Spontaneous activity in the developing gerbil auditory cortex in vivo involves GABAergic transmission.

A salient feature of the developing brain is that spontaneous oscillations (SOs) and waves may influence the emergence of synaptic connections. While GABA produces depolarization and may support SOs in the neurons of developing rodents, it elicits hyperpolarization and diminishes SOs in developing gerbil auditory cortex (ACx). Therefore, we asked whether SOs exist in developing gerbil ACx in vivo and if GABAergic involvement can be manipulated. In vivo extracellular recordings in P3-5 ACx revealed SOs with longer burst durations and shorter inter-event intervals compared to ACx SOs in slices. ACx was then validated by gross anatomical features and lesions created at the in vivo recording site that corresponded with the electrophysiological coordinates of thalamorecipient ACx in slices. Further, NeuroVue Red, a lipophilic dye loaded at the in vivo recording sites, stained anatomically identifiable fiber tracks between the ACx and the auditory thalamus, medial geniculate body (MG). Separately, to chronically perturb GABAergic role in SOs, P2-5 pups were administered daily with GABA(A) receptor blocker, bicuculline (BIC). We then recorded from P14-17 ACx neurons in slices generated after hearing onset. ACx neurons from BIC-administered pups exhibited spontaneous action potentials in contrast to subthreshold synaptic potentials in neurons from sham-injected animals. Finally, to elucidate whether the gap junction blocker mefloquine (MFQ) previously shown to dampen ACx SOs in slices affected GABAergic transmission, MFQ was acutely applied in P3-5 slices while spontaneous inhibitory postsynaptic currents (sIPSCs) were recorded. Whereas MFQ increased the amplitude and frequency of sIPSCs in ACx neurons, the broad-spectrum gap junction blocker carbenoxolone decreased sIPSC amplitudes only. Together, we show that P2-5 gerbil ACx can endogenously generate SOs in vivo. Persistence of activity in ACx in P14-17 slices from pups administered with BIC at P2-5 implies that inhibitory GABAergic activity linked with gap-junction participates in the maturation of ACx.

Developmental expression of endogenous oscillations and waves in the auditory cortex involves calcium, gap junctions, and GABA.

Neuronal oscillations and population waves (OWs) may be important for the maturation of neural circuits in the cortex and other developing areas of the CNS. We examined endogenous network activity by whole-cell and paired extracellular recordings in the thalamorecipient auditory cortex (ACx) in slices of gerbil pups during the first three postnatal weeks. Separately, we examined network ensemble correlates of the OWs using population intracellular free calcium (Ca2+) imaging in slices bulk-loaded with fura-2 AM. In slices devoid of physiological or pharmacological manipulations, spontaneous multi-neuronal bursts recorded extracellularly at the perirhinal cortex precede bursts simultaneously recorded at the ACx, suggesting their caudorostral propagation. OWs waned after postnatal day (P) 7, ceased following hearing onset (P12), and accompanied altered membrane properties. Population imaging from P2-5 slices with fura-2 AM revealed endogenously generated waves that spread from the perirhinal cortex toward the thalamorecipient ACx. Wave incidence varied between 5 waves/min to 0.4 waves/min. OWs were disrupted by treatment of slices with [Ca2+]i chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, the gap junction blocker mefloquine or the GABAA receptor blocker bicuculline. These results suggest that propagating activity involving calcium, gap junctions and GABAergic transmission exists in the gerbil ACx and it correlates with key developmental events in vivo. We speculate such activity may be integral to postnatal maturation of ACx.

GABA(B) and Trk receptor signaling mediates long-lasting inhibitory synaptic depression.

