Gonadotropin-releasing hormone (GnRH) modulates auditory processing in the fish brain

Gonadotropin-releasing hormone 1 (GnRH1) neurons control reproductive activity, but GnRH2 and GnRH3 neurons have widespread projections and function as neuromodulators in the vertebrate brain. While these extra-hypothalamic GnRH forms function as olfactory and visual neuromodulators, their potential effect on processing of auditory information is unknown. To test the hypothesis that GnRH modulates the processing of auditory information in the brain, we used immunohistochemistry to determine seasonal variations in these neuropeptide systems, and in vivo single-neuron recordings to identify neuromodulation in the midbrain torus semicircularis of the soniferous damselfish Abudefduf abdominalis. Our results show abundant GnRH-immunoreactive (-ir) axons in auditory processing regions of the midbrain and hindbrain. The number of extra-hypothalamic GnRH somata and the density of GnRH-ir axons within the auditory torus semicircularis also varied across the year, suggesting seasonal changes in GnRH influence of auditory processing. Exogenous application of GnRH (sGnRH and cGnRHII) caused a primarily inhibitory effect on auditory-evoked single neuron responses in the torus semicircularis. In the majority of neurons, GnRH caused a long-lasting decrease in spike rate in response to both tone bursts and playbacks of complex natural sounds. GnRH also decreased response latency and increased auditory thresholds in a frequency and stimulus type-dependent manner. To our knowledge, these results show for the first time in any vertebrate that GnRH can influence context-specific auditory processing in vivo in the brain, and may function to modulate seasonal auditory-mediated social behaviors.     

Tryptamine Induces Phosphoinositide Turnover and Modulates Adrenergic and Muscarinic Cholinergic Receptor Function in Cultured Cerebellar Granule Cells

Abstract: Tryptamine dose-dependently increased phosphoinositide (PI) hydrolysis by approximately fourfold in primary cultures of rat cerebellar granule cells (EC₅₀ = 56 µ M ). The PI response stimulated by tryptamine was dependent on the presence of extracellular Ca²⁺ and Na⁺. Tryptamine-induced PI breakdown could be partially inhibited by pretreatment with 4β-phorbol 12-myristate 13-acetate but not pertussis toxin. The presence of tryptamine markedly attenuated PI responses induced by norepinephrine (NE) and carbachol, with no apparent effect on the responses to 5-hydroxytryptamine and glutamate. The inhibition of NE- and carbachol-induced PI turnover by tryptamine was dose dependent with IC₅₀ values of ∼0.4 and ∼2.5 m M , respectively. Pretreatment of cells with tryptamine (0.5 m M ) also attenuated NE- and carbachol-induced PI turnover, but failed to affect 5-hydroxytryptamine- and glutamate-induced responses. Furthermore, ketanserin, atropine, and prazosin did not have any effect on inositol phosphate formation induced by tryptamine. These observations indicate that tryptamine markedly increased Ca²⁺- and Na⁺-dependent PI turnover in cerebellar neurons and selectively inhibited NE- and carbachol-induced PI hydrolysis.     

High-Affinity Uptake of Spermine by Slices of Rat Cerebral Cortex

Abstract: The accumulation of the polyamine spermine into 0.1-mm prisms of rat cerebral cortex has been investigated at both 37°C and at 4°C. Kinetic analysis of the temperature-sensitive portion of uptake indicates two high-aftinity saturable components together with an unsaturable component at high concentrations. The 'very high'– affinity saturable system ( K _m= 3.8 nM) was temperature- and sodium-dependent, and significantly reduced by metabolic inhibitors, findings that are consistent with an active transport system for spermine into brain tissue. The 'high'– affinity saturable component ( K _m= 0.44 μM) was sodium-dependent and inhibited by ouabain, but only partially susceptible to inhibition by 2,4-dinitrophenol and sodium cyanide. The significance of these results with respect to the function of spermine in the central nervous system is discussed.     

Distribution of serotonin-like immunoreactive neurons in the slug<Emphasis Type="Italic"> Limax valentianus</Emphasis>

Immunohistochemical techniques were used to study the distribution of serotonin-containing neurons in the nervous system of the slug Limax valentianus. Approximately 350 serotonin-like immunoreactive cell bodies were found in the central nervous system. These were located in the cerebral, pedal, visceral and right parietal ganglia. Most serotonin-like immunoreactive neurons had small cell bodies, which were aggregated into discrete clusters. A pair of previously identified metacerebral giant cells were found on the anterior side of the cerebral ganglion, and two additional pairs of uniquely identifiable, serotonin-like immunoreactive cells were found on the posterior side of the cerebral ganglion. The whole-mount maps of these stained neurons will be useful in further physiological and biochemical studies of olfactory learning at the cellular level in Limax valentianus.This study was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Science, Sports and Technology, Japan (nos. 12307053 and 13771353)     

Serotonin modulates responses to species-specific vocalizations in the inferior colliculus

Neuromodulators such as serotonin are capable of altering the neural processing of stimuli across many sensory modalities. In the inferior colliculus, a major midbrain auditory gateway, serotonin alters the way that individual neurons respond to simple tone bursts and linear frequency modulated sweeps. The effects of serotonin are complex, and vary among neurons. How serotonin transforms the responses to spectrotemporally complex sounds of the type normally heard in natural settings has been poorly examined. To explore this issue further, the effects of iontophoretically applied serotonin on the responses of individual inferior colliculus neurons to a variety of recorded species-specific vocalizations were examined. These experiments were performed in the Mexican free-tailed bat, a species that uses a rich repertoire of vocalizations for the purposes of communication as well as echolocation. Serotonin frequently changed the number of recorded calls that were capable of evoking a response from individual neurons, sometimes increasing (15% of serotonin-responsive neurons), but usually decreasing (62% of serotonin-responsive neurons), this number. A functional consequence of these serotonin-evoked changes would be to change the population response to species-specific vocalizations.     

