of the bag cellsan identified gioup of neurosecretory neurons in Aplysia, leleasesa substance that causes egg laying. A cytologic analysis ofthe ovotestis, before and after administration of the bag cellextract, suggests that one function of the bag cell substanceis to cause the small muscle cells that surround each follicleof the ovotestis to contract, thus expelling ripe oocytes fromthe ovotestis ind beginning them on then journey through theoviduet     

Complex behavior induced by egg-laying hormone in Aplysia

Previous studies have described a pattern of complex behavior that occurs in the marine mollusc Aplysia during egg laying. Egg laying and the behavior are initiated by a burst of impulse activity in the neuroendocrine bag cells of the abdominal ganglion or by injection of bag cell extract. To more precisely identify the factors responsible for inducing the behavior we injected animals with egg laying hormone (ELH), one of the neuropeptides secreted by the bag cells. We found that ELH causes a behavior pattern similar to what occurs during spontaneous egg laying. This includes a temporal pattern of head movements consisting of waves and undulations, followed near the beginning of egg deposition by a transition to head weaves and tamps and inhibition of locomotion. There was also a small decrease in respiratory pumping. Except for respiratory pumping, a similar pattern occurred in a second group of animals injected with atrial gland homogenate, which is presumed to induce bag cell activity, but not in controls. These results further implicate ELH in regulation of the behavior. We discuss possible sites of action of ELH and the neural mechanisms by which the behavior is controlled.Abbreviations ELH egg laying hormone - ASW artificial sea water     

Pheromones linked to sexual behaviors excite the appetitive phase of feeding behavior of Aplysia fasciata II. Excitation of C-PR, a neuron involved in the generation of appetitive behaviors

Pheromones presumably released by conspecifics amplify both the appetitive and the consummatory components of feeding in Aplysia. These effects can be mimicked by administering homogenate of the large hermaphroditic duct containing atrial gland tissue, as well as peptides from the bag cells. Identified cerebro-pedal regulator (C-PR) neuron is thought to command various behaviors that comprise the appetitive phase of feeding. In a reduced preparation, we investigated the effects on the C-PR of applying these substances to the rhinophores, the sensory organs which detect pheromones. Stimuli that excite feeding in the animal were also found to affect the C-PR. Large hermaphroditic duct homogenate caused a doubling in the firing rate of the C-PR, and amplified the response of the C-PR to other excitatory stimuli, such as touch of food to the rhinophores. Bag cell peptides (α, β and γ bag cell peptide, and egg-laying hormone) caused smaller increases in the firing rate of the C-PR. These data are consistent with the hypothesis that pheromones facilitate appetitive feeding behavior in part via their excitation of C-PR. Accepted: 28 November 1997     

Activity of neck-muscle motoneurones during eye cleaning behaviour in the cricket Gryllus campestris

The activity of neck-muscle motoneurones which control head movements during eye cleaning behaviour was recorded from motor nerves with chronically implanted electrodes in unrestrained crickets. We show that motoneurones of the dorso-ventral muscles displayed strong activity differences between both sides of the neck, with higher discharge frequencies either ipsi- or contralateral to the direction of the head movement. Motoneurones innervating dorsal-longitudinal muscles were equally active on both sides. A single excitatory motoneurone of one dorso-ventral muscle showed a discharge pattern unequivocally related to eye cleaning. Lesions of connectives revealed that this motoneurone is monitored by interneuronal pathways from the suboesophageal ganglion although the primary sensory axons eliciting eye cleaning, project into the prothoracic ganglion.     

Synthetic egg-laying hormone of Aplysia: quantitative studies of induction of egg laying in Stylocheilus and of the activation of Aplysia buccal neuron B16

1. Synthetic Aplysia egg-laying hormone (ELH-lysine-amide) elicited egg laying in Stylocheilus at a threshold dose of 0.5 microgram per recipient, estimated to be a concentration in the circulation of Stylocheilus of approximately 70 nM. 2. Threshold level and size of egg mass produced by ELH-lysine-amide and bag cell extracts (containing biological ELH) were not significantly different. Latency to lay of recipients of 0.5-4.0 micrograms ELH-lysine-amide (30 +/- 1 min) was significantly longer (P less than 0.05) than for bag cell extract recipients (21 +/- 1 min). 3. ELH-lysine-amide depolarized and activated action potentials in Aplysia buccal neuron B16 in high magnesium, low calcium medium. 4. The lowest concentration of ELH-lysine-amide to activate a supra-threshold response of left and right B16 neurons ranged from 250 nM to 1 microM. 5. Threshold levels for responses to synthetic ELH-lysine-amide and to biological ELH were approximately the same in both egg-laying assay and electrophysiological assay, indicating the likely identity of synthetic and biological ELH. However, the shorter egg-laying latency with bag cell extract suggests that there may be additional factors in the extract that facilitate egg laying.     