In many areas of the nervous system, excitatory and inhibitory synapses are reconfigured during early development. We have previously described the anatomical refinement of an inhibitory projection from the medial nucleus of the trapezoid body to the lateral superior olive in the developing gerbil auditory brain stem. Furthermore, these inhibitory synapses display an age-dependent form of long-lasting depression when activated at a low rate, suggesting that this process could support inhibitory synaptic refinement. Since the inhibitory synapses release both glycine and GABA during maturation, we tested whether GABA(B) receptor signaling could initiate the decrease in synaptic strength. When whole cell recordings were made from lateral superior olive neurons in a brain slice preparation, the long-lasting depression of medial nucleus of the trapezoid body-evoked inhibitory potentials was eliminated by the GABA(B) receptor antagonist, SCH-50911. In addition, inhibitory potentials could be depressed by repeated exposure to the GABA(B) receptor agonist, baclofen. Since GABA(B) receptor signaling may not account entirely for inhibitory synaptic depression, we examined the influence of neurotrophin signaling pathways located in the developing superior olive. Bath application of brain-derived neurotrophic factor or neurotrophin-3 depressed evoked inhibitory potentials, and use-dependent depression was blocked by the tyrosine kinase antagonist, K-252a. We suggest that early expression of GABAergic and neurotrophin signaling mediates inhibitory synaptic plasticity, and this mechanism may support the anatomical refinement of inhibitory connections.

Long-lasting inhibitory synaptic depression is age- and calcium-dependent.

The developmental refinement of excitatory synapses is often influenced by neuronal activity, and underlying synaptic mechanisms have been suggested. In contrast, few studies have asked whether inhibitory synapses are reorganized during development and whether this is accompanied by use-dependent changes of inhibitory synaptic strength. The topographic inhibitory projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) undergoes synapse elimination during development (Sanes and Takács, 1993). To determine whether there is an associated period of synaptic plasticity, whole-cell recordings were obtained from developing LSO neurons of gerbils in a brain slice preparation. In current-clamp recordings, low-frequency stimulation of the MNTB led to a decline in IPSP amplitude by 43%. In voltage-clamp recordings, hyperpolarized LSO neurons also exhibited a long-lasting depression of MNTB-evoked inhibitory synaptic currents (34%) after low-frequency stimulation. When LSO neurons were depolarized, low-frequency stimulation of the MNTB produced a significantly larger inhibitory synaptic depression (59%). This synaptic plasticity declined dramatically by postnatal days 17-19. Similar to well studied forms of excitatory synaptic plasticity, inhibitory depression depended on postsynaptic calcium. We propose that such activity-dependent synaptic depression may support the developmental rearrangement of inhibitory terminals as they compete with neighboring excitatory and/or inhibitory inputs.

Commissural and lemniscal synaptic input to the gerbil inferior colliculus.

The central nucleus of the inferior colliculus (ICC) receives direct inputs, bilaterally, from all auditory brain stem nuclear groups. To evaluate the contribution made to gerbil ICC neuron physiology by two major afferent pathways, we examined the synaptic responses evoked by direct stimulation of the commissure of the inferior colliculus (CIC) and the ipsilateral lateral lemniscus (LL). Frontal midbrain slices were obtained from postnatal day (P) 9-P19 gerbils, and whole cell recordings were made under current- (n = 22) or voltage-clamp (n = 52) conditions. Excitatory and inhibitory synaptic responses were characterized by sequentially exposing the slice to ionotropic glutamate receptor antagonists [6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) + aminophosphonpentanoic acid (AP-5), or kynurenic acid)], a gamma-aminobutryic acid type A receptor antagonist (bicuculline), and a glycine receptor antagonist (strychnine). In current clamp, LL stimulation typically produced a short latency depolarization followed by a longer duration hyperpolarization. The depolarization was abolished by AP-5 + CNQX, and the remaining inhibitory potential displayed either bicuculline or strychnine sensitivity. In voltage clamp, 79% of ICC neurons displayed synaptic currents after stimulation of each pathway. The synaptic currents were typically complex waveforms, and ionotropic glutamate receptor antagonists reduced inward currents at a holding potential of -80 mV in the majority of neurons. In addition, this treatment reduced outward synaptic currents at a holding potential of -20 mV, indicating that inhibitory interneuronal input was often activated by LL or CIC afferents. A minority of neurons had synaptic currents that were unaffected by glutamate receptor antagonists, but it was more common for CIC-evoked currents to be unaffected (38%) rather than LL-evoked currents (22%). The CIC provided a strong inhibitory input that was almost exclusively GABAergic, whereas the LL inhibition often included a glycinergic component. These experiments have shown that the CIC provides a major glutamatergic and GABAergic input to most ICC neurons. However, much of the inhibitory input from both the CIC and the LL appears to be mediated by interneuronal connections.