Sensitivity of transformed (phasic to tonic) motor neurons to the neuromodulator 5-HT

Long-term adaptation resulting in a 'tonic-like' state can be induced in phasic motor neurons of the crayfish, Procambarus clarkii, by daily low-frequency stimulation [Lnenicka, G.A., Atwood, H.L., 1985b. Long-term facilitation and long-term adaptation at synapses of a crayfish phasic motoneuron. J. Neurobiol. 16, 97-110]. To test the hypothesis that motor neurons undergoing adaptation show increased responses to the neuromodulator serotonin (5-HT), phasic motor neurons innervating the deep abdominal extensor muscles of crayfish were stimulated at 2.5 Hz, 2 h/day, for 7 days. One day after cessation of conditioning, contralateral control and conditioned motor neurons of the same segment were stimulated at 1 Hz and the induced excitatory post-synaptic potentials (EPSPs) were recorded from DEL(1) muscle fibers innervated by each motor neuron type. Recordings were made in saline without and with 100 nM 5-HT. EPSP amplitudes were increased by 5-HT exposure in all cases. Conditioned muscles exposed to 5-HT showed a 2-fold higher percentage of increase in EPSP amplitude than did control muscles. Thus, the conditioned motor neurons behaved like intrinsically tonic motoneurons in their response to 5-HT. While these results show that long-term adaptation (LTA) extends to 5-HT neuromodulation, no phenotype switch could be detected in the postsynaptic muscle. Protein isoform profiles, including the myosin heavy chains, do not change after 1 week of conditioning their innervating motor neurons.     

Neuromuscular response of the immature rat of ACTH/MSH 4--10

The adrenocorticotropin fragment ACTH/MSH 4--10 (0.1 ug/kg IP) effectively modulates the neuromuscular responses of 9 to 15 day old rats. Muscle (extensor digitorum longus) contraction amplitude is increased, fatigue is delayed and muscle half-relaxation time is shortened during 20 min of continuous in situ stimulation of a branch of the deep peroneal nerve (square wave shocks 10 Hz, duration 0.5 msec, strength supermaximal). No effect on contraction time is seen. There is no facilitation or change in any contraction parameter in rats older than two weeks (16 to 40 days) indicating that these older animals, like normal adult rats, are unaffected by the peptide. Immature rats, however, are even more sensitive than hypophysectomized adult rats [29] to the ameliorative action of ACTH/MSH 4-10. This early sensitivity to ACTH/MSH 4--10 corresponds to important developmental changes occurring in nerve and muscle during the most critical period in postnatal development, the first two weeks.     

Flight and fight: a comparative view of the neurophysiology and genetics of honey bee defensive behavior

Honey bee nest defense involves guard bees that specialize in olfaction-based nestmate recognition and alarm-pheromone-mediated recruitment of nestmates to sting. Stinging is influenced by visual, tactile and olfactory stimuli. Both quantitative trait locus (QTL) mapping and behavioral studies point to guarding behavior as a key factor in colony stinging response. Results of reciprocal F1 crosses show that paternally inherited genes have a greater influence on colony stinging response than maternally inherited genes. The most active alarm pheromone component, isoamyl acetate (IAA) causes increased respiration and may induce stress analgesia in bees. IAA primes worker bees for 'fight or flight', possibly through actions of neuropeptides and/or biogenic amines. Studies of aggression in other species lead to an expectation that octopamine or 5-HT might play a role in honey bee defensive response. Genome sequence and QTL mapping identified 128 candidate genes for three regions known to influence defensive behavior. Comparative bioinformatics suggest possible roles of genes involved in neurogenesis and central nervous system (CNS) activity, and genes involved in sensory tuning through G-protein coupled receptors (GPCRs), such as an arrestin (AmArr4) and the metabotropic GABA(B) receptor (GABA-B-R1).     

Epidermal Growth Factor in Synaptosomal Fractions of Mouse Cerebral Cortex

Using a specific and sensitive epidermal growth factor radioimmunoassay (EGF-RIA) we measured EGF concentrations in whole brain, cerebral cortex, and cerebral cortical synaptosomal (pinched-off presynaptic nerve terminals) fractions of 26-day-old mouse brain. The relative EGF concentration in synaptosomal fractions was significantly greater than the growth factor concentrations in whole brain or cerebral cortex. Intracerebral injection, in an amount of EGF, several-fold greater than whole brain EGF content, did not appreciably increase synaptosomal EGF concentration, suggesting that no artifact was involved. The high synaptosomal EGF content suggests a neurotransmitter or a neuromodulator role for EGF in the CNS.