Local hormonal modulation of neural activity in Aplysia

The abdominal ganglion of Aplysia provides a convenient experimental system for cellular studies on the roles of peptides as chemical messengers in the nervous system. There are indications that the bag cells, a group of neuroendocrine cells, synthesize and release egg laying hormone (ELH), a peptide with an apparent molecular weight of 6000. Our recent investigations indicate that a burst of impulse activity in the bag cells produces five types of long-lasting responses, some excitatory, others inhibitory, in 26 identified neurons and 2 identified cell clusters located near the bag cells in the abdominal ganglion. The responses have slow, smoothly graded onsets, and many of them result in modulation of neuronal activity for 3 hours or more. Physiological and ultrastructural data support the hypothesis that they are induced by a bag cell hormone (or hormones) that is released into vascular and interstitial spaces of the ganglion to act on the target neurons. Local application of purified ELH to one of the target neurons provides evidence that the bag cell effect is mediated by ELH. Many of the target neurons are known to be parts of neuronal circuits that control specific behavioral and homeostatic processes. Since egg laying is initiated by the bag cell discharge and is associated with a stereotyped behavior pattern lasting several hours, the actions of these peptide-secreting neurons on the central nervous system may serve to regulate certain elements of behavior and homeostasis during egg laying.     

Pre- and postnatal bisphenol A treatment results in persistent deficits in the sexual behavior of male rats, but not female rats, in adulthood

Perinatal administration of the endocrine disruptor bisphenol A (BPA) reportedly inhibits the sexual behavior of sexually naïve adult male rats. In order to evaluate the effects of BPA administration during early development on later reproductive behavior, we administered one of five doses of bisphenol A daily to pregnant female rats throughout gestation and lactation, and quantified the appetitive and consummatory sexual behaviors of the resultant male and female offspring over multiple sexual encounters in adulthood. Males receiving low dose perinatal BPA (50 μg/kg bw/day) showed persistent deficits in sexual behavior in adulthood. Males receiving the highest dose (5 mg/kg bw/day), however, were indistinguishable from controls with respect to consummatory sexual behaviors but showed decreased latencies to engage in those behaviors when sexually naïve, with significant non-linear, or U-shaped, dose-response relationships observed on the first and last day of testing. Adult female sexual behavior was not affected by early BPA administration at any dose tested. These results are consistent with previous reports that BPA exerts behavioral effects especially at low doses, and further indicates that BPA can cause lasting impairment of sexual behavior in males, but does not alter the normal development of female appetitive or consummatory sexual behaviors. To our knowledge, this is the first report indicating that adult sexual performance is impaired in sexually experienced animals following perinatal exposure to bisphenol A.     

The bag cell neurons ofAplysia

Egg laying in Aplysia involves a well-characterized series of behaviors that can last for several hours. The behaviors are controlled by two bilateral clusters of peptidergic neurons in the abdominal ganglion. Following brief stimulation, these neurons, which have been termed the bag cell neurons, undergo a sequence of changes in their excitability lasting many hours. The bag cell neurons have served as a model system for studying the molecular mechanisms involved in the synthesis, processing, and release of neuroactive peptides and in the regulation of prolonged changes in neuronal excitability.     

Peptide B induction of bag-cell activity in Aplysia: localization of sites of action to the cerebral and pleural ganglia