A developmental shift from GABAergic to glycinergic transmission in the central auditory system.

GABAergic and glycinergic circuits are found throughout the auditory brainstem, and it is generally assumed that transmitter phenotype is established early in development. The present study documents a profound transition from GABAergic to glycinergic transmission in the gerbil lateral superior olive (LSO) during the first 2 postnatal weeks. Whole-cell voltage-clamp recordings were obtained from LSO neurons in a brain slice preparation, and IPSCs were evoked by electrical stimulation of the medial nucleus of the trapezoid body (MNTB), a known glycinergic projection in adult animals. GABAergic and glycinergic components were identified by blocking transmission with bicuculline and strychnine (SN), respectively. In the medial limb of LSO, there was a dramatic change in the GABAergic IPSC component, decreasing from 78% at postnatal day 3 (P3)-P5 to 12% at P12-P16. There was an equal and opposite increase in the glycinergic component during this same period. Direct application of GABA also elicited significantly larger amplitude and longer duration responses in P3-P5 neurons compared with glycine-evoked responses. In contrast, MNTB-evoked IPSCs in lateral limb neurons were more sensitive to SN throughout development. Consistent with the electrophysiological observations, there was a reduction in staining for the beta2,3-GABAA receptor subunit from P4 to P14, whereas staining for the glycine receptor-associated protein gephyrin increased. Brief exposure to baclofen depressed transmission at excitatory and inhibitory synapses for approximately 15 min, suggesting a GABAB-mediated metabotropic signal. Collectively, these data demonstrate a striking switch from GABAergic to glycinergic transmission during postnatal development. Although GABA and glycine elicit similar postsynaptic ionotropic responses, our results raise the possibility that GABAergic transmission in neonates may play a developmental role distinct from that of glycine.

Deafferentation weakens excitatory synapses in the developing central auditory system.

Decreased excitatory synaptic activity during development often leads to pre- and postsynaptic atrophy, as assessed anatomically. The present study considers the effect of decreased excitatory transmission on the maturation of synaptic strength. Towards this end, cochlear nucleus neurons, which project to the ipsilateral lateral superior olive (LSO), were denervated in gerbils at postnatal day 7, before the onset of hearing. This manipulation was intended to disrupt spontaneous glutamatergic transmission in the LSO while sparing the glycinergic afferents from the medial nucleus of the trapezoid body (MNTB). Afferent-evoked synaptic activity was assessed 1-6 days after ablation in a brain slice preparation using whole-cell current- and voltage-clamp recordings. In control animals, ipsilaterally evoked excitatory postsynaptic potentials (EPSPs) were present in 91% of neurons tested, but were observed in only 60% of neurons following cochlea removal. The maximum EPSP amplitude was significantly smaller in manipulated neurons compared with controls, and this was accompanied by a higher incidence of ipsilaterally evoked inhibitory postsynaptic potentials (IPSPs). To study the efficacy of excitatory synapses in greater detail, voltage-clamp recordings were made in the presence of strychnine and AP-5 [D(O)-2-amino-5-phosphonopentanoic acid]. The minimum excitatory postsynaptic current (EPSC) amplitude, presumed to reflect the efficacy of a single glutamatergic afferent, was approximately 40% smaller in manipulated neurons. In contrast, MNTB-evoked IPSPs were similar in neurons from control and ablated animals. However, manipulated neurons often exhibited a rebound depolarization after a hyperpolarizing current pulse or an afferent-evoked IPSP. In 70% of manipulated neurons, synaptically evoked rebound depolarizations were reduced, but not eliminated, by glutamate receptor antagonists. The glycine receptor antagonist strychnine did eliminate the IPSP-associated depolarization in these neurons. Collectively, these results suggest that functional denervation of excitatory afferents decreases their synaptic efficacy as result of both cell loss as well as decreased strength of individual surviving synapses.

Developmental influence of glycinergic transmission: regulation of NMDA receptor-mediated EPSPs.

