L‐, N‐ and T‐ but neither P‐ nor Q‐type Ca2+ Channels Control Vasopressin‐Induced Ca2+ Influx in Magnocellular Vasopressin Neurones Isolated from the Rat Supraoptic Nucleus
Authors: Sabatier Nancy , Richard Philippe , Dayanithi Govindan 
Published: September 2004

The role of voltage‐dependent Ca2+ channels during vasopressin and oxytocin actions on their respective neurones has been analysed by measuring intracellular Ca2+ concentration ([Ca2+]i) in individual, freshly dissociated magnocellular neurones from rat supraoptic nucleus (SO) using microspectrofluorimetry.

Pre‐incubation of vasopressin‐sensitive neurones with Cd2+ (100μM), a non‐discriminatory high‐voltage‐activated Ca2+ channel antagonist, or Ni2+ (50μM), a blocker of T‐type Ca2+ current, reduced [Ca2+]i responses by 77 and 19%, respectively. When Cd2+ was given together with Ni2+, the response was blocked by 92%. Similarly, when Ni2+ was pre‐incubated with Cd2+, the response was blocked by ∼84%.

Exposure of vasopressin‐sensitive neurones to a specific Ca2+ channel blocker, nicardipine (L‐type) reduced vasopressin responses by 48% at 1μM and 62% at 5μM. Similarly, ω‐conotoxin GVIA (ω‐CgTX, N‐type; 500 nm) inhibited the response by 46% with a stronger inhibition (75%) at 800 nm. By contrast, neither ω‐agatoxin IVA (ω‐Aga IVA; 300 nm), which blocks both P‐ and Q‐type channels, nor synthetic ω‐conotoxin MVIIC (ω‐MVIIC; 100 or 500 nm), a Q‐type blocker, affected vasopressin‐induced [Ca2+]i responses. These antagonists, given together (nicardipine 5μM +ω‐CgTX 800 nm+ω‐Aga IVA 300 nm), decreased vasopressin‐induced [Ca2+]i responses by 76 %.

In vasopressin‐sensitive neurones, the presence of both nicardipine and ω‐CgTX, reduced the K+‐evoked [Ca2+]i increase by 61 %. This blockade was increased by a further 21 % with ω‐Aga IVA, suggesting that N‐, L‐ and P‐type channels contribute to the depolarization‐induced [Ca2+]i rise. In addition, ω‐MVIIC alone reduced the K+‐evoked [Ca2+]i release by 24%. Also the remaining K+ responses were further reduced by 60% when pre‐incubated with L‐ N‐ and P‐type blockers, suggesting the involvement of Q‐type channels.

In oxytocin‐sensitive neurones, the peak amplitude of the [Ca2+]i response was not affected by Cd2+ alone, by combined Cd2+ and Ni2+, or by the mixture of nicardipine, ω‐CgTX and ω‐Aga IVA. By contrast, the responses evoked by depolarization with K+ were blocked by Cd2+. Both nicardipine and ω‐CgTX blocked 65% of K+ response and an additional block of ∼18% was obtained with ω‐Aga IVA, suggesting the involvement of L‐, N‐ and P‐type channels. In combination, these antagonists strongly inhibited (∼80% reduction) the K+ responses. Further reduction to 18% was made by the Q‐type blocker ω‐MVIIC. Pre‐incubation with L‐, N‐ and P‐type blockers caused an additional block of 71 %.

Some supraoptic neurones (5–10%) responded to both vasopressin and oxytocin, with only the [Ca2+]i responses induced by vasopressin blocked (> 90% inhibition) by the mixture of Ca2+ channel antagonists.

In conclusion, both vasopressin and oxytocin magnocellular SO neurones have been shown to express T‐, L‐, N‐, P‐, Q‐ and R‐type Ca2+ channels in their somata. Our results show that the vasopressin‐induced [Ca2+]i increase in vasopressin‐sensitive neurones is mediated by L‐, N‐and T‐type Ca2+ channels and not by P‐ and Q‐type channels; Ca2+ channels are not involved in oxytocin action on oxytocin‐sensitive neurones and L‐, N‐, P‐ and Q‐type channels control the K+‐induced [Ca2+]i increase in SO neurones.