The bag cells of the marine mollusc Aplysia are model neuroendocrine cells involved in the initiation of egg laying and its associated behaviors, but the natural stimulus triggering bag-cell activity is not known. The atrial gland of A. californica, an exocrine organ in the reproductive tract, contains two structurally related peptides (A and B) which can induce an afterdischarge in vitro, and these peptides can be used to probe the central nervous system for sites where extrinsic excitatory input onto the bag-cell system might occur. These sites were identified in a series of lesion and ablation experiments. The entire central nervous system was removed from an animal and placed in a chamber with two compartments which could be independently perfused, allowing peptides (atrial gland extract or purified peptide B) to be selectively applied to specific regions of the nervous system while bag-cell activity was monitored using extracellular suction electrodes. Afterdischarges were consistently and reliably induced when peptides were applied to the cerebral ganglion, the pleural ganglia, the cerebropleural connectives, or the rostral 10-15% of the pleurovisceral connectives, provided that an intact neuronal pathway connected the site of peptide application with the bag cells. In contrast, afterdischarges were never observed when peptides were selectively applied to the buccal or pedal ganglia and only rarely observed when applied to the abdominal ganglion and caudal pleurovisceral connectives. These results support the hypothesis that bag-cell processes and/or neuron(s) presynaptically excitatory to the bag cells are located in the pleural and cerebral ganglia and narrow the region of the central nervous system where the critical initiator element(s) can be identified.     

Differential hormonal action of the bag cell neurons on the arterial system ofAplysia

Summary The peptide-secreting bag cell neurons ofAplysia californica activate a long-lasting, complex behavior called egg laying. During egg laying some organ systems (reproductive) are more active than others (digestive) suggesting that blood flow to these tissues may change in accordance with their activities during egg laying. To examine this possibility we used a semi-intact preparation of the three major arteries innervated by the abdominal ganglion. We found that electrically stimulated bursts of bag cell activity triggered a long-lasting (>1 h) increase in contractile activity in two arteries, the anterior and gastroesophageal, but did not affect contractions of the third (abdominal) artery. The arterial responses were not affected either in form or duration by denervation of the arteries, suggesting that the increase in contractile activity was mediated by hormonal actions of bag cell transmitters on vasoconstrictor muscles. In intact animals this differential action on the arterial system may cause a long-term decrease in blood flow to relatively inactive tissues (digestive and locomotory organs) while increasing circulation to tissues involved in egg production (ovotestis and oviduct).Abbreviations ASW artificial sea water - BCA bag cell activation - ELH egg laying hormone     

When do we eat? Ingestive behavior,survival, and reproductive success

The neuroendocrinology of ingestive behavior is a topic central to human health, particularly in light of the prevalence of obesity, eating disorders, and diabetes. The study of food intake in laboratory rats and mice has yielded some useful hypotheses, but there are still many gaps in our knowledge. Ingestive behavior is more complex than the consummatory act of eating, and decisions about when and how much to eat usually take place in the context of potential mating partners, competitors, predators, and environmental fluctuations that are not present in the laboratory. We emphasize appetitive behaviors, actions that bring animals in contact with a goal object, precede consummatory behaviors, and provide a window into motivation. Appetitive ingestive behaviors are under the control of neural circuits and neuropeptide systems that control appetitive sex behaviors and differ from those that control consummatory ingestive behaviors. Decreases in the availability of oxidizable metabolic fuels enhance the stimulatory effects of peripheral hormones on appetitive ingestive behavior and the inhibitory effects on appetitive sex behavior, putting a new twist on the notion of leptin, insulin, and ghrelin “resistance.” The ratio of hormone concentrations to the availability of oxidizable metabolic fuels may generate a critical signal that schedules conflicting behaviors, e.g., mate searching vs. foraging, food hoarding vs. courtship, and fat accumulation vs. parental care. In species representing every vertebrate taxa and even in some invertebrates, many putative “satiety” or “hunger” hormones function to schedule ingestive behavior in order to optimize reproductive success in environments where energy availability fluctuates.     

Role of NPY and its receptor subtypes in foraging, food hoarding, and food intake by Siberian hamsters