The influence of excitatory transmission on postsynaptic structure is well established in developing animals, but little is known about the role of synaptic inhibition. We addressed this issue in developing gerbils with two manipulations designed to decrease glycinergic transmission in an auditory nucleus, the lateral superior olive (LSO), before the onset of sound-evoked activity. First, contralateral cochlear ablation functionally denervated the glycinergic pathway from the medial nucleus of the trapezoid body (MNTB) to the LSO, while leaving the excitatory pathway intact. Second, continuous release of a glycine receptor antagonist, strychnine (SN), was used to decrease transmission. The strength of excitatory and inhibitory synapses was examined with whole-cell recordings from LSO neurons in a brain-slice preparation. The percentage of LSO neurons exhibiting MNTB-evoked IPSPs was reduced in both ablated and SN-treated animals. In those neurons displaying IPSPs, the amplitude was significantly reduced. This decrease was accompanied by an 8 mV depolarization in the IPSP equilibrium potential. In contrast, the ipsilaterally evoked EPSPs were of unusually long duration in experimental animals. These long-duration EPSPs were significantly shortened by hyperpolarizing the neuron to -90 mV or exposing them to aminophosphonopentanoic acid (AP-5), an NMDA receptor antagonist. Membrane hyperpolarization and AP-5 had little effect in control neurons. In addition, LSO neurons from ablated or SN-treated animals displayed broad rebound depolarizations after membrane hyperpolarization, and these were abolished in the presence of Ni2+. Because both cochlear ablation and SN-rearing were initiated before the onset of sound-evoked activity, the results suggest that spontaneous glycinergic transmission influences the development of postsynaptic properties, including the IPSP reversal potential, NMDA receptor function, and a Ca2+ conductance.

Synaptically evoked prolonged depolarizations in the developing auditory system.

1. Although synaptic transmission is known to influence many aspects of neuronal development, activity rates are quite low at early ages. The present study describes a long-lasting postsynaptic response to brief periods of synaptic stimulation that may underlie such an influence. Whole-cell patch clamp recordings were made from the lateral superior olive (LSO) in a brain slice preparation from early postnatal gerbils. 2. Stimulation of the excitatory afferent pathway from the cochlear nucleus elicited a prolonged depolarization (PD) in approximately 60% of the LSO neurons tested. Low frequency stimulation (1 Hz) was as effective as tetanic stimulation in producing PDs. These synaptically evoked depolarizations ranged in amplitude from 3 to 32 mV and recovered spontaneously after 0.5-35 min. 3. The LSO neuron input resistance declined during every PD episode and remained significantly lower even after the membrane potential had recovered. These PDs were partially reversed by 2 mM Ni(+2), but 1 microM tetrodotoxin and 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were ineffective. The metabotropic glutamate receptor agonist, trans-1-aminocyclopentane-1,3-dicarboxylic acid (40 microM), produced depolarizations that outlasted the exposure period by an average of 20 min and were also partially repolarized by 2 mM Ni(+2). In contrast, the depolarizations produced by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or N-methyl-D-aspartate decayed within a much shorter period of time. 4. To test whether in vivo discharge rates are, in fact, very low during development, spontaneous activity was recorded from neurons of the auditory midbrain in gerbils before and during the onset of sound-evoked responses. The average discharge rate of auditory neurons was quite low (X = 0.4 spikes/s), although many cells displayed brief periods of rapid discharge rate (X = 37 spikes/ s). Together, these results demonstrate a novel form of developmental plasticity elicited by low rates of glutamatergic transmission that may involve a metabotropic pathway and prolonged calcium influx.

Fine structure of the free-living parakeet pineal in relation to the breeding cycle.

