Low-threshold heat receptor in chick sensory neurons is upregulated independently of nerve growth factor after nerve injury.

In mammals, the cloned low-threshold heat receptor, vanilloid receptor subtype 1 (VR1), is involved in the genesis of thermal hyperalgesia after inflammation. However, there is evidence that VR1 is not involved in the thermal hyperalgesia that occurs after nerve injury. In search for other heat receptors which might be involved in this phenomenon, we previously demonstrated that chick dorsal root ganglion neurons, which are insensitive to capsaicin, respond to low-threshold heat. Here, we investigated whether expression of the low-threshold noxious heat receptor in chicks is regulated by nerve growth factor (NGF), as VR1 is in mammals. Heat (44 degrees C) responsiveness of isolated dorsal root ganglion neurons of chicks was investigated (i) under culture conditions for up to 4 days with and without NGF and (ii) after a tight ligation of the sciatic nerve for up to 6 days, using cobalt-uptake method. In every case, a significant upregulation in the proportion of heat-responsive neurons was observed. On the molecular level, there was an increase of chick VR1 mRNA level in dorsal root ganglion cells cultured for 3 days in medium lacking NGF. In rat dorsal root ganglion neurons cultured for 1-4 days without NGF, patch-clamp experiments revealed that after 1 day almost all neurons responding to heat also responded to capsaicin, whereas after 3-4 days, more than one-half of the heat-responsive neurons did not respond to capsaicin. These data suggest the existence of low-threshold heat receptors in chick dorsal root ganglion neurons, the expression of which is regulated independently of NGF.

Low-threshold heat response antagonized by capsazepine in chick sensory neurons, which are capsaicin-insensitive.

The heat-transducing receptor VR1 cloned from rat sensory neurons can be activated by both noxious heat and capsaicin. As the response of sensory neurons to capsaicin is species dependent, it is conceivable that the responses to noxious heat and to capsaicin are transduced by distinct receptors across different species. Therefore, we investigated responses to noxious heat from a capsaicin-insensitive (chick) and a capsaicin-sensitive (rat) species. In chick, whole-cell patch-clamp experiments in isolated dorsal root ganglion neurons revealed two populations of neurons with different thresholds to noxious heat, activated at approximately 43 degrees C and approximately 53 degrees C. In cobalt uptake experiments, the proportion of neurons showing a heat-induced response increased with increasing heat stimuli. Application of capsaicin (1-10 microM) did not result in inward currents or cobalt uptake. Rat neurons yielded comparable results in heat experiments, but were capsaicin-sensitive. Although chick neurons are insensitive to capsaicin, the competitive capsaicin antagonist capsazepine (1-10 microM) was effective in blocking heat-induced responses, verified by patch-clamp and cobalt uptake methods. The noncompetitive capsaicin antagonist ruthenium red (10 microM) reduced to almost nil the proportion of heat-responsive neurons identified with the cobalt uptake method. These findings suggest that chick DRG neurons express a low-threshold heat-transducing receptor with a pharmacological profile distinct from the low-threshold heat receptor VR1 cloned from rat DRG neurons. The data support the idea that there might be heat receptor subtypes with differences in the capsaicin binding site.

Processing of sensory signals by a non-spiking neuron in the leech.

The non-spiking neurons 151 are present as bilateral pairs in each midbody ganglion of the leech nervous system and they are electrically coupled to several motorneurons. Intracellular recordings were used to investigate how these neurons process input from the mechanosensory P neurons in isolated ganglia. Induction of spike trains (15 Hz) in single P cells evoked responses that combined depolarizing and hyperpolarizing phases in cells 151. The phasic depolarizations, transmitted through spiking interneurons, reversed at around -20 mV. The hyperpolarization had two components, both reversing at around -65 mV, and which were inhibited by strychnine (10 micromol l(-1)). The faster component was transmitted through spiking interneurons and the slower component through a direct P-151 interaction. Short trains (<400 ms) of P cell spikes (15 Hz) evoked the phasic depolarizations superimposed on the hyperpolarization, while long spike trains (>500 ms) produced a succession of depolarizations that masked the hyperpolarizing phase. The amplitude and duration of the hyperpolarization reached their maximum at the initial spikes in a train, while the depolarizations persisted throughout the duration of the stimulus train. Both phases of the response were relatively unaffected by the spike frequency (5-25 Hz). The non-spiking neurons 151 processed the sensory signals in the temporal rather than in the amplitude domain.

Basal acetylcholine release in leech ganglia depolarizes neurons through receptors with a nicotinic binding site.

The response of Retzius neurons, the main neuronal source of serotonin in the leech nervous system, to cholinergic agonists has been extensively investigated. In this study, we analyzed the effects of inhibiting the acetylcholinesterase (AChE) activity in the leech midbody ganglion on the electrophysiological activity of the Retzius neurons. Bath application of neostigmine and physostigmine (0.1-100 mumol l-1) produced, after a delay, a strong depolarization of the Retzius neurons with a dose-dependent amplitude and latency. The amplitude of this depolarization increased as the extracellular level of Ca2+ increased and decreased as the extracellular level of Ca2+ decreased. The response to neostigmine and physostigmine was inhibited by curare (100 mumol l-1), nicotine (10 mumol l-1), atropine (100 mumol l-1) and strychnine (100 mumol l-1), but was not affected by mecamylamine (100 mumol l-1) or hexamethonium (100 mumol l-1). Superfusion with solutions containing 100 mumol l-1 strychnine or atropine produced a progressive hyperpolarization of the Retzius neurons, while superfusion with 100 mumol l-1 curare did not. The hyperpolarization induced by atropine was inhibited in the presence of curare. Other neurons in the ganglion showed distinctive responses to the AChE inhibitors that were coincident with their responses to cholinergic agonists. The results suggest the existence of a basal level of acetylcholine (ACh) release in the leech ganglion that is powerfully counteracted by endogenous AChE activity. Under control conditions, this basal release appears to be sufficient to generate an ACh tonus that regulates the membrane potential of Retzius neurons. Since these neurons can support a sustained firing rate, which is dependent on the membrane potential, the results presented in this report suggest that the basal ACh tonus regulates the output of these neuromodulatory serotonergic neurons.