Fasting has widespread physiological and behavioral effects such as increases in arcuate nucleus neuropeptide Y (NPY) gene expression in rodents, including Siberian hamsters. Fasting also stimulates foraging and food hoarding (appetitive ingestive behaviors) by Siberian hamsters but does relatively little to change food intake (consummatory ingestive behavior). Therefore, we tested the effects of third ventricular NPY Y1 ([Pro(34)]NPY) or Y5 ([D-Trp(34)]NPY) receptor agonists on these ingestive behaviors using a wheel running-based food delivery system coupled with simulated burrow housing. Siberian hamsters had 1) no running wheel access and free food, 2) running wheel access and free food, or 3) foraging requirements (10 or 50 revolutions/pellet). NPY (1.76 nmol) stimulated food intake only during the first 4 h postinjection ( approximately 200-1,000%) and mostly in hamsters with a foraging requirement. The Y1 receptor agonist markedly increased food hoarding (250-1,000%), increased foraging as well as wheel running per se, and had relatively little effect on food intake (<250%). Unlike NPY, the Y5 agonist significantly increased food intake, especially in foraging animals ( approximately 225-800%), marginally increased food hoarding (250-500%), and stimulated foraging and wheel running 4-24 h postinjection, with the distribution of earned pellets favoring eating versus hoarding across time. Across treatments, food hoarding predominated early postinjection, whereas food intake tended to do so later. Collectively, NPY stimulated both appetitive and consummatory ingestive behaviors in Siberian hamsters involving Y1/Y5 receptors, with food hoarding and foraging/wheel running (appetitive) more involved with Y1 receptors and food intake (consummatory) with Y5 receptors.     

Lidocaine suppresses subthreshold oscillations by inhibiting persistent Na(+) current in injured dorsal root ganglion neurons

The aim of this study was to determine the effect and mechanism of low concentration of lidocaine on subthreshold membrane potential oscillations (SMPO) and burst discharges in chronically compressed dorsal root ganglion (DRG) neurons. DRG neurons were isolated by enzymatic dissociation method. SMPO, burst discharges and single spike were elicited by whole cell patch-clamp technique in current clamp mode. Persistent Na(+) current (I(NaP)) and transient Na(+) current (I(NaT)) were elicited in voltage clamp mode. The results showed that SMPO was suppressed and burst discharges were eliminated by tetrodotoxin (TTX, 0.2 micromol/l) in current clamp mode, I(NaP) was blocked by 0.2 micromol/l TTX in voltage clamp mode. SMPO, burst discharges and I(NaP) were also suppressed by low concentration of lidocaine (10 micromol/l) respectively. However, single spike and I(NaT) could only be blocked by high concentration of lidocaine (5 mmol/l). From these results, it is suggested that I(NaP) mediates the generation of SMPO in injured DRG neurons. Low concentration of lidocaine (10 micromol/l) suppresses SMPO by selectively inhibiting I(NaP), but not I(NaT), in chronically compressed DRG neurons.     

Neurobiology of sexual behavior

Recent advances in the neurobiology of sexual behavior have helped to refine our understanding of the neuroanatomical, neuroendocrine and neurochemical systems that modulate responses to sexual stimulation. Both appetitive and consummatory sexual behaviors have been studied in several laboratory species and in humans using traditional and novel behavioral paradigms. New knowledge has emerged concerning the role of hypothalamic, limbic and brainstem structures, neuropeptides, brain monoamines and nitric oxide in the control of partner preference, sexual desire, erection, copulation, ejaculation, orgasm and sexual satiety. Brain imaging of visually evoked sexual arousal in humans has also been examined.     

Head Movements Evoked in Alert Rhesus Monkey by Vestibular Prosthesis Stimulation: Implications for Postural and Gaze Stabilization

The vestibular system detects motion of the head in space and in turn generates reflexes that are vital for our daily activities. The eye movements produced by the vestibulo-ocular reflex (VOR) play an essential role in stabilizing the visual axis (gaze), while vestibulo-spinal reflexes ensure the maintenance of head and body posture. The neuronal pathways from the vestibular periphery to the cervical spinal cord potentially serve a dual role, since they function to stabilize the head relative to inertial space and could thus contribute to gaze (eye-in-head + head-in-space) and posture stabilization. To date, however, the functional significance of vestibular-neck pathways in alert primates remains a matter of debate. Here we used a vestibular prosthesis to 1) quantify vestibularly-driven head movements in primates, and 2) assess whether these evoked head movements make a significant contribution to gaze as well as postural stabilization. We stimulated electrodes implanted in the horizontal semicircular canal of alert rhesus monkeys, and measured the head and eye movements evoked during a 100ms time period for which the contribution of longer latency voluntary inputs to the neck would be minimal. Our results show that prosthetic stimulation evoked significant head movements with latencies consistent with known vestibulo-spinal pathways. Furthermore, while the evoked head movements were substantially smaller than the coincidently evoked eye movements, they made a significant contribution to gaze stabilization, complementing the VOR to ensure that the appropriate gaze response is achieved. We speculate that analogous compensatory head movements will be evoked when implanted prosthetic devices are transitioned to human patients.