Seasonal changes in the ultrastructure of the free-living Rose-Ringed Parakeet Psittacula krameri pineal were examined in relation to the sub-tropical environment and seasonal reproduction. Dark and light pinealocytes of the presumptive neuroendocrine cell line predominated, while supporting cells, ependymal cells, myelinated and non-myelinated nerve fibers with nerve endings, and regressed photoreceptor elements were also observed. Unlike in pineals of many animals, particularly mammals, the presence of dense-core vesicles (DCVs) with varying core density, and absence of clear vesicles and vacuoles with flocullent material, indicate the involvement of DCVs in the synthesis and secretion of pineals principle/s. In November (pre-breeding) when the day length registered a drop to LD 10:14, pinealocytes showed significantly decreased and smaller DCVs and mitochondria, nuclei with heterochromatin, and greater distribution of glycogen and lipid droplets, all indicating low pineal metabolic activity. During the shortest day regime from December to March, when the birds peaked breeding, the number and size of DCVs and mitochondria increased, and Golgi body-endoplasmic reticulum-lysosome complex (GERL) was very well defined. Images of DCVs suggested possible secretion of pineal principle/s by dissolution, and exocytosis. Coincidence of these features with peak gonadotrophic (circulating LH) and spermatogenic and testicular endocrine activity described previously suggested an active turnover of pineal products during this short day length regime when parakeets breed. In contrast, during the post-breeding season (April onwards), when the day-length increased to LD 13:11 and hypophyseal-gonadal function was down, nuclei and RER continued to show active profile, the Golgi body and associated complex were moderately seen, and the DCVs and mitochondria were significantly smaller and lesser. It is therefore probable that the pineal is an important relay to translate cues related to less drastic sub-tropical environmental change into DCV-linked neurohormones that in turn may be involved in modulating seasonal breeding in parakeets.

Sensory inhibition of flexion producing interneurons in lobster abdomen.

Mechanosensory stimulation of an abdominal swimmeret initiates a fictive extension which includes flexion inhibition. The role of flexion producing interneurons (FPIs) in this motor program was examined by recording from a pair of FPIs which excite flexor motor neuron f3. The afferent-FPI-f3 pathway includes at least 5 levels of neural activity. Since swimmeret stimulation hyperpolarizes the FPIs, and mechanosensory afferents are not known to be inhibitory, sensory interneurons must connect the afferent with the FPIs. The generalized receptive fields and long latencies of the FPI response support polysynaptic afferent-FPI connections. The poor correlation between f3FPI spikes and f3 EPSPs they evoke, and the 15 ms delay for initiation of these EPSPs, suggest that additional premotor interneurons are interposed between f3FPI and f3. Since restricted stimulation of swimmeret sensilla generates IPSPs in f3 without affecting f3FPI activity, f3FPI and f3 must be inhibited by different interneurons. Sensory evoked FPI inhibition contributes to the flexion inhibition component of the swimmeret evoked responses since hyperpolarization of f3FPI to block f3FPI spiking during spontaneous flexion activity decreases ongoing f3 spike discharge. Coupling between f3FPI and f3 activities during spontaneously initiated postural flexions supports this conclusion.

GABA mediated inhibition of abdominal postural flexion in lobster.

Swimmeret mechanostimulation initiates an abdominal extension program which includes flexion inhibition. Agonists and antagonists were used to examine the GABAergic nature of inhibitory responses recorded intracellularly from a flexion producing interneuron (FPI 303) and flexor motor neuron (f3) pair, and extracellularly from the other flexor efferents. The GABA antagonist picrotoxin (PTX) enhanced spontaneous flexion. As PTX levels increased, the swimmeret evoked response shifted from inhibition of flexion (less than 10 microM), to inhibition followed by excitation (10-30 microM), to flexion excitation (greater than or equal to 50 microM). The irreversibility of PTX effects, and the absence of bicuculline or baclofen induced changes in flexion activity, suggests that the receptors differ from mammalian GABA receptors. Both GABA and its agonist muscimol suppressed flexion activity and reduced intracellular potential amplitudes. Proof that PTX acts by binding the GABA receptor was obtained by observing that the addition of GABA or muscimol to preparations pretreated with PTX did not affect either spontaneous or swimmeret evoked activities, or intracellular potential amplitudes. These results imply involvement of GABAergic interneurons in the abdominal motor programs which inhibit flexion.

Environmental and hormonal factors in seasonal breeding in free-living male Indian rose-ringed parakeets (Psittacula krameri).

Seasonal changes in testicular activity, plasma luteinizing hormone (LH), estradiol (E2), testosterone (T), and 5 alpha-dihydrotestosterone (5 alpha-DHT) were related to pair bond formation, nest building, nest defense, and parental behavior in free-living Indian rose-ringed parakeets (Psittacula krameri) in northwest India. Spermatozoa production occurred between January and March when daylengths were short (10-12 hr) and ambient temperature was seasonally low (8-20 degrees C). At other times of the year the testes were regressed. Plasma LH levels increased during the prebreeding period (September-December) when the birds were forming pairs and selecting or defending nest sites. Plasma LH levels increased further between January and March and decreased to seasonal low values during the post breeding period between April and June when the birds were caring for young. Concentrations of plasma androgens and estrogens were similar during the prebreeding and postbreeding phases of the breeding cycle. During the breeding period, the ratios between plasma 5 alpha-DHT and testosterone and between plasma estradiol and testosterone increased. It is proposed that the absence of marked seasonal changes in plasma steroid levels is related to nest defense behavior which occurs during the prebreeding, breeding, and postbreeding phases of the breeding cycle. Winter breeding makes it possible for the parakeets to avoid competition with other birds for nesting sites, to avoid fledging young during the monsoon period, and to take advantage of the winter pea crop which provides the female with extra nutrients for egg production.

Environmental, dietary, and hormonal factors in the regulation of seasonal breeding in free-living female Indian rose-ringed parakeets (Psittacula krameri).

The roles of environmental, dietary, and hormonal factors in the timing of seasonal breeding were assessed in free-living female Indian rose-ringed parakeets, Psittacula krameri, in northwest India (22 degrees 2'N, 73 degrees E). The ovaries and oviducts began to enlarge in January, were fully developed in February, and began to regress in March. During this time there was no significant change in the concentration of plasma luteinizing hormone (LH) or estradiol. The concentration of plasma LH decreased (P less than 0.01) at the end of the breeding season. Pair bond formation occurred between September and December and was associated with an increase in levels of plasma LH but no change in plasma estradiol. Concentrations of plasma testosterone (T) and 5 alpha-dihydrotestosterone (5 alpha-DHT) did not vary significantly during the year and were similar to those in males except for higher values of 5 alpha-DHT and lower values of T during the pre- and postbreeding periods, respectively. The similar levels of plasma androgens in both sexes may be related to the equal roles that both sexes play in the defence of their nest holes. An analysis of crop sac contents showed that the birds fed chiefly on pigeon peas (Cajanus cajan) during the breeding season and on cereal grains at other times of the year. It is suggested that pigeon peas provide the extra nutrients, including calcium, required for egg production. Since pigeon peas ripen between November and March, the production of the crop may play a role in the timing of seasonal breeding. A further factor appears to be competition for nest sites. By breeding in winter, the parakeet avoids competing with other species which nest in holes.

Intersegmental modulation of abdominal postural responses initiated by mechanostimulation of the swimmeret in lobster.

In a multiganglionic preparation of the lobster abdominal nerve cord, composed of the first through fifth ganglia (A1-A5) and attached second swimmeret, tactile stimulation of the cuticular surface of the swimmeret initiates a postural motor program in A2 for abdominal extension, whereas deflection of feathered hair sensilla that fringe the swimmeret rami does not affect postural motor activity recorded from A2 (Kotak and Page, 1986a). This report demonstrates that partial isolation of A2 from adjacent abdominal ganglia by sectioning the A1-A2 or the A2-A3 connectives both increases the strength of the extension response evoked by cuticular stimulation and disinhibits a postural flexion inhibition response initiated by feathered hair stimulation. Complete isolation of A2, by cutting the A1-A2 and the A2-A3 connectives, further increases the strength of these postural responses. Intersegmental inhibition of these responses originates in the ganglia adjacent to A2, since mechanoresponsiveness of A2 is not affected by resection of a more distant connective (A3-A4). These results provide evidence for the presence in adjacent abdominal ganglia of intersegmental interneurons that regulate the access of swimmeret sensory activity to the postural motor neurons in A2.

Synaptic responses produced in lobster abdominal postural motor neurons by mechanical stimulation of the swimmeret.

1. Intracellular recordings were obtained from the somata of identified abdominal postural motor neurons in lobster to examine their subthreshold and suprathreshold responses to tactile stimulation of the swimmeret. 2. Pressure stimulation of the swimmeret surface evoked abdominal extension by producing tonic spiking in the extensor excitors and the synergistic flexor inhibitor (f5) and hyperpolarizing responses in the extensor inhibitor and antagonistic flexor excitors. These responses often continued for several seconds following the termination of the stimulus. The receptive fields of these motor responses extended over most of the swimmeret surface. 3. More localized tactile stimulation of the swimmeret surface elicited EPSPs in f5 and the extensor excitors, and IPSPs in the flexor excitors. The amplitude of these synaptic potentials decreased as the stimulus intensity was reduced. 4. Stimulation of feathered hair (both sexes) and smooth hair (female only) sensilla produced responses characteristic of extension whereas bristly spines on the male accessory lobe excited only two flexor excitors without affecting any of the other postural motor neurons. 5. Summed synaptic responses recorded from the motor neurons differed in their amplitudes and latencies according to the type of mechanoreceptor stimulated-cuticular receptors, feathered hairs or smooth hairs. Stimulation of the swimmeret cuticle produced the strongest responses (shortest latency, largest amplitude), while feathered hair stimulation initiated the weakest responses (longest latency, smallest amplitude). 6. The relatively long latencies (greater than 35 ms) and the complex form of the EPSPs and IPSPs indicate the involvement of multisynaptic interneuronal pathways in the reflex arcs.

Sexually dimorphic mechanosensitive swimmeret sensilla affect abdominal posture in the lobster.

The sensilla on the male and female second swimmerets are sexually dimorphic. Female swimmerets contain many long "smooth hairs" (long simple setae) on the coxa and rami. The endopodite of the male swimmeret has an accessory lobe covered with short "bristly spines" (serrate setae). In both sexes the swimmeret rami are lined by "feathered hairs" (plumose setae). The influence of mechanosensory stimulation of these sensilla upon abdominal tonic motor activity was analyzed in an in vitro swimmeret-nerve cord preparation. Movement of several clusters of smooth hairs produced an abdominal extension program by exciting the flexor inhibitor f5, inhibiting the flexor excitors, and activating several extensors. Stimulation of the male bristly spines excited the medium-sized flexor excitors f3 and f4. In both sexes the feathered hairs did not generate any response to mechanical stimulation. We infer that in nongravid females the smooth hairs are involved in receiving mechanosensitive cues to support abdominal extension. Bristly spines may contribute to postural adjustments that assist mating. The long latencies of these responses and their propagation to adjacent ganglia suggest that they are mediated by postural interneurons rather than by direct afferent terminations on postural motoneurons.

Tactile stimulation of the swimmeret alters motor programs for abdominal posture in the lobster Homarus americanus.

The influence of mechanosensory stimulation of a second segment swimmeret upon the abdominal postural program was examined in an isolated abdominal nerve cord-swimmeret preparation. The swimmeret was stimulated in several different ways to assess the extent of influence exerted on abdominal positioning. Localized tactile stimulation of the swimmeret surface with a mechanical probe usually generated flexion inhibition where the flexor inhibitor (f5) was activated while the small and medium flexor excitors were inhibited. Flexion inhibition was much stronger in females than males. In 50% of the animals a weak flexion excitation was seen. After 3-6 hours the response of one-third of these preparations changed to flexion inhibition. Strong manual stimulation of the swimmeret surface inhibited all of the flexor excitors (f1, f2, f3, f4, and f6) while exciting the inhibitor f5 and increasing extensor activity. Similar extension responses were observed in both sexes. Repeated tactile stimulation of the swimmeret surface elicited a response similar to that evoked during manual stimulation. The strongest extension response was produced at 2 Hz which falls within the normal range of swimmeret beating in intact lobsters. Similar extension responses were also obtained during spontaneous swimmeret beating and rhythmic manual movement of the swimmeret.(ABSTRACT TRUNCATED AT 250 WORDS)

Variations in ascorbic acid concentrations in the liver, gonads, kidney, and blood serum of feral blue rock pigeon (Columba livia gmelin) during the breeding and nonbreeding seasons.

To elucidate the possible involvement of ascorbic acid in gonadal steroidogenesis and role of gonadal steroids on the vitamin's turnover, ascorbic acid levels were quantified in the liver, gonads, kidney, and blood serum in both sexes of the Indian feral blue rock pigeon (Columba livia gmelin) during the breeding (March to April) and nonbreeding (June to July) seasons. Low concentrations were observed during the breeding season and significantly higher cencentrations during the nonbreeding season. These changes are discussed in relation to the synthesis and distribution of ascorbic acid, gonadal steroidogenesis, and probable influence of gonadal function on ascorbic acid levels.